U.S. patent application number 11/123985 was filed with the patent office on 2006-11-23 for system and method for electromagnetic navigation in the vicinity of a metal object.
Invention is credited to Chunwu Wu.
Application Number | 20060264732 11/123985 |
Document ID | / |
Family ID | 37101398 |
Filed Date | 2006-11-23 |
United States Patent
Application |
20060264732 |
Kind Code |
A1 |
Wu; Chunwu |
November 23, 2006 |
System and method for electromagnetic navigation in the vicinity of
a metal object
Abstract
A system and method for performing object localization based on
the emission of electromagnetic fields. The electromagnetic fields
are simultaneously emitted from different transmitters. One
electromagnetic field is emitted at a base frequency; the remaining
waves are emitted at frequencies that are harmonics of the base
frequency. The composite magnetic fields are measured by sensors.
The signal generated by each sensor is subject to a Fourier
analysis to determine the strengths of the individual
electromagnetic fields forming the composite electromagnetic field.
These individual measure field strength data are then used to
determine the position and orientation of the sensors relative to
the transmitters.
Inventors: |
Wu; Chunwu; (Kalamazoo,
MI) |
Correspondence
Address: |
INTEL. PROP./ RND;STRYKER CORPORATION
4100 EAST MILHAM AVE.
KALMAZOO
MI
49001-6197
US
|
Family ID: |
37101398 |
Appl. No.: |
11/123985 |
Filed: |
May 5, 2005 |
Current U.S.
Class: |
600/407 |
Current CPC
Class: |
A61B 2034/2051 20160201;
A61B 5/7257 20130101; A61B 90/36 20160201; A61B 34/20 20160201;
A61B 2090/397 20160201; A61B 5/062 20130101; A61B 5/06
20130101 |
Class at
Publication: |
600/407 |
International
Class: |
A61B 5/05 20060101
A61B005/05 |
Claims
1. A system for determining the position and orientation of an
object, said system comprising: at least two transmitters each said
transmitter capable of emitting a magnetic field, said transmitter
configured so that said transmitters emit navigation magnetic
fields at different frequencies, the frequencies being harmonic
frequencies of a base frequency; at lest two sensors, each said
sensor configured to simultaneously measure the navigation magnetic
fields emitted by said transmitters and to generate a sensor signal
reprehensive of a composite magnetic field measured by said sensor;
and a processor connected to said sensors to receive the sensor
signals and configured to, based on the sensor signals, compute
position and orientation data for said sensors relative to said
transmitters.
2. The system of claim 1, wherein said transmitters are configured
to emit magnetic fields at frequencies of 1,500 Hz or less.
3. The system of claim 1, wherein said processor is configured to:
for each sensor signal, perform a frequency component decomposition
process to determine the strengths of the individual magnetic
fields that comprise the magnetic field measured by said sensor
that generated the sensor signal; and based on the strengths of the
individual magnetic fields, compute position and orientation data
for said sensors relative to said transmitters.
4. The system of claim 3, wherein the frequency component
decomposition process said processor is configured to perform is a
Fourier transformation.
5. The system of claim 1, wherein there are said transmitter
assembly has three said transmitters.
6. The system of claim 1, wherein there are three said
receivers.
7. The system of claim 1, wherein: said transmitter assembly is
further configured so that at least one said transmitter emits a
surveillance magnetic field simultaneously with said navigation
magnetic field at a frequency less than the frequency at which the
navigation magnetic field is emitted said sensors measure the
surveillance magnetic field emitted with the navigation magnetic
field; said processor is further configured to: determine the field
strengths of the measured navigation magnetic field and the
surveillance magnetic field; based on the field strengths of the
measured magnetic field, generate data representative of corrected
navigation magnetic field strength measurements; and based on the
data representative of the corrected navigation magnetic field
strength measurements, compute the position and orientation data
for the said sensor relative to said transmitters.
8. The system of claim 1, wherein: one said transmitter emits
navigation magnetic fields at the base frequency; and the remaining
said transmitters emit navigation magnetic fields at harmonic
frequencies of the base frequency.
9. The system of claim 1, wherein: at least one of the harmonic
frequencies of the base frequency is a frequency at which
electromagnetic interference fields are present; and said
transmitters do not emit magnetic fields at the at least one
harmonic frequency of the base frequency at which magnetic
interference fields are present.
10. A system for determining the position and orientation of an
object, said system comprising: a transmitter assembly comprising
at least two transmitters, each said transmitter capable of
emitting magnetic fields, said transmitter assembly configured so
that: said transmitters simultaneously emit navigation magnetic
fields at different frequencies; at least one said transmitter
simultaneously emits at least one surveillance magnetic field, the
at least one surveillance magnetic field being emitted at a
frequency lower that the frequency of the navigation magnetic field
emitted by said at least one transmitter; at least two sensors,
each said sensor configured to simultaneously measure the magnetic
fields emitted by said transmitters and to generate a sensor signal
representative of the composite magnetic field emitted by said
sensor; and a processor connected to said sensors to receive the
sensor signals, said processor configured to: based on the sensor
signals, generate corrected navigation magnetic field strength
measurement data; and based on the corrected navigation magnetic
field strength measurement data, compute the position and
orientation of said sensors relative to said transmitters.
11. The system of claim 10, wherein said transmitter assembly is
further configured so that: emit the at least one surveillance
magnetic field at a base frequency; and emit the navigation
magnetic fields at harmonic frequencies of the base frequency.
12. The system of claim 10, wherein: said transmitter assembly is
further configured so that said at least one transmitter
simultaneously emits a plurality of surveillance magnetic fields at
different frequencies; and said processor, based on components of
said sensor signal representative of magnetic fields of the plural
simultaneously emitted surveillance magnetic fields, generates the
data representative of the corrected navigation magnetic field
strength measurements.
13. The system of claim 10, wherein: said transmitter assembly is
further configured so that said at least one transmitter
simultaneously emits a plurality of surveillance magnetic fields at
different frequencies; and said processor, based on components of
said sensor signal representative of magnetic field strengths of
the plural simultaneously emitted surveillance fields determines if
excessive spurious magnetic fields are present.
14. The system of claim 10, wherein said processor is further
configured to: based on the sensor signals, generate data
representative of the measured navigation magnetic field emitted by
said at least one transmitter and the measured surveillance
magnetic field simultaneously emitted by said at least one
transmitter; and based on the measured navigation magnetic field
data and the surveillance magnetic field data, determine if
excessive spurious magnetic fields are present.
15. A system for determining the position and orientation of an
object, said system comprising: a transmitter assembly comprising
at least two transmitters said transmitter assembly configured so
that: said transmitters simultaneously emit magnetic fields at
different frequencies; and at least one transmitter alternatingly
emits a low frequency magnetic field and a high frequency magnetic
field at least two sensors, each said sensor configured to
simultaneously measure the magnetic fields emitted by said
transmitters and to generate a sensor signal representative of a
composite magnetic field measured by said sensor; and a processor
connected to said sensors to receive said sensor signals and
configure to: based on the sensor signals, compute position and
orientation data for said sensors relative to said transmitters;
and based on the sensor signals representative of the low and high
frequency magnetic fields emitted by said at least one transmitter,
determine if spurious magnetic fields are present.
16. The system of claim 15, wherein said processor, based on the
sensor signals representative of the low and high frequency
magnetic fields emitted by said at least one transmitter, computes
the sensor position and orientation data.
17. The system of claim 15, wherein said transmitter is configured
so that: the low frequency magnetic field emitted by said at least
one transmitter is emitted at a base frequency; and the remaining
magnetic fields emitted by said transmitters are at frequencies
that are harmonics of said base frequency.
18. The system of claim 15, wherein, said processor does not use
the sensor signal based on the emitted low frequency magnetic field
to determine the sensor position and orientation data.
19. A method for determining the position or orientation of an
object, said method including the steps of: emitting a calibration
magnetic field pulse from a transmitter; measuring the strength of
the calibration magnetic field pulse over time with a sensor; based
on said measurement of the strength of the calibration magnetic
field pulse, determining a measurement start time; emitting a
navigation magnetic field pulse from the transmitter from a time
t.sub.NPS to a time t.sub.NPE; measuring the strength of the
navigation magnetic field pulse with the sensor from a measurement
start time t.sub.NPMS based on the determined measurement start
time to time t.sub.NPE, wherein time t.sub.NPS<time
t.sub.NPMS<time t.sub.NPE; based on said measuring of the
navigation magnetic field pulse, determining the strength of the
navigation magnetic field; and computing the position or
orientation of the object based on the determined strength of the
navigation magnetic field.
20. The method of claim 19, wherein the navigation magnetic field
pulse is emitted for a time less than the time the calibration
magnetic field pulse is emitted.
21. The method of claim 19, wherein, in said step of emitting a
navigation magnetic pulse, the navigation magnetic pulse is emitted
for a variable time as a function of the measurement start time
t.sub.NPMS.
22. A method of determining the position and orientation of an
object, said method including the steps of: emitting
electromagnetic fields from a plurality of transmitters; measuring
the strengths of the emitted electromagnetic fields with a
plurality of sensors; based on the strengths of the measured
electromagnetic fields, compute position and orientation data for
the sensors; and based on data generated as a result of said
computation of the position and orientation data, determine if
significant extraneous electromagnetic fields are present.
23. The method of claim 22, wherein: eigenvalues of a matrix based
on a measurements of the magnetic fields by said sensors are
generated; and in said step of determining if significant
extraneous electromagnetic fields are present, said determination
is made by comparing the eigenvalues to each other.
24. The method of claim 22, wherein: as part of said step of
computing position and orientation data for the sensors, a
rotational matrix that defines the orientation of said sensors is
generated; and in said step of determining if significant
extraneous magnetic fields are present, said determination is based
on an evaluation of the rotational matrix.
25. A method of determining the position and orientation of an
object, said method including the steps of: emitting
electromagnetic fields from a plurality of transmitters;
simultaneously measuring the strengths of the emitted
electromagnetic fields with a first set of a plurality of sensors
and with a second set of a plurality of sensors spaced from the
first set of sensors; based on the strengths of the measured
electromagnetic fields, compute position data and orientation data
for the for the first set of sensors; based on the strengths of the
measured electromagnetic fields, compute position data for the for
the second set of sensors and based on the position data for the
first and second sets of sensors, determine if significant
extraneous electromagnetic fields are present.
26. A method of determining the position and orientation of an
object, said method including the steps of: emitting
electromagnetic fields from a first set of a plurality of
transmitters; simultaneously with said emission of the magnetic
fields from the first set of transmitters, emit magnetic fields
from a second set of a plurality of transmitters spaced from the
first set of transmitters; simultaneously measuring the strengths
of the emitted electromagnetic fields with a plurality of sensors;
based on the strengths of the measured electromagnetic fields,
compute position and orientation data for the sensors, the position
and orientation data indicating the position and orientation of the
sensors relative to the first set of transmitters and relative to
the second set of sensors; and based on the position and
orientation data of said sensors relative to the first and second
sets of transmitters, determining if significant spurious
electromagnetic fields are present.
Description
FIELD OF THE INVENTION
[0001] This invention is generally related to a system and method
for determining the location of an object, such as the location of
an object in a surgical field. More specifically, this invention is
related to a system and method for determining the location of an
object using electromagnetic radiation and in an environment where
a metal object may be present.
BACKGROUND OF THE INVENTION
[0002] There are number of fields of human endeavor wherein it is
useful, if not necessary, to know precisely the location and
orientation of an object within a space. Surgery is one such field
in which this information is desirable. Surgical navigation systems
are available that enable medical personnel to know, with a high
degree of precession, the location and orientation of surgical
instrument or implant relative to a surgical site on the patient.
Often this information is used in surgical procedures to facilitate
the accurate removal and shaping of tissue. In an orthopedic
surgical procedure, the information provided by the surgical
navigation system ensures that an implant is precisely
positioned.
[0003] Surgical navigation systems and other position-locating
systems use different means to identify the locations and
orientations of the objects they track. A number of commercially
available surgical navigation systems rely on light tracking to
determine the position of the tracked object. Some systems for
include trackers that are attached to the objects being tracked.
Each tracker emits a number of light beams. Often light is emitted
in the infrared wavelengths. A static device, referred to as a
localizer, has light sensitive-receivers. Based on the locations
from which the individual light beams are received at a localizer,
a processor, also part of the system, determines both the position
and orientation of the tracker. Based on this information, the
position and orientation of the device attached to the tracker is
inferentially determined.
[0004] Often, at the start of a medical procedure, the position of
the patient's body tissue is mapped into a memory integral with the
processor. Based on these data and the inferential determination of
the tracked object, the surgical navigation system presents an
image on a display that indicates the position of the tracked
object relative to the body tissue. This allows a surgeon to
virtually "view" the position of the object that is otherwise be
concealed by overlying tissue.
[0005] In an orthopedic surgical procedure, a surgical navigation
system is also used to measure the range of motion of the body
limb(s) subject to the procedure. These measurement data facilitate
the fitting of the implant to the patient to increase the
likelihood of successful outcome of the procedure.
[0006] Light-based surgical navigation systems work reasonably well
for providing object location and orientation data in a surgical
setting. Nevertheless, there is a drawback associated with these
systems. A light-based navigation system requires a line-of-sight
between the light emitting components and the light-sensitive
localizer. If the line is broken, the ability of the system to
provide object position and location data may be interrupted. Thus,
medical personnel using such system must make a concerted effort to
keep their own body parts as well as other surgical devices from
entering into the space wherein such lines-of-sight may be
present.
[0007] If the breaking of a line-of-sight results in the
interruption of the generation of the object position and location
data, it may be necessary stop the procedure until the system can
again provide the data. Such delays reduce the overall efficiency
of the surgeon performing the procedure. Moreover, such delays can
increase the overall length of time it takes to perform the
procedure. This is counter to an objective of modern surgical
practice, to perform the procedure as quickly as possible. Surgeons
work to this goal to reduce the amount of time the patient is held
under anesthesia and his/her body is exposed and open to
infection.
[0008] Recently, there have been efforts to employ electromagnetic
field-sensing systems as surgical navigation systems. Generally,
this type of navigation system includes one or more transmitters
that emit electromagnetic fields. There is a sensor with one or
more antenna sensitive to the electromagnetic fields. To provide
both position and location information about an object, it is
typically necessary to transmit plural fields and monitor the
strength of each signal at plural antennae. Some of these
transmitters emit electromagnetic fields upon being energized by AC
drive signals. Others of these transmitters emit electromagnetic
fields upon being energized by DC pulse signals. Based on the
strength of the electromagnetic fields measured by the sensor, a
processor determines the position and orientation of the sensor
relative to the transmitter.
[0009] An electromagnetic navigation system does not require a
line-of-sight path between the transmitter and sensor. Thus a
surgeon could allow his/her arm to enter the space between the
system's transmitter and sensor without being concern that such
action will result in the interruption of the generation of the
object position and orientation-defining data.
[0010] Nevertheless, care must be taken when using an
electromagnetic navigation system, especially in a surgical
setting. This is because metal objects exposed to electromagnetic
waves from a first source, in turn, generate their own
electromagnetic waves. When ferrous metals, such cold rolled steel,
are exposed to magnetic waves, the metal itself becomes magnetized.
The metal, in turn, generates its own magnetic fields. This added
magnetic field is sensed by the sensor. This added magnetic field
thus introduces an error into the magnetic field measurements made
by the sensor.
[0011] Some metal, such as aluminum, copper, brass and 300 Series
stainless steel are non-ferrous. When this type of metal is exposed
to a changing magnetic field, a loop current, called an eddy
current, develops around the metal. The eddy current, which is
changes over time, generates its own magnetic field. This magnetic
field, like the magnetic field generated by a ferrous metal object,
can introduce an error into the magnetic field measurements made by
the sensor.
[0012] In surgery, it is often necessary to introduce one or more
metal instruments into the surgical in order to accomplish the
desired procedure. Many of these instruments have metal parts. For
the reasons discussed above, these instruments serve as sources of
supplemental magnetic fields that introduce errors into the
measurements made by the system sensor. These errors, in turn, can
result in the system generating position and orientation
information about the tracked object that may not be accurate. In a
surgical procedure, and most other procedures in which such
navigation is employed, such inaccuracies are wholly
unacceptable.
[0013] A number of proposed systems sense and/or correct for the
errors induced by the extraneous magnetic fields generated in the
environment wherein the tracking is performed by electromagnetic
field sensing. Some of these systems have transmitters that output
AC signals. Some of these systems have transmitters that generate
plural magnetic fields to each antenna. Systems wherein the
transmitter includes plural parallel-aligned antenna have also been
proposed. A disadvantage of many of these systems is that they
require their complementary processors to perform numerous
calculations in order to generate data representative of the
"adjusted", eddy current-effect free, strength of the sensed
magnetic fields.
[0014] Other proposed systems include providing the sensor unit
with a calibration sensor. These systems thus require the addition
of added component to device that it is desirable to keep as
compact as possible. Moreover, these systems similarly require
their processors to engage in numerous processing steps in order to
produce output data representative of adjusted strength of the
magnetic field.
[0015] Some of the proposed systems monitor the strength of the
magnetic fields generated due to the generation of DC pulse
currents. Some of these systems measure the magnetic field or the
integral of the change in the magnetic field,
.intg..differential.B/.differential.t, at a time after the magnetic
pulse is initially generated. The logic behind waiting this time
period to make the measurement is that effects of the eddy currents
will have attenuated to a nil level. One disadvantage of these
systems is that it delays when, during the signal processing cycle,
the magnetic field is measured. This delays when the processor is
able to determine object position and orientation. Also, given the
relatively long period in which the signal is emitted, these
systems can only provide updated sensor position and orientation
data at relatively slow frequencies.
[0016] Still others of these systems do not actually measure the
actual magnetic field, its rate of change or any related integrals.
These systems, instead, monitor the profile the strength magnetic
field generated as a consequence of the initial emission of the DC
pulse. Based on these measurements, a value representative of the
eddy current-free magnetic field is calculated. The logic behind
this process is that, since the effect of the eddy current
diminishes over time, the initial plot of field strength should, in
theory, serve as a basis for calculating the strength of the eddy
current free magnetic field. In practice, it has been found that
these calculations do not result in the determination of values
that accurately represent eddy current-free magnetic field
strength. Consequently, the accuracy of the object position and
location data produced from these adjusted magnetic field strength
data is open to question.
SUMMARY OF THE INVENTION
[0017] This invention is related to a new and useful system and
method for determining the position and orientation of an object by
using magnetic field sensing. The system and method of this
invention relatively quickly generates object location and
orientation data even in the presence of eddy current-induced
magnetic fields. The invention also provides a means for
determining whether or not extraneous magnetic fields, such as
those generated by ferrous metal objects, are present in the space
in which the object is being tracked. This notice makes it possible
to take the steps necessary to eliminate the presence of these
objects.
[0018] In one embodiment, the invention operates by simultaneously
transmitting AC signal-induced magnetic fields. The signals applied
to the transmitters that emit these fields are at a base frequency
and at frequencies that are harmonics of the base frequency. In
some versions of the invention, these signals are at low
frequencies, below 1,000 Hz. This minimizes, if not eliminates, the
effect of eddy current-induced magnetic fields.
[0019] In another version of the invention, the system
simultaneously emits two sets of magnetic fields. One set of fields
are emitted based on the application of relatively high frequency
drive signals. These fields are the navigation magnetic fields. The
second set of emitted fields is emitted based on lower frequency
drive signals. These fields are the surveillance magnetic fields.
In preferred versions of this embodiment of the invention, the
lowest frequency AC surveillance drive signal is the base signal
for both the surveillance and navigation drive signals. The
remaining surveillance and navigation drive signals are harmonics
of the base signal.
[0020] Based on the measured surveillance fields, the field
strength of the measured navigation magnetic fields are corrected
to compensate for the effects of the eddy current-induced fields.
The measured surveillance fields are also used to determine whether
or not the eddy-current induced magnetic fields are at an
unacceptable high level.
[0021] In another version of the invention, the strengths of the
navigation magnetic fields are, themselves, monitored to determine
whether or not the eddy current-induced signals have reached an
unacceptably high level.
[0022] Another embodiment of the system and method of this
invention emits DC pulse-induced magnetic fields. A period of each
magnetic field starting after the initial emission of the field is
measured. Based on the strength of the field during the measured
field, a value representative of the eddy current field-free
measurement of magnetic field strength is calculated.
[0023] Another embodiment of the system of this invention is the
invention has plural transmitters. The transmitters are in a fixed
spatial relationship. The sensor unit receives the magnetic fields
emitted by both transmitters. Based on the sensed magnetic fields,
the system is able to determine whether or not ferrous metal object
within space in which tracking is occurring affecting the ability
of the system to track.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] The invention is pointed out with particularity in the
claims. The above and further features and benefits of the
invention are described in the following detailed description taken
in conjunction with the accompanying drawings in which:
[0025] FIG. 1 is a diagrammatic illustration of the principles of
tracking an object based on magnetic field strength;
[0026] FIG. 1A is a perspective view of the components of a
surgical navigation system constructed in accordance with this
invention;
[0027] FIG. 2 is a block diagram of an transmitter assembly of the
system of this invention;
[0028] FIG. 3 is a block diagram of an idealized receiver assembly
of the system of this invention;
[0029] FIGS. 4A, 4B and 4C are plots of the base drive signals
applied to a first transmitter and two of its harmonics which are
applied as drive signals to, respectively, the second and third
transmitters as drive signals;
[0030] FIG. 4D is a plot of the composite magnetic field sensed at
a single sensor of the sensor assembly;
[0031] FIG. 4E is a plot of results of a Fourier transformation of
the signal of FIG. 4E;
[0032] FIG. 5 is a flow chart of a basic method of object
localization of this invention when AC signal-based magnetic fields
are emitted;
[0033] FIGS. 6A and 6B are plots of the surveillance drive signals
that are simultaneously applied to each transmitter in order
according the second AC embodiment of this invention;
[0034] FIGS. 6C, 6D and 6E are plots of the three navigation drive
signals that are separately applied simultaneously to,
respectively, the first, second and third transmitters as
navigation drive signals;
[0035] FIG. 6F is a plot of the composite magnetic field sensed by
a single sensor of the second AC embodiment of this invention;
[0036] FIG. 6G is a plot of results of a Fourier transformation of
the signal of FIG. 6F;
[0037] FIG. 7 is a flow chart of the process steps executed
according to the second method of object localization of this
invention when AC signal-based magnetic fields are emitted;
[0038] FIG. 7A is a flow chart of an alternative method of
determining the error matrix employed in the process of FIG. 7;
[0039] FIG. 8 is a graphical representation of the strength of eddy
current-induced magnetic fields as function of frequency of the
transmitter inducing the generation of the eddy current;
[0040] FIG. 9 is a block diagram of an alternative transmitter
assembly of AC signal magnetic field navigation system of this
invention;
[0041] FIG. 10 is a timing diagram illustrating the periods in
which the different navigation drive signals are applied to the
individual transmitters of the embodiment of the invention
illustrated in FIG. 9;
[0042] FIG. 11 is a flow chart of the process steps executed during
the operation of the system of this invention described with
respect to FIG. 9;
[0043] FIG. 12 is a graphical representation how, the strengths of
magnetic fields emitted by a common transmitter of the embodiment
of the invention illustrated in FIG. 9, over successive time
periods, vary as a function of the absence/presence of eddy
current-generating objects;
[0044] FIG. 13 is a plot of the strengths of magnetic fields
measured by a single sensor form three transmitters over time in a
DC-signal magnetic field navigation system of this invention;
[0045] FIG. 14 is a plot of the strength of single DC
pulse-generated magnetic field over time and, when, during the
period of the pulse, measurements of pulse strength are made
according to this invention;
[0046] FIG. 15 is a flow chart of the process steps executed by the
system of this invention to determine the strength of DC pulse
generated navigation magnetic fields when eddy current-induced
magnetic fields are also present;
[0047] FIG. 16A is a plot of the strength of the measured
navigation pulse over time when nominal eddy current-induced
magnetic fields are also present;
[0048] FIG. 16B is a plot of the strength of the measured
navigation pulse over time when essentially no eddy current-induced
magnetic fields are present;
[0049] FIG. 16C is the plot of the strength of the measured
navigation pulse over time when significant eddy current-induced
magnetic fields are present;
[0050] FIG. 17 is a flow chart of the process steps executed
according to one method of this invention to determine if spurious
electromagnetic fields are present;
[0051] FIG. 18 is a diagrammatic illustrate of a system of this
invention that determines if spurious electromagnetic fields are
present;
[0052] FIG. 19 is a block diagram of the sensor assembly of FIG.
18;
[0053] FIG. 20 is a flow chart of the process steps executed by the
system of FIG. 18 to determine if spurious electromagnetic fields
are present;
[0054] FIG. 21 is a diagrammatic illustrate of a system of this
invention that determines if spurious electromagnetic fields are
present;
[0055] FIG. 22 is a block diagram of the transmitter assembly of
FIG. 21;
[0056] FIG. 23 is a flow chart of the process steps executed by the
system of FIG. 21 to determine if spurious electromagnetic fields
are present;
[0057] FIG. 24 is a block diagram of an alternative system of this
invention; and
[0058] FIG. 25 is a timing diagram illustrating a pattern for
interleaving a surveillance signal with the navigation signals
according to this invention.
DETAILED DESCRIPTION
I. Basics of Magnetic Field Sensing Navigation
[0059] FIG. 1 is a basic illustration of how a magnetic field
sensing system is employed to determine the position and
orientation of a tracked object. In the surgical field, such
systems are called surgical navigation systems. In others fields of
endeavor, these systems are referred to as object tracking systems,
position measurement systems or object localization systems.
Substantially all magnetic field sensing navigation systems,
including system 20 of this invention, operate according to the
basic principles now described.
[0060] Generally, the system 20 includes a transmitter assembly 22
and a sensor assembly 24. Transmitter assembly 22 includes three
transmitters, 26, 28 and 30 in the Figures. The transmitters 26,
28, and 30 are typically in the form of coils. The transmitters 26,
28, 30 are, ideally, mutually orthogonal relative to each other. In
preferred systems, including systems of this invention,
transmitters 26, 28 and 30 are centered on a common point, point 31
in the Figures. Mathematically, transmitters 26, 28 and 30 are
represented by vectors {right arrow over (T)}1, {right arrow over
(T)}2 and {right arrow over (T)}3, respectively. For mathematic
derivation of this invention, we define a transmitter coordinate
system 23 seen in FIG. 1A that has its origin at point 31 and its
x-, y- and z-axis align along the vectors {right arrow over (T)}1,
{right arrow over (T)}2 and {right arrow over (T)}3 of the
transmitters 26, 28 and 30, respectively.
[0061] Sensor assembly 24 includes three sensors 32, 34, and 36
capable of monitoring the magnetic fields generated by the
transmitter assembly 22. The individual sensors may be coils,
flexgate transducers, magnetorsisitive sensors, Hall effect sensors
or any other devices capable of providing precision measurements of
magnetic fields. The individual sensors 32, 34 and 36 are, ideally,
mutually orthogonal from each other. Ideally, especially in the
system 20 of this invention, the sensors are also centered on a
common point, point 38 in the drawings. Mathematically, sensors 32,
34 and 36 are represented by vectors {right arrow over (S)}1,
{right arrow over (S)}2 and {right arrow over (S)}3, respectively.
For mathematic derivation of this invention, we define a sensor
coordinate system 25 (FIG. 1A) that has its origin at point 38 and
its x'-, y'- and z'-axis align along the vectors {right arrow over
(S)}1, {right arrow over (S)}2 and {right arrow over (S)}3 of the
sensors 32, 34 and 36, respectively.
[0062] A typical navigation system 20 is constructed so that the
transmitter assembly 22 is at a relatively fixed location. Thus,
transmitters 26-30 are housed in a relatively static unit.
Transmitter coordinate system 23, defined with position and
orientations of transmitters 26-30, for reference purposes, is the
system localizer. Sensor assembly 24 is attached to the object the
position and orientation of which is to be tracked. In FIG. 1A,
this object is a medical instrument 17. The unit containing sensors
32-36 is often referred to as the tracker. The sensor coordinate
system 25, that define the position and orientations of sensors
32-36, for reference purposes, is the system tracker. For a typical
navigation system, one uses one localizer 23. There are several
trackers 25 (one shown). Each tracker 25 is used to track an
individual object so that system tracks multiple objects. Thus, in
the depicted system of FIG. 1A, the system is employed to determine
the position of a surgical tool, instrument or implant relative to
a surgical site on a patient 18.
[0063] Connectors, not shown, attach the transmitter assembly 22 to
drive circuitry (not shown in FIG. 1). The measurements of magnetic
field strength made by sensors 32-36 are applied to a sensor
processor 52 (FIG. 3). Typically, this signal transfer is by a
wireless RF connection. Other wireless signal transfer mechanisms
such as infra-red connections may be used to establish some or all
of these connections. Wired connections may also be employed.
[0064] In some alternative navigation systems, including systems of
this invention, the transmitter assembly 22 is attached to the
object to be tracked and the sensor assembly 24 is at a fixed
location.
[0065] A primary goal of the navigation process is to determine the
geometrical relationship between the transmitter coordinate system
(localizer) 23 and the sensor coordinate system (tracker) 25. This
makes it possible to transform position and orientation data in the
sensor coordinate system (tracker) 25 to the transmitter coordinate
system (localizer) 23, and vise verse. Using these transformations,
all the objects tracked are placed in a common coordinate system
even if the objects shift positions and orientations. The
transformation that converts the coordinates in sensor coordinate
system to that in transmitter coordinate system is represented by a
translation vector {right arrow over (x)} and a rotational matrix
R. Here the vector {right arrow over (x)} is the vector from point
31, the origin of transmitter coordinate system (localizer) 23 to
point 38, the origin of sensor coordinate system (tracker) 25. This
is also the coordinates of point 38 referred to transmitter
coordinate system. The rotational matrix R represents the rotation
that aligns the x-, y- and z-axes of transmitter coordinate system
(localizer) 23 to the x'-, y'- and z'-axes of sensor coordinate
system (tracker) 25. If we use '.sub.x, '.sub.y, '.sub.z to
represent the unit vectors of, respectively, the x'-, y'- and
z'-axes of the sensor coordinate system, referenced to transmitter
coordinate system, rotational matrix R can be written as: R = [ e ^
x ' , e ^ y ' , e ^ z ' ] = [ e xx ' e yx ' e zx ' e xy ' e yy ' e
zy ' e xz ' e yz ' e zz ' ] ( 1 ) ##EQU1## Each matrix element
e'.sub.ij is a component of the unit vector of sensor axis j
projected to transmitter axis i.
[0066] Transmitters 26, 28 and 30 generate separate magnetic
fields. The magnetic fields emitted by transmitters 26, 28 and 30
are represented as vectors {right arrow over (B)}1, {right arrow
over (B)}2 and {right arrow over (B)}3, respectively. If each
transmitter 26, 28 and 30 is considered to be a magnetic dipole,
the particular magnetic field {right arrow over (B)}i emitted by
transmitter {right arrow over (T)}i present at location {right
arrow over (x)} is defined by the equation: B -> .times. i = 2
.times. ( x ^ T -> .times. i ) .times. x ^ - Ti x -> 3 ( 2 )
##EQU2## Here, {circumflex over (x)} is the unit vector along
vector x. The ".circle-solid." operator is the vector dot product
operator. The magnetic fields produced by transmitters 26, 28 and
30 are now referred to as navigation magnetic fields. Thus, the
magnetic fields collectively present at point 38 are described by
magnetic field matrix Bm, where: B ' .times. m = [ B -> '
.times. 1 B -> ' .times. 2 B -> ' .times. 3 ] = [ B ' .times.
1 x B ' .times. 2 x B ' .times. 3 x B ' .times. 1 y B ' .times. 2 y
B ' .times. 3 y B ' .times. 1 z B ' .times. 2 z B ' .times. 3 z ] (
3 ) ##EQU3## The superscript apostrophe indicates that the magnetic
fields are normalized to account for differences in magnetic
strength of each transmitter.
[0067] Each sensor 32, 34 and 36, measures the strength of each of
the three magnetic fields. Thus, at a given location and
orientation, the following measurements are obtained: [ m 11 m 21 m
31 m 12 m 22 m 23 m 13 m 23 m 33 ] ##EQU4## where m.sub.ij is the
strength of the magnetic field based on the field emitted from
transmitter i measured at sensor j. The above matrix is the
measurement matrix M. Each measured value m.sub.ij of magnetic
field strength is based on the magnetic field {right arrow over
(B)}i being measured by sensor {right arrow over (S)}j according to
the following formula: m.sub.ij={right arrow over (B)}i*{right
arrow over (S)}j (4) Each sensor vector {right arrow over (S)}j
aligns with the associated axis j, unit vector '.sub.j, in the
sensor coordinate system.
[0068] Thus, the nine measured magnetic field values are used to
solve for six unknowns, the three variables representative of
position vector {right arrow over (x)} and the three variables
representative of the orientation of the sensor coordinate system
(tracker) 25 and, therefore, the object to be tracked, relative to
the transmitter coordinate system (localizer) 23.
[0069] Algorithms such as those provided in U.S. Pat. No.
4,287,809, Helmut-Mounted Sighting System, issued 8 Sep. 1981, U.S.
Pat. No. 4,314,251, Remote Object Position And Orientation Locator,
issued 2 Feb. 1982 and U.S. Pat. No. 4,945,305, Device For
Quantitatively Measuring The Relative Position And Orientation Of
Two Bodies In The Presence Of Metals Utilizing Direct Current
Magnetic Fields, issued 31 Jul. 1990 are employed to, based on the
magnetic field measurements, determine the position and orientation
of the sensor assembly 24. Each of the above-cited documents is
incorporated herein by reference. By induction, this leads to the
knowledge of the position and orientation of the object attached to
the sensor assembly. Often, especially with a surgical navigation
system, this information is presented on a display 50 (FIG. 3).
[0070] In brief, though, it can be appreciated that, given Equation
4 above, the elements of the measurement matrix M' can be
determined according to the relationship: M ' = [ m 11 ' m 21 ' m
31 ' m 12 ' m 22 ' m 23 ' m 13 ' m 23 ' m 33 ' ] = [ B -> '
.times. 1 B -> ' .times. 2 B -> ' .times. 3 ] [ e ^ x ' , e ^
y ' , e ^ z ' ] = B ' .times. m T R ( 5 ) ##EQU5## Here, the
apostrophe of matrix M' indicates that the measurements of the
matrix are normalized to account for differences in the
efficiencies of the transmitters 26-30 and the differences in the
sensitivities of the sensors 32-36. Superscript T denotes matrix
transpose. The "" operator indicates matrix multiplication. Thus,
Equation 5 states that the measurements of the magnetic fields by
all sensors 32-36 is related to the ideal dipole at the location of
point 38, the origin of the sensor coordinate system (tracker) 38
by the rotational matrix R.
[0071] Moreover, Equation 5 also means that a common matrix A can
be calculated from either the magnetic field measurements or the
ideal dipole magnetic fields oriented at point 38. Matrix A is
formed as:
A=M'M'.sup.T=B'm.sup.TR[B'm.sup.TR].sup.T=B'm.sup.TRR.sup.TB'm=B'm.sup.TB-
'm (6) Thus, matrix A is the product of the transpose of a
3.times.3 matrix and the matrix itself. Matrix A is therefore real
and symmetric. Accordingly, matrix A can be diagonalized by
calculating its eigenvalues and eigenvectors: X.sup.TAX=.lamda. (7)
Here, .lamda. is a diagonal matrix with the eigenvalues as its
diagonal terms. Matrix X is the orthogonal transformation matrix
formed with all the orthornormal eigenvectors.
[0072] The eigenvector for the largest eigenvalue is the unit
vector {circumflex over (x)} in Equation 2 along vector {right
arrow over (x)} from point 31 (the original of the localizer) to
point 38 (the origin of the tracker). The largest eigenvalue,
.lamda..sub.max, is related to vector {right arrow over (x)} as: x
-> = 2 .lamda. max 3 ( 8 ) ##EQU6## Thus, from the eigenvalues
and associated eigenvectors of matrix A, unit vector {circumflex
over (x)} and |{right arrow over (x)}| the length of vector {right
arrow over (x)} are known. This makes it possible to obtain vector
{right arrow over (x)} from: {right arrow over (x)}=|{right arrow
over (x)}|{circumflex over (x)} (9)
[0073] From the determination of vector {right arrow over (x)}, the
position of point 38 relative to point 31 is known. From knowledge
of vector {right arrow over (x)}, Equations 2 and 3 are used to
calculate matrix B'm. Equation 5 is then employed to calculate the
rotational matrix R. This provides the orientation of the sensor
coordinate system (tracker) 25 relative to the orientation of
transmitter coordinate system (localizer) 23.
[0074] It should be further recognized that part of the signal
processing includes normalizing the measurements of magnetic field
strength. The magnetic field strength measurements are first
normalized to account for differences in strength of the magnetic
fields emitted by the individual transmitters 26, 28 and 30. The
measured magnetic field strength signals are further normalized to
account for variations in sensitivity of the individual sensors 32,
34 and 36. These normalized measurements of magnetic field strength
are the measurements upon which the position vector {right arrow
over (x)} and rotational matrix R are calculated.
II. Basic AC-Generated Magnetic Field Sensing System and Method of
the Invention
[0075] FIG. 2 illustrates the components of system 20 of this
invention that cause transmitters 26, 28 and 30 to emit magnetic
fields. Three drivers 39, 40, and 41 connected to the individual
transmitters 26, 28, and 30, respectively. Each driver 39, 40 and
41 generates a specific frequency AC signal to the transmitter 26,
28 and 30, respectively, to which the driver is connected.
[0076] The frequencies of the AC signals generated by the drivers
39, 40 and 41 are different from each other and in a specific
relationship. More particularly, one of the drivers 39, 40 or 41
generates a signal at a base frequency. The remaining two drivers
generate signals at frequencies that are harmonics of the base
frequency. By way of example, driver 39 generates the signal at the
base frequency, as represented by FIG. 4A, and the base frequency
is 25 Hz. Driver 40 generates a signal at 50 Hz as represented by
FIG. 4B. Driver 41 generates a signal at 75 Hz as represented by
FIG. 4C. In the Figures, the signals emitted by drivers 39, 40 and
41 are shown as being in phase. This is for purposes of
illustration. In practice, emitting the signals in phase simplifies
some of the signal processing. However, there is no requirement
that the signals or the navigation magnetic fields be emitted in
phase.
[0077] To ensure that the signals generated by the drivers 39, 40
and 41 are in a harmonic relationship, a common frequency generator
generates the base sin waves that are, in turn, amplified by the
driver. In the illustrated version of the invention, this function
is performed by a transmitter controller 48. This controller may,
for example, be a DSP controller that generates three sin waves
with defined frequencies. In FIG. 2, two conductors are shown as
extending from the transmitter controller 48 to each driver 39-41.
Two conductors are shown to illustrate that, in some versions of
the invention, transmitter controller 48 in addition to supplying
each driver 39-41 with a base signal at a select frequency also
asserts control signals that regulate the actuation of the
driver.
[0078] The output signals generated by sensors 32, 34, and 36 of
sensor assembly 24 are applied to a common sensor processor 52,
also part of system 20, seen in FIG. 3. While not shown, it should
be appreciated that, prior to the application of each sensor signal
to processor 52, the signal may be subjected to DC filtering to
eliminate DC components, band pass filtering to eliminate noise and
digitized. Each signal may, prior to or after digitization, may
also be subjected to a variable gain amplification. This
amplification may be performed to compensate for differences in
sensitivities of the individual sensors 32, 34 and 36. Performing
this individual amplification may eliminate the need to normalize
the measurements of magnetic field strength otherwise performed to
compensate for differences in sensor sensitivity.
[0079] Sensor processor 52 is also the component of the system 20
that, based on the measured signals representative of magnetic
field strength, generates the position and orientation data for the
tracked object. This is the data presented on display 50. In some
versions of the invention, for example, the version depicted in
FIG. 1A, a single hardware unit or subassembly collectively
functions as transmitter controller 48 and sensor processor 52. In
versions of the invention wherein the transmitter controller 48 and
sensor processor 52 are separate components, these components are
connected together. Generally, the transmitter controller 48
forwards data to the sensor processor 52 indicating the type of and
when electromagnetic fields are emitted. Based on these data, the
sensor processor 52 performs the processing steps discussed
below.
[0080] In the basic system 20 and method of this invention, drivers
39, 40 and 41 simultaneously output the individual AC signals to
their respective transmitters 26, 28, and 30. Transmitters 26, 28
and 30 therefore simultaneously emit navigation magnetic fields;
each transmitter emits a navigation magnetic field at a distinct
frequency, step 42 in FIG. 5. Each sensor 32, 34 and 36
simultaneously monitors the strength of a composite electromagnetic
field that is the sum of the individual fields, step 43. FIG. 4D is
representative of a sensor output signal representative of
composite measured electromagnetic field strength made by one of
the sensors 32, 34 or 36.
[0081] The signal representative of the strength of the composite
measured magnetic field generated by each sensor 32, 34 and 36 is
forwarded to the sensor processor 52. Sensor processor 52, in turn,
performs a Fast Fourier Transformation (FFT) on each of composite
signal, step 44. The coefficients b.sub.ij of the signal harmonics
yielded in this transformation represent the strengths of the
individual navigation magnetic fields that form the composite
signal. FIG. 4E represents the coefficients generated as a result
of the Fourier transformation of the signal of FIG. 4D. Thus, by
way of example, the FFT of the signal produced by the sensor 34,
the y-axis signal, yields three coefficients. The individual first,
second and third component coefficients b.sub.1y, b.sub.2y and
b.sub.3y are representative of the measurements, m.sub.1y, m.sub.2y
and m.sub.3y, of the strengths of {right arrow over (B1)}, {right
arrow over (B2)} and {right arrow over (B3)}, the three magnetic
fields measured by sensor 34. Sensor processor 52 thus sets the
measurements of magnetic field strength m.sub.ij based on the
calculated b.sub.ij coefficients, step 45. Once these field
strength determinations are made, sensor processor 52, in step 46,
uses these data to execute the position and orientation-determining
algorithms to determine the position and orientation of the sensor
coordinate system (tracker) 25 relative to the transmitter
coordinate system (localizer) 23.
[0082] When the system 20 and the method of this invention is
employed, the FFT provides fast means for determining the strengths
of the three magnetic fields sensed at each sensor 32, 34 and 36.
Since the navigation magnetic fields are emitted at a base signal
and its harmonics, the FFT quickly executes and the resultant
coefficients are accurate representations of measured magnetic
field strength. This makes it possible to, after the sensor
measurements are made, to quickly and accurately determine the
position and orientation of the sensor coordinate system (tracker)
25.
[0083] System 20 and the method of this invention is further
arranged so that, in preferred versions of the highest frequency
drive signal applied to any individual transmitter in 1,000 Hz or
less. In more preferred versions of the invention, the highest
frequency drive signal is at 300 Hz or less. In still more
preferred versions, the highest frequency drive signal is 150 Hz or
less. The application of these low frequency (100 Hz to 1 kHz)
and/or very low frequency drive signals (100 Hz or less) to the
transmitters 26-30 is that the transmitters, in turn, generate
navigation magnetic fields at corresponding low or very low
frequencies.
[0084] The reason it is desirable to emit the navigation magnetic
fields at these frequencies is that the strength of the magnetic
fields, {right arrow over (B)}.sub.e, developed as a consequence of
the generation of eddy currents is directly proportional to the
frequency of the magnetic field(s) in the environment wherein the
eddy-current producing object is located. It has been determined
that, when the magnetic field is emitted at a frequency of 100 Hz
or less, the effects of the eddy current-induced magnetic field is
often negligible.
[0085] Thus, by operating system 20 so the magnetic fields are
emitted at low or very low frequencies, most ideally at 100 Hz and
below, the likelihood that the presence of any eddy current-induced
magnetic field generating object in the space where the tracking is
occurring, the "navigation" space, is often nil.
[0086] Another preferred method of operating system 20 is to
operate the system so that none of the drivers generate signals at
line voltage 60 Hz or at frequency of one of its harmonics. This
avoids having to remove from the magnetic field measurements,
magnetic field components emitted by electrical equipment into the
navigation space space. Thus, if system 20 operates wherein the
base frequency of drive signal generated by one of the drivers 39,
40 or 41 is 30 Hz, the remaining two drivers should output drive
signals at 90 and 150 Hz, respectively. For international
operation, i.e., operation outside of North America, signals should
not be generated at 50 Hz or harmonics of this frequency.
III. AC-Generated Magnetic Field Sensing System and Method Capable
of Eddy Current Monitoring and Correction
[0087] In one alternative embodiment of this invention, system 20
both monitors and corrects for magnetic fields that develop as a
consequence of the generation of eddy currents.
[0088] This version of the invention employs the two additional
drivers 54 and 56 illustrated in FIG. 2. Drivers 54 and 56 generate
AC surveillance drive signals. Drivers 39, 40 and 41 thus output
the AC navigation drive signals. One of the drivers 54 or 56
generates a constant frequency signal referred to as the base
signal. The remaining drivers, the drivers the generate the
navigation drive signals, drivers 39-41, and the remaining
surveillance signal driver 56 or 54, output signals at harmonics of
the base drive signal. The signal output by the second surveillance
signal driver is at a frequency lower than the frequencies of the
signals output by the navigation signal drivers 39-41.
[0089] The signals output by surveillance signal drivers 54 and 56,
are at power levels lower than those at which navigation signal
drivers 39-41 output the navigation signals. Typically, the power
level of the signals produced by the surveillance signal drivers 54
and 56 is between 1/3 to 1/2 of the power levels of the signals
output by the navigation signal drivers 39-41.
[0090] By way of example, in one version of the invention,
surveillance driver 56 outputs a base signal of 20 Hz. The second
surveillance driver, driver 54 outputs a second surveillance signal
at 40 Hz. The three navigation signal drivers, drivers 39, 40 and
41, output signals at, 80, 100 and 140 Hz, respectively. Per the
above discussion, drivers 39-41 do not output signals at 60 Hz, or
its first harmonic, 120 Hz.
[0091] The pair of surveillance drive signals is simultaneously
applied in sequence to each of the transmitters 26, 28, and 30. In
FIG. 2, the application of the surveillance signals to the drivers
is shown through a multiplexer 58. Transmitter controller 48
generates control signals to the multiplexer 58 to establish to
which transmitter 26, 28 or 30 the surveillance drive signals are
sent simultaneously.
[0092] During the operation of this version of system 20, the pair
of surveillance signals is simultaneously added to an individual
one of the navigation drive signals for a select period of time,
step 68 of FIG. 7. In some versions of the invention, this period
may be between 5 and 500 ms. In more specific versions of the
system 20, the surveillance signals are added for a period of 50
ms. The actual time period in which the surveillance drive signals
are added varies inversely with the frequency of the navigation
drive signals. After being added to a first one of the navigation
drive signals, the surveillance drive signals are, in sequence,
added to the second and third navigation drive signals before the
cycle repeats. Thus for a set period of time, the magnetic field
measured be each sensor 32, 34 and 36 is a composite of the three
navigation magnetic fields and the two surveillance magnetic
fields. This is seen by FIGS. 6A-6F. Here, FIG. 6A represents the
base surveillance magnetic field emitted (the 20 Hz signal) by a
particular transmitter 26, 28 or 30 when the lowest frequency
surveillance drive signal is applied to the transmitter. FIG. 6B
represents the second surveillance magnetic field emitted (the 40
Hz signal) by the same transmitter. FIGS. 6C, 6D and 6E, represent
the three navigation magnetic fields emitted, respectively by the
transmitters, 26, 28 and 30, at respectively, 80, 100 and 140
Hz.
[0093] Also in step 68, the remaining transmitters 26, 28 and 30
emit the navigation magnetic fields they normally emit.
[0094] As with the basic version of the invention, each sensor 32,
34 and 36 generates an output signal that is the composite of the
received emitted magnetic fields, step 69. The composite signal of
FIG. 6F is a composite of the two surveillance magnetic fields and
the three navigation magnetic fields. The FFT is used to decompose
the output composite sensor signals, step 70. FIG. 6G represents
the five coefficients produced by the FFT of the. Here, the
coefficients of the first and second components of the composite
signal represent, respectfully, the strengths of the surveillance
magnetic fields. The coefficients of the third, fourth and fifth
components represent the strengths of the three navigation magnetic
fields.
[0095] In step 71, a two-step process is employed to calibrate the
magnetic field measurements. This calibration, it is understood, is
performed in addition to any normalization employed to compensate
for differences in the strengths of the emitted navigation magnetic
fields and the sensitivities of the sensors. In a first sub-step,
both the navigation magnetic field strength signals, the Mi
measurements, and the surveillance magnetic field strength
measurements, n.sub.kj measurements, are adjusted to account for
the fact that the sensor assembly 24 responds differently when
exposed to fields with appreciably different frequencies. For the
navigation magnetic field measurements, k=4, 5, or 7; this is
representative of the fact that the navigation magnetic fields are
emitted at frequencies that are, respectively at the fourth, fifth
and seventh harmonics of the base frequency. For the surveillance
magnetic fields, k=1 or 2; this is representative of the fact that
the surveillance magnetic fields are emitted at the base frequency
(unity) or its first harmonic.
[0096] Calibrated measurement signals, m'.sub.kj and n'.sub.kj are
produced according to the following formulas: m'.sub.kj=m.sub.kj
c.sub.kj (10) [0097] where c.sub.kj is the calibration factor for
the field k=4, 5 or 7 (the navigation magnetic fields) at sensor j
at which the measurement is made; j=1 2 or 3. n'.sub.kj=n.sub.kj
c.sub.kj (11) [0098] where c.sub.kj is the calibration factor for
field k; k=1 or 2, the surveillance magnetic fields at sensor j at
which the measurement is made; j=1, 2 or 3. The c.sub.kj values for
both the navigation and surveillance magnetic fields are determined
empirically prior to use of the system 20.
[0099] A second calibration is then performed on the initially
calibrated surveillance magnetic field measurements. This
calibration adjusts for the fact that the strengths of the
surveillance magnetic fields are lower than that of the navigation
magnetic fields. This calibration produces a finally adjusted
surveillance magnetic field strength measurement n''.sub.kj
according to the formula: n''.sub.kj=n'.sub.kj d.sub.k (12) where
d.sub.k is the power adjust factor for the specific surveillance
field and is the invert of the ratio in the difference between in
power output from the surveillance signal driver and a "normalized"
navigation signal driver, k=1 or 2.
[0100] These calibrated measurements are used to produce
eddy-current corrected magnetic field strength measurements, step
72 of FIG. 7. These corrected measurements are produced by first
generating error values for the measurements. These error values
are generated for the navigation magnetic field emitted by each
transmitter based on the surveillance magnetic fields
simultaneously emitted from the transmitter. For example, the error
factors for the measurements based on the navigation magnetic field
emitted by transmitter 30 (here k=7) are generated according to the
following formula:
.delta..sub.7j=m'.sub.7j-[(n''.sub.1j+n''.sub.2j)/2] (13) [0101]
where .delta..sub.7j is the error value for the magnetic field
emitted by transmitter 30 at j=1, 2 or 3, j being the sensor at
which the measurement of field strength is made.
[0102] The individual error values are based on the following
relationship. As set forth above, at low frequencies the eddy
current-induced magnetic field effect is proportional to frequency,
{right arrow over (B)}.sub.e .varies. |.omega.| (14) where .omega.
is the frequency of the magnetic field to which the object that
produces eddy current is exposed. At low frequencies, especially
frequencies of 500 Hz and less, and more especially at frequencies
of 300 Hz or less, there is a linear relationship between the
strength of the measured eddy current magnetic field, and the
frequency of the field to which the eddy current-producing object
is stored.
[0103] Thus, when no eddy current-induced magnetic fields are
present, the calibrated measured strengths of the magnetic fields
remain constant with frequency. This is represented by line 80 of
FIG. 8. On line 80, the strengths of the two surveillance magnetic
fields and the navigation magnetic field (here the field emitted
from transmitter 30) are represented by points 81, 82 and 83. Given
the equal strengths of these magnetic fields at a given point, the
error value, .delta..sub.7j, is zero.
[0104] However, in a situation where an eddy current-producing
object is in or near the navigation space, the added eddy
current-induced magnetic field is present. The strength of the eddy
current-induced magnetic field, in the above-discussed frequency
range, linearly increases with frequency as represented by line 84
of FIG. 8. Line 84, it should be understood, represents the
composite strength of the emitted surveillance or navigation
magnetic field and the eddy current-induced magnetic field. The
strengths of the calibrated measured surveillance fields,
represented by points 85 and 86 on line 84, are less than strength
of the calibrated measured navigation field, represented by point
87.
[0105] However, the corrected measurements of strength of the
surveillance magnetic fields when the system is in this condition,
are very close to the field strength measurements when there is no
eddy current-induced magnetic field. (In FIG. 8, the differences
are exaggerated for purposes of illustration.) Therefore, for the
purposes of generating error values, the (n''.sub.kj+n''.sub.kj)/2
component of the error value generation equation represents the
strength of the magnetic field that would be measured if the eddy
current-induced magnetic field is not present. This value is an
average of strengths of the two surveillance magnetic fields. This
value is represented in FIG. 8 by line segment 79.
[0106] The error value .delta..sub.kj thus represents the
difference between the measured strength of the navigation magnetic
field and a value representative of the field strength in an eddy
current-free environment.
[0107] Consequently, nine error values .delta..sub.ij are obtained.
In step 73, these values are subtracted from the normalized
measurements of magnetic field strength: [ m 11 ' m 21 ' m 31 ' m
12 ' m 22 ' m 23 ' m 13 ' m 23 ' m 33 ' ] - [ .delta. 11 .delta. 21
.delta. 31 .delta. 12 .delta. 22 .delta. 32 .delta. 13 .delta. 23
.delta. 33 ] = [ m ECC 11 m ECC 21 m ECC 31 m ECC 12 m ECC 22 m ECC
32 m ECC 13 m ECC 23 m ECC 33 ] ( 15 ) ##EQU7## Here, a matrix
element m'.sub.ij is a calibrated measurement of magnetic field
strength. A matrix element m.sub.ECCij is an eddy current-corrected
measurement of navigation signal magnetic field strength of the
field generated by transmitter i as measured by sensor j. The
matrix containing the .delta..sub.ij values is referred to as the
error matrix, .DELTA.. The calibrated and eddy current corrected
measurements of navigation signal magnetic field strength, the
m.sub.ECCij measurements, are then used in step 76 to determine
position and orientation of the sensor coordinate system (tracker)
25. In FIG. 7, this process is represented by step 76. Steps 74 and
75 that occur between steps 73 and 76 are discussed below.
[0108] In executing the above process, it is not recommended the
n''.sub.1j and n''.sub.2j calibrated measurements of surveillance
magnetic field strength be extrapolated to a 0 Hz a theoretical
zero eddy current-induced magnetic field measurement. This is
because the system error is assumed to be .+-..delta. at the
surveillance frequencies. If the surveillance magnetic field
strength is extrapolated to 0 Hz, the error value becomes
.+-.3.delta.. Thus, the noise is amplified three times before there
is error correction.
[0109] This version of the system and method of this invention is
further configured to detect when, the strength of the eddy
current-induced magnetic field is so strong that the error
correction process cannot be employed to attenuate its effect.
Specifically, as seen by plot 88 of FIG. 8, there may be situations
when the strength of the eddy current-induced magnetic field is so
great at frequencies of interest, field strength increases very
rapidly with frequency. As seen by the right end of plot 88, at the
higher range of the frequencies-of-interest, the increase in field
strength with frequency may even shift from linear to exponential.
When this occurs, as represented by points 89 and 90 on curve 88,
the difference measured strength of the two surveillance magnetic
fields is significant. Also, given the non-linear change in eddy
current strength with frequency, represented by point 91 on plot
88, the error correction values are not able to accurately produce
eddy-current-corrected measures of navigation magnetic field
strength.
[0110] Therefore, in this version of system 20 of this invention,
sensor processor 52 generates a monitor value a, step 74 of FIG. 7.
Monitor value a is representative of strength of the eddy
current-induced magnetic field measured by the sensor 32, 34 or 36.
The monitor value is calculated according to the formula: .sigma. =
1 3 .times. j = 1 3 .times. ( n 1 .times. j ' - n 2 .times. j ' ) (
16 ) ##EQU8## Monitor value .sigma. is thus the average difference
in surveillance field strength over the three sensors 32, 34 and
36. In preferred versions of this embodiment of the invention, the
monitor value .sigma. is calculated each time error matrix .DELTA.
is calculated. The monitor value .sigma. is compared to a
predetermined threshold value .kappa., step 75. The .kappa. value
is typically a value 2 to 3 times the rms error of the system. This
is because the above-discussed method for correcting for the
effects of eddy current-induced magnetic fields can normally be
corrected if the error they cause is within 2 to 3 times the rms
measurement error of the system. When the system 20 of this
invention is employed as a surgical navigation system, these are
the type of errors that would occur by the introduction of routine
surgical tools into the space where the object is being tracked. If
monitor value .sigma. is in this range, step 76 is executed.
[0111] However, there may be instances when a large object around
which eddy currents form is introduced into the tracking space. In
a surgical environment, this could happen, for example, if an
aluminum tray is introduced into the space. In this circumstance,
given the magnitude of the eddy current-induced magnetic field, the
above process steps may not be able to correct for the changes in
measured field strength. In this circumstance, a will be above
.kappa.. Once this state is detected, sensor processor 52, in step
77, actuates an alarm 56, also part of system 20. This provides
notice to surgical personnel that an object causing unacceptably
high eddy currents is in the tracking space and needs to be
removed.
[0112] In some versions of this embodiment of the invention, the
process steps executed to determine the monitor value are executed
before the steps used to determine the error values .delta..sub.ij.
This provides individuals using the system fast notification that a
significant eddy current-generating object is within the navigation
space.
IV. Second Alternative AC-Generated Magnetic Field Sensing System
and Method for Eddy Current Correction
[0113] In a second alternative AC-generated magnetic field sensing
system and method of this invention, the same basic transmitter
assembly 22 and sensor assembly 24 described with regard to the
first alternative system is employed. Here the error values are
used to produce a virtual transmitter location and orientation data
that is adjusted for the effects of any eddy current-induced
magnetic field.
[0114] In this method, Equation 13 above is used to calculate the
error correction values for one particular transmitter i. The error
correction values are then placed into vector form as: .delta.
-> i = [ .delta. i .times. .times. 1 .delta. i .times. .times. 2
.delta. i .times. .times. 3 ] ( 17 ) ##EQU9##
[0115] Then, the, last calculated rotational matrix R is used to
transform vector {right arrow over (.delta.)}.sub.i from the sensor
coordinate system to the fixed transmitter coordinate system by the
formula: {right arrow over (.delta.)}'.sub.i=R{right arrow over
(.delta.)}.sub.i (18) In FIG. 7A, the generation of these
transmitter coordinate system error vectors is represented by step
92.
[0116] Vector {right arrow over (.delta.)}'.sub.i is then used to
form a row in the error correction matrix, step 93. The formation
of a matrix using the three vectors {right arrow over
(.delta.)}'.sub.i, i=1, 2 and 3, thus forms a transmitter
coordinate system error matrix, .DELTA.'.
[0117] Then, in a step 94, the transmitter coordinate error matrix
.DELTA.' is converted into a sensor coordinate error matrix .DELTA.
according to the following formula:
.DELTA.=[R.sup.T(.DELTA.').sup.T].sup.T (19) Here, R.sup.T is the
transpose of the rotational matrix R and (.DELTA.').sup.T is the
transpose of matrix .DELTA.'.
[0118] Once the error matrix A is calculated, in a step 95 it is
employed in Equation 15 above to determine the m.sub.ECCij values
from which the position and orientation of the sensor assembly 24
are next determined.
[0119] In one preferred version of this embodiment of the
invention, the surveillance magnetic fields are based on 20 and 40
Hz signals. The navigation magnetic fields are based on AC signals
at 80, 160 and 240 Hz. The surveillance drive signals are applied
as a pair of signals to each transmitter 26, 28 and 30 in sequence,
for periods of approximately 1/3 second.
[0120] In this embodiment of the invention, the above process steps
transpose the error matrix so that it is based on the fixed
transformer assembly 22. The process is based on the assumptions
that (1) from second-to-second the maximum movement of the origin
of the sensor coordinate system (tracker) 25 is typically only
several centimeters and (2) the change in eddy current-induced
magnetic field strength in the navigation space typically does not
changer over the distance of this second-by-second movement of the
sensor assembly 24.
[0121] However, the measured surveillance magnetic field strength
for any field emitted from transmitter i as measured by sensor j,
n.sub.ij.sup.x is based on the following dot product relationship:
n.sub.ij.sup.x={right arrow over (B)}.sub.i.sup.x .sub.j (20) Here,
{right arrow over (B)}.sub.i.sup.x is the surveillance magnetic
field x emitted from transmitter i; .sub.j is unit vector of the
sensor j at which the measurement of field strength is made. Thus,
rotation of tracker, which causes a significant shift in .sub.j,
results in a like significant change in n.sub.ij.sup.x. In a
surgical procedure, as well as other fields of human endeavor, it
is quite simple to rotate the sensor assembly 24 between 20 and
30.degree. within a second. Such rotation significantly changes the
error values .delta..sub.ij forming error matrix .DELTA..
[0122] To correct for this problem, the above steps are executed to
first transform the errors caused by the eddy current-induced
magnetic field into the fixed coordinate system of the point
(localizer) 31. This transformation produces the fixed coordinate
system error matrix .DELTA.'. Then, Equation 19 is employed using
the most recently calculated rotational matrix R to determine the
error matrix .DELTA. for the sensors. Once the elements of matrix
.DELTA. are generated, Equation 15 is executed to determine the
eddy current-corrected measurements, the me.sub.ECCij values, for
the navigation magnetic fields.
[0123] This version of the invention makes it possible to perform
AC-signal navigation using magnetic fields generated at relatively
high frequencies, here, starting at 80 Hz, and still correct for
the low strength eddy current-induced magnetic fields.
V. Third Alternative AC-Generated Magnetic Field Sensing System and
Method
[0124] FIG. 9 illustrates an alternative system 20a of this
invention. System 20a includes the previously described
transmitters 26-30, drivers 39-41 and transmitter controller 48.
Drivers 39, 40 and 41 are configured to generate AC signals at,
respectively, 20 Hz, 40 Hz and 80 Hz.
[0125] System 20a is further configured so that each driver 39, 40
and 41 is selectively connectable to each one of the transmitters
26, 28 and 30. In FIG. 9, this selective connection is shown by the
output signals of the drivers 39-41 functioning as input signals to
a cross switch 49. The output signals from cross switch 49 are
applied to the transmitters 26, 28 and 30. Cross switch 49 is
configured to simultaneously apply a separate one of the output
signals from each driver 39, 40 and 41 to each of the transmitters
26, 28 and 30. Control signals asserted by transmitter controller
48a to cross switch 49 regulate, at any given instant, to which
transmitter 26, 28 or 30, each driver 39, 40 and 41 outputs its
driver signals.
[0126] The sequence in which the drive signals are applied to each
transmitter 26, 28 and 30 is illustrated in the timing diagram of
FIG. 10. During three successive periods, n, n+1 and n+2, the
following are the frequencies of the navigation magnetic fields
emitted by transmitter 26: 20 Hz; 80 Hz; and 40 Hz. Simultaneously;
navigation magnetic fields at the following frequencies are emitted
from transmitter 28: 40 Hz; 20 Hz; and 80 Hz. During the n, n+1 and
n+2 time periods, the navigation magnetic fields are emitted by
transmitter 30 at the following frequencies: 80 Hz; 40 Hz; and 20
Hz. After period n+2, the sequence repeats.
[0127] Steps 97 and 99 of FIG. 11, represent the emission of
magnetic fields at times n and n+1, respectively, from transmitter
26. Also, in steps 97 and 99, the navigation magnetic fields are
simultaneously emitted from transmitters 28 and 30.
[0128] During any given time period, the strengths of the three
navigation magnetic fields emitted by transmitters 26, 28 and 30
are simultaneously measured by sensors 32, 34 and 36. In FIG. 11,
these steps for time periods n and n+1 are represented by,
respectively, steps 98 and 100. Again, the FFT is used to generate
data representative of the strengths of the individual magnetic
fields, (step not shown). Based on these data, sensor processor 52
generates the data representative of the position and orientation
of the sensor coordinate system (tracker) 25 relative to the
transmitter coordinate system (localizer) 23, step 102.
[0129] Prior to step 102, sensor processor 52, based on the data
representative of magnetic strength, determines whether or not an
excessive eddy current-induced magnetic field is present. Sensor
processor 52 makes this determination based on the proportional
relationship between eddy currents and magnetic fields as
illustrated by FIG. 12. Specifically, as seen by horizontal plot
114, in situations wherein no or very low eddy currents are
present, the change in measured strength of the navigation magnetic
field emitted from a single transmitter 26, 28 or 30 as measured by
a single sensor 32, 34 or 36 only nominally changes with
frequency.
[0130] Therefore, the difference in strength of the magnetic field
emitted by a single transmitter 26, 28 or 30 as measured by a given
sensor 32, 34 or 36 does not appreciably change between time n when
the transmitter emits a magnetic field at 20 Hz and time n+1 when
the field is emitted at 80 Hz. This is represented by points 116
and 118 on plot 114. Here, point 116 represents the measured
magnetic field strength when a 20 Hz magnetic field is emitted by
transmitter 26 as measured by sensor 32, 34 or 36 at time n. Point
118, represents the strength of the magnetic field emitted by the
same transmitter 26 as measured by the same sensor at time n+1,
when the transmitter emits the field at 80 Hz. The strengths of the
measured magnetic field at successive time periods are essentially
equal, m'.sub.ij(n).apprxeq.m'.sub.ij(n+1) (21) It should be
understood that determination test is between the magnetic field
strength measurements that have been normalized, the m'.sub.ij
values, for differences in field strength of the different
frequency signals emitted by the common transmitter 26, 28 or 30.
The m'.sub.ij measurements are also normalized for any differences
in sensitivity of the sensor 32, 34 or 36 due to the sensor's
measuring magnetic fields at different frequencies.
[0131] When an object that causes large eddy currents to appear is
introduced into or near the navigation space, the strength of the
eddy current-induced magnetic fields appreciably increases with
frequency of the navigation magnetic fields. In FIG. 12, this is
represented by diagonal plot 120. Thus, when measurements of the
magnetic fields emitted at 20 Hz and 80 HZ in successive time
periods from the same transmitter 26, 28 or 30 are measured with
the same sensor 32,. 34 or 36, the field strength measured in the
later period will not be equal to the field strength measured in
the initial period, m'.sub.ij(n).noteq.m'.sub.ij(n+1) (22) In FIG.
12, this is seen in the difference in measured navigation magnetic
field strength along line 120 at points 122 and 124. Point 122
represents the strength of the magnetic field emitted by
transmitter 26, at time n when the field is based on a 20 Hz drive
signal, as measured by a particular sensor 32, 34 or 36. Point 124
represents the strength of the magnetic field emitted by the same
transmitter 26 as measured by the same sensor at time n+1 when the
field is based on an 80 Hz drive signal. Here, owing to the added
appreciable eddy current-induced magnetic field, the strengths of
the fields emitted from the same transmitter are appreciably
different even though nothing other than their frequencies has
significantly changed.
[0132] When an object that induces significant eddy currents is
present, there may be phase differences between the navigation
magnetic field and the eddy current-induced magnetic field. Owing
to these phase differences there may be times when the strength of
the second measured magnetic field is appreciably less than that of
the first, low-frequency-emitted magnetic field. Thus, the
determination of whether or not such object is present is not based
on simply determining if the strength of the second measured
magnetic field is greater than the strength of the first measured
magnetic field. Instead, as indicated by Equations 21 and 22, the
relevant process is to determine whether or not the two successive
measured navigation magnetic fields are of approximately equal
strength.
[0133] Thus, sensor processor 52 determines the normalized
strengths of the successive 20 Hz and 80 Hz magnetic fields emitted
by a single one of the transmitters 26, 28 or 30 as measured by a
single one of the sensors 39, 40 and 41 are substantially
different. After these determinations are made, in step 101, sensor
processor 52 performs the determination of Equation 21. If the
measured field strengths are similar, the determination tests true,
sensor processor 52 considers the navigation space to be in a state
where no significant eddy current-producing object is present. The
navigation process, step 102, is executed. Alternatively, if the
determination tests false, sensor processor 52 recognizes the
navigation space as being in a state in which significant eddy
current-induced magnetic fields are present. Sensor processor 52
then, in step 103, actuates alarm 56 so that surgical personnel can
clear the space of the object-of-concern.
[0134] An advantage of some versions of the above system is that it
is does not require a driver or drivers to provide supplemental
surveillance magnetic fields. Moreover, this embodiment of the
invention uses the same normalized data representative of magnetic
strength that is used to determine object position and orientation
that to determine if a significant eddy current-producing object is
present. This reduces the data processing needed to make this
latter evaluation. This frees sensor processor 52 to perform other
data calaculations.
[0135] Moreover, in the system of this version of the invention,
the presence of the significant eddy current-producing object is
based on magnetic field strength measurements made by the same
sensor in successive time periods. In the event point (tracker) 38
is rapidly moved, there is little likelihood such movement could
result in a series of successive field strength measurements that
do not accurately indicate the absence/presence of an eddy
current-producing object.
[0136] In order for the above system to operating the navigation
magnetic fields should be emitted at frequencies of 500 Hz or less.
Preferably, the navigation magnetic fields should be emitted at
frequencies of 300 Hz or less. In the most preferred versions of
the invention, the navigation magnetic fields should be emitted at
frequencies of 150 Hz or less. For operation, the frequency
difference between the two magnetic fields should be an order of
magnitude of at least 1.5. For improved operation, the order of
magnitude difference in frequency should be at least 2. Above the
low frequency range, above 100 Hz, the order of magnitude
difference between the two frequencies should be even greater, at
least 3 if not 4.
VI. DC Pulse-Generated Magnetic Field Sensing System of the
Invention
[0137] A DC pulse-generated magnetic field sensing navigation
system and method of this invention is now explained by initial
reference to the components of FIG. 1 and the plot of FIG. 13. In a
DC pulse-generated system, the transmitter controller 48 causes
each driver 39, 40 and 41 to emit a short DC-pulse to the
associated transmitter 26, 28 and 30, respectively. The pulses are
emitted sequentially. The strength of the magnetic field emitted by
each transmitter 26, 28 and 30 is simultaneously measured by each
sensor 32, 34 and 36. FIG. 13 represents the relative strengths of
magnetic field measurements made by a single one of the sensors 32,
34 or 36 of the signals received from each of the transmitters 26,
28 and 30. The magnetic fields measured in the absence of eddy
current-generating objects in or adjacent the navigation space is
represented by three solid line square wave plots, 130, 132 and
134.
[0138] In practice, there is typically a pause of approximately 3
to 15 ms between the emission of one set of DC magnetic pulses and
the next set of pulses. For reasons that are apparent below, these
pulses are referred to as "navigation" magnetic pulses. The
relatively long gap between the emission of each set of navigation
magnetic pulses allows the sensor assembly 24 to take measurements
of the background (earth) magnetic field.
[0139] When the pulses are emitted, drivers 39, 40 and 41 are
actuated sequentially so that the navigation magnetic pulses are
likewise sequentially individually emitted from the transmitters
26, 28 and 30. Drivers 39-41 are actuated so that a DC magnetic
field is emitted individual from each transmitter 26-30 for a
period of approximately 1 to 10 ms. There is an approximately 1 to
10 ms quiet period between when the first and second transmitters
26 and 28 emit magnetic fields and between when the second and
third transmitters 28 and 30 emit magnetic fields.
[0140] If an object around which eddy currents form is present,
eddy currents form during DC emission of the pulses as a result of
changes in the magnetic field, .differential.{right arrow over
(B)}.sub.i/.differential.t.noteq.0, where {right arrow over
(B)}.sub.i is the magnetic field emitted as a result of the
actuation of transmitter i. Thus, during periods in which the
navigation magnetic fields are rising and falling, eddy currents
are present.
[0141] These eddy currents induce there own magnetic fields that
affect the strengths of the navigation magnetic fields measured by
the sensors 32-36. In FIG. 13, dotted line pairs 138 and 139, 140
and 141, and 142 and 143 associated with square wave plots 130, 132
and 134, respectively, represent the measurements of magnetic field
strength when an eddy current-producing object is present.
[0142] The system and method of this invention is based on the
principle that the strength of the DC pulse generated field at time
t, M(t), is calculated according to the following formula: M
.function. ( t ) = A + i = 1 N .times. Bi .times. .times. exp
.function. ( - C i .times. t ) ( 23 ) ##EQU10## Constant A is the
magnetic field strength of the navigation magnetic pulse. Each
B.sub.iexp(-C.sub.it) component is an individual one of the
components that collectively comprise the eddy current-induced
magnetic field.
[0143] In practice, it has been found that, only in the initial
period after a transmitter 26, 28 or 30 emits a navigation magnetic
pulse do the second and higher order B.sub.iexp(-C.sub.it) eddy
current-induced components appreciably contribute to the magnetic
field. The initial period in which these higher order components
significantly contribute to the measured magnetic field is
typically 1 ms or less. After this period, the contribution of
these components goes to zero. After this period, constant A and
the first order Bi exp(-C.sub.1t) eddy current-induced component
collectively provide an accurate estimate of magnetic field
strength, over time, of the navigation pulse.
[0144] Further, if a magnetic pulse is emitted for an extended
period of time, t.fwdarw..infin., the B.sub.iexp(-C.sub.1t) first
order eddy-current-induced magnetic field component goes to zero.
The magnetic field strength at the sensor 32, 34 or 36, thus
becomes based solely on constant A, the magnetic field strength of
the navigation magnetic pulse.
[0145] Graphically, these relationships are illustrated in FIG. 14.
Solid plot 148 represents the measurement of a magnetic field
emitted from a transmitter 26, 28 or 30 as measured over time by a
sensor 32, 34 or 36. Dashed line plot 149 represents what would be
expected as the measurement of a navigation pulse in an eddy
current free environment. (Propagation delays not illustrated.)
During an initial period from the start of field emission, time
t.sub.S in FIG. 14, to a time at point 150 on plot 148 the field
strength measurement increases rapidly with time. This is due to
the fact that, initially, the second order and higher eddy
current-induced magnetic fields significantly contribute to the
composite magnetic field measured by the sensor.
[0146] After the time beyond point 150, only the first order eddy
current-induced magnetic field and the navigation magnetic field
itself contribute to the measured magnetic field. Again, the first
order eddy current induced magnetic field decays exponentially over
time. Thus, as the time the pulse is emitted increases, the only
magnetic field measured is that of the navigation magnetic pulse.
This is seen by the merger of plot 148 into plot 149.
[0147] The system and method of this invention takes advantage of
the above relationships according to the process steps set forth in
FIG. 15. As indicated in this Figure, an initial step 152 of this
process is the emission of a DC pulse-generated magnetic field from
a transmitter 26, 28 or 30. (Implicit in step 152 is the actuation
of the appropriate driver 39, 40 or 41.) This pulse, referred to as
a calibration pulse, is between 5 and 12 msec in length. More
particularly, this pulse is between 7 and 11 msec in length. Thus,
it should be appreciated that calibration pulses are emitted for
time periods longer than those in which navigation magnetic pulses
are emitted.
[0148] Simultaneously with the emission of calibration magnetic
field, in step 153, measurements of field strength are made by each
sensor 32-36. In a situation wherein and eddy currents are present,
the strength of the pulse over time has the profile of solid plot
148 of FIG. 14. Time t.sub.S is the start time of the pulse. Time
t.sub.E is the time at which the pulse is no longer emitted.
Measurements of strength of the calibration pulse are not needed
after time t.sub.E.
[0149] Then, in step 156, using an exponential curve-fitting
function, the measurements of magnetic field strength of the
calibration plot from a time t.sub.CS to time t.sub.E are used to
define a curve according to the following formula:
m.sub.CP(t)=A.sub.Cp+B.sub.CP exp(-C.sub.Cpt) (24)
[0150] Here, the end result m(t) is the calculated measurement of
magnetic field strength at time t; A, B and C are the constant and
coefficients of the exponential equation that define m(t).
Subscript ".sub.CP" indicates that these values based on
measurements of magnetic field strength for the calibration
pulse.
[0151] As seen graphically in FIG. 14, the start time, time
t.sub.CS, for the measurements from which A.sub.CP, B.sub.CP, and
C.sub.Cp are determined is after the pulse start time t.sub.S.
Typically, this delay is at least 1 msec. More often, this delay is
1.5 msec or more. On plot 148, point 154 is the start point on the
plot from which the curve fitting process is used to determine
A.sub.CP, B.sub.CP and C.sub.Cp. Specifically the curve fitting
process is used on the data from point 154 to the data collected
through time t.sub.E.
[0152] The constant A.sub.CP and coefficients B.sub.CP and C.sub.CP
determined in step 156 do not accurately define the strength of the
magnetic field be measured from time t.sub.S to a time before time
t.sub.CP. This is seen in FIG. 14 by a dotted plot 157. Plot 157
represents the calculated values of constant A.sub.CP and
coefficients B.sub.CP and C.sub.Cp for the period before time
t.sub.CS. Plot 147 it can be seen actually starts, M(t)=0, at a
time before time t.sub.S. Only starting at the time represented by
point 150 does the calculated measurement of magnetic field
strength correspond to the actual measurement of magnetic field
strength. (From point 150 to point 154 plot 147 overlaps plot
148.)
[0153] Thus, in a step 158, sensor processor 52, based on the
constant A.sub.CP and coefficients B.sub.CP and C.sub.Cp, generates
an equivalent of plot 157. These data are generated to determine
the time when the calculated measurement of magnetic field strength
corresponds to the actual measurement of magnetic field strength.
In other words, in step 158, the actual measurement of field
strength over time and constant A.sub.CP and coefficients B.sub.CP
and C.sub.Cp are used to determine the time when the measurement at
point 150 occurs. This time is referred to as the measurement start
time, time t.sub.MS.
[0154] The above process is performed for each transmitter 26-30.
(FIG. 15 only illustrates the process for a single transmitter.) A
measurement start time t.sub.MS.sup.ij is thus determined for each
transmitter i at each sensor j.
[0155] In a step 165, an evaluation is made to determine whether or
not excessive eddy current-induced magnetic fields are present.
Alternative processes by which this evaluation is performed are
discussed below.
[0156] For each transmitter 26-30 there is thus a
t.sub.MS.sub.MAX.sup.i which is the longest of the three
measurement start times t.sub.MS.sup.ij for the three sensors
32-36. In a step 166, this measurement start time is
determined.
[0157] The system is then ready to begin tracking of point
(tracker) 38. In this process, the system emits from each
transmitter a DC pulse generated navigation magnetic field. Step
168 of FIG. 15 represents the emission of single one of these
navigation magnetic fields. The system emits the pulse for time
period t.sub.PP.sup.i for a particular transmitter i according to
the following formula: t.sub.PP.sup.i=t.sub.MS.sub.MAX.sup.i+MC
(25) Constant MC is the time period over which sensor processor 52
needs magnetic field strength measurements in order to generate the
constant and coefficients of the exponential curve of Equation 24.
The MC time period is typically 1 or 2 ms. In situations in which
the eddy current induced magnetic field is large, and thus the
measurement start time t.sub.MS.sub.MAX.sup.i is larger than 1 ms,
the MC time period can be increased to 3 ms or longer.
[0158] In a step 169, the strength of the emitted navigation
magnetic field over time is measured by each sensor 32-36. More
particularly, in step 169, sensor processor 52, records the
measurements of magnetic field strength that are obtained from time
t.sub.MS.sub.MAX.sup.i to time t.sub.NPE for the emitted navigation
pulse. Steps 169, and subsequent steps 172, 175 and 176, represent
the process steps associated with the measurements made by a single
one of the sensors 32, 34 or 36.
[0159] Plot 170, of FIG. 16a illustrates the field strength of the
navigation magnetic pulse emitted in step 168 over time. Time
t.sub.NPS is the pulse start of the emission. Time t.sub.NPMS is
the start time for when the magnetic field measurements are taken;
the time when step 169 starts. It should be understood that for
each transmitter i, time t.sub.NPMS.sup.i is the previously
determined time t.sub.MS.sub.MAX.sup.i. Time t.sub.NPE is the end
time of the emission of the pulse, the time when steps 168 and 169
both terminate. The period between time t.sub.NPS and time
t.sub.NPE is the pulse period time period t.sub.PP.sup.i for the
transmitter i.
[0160] In FIG. 16a, a section 173 of plot 170, from time t.sub.NPS
to time t.sub.NPMS, the period in which magnetic field measurements
are not taken, is shown as a dotted line. Section 175 of plot 170,
shown as a solid line from time t.sub.NPMS to time t.sub.NPE,
represents the actual measurements taken of the emitted pulse.
Dashed plot 171 represents the strength of the navigation magnetic
field if no eddy current-generating objects are present.
[0161] In step 172, sensor processor 52, using the measurements of
field strength from step 169 determine the constant and
coefficients of Equation 24 that define section 175 of plot 170
from time t.sub.NPMS to time t.sub.NPE. Step 172 thus yields the
equation: m(t)=A.sub.NP+B.sub.NP exp(-C.sub.NPt) (26)
[0162] The subscript ".sub.NP" denotes navigation magnetic pulse.
In this equation, constant A.sub.NP is equal to the eddy current
effect-free strength of the navigation pulse. In other words,
constant A.sub.NP is equal to the maximum height, the field
strength if the magnetic field represented by plot 171 was measured
at sensor j. Thus, in a step 174 each m.sub.ij is set to the
associated A.sub.NP.sup.ij constant. These m.sub.ij values are
employed in the object tracking algorithms employ to determine the
position and orientation of point (tracker) 38, step 176.
[0163] An advantage of the above arrangement is that it is not
necessary to emit navigation magnetic pulses for relatively long
time periods. The pulses need only be emitted for a sufficient time
to obtain enough data so that curve fitting functions can be used
to determine constants A.sub.NP and coefficients B.sub.NP and
C.sub.Np. This is because these are all the data that are required
to determine the apparent strength of the eddy current-effect free
navigation magnetic pulse. Graphically this is seen in FIG. 16a.
Dotted plot 180 represents the extension of plot 170 after time
t.sub.E. Plot 180 is obtained by plotting out of Equation 26 as
t.fwdarw..infin.. Here it seen that, when a constant DC pulse
magnetic field is generated, over time, the effects of eddy
currents drop to zero. Therefore as seen in this Figure, constant
A.sub.NP is equivalent to the eddy current effect-free measurement
of magnetic field.
[0164] In this version of the invention, the measurements from
which the eddy current effect-free measurements of navigation pulse
magnetic field strength are calculated are measurements from which
it is possible to accurately determine constant A.sub.NP. Since
constant A.sub.NP is equal to the measure of strength of the
navigation magnetic pulse, this means this field strength
measurement is accurately calculated.
[0165] Moreover, the system and method of this invention, does not
rely of field strength measurements taken after all effects of the
eddy current-induced magnetic fields have essentially dropped to
zero (at a virtual time t=.infin.). Instead, the system and method
of this invention relies on field strength measurements available
soon after time t.sub.NPS. This means that the periods of the
navigation pulses can be relatively short, in the nature of 4 msec
or less or, sometimes less than 3 msec or even 2 msec or less.
[0166] Another benefit is gained in this embodiment of the
invention of the invention by periodically determining updating
when magnetic field measurements can be used to determine the
strength of the eddy current effect-free navigation pulses, the
t.sub.NPMS.sup.i navigation pulse measurement start times. This is
seen in the comparison of plot 186 of FIG. 16B with plot 170 of
FIG. 16A. Plot 186 represents the measure of magnetic field
strength over time when there are essentially no eddy currents
generating objects present. Initial section 187 of plot 186, shown
as a dotted line, in comparison to section 173 of plot 170, has an
appreciably stepper slope. Therefore section 188 of plot 186, shown
as a solid line, starts at a time sooner to the pulse start time
t.sub.NPS the start time of section 175 of plot 170. In FIG. 16B,
dashed line plot 190 represents the strength of the eddy
current-effect free navigation magnetic pulse.
[0167] Thus, the measurement start time t.sub.NPMS for plot 186,
relative to the pulse start time t.sub.NPS, is sooner than the
measurement start time for plot 170. Per Equation 25 above, when
this no/low eddy current environment is present, the time periods
of the navigation pulses can be shortened. This means the overall
frequency of the pulses can be increased to increase the rate at
which the object localization position and orientation data are
provided.
[0168] Alternatively, there may be instances when excessively
strong eddy current-induced magnetic fields are present. This
situation is represented by measurement of magnetic field strength
over time of calibration plot 202 of FIG. 16C. Here dashed plot 204
represents the strength of the measured calibration pulse if eddy
current-induced magnetic fields are absent from the navigation
space. In this Figure, section 206 of plot 202, shown as a dashed
line, represents the field strength measurements when the emission
of the electromagnetic pulse causes significant eddy
current-induced magnetic fields to be generated. Section 206 of
plot 202 has, in comparison to initial section 173 of plot 170, a
relatively shallow slope. Section 208 of plot 202, shown as a solid
line, represents the measured strength of the calibration pulse
after the effects of the second order and higher eddy currents have
appreciably subsided.
[0169] From FIG. 16C it can be seen the point in time in which the
second order and higher eddy current subside may start before after
time t.sub.CS, the time at which the measurements taken in step 152
start.
[0170] In a calibration sequence, the execution of steps 156 and
162 determine if there is a relatively long period of time between
start time t.sub.S of the calibration pulse and the calculated
measurement time t.sub.MS. This period may be so long that it is
not recommended to perform the object tracking
[0171] In practice, in step 165 of the calibration process, sensor
processor 52 executes one of two algorithms to determine if such a
condition exists. In a first process, sensor processor 52, based on
the constant and coefficients calculated in step 156, generates a
calculated determination to magnetic field strength at the time
when the calibration pulse is terminated, time t.sub.E. This
calculated value is compared to the measured value. If the two
values are essentially equal, it is clear that eddy current-induced
magnetic fields do not effect the overall measurement of magnetic
field strength to such an extent that the field strength of the
navigation pulse cannot be accurately determined. The calibration
and navigation processes, starting with step 166, can continue.
[0172] However, if there is a significant difference in the
calculated determination of magnetic field strength at the end of
the magnetic pulse, typically the calculated value is less than the
measured value, the eddy current-induced magnetic fields are
appreciably affecting the ability of the system to generate
accurate determinations of navigation pulse magnetic field
strength. In such a situation, the only way it may be possible to
make such a determination accurately is to appreciably lengthen the
time between when the navigation pulse is emitted, and the measured
magnetic field strength data are used to generate the pulse
strength measurement. Therefore, in step 165, if sensor processor
52 determines there is a significant difference between the
measured and calculated determination of magnetic field strength at
the end of the pulse, alarm 56 is asserted, step 184.
[0173] Alternatively, in step 165, sensor processor 52 compares the
deduced measurement start time t.sub.MS to a cutoff start time
t.sub.CUTOFF. Time t.sub.CUTOFF is a time that, in combination with
time constant MC equals the maximum time period, time t.sub.PP, for
which a navigation pulse can be emitted. This time period is based
on the minimum frequency at which navigation pulses can be emitted
in order for the system to provide sufficiently updated object
position and orientation data.
[0174] If t.sub.MS.ltoreq.t.sub.CUTOFF, then sensor processor 52
determines the effects of any eddy current-induced magnetic fields
are not strong enough to affect the localization process. If
t.sub.MS>t.sub.CUTOFF, sensor processor 52 interprets the
environment as being one in which eddy current-induced magnetic
fields of sufficient strength are present to effect the
localization process. If this determination is made, sensor
processor 52 asserts alarm 56, step 184 is executed.
[0175] Thus, the system and method of this version of the invention
provide a means to correct for the presence of eddy currents, run
at improved efficiency levels when such currents are not present
and make a determination when excessive eddy currents that effect
localization are present.
VII. First System and Method for Determining the Presence of
Ferromagnetic Objects
[0176] System 20 and method of this invention is also capable of
determining if a ferromagnetic object is so close to the navigation
space that the object is affecting the ability of the system to
accurately perform the localization process.
[0177] In a first embodiment of this process of this invention, as
depicted by step 210 of FIG. 17, the eigenvalues of matrix A of
Equation 6 above are generated. As discussed above, these
eigenvalues and the eigenvector are generated to determine the
eigenvalue and eigenvector from which vector {right arrow over (x)}
is determined. Step 210 produces three eigenvalues .lamda..
[0178] In, in step 212, the largest of three eigenvalues,
.lamda..sub.max, is compared to the remaining eigenvalues,
.lamda..sub.2 and .lamda..sub.3 multiplied by four (4). If
.lamda..sub.max.apprxeq.4.lamda..sub.2 and
.lamda..sub.max.gtoreq.4.lamda..sub.3 then sensor processor 52
determines that there are no spurious ferromagnetic object-induced
magnetic fields in the navigation space.
[0179] The above determination is based on the fact that, if only
navigation magnetic fields are present, the magnetic field at the
position directly on the dipole axis is twice as large as the
magnetic field at positions perpendicular to the dipole. Since the
eigenvalues are proportional to the square of their corresponding
magnetic fields, this results in the largest eigenvalue having a
value that is as four times as the eigenvalues of the remaining two
axes. So, if the evaluations of step 212 test true, then
significant extraneous magnetic fields are not present.
[0180] Alternatively, if appreciable extraneous magnetic fields
from ferromagnetic objects or other objects are present, the
relationships test for in step 212 will test false. Thus, if the
evaluations of step 212 test false, sensor processor 52 recognizes
the environment in one in which the appreciable extraneous magnetic
fields are present. If this state exists, the sensor processor 52
asserts alarm 56, step 214. The surgical personnel thus have notice
that they are to identify and remove the ferromagnetic object in
order for the localization process to proceed.
[0181] In step 216 a second test of the first version of this
feature of the invention is performed to determine whether or not
ferromagnetic object-induced magnetic fields are present. In step
216 it is determined whether or not the transpose of the rotational
matrix R is also the inverse of the rotational matrix, R R T = [ 1
0 0 0 1 0 0 0 1 ] ##EQU11## The above determination tests true if
excessive spurious magnetic fields are not present. If the above
determination tests false, such fields are present. Therefore, in
the event the determination tests false, sensor processor 52
executes step 214 and asserts alarm 56.
[0182] The system and method of this embodiment of the invention
for determining whether or not ferromagnetic object-induced
magnetic fields are present does not require the addition of
hardware to the system. Also, the base information from which the
determination is made, the eigenvalues and the rotational matrix,
are information already generated as part of the object position
and orientation determining process. Thus, only a small amount of
processing time is required to perform the determation process of
this embodiment of the invention.
VIII. First Alternative System and Method for Determining the
Presence of Ferromagnetic Objects
[0183] FIGS. 18 and 19 illustrate the structural features of a
second embodiment of a system 20b and method of this invention for
determining if ferromagnetic objects are present. This version of
the invention includes the tracker 220 of FIG. 18. Internal to
tracker 220 are two sensor assemblies 24a and 24b. Sensor assembly
24a includes three sensors 32a, 34a and 36a that are arranged
orthogonal to each other and centered at point 38a. Mathematically,
sensors 32a, 34a and 36a are represented as vectors {right arrow
over (S)}1, {right arrow over (S)}2 and {right arrow over (S)}3,
respectively. The point 38a and the three sensor vectors {right
arrow over (S)}1, {right arrow over (S)}2 and {right arrow over
(S)}3 define a first sensor coordinate system 25a. Sensor assembly
24b includes three sensors 32b, 34b and 36b arranged orthogonal to
each other and centered at point 38b. Mathematically, sensors 32b,
34b and 36b are represented as vectors {right arrow over (S)}4,
{right arrow over (S)}5 and {right arrow over (S)}6, respectively.
The point 38b and the three sensor vectors {right arrow over (S)}4,
{right arrow over (S)}5 and {right arrow over (S)}6 define a second
sensor coordinate system 25b.
[0184] Sensor assemblies 24a and 24b are rigidly mounted to tracker
220 so that the geometrical relationship between the two sensor
coordinate systems is fixed. Specifically, the distance x.sub.0
from point 38a to point 38b is fixed and the rotational matrix
R.sub.0 that rotates the second sensor coordinate system 25b to
align with the first sensor coordinate system 25a is fixed. The
distance x.sub.0 and matrix R.sub.0 are measured in manufacture and
stored in the tracker 220 or sensor processor 52a. As seen by FIG.
19, each sensor 32a-36b outputs its sensor signal to the common
sensor processor 52a.
[0185] This embodiment of the invention includes the basic
transmitter assembly 24, drivers 39-41 and transmitter controller
48 of FIGS. 1 and 2.
[0186] The method by which the presence of ferromagnetic objects is
determined according to this invention is represented by the flow
chart of FIG. 20. In step 222, transmitter assembly 24 emits the
navigation magnetic fields. Either AC drive signals or DC pulse
signals can be applied in this step to cause the emission of the
navigation magnetic fields (pulses). In a step 223, the navigation
magnetic fields are received by the sensors 32a-36b; the sensor
signals are outputted to sensor processor 52a.
[0187] In step 224, based on the measurements of the navigation
magnetic fields made by sensor assembly 24a, sensor processor 52a
determines the position of point 38a relative to point 31, vector
{right arrow over (x)}.sub.a of FIG. 18. Here sensor processor 52a
also determines the rotational matrix, R.sub.a of the first sensor
coordinate system 25a. In step 226, based on the navigation
magnetic fields made by sensor assembly 24b, sensor processor 52a
determines the position of point 38b relative to point 31, vector
{right arrow over (x)}.sub.b of FIG. 18. Here sensor processor 52a
also determines the rotational matrix, R.sub.b, of the second
sensor coordinate system 25b.
[0188] From position vectors {right arrow over (x)}.sub.a and
{right arrow over (x)}.sub.b, in step 228, sensor processor then
produces a calculated measurement of the distance between the ends
of the vectors, .parallel.{right arrow over
(x)}.sub.0.sup.CALC.parallel.. In step 228, sensor processor also
calculates a virtual rotational matrix R'.sub.a=R.sub.0R.sub.b
using the stored R.sub.0 that rotates sensor coordinate system 25b
to align with sensor coordinate system 25a. In step 229, the
distance .parallel.{right arrow over (x)}.sub.0.sup.CALC.parallel.
is compared to the known and stored distance x.sub.0 between points
38a and 38b. This comparison is made to determine if:
.parallel.{right arrow over
(x)}.sub.0.sup.CALC.parallel..apprxeq.x.sub.0 If this determination
tests false, alarm 56 is asserted, step 232 is executed.
[0189] In step 230, the virtual rotational matrix R'.sub.a is
compared to the real rotational matrix, R.sub.a, of the first
sensor coordinate system: R'.sub.a.apprxeq.R.sub.a If the above
determination tests true, sensor processor 52a considers the
environment to be one in which extraneous electromagnetic waves are
not effecting the measurements of the navigation magnetic fields
made by sensors 32a-36c. If the determination tests false, step 232
is executed to assert the alarm 56.
[0190] The above test are based on the principle that, if the
environment is one in which extraneous electromagnetic waves are
not effecting the measurements of the navigation magnetic fields
made by the sensors, the position and orientation data generated
based upon the sensor measurements accurately indicates the
position and location of the object being tracked. Therefore, the
virtual determination of vector {right arrow over
(x)}.sub.0.sup.CALC and matrix R'.sub.a should be equal to the
actual values determined for these variables. It should be
understood that data from only one sensor assembly 24a or 24b are
needed to provide the object position and orientation data.
[0191] Alternatively, if significant extraneous electromagnetic
fields are present, including fields generated by ferromagnetic
objects, these fields effect the measurements made by the two
sensor assemblies 24a and 24b. The effects of these spurious
magnetic fields cause inaccurate determinations of the positions of
points 38a and 38b. In this situation, the distance
.parallel.{right arrow over (x)}.sub.0.sup.CALC.parallel. between
vectors {right arrow over (x)}.sup.A and {right arrow over
(x)}.sup.B will not equal the distance x.sub.0 between relative to
each other and centered around a reference point 31b.
Mathematically, transmitters 26b, 28b and 30b are represented as
vectors {right arrow over (T)}4, {right arrow over (T)}5 and {right
arrow over (T)}6, respectively. The point 31a and the three
transmitter vectors {right arrow over (T)}1, {right arrow over
(T)}2 and {right arrow over (T)}3 define a first transmitter
coordinate system 23a. Transmitter assembly 22b includes three
transmitters 26b, 28b and 30b arranged orthogonally relative to
each other and centered around a reference point 31b.
Mathematically, transmitters 26b, 28b and 30b are represented as
vectors {right arrow over (T)}4, {right arrow over (T)}5 and {right
arrow over (T)}6, respectively. The point 31b and the three
transmitter vectors {right arrow over (T)}4, {right arrow over
(T)}5 and {right arrow over (T)}6 define a second transmitter
coordinate system 23b.
[0192] The two transmitter assemblies 22a and 22b are rigidly
attached to each other. Therefore, the geometrical relationship
between the first and second transmitter coordinate systems 23a and
23b, respectively, is fixed. This means mathematically it is
possible to derive a coordinate transformation formula that
transforms the coordinates in the first transmitter coordinate
system 23a to the second transmitter coordinate system 23b. This
formula is defined by a translation vector {right arrow over
(x)}.sub.0 and rotational matrix R.sub.0. Here {right arrow over
(x)}.sub.0 is the vector from point 31a to point 31b, and R.sub.0
is the matrix rotates and aligns vectors {right arrow over (T)}1,
{right arrow over (T)}2 and {right arrow over (T)}3 to vectors
{right arrow over (T)}4, {right arrow over (T)}5 and {right arrow
over (T)}6. Once the translation vector {right arrow over
(x)}.sub.0 and rotational matrix R.sub.0 are determined, the
coordinates {right arrow over (x)}.sub.b of the second transmitter
coordinate system for any position can be transformed to the
coordinates {right arrow over (x)}'.sub.a of the first transmitter
coordinate system for the same position by: tracker points 38a and
38b. Similarly, the rotational transformation between rotational
matrices R.sub.a and R.sub.b of the two sensor coordinate systems
will not equal the rigid geometrical relationship between them.
Thus, if either of the above determinations of steps 229 and 230,
test false, sensor processor 52a recognizes the environment as
being one in which, due to the presence of significant extraneous
electromagnetic fields, accurate tracking cannot be performed. In
this event, sensor processor performs step 232 to actuate alarm
56.
[0193] This method of determining the presence of appreciable
spurious electromagnetic fields, including those generated by
ferromagnetic objects, provides an almost immediate indication if
such fields are present.
IX. Second Alternative System and Method for Determining the
Presence of Ferromagnetic Objects
[0194] FIGS. 21 and 22 illustrate a third embodiment of the system
and method of this invention that determines whether or not
significant ferromagnetic object-induced magnetic fields are
adversely affecting the navigation process. System 20c of this
version of the invention includes the localizer 240. Localizer 240
has two transmitter assemblies 22a and 22b. Transmitter assembly
22a includes three transmitters 26a, 28a and 30a arranged
orthogonally relative to each other and centered around a reference
point 31a. Mathematically, transmitters 26a, 28a and 30a are
represented as vectors {right arrow over (T)}1, {right arrow over
(T)}2 and {right arrow over (T)}3, respectively.
[0195] Transmitter assembly 22b includes three transmitters 26b,
28b and 30b arranged orthogonally {right arrow over
(x)}'.sub.a=R.sub.0{right arrow over (x)}.sub.b+{right arrow over
(x)}.sub.0 (27) Any rotation matrix R.sub.b of transmitter
coordinate system 23b can also be transformed to the rotation
matrix R'.sub.a of transmitter coordinate system 23a by:
R'.sub.a=R.sub.0R.sub.b (28) Here we use superscript "'" to
indicate that {right arrow over (x)}'.sub.a and R'.sub.a are
transformed values, not the directly measured values from the first
transmitter coordinate system 23a. Since the two transmitter
assemblies are rigidly attached once the localizer unit is made in
manufacture, we can directly measure the translation vector {right
arrow over (x)}.sub.0 and rotational matrix R.sub.0 or use a
calibration procedure to derive them. It is also possible to use an
in-field calibration procedure to update them in case the rigid
relationship between the two transmitter assemblies is changed due
to shipping or other factors. Nevertheless, one determines the
translation vector {right arrow over (x)}.sub.0 and rotational
matrix R.sub.0 prior to surgical navigation and stored them in the
localizer for use during surgery.
[0196] In the illustrated AC drive signal version of the invention
of FIG. 22, drive signals are applied to each transmitter 26a-30b
by individual drivers. Specifically, drivers 39a, 40a and 41a,
respectively, provide the AC drive signals to transmitters 26a, 28a
and 30a. Drivers 39b, 40b and 41b, respectively, provide the AC
drive signals to transmitters 26b, 28b and 30b. In this version of
the invention, drivers 39a-41b drive signals at separate
frequencies. For example, in one version of the invention, Table 1
lists the frequencies of the drive signals emitted by the drivers.
TABLE-US-00001 TABLE 1 Frequencies of Drive Signals Emitted By The
Drivers of FIG. 22. Driver 39a 20 Hz Driver 40a 40 Driver 41a 80
Driver 39b 100 Driver 40b 140 Driver 41c 160
[0197] Transmitter assemblies 22a and 22b are mounted to localizer
240 so that points 31a and 31b are a known fixed distance x.sub.0
from each other.
[0198] System 20c of this version of the invention also includes
the basic sensor assembly 24 described with respect to FIGS. 1 and
3.
[0199] The method by which system 20c determines whether or not
excessive spurious electromagnetic fields are present in the
navigation space is now described by reference to the flow chart of
FIG. 23. In a step 246, the localizer also retrieves the stored
translation vector {right arrow over (x)}.sub.0 and rotational
matrix R.sub.0 which defines the geometrical relationship between
transmitter coordinate systems 23a and 23b. If these data are
generated by transmitter controller 48c, they are forwarded to
sensor processor 52, step not shown.
[0200] In a step 248, navigation magnetic fields are simultaneously
emitted from each of the transmitters 26a-30b. The fields are
simultaneously emitted as a consequence of transmitter controller
48c simultaneously actuating drivers 39a-41b. Also in step 248,
sensors 32, 34 and 36 simultaneously measure the strengths of the
six emitted navigation magnetic fields.
[0201] In step 250, sensor processor 52 performs an FFT on each of
the signals generated by sensors 32-36. The FFT for each sensor
signal generates six coefficients. The first, second and third
order coefficients represent the strengths of the measured fields
emitted by the transmitters of transmitter assembly 22a. The
fourth, fifth and sixth order coefficients represent the strengths
of the measured fields emitted by the transmitters of transmitter
assembly 22b.
[0202] Based on the nine measurements of field strength of the
navigation magnetic fields emitted by transmitter assembly 24a, in
step 252, using method described in Section I, sensor processor 52a
generates the translation vector {right arrow over (x)}.sub.a and
rotational matrix R.sub.a of coordinate transformation from sensor
coordinate system 25 to transmitter coordinate system 23a. In step
253, based the nine measurements of field strength of the
navigation magnetic fields emitted by transmitter assembly 22b, the
sensor processor 52a, using the method of Section I, generates the
translation vector {right arrow over (x)}.sub.b and rotational
matrix R.sub.b of coordinate transformation from sensor coordinate
system 25 to transmitter coordinate system 23b. Note the
translation vectors {right arrow over (x)}.sub.a and {right arrow
over (x)}.sub.b as well as the rotational matrices R.sub.a and
R.sub.b are for the same sensor assembly at point 38, but referred
to two different transmitter assemblies.
[0203] In step 254, using Equation (27), a virtual vector {right
arrow over (x)}'.sub.a for point 31 in transmitter coordinate
system 23a is calculated from the coordinate {right arrow over
(x)}.sub.b of the same point 31 in transmitter coordinate system
23b. Also in step 254, using Equation (28), a virtual rotational
matrix R'.sub.a for sensor axes {right arrow over (S)}.sub.1,
{right arrow over (S)}.sub.2, and {right arrow over (S)}.sub.3 in
transmitter coordinate system 23a is calculated from the rotational
matrix R.sub.b. of the same vectors in transmitter coordinate
system 23b.
[0204] In step 256, the equivalency {right arrow over
(x)}'.sub.1.apprxeq.{right arrow over (x)}.sub.1 is tested. In step
258, the equivalency R'.sub.1.apprxeq.R.sub.1 is tested. If both
equivalencies test true, the sensor processor 52 recognizes the
environment as being one in which no spurious magnetic fields are
adversely effecting the accurate measurements or determinations of
magnetic field strength.
[0205] However, if in either step 256 or step 258, the equivalency
tests false, sensor processor 52 recognizes the environment as
being one in which spurious magnetic fields are adversely affecting
the ability of system 20c to accurately track an object. If this
determination is made, in step 260, sensor processor actuates alarm
56.
[0206] System 20c and the accompanying method of this version of
the invention is based on principle that, as long as excessive
spurious electromagnetic fields are not present, the virtual
determinations of vector {right arrow over (x)}.sub.a and
rotational matrix R.sub.a based on vectors {right arrow over
(x)}.sub.0 and {right arrow over (x)}.sub.b and matrices R.sub.0
and R.sub.b should equal the actual determinations of these data
because they represent the same position and rotation in a same
coordinate system. However, if excessive spurious magnetic fields
are present, they unequally effect the navigation magnetic field
strength measurements made by sensor assembly 24 of the fields
emitted by the two different transmitter assemblies 22a and 22b. If
this event occurs, the virtual determinations of vector {right
arrow over (x)}.sub.a and rotational matrix R.sub.a derived from
measurements of transmitter assembly 22b will not equal the
equivalent versions of these data based on the measurements made of
the transmitter assembly 22a navigation magnetic fields by sensor
assembly 24.
[0207] System 20c and the companion method of this embodiment of
the invention do not require the mounting of additional components
in the tracker. Thus, this version of the invention does not
require in increase in tracker size in order to provide an
indication of whether or not excessive spurious electromagnetic
fields are present. Also, given that many navigation systems are
provided with plural trackers, the instillation of a single pair of
transmitter assemblies, in comparison to installing plural pairs of
sensor assemblies, one pair in each tracker, minimizes both system
complexity and cost.
[0208] It should be appreciated that the process steps executed by
system 20c do not have to be emitted each time the system is cycled
through a object localization sequence. It is anticipated, the
method will be employed only occasionally, for example, once every
one to three minutes. Thus, providing system 20c and the
accompanying method does not appreciably add to the overall amount
of processing time required to perform object localization.
X. Alternative Versions
[0209] It should be appreciated that the above describes the basic
features of a number of different embodiments of the system and
method of this invention. Other versions of the invention are
possible.
[0210] For example, while in the preferred versions of the
invention, each transmitter assembly has three transmitters and
each sensor assembly has three sensors, this is not required in all
versions of the invention. At a minimum, the system and method of
this invention requires at least two transmitters and at least two
sensors. In versions of the invention with two transmitters,
ideally the sensor assembly should have three sensors. In versions
of the invention wherein the sensor assembly only has two sensors,
ideally the transmitter assembly should have three transmitters. It
should be appreciated the ability of these versions of the system
to produce object position and orientation data may be more limited
than in the three transmitter and three sensor versions.
[0211] Similarly, there is no absolute requirement that the plural
transmitters 26-30 be mutually orthogonal or centered around a
common point. There is no similar requirement with regard to the
sensors 32-36 forming the sensor assembly. However, additional
processing steps are then required to generate the object position
and orientation data. More specifically, in situations where the
transmitters and/or sensors are not co centered or orthogonal,
analytical solutions for rotational matrix R and vector {right
arrow over (x)} such as Equations 5 and 9 will not be possible.
However, it is still possible to solve for rotational matrix R and
vector {right arrow over (x)} by modeling the systems based on
physics and solving the non-linear equations by iterative
algorithms. Examples of such algorithms are the Levenberg-Marquardt
Method, simulated annealing or a genetic algorithm.
[0212] Likewise, there is no requirement that in the versions of
the invention wherein the AC generated magnetic fields are based on
harmonic drive signals, the drive signals and emitted fields be low
frequency signals, below 1,500 Hz. As discussed above, this feature
of the invention minimizes the effect that any eddy
current-generating objects may have on the localization process.
However, there may be environments wherein the invention is
employed where this matter is not a concern. In these environments,
emitting the navigation magnetic fields at higher frequencies can
result in a more frequent updating of the object localization
data.
[0213] Also, in versions of the invention wherein AC generated
magnetic fields are emitted that are in a harmonic relationship,
there is no requirement that one of the fields be emitted at the
base frequency. For example, one could construct a version of this
invention wherein AC magnetic navigation fields are emitted at
frequencies of 40, 80 and 100 Hz. In this version of the invention,
since the emitted signals are harmonics of the 20 Hz base signal,
the FFT is used to decompose the composite signal received by each
sensor 32, 34 and 36 to determine the strengths of the individual
navigation magnetic fields received by the sensor.
[0214] In versions of the invention wherein surveillance magnetic
fields are emitted simultaneously with the navigation magnetic
fields, there is no requirement that any or all of the emitted
fields be emitted at a base frequency and its harmonics. As
discussed above, emitting fields that have this relationship
reduces the processing required to determine the relative strengths
of the signals.
[0215] Similarly, there is no requirement that, in the methods
described with respect to FIGS. 9-11, all signals are emitted at a
base frequency and its harmonics. However, for reasons discussed
above, it is relatively simple to determine the strengths of the
navigation magnetic fields of these versions of the invention.
[0216] Further, there is no requirement that plural surveillance
magnetic fields always be emitted. In some versions of the
invention, it may be possible to emit a single low frequency
surveillance magnetic fields while the navigation magnetic fields
are emitted at higher frequencies. In this version of the invention
the error value .delta..sub.ij for the signal emitted by
transmitter i and received by sensor j is equal to:
.delta..sub.ij=m'.sub.ij-n''.sub.ij (29) Here, m'.sub.ij is the
corrected measurement of navigation magnetic field strength;
n''.sub.ij is the corrected measurement of surveillance magnetic
field strength.
[0217] In this version of the invention, the corrected measurement
of the strength of the surveillance magnetic field, n''.sub.ij is
also compared to the corrected measurement of navigation field
strength, m'.sub.ij. Specifically, a test is made for the following
determination: m'.sub.ij.apprxeq.n''.sub.ij
[0218] If the determination tests true, then ambient eddy
current-induced magnetic fields do not have a significant effect on
the measurements of the navigation magnetic fields. If the
determination tests false, the ambient eddy current-induced
magnetic fields are possibly affecting the ability of the system to
accurately determine the strength the true strengths of the
navigation magnetic fields.
[0219] Thus, it is implicit from the above discussion that the
magnetic fields measurements upon which the strength comparisons
are made may not just be two navigation magnetic fields or two
surveillance magnetic fields. In some versions of the invention,
the comparisons of signal strength to determine the magnitude of
eddy current-induced magnetic fields may be between a navigation
magnetic field and a surveillance magnetic field.
[0220] An advantage of these versions of the invention is that they
eliminate the need to provide one of the surveillance signal
drivers.
[0221] Alternatively, when a single surveillance driver is
provided, this version of the invention can be combined with one of
the other versions of the invention that provides a method for
monitoring the strength of extraneous electromagnetic fields. The
combined assembly, with a single surveillance driver, both corrects
for low strength eddy current-induced magnetic fields and provides
an indication of the presence of excessive spurious electromagnetic
fields that prevent accurate object localization.
[0222] Another combined version of the invention is illustrated by
FIG. 24. Here system 20d has the two transmitters 22a and 22b of
system 20c. System 20d also has the previously described drivers
39a-41b. A switch 270 connects the output drive signal generated by
driver 39b to any one of transmitters 26a, 28a, 30a or 26b. A
switch 272 connect the output drive signal generated by driver 40b
to any one of the transmitters 26a, 28a 30a or 28b. The states of
switches 270 and 272 are set by transmitter controller 48d.
Transmitter controller 48d also selectively sets the frequencies of
the drive signals emitted by drivers 39b and 40b.
[0223] System 20d of this version of the invention operates in two
modes. Initially, and then periodically throughout the tracking
process, system 20d operates in a check mode. In the check mode,
transmitter controller 48d causes the drivers to output the drive
signals identical to those set forth in Table 1. Switch 270 is set
so the output signal generated by driver 39b is applied to
transmitter 26b. Switch 270 is set so that output signal generated
by driver 40b is applied to transmitter 28b. The system performs
the evaluation described with respect to system 20c above to
determine whether or not electromagnetic fields that affect the
localization process are present.
[0224] System 20d also operates in a correction mode. In the
correction mode, transmitter controller 48d causes drivers 39a-41a
to output the same drive signals as in the check mode. Driver 39b
is set to output a drive signal at 20 Hz. Driver 40b is set to
output a drive signal at 40 Hz. In this mode, driver 41b is
deactivated. Switches 270 and 272 are set so the output signals
generated by drivers 39b and 40b are applied simultaneously and in
sequence to each of transmitters 26a, 28a and 30a.
[0225] Thus, in the correction mode, drivers 39b and 40b function
as surveillance signal drivers. Each transmitter 26a, 28a and 30a
periodically emits both navigation magnetic field and the
surveillance magnetic fields. Sensor processor 52 then performs the
processing steps necessary to produce the eddy current-corrected
measurements of navigation magnetic field strength.
[0226] System 20d both corrects for minor eddy currents and
provides an indication when significant extraneous electromagnetic
fields are present.
[0227] Alternative versions of system 20a of this invention are
also possible. Thus, there is no requirement that in all versions
of the invention the low frequency magnetic field employed as part
of the navigation process and to determine the presence of large
eddy current-induced magnetic fields always precede the high
frequency magnetic field. In some versions of the invention, the
sequence in which these fields are emitted may be reversed.
[0228] Likewise, it should be appreciated that, in this version of
the invention, data describing the difference in signal strength
may be filtered over time in order to determine whether or not
appreciable eddy current-induced magnetic fields are present. This
filtering prevents a single instance of a significant difference in
magnetic field strength over two successive time periods from being
interpreted as indicating that an eddy current-generating object is
nearby. Such differences in measured field strength could also
occur if a tracker is rapidly rotated. The filtering of the
difference between successive measurements of field strength value
thus ensures that, only when the difference is continual over a
period of time, for example, approximately 1 second, does the
sensor processor 52 interpret the situation as one in which the
successive magnetic field measurements indicate the nearby presence
of an eddy current-producing object. In such situation, sensor
processor 52 then actuates the alarm 56, (step 103 is
executed.)
[0229] Also, there is no requirement that the determination of this
process, the test with regard to Equation 21, be performed on the
fields emitted by each transmitter 26-30. In some versions of the
invention it may only be necessary to test the successive different
frequency fields emitted by a single one of the transmitters. This
version of the invention may be simpler to construct or operate
than other versions.
[0230] Similarly, in another version of the system 20a embodiment
of the invention, the low frequency drive signal used as part of
the eddy current-induced field evaluation may not be one of the
navigation drive signals. In these versions of the invention, the
signal may come from a surveillance driver. During the operating of
this version of system, transmitter controller 48a, periodically
negates the application of the navigation drive signal to one or
more transmitters and momentarily inserts this surveillance drive
signal. The relative strengths of the two successive magnetic
fields emitted by the same transmitter are compared to determine
whether or not the significant eddy current-generating objects are
nearby.
[0231] FIG. 25 illustrates one sequence in which the surveillance
magnetic field is interleaved with the navigation fields. Here, at
time periods n, n+2, n+4 and n+6, navigation magnetic fields are
simultaneously emitted. The strength measurements of these fields
are the data from which sensor processor 52 performs the object
localization process. At time n+1, transmitter 26, instead of
emitting the 80 Hz navigation magnetic field, emits the 20 Hz
surveillance magnetic field. Based on the differences in measured
field strength between times n and n+1, sensor processor determines
an eddy current correction error correction value and whether or
not excessive eddy currents are present. At time n+3, the process
is repeated for the 100 Hz navigation magnetic field normally
emitted by transmitter 28. At time n+5, the process is repeated for
the 140 Hz magnetic field normally emitted by transmitter 30.
[0232] This version of the invention thus provides a means to
monitor and correct for the presence of eddy current-generating
objects while allowing signals above 50 Hz to be used to perform
the localization process.
[0233] It should likewise be appreciated that the systems and
methods of this invention for determining whether or not excessive
spurious electromagnetic fields, including the fields generated by
ferromagnetic objects, may be incorporated into the versions of
this invention that emit either AC or DC pulse magnetic fields.
[0234] It should further be understood that in versions of the
invention capable of detecting the presence of excessive spurious
magnetic fields, the systems may do more than actuate alarm 56. In
these embodiments of the invention, the system may also stop
generating data representing the position and orientation of the
object being tracked. This prevents personnel from taking action
based on localization data that may not be accurate.
[0235] Moreover, it is further understood that the hardware
components of the disclosed versions of the invention are meant to
be exemplary, not limiting. For example, in the illustrated version
of the system 20a embodiment of the invention, the three drivers
39, 40 and 41, each emit a fixed frequency drive signal. These
signals are selectively applied to the transmitters 26, 28 and 30
through cross switch 49. In an alternative version of the
invention, the drivers 39-41 emit variable frequency drive signals.
Each driver 39, 40 and 41 is connected to a separate one of the
transmitters 26, 28 and 30, respectively. Control, frequency
setting, signals are applied to the drivers 39-41 from the
transmitter controller 48a. Thus, based on control signals asserted
by the transmitter controller 48a, each driver 39, 40 and 41
generates a drive signal at an appropriate frequency that causes
the associated transmitter 26, 28 or 30 to emit a navigation
magnetic field at the appropriate frequency.
[0236] Alternatively, the DSP that functions as the transmitter
controller 58 during the appropriate time periods adds the
surveillance signals to each of the navigation signals. Each
composite signal is output to the appropriate driver 39, 40 or 41.
An advantage of these versions of the invention is that it
eliminates the need to provide multiplexer 58. Similarly, by
similarly configuring transmitter controller 48a to periodically
output different frequency sin waves to the individual drivers
39-41, the need to provide cross switch 49 is eliminated.
Transmitter controller 48d can likewise be configured in order to
eliminate switches 270 and 272.
[0237] Similarly some versions of the invention may have plural
identical localizers 23 each with its own transmitter assembly 22.
These versions of the invention are employed to track an object or
multiple objects throughout a space wider that can tracked with a
single localizer 23. When constructing these versions of the
invention, the spaces covered by the localizers 23 need to overlap
so that one can establish the geometric relationship of the
individual transmitter coordinate systems.
[0238] Also, it may be possible to practice the invention by
executing the various process steps in sequences different than
what has been described. For example, production of a monitor value
.sigma.of step 74 and the comparison of the monitor value to
.kappa. of step 75 may occur before the generation of error values
and the determination of the corrected magnetic field strength
measurements m.sub.ECCij of steps 72 and 73
[0239] Similarly, methods other than FFT may be used to decompose
the composite AC navigation magnetic fields into their individual
frequency components. These methods include known analog or digital
demodulation techniques.
[0240] Therefore, it is an object of the appended claims to cover
all such variations and modifications that come within the true
spirit and scope of this invention.
* * * * *