U.S. patent application number 11/276926 was filed with the patent office on 2006-11-30 for position and orientation tracking of transponder.
This patent application is currently assigned to General Electric Company. Invention is credited to Lewis J. Levine.
Application Number | 20060267759 11/276926 |
Document ID | / |
Family ID | 33435425 |
Filed Date | 2006-11-30 |
United States Patent
Application |
20060267759 |
Kind Code |
A1 |
Levine; Lewis J. |
November 30, 2006 |
Position and Orientation Tracking of Transponder
Abstract
One or more implementations are described herein for improved
instrument tracking. In a surgical navigation system, one
implementation stores a plan for an image guided procedure, before
conducting the procedure. This plan includes a path to be traversed
by a medical instrument during the procedure. An image of the
patient's anatomy displayed with a superimposed a pictorial
representation of the path on the image. A transpoder coupled to
the medical instrument and emits a signal while inside the
patient's body. A position and/or orientation of the transponder
(and the instrument) is determined based, at least in part, upon
the received transponder signal.
Inventors: |
Levine; Lewis J.; (Weston,
MA) |
Correspondence
Address: |
LEE & HAYES PLLC
SUITE 500
421 W RIVERSIDE
SPOKANE
WA
99201
US
|
Assignee: |
General Electric Company
Schenectady
NY
|
Family ID: |
33435425 |
Appl. No.: |
11/276926 |
Filed: |
March 17, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10611112 |
Jul 1, 2003 |
|
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11276926 |
Mar 17, 2006 |
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Current U.S.
Class: |
340/539.12 |
Current CPC
Class: |
A61B 5/06 20130101; A61B
5/062 20130101; A61B 2562/17 20170801 |
Class at
Publication: |
340/539.12 |
International
Class: |
G08B 1/08 20060101
G08B001/08 |
Claims
1. A method for improved instrument tracking, said method
comprising: in a surgical navigation system, storing a plan for an
image guided procedure, before conducting said procedure, said plan
comprising a path to be traversed by a medical instrument during
said procedure; displaying an image of a patient's anatomy on a
display and superimposing a pictorial representation of said path
on said image; receiving a signal at a receiver from a transponder,
said transponder being coupled to a medical instrument inside a
patient's body and emitting said transponder signal; determining a
position of said transponder based, at least in part, upon said
transponder signal.
2. The method of claim 1 further comprising transmitting data from
said transponder to said receiver.
3. The method of claim 2 wherein said data comprises information
identifying said transponder.
4. The method of claim 3 wherein said data distinguishes said
transponder from other transponders.
5. The method of claim 1 wherein said plan comprises the
administration of a therapeutic material.
6. The method of claim 5 further comprising facilitating
administration of said therapeutic material when said medical
instrument has traversed said path.
7. The method of claim 1 further comprising determining said plan
for said image guided procedure before conducting said
procedure.
8. The method of claim 1 further comprising: facilitating insertion
of said medical instrument inside a patient's body; facilitating
guiding of said medical instrument along said path inside the
patient's body so that the transponder substantially traverses said
path.
9. The method of claim 1 further comprising superimposing an
indication of said position of said transponder on said image.
10. The method of claim 1 further comprising superimposing an
indication of said position of said transponder on said image with
said superimposed pictorial representation of said path on said
image.
10. The method of claim 1, wherein said plan further comprises a
planned radition exposure dose, the method further comprising:
measuring an actual radiation exposure dose during an actual
image-guided medical procedure; comparing the planned radiation
exposure dose of a planned image-guided procedure to the actual
radiation exposure dose during the actual image-guided procedure;
updating the plan for an image guided procedure in response to the
comparison.
11. One or more processor-readable media having
processor-executable instructions that, when executed by a
processor of a surgical navigation system, performs acts
comprising: in a surgical navigation system, storing a plan for an
image guided procedure, before conducting said procedure, said plan
comprising a path to be traversed by a medical instrument during
said procedure; displaying an image of a patient's anatomy on a
display and superimposing a pictorial representation of said path
on said image; receiving a signal at a receiver from a transponder,
said transponder being coupled to a medical instrument inside a
patient's body and emitting said transponder signal; determining a
position of said transponder based, at least in part, upon said
transponder signal.
12. One or more media as recited in claim 11 further comprising
determining an orientation of said transponder based, at least in
part, upon said transponder signal.
13. One or more media as recited in claim 11 further comprising:
deriving data from said transponder; identifying said transponder
based upon data derived from said transponder.
14. One or more media as recited in claim 13, wherein said data
distinguishes said transponder from other transponders.
15. One or more media as recited in claim 11 wherein said plan
comprises the administration of a therapeutic material.
16. One or more media as recited in claim 11 further comprising
facilitating administration of said therapeutic material.
17. One or more media as recited in claim 11 further comprising
facilitating determination of said plan for said image guided
procedure before conducting said procedure.
18. One or more media as recited in claim 11 further comprising:
facilitating insertion of said medical instrument inside a
patient's body; facilitating guiding of said medical instrument
along said path inside the patient's body so that the transponder
substantially traverses said path.
19. One or more media as recited in claim 11 further comprising
superimposing an indication of said position of said transponder on
said image.
20. One or more media as recited in claim 11 further comprising
superimposing an indication of said position of said transponder on
said image with said superimposed pictorial representation of said
path on said image.
Description
BACKGROUND
[0001] The present invention generally relates to an
electromagnetic tracking system. In particular, the present
invention relates to an electromagnetic tracking system using a
single-coil wired or wireless transmitter.
[0002] Many medical procedures involve a medical instrument, such
as a drill, a catheter, scalpel, scope, stent or other tool. In
some cases, a medical imaging or video system may be used to
provide positioning information for the instrument, as well as
visualization of an interior of a patient. Typically, during the
course of a procedure, an instrument is guided by continuously
obtaining and viewing x-ray images that show the current location
of the instrument along with a portion of the patient's anatomy in
a region of interest. However, because repeated exposure to x-ray
radiation is harmful to medical personnel that perform image guided
procedures on a daily basis, many navigation systems have been
proposed that attempt to reduce exposure to x-ray radiation during
the course of a medical procedure.
[0003] For example, electromagnetically tracking the position of
medical instruments during a medical procedure is used as a way to
decrease exposure to x-ray radiation by decreasing the number of
x-ray images acquired during a medical procedure. Typically, an
electromagnetic tracking system employs a transmitter coil, a
transponder coil, and a receiver coil. The transmitter coil emits a
signal at a frequency that is picked up by the transponder coil.
The transponder coil emits a signal at the same frequency in
response to the transmitter signal. The signal from the transponder
is received at the receiver coil and the tracking system calculates
position information for the medical instrument with respect to the
patient or with respect to a reference coordinate system. During a
medical procedure, a medical practitioner may refer to the tracking
system to ascertain the position of the medical instrument when the
instrument is not within the practitioner's line of sight.
[0004] The tracking or navigation system allows the medical
practitioner to visualize the patient's anatomy and track the
position and orientation of the instrument. The medical
practitioner may then use the tracking system to determine when the
instrument is positioned in a desired location. Thus, the medical
practitioner may locate and operate on a desired or injured area
while avoiding other structures with less invasive medical
procedures.
[0005] Tracking systems are also used outside of the medical field
to track the position of items other than medical instruments. For
example, tracking technology is used in forensic and security
applications. Retail stores use tracking technology to prevent
theft of merchandise. In such cases, a passive transponder can be
located on the merchandise. A transmitter may be strategically
located within the retail facility. The transmitter emits an
excitation signal at a frequency that is designed to produce a
response from the transponder. When merchandise carrying a
transponder is located within the transmission range of the
transmitter, the transponder produces a response signal that is
detected by a receiver. The receiver then determines the location
of the transponder based upon characteristics of the response
signal.
[0006] Tracking systems are also often used in virtual reality
systems or simulators. For example, tracking systems are used to
monitor the position of a person in a simulated environment. A
transmitter emits an excitation signal and a transponder located on
the person produces a response signal. The response signal is
detected by a receiver. The signal emitted by the transponder is
then used to monitor the position of a person or object in a
simulated environment.
[0007] Electromagnetic tracking systems such as those presented
above may employ coils that act as the transmitters, transponders,
and receivers. Typically, an electromagnetic tracking system is
configured in an industry-standard coil architecture (ISCA). ISCA
uses three colocated orthogonal quasi-dipole transmitter coils and
three colocated quasi-dipole receiver coils. Other systems may use
three large, non-dipole, non-colocated transmitter coils with three
colocated quasi-dipole receiver coils. Another tracking system
architecture uses an array of six or more transmitter coils spread
out in space and one or more quasi-dipole receiver coils.
Alternatively, a single quasi-dipole transmitter coil may be used
with an array of six or more receivers spread out in space.
[0008] The ISCA tracker architecture uses a three-axis dipole coil
transmitter and a three-axis dipole coil receiver. Each three-axis
transmitter or receiver is built so that the three coils exhibit
the same effective area, are oriented orthogonally to one another,
and are centered at the same point. An example of a dipole coil
trio with coils in X, Y, and Z directions spaced approximately
equally about a center point is shown in FIG. 4. If the coils are
small enough compared to a distance between the transmitter and
receiver, then the coil may exhibit dipole behavior. Magnetic
fields generated by the trio of transmitter coils may be detected
by the trio of receiver coils. Using three approximately
concentrically positioned transmitter coils and three approximately
concentrically positioned receiver coils, for example, nine
parameter measurements may be obtained. From the nine parameter
measurements and a known position or orientation parameter, a
position and orientation calculation may determine position and
orientation information for each of the transmitter coils with
respect to the receiver coil trio with three degrees of
freedom.
[0009] As discussed earlier, the response signal emitted by the
transponder and the excitation signal emitted by the transmitter
are incident upon the receiving coil. Typically, in a tracking
system using a passive transponder, the excitation signal is much
larger than the response signal when both signals are received at
the receiver. Because the response signal is emitted at the same
frequency as the excitation signal and the response signal is much
smaller than the excitation signal, accurately separating and
measuring the response signal is difficult.
[0010] While current ISCA architectures track a trio of transmitter
coils with a trio of receiver coils, many instruments, such as
catheters or flexible ear, nose and throat instruments, require a
single small coil to be tracked. There is no known conventional
electromagnetic tracking system for tracking an instrument using a
single coil.
[0011] Additionally, with multiple instruments each containing
single coil transmitters emitting signals at the same frequency as
the transmitter signal, it becomes difficult to discern one
transponder signal from another. Thus, if multiple instruments are
used simultaneously during a procedure, it becomes difficult to
simultaneously track and identify each instrument. There is no
known conventional electromagnetic tracking system for tracking an
instrument using a single coil that allows for identification and
location of the individual coils.
[0012] Additionally, to optimize guidance of medical instruments
using transponders and receivers, and reduce trauma to a patient,
it may be desirable to predetermine a path to be traversed within a
patient's anatomy. If the path and therapeutic materials could be
entered and saved in a navigation system, medical personnel could
access the information regarding the predetermined path and guide a
medical instrument accordingly. There is no known conventional
surgical navigation system that allows for a pre-operative plan to
be entered and saved before beginning a procedure and can be viewed
and followed during the course of a procedure.
[0013] Thus, there is a need for an improved electromagnetic
tracking system using a single-coil wired or wireless
transmitter.
BRIEF SUMMARY
[0014] One or more implementations are described herein for
improved instrument tracking. In a surgical navigation system, one
implementation stores a plan for an image guided procedure, before
conducting the procedure. This plan includes a path to be traversed
by a medical instrument during the procedure. An image of the
patient's anatomy displayed with a superimposed a pictorial
representation of the path on the image. A transpoder coupled to
the medical instrument and emits a signal while inside the
patient's body, A position and/or orientation of the transponder
(and the instrument) is determined based, at least in part, upon
the received transponder signal.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0015] FIG. 1 illustrates a wireless tracker used in accordance
with an embodiment of the present invention.
[0016] FIG. 2 shows a printed circuit board used in accordance with
an embodiment of the present invention.
[0017] FIG. 3 depicts a flow diagram for a method for a position,
orientation and gain determination used in accordance with an
embodiment of the present invention.
[0018] FIG. 4 illustrates a dipole coil trio used in accordance
with an embodiment of the present invention.
[0019] FIGS. 5 and 6 depict a flow diagram for a method for
improved instrument tracking in a surgical navigation system used
in accordance with an embodiment of the present invention.
[0020] The foregoing summary, as well as the following detailed
description of certain embodiments of the present invention, will
be better understood when read in conjunction with the appended
drawings. For the purpose of illustrating the invention, certain
embodiments are shown in the drawings. It should be understood,
however, that the present invention is not limited to the
arrangements and instrumentality shown in the attached
drawings.
DETAILED DESCRIPTION
[0021] Certain embodiments of the present invention provide a
system and method for electromagnetic tracking using a single-coil
transponder. The system includes a single coil transponder emitting
a signal, a receiver receiving a signal from the single coil
transponder, and electronics for processing the signal received by
the receiver. The electronics determine a position of the single
coil transponder.
[0022] The transponder may be a wireless or wired transponder. The
single coil of the transponder may be a dipole. The transponder may
be battery-powered. Additionally, the transponder may be driven
with a continuous wave signal.
[0023] The receiver may be a printed circuit board. Additionally,
the receiver may be a twelve-receiver array. In an embodiment, the
receiver may be a twelve receiver circuit printed circuit board.
Four circuits may include single spiral coils. Eight circuits may
include pairs of spiral coils.
[0024] The electronics may identify the transponder and determine
position, orientation, and/or gain of the transponder. The
electronics may determine a ratio of mutual inductance between the
transponder and the receiver to determine the position of the
transponder. The electronics may also determine a ratio of currents
and/or magnetic fields produced at the transponder to determine the
position of the transponder.
[0025] Certain embodiments provide an improved instrument tracking
system including a single-coil wireless transponder, a printed
circuit board receiver array including a plurality of coils and
coil pairs, and tracker electronics for analyzing parameter(s)
between the transponder and the coils and coil pairs of the
receiver array to determine a position of the transponder in
relation to the receiver array. The parameters may include mutual
inductances and/or magnetic fields. The tracker electronics may
also determine a gain and/or an orientation of the transponder. In
an embodiment, reciprocity allows the coils of the receiver array
to be treated as transponder coils. The system may also include a
calibration coil for calibrating the receiver array.
[0026] In an embodiment, the printed circuit board receiver array
produces magnetic fields as follows: a mostly uniform field point
in an X direction; a field varying mostly with X, pointed in the X
direction; a field varying mostly with Y, pointed in the X
direction; a field varying mostly with Z, pointed in the X
direction; a mostly uniform field pointed in the Y direction; a
field varying mostly with X, pointed in the Y direction; a field
varying mostly with Y, pointed in the Y direction; a field varying
mostly with Z pointed in the Y direction; a mostly uniform field
pointed in the Z direction; a field varying mostly with X pointed
in the Z direction; a field varying mostly with Y pointed in the Z
direction; and a field varying mostly with Z pointed in the Z
direction.
[0027] Certain embodiments provide a method for improved instrument
tracking. The method includes driving a transponder coil at a
certain frequency to emit a signal and receiving the signal at an
array of receiver coils. The method also includes determining a
gain of the transponder coil and measuring a mutual inductance
between the transponder coil and an array of receiver coils. An
initial estimate of a position of the transponder coil is selected.
The initial estimate is adjusted using an error-minimizing routine
based on the mutual inductance. The initial estimate may be a
previous calculation result.
[0028] The method may also include calibrating the array of
receiver coils. Additionally, the method may include eliminating a
sign ambiguity of the gain of the transponder coil. A transponder
current may also be determined from the signal received at the
array of receiver coils.
[0029] In a certain embodiment, a method for electromagnetic
tracking includes driving an array of coils at different
frequencies, determining ratios of currents produced by the
different frequencies, measuring voltages generated at the
different frequencies, and calculating ratios of mutual inductances
between the array of coils and a single coil located remotely from
said array of coils. The method further includes estimating an
initial value for at least one of position, gain, and orientation
of the single coil and determining a best fit value for at least
one of the position, gain, and orientation of the single coil based
on the initial value and the ratios of mutual inductances. The
method may also include calibrating the array of coils.
[0030] FIG. 1 illustrates a wireless tracker 100 used in accordance
with an embodiment of the present invention. The wireless tracker
100 includes a transmitter 110, a wireless transponder 115, a
receiver 120, and tracker electronics 130. The transmitter 110
emits a transmitter signal. The wireless transponder 115 receives
the transmitter signal and emits a transponder signal. The
transponder signal may include data such as identification
information that may be used to associate a transponder signal with
a particular transponder. The receiver 120 detects the transmitter
signal and the transponder signal. The tracker electronics 130
analyzes the signals received by the receiver 120 to identify the
transponder 115 and determine a position of the transponder
115.
[0031] In an embodiment, the transponder 115 is a single-coil
wireless transponder. The wireless transponder 115 may be a
battery-powered wireless transponder or a passive transponder.
Alternatively, a single-coil wired transponder may be used in place
of or in addition to the wireless transponder 115.
[0032] During some medical procedures, portions of medical
instruments may be obscured or covered by portions of a patient's
anatomy. For example, a small incision may be made in a patient's
abdomen and a medical instrument such as a needle and trocar
inserted in the incision. After the needle and trocar is inserted
through the incision, the surgeon can not see the portion of the
needle and trocar that is within the patient's abdomen.
[0033] In order to guide the tip of the needle to a desired region
of interest, a transponder may be placed near the tip of the
needle. A transmitter can emit a transmitter signal that propagates
through the patient's anatomy. The transmitter signal impinges upon
the transponder located on the tip of the needle. In response, the
transponder emits a transponder signal. The transponder may include
a memory that stores data such as identification information that
distinguishes the transponder from other transponders. When the
transponder receives the transmitter signal, the transponder emits
a transponder signal that may include a portion of the data stored
in the memory.
[0034] A receiver receives the transponder signal. A tracking
system coupled to the receiver processes the transponder signal. If
the transponder signal contains identification data, the tracking
system can identify from which transponder the transponder signal
was emitted. The tracking system can also use the transponder
signal to calculate the location of the transponder. Consequently,
the transmitter, transponder, receiver, and tracking system can be
used to identify and locate portions of medical instruments during
a medical procedure and to aid in navigating the medical
instruments to regions of interest.
[0035] In an embodiment, the transponder may be a 23 mm glass
transponder with a read only memory of 64 bits and an operating
frequency of 134.2 kHz as manufactured by Texas Instruments. The 64
bit memory can be used to store unique identification data that
identifies the medical instrument to which the transponder is
attached and to distinguish the transponder and its corresponding
medical instrument from other transponders and their corresponding
medical instruments.
[0036] In an embodiment, the transponder coil is small enough that
the coil acts sufficiently like a dipole for tracking purposes. A
dipole may be described by position, orientation, and gain (or
strength). The position, orientation, and strength of the coil may
be determined as described below. Therefore, the position,
orientation, and gain of the wireless transponder coil and the
tracker electronics 130 may be determined without
characterization.
[0037] In an embodiment, the receiver 120 is a single 0.48 meter by
0.52 meter printed circuit board (PCB). The PCB may include 20
coils formed by copper tracks in the PCB, for example. The coils
may be connected in series pairs and/or used individually, for
example. In an embodiment, twelve separate conducting paths may be
present on the PCB (called the ANT-009 design). PCB coils may be
precisely made at a low cost. The ANT-009 PCB may be used as an
array of transmitters or as an array of receivers, for example.
FIG. 2 shows an embodiment of the ANT-009 PCB.
[0038] A transponder 115 with a driver may be used in place of a
transponder 115 and transmitter 110 combination. Rather than the
transponder 115 emitting a transponder signal after receiving a
transmitter signal from a transmitter 110, the transponder driver
may be used to provide a signal to the transponder 115 and cause it
to emit a transponder signal.
[0039] In an embodiment, receiver coils in the PCB are spread out
or distributed on the PCB. The distributed coils are susceptible to
electrostatic pickup. A Faraday shield may be used to block
electrostatic pickup from the PCB without affecting electromagnetic
signals received by the receiver 120.
[0040] Mutual inductance may be used in the electromagnetic
tracking system to identify the positions of components in the
system. Mutual inductance may allow the system to be divided into
two parts: coils and electronics 130. Determining mutual inductance
involves a physical design of the coils and a geometrical
relationship between the coils but not details of the electronics
130 used to measure the mutual inductance. Additionally, mutual
inductance does not depend on which coil receives an applied
current.
[0041] In addition to the electronics 130 used to measure mutual
inductance, a system including one transponder coil and one
receiver coil forms a four-terminal two-port network. A varying
current injected into one coil induces a voltage in the other coil.
The induced voltage V is proportional to the rate of change of the
applied current I: V=L.sub.m(dI/dt) (2), wherein L.sub.m represents
mutual inductance. L.sub.m is based on the geometry of the coils
(closed circuits). L.sub.m is a ratio independent of applied
current waveform or frequency. Thus, L.sub.m is a well-defined
property that may be measured with reasonable precision.
[0042] The position, orientation, and gain (POG) of the transponder
115 may be calculated with respect to a coordinate system of the
receiver 120. POG determinations employ reciprocity to generate
magnetic field models that treat PCB receiver coils as transponder
coils. Reciprocity indicates that a mutual inductance of a pair of
coils is independent of which coil is driven. By using pairs of
coils in series on the PCB, magnetic fields in XYZ directions and
with XYZ gradients are formed in a "sweet spot" in relation to the
PCB. For example, fields are formed 0.1-0.2 meters above the center
of the PCB. In an embodiment, the PCB includes 12 distinct single
coils and coil pairs. A variety of magnetic fields enhance
numerical stability of the POG calculation.
[0043] In an embodiment, the gain of the single transponder coil
may be determined with 6 or more receiver coils. In an embodiment,
a mutual inductance model provides 12 mutual inductances from the
transponder coil to each of the receiver coils as a function of
POG. First, an initial estimate of POG may be selected. For
example, a POG result from a previous measurement and calculation
cycle may be used as an initial estimate or seed for a POG
calculation. Then, an error-minimizing routine may be used to
adjust the POG estimate. The POG estimate is adjusted to minimize a
difference between measured and modeled mutual inductances.
[0044] If a sine wave is emitted by the transponder 115 and the
receiver 120 calculation is phase-locked to the transponder signal,
a sign of the transponder coil gain may not be determined. An
unknown sign of the transponder gain may create ambiguity in the
POG. For example, reversing the transponder coil end-for-end has no
effect on the POG. In an embodiment, tracking may start with the
transponder coil at an approximately determined POG. The POG may
then be tracked from cycle to cycle.
[0045] In an alternative embodiment, sign ambiguity of the
transponder gain may be eliminated. A phase or sign of the
transponder 110 sine wave may be determined directly with no memory
(e.g., without previous calculations). The phase may be determined
without a phase-locked loop.
[0046] A complex transponder current (tx_current) may be expressed
as a product of two factors:
tx_current=tx_current_magnitude*tx_current_phase (3), where
tx_current_magnitude is a magnitude of the transponder 115 current,
and tx_current_phase is a phase of the transponder 115 current. In
an embodiment, the magnitude of the transponder 115 current is
real, positive, and varies slowly. The magnitude of the transponder
current is proportional to the gain of the POG. Thus, transponder
current magnitude may be determined by a POG calculation. The
transponder current phase is a complex, unity magnitude value. The
phase is recalculated from newest receiver 120 signal data for each
cycle. Transponder current phase may be different for each cycle's
data.
[0047] In an embodiment, the largest magnitude received signal in a
12-receiver array is one of receivers 0, 5, and 11 of an array of 0
to 11. The three receiver coil boards 0, 5, and 11 have
approximately orthogonal directional responses. That is, if the
total signal is a reasonable size, at least one of the receiver
boards 0, 5, and 11 receives a signal that is not small. For a
receiver signal array, receiver signals 0, 5, and 11 may be tested
to determine which receiver signal is largest in magnitude. The
signal with the largest magnitude is designated
receiver_signal[r].
[0048] A denormalized transponder current phase may then be
calculated as follows: tx_current .times. _phase .times.
_denormalized = sign .times. receiver_signal .function. [ r ] i
.times. .times. 2 .times. .pi. , ( 4 ) ##EQU1## where the sign is
either +1 or -1. Then the current phase may be normalized and the
sign corrected: tx_current .times. _phase = tx_current .times.
_phase .times. _denormalized tx_current .times. _phase .times.
_denormalized . ( 5 ) ##EQU2## A transponder 115 complex current
may then be determined: tx_current=tx_current_mag*tx_current_phase
(6).
[0049] Without a second harmonic signal measurement, a sign may be
chosen for each cycle to maintain a consistent sign of the
receiver_signal[n] elements over time. In an embodiment, tracking
of the transponder 115 begins from a selected position, such as a
calibration position, to make an initial sign choice (+ or -). A
second harmonic current of the transponder coil may be generated
with an asymmetrical waveform including even harmonics and a CW
fundamental frequency. For example, a transponder coil driver may
output an asymmetrical square wave voltage (for example, 1/3, 2/3
duty cycle) to drive the coil in series with a tuning capacitor.
Alternatively, a diode (or a series combination of a diode and a
resistor, for example) may be connected in parallel with the coil
to generate even harmonics.
[0050] A harmonic frequency may be used to determine the sign of
the fundamental frequency. The harmonic may be amplitude modulated
with low-speed analog or digital data without affecting a tracking
function. The data may be characterization data, data from a
transducer mounted on the transponder 115, or other data, for
example.
[0051] In an embodiment, a low cost battery-powered transponder
driver and coil may be used. Cost may be reduced by not
characterizing the single coil of the transponder 115. The low cost
driver and single coil may be used in disposable applications, for
example.
[0052] If a transponder unit 115 is sealed, such as in medical
applications, activating or turning a unit "on and off" may present
difficulties. In an embodiment, a transponder driver includes a
silicon CMOS chip with an on-off flip-flop or latch circuit and a
photocell. A brief flash of light sets the flip-flop and activates
the driver. Once set, the flip-flop remains set independent of
illumination until a specific electromagnetic pulse resets the
flip-flop and turns the driver off. After manufacture and testing,
the driver-coil assembly may be packaged in a sealed, lightless
container, such as a container used for photographic film. The
packaged driver is turned off by applying an electromagnetic pulse.
When a user opens the package, ambient light turns on the driver.
The driver runs until receiving an electromagnetic pulse or until
energy in a driver battery is exhausted.
[0053] The transponder 115 may be driven by an oscillator powered
by direct current, for example. In an embodiment, the wired
transponder driver may be powered from a source of 3 volts at a
milliampere direct current. For example, photocells powered by
ambient light may power the driver. Alternatively, radio frequency
energy may be rectified to power the driver.
[0054] In one embodiment, a single transponder coil is located at
the tip of a catheter. A small silicon photocell is connected
across the coil. The photocell is illuminated with
amplitude-modulated light. The photocell powers a driver for the
transponder coil. Alternatively, two photocells may be connected in
antiparallel across the transponder coil. By alternately
illuminating each photocell, an alternating current may be
generated in the coil.
[0055] Alternate illuminations may be achieved using two optical
fibers (one to each photocell). Illumination may also be achieved
using one fiber to illuminate the photocells through filters of
different polarizations or different colors, for example. In
another embodiment, two photocells may be integrated on top of each
other. Each photocell may be sensitive to different wavelengths of
light.
[0056] An optically powered coil may have advantages over an
electrically powered coil. For example, optical fibers may be
smaller than electrical wires. Additionally, a catheter, for
example, with an optically powered coil has no electrical energy in
most of the length of the catheter. An electrically powered coil
may result in some electrical energy in the catheter.
[0057] In another embodiment, the receiver 120 may include an
array(s) of three-axis dipole wire-wound coil trios. Due to
inaccuracies in coil winding, the receiver 120 is characterized
before use in tracking. The wire-wound receiver coil arrangement
may have a better signal-to-noise ratio than a PCB coil, due to a
larger volume of copper in a wound coil of a given volume.
Additionally, POG seed algorithms may be used with characterized
receiver coils.
[0058] In an alternative embodiment, a battery-powered wireless
transponder driver receives a clock signal from the tracker
electronics 130 via a magnetic, radio frequency, ultrasonic, or
other signal generator. A clock signal may eliminate phase-locking
and ambiguity in the sign of the transponder gain.
[0059] In another embodiment, the wireless transponder 115 may be
combined with various wireless radio frequency identification
(RFID) schemes. RFID techniques allow for identification and/or
data transfer without contact between the transponder 115 and the
receiver 120. The wireless transponder 115 may be used with RFID
technology to transmit data to the receiver 120 and tracker
electronics 130.
[0060] As described above, a PCB may be used in an electromagnetic
tracking system, such as the wireless tracker 100. The following
discussion illustrates an embodiment of the PCB in more detail. The
PCB may be configured as a transponder coil array and be used to
track a single receiver coil against an array of twelve transponder
coils, for example. The PCB may also be configured as a receiver
coil array and used to track a single-coil transponder. The PCB may
be used as the receiver 120 in the wireless tracker 100 tracking
the single-coil transponder 115. Reciprocity allows coils in the
receiver coil array to be treated as transponder coils.
[0061] In an embodiment, the PCB is precisely manufactured, so a
magnetic field model of the PCB may be determined with sufficient
accuracy without characterization. A single coil transponder is
small enough to be modeled with sufficient accuracy as a dipole
with a position, orientation, and gain that are determined through
tracking without characterization. In an embodiment, the PCB does
not include curved traces. Magnetic fields may be more precisely
calculated with straight line segments.
[0062] The PCB board, such as the ANT-009 coil board described
above and shown in FIG. 2, may facilitate tracking around a small
volume "sweet spot" located over the center of the PCB. In an
embodiment, the board provides magnetic fields in the sweet spot
that are approximately as follows:
[0063] 1. a mostly uniform field pointed in the X direction;
[0064] 2. a field varying mostly with X pointed in the X
direction;
[0065] 3. a field varying mostly with Y pointed in the X
direction;
[0066] 4. a field varying mostly with Z pointed in the X
direction;
[0067] 5. a mostly uniform field pointed in the Y direction;
[0068] 6. a field varying mostly with X pointed in the Y
direction;
[0069] 7. a field varying mostly with Y pointed in the Y
direction;
[0070] 8. a field varying mostly with Z pointed in the Y
direction;
[0071] 9. a mostly uniform field pointed in the Z direction;
[0072] 10. a field varying mostly with X pointed in the Z
direction;
[0073] 11. a field varying mostly with Y pointed in the Z
direction; and
[0074] 12. a field varying mostly with Z pointed in the Z
direction.
The X and Y directions are in the plane of the PCB. The Z direction
is perpendicular to the plane of the PCB.
[0075] In an embodiment, the ANT-009 coil PCB includes twelve
separate electrical circuits. Four of the circuits include single
spiral coils. Eight of the circuits include pairs of spiral coils.
The single coils generate non-uniform fields. The non-uniform
fields generated by the single coils are generated mostly in the Z
direction at the sweet spot. Two coils in a pair of spiral coils
are positioned side-by-side. The coils are connected in series.
Opposing coils connected in series produce non-uniform fields
pointed mostly in the X and Y directions at the sweet spot. A
single large coil generates a mostly uniform Z field. A pair of
long narrow spirals on opposite edges of the PCB generates a mostly
uniform X field. Another pair of long narrow spirals on the other
pair of opposite edges of the PCB generates a mostly uniform Y
field.
[0076] The PCB utilizes an approximate nature of the "mostly
uniform" fields to produce an effect of the desired "varying
mostly" fields. The "mostly uniform" fields may have gradients. For
example, consider the Z-direction fields. One large coil generates
a "mostly uniform" Z field. Three small coils may be placed near
the origin of the PCB and offset from the origin along lines at
roughly 0 degrees, 120 degrees, and 240 degrees. The three small
coils generate smaller "mostly uniform" Z fields displaced from the
main "mostly uniform" Z field generated by the large coil. The
effects of the "mostly varying" fields may be produced by taking
sums and differences among the four fields discussed above. Fields
in the X and Y directions may be generated similarly. However,
connected pairs of series-opposing coils may be used instead of
single coils to generate fields in the X and Y directions. The
above fields may be calculated using a straight line segment field
model, for example.
[0077] In an embodiment, the tracker electronics 130 includes
twelve receiver coil drivers. The twelve coil drivers operate at
twelve different CW frequencies, for example. The twelve coil
drivers drive twelve receiver coil circuits on the receiver PCB.
Currents in the twelve receiver coil circuits are measured. In an
embodiment, current values are approximately determined. Then,
ratios of the currents are determined.
[0078] Current in the coils causes the receiver coil circuits to
emit magnetic fields. The magnetic fields induce voltages in a
single transponder coil at the twelve driver frequencies. The
tracker electronics 130 measures signals at the twelve
frequencies.
[0079] A mutual inductance between each receiver circuit and the
transponder coil is calculated. Mutual inductances between the
transponder 115 and receiver 120 are determined. In an embodiment,
mutual inductances are approximately determined. Then, ratios of
the twelve mutual inductances are determined. Six or more receiver
coils spread in a selected configuration and measurements of the
ratios of the mutual inductances to the transponder coil may be
used to calculate a position of the transponder coil, an
orientation (except roll) of the transponder coil, and a gain of
the transponder coil (a POG determination). The gain of the
transponder coil represents a scale factor that converts the mutual
inductance ratios into mutual inductance values (in Henries, for
example).
[0080] In an alternative embodiment, a single-receiver-coil version
PCB may be used to characterize three coils in an ISCA receiver or
transponder coil trio. The characterization process includes
separately tracking each of the three ISCA coils for position,
orientation, and gain. Then, the tracking data are combined into a
coil characterization format used by ISCA trackers, for
example.
[0081] FIG. 3 depicts a flow diagram for a method 300 for a POG
determination used in accordance with an embodiment of the present
invention. First, at step 310, receiver coils are driven at
different frequencies. Drivers produce currents in the receiver
coils. Then, at step 320, ratios of the currents produced in the
receiver coils are determined. The receiver coils generate magnetic
fields that induce voltages at different frequencies in the
transponder coil. At step 330, the signals induced at the
transponder coil are measured.
[0082] The voltages and currents produce mutual inductances between
the transponder coil and the receiver coils. At step 340, ratios of
the mutual inductances between the receivers and the transponder
are calculated.
[0083] Next, at step 350, an initial estimate, or seed, of
transponder position, orientation, and gain is obtained. The
estimate may be generated from prior mechanical knowledge of the
transponder POG, from a final POG estimate from a previous tracking
cycle, or from a direct calculation from the mutual inductance
measurements, for example.
[0084] Then, at step 360, a best-fit estimate of the POG to the
mutual inductance ratio measurements may be calculated. The
best-fit estimate may be calculated using a model of the
transponder-to-receiver mutual inductances and the seed POG values,
for example. The best fit calculation may be any of several
well-known solution fitting algorithms, such as least squares,
Powell, and Levenberg-Marquardt, for example.
[0085] The above calculations may also be performed with the PCB
configured as a twelve transponder coil board with a single
receiver coil. Additionally, the PCB may be configured with
different numbers of coils to function as a transponder and/or
receiver.
[0086] In an embodiment, electromagnetic tracking systems calibrate
receiver electronics to help ensure accurate positional
measurements, for example. A calibration coil may be placed
diagonally in a receiver coil assembly to provide approximately
equal mutual inductances from the calibration coil to each of the
receiver coils. The mutual inductances may be individually measured
during manufacture. The mutual inductance values measured during
manufacture may be stored in a characterization memory, for
example. The measured mutual inductances may be used during
tracking to calibrate the receiver electronics.
[0087] The PCB may include a calibration coil. The calibration coil
may improve the usefulness of the PCB as a receiver 120. In an
embodiment, the calibration coil is built on an inner layer or
layers of the printed circuit assembly. The calibration coil may
partially overlap existing coils in the assembly to produce desired
calibration coil to receiver coil mutual inductances. In an
embodiment, a single-turn calibration coil in a rectangle covering
approximately one corner quadrant of the PCB is used.
[0088] In an embodiment, the calibration coil is part of a single
PCB, rather than a separately fabricated addition. Thus, the
calibration coil is in approximately the same plane as the receiver
coils. Mutual inductances between the calibration coil and the
receiver coils may be fixed by a fabrication process and calculated
without measuring separate boards, for example. Alternatively, a
separate calibration module may be added to measure small mutual
inductances or mutual impedances separate from the coil
assembly.
[0089] Ratios of transponder 115 currents to a reference current in
the calibration coil may be determined, for example. The
calibration coil may have a defined mutual inductance with respect
to each receiver coil. The mutual inductances, combined with
measured current ratios, allow determination of
transponder-to-receiver mutual inductances from the measured
ratios. If a wireless transponder is used, current ratios may not
be measured. Another measurement, such as magnetic field ratios,
may be used with wireless transponders.
[0090] Thus, certain embodiments of the PCB provide transponder and
receiver coils that do not need precise characterization. Certain
embodiments use pairs of coils in series to generate magnetic
fields parallel to the plane of the PCB while reducing the number
of separate coil drivers used. For the ANT-009 coil board, 12
drivers are used. A separate-coil version of the ANT-009 may use 20
drivers. Additionally, the straight line segments of the PCB allow
use of an analytical model of a magnetic field due to a straight
line current segment. Furthermore, expressions for mutual
inductance between two straight line current segments may be used.
Certain embodiments of the PCB also provide for calibration of the
receiver and tracker electronics.
[0091] Certain embodiments of the present invention provide an
electromagnetic tracking system including a wired or wireless
transponder with a single-coil. In an embodiment, one receiver coil
assembly, whether PCB or wire-wound, may be used to simultaneously
track a plurality of wireless and/or wired transponders on
different frequencies.
[0092] Additionally, a pre-operative plan may be entered into a
surgical navigation workstation prior to performing a medical
procedure. The plan may be transcribed to a surgical navigation
workstation and followed during the course of the medical procedure
by guiding a medical instrument with a transponder and receiver.
For example, therapeutic materials may be associated with a
particular treatment placement device and a path to be traversed by
the placement device inside a patient may be entered into the
system. During the procedure, the placement device can be
manipulated by observing an image of the patient's anatomy and the
predetermined path, and guiding the placement device using a
transponder attached to the device so that it follows the
predetermined path. An image of the placement device following the
predetermined path can be displayed on a monitor or other display
device. Upon reaching the predetermined destination within the
patient's anatomy, the therapeutic materials can be dispersed in
accordance with the pre-operative plan.
[0093] The pre-operative plan can be updated with real-time
feedback during the course of the procedure to allow for changes in
the plan to be accommodated. For example, the placement of
radioactive seeds for such interventions as brachytherapy on the
prostate can be entered into the system as a pre-operative plan.
During the procedure, changes in anatomy or other conditions can be
accommodated for by allowing real-time feedback from a surgeon to
adjust the path of the medical instrument being guided within the
patient
[0094] This approach may be used, for example, in a form of cancer
therapy using radioactive seeds. The planning could be done in both
2D and 3D. With 3D, the isocontours of the radiation plan could be
checked against actual placement.
[0095] With this approach, placement and retrieval of the seeds
could be accomplished through navigation. Conventionally, this is
typically done by combining x-ray and ultrasound. X-rays are used
to see the seeds but not the soft tissue and ultrasound to see the
soft tissue and not the seeds. It is challenging is to register the
two conventional modalities. This is particularly so because the
perspective and distortion are different in each conventional
modality and it is difficult to pick up common landmarks. However,
this new approach overcomes those drawbacks of the conventional
approach.
[0096] FIG. 5 depicts a flow diagram for a method 500 for improved
instrument tracking in a surgical navigation system. This method
500 may be performed in software, hardware, or a combination
thereof. For ease of understanding, this method is delineated as
separate steps represented as independent blocks in FIG. 5;
however, these separately delineated steps should not be construed
as necessarily order dependent in their performance.
[0097] This described method 500 includes storing (at 502) a plan
for an image-guided procedure, before conducting the procedure. The
plan includes a path to be traversed by a medical instrument during
the procedure. The described method also includes displaying (at
504) an image of a patient's anatomy on a display and superimposing
a pictorial representation of the path on the image. In addition,
the described method also includes receiving (at 506) a transponder
signal at a receiver. The transponder being coupled to a medical
instrument inside a patient's body and emitting said transponder
signal. Furthermore, the described method includes determining (at
508) a position of the transponder based, at least in part, upon
the transponder signal.
[0098] FIG. 6 depicts a flow diagram for a method 600 for adjusting
the plan for an image-guided procedure while the procedure is
occurring, especially where such procedure involves exposure to
radiation. This method 600 may be performed in software, hardware,
or a combination thereof. For ease of understanding, this method is
delineated as separate steps represented as independent blocks in
FIG. 6; however, these separately delineated steps should not be
construed as necessarily order dependent in their performance.
[0099] This described method 600 employs the techniques described
above in method 500 and shown in FIG. 5. This described method 600
includes comparing (at 602) a planned radiation exposure dose of a
planned image-guided procedure to an actual radiation exposure dose
during the actual image-guided procedure. Of course, as part of the
comparison the actual exposure does is measured. In response to the
comparisons, the plan is updated (at 604). The actual may vary from
the plan for many reasons. For example, the practitioner may have
to place the seeds in different locations or take a different path
then what was planned because, for example, of aspects of the
anatomy not known at the time of planning. This is much like an
aircraft would alter is flight path in response to real-time
conditions such as weather.
[0100] With the updated plan, the practitioner would then have the
ability to check the accuracy of the plan, with its optimal
radiation dose, to the actual radiation dose, and would therefore
have additional information from which to assess the potential
clinical outcome of the treatment. For instance, if the actual
placement resulted in a lesser dose being delivered to the
treatment site, the practitioner could recommend an extension to
the overall treatment plan prior to retrieval of the seeds.
Premature removal of the seeds could result in sub-optimal dosing
of the treatment site, yielding a less effective treatment.
[0101] Attempting to gauge the effectiveness of placement through
imaging (X-ray and/or ultrasound) might not allow for a true or
accurate 3D positioning of the seeds and an inaccurate comparison
between planned and actual placement. Again, the result may be less
than optimal treatment (either in the form of too little dose to
the treatment site or too much dose to the healthy tissue).
[0102] While the invention has been described with reference to
certain embodiments, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted without departing from the scope of the invention. In
addition, many modifications may be made to adapt a particular
situation or material to the teachings of the invention without
departing from its scope. Therefore, it is intended that the
invention not be limited to the particular embodiment disclosed,
but that the invention will include all embodiments falling within
the scope of the appended claims.
* * * * *