U.S. patent application number 11/063094 was filed with the patent office on 2006-10-26 for reference pad for position sensing.
Invention is credited to Andres Claudio Altmann, Yaron Ephrath, Assaf Govari.
Application Number | 20060241397 11/063094 |
Document ID | / |
Family ID | 38068773 |
Filed Date | 2006-10-26 |
United States Patent
Application |
20060241397 |
Kind Code |
A1 |
Govari; Assaf ; et
al. |
October 26, 2006 |
Reference pad for position sensing
Abstract
A method for position tracking includes attaching a location pad
to a body of a subject and introducing into the body of the subject
a first position transducer. A procedure is performed on the body
of the subject using a medical tool, to which a second position
transducer is fixed. Magnetic fields are transmitted between the
location pad and first and second position transducers.
Responsively to the magnetic fields, first and second position
signals are generated. The signals are indicative of coordinates of
the first and second position transducers relative to the location
pad. The first and second position signals are processed so as to
determine a disposition of the tool relative to the first position
transducer.
Inventors: |
Govari; Assaf; (Haifa,
IL) ; Altmann; Andres Claudio; (Haifa, IL) ;
Ephrath; Yaron; (Karkur, IL) |
Correspondence
Address: |
PHILIP S. JOHNSON;JOHNSON & JOHNSON
ONE JOHNSON & JOHNSON PLAZA
NEW BRUNSWICK
NJ
08933-7003
US
|
Family ID: |
38068773 |
Appl. No.: |
11/063094 |
Filed: |
February 22, 2005 |
Current U.S.
Class: |
600/424 |
Current CPC
Class: |
A61B 2034/2051 20160201;
A61B 5/062 20130101; A61B 90/36 20160201; A61B 2090/3958 20160201;
A61B 34/20 20160201 |
Class at
Publication: |
600/424 |
International
Class: |
A61B 5/05 20060101
A61B005/05 |
Claims
1. A tracking system, comprising: a location pad, which is
configured to be attached to a body of a subject and to generate a
magnetic field within the body; a first position transducer, which
is adapted to be introduced into the body of the subject and
responsively to the magnetic field, to generate and transmit a
first position signal that is indicative of first coordinates of
the first position transducer relative to the location pad; a
second position transducer, which is fixed to a medical tool
adapted for performing a procedure on the body of the subject and
is adapted, responsively to the magnetic field, to generate and
transmit a second position signal that is indicative of second
coordinates of the second position transducer relative to the
location pad; and a system controller, which is coupled to receive
and process the first and second position signals so as to
determine a disposition of the tool relative to the first position
transducer.
2. The system according to claim 1, wherein the location pad is one
of at least first and second location pads, which are configured to
be attached to the body at respective locations and to generate
respective magnetic fields with respective ranges, and wherein the
first and second position transducers are adapted respectively to
generate the first and second position signals responsively to the
magnetic field within whose range they are located.
3. The system according to claim 2, wherein the system controller
is adapted to determine pad coordinates of the first location pad
relative to the second location pad, and thereby to register the
first and second coordinates in a common reference frame.
4. The method according to claim 3, wherein the first location pad
is adapted to generate a third position signal responsively to the
magnetic field of the second location pad, and wherein the system
controller is coupled to receive and process the third position
signal in order to determine the pad coordinates.
5. The system according to claim 4, and comprising a third location
pad, wherein the second location pad is adapted to generate a
fourth position signal responsively to a magnetic field of the
third location pad, and wherein the system controller is coupled to
receive and process the fourth position signal in order to register
the first, second and third location pads in the common reference
frame.
6. The system according to claim 2, wherein the first and second
location pads are mounted on a single unit, which is configured to
be fixed to the body of the subject.
7. The system according to claim 1, and comprising a third position
transducer, which is fixed in a frame of reference external to the
body of the subject and is adapted to generate a third position
signal responsively to the magnetic field, wherein the system
controller is coupled to receive and process the third position
signal so as to determine the first and second coordinates in the
external frame of the reference.
8. The system according to claim 1, wherein the location pad is
adapted to be affixed to a surface of the body of the subject.
9. The system according to claim 1, wherein the location pad
comprises a plurality of concentric, orthogonal magnetic field
generating coils.
10. The system according to claim 1, wherein at least one of the
first and second position transducers comprises one or more
transducer coils, which are adapted to sense the magnetic fields so
as to generate at least one of the first and second position
signals.
11. The system according to claim 1, and comprising a driving
antenna, which is adapted to radiate a radio frequency (RF)
electromagnetic field, and wherein the location pad comprises a
power coil, which is coupled to receive the RF electromagnetic
field and thereby to provide power for generating the magnetic
field.
12. The system according to claim 1, wherein the location pad
comprises an internal power source to provide power for generating
the magnetic field.
13. The system according to claim 1, wherein the first and second
position transducers comprise wireless transmitters for
communicating with the system controller.
14. The system according to claim 1, wherein the first position
transducer is coupled to communicate with the system controller via
a wired connection.
15. The system according to claim 14, and comprising a first wire
coupling the first position transducer to the location pad, and a
second wire coupling the location pad to the system controller.
16. A tracking system, comprising: a first position transducer,
which is adapted to be introduced into a body of a subject and to
generate a first magnetic field; a second position transducer,
which is fixed to a medical tool adapted for performing a procedure
on the body of the subject and is adapted to generate a second
magnetic field; a location pad, which is configured to be attached
to the body of the subject, to receive the first and second
magnetic fields, and to generate and transmit first and second
position signals that are indicative of respective first and second
coordinates of the first and second position transducers relative
to the location pad; and a system controller, which is coupled to
receive and process the first and second position signals so as to
determine a disposition of the tool relative to the first position
transducer.
17. The system according to claim 16, wherein the location pad is
one of at least first and second location pads, which are
configured to be attached to the body at respective locations, and
wherein the first and second location pads are adapted to receive
the first and second magnetic fields within respective ranges, so
that the first and second position signals are generated by the
location pad within whose range the first and second position
transducers are located.
18. The system according to claim 17, wherein the system controller
is adapted to determine pad coordinates of the first location pad
relative to the second location pad, and thereby to register the
first and second coordinates in a common reference frame.
19. The system according to claim 18, wherein the first location
pad is adapted to generate a third magnetic field, and wherein the
second location pad, responsively to the third magnetic field,
generates a third position signal, and wherein the system
controller is coupled to receive and process the third position
signal in order to determine the pad coordinates.
20. The system according to claim 19, and comprising a third
location pad, wherein the second location pad is adapted to
generate a fourth magnetic field, and wherein the third location
pad, responsively to the fourth magnetic field of the third
location pad, generates a fourth position signal, and wherein the
system controller is coupled to receive and process the fourth
position signal in order to register the first, second and third
location pads in the common reference frame.
21. The system according to claim 16, wherein the first and second
location pads are mounted on a single unit, which is configured to
be fixed to the body of the subject.
22. The system according to claim 16, and comprising a third
position transducer, which is fixed in a frame of reference
external to the body of the subject and is adapted to generate a
third magnetic field, wherein the location pad is adapted
responsively to the third magnetic field to generate a third
position signal, and wherein the system controller is coupled to
receive and process the third position signal so as to determine
the first and second coordinates in the external frame of the
reference.
23. The system according to claim 16, wherein the location pad is
adapted to be affixed to a surface of the body of the subject.
24. The system according to claim 16, wherein the location pad
comprises a plurality of concentric, orthogonal coils for receiving
the magnetic fields.
25. The system according to claim 16, wherein at least one of the
first and second position transducers comprises one or more
transducer coils, which are adapted to generate at least one of the
first and second magnetic fields.
26. The system according to claim 16, wherein the location pad is
adapted to transmit the first and second position signals over a
wireless connection to the system controller.
27. A method for position tracking, comprising: attaching a
location pad to a body of a subject; introducing into the body of
the subject a first position transducer; performing a procedure on
the body of the subject using a medical tool, to which a second
position transducer is fixed; transmitting magnetic fields between
the location pad and first and second position transducers;
responsively to the magnetic fields, generating first and second
position signals that are indicative of coordinates of the first
and second position transducers relative to the location pad; and
processing the first and second position signals so as to determine
a disposition of the tool relative to the first position
transducer.
28. The method according to claim 27, wherein transmitting the
magnetic fields comprises transmitting the magnetic fields from the
location pad, and wherein generating the first and second position
signals comprises generating the first and second position signals
responsively to the magnetic fields received at the first and
second position transducers.
29. The method according to claim 27, wherein transmitting the
magnetic fields comprises transmitting the magnetic fields from the
first and second position transducers, and wherein generating the
first and second position signals comprises generating the first
and second position signals responsively to the magnetic fields
received at the location pad from the first and second position
transducers.
30. The method according to claim 27, wherein attaching the
location pad comprises attaching first and second location pads to
the body, each of the first and second location pads having a
respective range, and wherein the first and second position signals
are indicative of the coordinates of the first and second position
transducers relative to one of the first and second location pads
within whose range the first and second position transducers are
located.
31. The method according to claim 30, and comprising determining
pad coordinates of the first location pad relative to the second
location pad, and thereby registering the first and second
coordinates in a common reference frame.
32. The method according to claim 31, wherein determining the pad
coordinates comprises generating a third position signal
responsively to a magnetic field transmitted between the first and
second location pads, and receiving and processing the third
position signal in order to determine the pad coordinates.
33. The method according to claim 32, wherein attaching the
location pad further comprises attaching a third location pad to
the body, and further comprising generating a fourth position
signal responsively to a further magnetic field transmitted between
the second and third location pads, and receiving and processing
the fourth position signal in order to register the first, second
and third location pads in the common reference frames.
34. The method according to claim 27, wherein attaching the
location pad comprises mounting the first and second location pads
on a single unit, which is configured to be fixed to the body of
the subject.
35. The method according to claim 27, and comprising fixing a third
position transducer in a frame of reference external to the body of
the subject, generating a third position signal responsively to a
magnetic field transmitted between the third position transducer
and the location pad, and processing the third position signal so
as to determine the first and second coordinates in the external
frame of the reference.
36. The method according to claim 35, wherein processing the third
position signal comprises monitoring movement of a part of the body
of the subject using the first and third position transducers.
37. The method according to claim 27, wherein attaching the
location pad comprises fixing the location pad to a surface of the
body of the subject.
38. The method according to claim 27, and comprising transmitting a
radio frequency (RF) electromagnetic field to provide power for
generating the magnetic fields.
39. The method according to claim 27, wherein processing the first
and second position signals comprises receiving the first and
second position signals over a wireless connection.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to tracking the
spatial coordinates of objects used during medical procedures, and
specifically to methods and devices for tracking the position and
orientation of medical tools and intrabody devices.
BACKGROUND OF THE INVENTION
[0002] Various methods and systems are known in the art for
tracking the coordinates of objects involved in medical
procedures.
[0003] For example, U.S. Pat. Nos. 5,391,199 and 5,443,489 to
Ben-Haim, whose disclosures are incorporated herein by reference,
describe systems wherein the coordinates of an intrabody probe are
determined using one or more field transducers, such as a Hall
effect device, coils, or other antennae carried on the probe. Such
systems are used for generating location information regarding a
medical probe or catheter. A sensor, such as a coil, is placed in
the probe and generates signals in response to externally-applied
magnetic fields. The magnetic fields are generated by magnetic
field transducers, such as radiator coils, fixed to an external
reference frame in known, mutually-spaced locations.
[0004] PCT Patent Publication WO 96/05768, U.S. Pat. No. 6,690,963,
and U.S. patent application Ser. No. 09/414,875, all to Ben-Haim et
al. (published as U.S. Patent Application Publication US
2002/0065455), whose disclosures are incorporated herein by
reference, describe a system that generates six-dimensional
position and orientation information regarding the tip of a
catheter. This system uses a plurality of sensor coils adjacent to
a locatable site in the catheter, for example near its distal end,
and a plurality of radiator coils fixed in an external reference
frame. These coils generate signals in response to magnetic fields
generated by the radiator coils, which signals allow for the
computation of six location and orientation coordinates.
[0005] U.S. Pat. No. 6,239,724 to Doron et al., whose disclosure is
incorporated herein by reference, describes a wireless, telemetry
system for providing coordinates of an intrabody object. The system
includes an implantable telemetry unit having (a) a first
transducer, for converting a power signal received from outside the
body into electrical power for powering the telemetry unit; (b) a
second transducer, for receiving a positioning field signal that is
received from outside the body; and (c) a third transducer, for
transmitting a locating signal to a site outside the body, in
response to the positioning field signal.
[0006] The above-mentioned U.S. patent application Ser. No.
10/029,473 to Govari, published as U.S. Patent Application
Publication 2003/0120150, describes a system wherein a wireless
transponder is fixed to an object. The transponder includes at
least one sensor coil, in which a signal current flows responsive
to the electromagnetic fields, and a power coil, which receives the
RF driving field and conveys electrical energy from the driving
field to power the transponder. The power coil also transmits an
output signal responsive to the signal current to a signal
receiver, which processes the signal to determine coordinates of
the object. In an embodiment of the invention, the object is a hip
joint.
[0007] U.S. Pat. No. 6,618,612 to Acker et al., whose disclosure is
incorporated herein by reference, describes a system wherein
reference field transducers are independently movable with respect
to one another to desired positions close to or mounted upon the
body. Calibration transducers determine the relative positions of
the field transducers with respect to one another after they are
located in their desired positions. From the detected fields, the
relative disposition of the probe with respect to the reference
field transducers is determined.
[0008] U.S. Pat. No. 6,332,089 to Acker et al., whose disclosure is
incorporated herein by reference, describes a system wherein a site
probe is placed within the body of a patient and an instrument
probe is guided within the body. One or more fields are transmitted
to or from each of the probes, which are adapted to detect each
such field. The relative disposition of the site probe and the
instrument probe is determined from the properties of the detected
fields. Furthermore, the instrument probe is directed toward the
site probe on the basis of the so determined relative disposition.
As described by Acker, relative disposition may mean relative
position and/or orientation.
[0009] U.S. Patent Application Publication 2004/0068178 to Govari,
whose disclosure is incorporated herein by reference, describes a
system wherein primary radiators are driven by a control unit to
track the positions of a plurality of secondary radiators with
respect to the primary radiators. The secondary radiators are
optionally movable, and are driven to track the position of the
probe with respect to the secondary radiators. A calculation is
then performed to determine the corresponding position of the probe
with respect to the fixed locations of the primary radiators.
SUMMARY OF THE INVENTION
[0010] Embodiments of the present invention provide magnetic
tracking systems for use in tracking the positions of objects
related to a medical procedure, such as medical tools and intrabody
devices. The system comprises one or more location pads attached to
the body and one or more position transducers that are inserted
into the body. In some embodiments, the location pads transmit
magnetic fields, which are received by the transducers. In other
embodiments, the transducers inside the body transmit magnetic
fields, which are received by the location pads. In both cases, the
received field amplitudes are used in determining the coordinates
of the transducers in the body relative to one or more of the
location pads.
[0011] Typically, each location pad is attached to the body surface
close to the area in which the position transducer is located. As a
result, accurate coordinates may be determined while transmitting
relatively weak magnetic fields, and interference of metal objects
with the tracking system is reduced. There is no limitation on
movement of the patient's body during the medical procedure, since
the location pad moves together with the body.
[0012] In some embodiments of the present invention, one of the
position transducers is fixed to a structure inside the body, and
another position transducer is attached to a surgical tool. Both
the fixed transducer and the tool transducer transmit or receive
magnetic fields to or from the same location pad. By processing the
received field amplitudes, the coordinates of the tool and the
fixed transducer relative to the location pad are determined, and
thus the coordinates of the tool relative to the fixed transducer
is known. The relative coordinates may be used to guide a medical
practitioner in manipulating the tool to perform a medical
procedure on the body structure to which the position transducer
inside the body is fixed.
[0013] In some embodiments, these systems are used in orthopedic
procedures, such as implantation of implants such as screws, nails,
rods or prosthetic joints. For this purpose, wireless or wired
magnetic position transducers may be inserted into the patient's
bone, into prosthetic implants and into tools used during surgery.
The tracking system determines the coordinates of the transducers,
and thus enables the surgeon to visualize the locations and
orientations of these elements while reducing or eliminating the
need for intraoperative X-ray imaging. Implanted position
transducers may also be used in post-operative follow-up. In other
embodiments, body-surface location pads are used in conjunction
with position transducers in body structures and devices used in
other medical procedures, such as endoscopy and cardiovascular
catheterization.
[0014] In embodiments of the present invention, the origin of the
coordinate system in which the transducer coordinates are found is
"floating," i.e., it moves with the patient without necessarily
having a fixed frame of reference in space. In some embodiments,
when multiple location pads are used, one of the pads may be chosen
as the primary, floating origin for the coordinate system. The
other pads may then contain position transducers, for use in
determining the coordinates of these pads relative to the primary
origin. The positions of all pads and transducers may thus be
monitored within a single coordinate system, based on the primary
floating origin.
[0015] Additionally or alternatively, the coordinates of the
location pads (and hence of the position transducers) may be
determined in a fixed, external frame of reference, such as an
operating table, by transmitting magnetic fields between the
location pads and a field generator or receiver in a known location
in the external frame of reference.
[0016] In some embodiments, implants, tools, and body surface pads
convey position information by wireless means to a wireless control
unit. The wireless unit is coupled to a computer that processes the
position signals and determines the relative dispositions. Power to
operate the transducers and pad coils may be provided by batteries
or by wireless inductive signals.
[0017] In further embodiments, to the location pads may take the
form of one or more rings or tapes, which are attached to the body
near the surgical site. Each ring or tape comprises multiple coils
spaced along its length. The coordinates of target objects are
determined by sensing the fields that are generated or received by
the coil that gives the strongest signal--typically whichever coil
is closest to the object in question.
[0018] There is therefore provided, in accordance with an
embodiment of the present invention, a tracking system,
including:.
[0019] a location pad, which is configured to be attached to a body
of a subject and to generate a magnetic field within the body;
[0020] a first position transducer, which is adapted to be
introduced into the body of the subject and responsively to the
magnetic field, to generate and transmit a first position signal
that is indicative of first coordinates of the first position
transducer relative to the location pad;
[0021] a second position transducer, which is fixed to a medical
tool adapted for performing a procedure on the body of the subject
and, responsively to the magnetic field, to generate and transmit a
second position signal that is indicative of second coordinates of
the second position transducer relative to the location pad;
and
[0022] a system controller, which is coupled to receive and process
the first and second position signals so as to determine a
disposition of the tool relative to the first position
transducer.
[0023] In some embodiments, the location pad is one of at least
first and second location pads, which are configured to be attached
to the body at respective locations and to generate respective
magnetic fields with respective ranges, and the first and second
position transducers are adapted respectively to generate the first
and second position signals responsively to the magnetic field
within whose range they are located. In some embodiments, the
system controller is adapted to determine pad coordinates of the
first location pad relative to the second location pad, and thereby
to register the first and second coordinates in a common reference
frame. In further embodiments, the first location pad is adapted to
generate a third position signal responsively to the magnetic field
of the second location pad, and the system controller is coupled to
receive and process the third position signal in order to determine
the pad coordinates. In still further embodiments, the system
includes a third location pad, and the second location pad is
adapted to generate a fourth position signal responsively to a
magnetic field of the third location pad, and the system controller
is coupled to receive and process the fourth position signal in
order to register the first, second and third location pads in the
common reference frame.
[0024] In some embodiments, the first and second location pads are
mounted on a single unit, which is configured to be attached to the
body of the subject.
[0025] In additional embodiments, a third position transducer is
fixed in a frame of reference external to the body of the subject
and is adapted to generate a third position signal responsively to
the magnetic field, and the system controller is coupled to receive
and process the third position signal so as to determine the first
and second coordinates in the external frame of the reference.
[0026] Typically, the location pad is adapted to be affixed to a
surface of the body of the subject.
[0027] In disclosed embodiments, the location pad includes a
plurality of concentric, orthogonal magnetic field generating
coils.
[0028] In some embodiments, at least one of the first and second
position transducers includes one or more transducer coils, which
are adapted to sense the magnetic fields so as to generate at least
one of the first and second position signals.
[0029] In a disclosed embodiment, a driving antenna is adapted to
radiate a radio frequency (RF) electromagnetic field, and the
location pad includes a power coil, which is coupled to receive the
RF electromagnetic field and thereby to provide power for
generating the magnetic field. Alternatively, the location pad
includes an internal power source to provide power for generating
the magnetic field.
[0030] Typically, the first and second position transducers include
wireless transmitters for communicating with the system
controller.
[0031] There is also provided, in accordance with an embodiment of
the present invention, a tracking system, including:
[0032] a first position transducer, which is adapted to be
introduced into a body of a subject and to generate a first
magnetic field;
[0033] a second position transducer, which is fixed to a medical
tool adapted for performing a procedure on the body of the subject
and which is adapted to generate a second magnetic field;
[0034] a location pad, which is configured to be attached to the
body of the subject, to receive the first and second magnetic
fields, and to generate and transmit first and second position
signals that are indicative of respective first and second
coordinates of the first and second position transducers relative
to the location pad; and
[0035] a system controller, which is coupled to receive and process
the first and second position signals so as to determine a
disposition of the tool relative to the first position
transducer.
[0036] Typically, the location pad is adapted to transmit the first
and second position signals over a wireless connection to the
system controller.
[0037] There is also provided, in accordance with an embodiment of
the present invention, a method for position tracking,
including:
[0038] attaching a location pad to a body of a subject;
[0039] introducing into the body of the subject a first position
transducer;
[0040] performing a procedure on the body of the subject using a
medical tool, to which a second position transducer is fixed;
[0041] transmitting magnetic fields between the location pad and
first and second position transducers;
[0042] responsively to the magnetic fields, generating first and
second position signals that are indicative of coordinates of the
first and second position transducers relative to the location pad;
and
[0043] processing the first and second position signals so as to
determine a disposition of the tool relative to the first position
transducer.
[0044] Typically, transmitting the magnetic fields includes
transmitting the magnetic fields from the location pad, and
generating the first and second position signals includes
generating the first and second position signals responsively to
the magnetic fields received at the first and second position
transducers.
[0045] Alternatively, transmitting the magnetic fields includes
transmitting the magnetic fields from the first and second position
transducers, and generating the first and second position signals
includes generating the first and second position signals
responsively to the magnetic fields received at the location pad
from the first and second position transducers.
[0046] The present invention will be more fully understood from the
following detailed description of the embodiments thereof, taken
together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] FIG. 1A is a schematic, pictorial illustration of a magnetic
tracking system used in surgery, in accordance with an embodiment
of the present invention;
[0048] FIG. 1B is a schematic, pictorial illustration of a magnetic
tracking system used in surgery, in accordance with an alternative
embodiment of the present invention;
[0049] FIG. 1C is a schematic, pictorial illustration of a magnetic
tracking system used in surgery, in accordance with another
alternative embodiment of the present invention;
[0050] FIGS. 2A and 2B are schematic, partly sectional
illustrations, showing insertion of implantable position
transducers into the bone of a patient, in accordance with an
embodiment of the present invention;
[0051] FIGS. 3A and 3B are schematic, pictorial illustrations
showing details of position transducers, in accordance with
embodiments of the present invention;
[0052] FIG. 4 is a schematic, pictorial illustration showing
details of a body surface pad, in accordance with an embodiment of
the present invention;
[0053] FIG. 5 is a schematic, pictorial illustration showing a
surgical tool and a position transducer used to track coordinates
of the tool, in accordance with an embodiment of the present
invention; and
[0054] FIG. 6 is a schematic, pictorial illustration showing an
alternate configuration of body surface pads, in accordance with an
embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
[0055] FIG. 1A is a schematic, pictorial illustration of a magnetic
tracking system 20 for use in surgery, in accordance with an
embodiment of the present invention. In the pictured embodiment, a
surgeon 22 is performing a procedure that involves maneuvering a
tool 24 to positions in contact with, or relative to, implantable
devices or probes 26 and 28, hereinafter referred to as implants 26
and 28. In the example of FIG. 1A, implants 26 and 28 have been
introduced into the body at a surgical site, which is located in a
leg 30 of a patient 32. In this example, implants 26 and 28 have
been introduced into the patient's tibia and femur near the knee,
for use in guiding the surgeon in performing a procedure on the
knee joint using tool 24. Exemplary methods and tools for use in
inserting the implants into the bone are described in U.S. patent
application No. (Applicant's Docket No. DEP-5482) entitled, "An
Instrument for Implanting a Sensor," filed on Feb. 22, 2005, which
is assigned to the assignee of the present application and whose
disclosure is incorporated herein by reference. This application of
the present invention is shown solely by way of example, however.
Other applications will be apparent to those skilled in the art and
are considered to be within the scope of the present invention.
[0056] Both tool 24 and implants 26 and 28 contain miniature,
wireless position transducers, which are described in detail
hereinbelow. In this embodiment, the transducers are wireless, but
the transducers may alternatively have wired connections for
electrical power and communications, as shown below in FIG. 1B.
Each transducer may be designed either to transmit or receive
magnetic fields. The fields are used in generating position signals
indicative of the transducer's location and orientation
coordinates, as described hereinbelow. Tracking system 20 thus
enables surgeon 22 to monitor the position of tool 24 relative to
implants 26 and 28 throughout a working volume that comprises the
space around and including the surgical site. Additional medical
devices and tools with position transducers similar to those of
implants 26 and 28 may also be used at additional locations in the
area of the surgical site. For example, the use of such position
transducers in a hip implant is shown in the above-mentioned U.S.
patent application Ser. No. 10/029,473.
[0057] Alternatively, although the embodiment shown in the figures
relates to orthopedic applications, the principles of the present
invention may similarly be applied in other types of medical
applications. For example, location pads 34 and 36 may be used in
determining the coordinates of position transducers in invasive
probes, such as catheters and endoscopes, which are inserted into
the cardiovascular system and other organs of the body.
[0058] The coordinates of the transducers in tool 24 and implants
26 and 28 are determined relative to location pads 34 and 36, which
are fixed to the body. The pads may conveniently be glued or
strapped on to the body surface, or held against the skin by some
other means. In the example shown in FIG. 1A, these pads are placed
on the patient's calf and thigh, in proximity to implants 26 and
28. Alternatively, the location pads may be held away from the skin
by support structures that are fixed to the body, so that the pads
move with the body part to which they are in proximity.
[0059] Location pads 34 and 36 comprise magnetic field transducers,
such as coils, which are used to transmit or receive magnetic
fields. In other words, if the transducers in implants 26 and 28
and in tool 24 are configured to receive magnetic fields, then
location pads 34 and 36 are configured as field generators.
Alternatively, the location pads may be configured to receive
fields generated by the position transducers in the implants and
the tool. For the sake of simplicity in the description that
follows, it is assumed that location pads 34 and 36 transmit the
magnetic fields, which are received by the transducers in implants
26 and 28 and in tool 24. The roles of transmitter and receiver may
be reversed in a straightforward manner, as will be apparent to
those skilled in the art.
[0060] Surgeon 22 is generally free to place location pads 34 and
36 at any convenient location in the vicinity of the surgical site,
as long as each of the pads is close enough to the implant (or
implants) with which it is to communicate during the procedure so
that the field strength remains sufficient to generate an
acceptable signal. Typically, it is desirable for this purpose that
the distance between pad and implant be kept to not more than about
20 cm, but greater or smaller distances may be appropriate
depending on the size and strength of the field transducers. For
example, 1 cm to 40 cm, and preferably a range from about 4 cm to
about 20 cm.
[0061] The field generator coils in pads 34 and 36 generate
electromagnetic fields at different, respective sets of frequencies
{.omega..sub.1} and {.omega..sub.2}. Typically, the sets comprise
frequencies in the approximate range of 100 Hz-30 kHz, although
higher and lower frequencies may also be used. The sets of
frequencies at which the coils radiate are set by a computer 38,
which serves as the system controller for system 20. The
frequencies {.omega..sub.1} and {.omega..sub.2} are set through
frequency scanning techniques for identifying optimal frequencies
respectively such as described in U.S. Pat. No. 6,373,240 which is
incorporated herein by reference. For the purposes of system 20,
pads 34 and 36 are placed in close proximity to the surgical site
so that minimal energy is needed to generate the magnetic field.
The pads are typically positioned such that the working volume of
the tracking system includes the entire area in which the surgeon
is operating. Furthermore, pads 34 and 36 are positioned so as not
to impede access to the surgical site.
[0062] At any instant in time, the applied magnetic fields induce
currents in coils contained in the transducers of tool 24 and of
implants 26 and 28. The induced currents comprise components at the
specific frequencies in sets {.omega..sub.1} and {.omega..sub.2}.
The respective amplitudes of these currents (or alternatively, of
time-varying voltages that may be measured across the transducer
coils) are dependent on the location and orientation of the
position transducer relative to the locations and orientations of
the field generator coils. In response to the induced currents or
voltages, signal processing and transmitter circuits in each
position transducer generate and transmit position signals that are
indicative of the location and orientation of the transducer.
[0063] These position signals are received by a wireless control
unit 40, which is coupled to computer 38. Alternatively, the
transducers of tool 24 and of implants 26 and 28 may be connected
by wire directly to computer 38, as shown in FIG. 1B. The computer
processes the received signals in order to calculate the relative
location and orientation coordinates of tool 24 and of implants 26
and 28. Hereinbelow, the relative location and/or orientation of
one object to another, determined in any or all of six dimensions,
is referred to as the relative disposition of the two objects. Of
the six dimensions, three dimensions represent the X, Y, and Z
coordinates of one object relative to the other. Three additional
dimensions represent the angular orientation of one object relative
to the other. Disposition in one dimension, for example, may mean
simply the distance between the origins of the two objects.
[0064] The disposition of the tool relative to each of the implants
is calculated based on the magnetic field that is generated by the
location pad on the limb in which the implant is located. In other
words, in the example shown in FIG. 1A, the disposition of the tool
relative to implant 26 is calculated based on the field generated
by location pad 34, while the disposition of the tool relative to
implant 28 is calculated based on the field generated by location
pad 36. Consequently, the disposition of the tool relative to each
of the implants (and hence of the bones in which the implants are
located) can be determined accurately notwithstanding motion of leg
30.
[0065] Optionally, one of the location pads may also comprise a
position transducer that receives the magnetic field generated by
the other location pad. The signals received by this transducer may
then be used by computer 38 in registering the separate, "floating"
coordinate systems of the two location pads. The registration may
be updated whenever leg 30 is moved. In this case, determination of
the coordinates of tool 24 in the frame of reference of either of
location pads 34 and 36 is sufficient to determine the disposition
of the tool relative to both of implants 26 and 28.
[0066] In embodiments in which the coordinate systems of multiple
location pads are mutually registered, computer 38 determines the
coordinates of tool 24 using the location pad that gives the most
accurate position signal. Typically, the coordinates of the tool
are determined based on the magnetic field that the tool transducer
receives with the least noise or interference. As the tool moves
through the working volume, a magnetic field signal from a first
pad may initially provide the greatest accuracy and is therefore
used to determine the relative disposition of the tool and the
implants. Subsequently, the field from a second pad may generate a
more accurate position signal, and the tracking process is
"handed-off," such that the disposition coordinates are now
determined based on the field from the second pad.
[0067] The coordinates are used by the computer in driving a
display 42, which shows the dispositions of the tool, screw and
other elements (such as prosthetic implants) to which position
transducers have been fixed.
[0068] Whereas system 20 is shown as comprising a specific
configuration of implants, tools, and body surface pads, in other
embodiments of the present invention, different numbers, types and
configurations of devices may used.
[0069] In other embodiments of the invention, as noted above, the
generation and reception of the magnetic fields are reversed such
that the coils in the implants and in the tool generate the
position-responsive magnetic fields, and the body surface pads
receive the fields. The relative disposition of the tool and either
of the implants is determined as above, by comparing the position
signals induced in pads 34 and 36 by the fields radiated from the
tool and the implant. In further embodiments, any or all of the set
of tools, implants, and pads may comprise transducers configured to
receive and to generate magnetic fields, such that there is
flexibility in selecting the coordinate system and the floating
origin.
[0070] Additionally or alternatively, a field transducer 46 may be
attached to a fixed frame of reference, such as an operating table
44 on which patient 32 is lying, and used as a fixed coordinate
reference. Magnetic fields transmitted between fixed field
transducer 46 and location pads 34, 36 on the patient's body or
implants 26, 28 or both may be used to register the floating origin
of the location pad coordinates or the implants respectively with
the fixed frame of reference. When fixed field transducer 46
interacts with the location pads, it can comprise a transducer of
the type which is used in implants 26, 28 and instrument 24. When
fixed field transducer 46 interacts with implants 26, 28, the
functional components of field transducer 46 by which a field is
generated can be similar to those in the location pads 36, 38.
[0071] A fixed field transducer can be used to monitor movement of
the patient (or of a part of the patient such as a limb). In
particular, it can be used to monitor movement during preparatory
steps, for example to determine the point about which a limb moves
(such as the center of rotation of the femur relative to the
acetabulum). It can also be used to monitor movement during a
procedure. In selecting the location of a fixed field transducer,
the range of movement of the patient (or of a part of the patient,
such as a limb) and the required signal strength for monitoring the
movement may be taken into account, to ensure that an adequately
strong signal is generated when the limb moves.
[0072] When a metal or other magnetically-responsive article is
brought into the vicinity of an object being tracked, such as
implant 26 or tool 24, the magnetic fields in this vicinity are
distorted. In the surgical environment shown in FIG. 1A, for
example, there can be a substantial amount of conductive and
permeable material, including basic and ancillary equipment
(operating tables, carts, movable lamps, etc.), as well as invasive
surgery apparatus (scalpels, scissors, etc., including tool 24
itself). The magnetic fields produced by the field generator coils
may generate eddy currents in such articles, and the eddy currents
then cause a parasitic magnetic field to be radiated. Such
parasitic fields and other types of distortion can lead to errors
in determining the position of the object being tracked.
[0073] In order to alleviate this problem, the elements of tracking
system 20 and other articles used in the vicinity of the tracking
system are typically made of non-metallic materials when possible,
or of metallic materials with low permeability and conductivity. In
addition, computer 38 may be programmed to detect and compensate
for the effects of metal objects in the vicinity of the surgical
site. Exemplary methods for such detection and compensation are
described in U.S. Pat. Nos. 6,147,480 and 6,373,240, as well as in
U.S. patent application Ser. Nos. 10/448,289, filed May 29, 2003,
and 10/632,217, filed Jul. 31, 2003, all of whose disclosures are
incorporated herein by reference.
[0074] FIG. 1B is a schematic, pictorial illustration showing an
alternative configuration of system 20, in accordance with another
embodiment of the present invention. This embodiment functions in a
manner that is substantially identical to the embodiment of FIG.
1A. In FIG. 1B, however, implants 26 and 28 and location pads 34
and 36 are connected by wires to an interface unit 47. The
interface unit connects the wires through to computer 38.
Alternatively, the wires may connect the implants to the location
pads, which then communicate (over wire or wireless connections)
with control unit 40 and/or computer 38. In the embodiment shown in
FIG. 1B, tool 24 still communicates with control unit 40 over a
wireless connection, although the tool may alternatively have a
wired connection.
[0075] Location pads 36 and 38 are marked with arrows 49, whose
purpose will now be explained. In this explanation, it is assumed
that the location pads transmit magnetic fields, which are sensed
by the transducers in implants 26 and 28; but the considerations in
the explanation are equally applicable to the reverse situation, in
which the implants transmit magnetic field to the location pads. As
noted above, the magnetic field transducers in the location pads
and implants typically comprise coils. The transmitting coils in
the location pads generate magnetic dipole fields, which induce
currents in the receiving coils of the implants. The currents are
indicative of the location and orientation of the receiving coils
relative to the transmitting coils. Because of the symmetry of the
dipole field, however, the measurement is ambiguous, i.e., the
point (x,y,z) will give the same current amplitudes as (-x,-y,-z).
This ambiguity may lead to errors in the determination of the
location of tool 24 relative to implants 26 and 28.
[0076] In order to resolve this ambiguity, the orientation of at
least one of the location pads relative to at least one of the
implants is fixed in advance. For this purpose, in the present
embodiment, each of the location pads is marked with an arrow 49,
which is aligned in a known direction with respect to the magnetic
field transducer on the location pad. Alternatively, other marks or
guides may be used in aligning the location pad transducer, as will
be apparent to those skilled in the art. The operator of system 20,
such as surgeon 22, places pads 34 and 36 on leg 30 in such a way
that arrows 49 on pads 34 and 36 point toward the locations of
implants 24 and 26. (In this case, both arrows point toward the
patient's knee joint.) Assuming arbitrarily that arrows 49 are
aligned along the positive X-axes of the respective pads, for
instance, it will then be known a priori that only coordinate
readings of the implant locations with positive X-coordinate values
can be correct. Thus, the orientation of the coordinate axes is
known, and all ambiguity is resolved.
[0077] Although arrows 49 are marked on both location pads 34 and
36 in the embodiment of FIG. 1B, it is in fact sufficient that the
orientation of one of the location pads be fixed in advance. In
this case, the disambiguated coordinates of both implants may be
determined initially with respect to this location pad. The
orientation of the axes of the other location pad may then be
determined correctly based on the known coordinates of the
implants. Alternatively, other means and methods of coordinate
disambiguation may be used in system 20.
[0078] FIG. 1C is a schematic, pictorial illustration showing
another alternative configuration of system 20, in accordance with
a further embodiment of the present invention. This embodiment
functions in a manner that is substantially identical to the
embodiment of FIG. 1B, except that in this case, implants 26 and 28
are connected by wires to respective location pads 34 and 36, which
are connected to interface unit 47.
[0079] The wires connected to implants 26 and 28 pass through soft
tissue between the bone in which the implants are located and the
surface of the skin above the bone. Typically, as a joint is
manipulated during an operation, there is relative lateral movement
between the bone and the surface of skin, especially in larger
patients. This relative movement results in an increase in the
distance through the soft tissue through which the wires connected
to implants 26 and 28 must pass. (The distance is smallest when the
path of the wire is perpendicular to the bone surface, and
increases with the angle of the wire path.) In order that the wire
be able to accommodate such movement, it is desirable to flex the
joint after inserting the implants into the bone in order to "pull"
loose wire into the soft tissue. The loose wire is then taped down
to prevent further movement.
[0080] FIG. 1C also shows a reference transducer 49, which is fixed
to operating table 44. Transducer 49 receives the magnetic fields
generated by location pads 36 and 38 or, alternatively, generates
magnetic fields that are received by the location pads (in a manner
similar to fixed field transducer 46 described above). Transducer
49 thus serves to measure the location and motion of leg 30 in the
fixed frame of reference of operating table 44.
[0081] FIG. 2A is a schematic, sectional illustration showing
implanted screw 48, adapted to perform the functions of the
above-described implants 26 and 28, in accordance with an
embodiment of the present invention. Screw 48 is implanted into a
bone 50, such as the femur of patient 32. To insert the screw,
surgeon 22 makes an incision through overlying soft tissue 52, and
then rotates the screw into bone 50 using tool 24, for example.
Alternatively, the screw may be inserted percutaneously, without
prior incision. Note that in the embodiment of FIG. 2A, screw 48
has no wired connection to elements outside the body. In
alternative embodiments, a wire connects screw 48 to external
units, such as computer 38. The configuration and operation of the
circuits in screw 48 are described hereinbelow with reference to
FIGS. 3A and 3B.
[0082] FIG. 2B is a schematic, sectional illustration showing
another position transducer device 54, which may similarly perform
the functions of implants 26 and 28, in accordance with an
alternative embodiment of the present invention. Device 54
comprises an implantable screw 56, which is coupled by wires 58 to
an external unit 60. Screw 56 is inserted into bone 50 in
substantially the same manner as is screw 48 (leaving wires 58 to
pass out of the patient's body through soft tissue 52). External
unit 60 may contain a battery or power coil to power signal
processing circuitry. When screw 56 comprises coils for generating
a magnetic field, external unit 60 may provide the power to these
coils. External unit 60 may also provide power for a wireless
transponder, or communication coil, used to transmit position
signals to wireless control unit 40. Alternatively, as noted above,
screw 56 may be connected by wires 58 to interface unit 47 or
directly to computer 38.
[0083] FIG. 3A is a schematic, pictorial illustration of a position
transducer 70 that is contained in screw 48, in accordance with an
embodiment of the present invention. Alternatively, transducer 70
may be contained in or otherwise attached to other types of
implants and invasive devices. Transducer 70 in this embodiment
comprises three sets of coils: transducer coils 72, power coils 74,
and a wireless transponder coil, or communication coil 76.
Alternatively, the functions of the power and communication coils
may be combined, as described in the above-mentioned U.S. patent
application Ser. No. 10/029,473. Further alternatively, although
communication coil 76 is shown in FIG. 3A to be wound in a plane
that is perpendicular to the longitudinal axis of screw 48, the
communication coil or antenna may alternatively be arranged along
the length of transducer 70, roughly parallel to the longitudinal
axis of the screw. Coils 72, 74 and 76 are coupled to electronic
processing circuitry 78, which is mounted on a suitable substrate
80, such as a flexible printed circuit board (PCB) Details of the
construction and operation of circuitry 78 are described in the
above-mentioned U.S. patent application Ser. No. 10/029,473 and in
the above-mentioned U.S. patent application 10/706,298.
[0084] Although for simplicity, FIG. 3A shows only a single
transducer coil 72 and a single power coil 74, in practice
transducer 70 typically comprises multiple coils of each type, such
as three transducer coils and three power coils. The transducer
coils are wound together, in mutually-orthogonal directions, on a
transducer core 82, while the power coils are wound together, in
mutually-orthogonal directions, on a power core 84. Alternatively,
the transducer and power coils may be overlapped on the same core,
as described, for example in U.S. patent application 10/754,751,
filed Jan. 9, 2004, whose disclosures are incorporated herein by
reference.
[0085] In another embodiment, not shown in the figures, transducer
coils 72 are non-concentric. The use of non-concentric coils is
described, for example, in the above-mentioned PCT Patent
Publication WO 96/05768 and in the corresponding U.S. patent
application Ser. No. 09/414,875. Alternatively, the position
transducer may comprise only a single transducer coil or two
transducer coils. Further alternatively, implants 26 and 28, and
tool 24 may comprise magnetic position transducers based on sensing
elements of other types known in the art, such as Hall effect
transducers or magneto-resistive components.
[0086] In operation, power coils 74 serve as a power source for
transducer 70. The power coils receive energy by inductive coupling
from an external driving antenna (which may be a part of wireless
control unit 40, shown in FIG. 1A). Typically, the driving antenna
radiates an intense electromagnetic field at a relatively high
radio frequency (RF), such as in the range of 13.5 MHz. The driving
field causes currents to flow in coils 74, which are rectified in
order to power circuitry 78. Although certain frequency ranges are
cited here by way of example, those skilled in the art will
appreciate that other frequency ranges may be used for the same
purposes.
[0087] Meanwhile, the magnetic fields generated by location pads 34
and 36 (FIG. 1A or 1B) induce time-varying signal voltages to
develop across transducer coils 72, as described above. Circuitry
78 senses the signal voltages, and generates output signals in
response thereto. The output signals may be either analog or
digital in form. Circuitry 78 drives communication coil 76 to
transmit the output signals to a receiving antenna typically in
wireless control unit 40). Additionally or alternatively, coil 76
may be used to receive control information, such as a clock signal,
from a transmitting antenna (not shown) outside the patient's
body.
[0088] As noted above, transducer coils 72 may alternatively be
driven to generate magnetic fields, which induce position signals
in the coils in the body-surface location pads. Further
alternatively, position transducer 70 may be connected by wire
directly to computer 38, bypassing wireless control unit 40.
Typically, in the wired configuration, communication coil 74 and
power coil 76 are not employed, and the respective communication
and power signals are transmitted by wire.
[0089] FIG. 3B is a schematic, pictorial illustration of a position
transducer 90, in accordance with another embodiment of the present
invention. Transducer 90 differs from transducer 70 in that
transducer 90 comprises a battery 92 as its power source, instead
of power coils 74. Battery 92 may be of any suitable type, either
single-use or rechargeable. In other respects, the operation of
transducer 90 is substantially similar to that of transducer 70, as
described above.
[0090] FIG. 4 is a schematic, pictorial illustration showing
details of a body-surface location pad 94, in accordance with an
embodiment of the present invention. Pad 94 is adapted to perform
the functions of location pads 34 and 36 (FIG. 1A). The pad
comprises a base 95, such as an adhesive patch, which permits the
pad to be fixed easily and securely to the body surface in any
desired location.
[0091] Body surface pad 94 comprises a magnetic field transducer
98, typically in the form of three coils wound in orthogonal
directions around a cubic magnetic core. Typically, each coil
comprises wire of approximately 250 microns in outer diameter,
wrapped several hundred turns around the core.
[0092] Alternatively, transducer 98 may comprise a smaller or
larger number of coils, and the coils may be concentric or
non-concentric. Transducer 98 may be used either to transmit or to
receive magnetic fields, depending on the configuration of the
magnetic sensing system. Typically, when transducer 98 is
configured to generate magnetic fields, the current in each coil is
approximately 100 mA. Alternatively, pad 94 may comprise magnetic
field transducers of other types. For example, when configured to
receive magnetic fields, pad 94 may comprise magnetic position
transducers based on sensing elements of other types known in the
art, such as Hall effect transducers or magneto-resistive
components.
[0093] Pad 94 further contains a battery 96, which powers a control
circuit 97. Alternatively, battery 96 is replaced by power coils
similar to power coils 74 (in FIG. 3A), which receive energy by
inductive coupling from an external driving antenna (typically in
wireless control unit 40). Further alternatively, pad 94 may be
connected by wire to an external power source, as shown in FIG.
1B.
[0094] Typically, control circuit 97 drives transducer 98 to
generate magnetic fields, as described above. Alternatively, when
transducer 98 is configured to receive magnetic fields, control
circuit 97 processes the signals generated by the transducer and
transmits position signals to control unit 40. Pad 94 comprises a
communication circuit 99 for transmitting position information to
control unit 40 and/or receiving instructions from the control
unit. The communication circuit may communicate with the control
unit by wireless link (as described above in reference to
transducer 70) or by a wire to the control unit.
[0095] In embodiments in which transducer 70 or 90 is configured to
generate magnetic fields, circuitry 78 and coils 72 typically
transmit a weak, narrowband signal at a precisely-controlled
frequency (for example, 5 kHz). In this case, transducer 98 may
comprise coils in a resonant circuit that is tuned for this
frequency with very high Q. Circuit 97 may comprise a narrowband
digital filter, with bandwidth as small as 20 Hz, in order to
detect the weak signals that are induced in transducer 98. As a
result, the receiver circuit will have very high inherent gain at
the transmitted frequency, and the receiver coils used in
transducer 98 may be relatively small. As a result, pad 94 itself
may be made sufficiently small and flexible to be conveniently
fixed to the body, as shown in FIGS. 1A and 1B. Similar principles
may be used to enhance system sensitivity, and thus reduce the size
of the body-surface location pad, when transducer 98 serves as the
field generator and coils 72 in transducer 70 or 90 receive the
fields.
[0096] Optionally, pad 94 also comprises a transducer coil 100,
which may be similar in construction and operation to transducer
coils 72 (FIG. 3A). When multiple location pads are applied to the
body surface, as shown in the embodiments of FIGS. 1A and 1B, a
second transducer coil 100 on one location pad may be used to
receive magnetic fields generated by field transducer coil 98 on
one or more other location pads. Alternatively, field transducer 98
itself may be used both to generate magnetic fields and to receive
magnetic fields generated by other location pads. The position
signals generated by transducer 98 (or by transducer 100) are
processed by computer 38 to determine the relative coordinates of
the two (or more) location pads in the tracking system. The
separate coordinate frames of the two location pads may thus be
registered with one another.
[0097] Further alternatively, when fixed reference transducer 46 is
utilized by system 20 (FIGS. 1A and 1B), magnetic fields
transmitted between this fixed reference transducer and the
location pads may be used to register the floating origin of the
location pad coordinates with the fixed frame of reference.
[0098] Optionally, the coordinate systems of multiple location pads
may be "chained." In other words, the coordinates of a first
location pad may be registered with the fixed reference transducer
by transmitting magnetic fields between the first location pad and
the fixed reference transducer, and the coordinates of a second
location pad may be registered with the first location pad by
transmitting magnetic fields between the two location pads. This
sort of chaining may be extended over a sequence of multiple
location pads, spaced along the patient's body, to register the
coordinate frames of all the pads with one another and, optionally,
with a fixed reference transducer. Chaining together multiple
location pads in this manner makes it possible to use smaller coils
in the location pads (since there will generally be at least one
coil that is relatively close to each transducer in the body).
Smaller coils have smaller detection volumes, and are therefore
less sensitive to field disturbance by metal objects that are not
in the immediate vicinity of the transducer.
[0099] FIG. 5 is a schematic, pictorial illustration showing
details of tool 24, in accordance with an embodiment of the present
invention. Tool 24 comprises a handle 101 and a shaft 102. A tool
transducer 104 fits snugly into a suitable receptacle inside handle
101. Transducer 104 comprises sensing and communication circuits
106, which are powered by a battery 108. Typically, circuits 106
comprise transducer coils, a communication coil and processing
circuitry, as in transducer 90 (FIG. 3B). The transducer coils are
similar to coils 72, generating position signals that are
indicative of the location and orientation of the sensor.
Alternatively, the transducer coils may be used to generate
magnetic fields which induce position signals in coils in the
body-surface location pads. The communication coil transmits
position signals to and/or receives control information from
wireless control unit 40. The operation of circuits 106 is thus
similar to that of the circuits in sensors 70 and 90, although
elements of circuits 106 may be made larger and consume greater
power than the corresponding elements in transducers 70 and 90. In
a further embodiment, circuits 106 may be connected by wire
directly to computer 38, bypassing wireless control unit 40.
[0100] Tool transducer 104 may be permanently housed inside tool
24, or the transducer may alternatively be removable (to replace
battery 108, for example). Because the geometry of tool 24 is
known, the location and orientation of handle 101, as indicated by
transducer 104, indicates precisely the location and orientation of
the distal tip of shaft 102. Alternatively, the tool transducer 104
may be miniaturized and may thus be contained inside shaft 102.
Optionally, the tool transducer 104 may be calibrated before use in
order to enhance the precision with which the shaft position is
measured.
[0101] FIG. 6 is a schematic, pictorial illustration showing body
surface pads 110 and 112, each comprising multiple magnetic field
transducers 114 in a ring configuration, in accordance with an
alternative embodiment of the present invention. The illustration
shows pads 110 and 112 mounted to leg 30 of patient 32. Each of
transducers 114 typically comprises coils that transmit and/or
receive magnetic fields, as described previously. Pads 110 and 112
have the form of a ring or tape, which can be attached to the
patient's body near the surgical site. One or more such rings or
tapes may be used for a medical procedure. In the present example,
it is assumed that pads 110 and 112 are used in determining the
disposition of tool 24 with respect to implants 26 and 28.
[0102] Each of transducers 114 is capable of covering only a
limited portion of the working volume of the medical procedure,
because of the small size and power of the transducers. The
relative disposition of the tool and an implant is calculated based
on the magnetic field that provides the greatest accuracy, which is
typically the signal received with the least noise. As the tool
moves through the working volume, a magnetic field signal from one
of transducers 114 may initially provide the greatest accuracy and
is therefore used to determine the relative disposition of tool and
the implant. Subsequently, the field from a second transducer may
become clearer (i.e., less noisy), and the tracking process is
"handed-off," such that the disposition is now determined based on
the field from the second pad. Each transducer may determine the
relative position of its neighbor, and the position of the
transducer unit and tool may then be determined in a constant frame
of reference by chaining together the sequence of transducer
coordinate vectors, as described above.
[0103] Although the embodiments described hereinabove relate
specifically to tracking systems that use time-varying magnetic
fields, the principles of the present invention may also be
applied, mutatis mutandis, in other sorts of tracking systems, such
as ultrasonic tracking systems, tracking systems based on DC
magnetic fields, and other tracking systems that are based on
electromagnetic radiation, such as optical tracking systems. It
will thus be appreciated that the embodiments described above are
cited by way of example, and that the present invention is not
limited to what has been particularly shown and described
hereinabove. Rather, the scope of the present invention includes
both combinations and subcombinations of the various features
described hereinabove, as well as variations and modifications
thereof which would occur to persons skilled in the art upon
reading the foregoing description and which are not disclosed in
the prior art.
* * * * *