Indian Patents
Title of Invention

DISTORTION-IMMUNE POSITION TRACKING USING REDUNDANT MEASUREMENTS
Abstract A method for tracking a position of an object includes using a field sensor associated with the object to measure field strengths of magnetic fields generated by two or more field generators, wherein a measurement of at least one of the field strengths is subject to a distortion. Rotation-invariant location coordinates of the object are calculated responsively to the measured field strengths. Corrected location coordinates of the object are determined by applying to the rotation- invariant location coordinates a coordinate correcting function so as to adjust a relative contribution of each of the measured field strengths to the corrected location coordinates responsively to the distortion in the measured field strengths.
Full Text DISTORTION-IMMUNE POSITION TRACKING USING REDUNDANT
MEASUREMENTS
FIELD OF THE INVENTION
The present invention relates generally to magnetic
position tracking systems, and particularly to methods
and systems for performing accurate position measurements
in the presence of field-distorting objects.
BACKGROUND OF THE INVENTION
Various methods and systems are known in the art for
tracking the coordinates of objects involved in medical
procedures. Some of these systems use magnetic field
measurements. For example, U.S. Patents 5,391,199 and
5,443,489, whose disclosures are incorporated herein by
reference, describe systems in which the coordinates of
an intrabody probe are determined using one or more field
transducers. 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.
Additional methods and systems that relate to
magnetic position tracking are also described, for
example, in PCT Patent Publication WO 96/057 68, U.S.
Patents 6, 690,963, 6,239,724, 6,618,612 and 6,332,08 9,
and U.S. Patent Application Publications 2002/0065455 Al,
2003/0120150 Al and 2004/0068178 Al, whose disclosures
are all incorporated herein by reference. These
BIO5136USNP

publications describe methods and systems that track the
position of intrabody objects such as cardiac catheters,
orthopedic implants and medical tools used in different
medical procedures.
It is well known in the art that the presence of
metallic, paramagnetic or ferromagnetic objects within
the magnetic field of a magnetic position tracking system
often distorts the system's measurements. The distortion
is sometimes caused by eddy currents that are induced in
such objects by the system's magnetic field, as well as
by other effects.
Various methods and systems have been described in
the art for performing position tracking in the presence
of such interference. For example, U.S. Patent 6,147,480,
whose disclosure is incorporated herein by reference,
describes a method in which the signals induced in the
tracked object are first detected in the absence of any
articles that could cause parasitic signal components.
Baseline phases of the signals are determined. When an
article that generates parasitic magnetic fields is
introduced into the vicinity of the tracked object, the
phase shift of the induced signals due to the parasitic
components is detected. The measured phase shifts are
used to indicate that the position of the object may be
inaccurate. The phase shifts are also used for analyzing
the signals so as to remove at least a portion of the
parasitic signal components.
2
BIO5136USNP

SUMMARY OF THE INVENTION
Embodiments of the present invention provide
improved methods and systems for performing magnetic
position tracking measurements in the presence of
metallic, paramagnetic and/or ferromagnetic objects
{collectively referred to as field-distorting objects)
using redundant measurements.
The system comprises two or more field generators
that generate magnetic fields in the vicinity of the
tracked object. The magnetic fields are sensed by a
position sensor associated with the object and converted
to position signals that are used to calculate the
position (location and orientation) coordinates of the
object. The system performs redundant field strength
measurements and exploits the redundant information to
reduce the measurement errors caused by the presence of
field-distorting objects.
The redundant measurements comprise field strength
measurements of magnetic fields generated by different
field generators and sensed by field sensors in the
position sensor. In an exemplary embodiment described
herein, nine field generators and three field sensing
coils are used to obtain 27 different field strength
measurements. The 27 measurements are used to calculate
the six location and orientation coordinates of the
tracked object, thus containing a significant amount of
redundant information.
In some embodiments, a rotation-invariant coordinate
correcting function is applied to the measured field
3
BIO5136USNP

strengths to produce a distortion-corrected location
coordinate of the tracked object. As will be shown
hereinbelow, the coordinate correcting function exploits
the redundant location information so as to reduce the
distortion level in the corrected location coordinate.
The coordinate correcting function can be viewed as
adjusting the relative contributions of the measured
field strengths to the corrected location coordinates
responsively to the respective level of the distortion
present in each of the measured field strengths. A
disclosed clustering process further improves the
accuracy of the coordinate correcting function by
defining different coordinate correcting functions for
different locations.
In some embodiments, the orientation coordinates of
the tracked object are calculated following the location
calculation. Other disclosed methods improve the accuracy
of the orientation calculation in the presence of
distortion, and compensate for non-concentricity of the
field sensors of the position sensor.
In some embodiments, the redundant field strength
measurements are used to identify one or more system
elements, such as field generators and/or field sensing
elements of the position sensor, which contribute
significant distortion. Field measurements associated
with these system elements are disregarded when
performing the position calculation. In some embodiments,
a distortion-contributing element may be deactivated.
4
BIO5136USNP

There is therefore provided, in accordance with an
embodiment of the present invention, a method for
tracking a position of an object, including:
using a field sensor associated with the object to
measure field strengths of magnetic fields generated by
two or more field generators, wherein a measurement of at
least one of the field strengths is subject to a
distortion;
calculating rotation-invariant location coordinates
of the object responsively to the measured field
strengths; and
determining corrected location coordinates of the
object by applying to the rotation-invariant location
coordinates a coordinate correcting function so as to
adjust a relative contribution of each of the measured
field strengths to the corrected location coordinates
responsively to the distortion in the measured field
strengths.
In some embodiments, the method includes inserting
the object into an organ of a patient, and determining
the corrected location coordinates of the object includes
tracking the position of the object inside the organ.
In an embodiment, the distortion is caused by a
field-distorting object subjected to at least some of the
magnetic fields, wherein the object comprises at least
one material selected from a group consisting of
metallic, paramagnetic and ferromagnetic materials.
In a disclosed embodiment, the method includes
performing calibration measurements of the magnetic
5
BIO5136USNP

fields at respective known coordinates relative to the
two or more field generators, and deriving the coordinate
correcting function responsively to the calibration
measurements. In another embodiment, the distortion is
caused by a movable field-distorting object, and
performing the calibration measurements includes
performing the measurements at different locations of the
field-distorting object. Additionally or alternatively,
deriving the coordinate correcting function includes
applying a fitting process to a dependence of the
calibration measurements on the known coordinates.
In yet another embodiment, applying the coordinate
correcting function includes applying a polynomial
function having coefficients including exponents of at
least some of the rotation-invariant location
coordinates.
In still another embodiment, applying the coordinate
correcting function includes identifying a distortion-
contributing element responsively to the measured field
strengths, and producing the coordinate correcting
function so as to disregard the measured field strengths
that are associated with the distortion-contributing
element.
In some embodiments, the field sensor includes one
or more field sensing elements, and identifying the
distortion-contributing element includes determining that
one or more of the field sensing elements and the field
generators are contributing to the distortion.
6
BIO5136USNP

In an embodiment, the method includes calculating
angular orientation coordinates of the object.
In another embodiment, the field sensor is used
within a working volume associated with the two or more
field generators, and determining the corrected location
coordinates includes:
dividing the working volume into two or more
clusters;
defining for each of the two or more clusters
respective two or more cluster coordinate correcting
functions; and
applying to each of the rotation-invariant location
coordinates one of the cluster coordinate correcting
functions responsively to a cluster in which the
rotation-invariant location coordinate falls.
Applying the cluster coordinate correcting functions
may include applying a weighting function so as to
smoothen a transition between neighboring clusters.
In yet another embodiment, the method includes
measuring the field strengths using two or more field
sensors having non-concentric locations, and compensating
for inaccuracies caused by the non-concentric locations
in the corrected location coordinates.
There is additionally provided, in accordance with
an embodiment of the present invention, a method for
tracking a position of an object, including:
using a field sensor associated with the object to
perform measurements of field strengths of magnetic
7
BIO5136USNP

fields generated by two or more field generators so as to
provide redundant location information, wherein at least
some of the field strength measurements are subject to a
distortion; and
determining location coordinates of the object
relative to the two or more field generators by applying
to the measurements a coordinate correcting function that
exploits the redundant location information so as to
reduce an impact of the distortion on the location
coordinates.
There is also provided, in accordance with an
embodiment of the present invention, a method for
tracking a position of an object, including:
using a field sensor, which includes one or more
field sensing elements associated with the object, to
measure field strengths of magnetic fields generated by
two or more' field generators, wherein a measurement of at
least one of the field strengths is subject to a
distortion;
identifying, responsively to the measured field
strengths, at least one distortion-contributing system
element, which is selected from a group consisting of the
one or more field sensing elements and the two or more
field generators; and
determining the position of the object relative to
the two or more field generators responsively to the
measured field strengths while disregarding field
measurements associated with the distortion-contributing
system element.
8
BIO5136USNP

In an embodiment, the method includes inserting the
object into an organ of a patient, and determining the
position of the object includes tracking the position of
the object inside the organ. In another embodiment, the
two or more field generators are associated with the
object, and the field sensor is located externally to the
organ. In yet another embodiment, identifying the
distortion-contributing system element includes accepting
an a-priori indication selected from a group consisting
of a characteristic direction of the distortion and an
identity of the distortion-contributing system element.
In still another embodiment, identifying the
distortion-contributing system element includes sensing a
presence of the distortion in the field measurements
associated with the distortion-contributing system
element. In an embodiment, the distortion-contributing
system element includes a pair of one of the field
sensing elements and one of the field generators. In
another embodiment, disregarding the field measurements
associated with the distortion-contributing system
element includes deactivating the distortion-contributing
system element.
There is further provide, in accordance with an
embodiment of the present invention, a system for
tracking a position of an object, including:
two or more field generators, which are arranged to
generate respective magnetic fields in a vicinity of the
object;
a field sensor associated with the object, which is
arranged to measure field strengths of the magnetic
9
BIO5136USNP

fields, wherein a measurement of at least one of the
field strengths is subject to a distortion; and
a processor, which is arranged to calculate
rotation-invariant location coordinates of the object
responsively to the measured field strengths, and to
determine corrected location coordinates of the object by
applying to the rotation-invariant location coordinates a
coordinate correcting function so as to adjust a relative
contribution of each of the measured field strengths to
the corrected location coordinates responsively to the
distortion in the measured field strengths.
There is additionally provided, in accordance with
an embodiment of the present invention, a system for
tracking a position of an object, including:
two or more field generators, which are arranged to
generate respective magnetic fields in a vicinity of the
object;
a field sensor associated with the object, which is
arranged to perform measurements of field strengths of
the magnetic fields so as to provide redundant location
information, wherein at least some of the field strength
measurements are subject to a distortion; and
a processor, which is arranged to determine location
coordinates of the object relative to the two or more
field generators by applying to the measurements a
coordinate correcting function that exploits the
redundant location information so as to reduce an impact
of the distortion on the location coordinates.
10
BIO5136USNP

There is also provided, in accordance with an
embodiment of the present invention, a system for
tracking a position of an object, including:
two or more field generators, which are arranged to
generate respective magnetic fields in a vicinity of the
object;
a field sensor, which is associated with the object
and includes one or more field sensing elements, which is
arranged to measure field strengths of the magnetic
fields, wherein a measurement of at least one of the
field strengths is subject to a distortion; and
a processor, which is arranged to identify
responsively to the measured field strengths a
distortion-contributing system element, which is selected
from a group consisting of the one or more field sensing
elements and the two or more field generators, and to
determine the position of the object relative to the two
or more field generators while disregarding field
measurements associated with the distortion-contributing
system element.
There is further provided, in accordance with an
embodiment of the present invention, a computer software
product used in a system for tracking a position of an
object, the product including a computer-readable medium,
in which program instructions are stored, which
instructions, when read by the computer, cause the
computer to control two or more field generators so as to
generate magnetic fields in a vicinity of the object, to
accept measurements of field strengths of the magnetic
fields performed by a field sensor associated with the
object, wherein a measurement of at least one of the
11
BIO5136USNP

field strengths is subject to a distortion, to calculate
rotation-invariant location coordinates of the object
responsively to the measured field strengths, and to
determine corrected location coordinates of the object by
applying to the rotation-invariant location coordinates a
coordinate correcting function so as to adjust a relative
contribution of each of the measured field strengths to
the corrected location coordinates responsively to the
distortion in the measured field strengths.
There is also provided, in accordance with an
embodiment of the present invention, a computer software
product used in a system for tracking a position of an
object, the product including a computer-readable medium,
in which program instructions are stored, which
instructions, when read by the computer, cause the
computer to control two or more field generators so as to
generate magnetic fields in a vicinity of the object, to
accept measurements of field strengths of the magnetic
fields performed by a field sensor associated with the
object, the measurements including redundant location
information, wherein at least some of the measurements
are subject to a distortion, and to determine location
coordinates of the object relative to the two or more
field generators by applying to the measurements a
coordinate correcting function that exploits the
redundant location information so as to reduce an impact
of the distortion on the location coordinates.
There is additionally provided, in accordance with
an embodiment of the present invention, a computer
software product used in a system for tracking a position
12
BIO5136USNP

of an object, the product including a computer-readable
medium, in which program instructions are stored, which
instructions, when read by the computer, cause the
computer to control two or more field generators so as to
generate magnetic fields in a vicinity of the object, to
accept measurements of field strengths of the magnetic
fields performed by a field sensor, which is associated
with the object and includes one or more field sensing
elements, wherein a measurement of at least one of the
field strengths is subject to a distortion, to identify
responsively to the measured field strengths a
distortion-contributing system element, which is selected
from a group consisting of the two or more field
generators and the one or more field sensing elements,
and to determine the position of the object relative to
the two or more field generators while disregarding field
measurements associated with the distortion-contributing
system element.
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
Fig. 1 is a schematic, pictorial illustration of a
system for position tracking and steering of intrabody
objects, in accordance with an embodiment of the present
invention;
Fig. 2 is a schematic, pictorial illustration of a
location pad, in accordance with an embodiment of the
present invention;
13
BIO5136USNP

Fig. 3 is a schematic, pictorial illustration of a
catheter, in accordance with an embodiment of the present
invention;
Fig. A is a flow chart that schematically
illustrates a method for position tracking in the
presence of field distortion, in accordance with an
embodiment of the present invention; and
Fig. 5 is a flow chart that schematically
illustrates a method for position tracking in the
presence of field distortion, in accordance with another
embodiment of the present invention.
DETAILED DESCRIPTION OF EMBODIMENTS
SYSTEM DESCRIPTION
Fig. 1 is a schematic, pictorial illustration of a
system 20 for position tracking and steering of intrabody
objects, in accordance with an embodiment of the present
invention. System 20 tracks and steers an intrabody
object, such as a cardiac catheter 24, which is inserted
into an organ, such as a heart 28 of a patient. System 20
also measures, tracks and displays the position (i.e.,
the location and orientation) of catheter 24. In some
embodiments, the catheter position is registered with a
three-dimensional model of the heart or parts thereof.
The catheter position with respect to the heart is
displayed to a physician on a display 30. The physician
uses an operator console 31 to steer the catheter and to
view its position during the medical procedure.
System 20 can be used for performing a variety of
intra-cardiac surgical and diagnostic procedures in which
14
BIO5136USNP

navigation and steering of the catheter is performed
automatically or semi-automatically by the system, and
not manually by the physician. The catheter steering
functions of system 20 can be implemented, for example,
by using the Niobe® magnetic navigation system produced
by Stereotaxis, Inc. (St. Louis, Missouri). Details
regarding this system are available at
www.stereotaxis.com. Methods for magnetic catheter
navigation are also described, for example, in U.S.
Patents 5,654,864 and 6,755,816, whose disclosures are
incorporated herein by reference.
System 20 positions, orients and steers catheter 24
by applying a magnetic field, referred to herein as a
steering field, in a working volume that includes the
catheter. An internal magnet is fitted into the distal
tip of catheter 24. ("Catheter 24 is shown in detail in
Fig. 3 below.) The steering field steers (i.e., rotates
and moves) the internal magnet, thus steering the distal
tip of catheter 24.
The steering field is generated by a pair of
external magnets 36, typically positioned on either side
of the patient. In some embodiments, magnets 36 comprise
electro-magnets that generate the steering field
responsively to suitable steering control signals
generated by console 31. In some embodiments, the
steering field is rotated or otherwise controlled by
physically moving (e.g., rotating) external magnets 36 or
parts thereof. The difficulties that arise from having
large metallic objects whose position may very over time,
15
BIO5136USNP

such as magnets 36, in close proximity to the working
volume will be discussed hereinbelow.
System 20 measures and tracks the location and
orientation of catheter 24 during the medical procedure.
For this purpose, the system comprises a location pad 40.
Fig. 2 is a schematic, pictorial illustration of
location pad 40, in accordance with an embodiment of the
present invention. Location pad 40 comprises field
generators, such as field generating coils 44. Coils 44
are positioned at fixed, known locations and orientations
in the vicinity of the working volume. In the exemplary
configuration of Figs. 1 and 2, location pad 40 is placed
horizontally under the bed on which the patient lies. Pad
40 in this example has a triangular shape and comprises
three tri-coils 42. Each tri-coil 42 comprises three
field generating coils 44. Thus, in the present example,
location pad 40 comprises a total of nine field
generating coils. The three coils 44 in each tri-coil 42
are oriented in mutually-orthogonal planes. In
alternative embodiments, location pad 40 may comprise any
number of field generators arranged in any suitable
geometrical configuration.
Referring to Fig. I, console 31 comprises a signal
generator 4 6, which generates drive signals that drive
coils 44. In the embodiments shown in Figs. 1 and 2, nine
drive signals are generated. Each coil 44 generates a
magnetic field, referred to herein as a tracking field,
responsively to the respective drive signal driving it.
The tracking fields comprise alternating current (AC)
16
BIO5136USNP

fields. Typically, the frequencies of the drive signals
generated by signal generator 46 (and consequently the
frequencies of the respective tracking fields) are in the
range of several hundred Hz to several KHz, although
other frequency ranges can be used as well.
A position sensor fitted into the distal tip of
catheter 24 senses the tracking fields generated by coils
44 and produces respective position signals, which are
indicative of the location and orientation of the sensor
with respect to the field generating coils. The position
signals are sent to console 31, typically along a cable
running through catheter 24 to the console. Console 31
comprises a tracking processor 48, which calculates the
location and orientation of catheter 24 responsively to
the position signals. Processor 48 displays the location
and orientation of the catheter, typically expressed as a
six-dimensional coordinate, to the physician using
display 30.
Processor 48 also controls and manages the operation
of signal generator 46. In some embodiments, field
generating coils 44 are driven by drive signals having
different frequencies, so as to differentiate between
their magnetic fields. Alternatively, the field
generating coils can be driven sequentially so that the
position sensor measures the tracking field originating
from a single coil 44 at any given time. In these
embodiments, processor 48 alternates the operation of
each coil 44 and associates the position signals received
from the catheter with the appropriate field generating
coil.
17
BIO5136USNP

Typically, tracking processor 48 is implemented
using a general-purpose computer, which is programmed in
software to carry out the functions described herein. The
software may be downloaded to the computer in electronic
form, over a network, for example, or it may
alternatively be supplied to the computer on tangible
media, such as CD-ROM. The tracking processor may be
integrated with other computing functions of console 31.
Fig. 3 is a schematic, pictorial illustration of the
distal tip of catheter 24, in accordance with an
embodiment of the present invention. Catheter 24
comprises an internal magnet 32 and a position sensor 52,
as described above. Catheter 24 may also comprise one or
more electrodes 56, such as ablation electrodes and
electrodes for sensing local electrical potentials.
Position sensor 52 comprises field sensing elements, such
as field sensing coils 60. In some embodiments, position
sensor 52 comprises three field sensing coils 60 oriented
in three mutually-orthogonal planes. Each coil 60 senses
one of the three orthogonal components of the AC tracking
field and produces a respective position signal
responsively to the sensed component. Sensor 52 and
electrodes 56 are typically connected to console 31 via
cables 64 running through the catheter.
It is well known in the art that metallic,
paramagnetic and ferromagnetic objects (collectively
referred to herein as field-distorting objects) placed in
an AC magnetic field cause distortion of the field in
their vicinity. For example, when a metallic object is
subjected to an AC magnetic field, eddy currents are
18
BIO5136USNP

induced in the object, which in turn produce parasitic
magnetic fields that distort the AC magnetic field.
Ferromagnetic objects distort the magnetic field by
attracting and changing the density and orientation of
the field lines.
In the context of a magnetic position tracking
system, when a field-distorting object is present in the
vicinity of position sensor 52, the tracking field sensed
by sensor 52 is distorted, causing erroneous position
measurements. The severity of the distortion generally
depends on the amount of field-distorting material
present, to its proximity to the position sensor and to
the field generating coils, and/or to the angle in which
the tracking field impinges on the field-distorting
object. In the system of Fig. 1, for example, external
magnets 36 typically contain a large mass of field-
distorting material and are located in close proximity to
the working volume. As such, external magnets 36 may
cause a significant distortion of the tracking field
sensed by the position sensor.
The methods and systems described hereinbelow are
mainly concerned with performing accurate position
tracking measurements in the presence of severe
distortion of the tracking magnetic field. The catheter
steering system of Fig. 1 is described purely as an
exemplary application, in which objects located in or
near the working volume of the position tracking system
cause a severe, time varying distortion of the tracking
field. However, embodiments of the present invention are
in no way limited to magnetic steering applications. The
19
BIO5136USNP

methods and systems described herein can be used in any
other suitable position tracking application for reducing
such distortion effects. For example, the methods and
systems described herein can be used to reduce field
distortion effects caused by object such as C-arm
fluoroscopes and magnetic resonance imaging (MRI)
equipment.
In alternative embodiments, system 20 can be used to
track various types of intrabody objects, such as
catheters, endoscopes and orthopedic implants, as well as
for tracking position sensors coupled to medical and
surgical tools and instruments.
DISTORTION REDUCTION METHOD USING REDUNDANT MEASUREMENT
INFORMATION
As noted above, system 20 comprises nine field
generating coils 4 4 that generate nine respective
tracking fields. Each of these fields is sensed by three
field sensing coils 60. Thus, the system performs a total
of 27 field projection measurements in order to calculate
the six location and orientation coordinates of catheter
24. It is evident that the 27 measurements contain a
significant amount of redundant information. This
redundant information can be used to improve the immunity
of the system to distortions caused by field-distorting
objects, such as external magnets 36.
The 27 field measurements can be viewed as vectors
in a 27-dimensional vector space. Each dimension of this
vector space corresponds to a pair of {field generating
coil 44, field sensing coil 60} . Because of the
20
BIO5136USNP

redundancy in the measurements, it is often possible to
determine a lower dimensionality sub-space of this vector
space that is invariant or nearly invariant to the field
distortions. The position tracking method described in
Fig. 4 below uses the redundant information present in
the field measurements to improve the accuracy of the
position measurements in the presence of such field
distortions.
In principle, the method first calculates three
location vectors that define the location of position
sensor 52 relative to the three tri-coils 42,
respectively. These location vectors are invariant to the
angular orientation of the position sensor and are
referred to as rotation invariants. The location vectors
are orientation-invariant since, as will be shown below,
they are calculated based on measured field intensity and
not based on the projection of the field strength onto
the field sensing coils.
The location vectors (rotation invariants) are
corrected by coordinate correcting functions, which
exploit the redundant measurement information to improve
field distortion immunity. The orientation coordinates of
the position sensor are then calculated to complete the
six-dimensional location and orientation coordinate of
the sensor. In some embodiments, the method of Fig. 4
also comprises calibration and clustering steps, as well
as a process for compensating for the non-concentricity
of coils 60 of position sensor 52.
21
BIO5136USNP



Although the method of Fig. 4 below refers to a
location pad comprising nine field generating coils
arranged in three mutually-orthogonal groups in tri-coils
42 and to a position sensor comprising three mutually-
orthogonal field sensing coils, this configuration is an
exemplary configuration chosen purely for the sake of
conceptual clarity. In alternative embodiments, location
pad 40 and position sensor 52 may comprise any number of
coils 44 and coils 60 arranged in any suitable
geometrical configuration.
Fig. 4 is a flow chart that schematically
illustrates a method for position tracking in the
presence of field distortion, in accordance with an
embodiment of the present invention. The method begins by
mapping and calibrating the tracking fields generated by
location pad 40, at a calibration step 100.
Typically, the calibration process of step 100 is
performed during the production of location pad 40, and
the calibration results are stored in a suitable memory
device coupled to the location pad- Calibration setups
that can be used for this purpose and some associated
calibration procedures are described, for example, in
U.S. Patent 6,335,617, whose disclosure is incorporated
herein by reference.
In the calibration process, a calibrating sensor
similar to position sensor 52 is scanned through multiple
locations in the three-dimensional working volume around
pad 40. At each location of the calibrating sensor, each
of the nine field generating coils 44 in pad 40 is driven
22
BIO5136USNP

to generate a respective tracking field, and the three
field sensing coils 60 of the calibrating sensor measure
this tracking field. The. sensed field strengths
associated with each location are recorded.
In some embodiments, the calibration process
comprises performing multiple field measurements at each
location of the calibrating sensor. Typically, some of
these measurements comprise free-space measurements
(i.e., measurements taken when the working volume and its
vicinity are free of field-distorting objects). Other
measurements are taken in the presence of field-
distorting objects, in the same positions they are
expected to have during the system operation. For
example, when the field-distorting objects comprise
external magnets 36 that are physically moved to steer
catheter 24, field measurements are performed while the
magnets are moved through their entire expected motion
range. Other field-distorting objects that may be
included in the calibration include, for example, a
fluoroscope used to irradiate the patient, as well as the
bed the patient lies on.
The calibration setup performs the field
measurements and records the measurement results along
with the associated known locations of the calibrating
sensor. In some embodiments, the calibration procedure is
carried out by a robot or other automatic calibration
setup that moves the calibration sensor across the
working volume around pad 40.
23

In some embodiments, every pad 40 being produced is
calibrated using the calibration procedure described
herein. Alternatively, such as when the production
process of pads 40 is sufficiently repeatable, the full
calibration procedure may be performed only on a single
location pad or a sample of pads and the results used to
calibrate the remaining pads. Further alternatively, a
sample of pads may be subjected to the full calibration
procedure. For the remaining pads, only differential
results, indicating the field strength differences
between free-space measurements and distorted
measurements, are recorded.
In some cases, the material composition, mechanical
structure and/or location of the field-distorting objects
is known. In such cases, the interference caused by these
objects can be modeled, and the model used as part of the
calibration measurements. In some cases, when multiple
field-distorting objects are present, calibration
measurements may be performed for each object separately.
The individual calibration measurements can then be
combined. Further additionally or alternatively, any
other suitable method of obtaining a set of calibration
measurements can be used.
The multiple field projection measurements, each
associated with a known location of the calibrating
sensor, are used to derive three rotation-invariant
coordinate correcting functions. The correcting functions
will later be applied during normal system operation. The
functions accept as input a set of raw field
measurements, as measured by position sensor 52. These
24

raw measurements may be distorted due to the presence of
field-distorting objects. The three functions produce
three respective corrected location coordinates of
position sensor 52 with respect to location pad 40. In
some embodiments, the correcting functions compensate for
distortion from field-distorting objects, as well as for
errors due to the fact that the tracking fields generated
by coils 44 deviate from ideal dipole fields. Modeling
the tracking fields as dipole fields is, however, not
mandatory.
In some embodiments, the coordinate correcting
functions are determined using a fitting process. The
fitting process determines the functions that best fit
the location coordinates measured during calibration step
100 above to the known location coordinates of the
calibrating sensor. Any suitable fitting method known in
the art can be used for this purpose, such as, for
example, polynomial regression methods.
Thus, the fitting process effectively causes the
coordinate correcting functions to adjust the relative
contribution of each raw location coordinate to the
corrected location coordinate responsively to the level
of distortion contained in the raw measurements. Raw
location coordinates having low distortion content are
likely to be emphasized, or given more weight, by the
fitting process. Raw location coordinates having high
distortion content are likely to be given less weight, or
even ignored.
25

The coordinate correcting functions can thus be
viewed as transforming the raw field measurements into a
sub-space that is as invariant as possible to the
distortion. Since the fitting process takes into
consideration the bulk of calibration measurements, the
sub-space is invariant to the distortion caused in
different field-distorting object geometries.
In some embodiments, the coordinate correcting
function can disregard field measurements associated with
one or more distortion-contributing system elements that
contribute a significant amount of distortion to the
calculation. Distortion-contributing elements may
comprise field generating coils 44, field sensing coils
60 and/or pairs of {coil 44, coil 60} . In these
embodiments, the function may ignore the measurements
related to the distortion-contributing elements, for
example by setting appropriate coefficients of the
coordinate correcting function to zero or otherwise
shaping the function to be insensitive to these elements.
In some embodiments, the distortion-contributing elements
can be switched off or otherwise deactivated.
The raw location coordinates are expressed as three
vectors denoted rtc, wherein tc=1...3 indicates an index of
the tri-coil 42 used in the measurement. Vector rtc
comprises three location coordinates {xtc,Ytc,Ztc}
indicating the location coordinates of the position
sensor, as calculated responsively to the tracking fields
generated by tri-coil tc. By convention, rtc is expressed
relative to a reference frame of location pad 40. An
26

exemplary mathematical procedure for calculating rtc
based on the measured field strengths, assuming an ideal
dipole field, is given in step 102 further below.
In some embodiments, the three coordinate correcting
functions comprise polynomial functions. In the
description that follows, each function comprises a
third-order polynomial of the location coordinates that
does not contain any cross-terms (i.e., the polynomial
may contain x, x2,x3,y, y2,y3,z, z2 and z3 terms but
not, for example, xy2 , xyz or y2 z terms) . The input to
the coordinate correcting functions can thus be expressed
as a 28-dimensional vector denoted In, which is defined
as In={l,r1,r2,r3,r12 ,r22 ,r32 ,r13 ,r23 ,r33 }
= {l,Xl,y1,Zl,X2,y2,Z2,X3,Y3,Z3,Xl2 'Yl2 Zl2 X22 Y22 Z22 X32 Y
32 ,z32 ,x13 Y13 ,z13 ,x23 ,Y23 Z23 X33 'Y33 'Z33 ) wherein the
first "1" term serves as an offset. The three coordinate
correcting functions have the form

wherein xcor yCOR and zcor respectively denote the
distortion-corrected x, y and z location coordinates of
position sensor 52, with respect to location pad 40.
Coefficients a...a28, b1-b28 and 71-728 denote the
27

coefficients of the polynomial functions. In the present
example, The fitting process described above comprises
fitting the values of the polynomial coefficients.
The three sets of coefficients can be arranged in a
coefficient matrix denoted Lcoeff, defined as

Using this representation, the corrected location
coordinates of the position sensor are given by
[3] rcor = {xcor' Ycor' zcor' = In Lcoeff
In order to further clarify the effectiveness of the
coordinate correcting functions, consider a particular
location of the calibration sensor. During the
calibration process of step 100, multiple field strength
measurements are performed at this particular location,
both in free space and in the presence of distortion from
different field-distorting objects, as expected to occur
during the normal operation of the system. The coordinate
correcting functions replace these multiple measurements
with a single corrected value, which best fits the known
location coordinate of the calibrating sensor.
The coordinate correcting functions effectively
exploit the redundant information contained in the 27 raw
location measurements to improve distortion immunity. For
example, since the intensity of a magnetic field decays
rapidly with distance {proportionally to 1/r3 ),
measurements performed using a tri-coil 42 that is
28

further away from the field-distorting object will
typically produce measurements containing less
distortion. In such cases, the fitting process will
typically give a higher weight to the measurements
associated with this lower distortion tri-coil when
calculating coefficients a1, bi and gi of the coordinate
correcting functions.
As another example, in many cases, the field
distortion is highly sensitive to the angle in which of
the magnetic field impinges on the field-distorting
object. Since the three field generating coils 44 in each
tri-coil 42 are mutually-orthogonal, there will typically
exist at least one coil 44 whose tracking field generates
little or no distortion. Again, the fitting process used
to calculate coefficients OCI, Bi and gi will typically
give a higher weight to the measurements associated with
this lower distortion coil 44.
In summary, calibration step 100 comprises mapping
the working volume around location pad 40, followed by
derivation of coordinate correcting functions that will
later on translate measured raw location coordinates to
distortion-corrected location coordinates of position
sensor 52.
Steps 102-110 below are. carried out by tracking
processor 48 during the normal operation of system 20,
whenever a position tracking measurement is desired.
Processor 48 calculates the rotation-invariant location
coordinates rtc (also referred to as the raw location
29

coordinates), at an invariant calculation step 102. As
noted above, the calculation that follows assumes that
the tracking fields generated by coils 44 are ideal
dipole fields.
For each tri-coil 42 having an index tc=1...3,
processor 48 calculates a field intensity matrix denoted
MtM, which is defined as

wherein Utc is a 3-by-3 matrix containing the field
strengths of the tracking fields generated by the three
field generating coils 44 of tri-coil tc, as measured by
the three field sensing coils 60 of position sensor 52.
Each matrix element (Utc)ij denotes the field strength
generated by the jth field generating coil 44 in tri-coil
tc, as sensed by the i field sensing coil 60 of sensor
52. Matrix Mtc is a 3-by-3 matrix comprising the inverse
of the magnetic moment matrix of tri-coil tc. The
operator () denotes matrix transposition.
Processor 48 now calculates |r| , which denotes the
radius-vector, or magnitude, of location vector rtC |r|

The direction of vector rtc is approximated by the
direction of the eigenvector of matrix MtM corresponding
to the largest eigenvalue. In order to determine this
eigenvector, processor 48 applies a singular value
30

decomposition (SVD) process, as is known in the art, to
matrix MtM:
[5] [U,W, ut] = SVD(MtM)
wherein u denotes the eigenvectors and w denotes the
eigenvalues of matrix MtM.
Let u (1) denote the eigenvector corresponding to the
largest eigenvalue. In order to resolve ambiguity, the z-
axis component of u(l) (by convention, the third
component of the eigenvector) is forced to be positive by
selecting the mirror image of the vector u(l) if
necessary. In other words, IF u (1).{0,0,l} u(1) . Finally, the raw location coordinate vector rtc is
estimated by
[6] rtc = ||r|| u(1) + ctc
wherein ctc denotes the location coordinate vector of
tri-coil tc in the coordinate system of location pad 40.
Tracking processor 48 typically repeats the process
of step 102 for all three tri-coils 42 of pad 40. The
output of step 102 is three vectors rtc tc=1...3, giving
the raw location coordinates of position sensor 52
relative to tri-coils 42. As noted above, the raw
location coordinates are uncorrected and may contain
distortion caused by field-distorting objects.
Processor 48 now calculates the distortion-corrected
location coordinates of sensor 52, at a corrected
coordinate calculation step 104. Processor 48 uses the
coordinate correcting functions calculated at calibration
31

step 100 above for this purpose. In the exemplary
embodiment described above, in which the functions
comprise third-order polynomials, the three coordinate
correcting functions are expressed in terms of matrix
Lcoeff, as defined in equation [2] above. In this
embodiment, vector rcor denoting the distortion-corrected
location coordinates of sensor 52 is given by
[7] rcor = In • Lcoeff
wherein In denotes the input vector of raw location
coordinates and their exponents, as described above. In
alternative embodiments, vector rcor is calculated by
applying the coordinate correction functions to the
measured raw location coordinates.
In some embodiments, tracking processor 48 applies a
clustering process to the location measurements, at a
clustering step 106. The accuracy of the coordinate
correcting functions can often be improved by dividing
the working volume into two or more sub-volumes, referred
to . as clusters, and defining different coordinate
correcting functions for each cluster.
Let N denote the number of clusters. In embodiments
in which the coordinate correcting functions are
expressed in terms of matrix Lcoeff, for example,
processor 48 calculates for each cluster c (c=l...N) a
cluster coefficient matrix denoted LCoeff-c at calibration
step 100 above. At step 104 above, processor 48
determines the cluster to which each raw location
coordinate measurement belongs, and applies the
32

appropriate cluster coefficient matrix to produce the
distortion-corrected location coordinates.
In some embodiments, the transitions between
neighboring clusters are smoothed using a weighting
function. In these embodiments, a prototype coordinate
denoted pc is defined for each cluster c, typically
located in the center of the cluster. Processor 48
calculates a weighted corrected coordinate denoted rw by
summing the corrected location coordinates calculated
using the coordinate correcting functions of each
cluster, weighted by the distance of the raw coordinate r
from the prototype coordinate pc of the cluster:

wherein a and t are constants used to appropriately shape
the weighting function.
In some embodiments, processor 48 verifies that the
raw location coordinate being processed is indeed located
inside the working volume mapped at step 100 above. This
validity check is sometimes desirable in order to ensure
that the coordinate correcting functions being used are
indeed valid for the coordinate in question. In some
embodiments, if the raw location coordinate is found to
be outside the mapped working volume, processor 4 8
33

notifies the physician of the situation, such as by
displaying the coordinate using a different color or icon
or by presenting an alert message. In some embodiments,
the raw coordinate is displayed without applying
correction. Alternatively, the measurement may be
discarded.
For example, in some embodiments, processor 48
produces a validity matrix denoted V during calibration
step 100. Matrix V comprises a three-dimensional bit
matrix, in which each bit corresponds to a three-
dimensional voxel (i.e., a unit volume, the three-
dimensional equivalent of a pixel) in the working volume
having a resolution denoted d. Each bit of matrix V is
set if the corresponding voxel coordinate is within the
mapped working volume, otherwise the bit is reset.
In order to preserve memory space, matrix V can be
represented as a two-dimensional array of 32-bit words.
The two indices of the array correspond to the x and y
coordinates of the voxel, and each bit in the indexed 32-
bit word corresponds to the z-axis coordinate of the
voxel. The following pseudo-code shows an exemplary
method for indexing matrix V in order to verify whether a
coordinate {x,y,zj is located within the valid working
volume:
{xlnx,ylnx,zlnxj = round[ ({x,y,z}-{xo,yo,zo})/d] ;
xlnx = Max [MinX,Min [MaxX,xInx] ;
ylnx = Max[MinY,Min [MaxY,xIny] ;
zlnx = Max[MinZ,Min[MaxZ,xInz];
valid = bitSet[V (xlnx,ylnx),zlnx];
34

wherein roundfx] denotes the integer closest to x, and
{xoryo,zo} denote the corner coordinates of the mapped
working volume. {xlnx,ylnx,zlnx} denote indices to matrix
V. MinX, MaxX, MinY, MaxY, MinZ, MaxZ denote range limits
of the x, y and z coordinates, respectively. If the
extracted valid bit is set, processor 48 concludes that
coordinate {x,y,z} is located within the mapped working
volume, and vice versa.
In some embodiments, two or more validity matrices
may be defined. For example, the boundary, or outskirts,
of the working volume may be mapped separately and
defined using a second validity matrix.
At this stage, processor 48 has calculated a
distortion-corrected location coordinate of position
sensor 52, typically expressed as a three-dimensional
coordinate. In order to obtain the complete six-
dimensional coordinate of the position sensor, processor
48 now calculates the angular orientation coordinates of
the position sensor, at an orientation calculation step
108.
In some embodiments, the orientation coordinates are
calculated using the relation
Mtc = R Btc
wherein Mtc denotes the inverse moment matrix described
above, R denotes a rotation matrix representing the
angular orientation of sensor 52 with respect to the
coordinate system of location pad 40, and Btc denotes the
measured magnetic field at coils 60 of sensor 52.
35


The measurements of Btc may contain distortion from
field-distorting objects, which may in turn affect the
estimation accuracy of matrix R. The estimation accuracy
may be improved by applying a symmetrical decomposition


process to R2. For example, let R2 = Rt R. Processor 48
applies a SVD process to R2 :
Processor 48 calculates an improved accuracy
rotation matrix denoted %, which is given by:
[13] R = R-S-1
Having calculated the distortion-corrected location
and orientation coordinates, processor 4 8 now has the
full six-dimensional coordinates of position sensor 52.
36

Until now it was assumed that field sensing coils 60
of position sensor 52 are concentric, i.e., have
identical location coordinates. In some cases, however,
sensor 52 is constructed so that coils 60 are not
concentric. This non-concentricity introduces an
additional inaccuracy into the distortion-corrected
coordinates. In some embodiments, tracking processor 48
compensates for the inaccuracies caused by the non-
concentricity of the field sensing coils, at a non-
concentricity compensation step 110.
For example, processor 48 may apply an iterative
compensation process to compensate for such inaccuracies.
Consider the tracking field denoted MEtcco, which is
generated by a coil co of tri-coil tc and measured by a'
non-concentric position sensor 52. Let vector r denote
the location coordinate of one of coils 60 of the sensor,
used as a reference coordinate, with respect to tri-coil
tc. Let rc1 and rc2 denote two vectors defining the
location offsets of the other two field sensing coils
with respect to the first (reference) coil. The tracking
field generated by coil co of tri-coil tc in sensor 52 is
given by:

wherein the second line of the field vector corresponds
to the reference coil. % denotes the improved accuracy
rotation matrix defined by equation [13] above.
37

Processor 48 improves the estimation of MEtc co by
iteratively repeating steps 104-108 above. At each
iteration step i+1, the measured field is given by

In some embodiments, processor 48 performs a
predetermined number of iteration steps. Alternatively, a
convergence threshold th is defined, and the iterative
process is repeated until
[16]

DISTORTION REDUCTION METHOD USING DIRECTIONAL SELECTION
As noted above, in some cases the distortion
introduced into a particular field strength measurement
is highly dependent on the mutual location and/or
orientation of the field generating coil used, the field
sensing coil used and the field-distorting object causing
the distortion. Therefore, when redundant field
measurements are performed using multiple field
generating coils 44 and field sensing coils 60 having
different locations and orientations, it is often
possible to identify one or more coil 44 and/or coil 60
that are dominant contributors of distortion. Discarding
the measurements related to these distortion-contributing
38

system elements may significantly reduce the total amount
of distortion in the position calculation.
Fig 5 is a flow chart that schematically
illustrates a method for position tracking in the
presence of field distortion, based on recognizing and
eliminating distortion-contributing elements, in
accordance with another embodiment of the present
invention. The method of Fig. 5 refers to a single
position tracking calculation, at a single position of
catheter 24 in the patient's body. This method can be
applied, of course, at multiple positions distributed
throughout the working volume of a position tracking
system.
The method begins with system 20 performing
redundant field measurements, at a measurement step 120.
Typically, multiple field strength measurements are taken
using different pairs of {field generating coil 44, field
sensing coil 60}. As noted above, the exemplary system
configuration of Figs. 1 and 2 comprises a total of 27
coil pairs, resulting in a maximum number of 27 redundant
field measurements.
Tracking processor 48 now identifies one or more
distortion-contributing measurements out of the redundant
field measurements, at an identification step 122. The
distortion-contributing measurements are characterized by
a high level of distortion. In some embodiments,
processor 48 may automatically detect and quantify the
level of distortion in the redundant field measurements.
Any suitable method may be used for this purpose, such
39

as, for example, methods described in U.S. Patent
6,147,480 cited above. Using the distortion-contributing
measurements, processor 48 identifies one or more
distortion-contributing system elements, which may
comprise field generating coils 44, field sensing coils
60 and/or pairs of {coil 44, coil 60} that are associated
with the distortion-contributing measurements.
Additionally or alternatively, the characteristic
direction of the distortion may be indicated to processor
48 a-priori. In some cases, the known direction of
distortion indicates to the processor which of coils 44
and/or coils 60 is particularly susceptible to the
distortion, and is therefore likely to comprise a
distortion-contributing element. Further alternatively,
the identity of a particular coil 44, coil 60 and/or pair
{coil 44, coil 60} that produces (or is likely to
produce) distortion-contributing measurements can be
indicated to the processor a-priori.
Tracking processor 4 8 calculates the position
coordinates of position sensor 52 (and of catheter 24)
while disregarding the measurements associated with the
distortion-contributing elements, at a position
calculation step 124. In some embodiments, the
measurements associated with a distortion-contributing
element are ignored or discarded from the position
calculation. Alternatively, a particular distortion-
contributing element can be switched off or otherwise
deactivated.
40

Processor 48 may use any suitable position tracking
method for calculating the position of sensor 52 (and of
catheter 24) in conjunction with the method of Fig. 5,
such as the method of Fig. 4 hereinabove, as well as
methods described in some of the publications cited
above.
In some embodiments, the method shown in Fig. 5
above can be similarly used in system configurations in
which the tracking fields are generated by catheter 24
and sensed by externally-located position sensors. In
these embodiments, signal generator 4 6 produces drive
signals that drive the field generators in catheter 24 to
produce the tracking fields. The external position
sensors sense the tracking fields. The sensed fields are
then used, in accordance with the appropriate method, to
determine a distortion-free position of catheter 24.
Although the embodiments described herein mainly
refer to improving the distortion immunity of medical
position tracking and steering systems, these methods and
systems can be used in additional applications, such as
for reducing the distortion caused by the operating room
table, fluoroscopy equipment, MRI equipment and/or any
other field-distorting object.
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 sub-combinations of the various features described
41

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.
42

CLAIMS
1. A method for tracking a position of an object,
comprising:
using a field sensor associated with the object to
measure field strengths of magnetic fields generated by
two or more field generators, wherein a measurement of at
least one of the field strengths is subject to a
distortion;
calculating rotation-invariant location coordinates
of the object responsively to the measured field
strengths; and
determining corrected location coordinates of the
object by applying to the rotation-invariant location
coordinates a coordinate correcting function so as to
adjust a relative contribution of each of the measured
field strengths to the corrected location coordinates
responsively to the distortion in the measured field
strengths.
2. The method according to claim 1, and comprising
inserting the object into an organ of a patient, wherein
determining the corrected location coordinates of the
object comprises tracking the position of the object
inside the organ.
3. The method according to claim 1, wherein the
distortion is caused by a field-distorting object
subjected to at least some of the magnetic fields,
wherein the object comprises at least one material
selected from a group consisting of metallic,
paramagnetic and ferromagnetic materials.
43

4. The method according to claim 1, and comprising
performing calibration measurements of the magnetic
fields at respective known coordinates relative to the
two or more field generators, and deriving the coordinate
correcting function responsively to the calibration
measurements.
5. The method according to claim 4, wherein the
distortion is caused by a movable field-distorting
object, and wherein performing the calibration
measurements comprises performing the measurements at
different locations of the field-distorting object.
6. The method according to claim 4, wherein deriving
the coordinate correcting function comprises applying a
fitting process to a dependence of the calibration
measurements on the known coordinates.
7. The method according to claim 1, wherein applying
the coordinate correcting function comprises applying a
polynomial function having coefficients comprising
exponents of at least some of the rotation-invariant
location coordinates.
8. The method according to claim 1, wherein applying
the coordinate correcting function comprises identifying
a distortion-contributing element responsively to the
measured field strengths, and producing the coordinate
correcting function so as to disregard the measured field
44

strengths that are associated with the distortion-
contributing element.
9. The method according to claim 8, wherein the field
sensor comprises one or more field sensing elements, and
wherein identifying the distortion-contributing element
comprises determining that one or more of the field
sensing elements and the field generators are
contributing to the distortion.
10. The method according to claim 1, and comprising
calculating angular orientation coordinates of the
object.
11. The method according to claim 1, wherein the field
sensor is used within a working volume associated with
the two or more field generators, and wherein determining
the corrected location coordinates comprises:
dividing the working volume into two or more
clusters;
defining for each of the two or more clusters
respective two or more cluster coordinate correcting
functions; and
applying to each of the rotation-invariant location
coordinates one of the cluster coordinate correcting
functions responsively to a cluster in which the
rotation-invariant location coordinate falls.
12.' The method according to claim 11, wherein applying
the cluster coordinate correcting functions comprises
45

applying a weighting function so as to smoothen a
transition between neighboring clusters.
13. The method according to claim 1, and comprising
measuring the field strengths using two or more field
sensors having non-concentric locations, and compensating
for inaccuracies caused by the non-concentric locations
in the corrected location coordinates.
14. A method for tracking a position of an object,
comprising:
using a field sensor associated with the object to
perform measurements of field strengths of magnetic
fields generated by two or more field generators so as to
provide redundant location information, wherein at least
some of the field strength measurements are subject to a
distortion; and
determining location coordinates of the object
relative to the two or more field generators by applying
to the measurements a coordinate correcting function that
exploits the redundant location information so as to
reduce an impact of the distortion on the location
coordinates.
15. A method for tracking a position of an object,
comprising:
using a field sensor, which comprises one or more
field sensing elements associated with the object, to
measure field strengths of magnetic fields generated by
two or more field generators, wherein a measurement of at
46

least one of the field strengths is subject to a
distortion;
identifying, responsively to the measured field
strengths, at least one distortion-contributing system
element, which is selected from a group consisting of the
one or more field sensing elements and of the two or more
field generators; and
determining the position of the object relative to
the two or more field generators responsively to the
measured field strengths while disregarding field
measurements associated with the distortion-contributing
system element.
16. The method according to claim 15, and comprising
inserting the object into an organ of a patient, wherein
determining the position of the object comprises tracking
the position of the object inside the organ.
17. The method according to claim 16, wherein the two or
more field generators are associated with the object, and
wherein the field sensor is located externally to the
organ.
18. The method according to claim 15, wherein
identifying the distortion-contributing system element
comprises accepting an a-priori indication selected from
a group consisting of a characteristic direction of the
distortion and an identity of the distortion-contributing
system element.
47

19. The method according to claim 15, wherein
identifying the distortion-contributing system element
comprises sensing a presence of the distortion in the
field measurements associated with the distortion-
contributing system element.
20. The method according to claim 15, wherein the
distortion-contributing system element comprises a pair
of one of the field sensing elements and one of the field
generators.
21. The method according to claim 15, wherein
disregarding the field measurements associated with the
distortion-contributing system element comprises
deactivating the distortion-contributing system element.
22. A system for tracking a position of an object,
comprising:
two or more field generators, which are arranged to
generate respective magnetic fields in a vicinity of the
object;
a field sensor associated with the object, which is
arranged to measure field, strengths of the magnetic
fields, wherein a measurement of at least one of the
field strengths is subject to a distortion; and
a processor, which is arranged to calculate
rotation-invariant location coordinates of the object
responsively to the measured field strengths, and to
determine corrected location coordinates of the object by
applying to the rotation-invariant location coordinates a
coordinate correcting function so as to adjust a relative
48

contribution of each of the measured field strengths to
the corrected location coordinates responsively to the
distortion in the measured field strengths.
23. The system according to claim 22, wherein the object
is adapted to be inserted into an organ of a patient, and
wherein the processor is arranged to track the position
of the object inside the organ.
24. The system according to claim 22, wherein the
distortion is caused by a field-distorting object
subjected to at least some of the magnetic fields,
wherein the object comprises at least one material
selected from a group consisting of metallic,
paramagnetic and ferromagnetic materials.
25. The system according to claim 22, wherein the
coordinate correcting function is determined by
performing calibration measurements of the magnetic
fields at respective known coordinates relative to the
two or more field generators, and deriving the coordinate
correcting function responsively to the calibration
measurements.
26. The system according to claim 25, wherein the
distortion is caused by a movable field-distorting
object, and wherein the calibration measurements comprise
measurements taken at different locations of the field-
distorting object.
49

27. The system according to claim 25, wherein the
processor is arranged to apply a fitting process to a
dependence of the calibration measurements on the known
coordinates, so as to derive the coordinate correcting
function.
28. The system according to claim 22, wherein the
coordinate correcting function comprises a polynomial
function having coefficients comprising exponents of at
least some of the rotation-invariant location
coordinates.
29. The system according to claim 22, wherein the
processor is arranged to identify a distortion-
contributing element responsively to the measured field
strengths, and to produce the coordinate correcting
function so as to disregard the measured field strengths
that are associated with the distortion^contributing
element.
30. The system according to claim 29, wherein the field
sensor comprises one or more field sensing elements, and
wherein the processor is arranged to identify that at
least one element selected from a group consisting of the
field sensing elements and the field generators is
contributing to the distortion, so as to identify the
distortion-contributing system element.
50

31. The system according to claim 22, wherein the
processor is further arranged to calculate angular
orientation coordinates of the object.
32. The system according to claim 22, wherein the field
sensor is used within a working volume associated with
the two or more field generators, and wherein the
processor is arranged to divide the working volume into
two or more clusters, to define for each of the two or
more clusters respective two or more cluster coordinate
correcting functions, and to apply to each of the
rotation-invariant location coordinates one of the
cluster coordinate correcting functions responsively to a
cluster in which the rotation-invariant location
coordinate falls.
33. The system according to claim 32, wherein the
processor is further arranged to apply a weighting
function so as to smoothen a transition between
neighboring clusters.
34. The system according to claim 22, and comprising two
or more field sensors having non-concentric locations,
wherein the processor is arranged to compensate for
inaccuracies caused by the non-concentric locations in
the corrected location coordinates.
35. A system for tracking a position of an object,
comprising:
51

two or more field generators, which are arranged to
generate respective magnetic fields in a vicinity of the
object;
a field sensor associated with the object, which is
arranged to perform measurements of field strengths of
the magnetic fields so as to provide redundant location
information, wherein at least some of the field strength
measurements are subject to a distortion; and
a processor, which is arranged to determine location
coordinates of the object relative to the two or more
field generators by applying to the measurements a
coordinate correcting function that exploits the
redundant location information so as to reduce an impact
of the distortion on the location coordinates.
36. A system for tracking a position of an object,
comprising:
two or more field generators, which are arranged to
generate respective magnetic fields in a vicinity of the
object;
a field sensor, which is associated with the object
and comprises one or more field sensing elements, which
is arranged to measure field strengths of the magnetic
fields, wherein a measurement of at least one of the
field strengths is subject to a distortion; and
a processor, which is arranged to identify
responsively to the measured field strengths a
distortion-contributing system element, which is selected
from a group consisting of the one or more field sensing
elements and the two or more field generators, and to
determine the position of the object relative to the two
or more field generators while disregarding field
52

measurements associated with the distortion-contributing
system element.
37. The system according to claim 36, wherein the object
is adapted to be inserted into an organ of a patient, and
wherein the processor is arranged to track the position
of the object inside the organ.
38. The system according to claim 36, wherein the two or
more field generators are associated with the object, and
wherein the field sensor is located externally to the
organ.
39. The system according to claim 36, wherein the
processor is arranged to accept an a-priori indication
selected from a group consisting of a characteristic
direction of the distortion and an identity of the
distortion-contributing system element.
40. The system according to claim 36, wherein the
processor is arranged to identify the distortion-
contributing system element by sensing a presence of the
distortion in the field measurements associated with the
distortion-contributing system element.
41. The system according to claim 36, wherein the
distortion-contributing system element comprises a pair
of one of the field sensing elements and one of the field
generators.
53

42. The system according to claim 36, wherein the
processor is arranged to deactivate the distortion-
contributing system element.
43. A computer software product used in a system for
tracking a position of an object, the product comprising
a computer-readable medium, in which program instructions
are stored, which instructions, when read by the
computer, cause the computer to control two or more field
generators so as to generate magnetic fields in a
vicinity of the object, to accept measurements of field
strengths of the magnetic fields performed by a field
sensor associated with the object, wherein a measurement
of at least one of the field strengths is subject to a
distortion, to calculate rotation-invariant location
coordinates of the object responsively to the measured
field strengths, and to determine corrected location
coordinates of the object by applying to the rotation-
invariant location coordinates a coordinate correcting
function so as to adjust a relative contribution of each
of the measured field strengths to the corrected location
coordinates responsively to the distortion in the
measured field strengths.
44. A computer software product used in a system for
tracking a position of an object, the product comprising
a computer-readable medium, in which program instructions
are stored, which instructions, when read by the
computer, cause the computer to control two or more field
generators so as to generate magnetic fields in a
vicinity of the object, to accept measurements of field
54

strengths of the magnetic fields performed by a field
sensor associated with the object, the measurements
comprising redundant location information, wherein at
least some of the measurements are subject to a
distortion, and to determine location coordinates of the
object relative to the two or more field generators by
applying to the measurements a coordinate correcting
function that exploits the redundant location information
so as to reduce an impact of the distortion on the
location coordinates.
45. A computer software product used in a system for
tracking a position of an object, the product comprising
a computer-readable medium, in which program instructions
are stored, which instructions, when read by the
computer, cause the computer to control two or more field
generators. so as to generate magnetic fields in a
vicinity of the object, to accept measurements of field
strengths of the magnetic fields performed by a field
sensor, which is associated with the object and comprises
one or more field sensing elements, wherein a measurement
of at least one of the field strengths is subject to a
distortion, to identify responsively to the measured
field strengths a distortion-contributing system element,
which is selected from a group consisting of the two or
more field generators and the one or more field sensing
elements, and to determine the position of the object
relative to the two or more field generators while
disregarding field measurements associated with the
distortion-contributing system element.

A method for tracking a position of an object
includes using a field sensor associated with the object
to measure field strengths of magnetic fields generated
by two or more field generators, wherein a measurement of
at least one of the field strengths is subject to a
distortion. Rotation-invariant location coordinates of
the object are calculated responsively to the measured
field strengths. Corrected location coordinates of the
object are determined by applying to the rotation-
invariant location coordinates a coordinate correcting
function so as to adjust a relative contribution of each
of the measured field strengths to the corrected location
coordinates responsively to the distortion in the
measured field strengths.

Documents:

http://ipindiaonline.gov.in/patentsearch/GrantedSearch/viewdoc.aspx?id=H1Yum7ZtVhn//bVnMZb3dg==&loc=wDBSZCsAt7zoiVrqcFJsRw==


« Previous Patent
Next Patent »
Patent Number 279397
Indian Patent Application Number 974/KOL/2007
PG Journal Number 04/2017
Publication Date 27-Jan-2017
Grant Date 19-Jan-2017
Date of Filing 09-Jul-2007
Name of Patentee BIOSENSE WEBSTER, INC.
Applicant Address 3333 DIAMOND CANYON ROAD, DIAMOND BAR CA
Inventors:
# Inventor's Name Inventor's Address
1 ASSAF GOVARI VITZO 1, HAIFA 34400
2 MEIR BAR-TAL ACHIRTA 17, ZICHRON YA'ACOV 30900
PCT International Classification Number G01R 19/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 11/462,733 2006-08-07 U.S.A.