U.S. patent application number 11/672562 was filed with the patent office on 2008-08-14 for impedance registration and catheter tracking.
Invention is credited to Doron Harlev, Adam Pidlisecky.
Application Number | 20080190438 11/672562 |
Document ID | / |
Family ID | 39682334 |
Filed Date | 2008-08-14 |
United States Patent
Application |
20080190438 |
Kind Code |
A1 |
Harlev; Doron ; et
al. |
August 14, 2008 |
IMPEDANCE REGISTRATION AND CATHETER TRACKING
Abstract
Methods and systems are disclosed for determining information
about a position of an object within a distribution of materials
having different complex conductivities. The method includes: (i)
causing current to flow in the distribution; (ii) measuring an
electrical signal at each of multiple locations in the distribution
of materials in response to the current flow; (iii) providing
spatial information about the distribution of materials with
respect to a first reference frame, the spatial information
indicative of regions of different complex conductivity in the
distribution of materials; and (iv) determining the position of the
object with respect to the spatial information about the
distribution of materials based on measured electrical signals and
the spatial information. In certain embodiments, the object is a
catheter inserted into a patients heart cavity for cardiac
mapping.
Inventors: |
Harlev; Doron; (Cambridge,
MA) ; Pidlisecky; Adam; (Salem, MA) |
Correspondence
Address: |
FISH & RICHARDSON PC
P.O. BOX 1022
MINNEAPOLIS
MN
55440-1022
US
|
Family ID: |
39682334 |
Appl. No.: |
11/672562 |
Filed: |
February 8, 2007 |
Current U.S.
Class: |
128/898 |
Current CPC
Class: |
A61B 90/36 20160201;
A61B 2034/2051 20160201; A61B 5/0538 20130101; A61B 5/6859
20130101; A61B 5/068 20130101; A61B 2017/00026 20130101; A61B
2017/00053 20130101; A61B 34/20 20160201; A61B 5/0044 20130101;
Y02A 90/26 20180101; Y02A 90/10 20180101; A61B 5/0536 20130101;
A61B 2017/00243 20130101 |
Class at
Publication: |
128/898 |
International
Class: |
A61B 19/00 20060101
A61B019/00 |
Claims
1. A method for determining information about a position of an
object within a distribution of materials having different complex
conductivities, the method comprising: causing current to flow in
the distribution; measuring an electrical signal at each of
multiple locations in the distribution of materials in response to
the current flow; providing spatial information about the
distribution of materials with respect to a first reference frame,
the spatial information indicative of regions of different complex
conductivity in the distribution of materials; and determining the
position of the object with respect to the spatial information
about the distribution of materials based on the measured
electrical signals and the spatial information.
2. The method of claim 1, wherein determining the position of the
object relative to the first reference frame comprises using an
optimization algorithm that minimizes differences between the
measured electrical signals and predicted signals determined from
the spatial information about the distribution of materials as a
function of the relative position.
3. The method of claim 2, wherein the optimization algorithm
further determines a conductivity value for each of one or more of
the materials in the distribution of materials.
4. The method of claim 1, wherein the distribution of materials
comprises a patient's heart cavity and the object is a catheter
inserted into the patient's heart.
5. The method of claim 4, wherein the spatial information about the
distribution of materials is based on one or more of: a computed
tomography (CT) image; a magnetic resonance imaging (MRI) image; a
fluoroscopic rotational angiography image; and an ultrasound
image.
6. The method of claim 4, wherein at least some of the electrodes
that cause the current to flow are located on different regions of
the catheter.
7. The method of claim 4, wherein the catheter comprises spatially
distributed electrodes to measure at least some of the electrical
signals produced in response to the injected current.
8. The method of claim 7, wherein the electrodes on the catheter
are further used to measure electrical signals indicative of
cardiac electrical activity.
9. The method of claim 8, wherein the current is injected at
frequencies spaced from those corresponding to the cardiac
electrical activity and the determining comprises frequency
processing the measured electrical signal to distinguish electrical
signals responsive to the injected current from those corresponding
to cardiac electrical activity.
10. The method of claim 4, further comprising repeating the
causing, measuring, and determining steps to track the position of
the catheter in the heart with respect to the first reference
frame.
11. The method of claim 4, wherein the catheter further comprises
at least one tracking element whose position in a second reference
frame is detectable by an independent tracking system.
12. The method of claim 11, wherein the method further comprises
using the determined information about the position of the catheter
to register the first and second reference frames.
13. The method of claim 4, wherein the spatial information
corresponds to a specific point in a cardiac cycle.
14. The method of claim 13, further comprising synchronizing the
injecting and the measuring with respect to the cardiac cycle.
15. The method of claim 4, wherein the spatial information
corresponds to an average of the geometrical configuration of the
heart cavity over multiple cardiac cycles.
16. A method for determining a transformation for registering first
and second reference frames for a distribution of materials, the
method comprising: causing current to flow in the distribution;
measuring an electrical signal at each of multiple locations in the
distribution of materials in response to the current flow;
providing spatial information about the distribution of materials
with respect to the first reference frame, the spatial information
indicative of regions of different complex conductivity in the
distribution of materials; providing positions in the second
reference frame for the multiple locations at which the electrical
signals are measured; and determining the transformation based on
the measured electrical signals, the spatial information about the
distribution of materials, and the positions in the second
reference frame for the multiple locations at which the electrical
signals are measured.
17. The method of claim 16, wherein the distribution of materials
comprises a patient's heart cavity, wherein at least some of the
electrical signals are measured by electrodes on a catheter
inserted into the heart cavity, and wherein the second reference
frame corresponds to coordinates provided by a tracking system for
the catheter.
18. A system for determining information about a position of an
object within a distribution of materials having different complex
conductivities, the system comprising: electronics for causing
current to flow in the distribution; electronics for measuring an
electrical signal at each of multiple locations in the distribution
of materials in response to the current flow; and an electronic
processor coupled to current causing and signal measuring
electronics, wherein the electronic processor is configured to
determine the position of the object with respect to spatial
information about the distribution of materials based on the
measured electrical signals and the spatial information, wherein
the spatial information is indicative of regions of different
complex conductivity in the distribution of materials with respect
to a first reference frame.
19. The system of claim 18, wherein the determination of the
position of the object relative to the first reference frame by the
electronic processor comprises using an optimization algorithm that
minimizes differences between the measured electrical signals and
predicted signals determined from the spatial information about the
distribution of materials as a function of the relative
position.
20. The system of claim 19, wherein the optimization algorithm
further determines a conductivity value for each of one or more of
the materials in the distribution of materials.
21. The system of claim 18, wherein the distribution of materials
comprises a patient's heart cavity, the object is a catheter
configured to be inserted into the patient's heart, and wherein the
system includes the catheter.
22. The system of claim 21, wherein the spatial information about
the distribution of materials is based on one or more of: a
computed tomography (CT) image; a magnetic resonance imaging (MRI)
image; a fluoroscopic rotational angiography image; and an
ultrasound image.
23. The system of claim 21, wherein the catheter comprises current
injecting electrodes coupled to the current causing electronics for
causing the current to flow.
24. The system of claim 23, further comprising a second catheter
comprising additional current injecting electrodes coupled to the
current causing electronics.
25. The system of claim 21, wherein the catheter comprises
spatially distributed electrodes coupled to the measuring
electronics to measure at least some of the electrical signals
produced in response to the injected current.
26. The system of claim 25, wherein the electronic processor is
further configured to use electrical signals measured by the
electrodes on the catheter to determine information about cardiac
electrical activity.
27. The system of claim 26, wherein the current causing electronics
causes the current to be injected at frequencies spaced from those
corresponding to the cardiac electrical activity, and wherein the
measuring electronics is configured to frequency process the
measured electrical signal to distinguish electrical signals
indicative of cardiac electrical activity from those responsive to
the injected current.
28. The system of claim 23, wherein the surface of one or more of
the current injection electrode has a coating to reduce its
electrical impedance with respect to blood in the heart cavity.
29. The system of claim 23, wherein the current injection
electrodes are positioned at opposite ends of a deployed
configuration for the catheter with respect to each of multiple
axes.
30. The system of claim 21, wherein the electronic processor is
configured to track the position of the catheter in the heart with
respect to the first reference frame in response to the current
injection and signal measuring.
31. The system of claim 21, further comprising at least one
tracking element coupled to the catheter and an independent
tracking system coupled to the electronic processor for providing
the position of the tracking element in a second reference
frame.
32. The system of claim 31, wherein the electronic processor is
further configured to use the determined information about the
position of the catheter to register the first and second reference
frames.
33. The system of claim 21, wherein the electronic processor is
configured to process the measured signals for each of multiple
catheter locations within the heart, and wherein the position of
the catheter is determined based on the measured electrical signals
for all of the multiple catheter locations, the spatial information
about the distribution of materials, and relative changes in the
position of the catheter corresponding to the multiple
locations.
34. The system of claim 21, wherein the spatial information
corresponds to a specific point in a cardiac cycle.
35. The system of claim 21, wherein the electronics are configured
to synchronize the current injection and the signal measuring with
respect to the cardiac cycle.
36. The system of claim 21, wherein the spatial information
corresponds to an average of the geometrical configuration of the
heart cavity over multiple cardiac cycles.
37. A system for determining a transformation for registering first
and second reference frames for a distribution of materials, the
system comprising: electronics for causing current to flow in the
distribution; electronics for measuring an electrical signal at
each of multiple locations in the distribution of materials in
response to the current flow; and an electronic processor coupled
to the current causing and signal measuring electronics and
configured to determine the transformation based on the measured
electrical signals, a spatial information about the distribution of
materials with respect to the first reference frame, and positions
in the second reference frame for the multiple locations at which
the electrical signals are measured, wherein the spatial
information about the distribution of materials with respect to the
first reference frame is indicative of regions of different complex
conductivity in the distribution of materials.
38. The system of claim 37, wherein the distribution of materials
comprises a patient's heart cavity, and wherein the system further
comprises a catheter configured for insertion into the heart cavity
and an independent tracking system for the catheter, wherein at
least some of the electrical signals are measured by electrodes on
the catheter and wherein the second reference frame corresponds to
coordinates provided by the tracking system for the catheter.
39. The method of claim 1, wherein the spatial information
comprises a conductivity value for each of the regions of different
complex conductivity in the distribution of materials.
40. The system of claim 18, wherein the spatial information
comprises a conductivity value for each of the regions of different
complex conductivity in the distribution of materials.
Description
TECHNICAL FIELD
[0001] This invention relates to determining the position of an
object, such as tracking the position of a catheter in a patient's
heart cavity, and to the registration of a representation of a
space, such as a 3D representation of the patient's heart cavity,
to a coordinate system used to track the catheter.
BACKGROUND
[0002] Use of minimally invasive procedures, such as catheter
ablation, to treat a variety of heart conditions, such as
supraventricular and ventricular arrhythmias, is becoming
increasingly more prevalent. Such procedures involve the mapping of
electrical activity in the heart, such as at various locations on
the endocardium surface ("cardiac mapping"), to identify the site
of origin of the arrhythmia followed by a targeted ablation of the
site. To perform such cardiac mapping a catheter with one or more
electrodes can be inserted into the patient's heart chamber.
[0003] Under some circumstances, the location of the catheter in
the heart chamber is determined using a tracking system. One type
of a tracking system that may be used to track the location of the
catheter inside the heart chamber is an independent tracking system
based on the use of magnetic or electric fields to sense and track
the location of the catheter. The location of a catheter tracked
using such an independent tracking system is thus provided in terms
of the tracking system's coordinates system.
[0004] The matching of the catheter's location according to the
independent tracking system and a coordinate system corresponding
to some type of 3D anatomical representation of the heart cavity,
is called coordinate system registration.
SUMMARY
[0005] In general, in one aspect, a method is disclosed for
determining information about a position of an object within a
distribution of materials having different complex conductivities.
The method includes: (i) causing current to flow in the
distribution; (ii) measuring an electrical signal at each of
multiple locations in the distribution of materials in response to
the current flow; (iii) providing spatial information about the
distribution of materials with respect to a first reference frame,
the spatial information indicative of regions of different complex
conductivity in the distribution of materials; and (iv) determining
the position of the object with respect to the spatial information
about the distribution of materials based on measured electrical
signals and the spatial information.
[0006] Embodiments of the method may include any of the following
features.
[0007] The regions of different complex conductivity can include
regions of having different real parts of the complex conductivity,
regions having different imaginary parts of the complex
conductivity, or regions having different real parts and different
imaginary parts of the complex conductivity. Similarly, the
measured electrical signals can include information about the
amplitude, phase, or both.
[0008] The spatial information can be indicative of a position
dependent complex conductivity throughout the distribution of
materials, the position dependent complex conductivity including a
conductivity value for each of the different materials in the
distribution.
[0009] Causing the current to flow in the distribution can include
causing the current to flow between each of multiple pairs of
electrodes. The electrical signals can then be measured for each
pair of electrodes that cause the current to flow. At least some of
the electrodes that cause current to flow can be located on
different regions of the object. At least some of the electrical
signals can be measured by corresponding electrodes on the
object.
[0010] Determining the position of the object with respect to the
spatial information about the distribution of materials can include
determining coordinates for the position of the object in the first
reference frame. The method can further include repeating the
causing, measuring, and determining steps to track the position of
the object in the first reference frame as it moves through the
distribution of materials.
[0011] Determining the position of the object with respect to the
spatial information can include tracking movement of the object in
a second reference frame and determining a transformation for
registering the first reference frame with the second reference
frame.
[0012] Determining the position of the object relative to the first
reference frame can include determining a location of a point of
the object in the first reference frame and an orientation of the
object in the first reference frame.
[0013] Determining the position of the object relative to the first
reference frame can include using an optimization algorithm that
minimizes differences between the measured electrical signals and
predicted signals determined from the spatial information about the
distribution of materials as a function of the relative position.
Moreover, the optimization algorithm can further determine a
conductivity value for each of one or more of the materials in the
distribution of materials.
[0014] In certain embodiments, the distribution of materials
includes a patient's heart cavity and the object is a catheter
inserted into the patient's heart.
[0015] The spatial information about the distribution of materials
can be based on one or more of: a computed tomography (CT) image; a
magnetic resonance imaging (MRI) image; a fluoroscopic rotational
angiography image; and an ultrasound image.
[0016] At least some of the electrodes that cause the current to
flow can be located on different regions of the catheter.
Alternatively, or in addition, at least some of the electrodes that
cause the current to flow can be located on a second catheter
positioned in the patient's heart cavity.
[0017] The catheter can include spatially distributed electrodes to
measure at least some of the electrical signals produced in
response to the injected current. The electrodes on the catheter
can be further used to measure electrical signals indicative of
cardiac electrical activity. For example, the current can be
injected at frequencies spaced from those corresponding to the
cardiac electrical activity. The method can further include
frequency processing the measured electrical signal to distinguish
electrical signals indicative of cardiac electrical activity from
those responsive to the injected current.
[0018] The information about electrical activity in the heart
cavity can be based on the measured electrical signals, the spatial
information about the heart cavity, and the determined relative
position of the catheter. The method can further include displaying
the information about the electrical activity in the heart on a
representation of the patient's heart. The method can further
include treating a patient's heart condition based on the displayed
information about the electrical activity in the heart.
[0019] The catheter can include current injection electrodes for
injecting current into the patient's heart cavity that are
different from the electrodes used measure electrical signals. The
surface area of each current injection electrode can be larger than
the surface area of each electrode used to measure an electrical
signal. The surface of each current injection electrode can have a
coating to reduce its electrical impedance with respect to blood in
the heart cavity. More generally, every electrode on the catheter
can include such a coating.
[0020] The catheter can include multiple pairs of current injection
electrodes. For example, the current injection electrodes can be
positioned at opposite ends of a deployed configuration for the
catheter with respect to each of multiple axes. The catheter can be
deployable in a rigid configuration. Furthermore, the catheter can
be configured for non-contact deployment in each of multiple
locations within the heart.
[0021] The method can further include repeating the causing,
measuring, and determining steps to track the position of the
catheter in the heart with respect to the first reference
frame.
[0022] The catheter can further include at least one tracking
element whose position in a second reference frame is detectable by
an independent tracking system. For example, the method can further
include using the determined information about the position of the
catheter to register the first and second reference frames.
Registering the first and second reference frame can include
determining a transformation that maps the locations of measuring
electrodes in the first reference frame to the locations of
measuring electrodes in the second reference frame.
[0023] The method further includes repeating the causing and
measuring steps as the catheter is moved to each of multiple
locations within the heart, wherein the position of the object is
determined based on the measured electrical signals for all of the
multiple catheter locations, the spatial information about the
distribution of materials, and relative changes in the position of
the catheter corresponding to the multiple locations.
[0024] The spatial information corresponds to an average of the
geometrical configuration of the heart cavity over multiple cardiac
cycles. Alternatively, the spatial information can corresponds to a
specific point in a cardiac cycle. The method can further include
synchronizing the injecting and the measuring with respect to the
cardiac cycle.
[0025] In general, in another aspect, a method is disclosed for
determining a transformation for registering first and second
reference frames for a distribution of materials. The method
includes: (i) causing current to flow in the distribution; (ii)
measuring an electrical signal at each of multiple locations in the
distribution of materials in response to the current flow; (iii)
providing spatial information about the distribution of materials
with respect to the first reference frame, the spatial information
indicative of regions of different complex conductivity in the
distribution of materials; (iv) providing positions in the second
reference frame for the multiple locations at which the electrical
signals are measured; and (v) determining the transformation based
on the measured electrical signals, the spatial information about
the distribution of materials, and the positions in the second
reference frame for the multiple locations at which the electrical
signals are measured. For example, the distribution of materials
may include a patient's heart cavity, wherein at least some of the
electrical signals are measured by electrodes on a catheter
inserted into the heart cavity, and wherein the second reference
frame corresponds to coordinates provided by a tracking system for
the catheter.
[0026] Embodiments of the method may include any features described
above in connection with the first method.
[0027] In general, in another aspect, a system is disclosed for
determining information about a position of an object within a
distribution of materials having different complex conductivities
The system includes: (i) electronics for causing current to flow in
the distribution; (ii) electronics for measuring an electrical
signal at each of multiple locations in the distribution of
materials in response to the current flow; and (iii) an electronic
processor coupled to current causing and signal measuring
electronics, wherein the electronic processor is configured to
determine the position of the object with respect to spatial
information about the distribution of materials based on the
measured electrical signals and the spatial information, wherein
the spatial information is indicative of regions of different
complex conductivity in the distribution of materials with respect
to a first reference frame.
[0028] Embodiments of the system may include any of the following
features.
[0029] The regions of different complex conductivity can include
regions of having different real parts of the complex conductivity,
regions having different imaginary parts of the complex
conductivity, or regions having different real parts and different
imaginary parts of the complex conductivity. Similarly, the
measured electrical signals can include information about the
amplitude, phase, or both of the respective electrical signals.
[0030] The spatial information can be indicative of a position
dependent complex conductivity throughout the distribution of
materials, the position dependent complex conductivity including a
conductivity value for each of the different materials in the
distribution.
[0031] The object can include electrodes coupled to the measuring
electronics for measuring the electrical signals.
[0032] The object can include electrodes that cause at least some
of the current to flow. For example, the object can include
multiple pairs of current injecting electrodes coupled to the
current causing electronics for causing the current to flow between
each pair of the current injecting electrodes. The object can also
include measuring electrodes positioned on the object and coupled
to the measuring electronics to measure the electrical signals in
response to current flow between each pair of current injecting
electrodes.
[0033] The determination of the position of the object relative to
the first reference frame by the electronic processor includes
determining a location of a point of a point of the object in the
first reference frame and an orientation of the object in the first
reference frame.
[0034] The determination of the position of the object with respect
to the spatial information about the distribution of materials by
the electronic processor can include determining coordinates for
the position of the object in the first reference frame.
[0035] The electronic processor can be configured to track the
position of the object in the first reference frame as it moves
through the distribution of materials in response to additional
current flow and electrical signal measurements by the
electronics.
[0036] The determination of the position of the object with respect
to the spatial information by the electronic processor can include
tracking movement of the object in a second reference frame and
determining a transformation for registering the first reference
frame with the second reference frame.
[0037] The determination of the position of the object relative to
the first reference frame by the electronic processor can include
using an optimization algorithm that minimizes differences between
the measured electrical signals and predicted signals determined
from the spatial information about the distribution of materials as
a function of the relative position. The optimization algorithm can
further determine a conductivity value for each of one or more of
the materials in the distribution of materials.
[0038] In certain embodiments, the distribution of materials
includes a patient's heart cavity, and the object is a catheter
configured to be inserted into the patient's heart, and wherein the
system includes the catheter.
[0039] The spatial information about the distribution of materials
can be based on one or more of: a computed tomography (CT) image; a
magnetic resonance imaging (MRI) image; a fluoroscopic rotational
angiography image; and an ultrasound image.
[0040] The catheter can include current injecting electrodes
coupled to the current causing electronics for causing the current
to flow. The system can further include a second catheter including
additional current injecting electrodes coupled to the current
causing electronics. The catheter can include spatially distributed
electrodes coupled to the measuring electronics to measure at least
some of the electrical signals produced in response to the injected
current. The current injection electrodes on the catheter for
injecting current into the patient's heart cavity can be the same
or different from the electrodes used measure the electrical
signals.
[0041] The surface area of each current injection electrode can be
larger than the surface area of each electrode used to measure an
electrical signal.
[0042] The surface of each current injection electrode can have a
coating to reduce its electrical impedance with respect to blood in
the heart cavity. More generally, all of the catheter electrodes
can include such a coating.
[0043] The catheter can include multiple pairs of current injection
electrodes. For example, the current injection electrodes can be
positioned at opposite ends of a deployed configuration for the
catheter with respect to each of multiple axes.
[0044] The catheter can be configured for non-contact deployment in
each of multiple locations within the heart.
[0045] The electronic processor can be further configured to use
electrical signals measured by the electrodes on the catheter to
determine information about cardiac electrical activity. For
example, the current causing electronics can cause the current to
be injected at frequencies spaced from those corresponding to the
cardiac electrical activity. The measuring electronics can then be
configured to frequency process the measured electrical signal to
distinguish electrical signals indicative of cardiac electrical
activity from those responsive to the injected current.
[0046] The information about cardiac electrical activity can be
based on the measured electrical signals, the spatial information
about the heart cavity, and the determined relative position of the
catheter. The electronic processor can be further configured to
display the information about the electrical activity in the heart
on a representation of the patient's heart. The system can further
include an ablation catheter for treating a patient's heart
condition based on the displayed information about the cardiac
electrical activity.
[0047] The electronic processor can be configured to track the
position of the catheter in the heart with respect to the first
reference frame in response to the current injection and signal
measuring.
[0048] The system can further include at least one tracking element
coupled to the catheter and an independent tracking system coupled
to the electronic processor for providing the position of the
tracking element in a second reference frame. For example, the
electronic processor can further be configured to use the
determined information about the position of the catheter to
register the first and second reference frames. Registering the
first and second reference frames can include determining a
transformation that maps the locations of the measuring electrodes
in the first reference frame to the locations of the measuring
electrodes in the second reference frame.
[0049] The electronic processor can be configured to process the
measured signals for each of multiple catheter locations within the
heart, wherein the position of the catheter is determined based on
the measured electrical signals for all of the multiple catheter
locations, the spatial information about the distribution of
materials, and relative changes in the position of the catheter
corresponding to the multiple locations.
[0050] The spatial information can correspond to an average of the
geometrical configuration of the heart cavity over multiple cardiac
cycles. Alternatively, the spatial information can correspond to a
specific point in a cardiac cycle. For example, the electronics can
be configured to synchronize the current injection and the signal
measuring with respect to the cardiac cycle.
[0051] In general, in another aspect, a system is disclosed for
determining a transformation for registering first and second
reference frames for a distribution of materials. The system
includes: (i) electronics for causing current to flow in the
distribution; (ii) electronics for measuring an electrical signal
at each of multiple locations in the distribution of materials in
response to the current flow; and (iii) an electronic processor
coupled to the current causing and signal measuring electronics and
configured to determine the transformation based on the measured
electrical signals, a spatial information about the distribution of
materials with respect to the first reference frame, and positions
in the second reference frame for the multiple locations at which
the electrical signals are measured. The spatial information about
the distribution of materials with respect to the first reference
frame is indicative of regions of different complex conductivity in
the distribution of materials. In certain embodiments, the
distribution of materials includes a patient's heart cavity,
wherein the system further includes a catheter configured for
insertion into the heart cavity and an independent tracking system
for the catheter, wherein at least some of the electrical signals
are measured by electrodes on the catheter and wherein the second
reference frame corresponds to coordinates provided by the tracking
system for the catheter.
[0052] Embodiments of the system may further include any of the
features described above in connection with the first system.
[0053] As used herein, the "position" of an object means
information about one or more of the 6 degrees of freedom that
completely define the location and orientation of a
three-dimensional object in a three-dimensional coordinate system.
For example, the position of the object can include: three
independent values indicative of the coordinates of a point of the
object in a Cartesian coordinate system and three independent
values indicative of the angles for the orientation of the object
about each of the Cartesian axes; or any subset of such values.
[0054] As used herein, "heart cavity" means the heart and
surrounding tissue.
[0055] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention belongs. In case
of conflict with documents incorporated herein by reference, the
present document controls.
[0056] The details of one or more embodiments of the invention are
set forth in the accompanying drawings and the description below.
Other features, objects, and advantages of the invention will be
apparent from the description and drawings, and from the
claims.
DESCRIPTION OF THE DRAWINGS
[0057] FIG. 1 is a schematic diagram of an exemplary automatic
registration system.
[0058] FIGS. 2a-2c show perspective, end, and side views,
respectively, of a deployed catheter with multiple current
injection electrodes (CIE) and multiple potential measuring
electrodes (PME).
[0059] FIG. 3 is a schematic diagram of an analog implementation of
a signal generation module (SGM) and signal acquisition module
(SAM) for an electronics module coupled to the multi-electrode
catheter.
[0060] FIG. 4 is a schematic diagram of a digital implementation of
a signal generation module (SGM) and signal acquisition module
(SAM) for an electronics module coupled to the multi-electrode
catheter.
[0061] FIG. 5 is a flow diagram of an exemplary embodiment of an
automatic registration procedure using an independent tracking
system for the catheter.
[0062] FIG. 6 is a schematic diagram of conductivities assigned to
different structures in the heart cavity.
[0063] FIG. 7 is a schematic diagram of potential field lines
produced by current injection electrodes (CIE) activated in a
patient's heart cavity, and potential measuring electrodes on a
catheter used to measure the potential field at different locations
to infer information about the position of the catheter within the
heart cavity.
[0064] FIGS. 8a and 8b are exemplary illustrations of electrode
measurements taken at a single catheter location (FIG. 8a) and at
multiple catheter locations (FIG. 8b).
[0065] FIG. 9 is a flow diagram of another exemplary embodiment of
a registration procedure for determining the position of a catheter
in a patient's heart cavity.
[0066] FIG. 10 is exemplary illustration of electrode measurements
taken at a single catheter location.
[0067] FIGS. 11a-c are exemplary schematic diagrams of different
arrangements for positioning current injection electrodes (CIEs)
and potential measuring electrodes (PMEs) with respect to a
patient's heart cavity.
[0068] FIG. 12 is a flow diagram of an exemplary embodiment for
cardiac mapping using a multi-electrode catheter.
[0069] FIG. 13 is a schematic diagram of a timing sequence for
synchronizing operation of the current injecting electrodes with
the heart cycle.
[0070] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0071] Overview
[0072] Embodiments disclosed herein include a method and system for
determining the position of a catheter in a patient's heart cavity.
For example, the catheter may be configured with multiple
electrodes and used for cardiac mapping, such as described in
commonly owned patent application Ser. No. 11/451,898, entitled
"NON-CONTACT CARDIAC MAPPING, INCLUDING MOVING CATHETER AND
MULTI-BEAT INTEGRATION" and filed Jun. 13, 2006, application Ser.
No. 11/451,908, entitled "NON-CONTACT CARDIAC MAPPING, INCLUDING
PREPROCESSING" and filed Jun. 13, 2006, and application Ser. No.
11/451,871 entitled "NON-CONTACT CARDIAC MAPPING, INCLUDING
RESOLUTION MAP" and filed Jun. 13, 2006, the contents of which are
incorporated herein by reference. Generally, cardiac mapping
involves determining information about the electrical activity of a
patients heart (e.g., at different locations of the endocardium
surface) based on electrical signals measured by the multiple
electrodes of the catheter. To perform such cardiac mapping, the
position of the catheter (or more generally the positions of the
catheter electrodes) within the heart cavity should be known.
[0073] To determine the position of the catheter in the patient's
heart cavity, certain embodiments disclosed herein cause electrical
current to flow within the heart cavity. The current may originate
from electrodes on the catheter itself and/or from one or more
other sources that may be internal or external to the heart cavity.
The potential fields generated by the injected current will depend
on the conductivity profile within the heart cavity. For example,
blood and heart muscle have different conductivities. The potential
fields are measured at multiple locations within the heart cavity.
For example, electrodes on the catheter can be used to measure the
potentials. The potentials measured by the electrodes on the
catheter will depend on the position of the catheter within the
heart cavity. Moreover, when current is injected from one or more
electrodes on the catheter, the resulting potential fields will
also depend on the on the position of the catheter within the heart
cavity. Accordingly, measurements made by the catheter electrodes
can be used to infer information about the position of the catheter
in the heart cavity.
[0074] To accurately determine the position of the catheter within
the heart cavity, information about the measured potentials
produced in response to the injected current is combined with
separately acquired spatial information about the patient's heart
cavity (e.g., magnetic resonance imaging (MRI) or computed
tomography (CT) image slices of the patient's heart). Such spatial
information is used to determine a 3D representation of the
patient's heart cavity, including its conductivity profile. Based
on this separately acquired conductivity profile, the expected
potentials measured for different positions of the catheter within
the heart cavity can be calculated and compared to the actual
measured potentials to accurately determine the position of the
catheter within the heart cavity.
[0075] In some embodiments, potentials measured in response to the
injected current can be used to continuously monitor the position
of the catheter in the heart cavity, even as it is moved within the
heart cavity. In other embodiments, an independent tracking system
is used to monitor the position of the catheter in another
coordinate system, and the information about the position of the
catheter determined from the measured potentials in response to the
injected current and the separately acquired spatial information
about the patient's heart cavity is used to register the coordinate
system of the independent tracking system to that of the 3D
representation of the patient's heart cavity.
[0076] In the above discussion and in the details that follow, the
focus is on determining the position of a catheter in a heart
cavity. However, this is only an exemplary application. In other
cases, the method and systems generally disclosed herein can be
applied to determining the position of any object within any
distribution of materials characterized by a conductivity profile,
or to register the position of that object as measured by some
independent tracking system with a 3D representation of that
distribution of materials.
[0077] Furthermore, while in some of the specific embodiments that
follow the signals measured by the object electrodes correspond to
the relative strength (i.e., amplitude) of the measured electrical
signal (e.g., potential), further embodiments may also analyze the
phase of the measured signal, either alone or in combination with
the amplitude of the measured signal. The phase of the measured
signal is indicative of spatial variations in the imaginary part of
the complex conductivity (e.g., permittivity) in the distribution
of materials.
[0078] Representative System
[0079] FIG. 1 shows a schematic diagram of an exemplary embodiment
of an automatic registration system 100 that includes an optional
tracking system 180 to facilitate the tracking and registration of
a catheter 110 inside the heart cavity of a patient 101. The
catheter 110 is a moveable catheter 110 having multiple spatially
distributed electrodes. The catheter is used by a physician 103 to
perform various medical procedures, including cardiac mapping.
During performance of the automatic registration procedure, and
subsequently when the catheter acquires signals that are used to
perform cardiac mapping, the catheter 110 is displaced to at least
one location within the heart chamber into which the catheter 110
was inserted.
[0080] In some embodiments the catheter 110 is fitted with various
types of electrodes that are configured to perform various
functions. For example, the catheter 110 may include at least one
pair of current injection electrodes ("CIEs") configured to inject
electrical current into the medium in which the catheter 110 is
disposed. The catheter 110 may also include multiple potential
measuring electrodes ("PMEs") configured to measure the potentials
resulting from the current injected by the current injection
electrodes. In certain embodiments, the potential measuring
electrodes are also used for cardiac mapping.
[0081] FIGS. 2a-c show different views for one embodiment of the
catheter 110, which includes a base sleeve 112, a central
retractable inner member 114, and multiple splines 116 connected to
base sleeve 112 at one end and inner member 114 at the other end.
When inner member 114 is in an extended configuration (not shown),
splines 116 are pulled tight to the inner member so that catheter
110 has a narrow profile for guiding it through blood vessels. When
inner member 114 is retracted (as shown in FIGS. 2a-b), splines 116
are deployed and pushed into an outward "olive" shaped
configuration for use in the heart cavity. As explained in more
detail below, the splines 116 each carry electrodes, so when the
inner member is in the retracted configuration, the electrode are
deployed in the sense that they are distributed over a greater
volume.
[0082] Other known configurations may be used to deploy
multi-electrode catheter 110 For example, the catheter may use a
balloon, shape memory material such as Nitinol, or a polymer or
other stiffening material to selectively deploy the catheter and
its electrode into a desired configuration when in the patient's
heart cavity. In further embodiments, the catheter geometry may be
fixed, in which case it has the some configuration in the heart
cavity as in the blood vessels leading to the heart cavity.
[0083] Returning to the specific catheter embodiment of FIGS.
2a-2c, FIG. 2a shows a perspective view of catheter 110, FIG. 2b
shows an end-on view of catheter 110, and FIG. 2c shows a side view
of catheter 110, all with the catheter in its deployed
configuration. Each spline includes multiple potential measuring
electrodes (PME) 118, and every other spline includes a current
injection electrode (CIE) 119 at its most-outward position. Current
injection electrodes (CIE) 119 are also included on sleeve 112 at
the base of the splines and on the front tip of inner member 114
where the splines meet. Accordingly, in the presently described
embodiment, there are three pairs of CIEs, each generally defining
one axis in a Cartesian coordinate system.
[0084] The purpose of the CIEs is to inject current into the heart
cavity. For example, each CIE pair can define a source and sink
electrode, respectively, for injecting current into the heart
cavity. More generally, however, current may be injected in the
heart cavity from multiple electrodes relative to a ground
electrode. The purpose of the PMEs is to measure potentials in the
heart cavity in response to the current provided by the CIEs. The
PMEs can also be used for cardiac mapping.
[0085] In preferred embodiments, the current injecting electrodes
119 are generally mounted at different regions of the catheter 110
so as to maximize the information collected by multiple
configurations. CIE pairs that are oriented orthogonally relative
to each other produce less correlated measurements, which in turn
increase resolution. In addition, electrode pairs that are distant
from each other also produce less correlated measurements which
increase resolution. This is why in the preferred embodiment of
catheter 110 shown in FIGS. 2a-2c, the CIE electrodes 119 are
aligned as pairs on three orthogonal axes.
[0086] In some embodiments, like that shown in FIGS. 2a-2c,
multiple CIE electrode pairs are employed so that a large sample of
measured potentials in the heart cavity can be obtained to thereby
improve the robustness and accuracy of the registration procedure.
At some given time, any two electrodes from the CIE electrodes can
be selected and activated so that one of the selected electrodes
acts as the source electrode and the other electrode acts as a sink
electrode. A control mechanism in electrical communication with the
CIEs enables selection of any two electrodes to serve as the
activated source/sink pair at a particular time. After that
selected pair has been activated, and the resulting potentials in
the heart cavity are measured by the multiple potential measuring
electrodes, the pair of CIEs can be deactivated, and another pair
of CIEs is selected to cause another electric field to be formed
inside the heart cavity. Thus, the control mechanism regulates the
selection and activation of the CIEs to cause a temporal sequence
of injected currents to be created at different time instances,
which in turn results in a temporal sequence of different electric
fields formed inside the heart chamber in which the catheter 110 is
deployed. The control mechanism electrically couples a signal
generator to the selected electrodes. Selection of the particular
electrodes to be activated can be based on a pre-determined
sequence that is stored in a memory module connected to a central
processor connected to the catheter 110, or it can be based on
user-controlled signals that are electrically relayed to the
control mechanism to cause the desired activation of the CIEs.
Moreover, in further embodiments, more than a single pair of CIEs
can be simultaneously activated to inject current into the heart
cavity.
[0087] Referring again to FIG. 1, system 100 includes an
electronics module 140 coupled to processing unit 120 for
controlling the electrodes on catheter 110, including a signal
generation module for injecting current into the heart cavity
through the CIEs and a signal acquisition module for measuring
potentials through the PMEs. The electronics module 140 can be
implemented using analog or digital electronics, or a combination
of both. Such exemplary configurations, which are intended to be
non-limiting, are now described.
[0088] Referring to FIG. 3, the signal generation and acquisition
modules are implemented using analog hardware. The signal
generation module (SGM) depicted supports 8 CIEs defining 4
source/sink electrode pairs, where SRC refers to a source electrode
and SNK refers to a sink electrode. For the purpose of this
example, each pair is driven using a 10 kHz oscillating 1 mA
current source. A selector switch is used to select each of the
pairs sequentially based on control signals provided by the
processing unit or other control logic. Each channel in the signal
generation module is connected to a current injecting electrode. In
this case the source and sink electrodes are pre-selected
permanently such that each electrode is always either a source or a
sink, although this need not be the case in other embodiments
[0089] The signal acquisition module (SAM) buffers and amplifies
the signals as they are collected by the potential measuring
electrodes. The buffer prevents the acquisition system from loading
the signals collected by the electrodes. After buffering and
amplification, the signals are split and filtered into two
channels, one for detecting the registration signal (i.e., the
signals produced in response to the CIEs) and one for detecting the
signal generated by the heart's electrical activation (i.e.,
cardiac mapping). Because the heart's electrical activity is
primarily below 2 kHz, a low pass filter (LPF) is used to separate
the cardiac mapping potential signals from those produced in
response to the CIEs. The low pass filter may be implemented as an
active filter responsible for both filtering and amplification. The
signal is then sampled by an analog to digital converter. To
support bandwidth and resolution requirements the converter may
sample at >4 kHz at 15 bits per sample. After sampling, the
signals are passed to the processing unit for further analysis.
Both the LPF and A/D may be configured such that the filter and
sample frequency can be changed by software control (not
drawn).
[0090] The second channel following the input buffer detects the
registration signal. In this embodiment, the detection is
implemented using a lock-in amplifier approach to detect amplitude.
It should be appreciated that other implementation can be used to
accomplish the same task. In this channel the signal is first
filtered using a band pass filter (BPF) whose pass band frequency
is centered on the 10 kHz generated by the SGM. Following the BPF,
the signal is multiplied by the same 10 kHz signal generated by the
SGM using a mixer. As a result, the signal is down converted to DC
such that its value following the down conversion is proportional
to its amplitude before the down-conversion. The signal is then
filtered using a very narrow LPF of roughly 100 Hz. The filter
bandwidth has two effects. On the one hand, the narrower the filter
the better noise performance will be. On the other hand, the wider
the filter, the more registration updates are available per second.
A filter setting of 100 Hz provides excellent noise performance
with a location update rate of roughly 20 Hz. After filtering, the
signal is amplified and sampled by an analog to digital converter.
The converter in this case may sample at 200 Hz using 15 bits per
sample. After sampling, the signals are passed to the processing
unit for further analysis. As before, the channel properties can be
configured to be changed by software control (not drawn).
[0091] Referring to FIG. 4, the signal generation and acquisition
modules have a digital implementation. The SGM generates the
required signals using an array of n digital to analog converters
(D/A). In a preferred embodiment n=6. It should be appreciated that
instead of n D/As it is possible to use fewer D/As and a
multiplexed sample and hold amplifier. The signals generated by the
D/As are controlled and timed by the processing unit. In one
embodiment, the signals may mimic those described in the analog
implementation whereby a sinusoidal signal is switched between
electrodes. In other embodiments, however, the digital
implementation provides more flexibility in that more complex
signals (e.g. different frequencies, simultaneous activation of
multiple electrodes) may be driven. After the conversion to an
analog signal, the signals are buffered by an amplifier capable of
driving the necessary current (<2 mA) at relevant frequencies
(<30 kHz). After buffering, a processor controlled switch is
used to support a high impedance mode. This is necessary in order
to block a particular electrode from acting as a source or a sink
at a particular time.
[0092] In the SAM hardware, an input stage amplifies and buffers
the signal. Following amplification the signal is low pass filtered
in a wide enough band such that both the heart's electrical
activity (<2 kHz) and signals generated by the SGM are kept
inside the filtered band. In FIG. 4 the frequency band is 15 kHz.
Following the filter, the signal is sampled above Nyquist frequency
(>30 kHz) at 15 bits per sample. The sampled signals are then
transferred to the processing unit which uses digital signal
processing (DSP) techniques to filter the two channels in each
electrode and down-convert the registration signal
appropriately.
[0093] A relatively small number of CIEs can result in a relatively
large number of possible electrode pair combinations that can be
activated to enable different potential field configurations to be
formed inside the heart cavity, in which the catheter 110 is
deployed and thus enhance the robustness of the registration
procedure. For example, six (6) electrodes mounted on the catheter
110 can be paired into fifteen (15) combinations of different
source/sink pairs, thus resulting in fifteen different potential
fields, for a particular potential value, formed inside the medium.
As noted above, to achieve high robustness of the registration
procedure, the various source/sink electrodes disposed on the
catheter 110 may be mounted at different regions of the catheter.
For example, one useful configurations corresponds to that shown in
FIGS. 2a-2c in which the six (6) CIEs include a pair of CIEs align
along each of three orthogonal axes.
[0094] The potential measuring electrodes, configured to measure
the electrical signals in the distribution of materials (e.g., the
intracardiac blood) at the locations in which those electrodes are
situated, are generally distributed substantially uniformly on the
catheter 110. Preferably, the current injecting electrodes are
designed to have low impedance at the interface between electrode
and blood. The impedance between electrodes and blood is determined
by the surface area of the electrode and electrode material. The
larger the surface area, the lower the impedance. In one preferred
embodiment, the potential measuring electrodes would have
dimensions of 100 .mu.m.times.100 .mu.m, yielding a surface area of
a surface area of 10,000 .mu.m.sup.2, while the current injecting
electrodes would have dimensions of 1 mm.times.1 mm, yielding a
surface area 1 mm.sup.2. The larger surface area for CIEs is
preferred in order to reduce their impedance at the interface to
blood and allow the injection of current. The PMEs are less
sensitive to blood interface impedance because they are performing
the measurement with very high input impedance. Accordingly,
reducing interface impedance for the PMEs is generally not as
important as reducing it for the CIEs. Specialized coatings such as
Platinum Black, Iridium Oxide and Titanium Nitride may also be used
to reduce impedance of electrodes for a given surface area. For
example, such coatings may be applied to one or more of the CIEs,
one or more of the PMEs, or all of the catheter electrodes.
[0095] In some embodiments, sixty-four (64) potential measuring
electrodes are used. The exact number of potential measuring
electrodes that are employed depends on the dimensions of the
catheter 110 and on the desired accuracy of the registration
procedure. In order to register the catheter with 6 degrees of
freedom, a minimum of 6 data points are required, and thus at least
six electrodes are necessary. However, given that the numerical
problem is ill-conditioned, additional electrodes help better
condition the problem, and consequently improve accuracy. In
embodiments in which the sensing electrodes serve in the dual
capacity of registration electrodes and cardiac mapping electrodes,
the number of sensing electrodes to be used will also depend on the
desired accuracy of the physiological information at the
endocardium surface that is to be reconstructed.
[0096] As noted above, the PMEs on catheter 110 can also used for
cardiac mapping, such as that described in commonly owned patent
application Ser. No. 11/451,898, entitled "NON-CONTACT CARDIAC
MAPPING, INCLUDING MOVING CATHETER AND MULTI-BEAT INTEGRATION" and
filed Jun. 13, 2006, the contents of which are incorporated herein
by reference. As also noted above, because the frequency of the
current injected by CIEs (e.g., 10 kHz) is much higher than the
frequency of the electrical activity of the patient's heart, the
signal acquisition module can separate signals measured by the PMEs
based on frequency to distinguish registration signals from cardiac
mapping signals (e.g., frequencies higher than 1 kHz, and lower
than 1 kHz, respectively.) Furthermore, in additional embodiments,
catheter 110 may include separate electrodes used only for cardiac
mapping.
[0097] As further shown in FIG. 1, the registration system 100
includes the image acquisition and preparation module 130. The
acquisition and preparation module 130 receives volumetric images
(e.g., CT, MRI or ultrasound images taken by a scanner apparatus)
of the torso, and processes them to provide spatial information
about the patient's heart cavity (or, in other non-cardiac
applications, spatial information may be derived from other imaging
means about some other distribution of materials.) The spatial
information thus provided can subsequently be processed to include
conductivity information relating to the heart chamber (or
distribution of materials), to thereby provide spatial information
that is indicative of position-dependent conductivity throughout
the heart cavity. Thus, module 130 processes the volumetric images
to provide a 3D representation of the heart cavity. The generated
3D representation of the heart cavity is then used to perform the
registration operation, as will be described in greater detail
below. Additionally, mapping of the data acquired by the multiple
mapping electrodes of catheter 110 is subsequently performed with
reference to the 3D representation of the heart cavity.
[0098] The registration system 100 further includes the processing
unit 120 which performs several of the operations pertaining to the
automatic registration procedure, including the determination of
catheter electrode locations that result in the best fit between
the measured signals and those calculated for different positions
of the catheter in view of the conductivity profile corresponding
to the 3D representation of the heart cavity provided by module
130. Additionally, the processing unit 120 can subsequently also
perform the cardiac mapping procedure, including a reconstruction
procedure to determine the physiological information at the
endocardium surface from measured signals, and may also perform
post-processing operations on the reconstructed physiological
information to extract and display useful features of the
information to the operator of the system 100 and/or other persons
(e.g., a physician).
[0099] Although the position of the catheter may be determined on
an ongoing basis as the catheter is moved within the heart cavity
based only on the signals measured by the PMEs (and the separately
acquired conductivity profile), optionally, an independent tracking
system may be used track the location of the catheter 110 as it is
moved inside the heart chamber. Thus, as shown in FIG. 1, the
registration system 100 may include an independent tracking system
180 that provides the 3D spatial coordinates of the catheter and/or
its multiple electrodes with respect to the tracking system's
coordinate system. The information from the PMEs produced in
response to the CIEs can be used to register the coordinate system
of the tracking system to the 3D representation of the heart cavity
provided by module 130.
[0100] In some embodiments, independent tracking system 180 is a
conventional tracking system based on tracking electric or magnetic
signals generated externally by the independent tracking system 180
and detected by one or more tracking elements, such as sensors,
affixed to the catheter 110. Alternatively, tracking elements such
as emitters or beacons affixed to the catheter may emit electric or
magnetic signatures that are detected by the independent tracking
system 180, and used to determine the location of the emitters, and
thus the location and orientation of the catheter 110. For example,
a collection of miniaturized coils oriented to detect orthogonal
magnetic fields and forming a sensor can be placed inside the
catheter to detect the generated magnetic fields. The independent
tracking system 180 is generally disposed outside the patient's
body at a distance that enables the system 180 to either generate
radiation of suitable strength (i.e., generate signals whose
amplitude will not harm the patient or otherwise interfere with the
operation of other apparatus disposed in the near vicinity of the
sensing and tracking system 180), or detect magnetic or electric
radiation emitted by the emitters affixed to the catheter 110.
[0101] In some embodiments, the location of the electrodes relative
to the catheter 110 is fixed and known, and thus the only
information that needs to be determined is the location and
orientation of the catheter 110 in the 3D space defined by the
heart cavity. Specifically, a sensor that is affixed to the
catheter 110 may be used to determine the location and orientation
of the catheter. In other embodiments the location and orientation
of the various electrodes relative to the catheter may vary, and
accordingly, in such embodiments multiple tracking elements
attached proximate to the various electrodes or the electrodes
themselves in case of impedance tracking may be used to facilitate
the determination of the location of the catheter and/or its
electrodes.
[0102] Alternatively and/or additionally, the independent tracking
system may be based on ultrasound, impedance or fluoroscopy
tracking. In impedance and fluoroscopy tracking it is possible to
locate the electrode location without necessitating dedicated
sensors. In the case of impedance, electrical potential generated
by electric field generators are detected by the existing
electrodes. In case of fluoroscopy, electrode location may be
detected by an image processing scheme that identifies and tracks
the electrodes and/or opaque markers located on the catheter.
[0103] The signals acquired by the various electrodes of catheter
110 during the registration procedure and/or the mapping procedure
are passed to the processing unit 120 via electronics module 140.
As described above, electronics module 140 can be used to amplify,
filter and continuously sample intracardiac potentials measured by
each electrode.
[0104] In some embodiments, the electronics module 140 is
implemented by use of integrated components on a dedicated printed
circuit board. In other embodiments, some of the signal
conditioning tasks may be implemented on a CPU, FPGA or DSP after
sampling. To accommodate safety regulations, the signal
conditioning module is isolated from high voltage power supplies.
The electronics module is also protected from defibrillation shock,
and interference caused by nearby pacing or ablation.
[0105] The processing unit 120, image acquisition and preparation
module 130 shown in FIG. 1 is a processor-based device that
includes a computer and/or other types of processor-based devices
suitable for multiple applications. Such devices can include
volatile and non-volatile memory elements, and peripheral devices
to enable input/output functionality. Such peripheral devices
include, for example, a CD-ROM drive and/or floppy drive, or a
network connection, for downloading related content to the
connected system. Such peripheral devices may also be used for
downloading software containing computer instructions to enable
general operation of the respective unit/module, and for
downloading software implemented programs to perform operations in
the manner that will be described in more detailed below with
respect to the various systems and devices shown in FIG. 1.
Alternatively, the various units/modules may be implemented on a
single or multi processor-based platform capable of performing the
functions of these units/modules. Additionally or alternatively,
one or more of the procedures performed by the processing unit 120
and/or image acquisition module 130 and/or electronics module 140
may be implemented using processing hardware such as digital signal
processors (DSP), field programmable gate arrays (FPGA),
mixed-signal integrated circuits, ASICS, etc. The electronics
module 140 is typically implemented using analog hardware augmented
with signal processing capabilities provided by DSP, CPU and FPGA
devices.
[0106] As additionally shown in FIG. 1, the registration system 100
includes peripheral devices such as printer 150 and/or display
device 170, both of which are interconnected to the processing unit
120. Additionally, the registration system 100 includes storage
device 160 that is used to store data acquired by the various
interconnected modules, including the volumetric images, raw data
measured by electrodes and the resultant endocardium representation
computed there from, the reconstructed physiological information
corresponding to the endocardium surface, etc.
[0107] Registration using Independent Tracking System
[0108] FIG. 5 is a flow diagram providing a top-level depiction of
the various procedures performed by the system 100 in the course of
performing the automatic registration of the representation of the
endocardium surface of the heart. In the embodiment shown in FIG.
5, the registration procedure is generally performed with the aid
of an independent tracking system.
[0109] In step 506, catheter 110 is positioned in the heart cavity
and in step 508 the position of the catheter is measured using the
independent tracking system 180. In step 510, the CIEs are used to
inject current into the heart cavity, and in step 512 the PMEs
measure potentials in the heart cavity in response to the injected
current. Although not explicitly depicted in FIG. 5, the steps 510
and 512 can be repeated for different combinations of the CIEs.
Furthermore, as noted by step 518, the process can be repeated for
different positions of the catheter, the relative positions of
which can be tracked by independent tracking system 180. Details of
these steps are described further below.
[0110] In a separate step, the registration system 100 obtains at
step 502 the 3D image of the heart cavity (e.g., partial or
complete anatomy of the patient's torso, including the patient's
heart) from acquired volumetric cardiac representations of a
patient's heart.
[0111] The volumetric representation may be acquired using a number
of sources such as computed tomography (CT), magnetic resonance
imaging (MRI), ultrasound, fluoroscopic rotational angiography,
etc. In each imaging modality the patient may be injected with a
contrast agent to enhance the boundary between tissue and blood. In
addition, the volume is acquired over multiple phases of the
heart's mechanical contraction. In order to obtain a particular
phase of the mechanical contraction (e.g. end diastole), an ECG
signal may be acquired in parallel in order to gate volume
acquisition at the correct phase.
[0112] Various structures in the heart cavity are identified and
delineated using a segmentation scheme. These structures include
cardiac tissue and related vessels, blood, general torso tissue and
lungs. After segmentation of the structures of interest, a choice
is made regarding assigning of conductivity values to the
structures of interest. Given suitable a-priori knowledge of the
different characteristics of the conductivity of the medium in
which the catheter 110 is inserted, as well as the surrounding
structures, the conductivities can be assigned prior to the
registrations process. Specifically, the respective conductivity
values (or, equivalently, the resistivity values) of the
intracardiac blood (i.e., the blood occupying the heart chamber)
and that of the cardiac muscle are substantially uniform throughout
the endocardium surface. For example, the resistivity of the
intracardiac blood is 1.6 .OMEGA.m and that of the myocardium
averages 5.6 .OMEGA.m. The heart is surrounded by the lungs whose
resistivity is assigned 15 .OMEGA.m. Other torso tissue is assumed
to have a conductivity similar to cardiac muscle 5.6 .OMEGA.m.
Thus, the conductivity, or resistivity values of the blood and/or
cardiac muscles for the particular patient with respect to which
cardiac mapping is to be performed can be determined in advance,
and these determined values can be used to perform the registration
procedure, as described herein. FIG. 6 is an illustration of
respective conductivity values assigned to the various tissues and
media. Thus, as shown, the myocardium is assigned a conductivity of
.sigma..sub.1, intracardiac blood is assigned a conductivity of
.sigma..sub.2 and the lungs are assigned a conductivity
.sigma..sub.3.
[0113] However, in the absence of this a-priori knowledge, the
conductivities of the structures of interest can be determined as
part of the registration process. For the embodiment described
herein, we consider the case that includes a-priori knowledge of
the conductivity structure. It is recognized, however, that in the
absence of this knowledge, the optimization problem would simply
contain additional model parameters that describe the conductivity
of the various structures of interest. Thus, solving for the
additional conductivity parameters as part of the registration
process can be done in a similar manner to the embodiment described
herein.
[0114] Turning back to FIG. 5, having generated the 3D
representation of the heart cavity and assigned, at step 504,
appropriate electrical conductivity values, determination of the
location of catheter 110 with respect to the generated 3D
representation of the heart cavity is performed. This is
accomplished by comparing observed values of electrical signals
measured in step 512 to computed values (i.e., theoretical values)
of those signals based on the heart cavity conductivity
representation. Particularly, the measured potential field due to
an input current from a dipole source is related via the
conductivity structure of the medium by the continuous form of
Ohm's law:
.gradient.-.sigma..gradient..phi.=I(.delta.(r-r.sub.s+)-.delta.(r-r.sub.-
s-)). (1)
[0115] Equation 1 is the partial differential equation that relates
the potential field (.phi.) to the input current (I), from a
dipole, through the conductivity structure of the medium (.sigma.).
In Equation (1), r.sub.s+ and r.sub.s- are the locations of the
source and sink current sources, respectively, and
.delta.(r-r.sub.s) is the dirac delta function, centered at the
current source or sink location. Thus, to compute the potential
field .phi., it is necessary to know the conductivity values
(denoted .sigma.) in the medium in which the potential field is
computed.
[0116] With regard to step 506, the catheter 110 is typically
inserted into the heart chamber via a suitable blood vessel leading
to the heart chamber. In some embodiments, the electrodes of the
catheter 110 are bundled into a compact configuration that enables
the catheter 110 to be delivered to the heart chamber with minimal
obstruction. Once inside the heart chamber, the electrodes of the
catheter are deployed into a specified electrode arrangement
relative to the catheter 110.
[0117] With the catheter 110 inserted into the patient's heart
chamber and placed into a particular position within the heart
chamber, information about the particular position can next be
determined using the CIEs and PMEs disposed on the catheter 110. A
pair of CIEs is selected as a source/sink pair by electronics
module 140 to inject current into the heart cavity. One of the
electrodes of the selected pair serves as the source electrode, and
accordingly that electrode is activated by applying a voltage
source to the source electrode. The other electrode serves as the
sink electrode, and is thus set to a lower potential level than the
source electrode. The other sink/source electrodes disposed on the
catheter 110 are electrically deactivated and held at high
impedance.
[0118] The selected pair of source/sink electrode thus becomes
active and imparts current, at step 510, into the intracardiac
blood medium in which the catheter 110 is disposed. As shown in
FIG. 7, causing a current to flow in the distribution of materials
constituting the medium in which the catheter is disposed (in this
case, the intracardiac blood) results in the formation of potential
fields in the medium.
[0119] In response to current flow between the pair of selected
source/sink electrodes, the PMEs distributed at multiple locations
on the catheter 110 measure, at step 512, the resultant potential
field present at the those multiple locations. The measured
potentials are recorded, along with other information associated
with the measurement, including, for example, the identity and/or
location of the activated sink/source electrodes that imparted the
current through the medium. Additionally, the location of the
catheter 110 in relation to the coordinate system of the
independent tracking system 180 is likewise recorded at step
508.
[0120] After the measurement of the potential fields caused by the
activated source/sink electrodes has been completed and recorded,
the pair of source/sink electrodes is de-activated, and another
pair of source/sink electrodes is subsequently activated to cause
potential fields corresponding to the next activated pair of
source/sink electrodes to form. Although a new pair combination of
electrodes is selected and activated, so as to cause an electric
field that is different from the potential field previously formed,
one of the electrodes may be an electrode that was previously
selected in a preceding source/sink electrode pair selection.
[0121] The number of different source/sink electrode pair
combinations that may used for any one given location of the
catheter 110 depends on the desired balance between accuracy and
robustness of the measurement to determine the positional
information of the catheter 110, and the computation complexity and
volume of data that can be handled, as well as the processing time
required to process the acquired data and compute the desired
positional information.
[0122] In one embodiment, each source/sink pair generates an
oscillating current of 1 mA at 10 kHz while the switching between
different pairs occurs at 100 Hz. It would be appreciated that
other values of current amplitude, frequency and switching
frequency may be used. The amount of current is chosen such that it
is sufficient for signal detection, but low enough such that it
does not affect cardiac tissue. The frequency is chosen such that
it is high enough so that it can easily be filtered from the
intracardiac signals (which are typically <2 kHz), and low
enough so as to minimize cross talk between PME signals. The
switching frequency between pairs is chosen such that multiple
scans of all pairs can be accomplished while the heart may be
assumed to be in a stable mechanical configuration (.about.100 mS).
In the case of 100 Hz switching and three source/sink pairs, at
least 3 scans of all pairs may be accomplished while the heart is
in a stable mechanical configuration.
[0123] Since the heart beats during the automatic registration
process, it is preferable to perform the steps 508, 510, and 512
while the heart is in a mechanical cycle corresponding to the one
represented by conductivity representation 504. In certain
embodiments, ECG gating can be used to control the timing of the
activation of the CIEs. Initially, an alignment algorithm is
employed to detect the R wave from the ECG signal. Following a
delay, each source/sink pair is activated in sequence. FIG. 13
shows the timing for a sequence of 4 CIE pairs. The initial delay
is adjustable and is used to align the timing of the R wave with
that of the mechanical phase in the chamber of interest. The four
CIE pairs are then activated in sequence over a time period that is
small relative to the period of the heart cycle.
[0124] Instead of ECG, other signals may be used to synchronize on
the heart's mechanical contraction. For examples, the signals
measured by the PME's at the frequency injected by the CIE
continuously throughout the heart's cycle may be used in a manner
similar to a conductance catheter or plethysmograph used to detect
stroke volume. More specifically, the signal represented by
S ( t ) = n = 1 N ( PME n ( t ) ) 2 ##EQU00001##
where N is the number of PME's may be used for the synchronization.
The amplitude of the PME signals that result from the injected CIE
current is modulated by the heart's contraction due to the
conductivity contrast between myocardium and blood. This modulation
is synchronized with the heart's mechanical contraction. Signal
S(t) may be used to in a manner identical to ECG gating shown in
FIG. 13.
[0125] Other signals such as intracardiac electrograms, and
pressure can also be used to synchronize on the heart's mechanical
contraction.
[0126] In other embodiments, the potential field inside the
patient's heart chamber may be created by having more than two CIEs
activated simultaneously. For example, in some circumstances, two
or more CIEs may be simultaneously coupled to signal sources as
described in FIG. 4 to cause those two or more electrodes to inject
current into the intracardiac blood in which the catheter 110 is
disposed. In addition, in yet other embodiments multiple orthogonal
frequencies may be introduced simultaneously in multiple
electrodes.
[0127] Once measurements of the various potential fields resulting
from activation of one or more source/sink electrode pairs at a
particular location of the catheter 110 have been completed, the
catheter 110 is moved, at step 518, to a new location within the
heart chamber. At the new location of the catheter 110, a
sequential activation of source/sink electrode pair is performed
again, thus repeating the sequence of operation discussed above in
relation to 508, 510 and 512. As a result, the potential fields
formed by the source/sink electrodes of the catheter 110 and
measured by the PMEs is performed at multiple locations of the
catheter within the heart chamber. The displacement of the catheter
110 to multiple locations within the heart chamber and the
subsequent sequence of measurement performed at each of those
location effectively results in the implementation of a mega
catheter having a number of electrodes that is proportional to the
product of the actual number of physical electrodes mounted on the
catheter and the number of locations to which the catheter is
moved. In some embodiments, the sequence of source/sink electrode
pair activations is constant such that at every location the same
pairs of electrodes are activated, at the same order of activation.
However, different activation sequences for the pairs of
source/sink electrodes at different locations of the catheter 110
within the heart chamber may be implemented.
[0128] Once measurements of the potential fields for different CIE
pair combinations and/or at different locations of the catheter 110
inside the heart chamber have been performed, an optimization
routine (e.g., non-linear optimization routine) is applied, at step
514, to the sets of recorded measurements to determine the position
of the catheter 110 relative to the 3D representation of the
endocardium surface. Specifically, the optimization procedure
applied at step 514 seeks to find the electrode positions within
the heart cavity that minimized the alignment error between the
observed potential values measured by the PMEs, and the
theoretically derived potential values.
[0129] In order to formulate the optimization problem, we first
define the vector form of Equation 1:
(DS(.sigma.)G)u=A(.sigma.)u=q. (2)
[0130] In Equation (2), D and G are matrices representing 3D
divergence and gradient operators, respectively, S(.sigma.) is a
matrix containing the conductivity values, u is a vector containing
the potentials, A(.sigma.) is the complete forward operator matrix
and q is a vector containing the locations of the positive and
negative current sources. Although the location of the CIEs with
respect to the independent tracking system is known, the locations
of those electrodes with respect to the coordinate system of the 3D
representation of the heart cavity is not known, and thus needs to
be determined.
[0131] It is to be noted that the above formulation expressed in
Equation (2) is a differential equation formulation of the forward
problem. However, the forward problem can also be solved using an
integral equation solution, and thus in some embodiments the
desired results may be obtained using integral equation solutions.
As described herein, the optimization problem will be explained in
terms of using a differential formulation for the forward
problem.
[0132] As can be seen from Equation (2), the theoretical
potentials, for the given conductivity model assigned at step 504,
can be determined according to:
u=A(.sigma.).sup.31 1q. (3)
[0133] Equation (3) yields the potential everywhere in the 3D
volume defined by the 3D representation of the heart cavity, and
therefore provides the theoretical potentials in the heart cavity's
frame of reference for a given location of the CIEs. However,
because the actual measured data against which the optimization
procedure will be performed corresponds to a small subset of
potentials (namely, the potential measured at the limited number of
PME locations), the set of theoretical potential values can be
reduced to a subset of the available computed theoretical
potentials. Accordingly, a projection matrix, Q, is defined for
selecting data points from the volume, for particular locations
inside the volume. Those particular locations associated with the
projection matrix Q correspond to the PME locations that measure
the potential fields created by injecting current using the CIEs.
Applying the matrix Q to the vector u representing all the
potential over the entire volume of the 3D representation of the
heart cavity yields the following equation for expressing a subset
of potential values at specific catheter locations:
d=Qu=QA(.sigma.).sup.-1q. (4)
[0134] The term d in Equation (4) represents the theoretical data
at the limited number of locations corresponding to the PMEs. The
locations of the catheter's electrodes, as used in the Q term are
defined with respect to the external frame of reference (namely,
the frame of reference of independent tracking system 180), which
is denoted .OMEGA..sub.e. The frame of reference of the 3D
representation of the heart cavity is denoted .OMEGA..sub.I. For
convenience, the origin of the 3D representation of the heart
cavity representation's frame of reference, .OMEGA..sub.I, is
defined as the centroid of the chamber of interest.
[0135] As noted, the data set that was obtained at the iterative
measurements performed at steps 508, 510, and- 512 is a composite
of the potential field measurements performed at multiple catheter
locations. As previously explained, obtaining measurements at
multiple catheter locations within the heart chamber is effectively
equivalent to having a mega catheter having a number of electrodes
that is proportional to the product of the number of actual
physical electrodes and the number of locations to which the
catheter is moved. Moreover, the relative changes in catheter
position can be tracked by independent tracking system 180. Thus,
to register the 3D representation of the endocardium surface with
respect to all of these data sets simultaneously, the data sets
obtained at those multiple locations are viewed as data
measurements from a single experiment ostensibly performed by a
single, giant catheter. FIGS. 8a-b are illustrations of electrode
measurements taken at a single (FIG. 8a) and multiple (FIG. 8b)
catheter locations, respectively. The accuracy of the optimization
procedure performed on measurements obtained from a single catheter
location is affected by the number, and spatial span, of the
electrodes partaking in the measurement process, and therefore the
accuracy of the optimization procedure may be susceptible to
measurement errors. On the other hand, when the catheter 110 is
moved to multiple locations, the effective number of electrodes
partaking in the measurement process and the spatial span of the
measurements taken are increased, thereby improving the accuracy of
the optimization.
[0136] To use all the data simultaneously, another coordinate
system, denoted .OMEGA..sub.e, is defined for the catheter, where a
given electrode location is defined as follows:
r.sub.c.sub.i=r.sub.e.sub.i- r.sub.e.sub.i (5)
[0137] Where r.sub.e.sub.i is the location of the centroid
corresponding to the composite of all the measurements taken at the
various catheter locations. Given this new coordinate system, a
corresponding projection matrix, {tilde over
(Q)}(r.sub.i,.phi.,.theta.,.gamma.), is defined in which r.sub.i is
the location of the centroid of the mega catheter measured in the
.OMEGA..sub.i frame of reference. .phi.,.theta. and .gamma. denote
the yaw, pitch and roll, respectively, of the virtual mega catheter
with respect to the .OMEGA..sub.i frame of reference (i.e., the
coordinate system of the 3D representation of the endocardium
surface). Additionally, a new source vector, {tilde over
(q)}(r.sub.i,.phi.,.theta.,.gamma.), corresponding to the new
coordinate system, .OMEGA..sub.c, is also defined. Having defined
the new coordinate system, .OMEGA..sub.c, for the virtual mega
catheter, Equation (4) can thus be expressed as follows:
d(m)=d(r.sub.i,.phi.,.theta.,.gamma.)={tilde over
(Q)}A(.sigma.).sup.-1{tilde over (q)} (6)
[0138] Equation (6) yields the computed theoretical potential
values data as a function of the mega catheter's location with
respect the 3D representation of the endocardium surface's frame of
reference. It is to be noted that another equivalent formulation of
the relationship between the measured potential field values and
the computed theoretical values would be to consider the mega
catheter as fixed, and view the conductivity matrix, S, as a
function of m. Both these equivalent formulations would produce the
same results.
[0139] Given equation (6) we can define an objective function,
.PHI.(m), that enables the determination of m, and consequently the
location of the mega catheter with respect to the 3D endocardium
surface representation:
.PHI.(m)=.parallel.d(m)-d.sub.obs.parallel..sup.2 (7)
where d.sub.obs is the vector of observed potential field data
measured by the electrodes of the catheter. In this case, the
vector of observed data corresponds to the separate measurements
performed for each of the CIE pairs that were sequentially
activated to generate corresponding potential fields inside the
intracardiac blood medium. The observed data pertains to
measurements performed by the mega catheter (i.e., over the
multiple locations to which the catheter 110 was displaced during
the course of conducting the measurements), and not merely by the
catheter positioned in a single location of the catheter. It should
be again noted that what enables measurements from the mega
catheter to be used as opposed to measurements from a single
catheter location is the fact that the independent tracking system
180 is employed, which enables the relative positions of the
catheter 110 with respect to the various locations to which the
catheter 110 is displaced to be determined.
[0140] By minimizing equation (7), a model, m, is determined that
leads to the best fit of the observed data (i.e., the measured
potential fields) in a least-squares sense. While the above
optimization is cast in terms of minimizing the L.sub.2 norm of the
residuals, i.e., the sum of the squares, other error metrics may be
used to perform the best fit of the observed data to the
theoretical data, such as any standard L.sub.n, norm or a modified
norm such as the Huber norm.
[0141] Solution of Equation (7) yields the locations of the mega
catheter electrodes, with respect to the frame of reference of the
3D representation of the endocardium surface, that result in the
best fit between the theoretical data and the observed data. As
will become apparent below, with these electrode location values
now determined, the transformation parameters to transform
coordinates in the frame of reference of independent tracking
system 180 to the frame of reference of the 3D representation of
the endocardium surface may subsequently be computed.
[0142] Minimizing Equation (7) requires a non-linear optimization
approach. There are multiple techniques that may be used to arrive
at a solution. In general there are two classes of techniques that
may be used: stochastic and deterministic. Stochastic optimization
techniques, such as simulated annealing and genetic algorithms,
involve stochastically guided searches of the model space to find a
suitable minimum. Deterministic approaches, such as Gauss-Newton
approach, Levenberg-Marquardt approach and the Newton method,
involve solving a linearized version of the non-linear problem
multiple times in order to achieve a suitable solution.
[0143] For illustration purposes, described herein is as an example
of a deterministic approach, namely, the Gauss-Newton approach,
which can be used to solve Equation (7). However, other non-linear
solution techniques may be used to solve the above optimization
problem.
[0144] Given the objective function defined Equation (7), the
Gauss-Newton approach is used to identify the model that leads to
the best fit of the observed data to the theoretical data. A
starting guess, m.sub.i, is first defined. In practice, this
starting guess would usually be a vector of zeros. Equation (7) is
then linearized about this model to yield:
.PHI. ( m ) = 1 2 ( d + .differential. d .differential. m .delta. m
) - d obs 2 ( 8 ) ##EQU00002##
[0145] To obtain the minimum of Equation (8), the derivative with
respect to the model is taken, and set it to zero. This yields the
following Gauss-Newton equation:
( J T J ) H .delta. m = - ( J T ( Q ~ A - 1 q ~ - d obs ) ) g ( 9 )
##EQU00003##
[0146] where J is the Jacobian, or sensitivity matrix,
( .differential. d i .differential. m j ) , ##EQU00004##
g is the gradient of the objective function, and H is the
approximation to the Hessian. Equation (9) is solved to yield the
so-called model-update, .delta.m, which represents an incremental
change in the model, which decreases the value of the objective
function. The model-update is then used to generate a new
model:
m.sub.i+1=m.sub.i+.delta.m (10)
where m.sub.i+1 is the new model.
[0147] Because Equation (9) resulted from a linearization about the
model, m.sub.i, the resulting model, m.sub.i+1, is likely not the
local minimum of Equation (7). Therefore, another Gauss-Newton
iteration is performed, this time linearizing Equation (7) about
the model m.sub.i+1. This iterative process is repeated until the
objective function has decreased to the noise level of the
data.
[0148] Aligning the mega catheter with the 3D representation of the
endocardium surface image is done by applying the following
transform:
r.sub.i=Rr.sub.c+.DELTA.r (11)
[0149] where r.sub.i and r.sub.c are coordinates in the
.OMEGA..sub.i and .OMEGA..sub.c reference frames, .DELTA.r is the
translation vector that can be derived from m, where
m=(.DELTA.r,.phi.,.theta.,.gamma.), and R is the Euler rotation
matrix defined as follows:
R = B C D where : B = [ 1 0 0 0 cos .gamma. sin .gamma. 0 - sin
.gamma. cos .gamma. ] ; C = [ cos .theta. 0 - sin .theta. 0 1 0 sin
.theta. 0 cos .theta. ] ; D = [ cos .phi. sin .phi. 0 - sin .phi.
cos .phi. 0 0 0 1 ] ( 12 ) ##EQU00005##
[0150] Following the application of equation (11), the coordinates
of each catheter electrode in the .OMEGA..sub.i reference frame
(i.e., the reference frame of the 3D representation of the
endocardium surface) are determined from the catheter's electrodes'
coordinates in the .OMEGA..sub.c. As previously explained, the
catheter coordinates in the .OMEGA..sub.c frame of reference are
determined using the coordinates of the catheter in the
.OMEGA..sub.e frame of reference (i.e., the frame of reference of
the tracking system) using, for example, Equation (5).
[0151] As noted, the procedure described in relation to Equations
(7)-(11) yields the locations of the electrodes of the mega
catheter with respect the frame of reference of the 3D
representation of endocardium surface, and thus enables aligning
the catheter's frame of reference with the frame of reference of
the 3D representation of the endocardium surface with respect to
the catheter current location (or, more specifically, with respect
to the constellation of measurements obtained from the mega
catheter). The next stage of the automatic registration process
involves calculating the transform between .OMEGA..sub.i and
.OMEGA..sub.e so that the coordinates of the catheter 110 in the
frame of reference of the 3D representation of the endocardium
surface can be obtained from knowledge of the current coordinates
of the catheter 110 as indicated by the independent tracking system
180. The objective function for registering the two data sets can
be described as follows:
.PSI. ( R , .DELTA. r ) = min R , .DELTA. r r e - R r i - .DELTA. r
2 ( 13 ) ##EQU00006##
where r.sub.i and r.sub.e are coordinates in the .OMEGA..sub.i and
.OMEGA..sub.e reference frames. Once again, .DELTA.r is the
translation vector and R is the Euler rotation matrix. Unlike
equation (11), where R and .DELTA.r were known, Equation (13) seeks
to find the R and .DELTA.r that minimize the expression provided in
Equation (13).
[0152] This optimization problem can also be solved using a variety
of non-linear optimization techniques. As an illustrative example
follows.
[0153] Firstly, the two sets of coordinates, r.sub.i and r.sub.e
presented in Equation (5) are modified so that the origin of the
each point cloud is the centroid of the point cloud. Thus, Equation
(13) can be re-written as:
.PSI. ( R ) = min R r ~ e - R r ~ i 2 ( 14 ) ##EQU00007##
where {tilde over (r)}.sub.e and {tilde over (r)}.sub.i are the
point cloud coordinates, shifted about their respective
centroids.
[0154] Accordingly, at step 514 of procedure 500, the coordinates
of multiple catheter locations, determined during the multiple
iterations performed with respect to 508, 510, and-512 of procedure
500, are provided. These coordinates are provided both in terms of
the frame of reference of the independent tracking system and in
terms of the frame of reference of the 3D representation of the
heart cavity (as provided by step 504 of procedure 500). With these
coincident point clouds of catheter locations provided in terms of
the frame of reference of the independent tracking system and the
frame of reference of the 3D representation of the heart cavity,
the rotational matrix R that minimizes the difference between
R{tilde over (r)}.sub.i and {tilde over (r)}.sub.e is determined.
The optimal solution to equation (14) can be determined using the
singular value decomposition (SVD) of the correlation matrix
between {tilde over (r)}.sub.e and {tilde over (r)}.sub.i. The
correlation matrix is defined as:
H={tilde over (r)}.sub.i{tilde over (r)}.sub.e.sup.T (15)
[0155] Using the notation of H=U.LAMBDA.V.sup.T for the SVD, the
optimal solution for R is thus presented as:
R=VU.sup.T (16)
[0156] Once R has been obtained, .DELTA.r is computed as:
.DELTA.r= r.sub.e-R r.sub.i (17)
[0157] Where r.sub.e and r.sub.i are the locations of the centroids
of the two respective point clouds. Because R and .DELTA.r are now
known, Equation (11) is applied to the coordinates of the 3D
representation of the endocardium surface to yield the new series
of coordinates in the .OMEGA..sub.e reference frame of the
independent tracking system. The independent tracking system can
now be used to track the catheter with respect to a visual
representation of the heart, and obtain, at step 516, successive
catheter locations of catheter 110 in terms of the 3D
representation of the heart cavity given the coordinates of the
catheter as indicated by the independent tracking system 180. It
should be noted the frame of reference transformation can be
performed in the opposite direction, i.e., projecting from
.OMEGA..sub.e to .OMEGA..sub.i. One drawback of such an approach is
that every time the catheter 110 is moved, Equation (11) must be
applied to the catheter location. However this is an instantaneous
calculation, so in practice either approach would be suitable.
[0158] Catheter Tracking without Independent Tracking System
[0159] FIG. 9 is a flow diagram of another exemplary embodiment of
a registration procedure 600 that is performed without using any
independent tracking system. Because the tracking system is not
utilized, the positions of spatial locations of the catheter 110
relative to each other cannot be ascertained. In other words,
without a tracking system to track the relative movement of the
catheter 110, no information is available on the distance and
direction that the catheter 110 moved from a first position to a
second position inside the heart chamber. Accordingly, because
relative positions of the catheter with respect to different
locations in the heart chamber cannot be directly determined (i.e.,
without having to perform optimization computations), an approach
that uses a composite of measurements taken at different locations
of the catheter 110, as was performed in relation to procedure 500,
is not used. Consequently, while the spatial positioning of the
catheter 110 with respect to the coordinate system of the 3D
representation of the heart cavity can be determined for a
particular location, once the catheter 110 is moved to a different
location within the heart chamber, the procedure 600 has to be
performed anew to determine the spatial positioning of the new
location of the catheter 110 relative to the 3D representation of
the heart cavity This approach has the advantage of negating the
need for additional equipment (i.e., the independent tracking
system).
[0160] Particularly, as shown in FIG. 9, a 3D representation of the
patient's heart cavity is constructed, at step 602, from volumetric
data in a manner similar to that outlined with respect to the
construction of the 3D representation performed at step 502 of
procedure 500. The conductivity values at various locations of the
constructed 3D representation of the heart cavity for various
tissues are then assigned at step 604 in a manner similar to the
conductivity assignment performed at step 504 of procedure 500.
[0161] At step 606, the catheter 110 is inserted into the patient's
heart chamber, and the catheter 110 is moved to some unknown
position. Subsequently, at step 608 the control mechanism of
electronics module 140 commences an activation sequence of the CIEs
of the catheter 110 to cause current injection into the
intracardiac blood medium in which the catheter 110 is disposed, in
a manner similar to that performed at 510 of procedure 500. Here
too, the control mechanism, or alternatively, the operator of
system 100, causes current to be injected in an ordered sequential
manner. Consequently, potential fields, corresponding to the
various source/sink electrode configurations activated, are formed
inside the patient's heart chamber.
[0162] The PMEs of the catheter 110 measure, at 610, the potential
fields present at the electrodes' positions, and the measured
potentials are amplified and sampled by the electronics module 140
of system 100. Thus, at the catheter's position inside the
patient's heart chamber, multiple sets of potential field
measurements, corresponding to the respective source/sink electrode
configurations that were activated to create those potential
fields, are recorded and constitute the observed raw data that
subsequently is used to determine the location of the catheter 110
relative to 3D representation of the heart cavity.
[0163] Accordingly, as further shown in FIG. 9, at step 612 the
measured potentials are processed by an optimization routine, such
as any one of the various non-linear technique that may be used
with respect to the non-linear optimization techniques performed in
procedure 500, to determine the position of the catheter 110 with
respect to the 3D representation of the heart cavity. The
processing operation and optimization procedure performed by
procedure 600 at step 612 are similar to the operations described
above in relation to Equations (6)-(12). Briefly, an objective
function is defined that searches for the coordinates of the
catheter 110 (and by extension, the coordinates of the electrodes
mounted on the catheter) with respect to the 3D representation of
the heart cavity, that would provide the best fit match between the
observed data sets of the potential measurements (i.e., for each of
activated CIE) and the corresponding theoretical potential
values.
[0164] Particularly, the theoretical potential values at the
electrodes used to measure the potential field created in the heart
chamber as a result of the current injected by activated
source/sink electrodes can be expressed as:
d(m)=d(r.sub.i,.phi.,.theta.,.gamma.)={tilde over
(Q)}A(.sigma.).sup.-1{tilde over (q)} (18)
[0165] In the projection matrix, {tilde over
(Q)}(r.sub.i,.phi.,.theta.,.gamma.), r.sub.i is location of the
centroid of the catheter measured in the .OMEGA..sub.i frame of
reference. .phi.,.theta. and .gamma. denote the yaw, pitch and
roll, respectively, of the catheter 110 with respect to the
.OMEGA..sub.i frame of reference. The source vector, {tilde over
(q)}(r.sub.i,.phi.,.theta.,.gamma.), corresponds to the locations
of the source/sink electrodes injecting the current relative to the
centroid r.sub.i. FIG. 10 shows a schematic representation of the
locations of the electrodes relative to the origin of the frame of
reference of the 3D representation of the heart cavity, and
relative to the centroid r.sub.i representing the center of the
object on which the CIEs are mounted. Equation (18) yields the
theoretical data as a function of the catheter location with
respect to the frame of reference of the 3D representation of the
heart cavity. As noted with respect to procedure 500, an equivalent
formulation of the problem represented by Equation (18) would be to
consider the catheter as fixed, and view the conductivity matrix,
S, as a function of m. Under those circumstances, this alternative
formulation would produce the same results as the formulation of
Equation (18).
[0166] With the formulation of Equation (18), the objective
function that will be used to determine the location of the
catheter 110 with respect to the 3D representation of the heart
cavity is defined as:
.PHI.(m)=.parallel.d(m)-d.sub.obs.parallel..sup.2 (19)
[0167] By minimizing Equation (19), the model m that leads to the
best fit of the observed data in a least-squares sense is
determined. The optimization techniques that are applied to
determining the best-fit model m are similar to those describe in
relation to procedure 500.
[0168] After the procedure 600 has determined the position of the
catheter 110 with respect to the 3D representation of the heart
cavity, a cardiac mapping procedure performed using the catheter
110 can be performed, as described above. Upon the completion of
the mapping procedure at the current location of the catheter 110,
the catheter may be moved to a new location. Unlike procedure 500,
which generates a transformation function that may be subsequently
used at other catheter locations to determine the position of the
catheter 110 relative to the 3D representation of the endocardium
surface, once the catheter 110 is moved to a new location, the
procedure 600 is repeated to determine the new location of the
catheter 110.
[0169] Additional Electrode Configurations
[0170] In the specific embodiments describe above, both the CIEs
and PMEs are on the same catheter. Other configurations are also
possible as shown in FIGS. 11a-c. For example, while FIG. 11a shows
catheter 110a in a patients heart chamber 108 as having both CIEs
(specifically, CIE1, CIE2, CIE3, and CIE4) and PMEs (specifically,
PME 1 . . . n), FIGS. 11b and 11c show configurations with two (2)
catheters, the second of which is anchored within the heart so as
not to move relative to the heart chamber and includes some or all
of the CIEs, while the other catheter is movable and includes the
PMEs (which may also be used for cardiac mapping).
[0171] Referring to FIG. 11b, the first movable catheter 110b
includes the PMEs (specifically, PME 1 . . . n), but not CIEs, all
of which are located on the second catheter 111b, The second
catheter may be anchored in structures such as the coronary sinus,
atrial appendages, or a ventricular apex. The second catheter may
be a linear catheter with CIEs spaced apart from each other along a
linear line. The first catheter would contain all PME which would
be distributed somewhat uniformly on a 3D surface.
[0172] Referring to FIG. 11c, the first movable catheter 110c
includes all of the PMEs (which may also be used for cardiac
mapping) and some of the CIEs (specifically CIE11, CIE12, CIE13,
and CIE14), and the second catheter includes the remaining CIEs
(specifically, CIE21, CIE22, CIE23, CIE24). Accordingly,
source/sink electrode pairs may be used across the two catheters.
In this manner one of the current injecting electrodes can anchored
with respect to the heart while the other is moving.
[0173] In general, the various electrode types may be mounted on
multiple objects that are deployed in a heart chamber or
surrounding structures. Moreover, in further embodiments, rather
than injecting the currents using a second catheter (as in FIGS.
11b and 11c), cutaneous patches may be used on the body surface to
inject current into the heart cavity from outside the heart
cavity.
[0174] In yet further embodiments, one or more of the electrodes on
the catheter can be driven by electronics module 140 to function as
both a CIE and a PME. For example, when it is desired to use an
electrode as both PME and CIE, the electrode is connected to both a
signal acquisition module and a signal generation module. For
example, for the electronics module depicted in FIG. 4, when the
electrode is not used as a CIE to drive a current, the switch in
the signal generation module corresponding to the respective
electrode is opened. Accordingly, time division multiplexing
schemes in the driving electronics of module 140 can be used to
operate a given catheter electrode as either a CIE or a PME. In yet
another example, the electronics module can drive a given electrode
so that it functions as a CIE at high frequencies and a PME at low
frequencies (such as might be useful for cardiac mapping.)
[0175] Complex Conductivity
[0176] As noted above, the measurements collected at the PMEs as a
result of current injected by the CIE are generally affected by the
complex conductivity, or admittivity, distribution of the medium.
While the specific embodiment discussed above focus on the real
part of the conductivity which affects the amplitude measured by
the PMEs, additional information can also be obtained by accounting
for the real part (conductivity) and imaginary part (permittivity)
of the medium's complex conductivity, which affects the amplitude
and phase of the signal measured by the PME. In this manner, the
use of both amplitude and phase, or phase alone may also be used
for tracking and/or automatic registration. Use of the imaginary
part of the complex conductivity is of particular importance in
material distributions where the permittivity contrast exceeds that
of the conductivity contrast.
[0177] To modify the mathematical formalism for the specific
embodiments described above to account for imaginary part of the
complex conductivity, the forward problem expressed in Equation (1)
is changed. Specifically, Equation (1) is modified as follows:
.gradient.-.sigma.*.gradient..phi.*=I(.delta.(r-r.sub.s+)-.delta.(r-r.su-
b.s-)). (20)
where .sigma.* and .phi.* represent the complex conductivity and
complex potential, respectively. The complex conductivity is
defined as: .sigma.*=.sigma.+i.omega..epsilon., where .sigma. is
the real component of conductivity (as in Equation 1), .omega. is
the frequency of the current source, and .epsilon. is the
electrical permittivity. From Equation (20), one can obtain a
corresponding discretized system, analogous to Equation (2), that
accounts for the complex conductivity and potential. The
optimization approaches described above can then be applied to this
complex discretized system to perform the complex impedance
registration.
[0178] Post Registration/Tracking Operation
[0179] Once the registration procedure is completed, the cardiac
mapping (e.g., non-contact mapping) of electro-physiological
information about the endocardium surface, as well as other
post-registration operations may be performed. A description of the
mapping and other post-operations procedures that may be performed
are provided for example, in application Ser. No. 11/451,871,
entitled "NON-CONTACT CARDIAC MAPPING, INCLUDING RESOLUTION MAP,"
and filed Jun. 13, 2006, the content of which is hereby
incorporated by reference in its entirety, as well as application
Ser. Nos. 11/451,898, and 11/451,908, referred to above.
[0180] Briefly, and with reference to FIG. 12, the catheter 110 may
be moved to a first location within the heart chamber, at step 902,
in which the first set of measurement by the catheter's multiple
mapping electrodes is performed. Control of the catheter's movement
and location within the heart chamber is performed manually by the
operator manipulating the catheter 110. Alternatively, the movement
of the catheter 110 within the heart chamber may be automated by
use of techniques such as magnetic (see, e.g., Stereotaxis, Inc. of
St. Louis, Mo.) or robotic (see, e.g., Hansen Robotics, Inc.)
navigation. Catheter manipulation may be used to cause the catheter
to follow a pre-determined displacement route to collect data at
locations that may be considered to be of higher interest than
others. For example, in some embodiments, the catheter 110 may be
moved at specified displacement intervals in an area of the heart
chamber that is known to have abnormal cardiac activity.
[0181] The 3D location of the catheter 110, and/or to its multiple
electrodes, is then determined using one of the techniques
discussed above. If a tracking system, such as the independent
tracking system 180, is used, the coordinate system transformation
function between the tracking system 180 frame of reference and the
3D representation of the heart cavity as determined, for example,
at step 516 of procedure 500, is applied to the coordinates of the
catheter 110 identified by the independent tracking system 180. If
an independent tracking system is not used to facilitate
determining the location of the catheter 110 in the patient's heart
chamber, the location of the catheter 110 in relation to the 3D
representation of the endocardium surface is determined by
performing, for example, procedure 600 as described herein.
[0182] At its current location, the multiple mapping electrodes of
the catheter 110 (which, as previously noted, may be the same as
the PMEs used during the tracking process as implemented either
through procedure 500 or procedure 600) acquire signals resulting
from the heart's electrical activities (at 904).
[0183] The mapping system (which may be implemented using the same
hardware used to implement registration system 100) generates
reconstruction transformation functions, at step 906, to be applied
on the acquired signals to reconstruct the electro-physiological
information at the endocardium surface. The generated
reconstruction transformation functions may be based, among other
things, on pre-computed reconstruction transformation functions
that were previously determined (generally prior to insertion of
the catheter 110 into the patient's heart chamber), and the
catheter's location relative to the endocardium surface. Thus, in
some embodiments, for every location of the catheter 110 at which
raw data is acquired, a corresponding set of reconstructed
electro-physiological information is computed.
[0184] After the raw data corresponding to the heart's electrical
activity has been acquired, recorded and processed using
reconstruction transformation function(s) to obtain reconstructed
electro-physiological information at the endocardium surface (also
at step 906), a determination is made, at step 908, whether there
are additional locations within the heart chamber to which the
catheter 110 is to be moved. If there are additional locations in
the heart chamber to which the catheter 110 needs to be moved the
catheter is moved, using manual or automatic control, to the next
location in the heart chamber, whereupon the operation described in
relation to the steps 902-906 in FIG. 12 are performed for that
next location.
[0185] To enhance the quality of the reconstructed
electro-physiological information at the endocardium surface, in
some embodiments the catheter 110 is moved to more than three
locations (for example, more than 5, 10, or even 50 locations)
within the heart chamber. Further, the spatial range over which the
catheter is moved may be larger than one third (1/3) of the
diameter of the heart cavity (for example, larger than 35%, 40%,
50% or even 60% of the diameter of the heart cavity).
[0186] In some embodiments, a composite set of
electro-physiological information can be generated by selecting
from multiple sets of reconstructed electro-physiological
information portions of the reconstructed information. Selecting
which portions of reconstructed information to use can be based on
resolution maps that are indicative of the quality of the
reconstructed information for a particular portion or set of the
reconstructed electro-physiological information. Other criteria and
techniques for selecting suitable portions of data to reconstruct a
composite set of electro-physiological information may be used.
[0187] In some embodiments, one (or more) composite reconstruction
transformation function is computed that is applied collectively to
the raw data acquired at multiple locations to generate a resultant
composite set of reconstructed electro-physiological information
based on a substantial part of the data acquired. Such a
transformation function represents a "mega transformation function"
that corresponds to a "mega catheter," whose effective number of
electrodes and electrode span is related to the number of locations
to which the catheter was moved within the heart chamber. Under
those circumstances the generation of the composite reconstruction
transformation function is deferred until data is collected from
the catheter's multiple locations.
[0188] Alternatively, in some embodiments, the "mega transformation
function" and "mega catheter" may be updated on an ongoing basis to
take into account a given relevant measurement window. This window
may be a fixed number of measurements such that the arrival of new
measurements displaces measurements that were obtained before the
time window. This yields a constantly updating moving average.
[0189] In some embodiments, signals are measured throughout a heart
beat cycle (for example, a measurement can be made at each catheter
electrode at each of multiple, different phases of a single beat
heart cycle).
[0190] Yet in further embodiments the reconstructed set of
electro-physiological information is computed based on measurements
taken over one or more heart beats. In the latter situation, the
catheter is moved to a particular location, and acquires multiple
sets of raw data over several heart beats. The acquired data is
averaged, and the reconstruction process is applied to the averaged
values. If the data is acquired over B heart beats (i.e., B
measurements), an improvement in the signal-to-noise ratio
proportional to {square root over (B)} is obtained. The timing of
the measurement operation is generally synchronized to ensure that
measured data is acquired at approximately the same phase of the
heart cycle.
[0191] If it is determined at 908 that there are no additional
locations within the heart chamber at which data needs to be
collected, then the non-contact mapping system may perform at 910
post-processing operations on the reconstructed
electro-physiological information to extract clinically useful
data. As noted, in some embodiments the mapping system produces a
composite reconstructed set of electro-physiological information.
Post processing operation are performed, under those circumstances,
on the composite set of reconstructed electro-physiological
information. In some circumstances where the non-contact mapping
system produces multiple reconstructed sets of
electro-physiological information for the raw data collected at
each location in the heart chamber to which the catheter 110 was
moved, the post processing operations are performed individually on
one or more sets of reconstructed electro-physiological
information.
[0192] In some embodiments, the post processing may involve nothing
further then selecting a format for outputting (e.g., displaying)
the reconstructed potentials to a user. In other embodiments, the
post-processing may involve significant further mathematical
manipulation of the reconstructed potentials to provide additional
types of electro-physiological information.
[0193] The reconstructed electro-physiological information and/or
sets of post-processed data are then displayed at 912. The
information, be it the reconstructed electro-physiological
information or any data resulting from the post-processing
performed at 910, is displayed on a 3D graphical rendering of the
3D representation of the endocardium surface generated from the
same data set acquired at 602 or at 502.
[0194] One of the post-processing operations performed on the
reconstructed set(s) of electro-physiological information can
include the generation of a resolution map. Such a resolution map
indicates the spatial resolution of electro-physiological
information at points on the endocardium surface, thereby providing
a measure of the reliability and accuracy of the information at
various points on the endocardium surface. The resolution map may
also be used to form a composite set of reconstructed
electro-physiological information by associating with individual
sets of acquired raw data and/or individual sets of reconstructed
electro-physiological information corresponding resolution maps. A
resultant composite set is then formed by selecting portions of
acquired raw data (or reconstructed information) whose reliability
or accuracy, as indicated by the resolution map corresponding to
the set from which the data is selected, is sufficiently high.
Resolution maps may be used with any form of post-processing
operation including all modes listed below. Strictly speaking,
information about the resolution maps can be determined prior to
obtaining the reconstructed potential data; however, herein we
generally refer to the generation and display of the resolution map
as "post-processing" because such information is typically
presented to the user after at least some of the potentials are
reconstructed.
[0195] Another type of post-processing operation that may be
performed includes the generation of isopotential maps.
Particularly, where the reconstructed electro-physiological
information pertains to electrical potentials, the reconstructed
potentials may be color coded and superimposed on the 3D
endocardial representation. Isopotential maps are the reconstructed
potentials computed for every sampled time instance for a set of
data acquired over a single or multiple heart beats.
[0196] Yet another type of post-processing operation includes the
generation of timing maps (such as activation time maps). The
timing maps provide information on the time-dependent behavior of
the heart's electrical activity. Particularly, the activation map
indicates at what point in time particular points on the
endocardium surface experience a change in their electrical
activity. For example, the activation map could identify the point
in time at which particular cells on the endocardium surface
experienced depolarization. Another type of timing map may be an
iso-duration map where the amount of time certain tissue has been
active for is detected. Timing maps may be computed from the
reconstructed potentials over a single or multiple heart beats.
Timing maps may be determined and displayed for one or more points
on the endocardium surface representation.
[0197] Another type of post processing operation that may be
performed at 910 is the generation of voltage maps. Voltage maps
can be used to display characteristics of voltage amplitude in a
given area. The voltage maps may be computed from the reconstructed
potentials over a single or multiple heart beats. Useful voltage
map information that may be determined and displayed for one or
more points on the endocardium surface representation includes the
maximum amplitude, or root mean square potential values.
[0198] Another type of post-processing operation is the generation
of a difference map. The difference map provides information
regarding the effectiveness of the clinical procedure (e.g.,
ablation) performed on the patient to ameliorate the symptoms of
arrhythmias. The difference map compares the electrical behavior of
the heart, as reflected from two or more voltage maps generated
before and after the performance of the particular clinical
procedure.
[0199] A further type of post processing operation is the
generation of frequency maps. Frequency mapping, and more generally
spectral analysis, are used to identify on the endocardium surface
localized sites of high-frequency activity during fibrillation.
Frequency maps are computed by acquiring multiple sets of
reconstructed information over a particular time interval which
includes a single or multiple heart beats. The acquired raw data is
then used to obtain the frequency representation of that data.
Specific information (e.g., dominant frequency components) from the
frequency representation is subsequently identified, and that
identified information may be displayed.
[0200] Other types of post-processing information may likewise be
performed at 910.
Other Embodiments
[0201] The methods and systems described herein are not limited to
a particular hardware or software configuration, and may find
applicability in many computing or processing environments. The
methods and systems can be implemented in hardware, or a
combination of hardware and software, and/or can be implemented
from commercially available modules applications and devices. Where
the implementation of the systems and methods described herein is
at least partly based on use of microprocessors, the methods and
systems can be implemented in one or more computer programs, where
a computer program can be understood to include one or more
processor executable instructions. The computer program(s) can
execute on one or more programmable processors, and can be stored
on one or more storage medium readable by the processor (including
volatile and non-volatile memory and/or storage elements), one or
more input devices, and/or one or more output devices. The
processor thus can access one or more input devices to obtain input
data, and can access one or more output devices to communicate
output data. The input and/or output devices can include one or
more of the following: Random Access Memory (RAM), Redundant Array
of Independent Disks (RAID), floppy drive, CD, DVD, magnetic disk,
internal hard drive, external hard drive, memory stick, or other
storage device capable of being accessed by a processor as provided
herein, where such aforementioned examples are not exhaustive, and
are for illustration and not limitation.
[0202] The computer program(s) can be implemented using one or more
high level procedural or object-oriented programming languages to
communicate with a computer system; however, the program(s) can be
implemented in assembly or machine language, if desired. The
language can be compiled or interpreted. The device(s) or computer
systems that integrate with the processor(s) can include, for
example, a personal computer(s), workstation (e.g., Sun, HP),
personal digital assistant (PDA), handheld device such as cellular
telephone, laptop, handheld, or another device capable of being
integrated with a processor(s) that can operate as provided herein.
Accordingly, the devices provided herein are not exhaustive and are
provided for illustration and not limitation.
[0203] References to "a microprocessor" and "a processor", or "the
microprocessor" and "the processor," can be understood to include
one or more microprocessors that can communicate in a stand-alone
and/or a distributed environment(s), and can thus be configured to
communicate via wired or wireless communications with other
processors, where such one or more processor can be configured to
operate on one or more processor-controlled devices that can be
similar or different devices. Furthermore, references to memory,
unless otherwise specified, can include one or more
processor-readable and accessible memory elements and/or components
that can be internal to the processor-controlled device, external
to the processor-controlled device, and can be accessed via a wired
or wireless network using a variety of communications protocols,
and unless otherwise specified, can be arranged to include a
combination of external and internal memory devices, where such
memory can be contiguous and/or partitioned based on the
application. Accordingly, references to a database can be
understood to include one or more memory associations, where such
references can include commercially available database products
(e.g., SQL, Informix, Oracle) and also proprietary databases, and
may also include other structures for associating memory such as
links, queues, graphs, trees, with such structures provided for
illustration and not limitation.
[0204] A number of embodiments of the invention have been
described. Nevertheless, it will be understood that various
modifications may be made without departing from the spirit and
scope of the invention. For example, as noted above, while the
discussion above focused on the automatic registration of the
coordinate system of a representation of the heart to the
coordinate system of an object inserted into the medium enclosed
within the heart (namely, the intracardiac blood), the procedures
and systems described herein may also be adapted to be used for
registering the coordinate system of representations of other
objects that can be characterized as a distribution of materials
having different conductivities.
[0205] Furthermore, while it is generally preferred that complete
information about the position of the object is determined, such as
the location of a point of the object and the orientation of the
object with respect to that point; in other embodiments, the
determined position for the object may include fewer than all of
these degrees of freedom.
[0206] Accordingly, other embodiments are within the scope of the
following claims.
* * * * *