U.S. patent application number 12/183688 was filed with the patent office on 2010-02-04 for navigation system for cardiac therapies using gating.
Invention is credited to Monte R. Canfield, Kenneth Gardeski, Michael R. Neidert.
Application Number | 20100030061 12/183688 |
Document ID | / |
Family ID | 41361884 |
Filed Date | 2010-02-04 |
United States Patent
Application |
20100030061 |
Kind Code |
A1 |
Canfield; Monte R. ; et
al. |
February 4, 2010 |
NAVIGATION SYSTEM FOR CARDIAC THERAPIES USING GATING
Abstract
An image guided navigation system for navigating a region of a
patient which is gated using ECG signals to confirm diastole. The
navigation system includes an imaging device, a tracking device, a
controller, and a display. The imaging device generates images of
the region of a patient. The tracking device tracks the location of
the instrument in a region of the patient. The controller
superimposes an icon representative of the instrument onto the
images generated from the imaging device based upon the location of
the instrument. The display displays the image with the
superimposed instrument. The images and a registration process may
be synchronized to a physiological event.
Inventors: |
Canfield; Monte R.; (Center
City, MN) ; Neidert; Michael R.; (Salthill, IE)
; Gardeski; Kenneth; (Plymouth, MN) |
Correspondence
Address: |
MEDTRONIC, INC.
710 MEDTRONIC PARKWAY NE
MINNEAPOLIS
MN
55432-9924
US
|
Family ID: |
41361884 |
Appl. No.: |
12/183688 |
Filed: |
July 31, 2008 |
Current U.S.
Class: |
600/413 |
Current CPC
Class: |
A61B 2018/00839
20130101; A61B 6/032 20130101; A61B 34/20 20160201; A61M 2230/04
20130101; A61B 8/0841 20130101; A61B 2090/3735 20160201; A61B
5/4839 20130101; A61B 2018/00577 20130101; A61B 5/036 20130101;
A61M 25/09 20130101; A61B 5/7285 20130101; A61B 18/1492 20130101;
A61M 25/0108 20130101; A61B 2090/374 20160201; A61B 2090/3784
20160201; A61B 6/5247 20130101; A61B 6/5264 20130101; A61N 1/372
20130101; A61B 8/543 20130101; A61B 2090/3764 20160201; A61B 8/12
20130101; A61B 6/037 20130101; A61B 2034/2065 20160201; A61B 6/541
20130101; A61B 2034/2063 20160201; A61B 6/5288 20130101; A61B 5/349
20210101; A61B 5/352 20210101; A61B 2017/00703 20130101; A61B
5/0066 20130101 |
Class at
Publication: |
600/413 |
International
Class: |
A61B 5/055 20060101
A61B005/055 |
Claims
1. An image guided navigation system for guiding an instrument
through a region of a patient, said image guided navigation system
comprising: an anatomical gating device operable to sense a first
physiological event and a second physiological event, the first
physiological event being different from the second physiological
event; an imaging device operable to capture image data of the
region of the patient in response to the second physiological event
by comparing one characteristic of the first physiological event
with respect a first threshold and when a second characteristic of
said second physiological event to a second threshold; a tracking
device operable to track the position of the instrument in the
region of the patient; a controller in communication with said
anatomical gating device, said imaging device and said tracking
device and operable to register said image data with the region of
the patient in response to the first and second physiological
events, said controller further operable to superimpose an icon
representing the instrument onto the image data of the region of
the patient based upon the position tracked by said tracking
device; and a display operable to display the image data of the
region of the patient with the superimposed icon of the
instrument.
2. The image guided navigation system for guiding an instrument
through a region of a patient as set forth in claim 1, wherein the
first physiological event is the generation of an R-wave during a
cardiac cycle and the second physiological event is diastole during
a cardiac cycle.
3. The image guided navigation system for guiding an instrument
through a region of a patient as set forth in claim 2, the
anatomical gating device generates a diastolic detection window for
confirming whether the patient is in diastole before capturing
image data.
4. The image guided navigation system for guiding an instrument
through a region of a patient as set forth in claim 3, wherein the
anatomical gating device is operable to confirm diastole during the
diastolic detection window by comparing at least one characteristic
of the ECG signal to one previously stored characteristic being
selected from the group consisting of the presence of an R-wave,
slew and DC offset.
5. The image guided navigation system for guiding an instrument
through a region of a patient as set forth in claim 3, wherein the
diastolic detection window extends from about 62% to about 80% of
the duration of the R-R interval following the detection of an
R-wave.
6. The image guided navigation system for guiding an instrument
through a region of a patient as set forth in claim 3, wherein the
diastolic detection window is centered at about 70% of the duration
of the R-R interval following the detection of an R-wave during
normal sinus rhythms.
7. The image guided navigation system for guiding an instrument
through a region of a patient as set forth in claim 3, wherein the
diastolic detection window is centered at about 45% of the duration
of the R-R interval following R-wave detection during
arrhythmias.
8. The image guided navigation system for guiding an instrument
through a region of a patient as set forth in claim 1, wherein the
first physiological event is associated with the presence of an
R-wave of an ECG signal, the anatomical gating device operable to
determine the presence of an R-wave by comparing at least one
characteristic of the ECG signal to at least one corresponding
threshold, the at least one characteristic selected from the group
consisting of slew, turning point and amplitude.
9. The image guided navigation system for guiding an instrument
through a region of a patient as set forth in claim 1, wherein the
anatomical gating device is operable to detect the onset of systole
by calculating slew, amplitude and turning point of an ECG signal
to determine the presence of an R-wave in the ECG signal, and
generate a diastolic detection window in response to the
variability of the interval between adjacent R-waves in the ECG
signal.
10. The image guided navigation system as defined in claim 1
wherein said imaging device is selected from a group of 2D, 3D or
4D imaging devices comprising a C-arm fluoroscopic imager, a
magnetic resonance imager (MRI), a computed tomography (CT) imager,
a positron emission tomography (PET) imager, an isocentric
fluoroscopy imager, a bi-plane fluoroscopy imager, an ultrasound
imager, a multi-slice computed tomography (MSCT) imager, positron
emission tomography-computed tomography (PET/CT), high definition
computed tomography (HDCT), dual source computed tomography, a
high-frequency ultrasound (HIFU) imager, an optical coherence
tomography (OCT) imager, an intra-vascular ultrasound imager
(IVUS), an ultrasound imager, an intra-operative CT imager, an
intra-operative MRI imager, a single photon emission computer
tomography (SPECT) imager, and a combination thereof.
11. The image guided navigation system as defined in claim 1,
wherein said instrument is operable to deliver a therapy to the
patient, the therapy is selected from a group comprising lead
placement, drug delivery, gene delivery, cell delivery, ablation,
mitral valve repair, aortic valve repair, leadless pacemaker
placement, leadless pressure sensor placement, and a combination
thereof.
12. The image guided navigation system as defined in claim 1,
wherein said instrument is selected from a group comprising a
catheter, a guide wire, a stylet, an insert, a needle and a
combination thereof.
13. Method for image guiding an instrument in a region of a
patient, the method comprising: identifying a first physiological
event; comparing at least one characteristic of the first
physiological event to a first predetermined threshold; identifying
a second physiological event different from the first physiological
event; comparing at least one characteristic of the second
physiological event to a second predetermined threshold; capturing
image data during the second physiological event when the at least
one characteristic of the first physiological event exceeds the
first threshold and when the at least one characteristic of the
second physiological event exceeds the second threshold;
registering the captured image data to the patient during the
second physiological event; and displaying the location of the
instrument on the image data of the region of the patient by
superimposing an icon of the instrument on the image data.
14. The method for guiding an instrument to a region of a patient
as set forth in claim 13, wherein the identification of a first
physiological event is the identification of an R-wave during a
cardiac cycle.
15. The method for guiding an instrument to a region of a patient
as set forth in claim 14, wherein identifying a second
physiological event is the identification of diastole during a
cardiac cycle.
16. The method for guiding an instrument to a region of a patient
as set forth in claim 15, wherein the identification of an R-wave
during a cardiac cycle includes comparing at least one
characteristic of an ECG signal to a corresponding threshold, the
at least one characteristic selected from the group consisting of
slew, turning point and amplitude.
17. The method for guiding an instrument to a region of a patient
as set forth in claim 15, wherein the identification of diastole
during the cardiac cycle includes comparing at least one
characteristic of an ECG signal to a previously stored
characteristic selected from the group consisting of the presence
of an R-wave, slew and DC offset.
18. A method for image guiding an instrument in a region of a
patient, said method comprising: generating a gating signal from an
anatomical gating device, the gating signal responsive to the
presence of arrhythmias and ectopic beats; capturing image data
during diastole in response to the gating signal generated by the
anatomical gating device; registering the captured image data to
the patient; and displaying the location of the instrument on the
image data of the region of the patient by superimaging an icon of
the instrument on the image data.
19. The method for image guiding an instrument in a region of a
patient as set forth in claim 18, further comprising detecting at
the onset of systole by calculating slew, amplitude and turning
point of an ECG signal to determine the presence of an R-wave in
the ECG signal, and generating a diastolic detection window in
response to the variability of the interval between adjacent
R-waves in the ECG signal.
20. The method for image guiding an instrument in a region of a
patient as set forth in claim 19, wherein generating a diastolic
detection window includes comparing the turning point of the ECG
signal to a predetermined threshold.
21. The method for image guiding an instrument in a region of a
patient as set forth in claim 19, wherein generating a diastolic
detection window includes comparing the amplitude of the ECG signal
to a predetermined threshold.
22. The method for image guiding an instrument in a region of a
patient as set forth in claim 20, wherein the diastolic detection
window is centered at about 45% to about 70% of the duration of the
R-R interval following the onset of an R-wave.
23. The method for image guiding an instrument in a region of a
patient as set forth in claim 20, wherein the width of the
diastolic detection window is about 75 milliseconds.
24. The method for image guiding an instrument in a region of a
patient as set forth in claim 18, wherein generating a gating
signal from the anatomical gating device includes determining
whether the onset of an R-wave has occurred.
25. The method for image guiding an instrument in a region of a
patient as set forth in claim 24, wherein displaying the location
of the instrument on the image data includes displaying a
previously acquired image of the instrument when the onset of an
R-wave has occurred.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to image guided
surgery, and more specifically, to systems and methods for using
one or more medical images to assist in navigating an instrument
through internal body structures, in particular for navigating a
catheter in a moving body structure, such as the heart, during a
surgical procedure.
BACKGROUND OF THE INVENTION
[0002] Image guided medical and surgical procedures utilize patient
images obtained prior to or during a medical procedure to guide a
physician performing the procedure. Recent advances in imaging
technology, especially in imaging technologies that produce two,
three, and four dimensional images, such as computed tomography
(CT), magnetic resonance imaging (MRI), isocentric C-arm
fluoroscopic imaging, positron emission tomography (PET), and
ultrasound imaging (US), has increased the interest in image guided
medical procedures.
[0003] At present, cardiac catheterization procedures are typically
performed with the aid of fluoroscopic images. Two-dimensional
fluoroscopic images taken intra-procedurally allow a physician to
visualize the location of a catheter being advanced through
cardiovascular structures. However, use of such fluoroscopic
imaging throughout a procedure exposes both the patient and the
operating room staff to radiation, and exposes the patient to
contrast agents. Therefore, the number of fluoroscopic images taken
during a procedure is preferably limited to reduce the radiation
exposure to the patient and staff.
[0004] An image guided surgical navigation system that enables the
physician to see the location of an instrument relative to a
patient's anatomy, without the need to acquire real-time
fluoroscopic images throughout the surgical procedure is generally
disclosed in U.S. Pat. No. 6,470,207, entitled "Navigational
Guidance Via Computer-Assisted Fluoroscopic Imaging," issued Oct.
22, 2002, which is incorporated herein by reference in its
entirety. In this system, representations of surgical instruments
are overlaid on pre-acquired fluoroscopic images of a patient based
on the position of the instruments determined by a tracking
sensor.
[0005] Other types of procedures include the use of
electro-physiologic mapping catheters to map the heart based on
measured electrical potentials. Such mapping catheters are useful
in identifying an area of tissue that is either conducting normally
or abnormally, however, some mapping catheters may not aid in
actually guiding a medical device to a targeted tissue area for
medical treatment.
[0006] Other procedures that could benefit from a navigation system
include cardiac lead placement. Cardiac lead placement is important
in achieving proper stimulation or accurate sensing at a desired
cardiac location. Endocardial is one type of lead placement
procedure that is an internal procedure where coronary vein leads
are generally implanted with the use of a guide catheter and/or a
guide wire or stylet to achieve proper placement of the lead.
Epicardial is another type of procedure that is an external
procedure for cardiac lead placement that may also benefit from
this navigation system. A coronary vein lead may be placed using a
multi-step procedure wherein a guide catheter is advanced into the
coronary sinus ostium and a guide wire is advanced further through
the coronary sinus and great cardiac vein to a desired cardiac vein
branch. Because the tip of a guide wire is generally flexible and
may be preshaped in a bend or curve, the tip of the guide wire can
be steered into a desired venous branch. The guide wire tip is
directed with a steerable guide catheter, and with the appropriate
pressure, it is manipulated into the desired vein branch.
[0007] A cardiac lead may therefore be advanced to a desired
implant location using a guide wire extending entirely through the
lead and out its distal end. Cardiac leads generally need to be
highly flexible in order to withstand flexing motion caused by the
beating heart without fracturing. A guide wire provides a flexible
lead with the stiffness needed to advance it through a venous
pathway. Leads placed with the use of a guide wire are sometimes
referred to as "over-the-wire" leads. Once the lead is placed in a
desired location, the guide wire and guide catheter may be removed.
A guide wire placed implantable lead is disclosed in U.S. Pat. No.
6,192,280, entitled "Guide wire Placed Implantable Lead With Tip
Seal," issued Feb. 20, 2001. A coronary vein lead having a flexible
tip and which may be adapted for receiving a stylet or guide wire
is disclosed in U.S. Pat. No. 5,935,160, entitled "Left Ventricular
Access Lead for Heart Failure Pacing", issued Aug. 10, 1999, each
of which are hereby incorporated by reference.
[0008] Also, pacing lead procedures currently performed today for
use in heart failure treatment are not optimized. In this regard,
the lead placement is not optimized due to the lack of having
real-time anatomic information, navigation and localization
information, hemo-dynamic data, and electro-physiological data.
Currently, pacing leads are simply "stuffed" into the heart without
any optimization being performed due to lack of information that
can be used for this optimization.
[0009] Advancement of a guide catheter or an over-the-wire lead
through a vessel pathway and through cardiac structures requires
considerable skill and can be a time-consuming task. This type of
procedure also exposes the patient to an undesirable amount of
radiation exposure and contrast agent. Therefore, it is desirable
to provide an image guided navigation system that allows the
location of a guide catheter being advanced within the
cardiovascular structures for lead placement to be followed in
either two, three, or four dimensional space in real time. It is
also desirable to provide an image guided navigation system that
assists in navigating an instrument, such as a catheter, through a
moving body structure or any type of soft tissue.
[0010] With regard to navigating an instrument through a moving
body structure, difficulties arise in attempting to track such an
instrument using known tracking technology as the instrument passes
adjacent or through a moving body structure, since the virtual
representation of the instrument may be offset from the
corresponding anatomy when superimposed onto image data.
Accordingly, it is also desirable to acquire image data and track
the instrument in a synchronized manner with the pre-acquired image
using gating or synchronization techniques, such as ECG gating or
respiratory gating.
[0011] Other difficulties with cardiac procedures include annual
check-ups to measure early indications for organ rejection in heart
transplant patients. These indicators include white blood cells,
chemical change, blood oxygen levels, etc. During the procedure, an
endovascular biopsy catheter is inserted into the heart and
multiple biopsies are performed in the septum wall of the heart.
Again, during this procedure, radiation and contrast agent is
utilized to visualize the biopsy catheter, thereby exposing both a
patient and the doctor to potential excess radiation and contrast
agents during the procedure. As such, it would also be desirable to
provide an image guided navigation system that assists in
performing this type of procedure in order to reduce radiation and
contrast agent exposure.
SUMMARY OF THE INVENTION
[0012] A navigation system is provided including a catheter
carrying single or multiple localization sensors, a sensor
interface, a user interface, a controller, and a visual display.
Aspects of the present invention allow for the location of a
catheter advanced within an internal space within the human body,
for example within the cardiovascular structures, to be identified
in two, three or four dimensions in real time. Further aspects of
the present invention allow for accurate mapping of a tissue or
organ, such as the heart or other soft tissue, and/or precise
identification of a desired location for delivering a medical lead,
or other medical device or therapy, while reducing the exposure to
fluoroscopy normally required during conventional catheterization
procedures. These types of therapies include, but are not limited
to, drug delivery therapy, cell delivery therapy, ablation,
stenting, or sensing of various physiological parameters with the
appropriate type of sensor. In cardiac applications, methods
included in the present invention compensate for the effects of
respiration and the beating heart that can normally complicate
mapping or diagnostic data. Aspects of the present invention may be
tailored to improve the outcomes of numerous cardiac therapies as
well as non-cardiac therapies, such as neurological, oncological,
or other medical therapies, including lung, liver, prostate and
other soft tissue therapies, requiring the use of a catheter or
other instrument at a precise location.
[0013] The steerable catheter provided by the present invention
features at least one or more location sensors located near the
distal end of an elongated catheter body. The location sensors may
be spaced axially from each other and may be electromagnetic
detectors. An electromagnetic source is positioned externally to
the patient for inducing a magnetic field, which causes voltage to
be developed on the location sensors. The location sensors may each
be electrically coupled to twisted pair conductors, which extend
through the catheter body to the proximal catheter end. Twisted
pair conductors provide electromagnetic shielding of the
conductors, which prevents voltage induction along the conductors
when exposed to the magnetic flux produced by the electromagnetic
source. Alternatively, the sensors and the source may be reversed
where the catheter emits a magnetic field that is sensed by
external sensors.
[0014] By sensing and processing the voltage signals from each
location sensor, the location of the catheter tip with respect to
the external sources and the location of each sensor with respect
to one another may be determined. The present invention allows a
two, three, or four-dimensional reconstruction of several
centimeters of the distal portion of the catheter body in real
time. Visualization of the shape and position of a distal portion
of the catheter makes the advancement of the catheter to a desired
position more intuitive to the user. The system may also provide a
curve fitting algorithm that is selectable based upon the type of
catheter used, and based upon the flexibility of the catheter,
based upon a path finding algorithm, and based upon image data.
This enables estimated curved trajectories of the catheter to be
displayed to assist the user.
[0015] The location sensor conductors, as well as conductors
coupled to other physiological sensors present, are coupled to a
sensor interface for filtering, amplifying, and digitizing the
sensed signals. The digitized signals are provided via a data bus
to a control system, embodied as a computer. Programs executed by
the control system process the sensor data for determining the
location of the location sensors relative to a reference source. A
determined location is superimposed on a two, three, or
four-dimensional image that is displayed on a monitor. A
user-interface, such as a keyboard, mouse or pointer, is provided
for entering operational commands or parameters.
[0016] In one embodiment, an image guided navigation system for
guiding an instrument through a region of the patient includes an
anatomic gating device, an imaging device, a tracking device, a
controller, and a display. The anatomic gating device senses a
first and second physiological event. The imaging device captures
image data in response to the first and second physiological event.
The tracking device tracks the position of the instrument in the
region of the patient. The controller is in communication with the
anatomic gating device, the imaging device and the tracking device,
and registers the image data with the region of a patient in
response to the first and second physiological event. The
controller also superimposes an icon representing the instrument
onto the image data, based on the tracked position. The display
shows the image data of the region of the patient with the
superimposed icon of the instrument.
[0017] Further areas of applicability of the present invention will
become apparent from the detailed description provided hereinafter.
It should be understood that the detailed description and specific
examples, while indicating the preferred embodiments of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The present invention will become more fully understood from
the detailed description and the accompanying drawings,
wherein:
[0019] FIG. 1 is a diagram of a catheter navigation system
according to the teachings of the present invention;
[0020] FIGS. 2a and 2b are diagrams representing undistorted and
distorted views from a fluoroscopic C-arm imaging device;
[0021] FIG. 3 is a logic block diagram illustrating a method for
navigating a catheter during cardiac therapy;
[0022] FIG. 4 is a logic block diagram illustrating the R-wave
detector associated with the method for navigating a catheter
during cardiac therapy as shown in FIG. 3;
[0023] FIG. 5 is a logic block diagram illustrating the diastole
detector associated with the method for navigating a catheter
during cardiac therapy as shown in FIG. 3; and
[0024] FIG. 6 is a logic block diagram illustrating the gating
phase of the diastole detector illustrated in FIG. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0025] The following description of the preferred embodiment(s) is
merely exemplary in nature and is in no way intended to limit the
invention, its application, or uses. As indicated above, the
present invention is directed at providing improved,
non-line-of-site, image-guided navigation of an instrument, such as
a catheter, balloon catheter, implant, lead, stent, needle, guide
wire, insert, and/or capsule, that may be used for physiological
monitoring, delivering a medical therapy, or guiding the delivery
of a medical device in an internal body space, such as the heart or
any other region of the body.
[0026] FIG. 1 is a diagram illustrating an overview of an
image-guided catheter navigation system 10 for use in
non-line-of-site navigating of a catheter during cardiac therapy or
any other soft tissue therapy. It should further be noted that the
navigation system 10 may be used to navigate any other type of
instrument or delivery system, including guide wires, needles, drug
delivery systems, cell delivery systems, gene delivery systems, and
biopsy systems. Moreover, these instruments may be used for cardiac
therapy or any other therapy in the body or be used to navigate or
map any other regions of the body, such as moving body structures.
However, each region of the body poses unique requirements to
navigate, as disclosed herein. For example, the navigation system
10 addresses multiple cardiac, neurological, organ and other soft
tissue therapies, including drug delivery, cell transplantation,
gene delivery, electro-physiology ablations, revascularization,
biopsy guidance, mitral valve repair, aortic valve repair, leadless
pacemaker placement, leadless pressure sensor placement, and
virtual echography imaging.
[0027] The navigation system 10 may include an imaging device 12
that is used to acquire pre-operative or real-time images of a
patient 14. The imaging device 12 is a fluoroscopic x-ray imaging
device that may include a C-arm 16 having an x-ray source 18, an
x-ray receiving section 20, an optional calibration and tracking
target 22, and optional radiation sensors 24. The calibration and
tracking target 22 includes calibration markers 26 (see FIGS.
2a-2b), further discussed herein. A C-arm controller 28 captures
the x-ray images received at the receiving section 20 and stores
the images for later use. The C-arm controller 28 may also control
the rotation of the C-arm 16. For example, the C-arm 16 may move in
the direction of arrow 30 or rotates about the long axis of the
patient, allowing anterior or lateral views of the patient 14 to be
imaged. Each of these movements involve rotation about a mechanical
axis 32 of the C-arm 16. In this example, the long axis of the
patient 14 is substantially in line with the mechanical axis 32 of
the C-arm 16. This enables the C-arm 16 to be rotated relative to
the patient 14, allowing images of the patient 14 to be taken from
multiple directions or about multiple planes. An example of a
fluoroscopic C-arm x-ray imaging device 12 is the "Series 9600
Mobile Digital Imaging System," from OEC Medical Systems, Inc., of
Salt Lake City, Utah. Other exemplary fluoroscopes include bi-plane
fluoroscopic systems, ceiling fluoroscopic systems, cath-lab
fluoroscopic systems, fixed C-arm fluoroscopic systems, isocentric
C-arm fluoroscopic systems, 3D fluoroscopic systems, etc.
[0028] In operation, the imaging device 12 generates x-rays from
the x-ray source 18 that propagate through the patient 14 and
calibration and/or tracking target 22, into the x-ray receiving
section 20. The receiving section 20 generates an image
representing the intensities of the received x-rays. Typically, the
receiving section 20 includes an image intensifier that first
converts the x-rays to visible light and a charge coupled device
(CCD) video camera that converts the visible light into digital
images. Receiving section 20 may also be a digital device that
converts x-rays directly to digital images, thus potentially
avoiding distortion introduced by first converting to visible
light. With this type of digital C-arm, which is generally a flat
panel device, the optional calibration and/or tracking target 22
and the calibration process discussed below may be eliminated.
Also, the calibration process may be eliminated or not used at all
for cardiac therapies. Alternatively, the imaging device 12 may
only take a single image with the calibration and tracking target
22 in place. Thereafter, the calibration and tracking target 22 may
be removed from the line-of-sight of the imaging device 12.
[0029] Two-dimensional fluoroscopic images taken by the imaging
device 12 are captured and stored in the C-arm controller 28.
Multiple two-dimensional images taken by the imaging device 12 may
also be captured and assembled to provide a larger view or image of
a whole region of a patient, as opposed to being directed to only a
portion of a region of the patient. For example, multiple image
data of a patient's leg may be appended together to provide a full
view or complete set of image data of the leg that can be later
used to follow a contrast agent, such as Bolus tracking. These
images are then forwarded from the C-arm controller 28 to a
controller or work station 34 having a display 36 and a user
interface 38. The work station 34 provides facilities for
exhibiting on the display 36, and saving, digitally manipulating,
or printing a hard copy of the received images. The user interface
38, which may be a keyboard, mouse, touch pen, touch screen or
other suitable device, allows a physician or user to provide inputs
to control the imaging device 12 via the C-arm controller 28, or
adjust the display settings of the display 36. The work station 34
may also direct the C-arm controller 28 to adjust the rotational
axis 32 of the C-arm 16 to obtain various two-dimensional images
along different planes in order to generate representative
two-dimensional and three-dimensional images. When the x-ray source
18 generates the x-rays that propagate to the x-ray receiving
section 20, the radiation sensors 24 sense the presence of
radiation, which is forwarded to the C-arm controller 28 to
identify whether or not the imaging device 12 is actively imaging.
This information is also transmitted to a coil array controller 48,
further discussed herein. Alternatively, a person or physician may
manually indicate when the imaging device 12 is actively imaging or
this function can be built into the x-ray source 18, x-ray
receiving section 20, or the control computer 28.
[0030] Fluoroscopic C-arm imaging devices 12 that do not include a
digital receiving section 20 generally require the optional
calibration and/or tracking target 22. This is because the raw
images generated by the receiving section 20 tend to suffer from
undesirable distortion caused by a number of factors, including
inherent image distortion in the image intensifier and external
electromagnetic fields. An empty undistorted or ideal image and an
empty distorted image are shown in FIGS. 2a and 2b, respectively.
The checkerboard shape, shown in FIG. 2a, represents the ideal
image 40 of the checkerboard-arranged calibration markers 26. The
image taken by the receiving section 20, however, can suffer from
distortion, as illustrated by the distorted calibration marker
image 42, shown in FIG. 2b.
[0031] Intrinsic calibration, which is the process of correcting
image distortion in a received image and establishing the
projective transformation for that image, involves placing the
calibration markers 26 in the path of the x-ray, where the
calibration markers 26 are opaque or semi-opaque to the x-rays. The
calibration markers 26 are rigidly arranged in pre-determined
patterns in one or more planes in the path of the x-rays and are
visible in the recorded images. Because the true relative position
of the calibration markers 26 in the recorded images are known, the
C-arm controller 28 or the work station or computer 34 is able to
calculate an amount of distortion at each pixel in the image (where
a pixel is a single point in the image). Accordingly, the computer
or work station 34 can digitally compensate for the distortion in
the image and generate a distortion-free or at least a
distortion-improved image 40 (see FIG. 2a). A more detailed
explanation of exemplary methods for performing intrinsic
calibration is described in the references: B. Schuele, et al.,
"Correction of Image Intensifier Distortion for Three-Dimensional
Reconstruction," presented at SPIE Medical Imaging, San Diego,
Calif., 1995; G. Champleboux, et al., "Accurate Calibration of
Cameras and Range Imaging Sensors: the NPBS Method," Proceedings of
the IEEE International Conference on Robotics and Automation, Nice,
France, May, 1992; and U.S. Pat. No. 6,118,845, entitled "System
And Methods For The Reduction And Elimination Of Image Artifacts In
The Calibration Of X-Ray Imagers," issued Sep. 12, 2000, the
contents of which are each hereby incorporated by reference.
[0032] While the fluoroscopic imaging device 12 is shown in FIG. 1,
any other alternative 2D, 3D or 4D imaging modality may also be
used. For example, any 2D, 3D or 4D imaging device, such as
isocentric fluoroscopy, bi-plane fluoroscopy, ultrasound, computed
tomography (CT), multi-slice computed tomography (MSCT), magnetic
resonance imaging (MRI), high frequency ultrasound (HIFU), positron
emission tomography (PET), positron emission tomography-computed
tomography (PET/CT), high definition computed tomography (HDCT),
dual source computed tomography, optical coherence tomography
(OCT), intra-vascular ultrasound (IVUS), ultrasound,
intra-operative CT or MRI, may also be used to acquire 2D, 3D or 4D
pre-operative or real-time images or image data of the patient 14.
The images may also be obtained and displayed in two, three or four
dimensions. In more advanced forms, four-dimensional surface
rendering of the heart or other regions of the body may also be
achieved by incorporating heart data or other soft tissue data from
an atlas map or from pre-operative image data captured by MRI, CT,
or echocardiography modalities. A more detailed discussion on
optical coherence tomography (OCT), is set forth in U.S. Pat. No.
5,740,808, issued Apr. 21, 1998, entitled "Systems And Methods For
Guilding Diagnostic Or Therapeutic Devices In Interior Tissue
Regions" which is hereby incorporated by reference.
[0033] Image datasets from hybrid modalities, such as positron
emission tomography (PET) combined with CT, or single photon
emission computer tomography (SPECT) combined with CT, could also
provide functional image data superimposed onto anatomical data to
be used to confidently reach target sights within the heart or
other areas of interest. It should further be noted that the
fluoroscopic imaging device 12, as shown in FIG. 1, provides a
virtual bi-plane image using a single-head C-arm fluoroscope 12 by
simply rotating the C-arm 16 about at least two planes, which could
be orthogonal planes to generate two-dimensional images that can be
converted to three-dimensional volumetric images. By acquiring
images in more than one plane, an icon representing the location of
a catheter or other instrument, introduced and advanced in the
patient 14, may be superimposed in more than one view on display 36
allowing simulated bi-plane or even multi-plane views, including
two and three-dimensional views.
[0034] These types of imaging modalities may provide certain
distinct benefits and disadvantages for their use. For example,
magnetic resonance imaging (MRI) is generally performed
pre-operatively using a non-ionizing field. This type of imaging
provides very good tissue visualization in three-dimensional form
and also provides anatomical and functional information from the
imaging. MRI imaging data is generally registers and compensates
for motion correction using dynamic reference frames that are
discussed herein.
[0035] Positron emission tomography (PET) imaging is generally a
pre-operative imaging procedure that exposes the patient to some
level of radiation to provide a 3D image. PET imaging provides
functional information and also generally requires registration and
motion correction using dynamic reference frames.
[0036] Computed tomography (CT) imaging is also generally a
pre-operative technique that exposes the patient to a limited level
of radiation. CT imaging, however, is a very rapid imaging
procedure. A multi-slice CT system provides 3D images having good
resolution and anatomical information. Again, CT imaging is
generally registered and needs to account for motion correction via
dynamic reference frames.
[0037] Fluoroscopy imaging is generally an intra-operative imaging
procedure that exposes the patient to certain amounts of radiation
to provide either two-dimensional or rotational three-dimensional
images. Fluoroscopic images generally provide good resolution and
anatomical information. Fluoroscopic images can be either manually
or automatically registered and also need to account for motion
correction using dynamic reference frames.
[0038] Ultrasound imaging is also generally an intra-operative
procedure using a non-ionizing field to provide either 2D, 3D, or
4D imaging, including anatomical and blood flow information.
Ultrasound imaging provides automatic registration and does not
need to account for any motion correction.
[0039] The navigation system 10 further includes an electromagnetic
navigation or tracking system 44 that includes a transmitter coil
array 46, the coil array controller 48, a navigation probe
interface 50, an electromagnetic catheter 52 or any other type of
instrument and a dynamic reference frame 54. Further, it should
further be noted that the entire tracking system 44 or parts of the
tracking system 44 may be incorporated into the imaging device 12,
including the work station 34 and radiation sensors 24.
Incorporating the tracking system 44 will provide an integrated
imaging and tracking system. Any combination of these components
may also be incorporated into the imaging device 12, which again
can include a fluoroscopic C-arm imaging device or any other
appropriate imaging device.
[0040] The transmitter coil array 46 is shown attached to the
receiving section 20 of the C-arm 16. However, it should be noted
that the transmitter coil array 46 may also be positioned at any
other location as well. For example, the transmitter coil array 46
may be positioned at the x-ray source 18, within or atop the OR
table 56 positioned below the patient 14, on siderails associated
with the table 56, or positioned on the patient 14 in proximity to
the region being navigated, such as on the patient's chest. The
transmitter coil array 46 includes a plurality of coils that are
each operable to generate distinct electromagnetic fields into the
navigation region of the patient 14, which is sometimes referred to
as patient space. Representative electromagnetic systems are set
forth in U.S. Pat. No. 5,913,820, entitled "Position Location
System," issued Jun. 22, 1999, and U.S. Pat. No. 5,592,939,
entitled "Method and System for Navigating a Catheter Probe,"
issued Jan. 14, 1997, each of which are hereby incorporated by
reference.
[0041] The transmitter coil array 46 is controlled or driven by the
coil array controller 48. The coil array controller 48 drives each
coil in the transmitter coil array 46 in a time division multiplex
or a frequency division multiplex manner. In this regard, each coil
may be driven separately at a distinct time or all of the coils may
be driven simultaneously with each being driven by a different
frequency. Upon driving the coils in the transmitter coil array 46
with the coil array controller 48, electromagnetic fields are
generated within the patient 14 in the area where the medical
procedure is being performed, which is again sometimes referred to
as patient space. The electromagnetic fields generated in the
patient space induces currents in sensors 58 positioned in the
catheter 52. These induced signals from the catheter 52 are
delivered to the navigation probe interface 50 and subsequently
forwarded to the coil array controller 48. The navigation probe
interface 50 provides all the necessary electrical isolation for
the navigation system 10. The navigation probe interface 50 also
includes amplifiers, filters and buffers required to directly
interface with the sensors 58 in catheter 52. Alternatively, the
catheter 52 may employ a wireless communications channel as opposed
to being coupled directly to the navigation probe interface 50.
[0042] The catheter 52 may be equipped with at least one, and
generally multiple, localization sensors 58. The catheter 52 may
also be a steerable catheter that includes a handle at a proximal
end and the multiple location sensors 58 fixed to the catheter body
and spaced axially from one another along the distal segment of the
catheter 52. The catheter 52, as shown in FIG. 1, includes four
localization sensors 58. The localization sensors 58 are generally
formed as electromagnetic receiver coils, such that the
electromagnetic field generated by the transmitter coil array 46
induces current in the electromagnetic receiver coils or sensors
58. The catheter 52 may also be equipped with one or more sensors,
which are operable to sense various physiological signals. For
example, the catheter 52 may be provided with electrodes for
sensing myopotentials or action potentials. An absolute pressure
sensor may also be included, as well as other electrode sensors.
The catheter 52 may also be provided with an open lumen to allow
the delivery of a medical device or pharmaceutical/cell/gene
agents. For example, the catheter 52 may be used as a guide
catheter for deploying a medical lead, such as a cardiac lead for
use in cardiac pacing and/or defibrillation or tissue ablation. The
open lumen may alternatively be used to locally deliver
pharmaceutical agents, cell, or genetic therapies. A representative
catheter which may be used is that which is disclosed in U.S.
Patent Publication No. 2004/0097805 entitled "Navigation System for
Cardiac Therapies", filed Jul. 14, 2003, which is hereby
incorporated by reference.
[0043] In an alternate embodiment, the electromagnetic sources or
generators may be located within the catheter 52 and one or more
receiver coils may be provided externally to the patient 14,
forming a receiver coil array similar to the transmitter coil array
46. In this regard, the sensor coils 58 would generate
electromagnetic fields, which would be received by the receiving
coils in the receiving coil array similar to the transmitter coil
array 46. Other types of localization sensors or systems may also
be used, which may include an emitter, which emits energy, such as
light, sound, or electromagnetic radiation, and a receiver that
detects the energy at a position away from the emitter. This change
in energy, from the emitter to the receiver, is used to determine
the location of the receiver relative to the emitter. Other types
of tracking systems include optical, acoustic, electrical field, RF
and accelerometers. Accelerometers enable both dynamic sensing due
to motion and static sensing due to gravity. An additional
representative alternative localization and tracking system is set
forth in U.S. Pat. No. 5,983,126, entitled "Catheter Location
System and Method," issued Nov. 9, 1999, which is hereby
incorporated by reference. Alternatively, the localization system
may be a hybrid system that includes components from various
systems.
[0044] The dynamic reference frame 54 of the electromagnetic
tracking system 44 is also coupled to the navigation probe
interface 50 to forward the information to the coil array
controller 48. The dynamic reference frame 54 is a small magnetic
field detector that is designed to be fixed to the patient 14
adjacent to the region being navigated so that any movement of the
patient 14 is detected as relative motion between the transmitter
coil array 46 and the dynamic reference frame 54. This relative
motion is forwarded to the coil array controller 48, which updates
registration correlation and maintains accurate navigation, further
discussed herein. The dynamic reference frame 54 can be configured
as a pair of orthogonally oriented coils, each having the same
center or may be configured in any other non-coaxial coil
configuration. The dynamic reference frame 54 may be affixed
externally to the patient 14, adjacent to the region of navigation,
such as on the patient's chest, as shown in FIG. 1 or on the
patient's back. The dynamic reference frame 54 can be affixed to
the patient's skin, by way of a stick-on adhesive patch. The
dynamic reference frame 54 may also be removably attachable to
fiducial markers 60 also positioned on the patient's body as
further discussed herein.
[0045] Alternatively, the dynamic reference frame 54 may be
internally attached, for example, to the wall of the patient's
heart or other soft tissue using a temporary lead that is attached
directly to the heart. This provides increased accuracy since this
lead will track the regional motion of the heart. Gating, as
further discussed herein, will also increase the navigational
accuracy of the system 10. An exemplary dynamic reference frame 54
and fiducial marker 60, is set forth in U.S. Pat. No. 6,381,485,
entitled "Registration of Human Anatomy Integrated for
Electromagnetic Localization," issued Apr. 30, 2002, which is
hereby incorporated by reference. It should further be noted that
multiple dynamic reference frames 54 may also be employed. For
example, an external dynamic reference frame 54 may be attached to
the chest of the patient 14, as well as to the back of the patient
14. Since certain regions of the body may move more than others due
to motions of the heart or the respiratory system, each dynamic
reference frame 54 may be appropriately weighted to increase
accuracy even further. In this regard, the dynamic reference frame
54 attached to the back may be weighted higher than the dynamic
reference frame 54 attached to the chest, since the dynamic
reference frame 54 attached to the back is relatively static in
motion.
[0046] The catheter and navigation system 10 further includes a
gating device or an ECG or electrocardiogram 62, which is attached
to the patient 14, via skin electrodes 64, and in communication
with the coil array controller 48. Respiration and cardiac motion
can cause movement of cardiac structures relative to the catheter
52, even when the catheter 52 has not been moved. Therefore,
localization data may be acquired on a time-gated basis triggered
by a physiological signal. For example, the ECG or EGM signal may
be acquired from the skin electrodes 64 or from a sensing electrode
included on the catheter 52 or from a separate reference probe. As
will be discussed more fully below, a characteristic of this signal
may be used as to gate or trigger image acquisition during the
imaging phase with the imaging device 12. By event gating at a
point in a cycle the image data and/or the navigation data, the
icon of the location of the catheter 52 relative to the heart at
the same point in the cardiac cycle may be displayed on the display
36, further discussed herein.
[0047] Additionally or alternatively, a sensor regarding
respiration may be used to trigger data collection at the same
point in the respiration cycle. Additional external sensors can
also be coupled to the navigation system 10. These could include a
capnographic sensor that monitors exhaled CO.sub.2 concentration.
From this, the end expiration point can be easily determined. The
respiration, both ventriculated and spontaneous causes an
undesirable elevation or reduction, respectively, in the baseline
pressure signal. By measuring systolic and diastolic pressures at
the end expiration point, the coupling of respiration noise is
minimized. As an alternative to the CO.sub.2 sensor, an airway
pressure sensor can be used to determine end expiration.
[0048] Briefly, the navigation system 10 operates as follows. The
navigation system 10 creates a translation map between all points
in the radiological image generated from the imaging device 12 and
the corresponding points in the patient's anatomy in patient space.
After this map is established, whenever a tracked instrument, such
as the catheter 52 or pointing device is used, the work station 34,
in combination with the coil array controller 48 and the C-arm
controller 28, uses the translation map to identify the
corresponding point on the pre-acquired image, which is exhibited
on display 36. This identification is known as navigation or
localization. An icon representing the localized point or
instruments are shown on the display 36 within several
two-dimensional image planes, as well as on three and four
dimensional images and models.
[0049] To enable navigation, the navigation system 10 must be able
to detect both the position of the patient's anatomy and the
position of the catheter 52 or other surgical instrument. Knowing
the location of these two items allows the navigation system 10 to
compute and display the position of the catheter 52 in relation to
the patient 14 on the radiological images. The tracking system 44
is employed to track the catheter 52 and the anatomy
simultaneously.
[0050] The tracking system 44 essentially works by positioning the
transmitter coil array 46 adjacent to the patient space to generate
a low-energy magnetic field generally referred to as a navigation
field. Because every point in the navigation field or patient space
is associated with a unique field strength, the electromagnetic
tracking system 44 can determine the position of the catheter 52 by
measuring the field strength at the sensor 58 location. The dynamic
reference frame 54 is fixed to the patient 14 to identify the
location of the patient in the navigation field. The
electromagnetic tracking system 44 continuously recomputes the
relative position of the dynamic reference frame 54 and the
catheter 52 during localization and relates this spatial
information to patient registration data to enable image guidance
of the catheter 52 within the patient 14.
[0051] Patient registration is the process of determining how to
correlate the position of the instrument or catheter 52 on the
patient 14 to the position on the diagnostic or pre-acquired
images. To register the patient 14, the physician or user may use
point registration by selecting and storing particular points from
the pre-acquired images and then touching the corresponding points
on the patient's anatomy with a pointer probe 66. The navigation
system 10 analyzes the relationship between the two sets of points
that are selected and computes a match, which correlates every
point in the image data with its corresponding point on the
patient's anatomy or the patient space. The points that are
selected to perform registration are the fiducial arrays or
landmarks 60. Again, the landmarks or fiducial points 60 are
identifiable on the images and identifiable and accessible on the
patient 14. The landmarks 60 can be artificial landmarks 60 that
are positioned on the patient 14 or anatomical landmarks that can
be easily identified in the image data. The system 10 may also
perform registration using anatomic surface information or path
information, further discussed herein. The system 10 may also
perform 2D to 3D registration by utilizing the acquired 2D images
to register 3D volume images by use of contour algorithms, point
algorithms or density comparison algorithms, as is known in the
art. An exemplary 2D to 3D registration procedure, as set forth in
U.S. Patent Publication No. 2004/0215671, entitled "Method and
Apparatus for Performing 2D to 3D Registration," which is hereby
incorporated by reference. The registration process may also be
synched to an anatomical function, for example, by the use of the
ECG device 62, further discussed herein.
[0052] In order to maintain registration accuracy, the navigation
system 10 continuously tracks the position of the patient 14 during
registration and navigation. This is because the patient 14,
dynamic reference frame 54, and transmitter coil array 46 may all
move during the procedure, even when this movement is not desired.
Therefore, if the navigation system 10 did not track the position
of the patient 14 or area of the anatomy, any patient movement
after image acquisition would result in inaccurate navigation
within that image. The dynamic reference frame 54 allows the
electromagnetic tracking system 44 to register and track the
anatomy. Because the dynamic reference frame 54 is attached to the
patient 14, any movement of the anatomy or the transmitter coil
array 46 is detected as the relative motion between the transmitter
coil array 46 and the dynamic reference frame 54. This relative
motion is communicated to the coil array controller 48, via the
navigation probe interface 50, which updates the registration
correlation to thereby maintain accurate navigation.
[0053] Turning now to FIG. 3, a logic flow diagram illustrating an
exemplary operation of the navigation system 10 is set forth in
further detail. First, should the imaging device 12 or the
fluoroscopic C-arm imager 12 not include a digital receiving
section 20, the imaging device 12 is first calibrated using the
calibration process 68. The calibration process 68 begins at block
70 by generating an x-ray by the x-ray source 18, which is received
by the x-ray receiving section 20. The x-ray image 70 is then
captured or imported at import block 72 from the C-arm controller
28 to the work station 34. The work station 34 performs intrinsic
calibration at calibration block 74, as discussed above, utilizing
the calibration markers 26, shown in FIGS. 2a and 2b. This results
in an empty image being calibrated at block 76. This calibrated
empty image is utilized for subsequent calibration and
registration, further discussed herein.
[0054] Once the imaging device 12 has been calibrated, the patient
14 is positioned within the C-arm 16 between the x-ray source 18
and the x-ray receiving section 20. The navigation process begins
at decision block 78 where it is determined whether or not an x-ray
image of the patient 14 has been taken. If the x-ray image has not
been taken, the process proceeds to block 80 where the x-ray image
is generated at the x-ray source 18 and received at the x-ray
receiving section 20. In some embodiments, when the x-ray source 18
is generating x-rays, the radiation sensors 24 identified in block
82 may activate to identify that the x-ray image 80 is being taken.
This enables the tracking system 44 to identify where the C-arm 16
is located relative to the patient 14 when the image data is being
captured. In some embodiments, however, the tracking system 44 may
not need to be triggered by the radiation sensors 24.
[0055] The process then proceeds to decision block 84 where it is
determined if the x-ray image acquisition will be gated to
physiological activity of the patient 14. If so, the image device
12 will capture the x-ray image at this desired gating time. For
example, the physiological change may be the beating heart, which
is identified by ECG gating at block 86. The ECG gating enables the
x-ray image acquisition to take place at the end of diastole at
block 88, as will be more fully discussed below. Diastole is the
period of time between contractions of the atria or the ventricles
during which blood enters the relaxed chambers from systemic
circulation and the lungs. Diastole is often measured as the blood
pressure at the instant of maximum cardiac relaxation. ECG gating
of myocardial injections also enables optimal injection volumes and
injection rates to achieve maximum cell retention. The optimal
injection time period may go over one heart cycle. During the
injection, relative motion of the catheter tip to the endocardial
surface needs to be minimized. Conductivity electrodes at the
catheter tip may be used to maintain this minimized motion. Also,
gating the delivery of volumes can be used to increase or decrease
the volume delivered over time (i.e., ramp-up or ramp-down during
cycle). Again, the image may be gated to any physiological change
like the heartbeat, respiratory functions, etc. The image acquired
at block 88 is then imported to the work station 34 at block 90. If
it is not desired to physiologically gate the image acquisition
cycle, the process will proceed from the x-ray image block 80
directly to the image import block 90.
[0056] Once the image is received and stored in the work station
34, the process proceeds to calibration and registration at block
92. First, at decision block 94, it is determined whether the
imaging device 12 has been calibrated, if so, the empty image
calibration information from block 76 is provided for calibration
registration at block 92. The empty image calibration information
from block 76 is used to correct image distortion by establishing
projective transformations using known calibration marker locations
(see FIGS. 2a and 2b). Calibration registration 92 also requires
tracking of the dynamic reference frame 54. In this regard, it is
first determined at decision block 96 whether or not the dynamic
reference frame is visible, via block 98. With the dynamic
reference frame 54 visible or in the navigation field and the
calibration information provided, the work station 34 and the coil
array controller 48, via the navigation probe interface 50 performs
the calibration registration 92 functions. In addition to
monitoring the dynamic reference frame 54, the fiducial array or
landmarks 60 may also be used for image registration.
[0057] Once the navigation system 10 has been calibrated and
registered, navigation of an instrument, such as the catheter 52 is
performed. In this regard, once it is determined at decision block
100 that the catheter 54 is visible or in the navigation field at
block 102, an icon representing the catheter 52 is superimposed
over the pre-acquired images at block 104. Should it be determined
to match the superimposed image of the catheter 52 with the motion
of the heart at decision block 106, ECG gating at block 108 is
performed. The catheter 52 may then be navigated, via navigation
block 110 throughout the anatomical area of interest in the patient
14.
[0058] The ECG gating at blocks 86 and 108 will now be fully
described with respect to FIGS. 4-7. The ECG gating signals
generated by blocks 86 and 108 may use different characteristics of
the ECG signal to generate gating signal, and may include the use
of an R-wave detector 112 as shown in FIG. 4, or a diastole
detector 140 as shown in FIGS. 5 and 6. In addition, the ECG gating
signals generate at blocks 86 and 108 may include the use of an
onset R-wave detector as shown in FIG. 7. Each of these detectors
for ECG gating at blocks 86 and 108 will now be described in
greater detail.
[0059] As discussed above, the ECG gating performed at blocks 86
and 108 may include an R-wave detector 112 as illustrated in FIG.
4. The R-wave detector 112 has two phases: a learning phase 114 and
a detection phase 116. In this regard, the threshold
characteristics associated with the ECG signals from the patient
are initially acquired during the learning phase 114. Once the
learning phase 114 has been completed, the detection phase 116
associated with the R-wave detector 112 is performed which
generates a gating signal by comparing the characteristics of the
current ECG signal to the thresholds calculated during the learning
phase 114.
[0060] As shown in FIG. 4, the learning phase 114 of the R-wave
detector 112 includes block 118 in which the ECG signal is
initially acquired. Once the ECG signal is acquired at block 118,
the learning phase 114 calculates certain characteristics of the
ECG signal at block 120 including slew, turning point and
amplitude. The slew of the ECG signal is the slope of the ECG
signal taken by selecting 10 samples within a 25 millisecond
window. The turning point represents the running sum of 20 samples
taken at a rate of 400 samples per second and represents a near
term extrema of the ECG signal. The amplitude determined at block
120 is simply the amplitude of a ECG signal.
[0061] Once the slew, turning point and amplitude of the ECG signal
are calculated at the block 120, the learning phase 114 of the
R-wave detector 112 adds these values to a threshold database at
block 122. The threshold database contains a 10 period moving
average of the slew, turning point and amplitude of the ECG
signals. Once the values for slew, turning point and amplitude have
been added to the threshold database at block 122, the learning
phase 114 determines whether the learning phase is completed at
block 124 by calculating whether 10 cardiac cycles have occurred
since the learning phase 114 began. If 10 cardiac cycles have
occurred, then the R-wave detector 112 initiates the detection
phase 116. However, if fewer then 10 cardiac cycles have occurred,
the learning phase 114 acquires another ECG signal at block
118.
[0062] As discussed above, if the learning phase 114 has obtained
information from 10 cardiac cycles, the R-wave detector 112 then
initiates the detection phase 116. As shown in FIG. 4, the
detection phase 116 initially acquires the current ECG signal at
block 126. After the current ECG signal is acquired at block 126,
the detection phase 116 then calculates the slew, turning point and
amplitude of the current ECG signal at block 128. The slew, turning
point and amplitude are calculated in the same manner as discussed
above with respect to block 120.
[0063] After the slew, turning point and amplitude have been
calculated at block 128, the detection phase 116 then determines
whether the slew of the current ECG signal is greater than a
threshold at block 130. In this regard, the slew threshold may be
about 0.9 times the average slew that is stored in the threshold
database at block 122 during the learning phase 114. It will be
understood, however, that other values for the slew threshold may
be used. If the slew of the current ECG signal is greater than the
threshold as determined at block 130, then the detection phase 116
determines whether the amplitude of the current ECG signal is
greater than an amplitude threshold. In this regard, the detection
phase 116 determines whether the amplitude of the current ECG
signal is greater than about 0.9 times the average amplitude of the
ECG signal stored in the threshold database at block 122 during the
learning phase 114. If the amplitude of the current ECG signal is
less than the amplitude threshold as compared at block 132, a new
sample is acquired at block 126. However, if the amplitude of the
current ECG signal is greater than the amplitude threshold as
determined by block 132, then the detection phase 116 compares at
block 134 the turning point of the current ECG signal to the
turning point threshold. In this regard, the detection phase 116
determines whether the turning point of the current ECG signal is
greater or lesser than the turning point threshold stored in the
threshold database at block 122. If the turning point of the
current ECG signal is less than the turning point threshold
determined at block 134, then the detection phase 116 assumes that
the current ECG signal does not contain an R-wave and therefore a
new sample is acquired at block 126.
[0064] However, if the turning point of the current ECG sample is
greater than the turning point threshold as determined at block
134, then the detection phase 116 assumes that the ECG signal
contains an R-wave. When this occurs, a gating signal is generated
by block 138 following a delay period from onset of the R-wave that
was detected. For example, the detection phase 116 may generate a
gating signal after a delay of 70% of the interval between adjacent
R-waves (hereinafter the "R-R interval"). In this case, if the
temporal spacing between two adjacent R-waves is 670 milliseconds,
a gating signal may be generated after a delay of approximately 469
milliseconds after the R-wave that was detected. However, other
delay periods may be suitable.
[0065] As discussed above, the ECG gating at blocks 86 and 108 may
also include the use of a diastole detector 140 as illustrated in
FIGS. 5 and 6. The diastole detector 140 has a learning phase 142
and a gating phase 144. The learning phase 142 of the diastole
detector 140 calculates the mean and standard deviation of the R-R
interval as will be fully discussed below. The gating phase 144 of
the diastole detector 140 is used for confirming that a diastolic
region of the ECG signal is present before causing a gating signal
to be generated at blocks 86 and 108. The learning phase 142 and
the gating phase 144 of the diastole detector 140 will now be
described in greater detail.
[0066] In the learning phase 142 of the diastole detector 140, the
presence of an R-wave is first detected at block 146. In this
regard, the R-wave detector 112, as shown in FIG. 4, may be used
for detecting the presence of an R-wave. However, other suitable
R-wave detectors may be used. After the R-wave is detected at block
146, the time interval between the current R-wave and immediately
proceeding R-wave is calculated at block 148. After the R-R
interval has been calculated at block 148, the learning phase 142
determines whether the R-R interval is too short at block 150
(e.g., when arrhythmias may have occurred). In one embodiment, the
R-R interval may be too short if the R-R interval is less than
about 300-350 milliseconds. If the R-R interval is too short as
determined by block 150, the learning phase 142 then waits until
the next R-wave occurs as indicated by block 146. If the R-R
interval is sufficiently long as determined by block 150, the
learning phase 142 then adds the R-R interval as well as the slew
to the interval database at block 152. Once the R-R interval is
added to the interval database, the learning phase 142 then
calculates the mean and standard deviation of the R-R intervals
stored in the database, as well as determines the minimum slew of
the slew data of the ECG signals that have been evaluated during
the learning phase 142. After the mean and standard deviation of
the R-R interval are calculated, the diastole detector 140
determines whether the learning phase is complete at block 156. The
learning phase may be determined to be complete after it has
processed 10 cardiac cycles of sufficient length, as described
above.
[0067] Once the learning phase 156 is complete, the gating phase of
the diastole detector 140 is initiated. In this regard, the gating
phase 144 initially determines the location of a diastolic
detection window. The diastolic detection window is the region of
the ECG signal in which the heart is believed to be in diastole,
and in which ECG signals are evaluated to confirm the heart is in
diastole. The width of the diastolic detection window may be about
75 milliseconds, though other suitable widths may be used. The
location of the diastolic detection window with respect to the
current ECG signal may be determined by the variation of the R-R
interval calculated during the learning phase 142. In this regard,
if there is a relatively high standard deviation in the R-R
interval as calculated at block 154, there is a relatively high
likelihood of that the ECG signal may contain arrhythmias or
ectopic beats. Under these circumstances, it may be desirable to
center the diastolic detection window at approximately 45% of the
R-R interval following the onset of an R-wave. In contrast, if the
standard deviation of the R-R interval calculated at block 154 is
relatively small, then it is likely that the ECG signal corresponds
to normal sinus rhythms. In this case, the diastolic detection
window may be centered later in the cardiac cycle, such as at 70%
(or about 62% to about 80%) of the duration R-R interval following
the detection of an R-wave. It is to be understood, however, that
the diastolic detection window may be centered at other suitable
locations.
[0068] After the location of the diastolic detection window is
determined at 158, the gating phase 144 detects the presence of an
R-wave block 160 (such as by using the R-wave detector 112). Once
an R-wave is detected at block 160, the current ECG signals located
in the diastolic detection window are recorded during the diastolic
detection window by waiting until the diastolic detection window
opens by means of blocks 162 and 164. Once the diastolic detection
window has opened, the gating phase 144 determines whether an
R-wave has occurred within the diastolic detection window at block
166. This may be performed using the R-wave detector 112 shown in
FIG. 4, though other suitable R-wave detectors may be used. If an
R-wave has occurred during the diastolic detection window, the
gating phase 144 may prevent the generation of a gating signal at
blocks 86 and 108 by waiting until the occurrence of the next
R-wave as indicated by block 160. If no R-wave has been detected
during the diastolic detection window as determined by block 166,
then the gating phase 144 may determine whether the slew associated
with the current ECG signal is relatively high. In this regard, the
gating phase 144 may select 30 samples (at a rate of 400 samples
per second) and determine whether the slew is greater than 5 times
the minimum slew as calculated at block 154. If the slew of the ECG
signal is sufficiently high, the gating phase 144 may not cause the
generation of a gating signal from blocks 86 and 108 but waits
until the next R-wave is detected at block 160.
[0069] If the slew of the current ECG signal is not high as
determined at block 168, the gating phase 144 may determine whether
there is a DC offset associated with the current ECG signal. If
there is a DC offset to the current ECG signal, then the gating
phase 144 may also not cause the generation of a gating signal at
blocks 86 and 108 but waits until another R-wave is detected at
block 160. If there is no DC offset, then the gating phase 144 may
trigger the generation of a gating signal at blocks 86 and 108 as
indicated by block 172.
[0070] It will be appreciated that the check for DC offset at block
170 may occur during normal sinus rhythms and may not generally be
necessary when arrhythmias may be present. In addition, also during
normal sinus rhythms, once a gating signal is generated by blocks
86 and 108, the gating phase 144 may prevent the generation of
another gating signal from blocks 86 and 108 for a period of time
(such as, for example, 70% of the R-R interval following the next
R-wave).
[0071] As discussed above, the ECG gating at blocks 86 and 108 can
also include an onset R-wave detector 174 as illustrated in FIG. 7.
The onset R-wave detector 174 is used for sensing the onset of an
R-wave during a cardiac cycle, and causing the navigation system to
display the previously acquired image of the catheter when the
heart was in diastole. As indicated by block 176, the onset R-wave
detector 174 initially acquires the ECG signal as well as the
catheter image. Once the ECG signal and catheter image has been
acquired at block 176, the onset R-wave detector 174 may calculate
the slew and amplitude characteristics of the ECG signal at block
178. In addition, the onset R-wave detector 174 may store the
catheter image in a frame buffer at block 180. If the slew or the
amplitude of the ECG signal exceed respectively thresholds (as
determined by blocks 182 and 184), the onset R-wave detector 174
determines that R-wave onset has occurred and, therefore, retrieves
the previous catheter image from the frame buffer for use to
register the image of the catheter as illustrated by block 186. If
either of the slew or the amplitude are not greater than their
respective thresholds as determined by block 182 and 184, then the
current image is used for purposes of registration and a new ECG
signal and catheter image are then obtained at block 176.
[0072] The description of the invention is merely exemplary in
nature and, thus, variations that do not depart from the gist of
the invention are intended to be within the scope of the invention.
Such variations are not to be regarded as a departure from the
spirit and scope of the invention.
* * * * *