U.S. patent application number 11/932227 was filed with the patent office on 2008-05-22 for mri compatible medical leads with band stop filters.
This patent application is currently assigned to Surgi-Vision, Inc.. Invention is credited to Ergin Atalar, Robert Susil.
Application Number | 20080119919 11/932227 |
Document ID | / |
Family ID | 37235464 |
Filed Date | 2008-05-22 |
United States Patent
Application |
20080119919 |
Kind Code |
A1 |
Atalar; Ergin ; et
al. |
May 22, 2008 |
MRI COMPATIBLE MEDICAL LEADS WITH BAND STOP FILTERS
Abstract
Herein is disclosed an MRI compatible medical lead with band
stop filters in communication with a respective lead conductor and
electrode tuned to resonate at selected (MRI) high frequencies.
Inventors: |
Atalar; Ergin; (Columbia,
MD) ; Susil; Robert; (Baltimore, MD) |
Correspondence
Address: |
MYERS BIGEL SIBLEY & SAJOVEC
PO BOX 37428
RALEIGH
NC
27627
US
|
Assignee: |
Surgi-Vision, Inc.
|
Family ID: |
37235464 |
Appl. No.: |
11/932227 |
Filed: |
October 31, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10123534 |
Apr 15, 2002 |
|
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11932227 |
|
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60283725 |
Apr 13, 2001 |
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Current U.S.
Class: |
607/116 |
Current CPC
Class: |
A61N 1/056 20130101;
A61N 1/3718 20130101; A61N 1/086 20170801; A61B 5/283 20210101;
A61N 1/05 20130101; A61B 18/14 20130101 |
Class at
Publication: |
607/116 |
International
Class: |
A61N 1/05 20060101
A61N001/05 |
Goverment Interests
GOVERNMENT GRANTS
[0002] This invention was made, in part, with government support
under grant numbers RO1 HL 57483 and RO1 HL61672 from the National
Institutes of Health. The United States government has certain
rights to this invention.
Claims
1. An MRI compatible medical diagnostic or therapeutic device,
comprising: a medical device including a lead wire extending
therefrom, the device adapted for contact with biological cells;
and a band stop filter associated with the lead wire for
attenuating current flow through the lead wire at a selected
frequency or frequency band, wherein the band stop filter comprises
a capacitor in parallel with an inductor, said parallel capacitor
and inductor placed in series with the lead wire, wherein values of
capacitance and inductance are selected such that the band stop
filter is resonant at the selected frequency or frequency band.
2. The device of claim 1, wherein the band stop filter attenuates
current flow through the lead wire at selected MRI frequencies.
3. The device of claim 1, wherein the band stop filter is proximate
an implantable electrode.
4. A process for attenuating current flow through a lead wire for a
medical device at a selected frequency, comprising the steps of:
selecting a capacitor which is resonant at the selected frequency;
selecting an inductor which is resonant at the selected frequency;
using the capacitor and the inductor to form a tank filter circuit;
and placing the tank filter circuit in series with the lead
wire.
5. The process of claim 4, including the step of configuring the
tank filter circuit to attenuate current flow through the lead wire
along a range of selected frequencies.
6. The process of claim 4, wherein the range of selected
frequencies includes a plurality of MRI pulse frequencies.
7. The process of claim 4, including the step of disposing the tank
filter circuit at the distal end portion of the lead wire.
8. The process of claim 6, including the step of placing the tank
filter circuit proximate an electrode.
9. The process of claim 8, wherein the electrode is held by a probe
or catheter.
10. A medical device comprising: a probe or catheter having
electrodes adapted to contact biological cells and at least one
conductor in communication with each electrode; and at least one
band stop filter associated with the electrodes for attenuating
current flow through the probe or catheter at a selected frequency
or frequency range, wherein the band stop filter comprises a
capacitor in parallel with an inductor, said parallel capacitor and
inductor placed in series with at least one of the electrodes,
wherein the band stop filter is resonant at the selected frequency
or frequency range, and wherein the band stop filter is configured
to attenuate current flow through the conductors at the selected
frequency or frequency range.
11. The device of claim 10, wherein the range of selected
frequencies includes a plurality of MRI pulse frequencies.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of U.S. patent
application Ser. No. 10/123,534, filed Apr. 15, 2002, which claims
the benefit of priority of U.S. Provisional Patent Application Ser.
No. 60/283,725, filed Apr. 13, 2001. The aforementioned
applications are incorporated herein in their entireties by this
reference.
BACKGROUND
[0003] The disclosed systems and methods relate in general to
ablation and electrophysiologic diagnostic and therapeutic
procedures, and in particular to systems and methods for
performing, guiding, and providing visualization of such
procedures.
[0004] Atrial fibrillation and ventricular tachyarrhythmias
occurring in patients with structurally abnormal hearts are of
great concern in contemporary cardiology. They represent the most
frequently encountered tachycardias, account for the most morbidity
and mortality, and, despite much progress, remain therapeutic
challenges.
[0005] Atrial fibrillation affects a larger population than
ventricular tachyarrhythmias, with a prevalence of approximately
0.5% in patients 50-59 years old, increasing to 8.8% in patients in
their 80's. Framingham data indicate that the age-adjusted
prevalence has increased substantially over the last 30 years, with
over 2 million people in the United States affected. Atrial
fibrillation usually accompanies disorders such as coronary heart
disease, cardiomyopathies, and the postoperative state, but occurs
in the absence of any recognized abnormality in 10% of cases.
Although it may not carry the inherent lethality of a ventricular
tachyarrhythmia, it does have a mortality twice that of control
subjects. Symptoms which occur during atrial fibrillation result
from the often rapid irregular heart rate and the loss of
atrio-ventricular (AV) synchrony. These symptoms, side effects of
drugs, and most importantly, thrombo-embolic complications in the
brain (leading to approximately 75,000 strokes per year), make
atrial fibrillation a formidable challenge.
[0006] Two strategies have been used for medically managing
patients with atrial fibrillations. The first involves rate control
and anticoagulation, and the second involves attempts to restore
and maintain sinus rhythm. The optimal approach is uncertain. In
the majority of patients, attempts are made to restore sinus rhythm
with electrical or pharmacologic cardioversion. Current data
suggest anticoagulation is needed for 3 to 4 weeks prior to and 2
to 4 weeks following cardioversion to prevent embolization
associated with the cardioversion. Chronic antiarrhythmic therapy
may be indicated once sinus rhythm is restored. Overall,
pharmacologic, therapy is successful in maintaining sinus rhythm in
30 to 50% of patients over one to two years of follow-up. A major
disadvantage of antiarrhythmic therapy is the induction of
sustained, and sometimes lethal, arrhythmias (proarrhythmia) in up
to 10% of patients.
[0007] If sinus rhythm cannot be maintained, several approaches are
used to control the ventricular response to atrial fibrillation.
Pharmacologic agents which slow conduction through the AV node are
first tried. When pharmacologic approaches to rate control fail, or
result in significant side effects, ablation of the AV node, and
placement of a permanent pacemaker may be considered. The
substantial incidence of thromboembolic strokes makes chronic
anticoagulation important, but bleeding complications are not
unusual, and anticoagulation cannot be used in all patients.
[0008] In addition to medical management approaches, surgical
therapy of atrial fibrillation has also been performed. The
surgical-maze procedure, developed by Cox, is an approach for
suppressing atrial fibrillation while maintaining atrial functions.
This procedure involves creating multiple linear incisions in the
left and night atria. These surgical incisions create lines that
block conduction and compartmentalize the atrium into distinct
segments that remain in communication with the sinus node. By
reducing the mass of atrial tissue in each segment, the mass of
atrial tissue is insufficient to sustain the multiple reentrant
rotors, which are the basis for atrial fibrillation. Surgical
approaches to the treatment of atrial fibrillation result in an
efficacy of >95% and a low incidence of complications. However,
despite these encouraging results, this procedure has not gained
widespread acceptance because of the long duration of recovery and
risks associated with cardiac surgery.
[0009] Invasive studies of the electrical activities of the heart
(electrophysiologic studies) have also been used in the diagnosis
and therapy of arrhythmias. Focal atrial tachycardias, AV-nodal
reentrant tachycardias, accessory pathways, atrial flutter, and
idiopathic ventricular tachycardia can be cured by selective
destruction of critical electrical pathways with radiofrequency
(RF) catheter ablation. Electrophysiologists have attempted to
replicate the maze procedure using RF catheter ablation. The
procedure is arduous, requiring general anesthesia and procedure
durations often greater than 12 hours, with exposure to ionizing
x-ray irradiation for over 2 hours. Some patients have sustained
cerebrovascular accidents. One of the main limitations of the
procedure is the difficulty associated with creating and confirming
the presence of continuous linear lesions in the atrium. If the
linear lesions have gaps, then activation can pass through the gap
and complete a reentrant circuit, thereby sustaining atrial
fibrillation or flutter. This difficulty contributes significantly
to the long procedure durations discussed above.
[0010] Creating and confirming continuous linear lesions and
morbidity could be facilitated by improved minimally-invasive
techniques for imaging lesions created in the atria. Such an
imaging technique may allow the procedure to be based purely on
anatomic findings.
[0011] The major technology for guiding placement of a catheter is
x-ray fluoroscopy. For electrophsiologic studies and ablation,
frame rates of 7-15 per second are generally used which allows an
operator to see x-ray-derived shadows of the catheters inside the
body. Since x-rays traverse the body from one side to the other,
all of the structures that are traversed by the x-ray beam
contribute to the image. The image, therefore is a superposition of
shadows from the entire thickness of the body. Using one
projection, therefore, it is only possible to know the position of
the catheter perpendicular to the direction of the beam. In order
to gain information about the position of the catheter parallel to
the beam, it is necessary to use a second beam that is offset at
some angle from the original beam, or to move the original beam to
another angular position. Since x-ray shadows are the superposition
of contributions from many structures, and since the discrimination
of different soft tissues is not great, it is often very difficult
to determine exactly where the catheter is within the heart. In
addition, the borders of the heart are generally not accurately
defined, so it is generally not possible to know if the catheter
has penetrated the wall of the heart, and lesions are invisible
under x-ray fluoroscopy. Thus, it is very difficult to discern
whether tissue has been adequately ablated. The intracardiac
electrogram may be used to guide the catheters to the proper
cardiac tissue.
[0012] Intracardiac ultrasound has been used to overcome
deficiencies in identifying soft tissue structures. With ultrasound
it is possible to determine exactly where the walls of the heart
are with respect to a catheter and the ultrasound probe, but the
ultrasound probe is mobile, so there can be doubt where the
absolute position of the probe is with respect to the heart.
[0013] Neither x-ray fluoroscopy nor intracardiac ultrasound have
the ability to accurately and reproducibly identify areas of the
heart that have been ablated.
[0014] A system known as "non-fluoroscopic electro-anatomic
mapping" (U.S. Pat. No. 5,391,199 to Ben-Haim), was developed to
allow more accurate positioning of catheters within the heart. That
system uses weak magnetic fields and a calibrated magnetic field
detector to track the location of a catheter in 3-space. The system
can mark the position of a catheter, but the system relies on
having the heart not moving with respect to a marker on the body.
The system does not obviate the need for initial placement using
x-ray fluoroscopy, and cannot directly image ablated tissue.
[0015] Magnetic resonance imaging (MRI) is a known imaging
technique which uses high-strength magnetic and electric fields to
image the body. A strong static magnetic field orients the magnetic
moments of the hydrogen nuclei creating a bulk nuclear
magnetization. RF magnetic field pulses with frequency tuned to the
resonant or "Larmor" frequency in the presence of the static field,
change the spatial orientation of the bulk magnetization of the
nuclei, as known to those skilled in the art. In addition,
time-varying gradient magnetic fields applied in the three
Cartesian directions (X, Y, Z) are used for spatial encoding of the
signals from the tissue. The magnitude of a gradient magnetic field
is such as to cause the main static magnetic field to vary linearly
as a function of the respective spatial coordinate in the magnet.
As a result of the addition of the static and gradient magnetic
fields, the local Larmor resonance frequency, is spatially encoded.
The information corresponding to the strength of the magnetization
at each point in space can then be decoded by means of
reconstruction techniques known to those skilled in the art,
permitting the imaging of tissues in three-dimensional space.
[0016] MRI has been used to guide procedures in which RF energy is
applied to non-contractile organs such as the brain, liver and
kidneys to ablate tumors. However, these systems are not suitable
for use in the heart. U.S. Pat. No. 5,323,778 to Kandarpa et al.
discloses a method and apparatus for MRI and tissue heating. There
is no provision in the disclosed probe for measuring electrical
signals, and it is unclear how much resolution is provided by the
probe.
SUMMARY
[0017] The systems and methods disclosed herein enhance the art by,
among other things, facilitating the performance of multiple
functions during electrophysiological interventions with MRI. An
embodiment provides an instrument that can be easily visualized
and/or tracked in an MR or other image. An embodiment provides an
instrument that can be easy to maneuver. An embodiment provides an
instrument that can facilitate high resolution imaging of a target
area. An embodiment provides an instrument that can record an
intracardiac electrogram. An embodiment provides an instrument that
can deliver RF energy for ablation of the desired tissue near the
instrument tip.
[0018] An embodiment provides an improved multi-functional systems
and methods for guiding and/or providing visualization during
electrophysiologic procedures.
[0019] In an embodiment, the disclosed systems and methods
facilitate guiding or visualizing ablation procedures suitable for
use in, e.g., the heart and other structures.
[0020] In an embodiment, the disclosed systems and methods include
catheters that are easy to maneuver and/or track.
[0021] In an embodiment, the disclosed systems and methods
facilitate imaging ablation lesions with increased resolution of
the target area and reliability.
[0022] In an embodiment, the disclosed systems and methods
facilitate delivering RF energy for ablation of tissue, including
but not limited to pathologic tissue.
[0023] In an embodiment, the disclosed systems and methods
facilitate recording and monitoring electrical potentials,
including but not limited to physiologic bio-potentials.
[0024] In an embodiment, the disclosed systems and methods
facilitate determining the position of, e.g., target tissue or an
ablation instrument.
[0025] In an embodiment, the disclosed systems and methods
facilitate guiding the delivery of RF ablation energy.
[0026] In an embodiment, the disclosed systems and methods
facilitate the integration of imaging and ablation in a single
instrument.
[0027] In an embodiment, the disclosed systems and methods
facilitate using magnetic resonance imaging to increase the safety
and accuracy of electrophysiologic procedures.
[0028] In an embodiment, the disclosed systems and methods provide
an invasive multi-functional electrophysiology and imaging antenna
catheter which includes an RF antenna for receiving magnetic
resonance signals, a loop antenna for device tracking, an ablation
tip for delivering RF energy for ablation procedures, and
diagnostic electrodes for receiving physiological electrical
potentials. The combined electrophysiology and imaging antenna
catheter can be used in combination with a MRI scanner to, for
example, guide, perform, and/or provide visualization during
electrophysiologic diagnostic or therapeutic procedures.
[0029] In an embodiment, the disclosed systems and methods can be
particularly applicable to ablation of atrial and ventricular
arrhythmias, and in such embodiments may be used as an intracardiac
device to both deliver energy to selected areas of tissue and
visualize the resulting ablation lesions, thereby greatly
simplifying production of continuous linear lesions. Additionally,
the disclosed systems and methods can be used as active tracking
devices by means of a "loop antenna" that receives MRI signals
excited by the scanner. Gradient echoes are then generated along
three orthogonal axes to frequency encode the location of the coil
and thus provide the three-dimensional (3D) space coordinates of
the electrode tip. These numeric coordinates can then be used to
control the imaging plane of the scanner, thereby allowing accurate
imaging slices to be prescribed to target anatomy for RF therapy.
Low resolution images can be obtained indicating catheter location
relative to the body by combining the MRI signals from the loop
with those of the conventional external MRI detector coil. In
addition, high resolution images can be obtained using the signals
from the loop antenna. In another embodiment, the loop antenna
utilized in the combined electrophysiology and imaging catheter for
receiving MRI signals is connected to form a "loopless" type
antenna. High-resolution images from the antenna may be combined
with low-resolution images from surface coils of the MR scanner to
produce a composite image. The disclosed systems and methods
further include embodiments useful for guiding electrophysiologic
diagnostic and therapeutic procedures other than ablation. An RF
filtering system is provided for suppressing the MRI signal while
not attenuating the RF ablative current. Steering means may be
provided for steering the invasive catheter under MR guidance,
Steering could be facilitated by proving a probe with a pull wire.
In addition, the disclosed systems and methods include acquisition
of local physiological bio-potential measurements using a
multi-electrode catheter, which permits, via MRI guidance, active
tracking of the location of each electrode. A central novel feature
of the disclosed systems and methods is that because
bio-potentials, RF ablation, and MRI are each performed over
different frequency ranges, frequency-dependent circuit elements
can be used to change the catheter's electrical structure enabling
it to provide multiple electrical structures within a single
physical structure.
[0030] In an embodiment, a probe or catheter includes a first
electrode disposed at least partially on the probe surface, a
second electrode disposed at least partially on the probe surface,
a first conductor electrically coupled to the first electrode, a
second conductor electrically coupled to the second electrode, and
a reactive element electrically coupling the first conductor and
the second conductor.
[0031] In an embodiment, a magnetic resonance imaging probe
includes a coaxial cable having an inner conductor and an outer
shield, a first split ring electrode electrically coupled to the
inner conductor, and a second split ring electrode electrically
coupled to the outer conductor, a first center split ring electrode
electrically coupled to the first split ring electrode and to a
first conductor, a second center split ring electrode electrically
coupled to the first center split ring electrode and to the second
split ring electrode, and also coupled to a second conductor.
[0032] In an embodiment, a magnetic resonance imaging probe
includes a first electrode disposed on the probe surface, a second
electrode disposed on the probe surface, a first conductor
electrically coupled to the first electrode through a reactance, a
second conductor electrically coupled to the second electrode
through a reactance, and a frequency-dependent reactive element
electrically coupling the first conductor and the second conductor,
such that high-frequency energy is conducted between the first
conductor and the second conductor.
[0033] In an embodiment, a system for magnetic resonance imaging
includes a magnetic resonance imaging probe, having a first
electrode disposed on the probe surface, a second electrode
disposed on the probe surface, a first conductor electrically
coupled to the first electrode through a reactance, a second
conductor electrically coupled to the second electrode through a
reactance, and a frequency-dependent reactive element electrically
coupling the first conductor and the second conductor, such that
high-frequency energy is conducted between the first conductor and
the second conductor; and an interface electrically coupled to the
probe, the interface having a tuning/matching/decoupling circuit
and a signal splitting circuit; and an MRI scanner electrically
coupled to the interface.
[0034] In an embodiment, a magnetic resonance imaging probe
includes a coaxial cable having an inner conductor and an outer
shield, and a split ring electrode having a first portion and a
second portion, the first portion being electrically coupled to the
inner conductor, and the second portion being electrically coupled
to the outer shield.
[0035] In an embodiment, a magnetic resonance imaging probe may
include a coaxial cable having an inner conductor and an outer
shield, a first split ring electrode electrically coupled to the
inner conductor, and a second split ring electrode electrically
coupled to the outer conductor; wherein the first split ring and
the second split ring are electrically coupled by a first reactive
element.
[0036] In an embodiment, the reactive element can conduct a high
frequency signal between the first conductor and the second
conductor. In an embodiment, the high frequency signal has a
frequency higher than about 10 MegaHertz (MHz). In an embodiment,
the reactive element can conduct a signal having magnetic resonance
imaging frequency energy between the first conductor and the second
conductor.
[0037] In an embodiment, at least one of the first conductor and
the second conductor can conduct a low frequency signal to at least
one of the first electrode and the second electrode. In an
embodiment, the low frequency signal has a frequency of up to about
500 kiloHertz (kHz). In an embodiment, the low frequency is in the
range from about 100 Hertz (Hz) to about 1 kHz. In an embodiment,
the frequency is about 100 kHz.
[0038] In an embodiment, the reactive element conducts a signal
having ablation frequency energy to at least one of the first
electrode and the second electrode. In an embodiment, the reactive
element conducts a signal having biopotential recording frequency
energy to at least one of the first electrode and the second
electrode.
[0039] In an embodiment, the probe further comprises a lumen.
[0040] In an embodiment, the reactive element comprises at least
one of a high-pass filter, a low-pass filter, a band-pass filter,
and a capacitor. In an embodiment, the reactance comprises at least
one of an inductor and an LC circuit.
[0041] In an embodiment, the first conductor couples to the first
electrode through a reactance. In an embodiment, at least one of
the first conductor, the second conductor, the first electrode, and
the second electrode comprises at least one of a magnetic resonance
compatible material, a superelastic material, copper, gold, silver,
platinum, iridium, MP35N, tantalum, titanium, Nitinol, L605,
gold-platinum-iridium, gold-copper-iridium, and gold-platinum.
[0042] In an embodiment, the first conductor and the second
conductor are electrically coupled to a tuning/matching/decoupling
circuit. In an embodiment, the first conductor and the second
conductor are electrically coupled to a signal splitting circuit.
In an embodiment, the first conductor and the second conductor are
electrically coupled by at least one capacitor.
[0043] An embodiment may further include a third conductor
electrically coupled to a third electrode, and a fourth conductor
electrically coupled to a fourth electrode, wherein a first signal
having high frequency energy is conducted between the first
conductor and the second conductor through the reactive element,
and a second signal having low frequency energy is conducted to at
least one of the third electrode and the fourth electrode.
[0044] An embodiment may further include a shaft, the shaft
including at least one of Kevlar, nylon, Teflon, polyethylene,
polyolefin, PTFE, polyurethane, PEBAX, braided Kevlar, and braided
nylon. In an embodiment, the probe surface is covered by a
lubricious coating.
[0045] In an embodiment, the probe has an outer diameter in the
range of about 1 French to about 15 French. In an embodiment, the
probe has a length in the range of about 50 cm to about 200 cm. In
an embodiment, the probe further comprises a pull wire.
[0046] In an embodiment, the first conductor, the reactive element,
and the second conductor form a loop antenna. In an embodiment, the
first conductor, the reactive element, and the second conductor
form a loopless antenna.
[0047] In an embodiment, the inner conductor and the outer shield
are electrically coupled by a reactive element. In an embodiment,
the reactive element comprises at least one of a high-pass filter,
a low-pass filter, a band-pass filter, and a capacitor. In an
embodiment, the inner conductor and the outer shield are
electrically coupled by a second reactive element. In an
embodiment, the second reactive element comprises at least one of a
high-pass filter, a low-pass filter, a band-pass filter, and a
capacitor. In an embodiment, the first reactive element comprises
at least one of a high-pass filter, a low-pass filter, a band-pass
filter, and a capacitor.
[0048] In an embodiment, a method for simultaneously imaging and
ablating a tissue can include exposing the tissue to a magnetic
field, the field having a static component and a gradient
component, placing a probe adjacent to the tissue, the probe
including a first electrode disposed at least partially on the
probe surface, a second electrode disposed at least partially on
the probe surface, a first conductor electrically coupled to the
first electrode, a second conductor electrically coupled to the
second electrode, and a frequency-dependent reactive element
electrically coupling the first conductor and the second conductor,
such that high-frequency energy is conducted between the first
conductor and the second conductor, and low frequency energy is
conducted to at least one of the first electrode and the second
electrode; directing low-frequency energy to the probe, the low
frequency energy being conducted to the tissue by at least one of
the first electrode and the second electrode; and receiving
high-frequency energy from at least one of the first conductor and
the second conductor for imaging at least one of the probe and the
tissue.
[0049] In an embodiment, a method for simultaneously imaging a
tissue and measuring a bioelectric potential in the tissue may
include exposing the tissue to a magnetic field, the field having a
static component and a gradient component, placing a probe adjacent
to the tissue, the probe including a first electrode disposed at
least partially on the probe surface, a second electrode disposed
at least partially on the probe surface, a first conductor
electrically coupled to the first electrode, a second conductor
electrically coupled to the second electrode, and a
frequency-dependent reactive element electrically coupling the
first conductor and the second conductor, such that high-frequency
energy is conducted between the first conductor and the second
conductor, and low frequency energy is conducted to at least one of
the first electrode and the second electrode; receiving
low-frequency energy from the probe, the low frequency energy being
conducted from at least one of the first electrode and the second
electrode; and receiving high-frequency energy from at least one of
the first conductor and the second conductor for imaging at least
one of the probe and the tissue.
[0050] In an embodiment, a method for simultaneously imaging a
tissue, ablating the tissue, and measuring a bioelectric potential
in the tissue may include exposing the tissue to a magnetic field,
the field having a static component and a gradient component,
placing a probe adjacent to the tissue, the probe including a first
electrode disposed at least partially on the probe surface, a
second electrode disposed at least partially on the probe surface,
a first conductor electrically coupled to the first electrode, a
second conductor electrically coupled to the second electrode, and
a frequency-dependent reactive element electrically coupling the
first conductor and the second conductor, such that high-frequency
energy is conducted between the first conductor and the second
conductor, and low-frequency and medium-frequency energy is
conducted to at least one of the first electrode and the second
electrode; receiving low-frequency energy from the probe, the low
frequency energy being conducted from at least one of the first
electrode and the second electrode, directing medium-frequency
energy to the probe, the medium-frequency energy being conducted to
the tissue by at least one of the first electrode and the second
electrode, and receiving high-frequency energy from the probe, the
high-frequency energy having magnetic resonance imaging data.
[0051] In an embodiment, a method for simultaneously imaging and
treating a tissue may include exposing the tissue to a magnetic
field, the field having a static component and a gradient
component, placing a probe adjacent to the tissue, the probe
including a first electrode disposed at least partially on the
probe surface, a second electrode disposed at least partially on
the probe surface, a first conductor electrically coupled to the
first electrode, a second conductor electrically coupled to the
second electrode, and a frequency-dependent reactive element
electrically coupling the first conductor and the second conductor,
such that high-frequency energy is conducted between the first
conductor and the second conductor; delivering a therapy to the
tissue, and receiving high-frequency energy from the probe, the
high-frequency energy having magnetic resonance imaging data.
[0052] In an embodiment, the therapy may include at least one of
ablation energy, heat, ultrasound energy, a substance discharged
through a lumen of the probe, and monitoring the delivering.
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] Embodiments of the disclosed systems and methods will be
apparent from the following more particular description of
preferred embodiments as illustrated in the accompanying drawings,
in which some reference characters refer to the same parts
throughout the various views. The drawings are not necessarily to
scale, emphasis instead being placed upon illustrating principles
of the disclosed systems and methods.
[0054] FIGS. 1A-1G show physical and electrical diagrams of a probe
or catheter.
[0055] FIGS. 2A-2I show physical and electrical diagrams of a probe
or catheter and of an interface.
[0056] FIG. 3 shows a block diagram illustrating a system for
magnetic resonance imaging.
[0057] FIG. 4 shows data comparing the signal-to-noise ratio
performance of some exemplary embodiments.
[0058] FIG. 5 shows imaging and/or tracking data according to an
embodiment,
[0059] FIG. 6 shows imaging and/tracking data and biopotential data
according to an embodiment.
DETAILED DESCRIPTION
[0060] The disclosed systems and methods in an embodiment use MRI
to allow multi-functional catheters to be placed without radiation,
and provides very accurate localization of catheter tips in 3D
space. With current MRI scanners, resolution is limited by the
distance the RF coil is from the volume of tissue being imaged. The
signal from any particular imaging volume is picked up by an
external detector coil. The gradients select a volume inside the
body for imaging, but the coil outside the body picks up the signal
from the imaging volume as well as the noise from all of the sample
regions that are within its range. The farther the surface coil is
from the imaging volume, the more noise can be present.
[0061] MRI has been proposed as an alternative imaging modality for
guiding and monitoring EP procedures and offers several advantages
over other modalities for electrophysiologic intervention. For
example, the heart and endocardial landmarks can be visualized
throughout the procedure. This anatomical information facilitates
the identification of ablation sites for arrhythmias such as atrial
flutter and fibrillation. In addition, cardiac motion and flow
dynamics can be monitored during the procedure. This enables acute
assessment of cardiac function as the intervention is performed.
Furthermore, MRI does not employ ionizing radiation, such as
x-rays, and is generally considered to be a minimal risk
procedure.
[0062] MRI can facilitate the visualization of ablated tissue
during the procedure. Because lesions are typically invisible under
x-ray fluoroscopy, it is currently difficult to discern whether
tissue has been completely ablated, is only temporarily stunned
(i.e., has been subjected to reversible thermal injury), or indeed
has not been treated at all. By actually visualizing the tissue
lesion, MRI enables positive confirmation of tissue treatment. This
can be especially important for procedures that benefit from
continuous lines of ablation, such as treatment for atrial flutter,
atrial fibrillation, and isolation of pathologic tissue in the
pulmonary veins. Imaging of ablated tissue may allow what are now
long and difficult procedures to be performed on a more routine and
efficient basis.
[0063] In an embodiment, an internal catheter receiving
coil/antenna may be incorporated into an electrophysiologic RF
ablation catheter that also includes electrodes for recording
bio-potentials. Because the receiving coil/antenna is closer to the
imaging volume being targeted for ablation and may have a limited
range of detection, the MRI signal is increased and the noise from
remote regions reduced, thereby providing an enhanced
signal-to-noise ratio (SNR), which enables higher strength MRI
gradients to be applied, thereby improving resolution where it is
needed most.
[0064] In an embodiment, MRI can be used to facilitate catheter
ablation of atrial fibrillation by guiding creation of continuous
linear ablation lesions, and confirming that a complete linear
lesion has been created (line of block) in combination with
measurements of bio-potential. The visualization of areas of
ablation and measurements of bio-potential may allow a reduction in
the number of lesions needed, and may also reduce the number of
recurrences, by more accurately ablating the arrhythmias.
[0065] A reactive element can include an electrical component such
as a capacitor, a resistor, an inductor, a diode, or various
combinations of these. A reactive element can be a filter, such as
a lowpass filter, a highpass filter, a bandpass filter, or a
bandstop filter. A reactance can be a reactive element.
[0066] A high frequency signal or high frequency energy can include
energy that has a frequency of 10 MegaHertz (MHz) or higher. It can
include energy that has a frequency of 60 MHz or higher. It can
include energy that has a frequency of about 63.9 MHz.
[0067] A low frequency signal or low frequency energy can include
energy that has a frequency of less than 10 MHz. It can include
energy that has a frequency of 500 kiloHertz (kHz) or less. It can
include energy that has a frequency of 100 kHz or less. It can
include energy that has a frequency of 1 kHz or less. It can
include energy that has a frequency of 100 Hertz (Hz) or less.
[0068] Ablation frequency energy can include energy that has a
frequency of less than 10 MHz. It can include energy that has a
frequency of 500 kiloHertz (kHz) or less. It can include energy
that has a frequency of 100 kHz or less. It can include energy that
has a frequency of 1 kHz or less. It can include energy that has a
frequency of 100 Hertz (Hz) or less.
[0069] Biopotential recording frequency energy can include energy
that has a frequency of less than 10 MHz. It can include energy
that has a frequency of 500 kiloHertz (kHz) or less. It can include
energy that has a frequency of 100 kHz or less. It can include
energy that has a frequency of 1 kHz or less. It can include energy
that has a frequency of 100 Hertz (Hz) or less. It can include
energy that has a frequency between about 100 Hz and about 1
kHz.
[0070] Medium frequency energy can include energy that has a
frequency of less than 10 MHz and greater that 50 kHz. It can
include energy that has a frequency between 500 kiloHertz (kHz) and
50 kHz. It can include energy that has a frequency between 200 kHz
and 50 kHz. It can include energy that has a frequency of 100
kHz.
[0071] A catheter can be a probe. A probe can be a catheter.
[0072] In an embodiment, a lubricious material can include at least
one of polyvinylpyrrolidone, polyacrylic acid, hydrophilic
substance, or silicone.
[0073] FIG. 1A depicts the physical structure of a probe or
catheter according to an embodiment. A probe 10 can be about 10 cm
to about 1000 cm long. The probe 10 can be from about 50 cm to
about 200 cm long. The probe can be 100 cm long. The probe 10 can
be a catheter. The probe 10 may have two surface electrodes 14, 15.
The surface electrodes 14, 15 may be ring electrodes. The, are
located at the distal end, 15, and two wire leads, 4 and 5, run the
length of the catheter. The electrical structure of a conventional
electrophysiology catheter is shown in FIG. 1b. Note that the
electrical leads 4 and 5, terminate in respective surface
electrodes, 14 and 15. This arrangement can be less preferred for
MRI studies because of (i) potential safety issues arising from
heating induced in the wires and electrodes by the MRI excitation
field; (ii) application of RF energy for ablation may result in
significant degradation and interference during MRI scanning; and
(iii) the elements are not tuned to the MRI frequency and generally
will not provide improved SNR and MRI resolution performance.
Interfering signals and noise not related to MRI detection should
be limited, as should be currents induced directly in the
conductors by the time-dependent MRI fields. MRI tracking may be
facilitated be creating a small, confined region of high SNR.
[0074] It is known to those skilled in the art that long, flexible
loop antennas produce local regions of high signal and are
therefore ideal for active catheter tracking (For example, see
Atalar E, Bottomley P A, Ocali O, Correia L C, Kelemen M D, Lima J
A, Zerhouni E A. High resolution intravascular MRI and MRS by using
a catheter receiver coil. Magn Reson Med. 1996; 36:596-605). For a
catheter structure with two electrical leads, a loop antenna
configuration can be produced by short-circuiting the distal end of
the catheter. While this solution may be theoretically ideal, it is
unacceptable for electrophysiology interventions because in order
to perform ablation and to record of bio-potentials, multiple,
electrically isolated wire leads are preferred.
[0075] In an embodiment, a capacitor 20, is placed at the distal
end 1, of the catheter (FIG. 1c). In another preferred embodiment,
in addition to the capacitor 20 at the distal end of the catheter,
RF reactances 24 and 25 are applied in series with each of the
surface electrodes 14 and 15, in order to improve the safety of the
device for MRI applications. The component values of the reactances
24 and 25 are adjusted or tuned such as to produce a high impedance
at the MRI frequency, but low impedance at the frequency used for
RF ablation and/or measuring bio-potentials. More specifically, in
one embodiment, each of the reactances 24 and 25 may include a
non-magnetic RF inductor. In another preferred embodiment, each of
the reactances 24 and 25 may include an RF inductor-capacitor pair,
preferably non-magnetic, connected in parallel to form a parallel
resonant LC circuit with resonant tuned to substantially equal to
the MRI frequency, for example, 63.9 MHz with a static magnetic
field of 1.5 Tesla. By this means each LC pair provides a high
impedance at the MRI frequency. The LC circuit can be formed by
winding a wire coil around a ceramic chip capacitor. For maximum
effectiveness, the LC circuit should be shielded with conducting
foil or the like.
[0076] To understand the function of the catheter, it is useful to
consider the electrical appearance of the device at both low and
high frequency ranges. First, at low frequencies, the capacitor has
a large impedance and behaves approximately as an open circuit. The
RF inductors have low impedance and appear as short circuits.
Therefore, the low frequency structure (FIG. 1d) of the catheter
behaves the same as the conventional structure (FIG. 1a). The
electrodes can thus be used for monitoring bio-potentials, and
applying RF ablation at frequencies, for example, that are lower
than the MRI frequency (for example, less than about 15 MHz for RF
ablation vs. 63.9 MHz for MRI at 1.5 Tesla). However, at high
frequencies or at the MRI frequency, the structure is quite
different. The impedance of the capacitor at high frequencies is
small and is depicted as a short circuit, 21 in FIG. 1e. The RF
inductors have a large impedance, behaving approximately as open
circuits 34, 35. Note that the use of LC resonant reactances 24 and
25 can provide a much higher impedance at the MRI frequency,
depending on the circuit Quality factor (Q). The net result is that
rather than a two-lead catheter, the catheter now acts as a long
loop MRI receiver, 40, which is ideal for tracking. Furthermore,
the two catheter electrodes, 14, 15 have been decoupled from the
rest of the circuit as depicted in FIG. 1e. Note that the two
electrical structures shown in FIG. 1d and FIG. 1e exist
simultaneously, but at different frequency ranges. Therefore, this
structure enables the same leads to be used for high resolution MR
Imaging, the recording of physiological potentials, and/or RF
ablation.
[0077] In the preferred embodiment, the proximal ends 4 and 5 of
the catheter are connected to an MRI tuning, matching and
decoupling circuit 99 and signal splitting circuit 100 as
exemplified in FIG. 2. FIG. 2a shows a photograph of the circuitry
in a prototype system. In the MRI tuning, matching and decoupling
circuit 99 (FIG. 2b), reactive elements 70, 71, 74, and 77 are
provided such that the catheter loop 40 shown electrically in FIG.
1e and which has an electrical inductance, is tuned to resonate at
the MRI frequency when the catheter is engaged in the sample of
interest such as the body. In addition, the reactive elements are
adjusted so that the impedance at the catheter MRI output
connections 84 and 85 is matched approximately to the
characteristic impedance of the transmission cable used to connect
the catheter to the receiver input of the MRI system. For example,
a 50.OMEGA. coaxial cable with BNC type connectors. Alternatively,
the catheter MRI output connections 84 and 85 can be connected
directly to the receiver input of the MRI system, for example, to
the input of a preamplifier. In this case, the impedance should be
adjusted to the value that corresponds to the preamplifier input
impedance that results in an optimum or near-optimum preamplifier
noise figure.
[0078] It will be understood that the matching circuit shown in
FIG. 2b has the advantage of providing a balanced input, analogous
to that provided by a balun transformer, by virtue of the two
capacitors 74 and 75 connected to both sides of the catheter leads.
Suitable tuning and matching circuits for catheter devices are
known to those skilled in the art, for example, as described in
Atalar et al cited above, and in Ocali O, Atalar E. entitled
"Intravascular magnetic resonance imaging using a loopless catheter
antenna" in Magn Reson Med. 1997; 37:112-8, and it will be
understood that other matching and tuning circuits that are
routinely used for matching and tuning MRI detector coils may be
alternatively used and are within the scope of, and foreseen by,
the disclosed systems and methods.
[0079] In addition, the MRI tuning, matching, decoupling and signal
splitting circuit (FIG. 2B) shows, e.g., decoupling element 90
including a low noise PIN diode, connected across the output
conductors 84 and 85. During MRI excitation by an external transmit
coil, a DC bias voltage is provided by the MRI scanner across the
coil input causing the PIN diode to conduct. During conduction, the
tuning elements are shorted-out, which results in detuning of the
catheter loop 40, and high impedance, thereby limiting those RF
currents induced at the MRI frequency in the loop.
[0080] In previous studies, concerns have been raised about the
safety of using metallic structures in MR scanners. Radiofrequency
energy (MHz)--transmitted from the scanner in order to generate the
MR signal--can be deposited around the interventional device. This
results in high electrical fields around the instrument and local
tissue heating. This heating tends to be most concentrated at the
ends of the electrical structure. This safety issue can be
addressed using the disclosed systems and methods. The concern is
that the surface ring electrodes, which directly contact the
tissue, can cause local tissue burns. The electrodes need to be
cut/removed from the circuit in the megahertz frequency range. This
can be accomplished with an inductor circuit element (placed in
series) between the lead wires and the surface electrodes. With
this design, the electrical end of the leads (in the megahertz
range) are buried inside of the catheter and as a result, the
concentrated electric fields are also located inside of the
catheter, instead of in the tissue. This results in a significant
reduction in unwanted tissue heating. A more effective way to `cut`
the surface electrodes from the rest of the circuit could be to use
a resonant circuit in place of the inductors. This resonant circuit
could include an inductor in parallel with a capacitor (an `LC
circuit`). If this LC circuit is tuned to the MR frequency, it can
present a very high impedance at this frequency. This can
effectively cut the surface electrodes and reduce unwanted heating.
For maximal effectiveness, the LC circuit should be shielded. The
LC circuit may be placed distal to the electrodes and allowing the
electrodes to be visualized.
[0081] In order to monitor physiologic bio-potentials and/or
deliver RF energy for ablation, a splitting circuit is required. In
FIG. 2b, leads 64 and 65 of splitting circuit 100 are connected
across the catheter output leads 4 and 5. These leads are connected
via RF filters 67 and 68 to either the bio-potential monitoring
device and/or the RF energy source for ablation at 94 and 95. The
purpose of RF filters 67 and 68 is to prevent spurious electrical
noise signals that are at least at or near the MRI frequency, from
passing from either the bio-potential monitoring device or the RF
energy source for ablation to the catheter system 40. In addition
the RF filters can stop RF signals induced by the MRI scanner, for
example during excitation, from being input to the bio-potential
monitoring device or the RF ablation energy source. The RF filters
67 and 68 can be of a number of types known to those skilled in the
art, including but limited to those described in U.S. patent
application Ser. No. 09/428,090, filed Nov. 4, 1999, of which this
application is a continuation in part, and in Lardo A C, McVeigh E
R, Jumrussirikul P, Berger R D, Calkins H, Lima J, Halperin H R,
"Visualization and temporal/spatial characterization of cardiac
radiofrequency ablation lesions using magnetic resonance imaging",
Circulaton 2000; 102:698-705. For example, RF filters 67 and 68 can
be low pass filters with a cut-off frequency chosen to lie between
the MRI frequency of the catheter loop 40, and a lower RF frequency
of the RF ablation device so that any signals that arise at the MRI
frequency on lines 64 and 65 are significantly attenuated and
effectively eliminated. Alternatively, in another preferred
embodiment as illustrated in FIG. 2b, filters 67 and 68 are
parallel LC resonant circuits with resonant frequencies adjusted to
substantially match that of the MRI frequency. By this means, the
impedance of each filter can be rendered a very high value at the
MRI frequency depending on the Q of the circuit, thereby stopping
or substantially eliminating signals arising on lines 64, 65, 94,
95.
[0082] In summary, it will be seen that, with the filtering and
connections in place as described, low frequency physiological
bio-potentials can pass unimpeded from surface electrodes 14 and 15
to bio-potential measuring device connected at outputs 94 and 95.
Similarly RF energy for ablation applied at a frequency that is
different from the MRI frequency to leads 94 and 95, can be
delivered unimpeded to electrodes 14 and 15 when they are used for
ablation. Any noise or other signals present at the MRI frequency
can be substantially attenuated or effectively eliminated from
lines 64, 65, 94, 95. From the MRI standpoint, electrodes 14 and 15
are deactivated and disconnected. The catheter behaves as a loop
antenna 40 tuned to the MRI frequency and matched to the MRI system
input, 84 and 85. During MRI excitation, or in fact at any point
during the procedure when it is desirable to do so, the antenna can
be deactivated by a DC bias voltage applied across inputs 84, 85,
by virtue of the decoupling means 90.
[0083] In this embodiment, it will be apparent also that the entire
area of the loop antenna 40 extending from the distal portion of
the catheter 11 near the electrodes, to the points where the
catheter is connected to the MRI tuning, matching, decoupling and
splitting circuits 99 and 100, is MRI active. However, a number of
situations may arise where it is desirable to limit the length of
sensitivity and/or improve the tuning properties of the loop
antenna. In particular, the loop 40 is tuned by capacitor 20 in
conjunction with the tuning and matching elements 71, 74, and 75 in
tuning and matching circuit 99 at each end of a catheter that may
be 100 cm or more long. The extended length of this loop may render
it susceptible to stray capacitance arising between the leads of
the catheter, 4 and 5, and the surrounding sample and/or
environment. The effect of this stray capacitance is generally to
add capacitance to the effective LC resonant circuit formed by the
loop antenna. If the detuning produced by this effect is
significant, it is preferably offset by re-adjustment of the tuning
and matching circuit 99. Under situations where the catheter is
inserted to different lengths in the sample, different amounts of
stray capacitance may occur resulting in variable performance
and/or a need for retuning which is not convenient during a
procedure or study. In addition there are losses associated with
the stray capacitances which can degrade performance as an MRI
antenna.
[0084] In an embodiment, the effect of the stray capacitance is
reduced relative to the total capacitance needed to tune the
circuit by adding additional capacitance at other locations. In
order not to impede passage of bio-potential and/or RF ablation
signals, the capacitors are placed between leads 4 and 5 analogous
to capacitor 20. In one embodiment, depicted in FIG. 2C, tuning
capacitor 20' is connected across 4 and 5 creating distal portions
4'' and 5'' and proximal portions 4' and 5'. The entire loop 40 is
tuned to resonance via circuitry 99 as above, but now with the 2
capacitors, 20 and 20' present. In another preferred embodiment the
placement of capacitor 20' is used to shorten the effective length
of loop 40 to a smaller loop 40'' of working length L, thereby
improving MRI performance by reducing stray capacitance effects and
the amount of noise picked-up from the sample. In this embodiment,
a section of shielded cable, such as coaxial cable 41 may be used
to form the proximal portion 40' of the loop 40. By this means,
stray capacitance and noise pick-up associated with the loop
section 40' is substantially eliminated. This has no effect on the
measurement of bio-potentials or application of RF ablation. In yet
another exemplary embodiment shown in FIG. 2d, additional
capacitors 20b, 20c, 20d are connected between the catheter leads 4
and 5 to further distribute the MRI tuning capacitance along the
MRI-active portion of the catheter L with shielded cable portion 41
connecting this portion to circuitry 99 and 100 described above. In
an embodiment, at least one tuning capacitor is provided. In this
embodiment, loop 40'' is tuned as before. The number of capacitors
that can be added in this way is limited by the inability to tune
the circuit if the capacitance becomes too large. In addition the
capacitance is preferably limited such that bio-potential and/or RF
ablation signals are transmitted without significant impediment
between electrodes 14 and 15, and input/output connections 94 and
95.
[0085] While the catheter devices that are illustrated in FIG. 1
and FIG. 2c and FIG. 2d are depicted with two electrodes 14 and 15,
this is not intended to be a limitation. Conventional catheters
with different numbers of electrodes, for example with 1-7
electrodes for intra-cardiac use, are available. The
multi-functional MRI probe or catheter disclosed herein can be
extended to catheters with tips with different numbers of
electrodes by placing the capacitor 20 immediately proximal to the
plurality of electrodes, in the case of two or more electrodes. In
such embodiments, the capacitor is connected across the lead 4a
that goes to the proximal electrode 15a, and to any of the leads
connected to a more distal electrode such as 14b, as depicted in
FIG. 2e. A loop antenna is thereby formed from leads 4a and 5a,
which are connected to the circuitry 99 and 100 shown in FIG. 2b,
as above. Because the other leads, e.g., leads 7, 8, can be used
for measuring bio-potentials, and/or delivering RF energy for
ablation thereby they may be filtered and each connected to a
filter circuit 100. In addition, performance and safety are
improved by providing reactances 24a, 24b, 25a, 25b etc, one for
each electrode, of the same form and design as reactances 24 and 25
in FIG. 1C3, 1F, or 1G.
[0086] FIG. 2F depicts an embodiment in which a probe or catheter
may have a single electrode, 15b. A loopless antenna, as described
by and Atalar cited above, is formed by providing a shielded
coaxial cable length 41'' for the catheter section, with an
unshielded central conductor portion extending to the single
electrode 15b as shown in FIG. 2f. In this single electrode
embodiment, the active or "whip" portion of the antenna extends
from the electrode over the extending portion 4b, with the proximal
end of 4b, and the shield of the coaxial cable fed to inputs 4 and
5 of circuits 99 and 100 shown in FIG. 2b.
[0087] In an embodiment, the electrophysiology catheter can be made
to function in the mode of a loopless antenna described by Ocali
and Atalar cited above, with catheters employing a plurality of
electrodes. In a preferred embodiment represented by FIG. 1b, the
distal end 4a of the catheter and distal electrode 14 form the MRI
active end of a loopless antenna, and the connecting portions 4 and
5 form a cable portion of same. In this embodiment leads 4 and 5
are preferably formed by a shielded cable section which extends
from tuning/matching/decoupling/splitting circuitry 99 and 100 to
proximal electrode 15, analogous to the use of cable section 41
used in FIG. 2d described above. In particular, lead 5 is formed by
the outer shielding of the coaxial cable and is terminated by the
ring electrode 15. The outer shielding of the coaxial cable is
insulated from the sample and is only exposed to the sample at ring
electrode 15. Similarly, section 4a connected to electrode 14 is
electrically insulated so that it is only exposed to the sample via
contact electrode 14. Tuning of the loopless antenna via circuitry
99 is as described by Ocali and Atalar cited above, and the
splitting circuitry remains unchanged. Note that in this embodiment
reactive elements 23-26 may be omitted. In another embodiment,
capacitor 20 may be omitted also.
[0088] During MRI the conducting loops formed by electrodes 14 and
15 in loopless antenna embodiments of the disclosed systems and
methods, may introduce decoupling artifacts, or give rise to local
heating due to currents induced in the loops by the MRI excitation
field. This problem is mainly limited to distal electrode 14
because electrode 15 is attached to the cable shield. In a further
embodiment these problems are minimized by electrically cutting the
circular electrodes so that they cannot make continuous loops while
maintaining continuous connections between the electrode and lead
4a and 5. This will not curtail their performance for measuring
bio-potentials or RF ablation. In another embodiment, these
problems are minimized by reducing the diameter of the electrodes
and by forming the electrodes with a solid area of conductor
covering the electrode diameter.
[0089] Loopless antenna embodiments have an advantage of shielding
essentially the entire length of the catheter up to the proximal
electrode, from stray capacitance and noise pick-up from the
sample. In addition it provides the advantage of an MRI capability
with an improved range beyond the proximal electrode 15 to the most
distal portion of the catheter, the distal electrode, thereby
improving image quality and resolution even closer to the target
area.
[0090] Embodiments wherein the catheter electrodes are split to
form rings as described above and which share the advantages of
shielding a substantial portion of the catheter length and
maximizing the imaging capability at the distal end of the catheter
are further described and illustrated in FIGS. 2g, 2h, and 2i. In
these embodiments, the electrode rings themselves form loop
antennas, and a portion of the catheter is formed by a section of
coaxial cable 41'', analogous to FIGS. 2d and 2f. A tuning
capacitor 20a is connected across the distal end of the coaxial
cable section 41a. In this case, the value of the capacitor is
chosen, in conjunction with capacitor(s) 29(a, b, etc), such as to
tune the loop formed by the distal electrode(s) to resonate at the
MRI frequency. One end of each split catheter ring is connected to
the next electrode via capacitors, 29, 29a, 29b etc, so that the
electrodes together effectively form a helical solenoid with turns
that are spaced by the separation of the electrodes. The end of the
split ring of the distal electrode is connected back to the other
conductor of the coaxial cable, depicted as 5b.
[0091] FIG. 2G depicts a single electrode loop antenna of this form
for use as a multi-functional device. FIG. 2H depicts a
two-electrode device. The capacitor 29 can block low frequency
currents so that bio-potentials can be measured between the
electrodes without being shorted out. For RF ablation, the
capacitor is chosen to have a high impedance at the RF ablation
frequency, which is preferably much lower than the MRI frequency
(e.g., <10 MHz compared to about 64 MHz for MRI at 1.5 Tesla) to
avoid interfering with MRI signals detection. FIG. 2i exemplifies a
four electrode device. In this case, the two center electrodes are
connected via additional leads 7a, 8a, which need to be connected
to filter circuitry 100, as described for FIG. 2e. In FIGS. 2g, 2h,
2I, reactances are preferably connected on the DC lines connecting
each electrode, as in 24a, 24b, 25a, etc in FIG. 2e and described
above in reference to FIGS. 1C3, 1F, and 1G. While these examples
are described in detail some embodiments, it will be seen that this
arrangement can be extended to other numbers of electrodes on the
catheters, of which it is the intent of the present application to
include by reference herein.
[0092] The multi-functional catheter or probe systems and methods
disclosed herein may be constructed so as to be fully
MRI-compatible. Specifically, it's design and materials are
selected such that (1) the image is not significantly distorted by
the device; (2) the MRI electromagnetic fields do not alter the
normal functioning of the device; (3) cardiac arrythmias or other
nerve stimulation affects are not produced by the device, and (4)
no damage to the tissue is produced by RF energy received from the
MRI scanner. The presence of even small amounts of magnetic
material in the imaging fields can produce substantial amounts of
image distortion. This distortion is caused by perturbation of the
imaging magnetic field. The most distortion is caused by
ferromagnetic materials (iron, nickel, cobalt). Little if any
distortion is produced by materials that do not become
significantly magnetized (low magnetic susceptibility) by the MRI
magnetic field. Metals which do not produce significant
magnetization include copper, gold, platinum and aluminum among
others. Many plastics and synthetic fibers are entirely
non-magnetic and do not distort the images.
[0093] FIG. 1C shows an embodiment in which the catheter or probe
is constructed based on gold-tipped, 1-15 French, most likely 7
French, MRI-compatible two-electrode ablation catheter.
MRI-compatible electrode components 14 and 15, such as those used
in commercial ablation catheters, or electrodes including, e.g.,
safe, bio-compatible, minimally corrosive materials such as gold,
platinum and the like are suitable for this purpose. Leads 4 and 5
are formed from insulated conducting wire, an insulated section of
flexible printed circuit board, or twin lead cable. The conductor
separation should be maintained substantially constant along the
length of the catheter shaft in order to promote uniform signal
sensitivity and to minimize variability in tuning, which can be
accomplished by twin lead cable and flexible printed circuit board,
bonding an insulated wire pair together, using a twisted pair, by a
multi-lumen tube to house the electrodes, by a composite tube
formed with multiple lumens for the electrodes, or by separating
the pair with spacers at fixed intervals with mechanisms known in
the art. The conductor leads can generally be fixed to the
electrodes and/or the electrical components by standard soldering
or welding techniques. Ceramic chip capacitors and (non-ferrite) RF
inductors wound from small diameter copper wire are preferably used
for the electrical components throughout.
[0094] The shaft of the catheter is typically formed of a polymer
tubing that is flexible and can be torqued within the body
cavities, such as Teflon, polyolefin, polyethylene, pebax,
polyurethane, or PTFE. The properties of the materials are such
that they enable the device to be easily steered under MRI
guidance. The diameter of the tubing should be in the range 0.8-5
mm used typically for electrophysiology procedures, or in the range
1-15 French. The total length of the catheter is in the range 0.6-2
m, but typically at least 1 m. In embodiments designed for cardiac
ablation applications, the length of the invasive portion of the
device is preferably at least 1.2 m long so that the tip can be
placed into the heart from the femoral artery or vein, and the
diameter of the device is approximately 2.5 mm. In the embodiment
shown in FIG. 2c and FIG. 2d, section 41 corresponding to loop 40'
can be formed from coaxial cable of diameter less than that of the
shaft, and connected to circuitry 99 and 100 via standard small
gauge RF-type connectors as are known and used by those skilled in
the art.
[0095] In the loopless antenna embodiment corresponding to FIG. 1b
wherein the entire proximal length of leads 4 and 5 up to electrode
15 are formed by a coaxial cable type configuration, similar
coaxial cable of diameter less than that of the shaft, and
connected to circuitry 99 and 100 via standard small gauge RF-type
connectors can be used. Alternatively, the leads can be formed by
flexible, torqueable insulated cable replacing the shaft
altogether. In this version of the loopless antenna embodiment, the
insulating shaft 10 (FIG. 1a) is replaced by the external
insulation on the shaft. In yet another preferred embodiment,
torqueability and maneuverability are enhanced by using coaxial
cable formed by a nitinol hypotube. Similarly, the shaft that forms
the coaxial cable may be constructed with an inner core that is
formed from nitinol, and plated with alternating layers of gold and
silver. A layer of insulation made out of FEP or PET could be used
to separate the inner core from the outer conductor.
[0096] FIG. 3 shows a block diagram illustrating the operation of
an MRI scanner system which may be used in connection with the
disclosed systems and methods. A magnet is provided for creating
the magnetic field for inducing magnetic resonance. Within the
magnet are X, Y, and Z gradient coils for producing a gradient in
the static magnetic field in three orthogonal directions. Within
the gradient coils is an external RF excitation coil. The external
RF excitation coil produces the magnetic field to excite the MRI
signals in the body. A computer is provided for controlling all
components in the MRI scanner. This includes the RF frequency
source, spectrometer and pulse programmer. The pulse programmer
generates a carefully-controlled time-sequence of shaped and/or
phase or frequency-modulated RF pulses that are delivered to the RF
power amplifier. The RF power amplifier has pulse power of 1-20 kW
which is applied to the external RF excitation coil. The computer
also controls the gradient magnetic field by providing a sequence
of modulated pulses that are synchronous with the RF pulse
sequence, to gradient power amplifiers, which in turn activate the
X, Y, and Z gradient magnetic field coils in the magnet. Signals
detected by receiver coils in response to the applied RF/gradient
imaging sequences, including those detected in the aforementioned
multi-functional MRI catheter system, are first input to a receiver
preamplifier. These signals are amplified, phase sensitive
detected, for example, by converting to digital signals and being
fed to a digital receiver. The digital image data are then
reconstructed in the computer and displayed as images on a monitor
or the like.
[0097] It is important that the location of the tip of the catheter
can be accurately determined by MRI. A number of modes of
localization can be used. Because the catheter is a receiver it can
be used to directly image the tissue around it. This image can be
viewed on with high resolution employing a probe or catheter as
disclosed herein, or, it can be viewed at low resolution as an
overlay on a large field-of-view "scout" image obtained with an
auxiliary coil outside the body. The location of the catheter in
the body can be tracked by the bright line of signal moving in the
scout image. The scout image can be updated at an interval set by
the user to compensate for patient motion. An interactive control
can allow the physician to "zoom in" towards the bright catheter,
finally resulting in a high-resolution image in the area of the
distal catheter and tip. The "zoom" function can be achieved with
interactive control of the imaging gradients.
[0098] Some exemplary embodiments may include a combined
multi-functional MRI catheter; a multi-functional MRI catheter
connected and used in conjunction with matching/tuning/decoupling
circuit means and splitting circuit means; a multi-functional MRI
catheter used with said circuit means in conjunction with a
bio-potential monitoring device and/or a standard RF generator for
use in ablation connected at inputs 94 and 95 of FIG. 2b; and any
of a combined multi-functional MRI catheter and said circuit means
used in conjunction with one or more of a bio-potential monitoring
device, an RF generator for ablation, and an MRI scanner. In such a
system embodiment, the MRI scanner has a receiver whose inputs are
connected to inputs 84 and 85.
[0099] The probe or catheter disclosed herein can be used in
combination with an MRI scanner such that RF energy can be
delivered to selected areas of tissue with the electrodes of the
multi-functional catheter, bio-potentials measured with the
electrodes, the tissue imaged with the antenna portion of the
catheter. RF lesions, target tissue, catheter location and tracking
may thus be visualized with the use of external detector coils or
the catheter-antenna, or using external detector coils in
conjunction with the catheter-antenna. This image visualization and
bio-potential measurements can be used for (1) precise titration of
therapy, (2) the ability to test the length and depth of lesions
from new ablation-energy sources, and (3) accurate assessment of
the success of making lines of ablation that block conduction.
[0100] MRI can also be used to guide other procedures. In
Cardiology, accurate anatomic information, combined with electrical
measurements, allows improved study of the pathophysiology of
arrhythmias, stunning, remodeling, and tachycardia-induced
myopathy. Outside of Cardiology, MRI-guided ablation of tumors such
as metastatic liver disease, brain tumors, and prostate cancer, may
allow treatment with less morbidity and less cost than conventional
open surgery.
[0101] As an example of the embodiment depicted in FIG. 1c, a 100
cm long prototype catheter device for use in a 1.5 Tesla MRI system
was constructed from electrode components taken from a gold-tipped,
7 French, MR-compatible two-electrode ablation catheter (Bard Inc.,
Murray Hill, N.J.). The two wire leads were formed from 30 gauge
insulated copper wire (0.25 mm wall thickness--AlphaWire Co.
Elizabeth, N.J.) with a conductor separation held constant at 1.3
mm. For the shaft of the catheter, polyolefin tubing (3.36 mm
diameter with 0.5 mm wall thickness--AlphaWire Co. Elizabeth, N.J.)
was used yielding a final catheter size of 10 French. A 500 pF chip
capacitor (1.4.times.1.4.times.1.45 mm) was placed between the
catheter leads 2 cm from the catheter tip (American Technical
Ceramics Corp. Huntington Station, N.Y.). RF tip chokes 24 and 25
were not used in this implementation. This catheter was connected
to circuitry shown in FIG. 2b. The catheter and circuitry are
pictured in FIG. 2a.
[0102] Capacitors could have values in the range of about 1-1000
pF. Inductors could have values in the range of about 100-1000 nH.
In an embodiment, the capacitor C and inductor L values can be
related by the equation L=1/(w.sup.2C), where w=2.pi.B, and where B
is the resonance frequency, for example, about 64 MHz.
[0103] FIG. 4 shows the SNR performance of this example. SNR curve
300 represents that of a probe or catheter used as a standard
electrophysiology catheter as in FIG. 1C. SNR curve 310 represents
that of a probe with an open circuit such as in FIG. 1B. SNR curve
320 represents that of a probe in which the loop 40 is shorted, as
in FIG. 1E. These data suggest that use of the example probe or
catheter described above can produce a large gain in SNR compared
with a standard catheter. In addition, the prototype catheter has
comparable performance to the design model, FIG. 1e, which, unlike
the prototype, cannot be used for electrophysiology
applications.
[0104] FIG. 5 exemplifies visualization and tracking of a prototype
catheter. Shown are selected images from a 10-second catheter push
acquired with 10 frame/sec, real-time MRI and overlaid on a static,
slice-selective roadmap image obtained with a conventional external
MRI coil. The location of the catheter is readily visualized by the
bright line of signal. Annotations indicate the left ventricle
(LV), right ventricle (RV), chest wall, and the superior vena cava
(SVC). The catheter starts in the right ventricle and is then
pulled up into the right atrium (Panels a & b). As the catheter
is subsequently pushed, the tip stays in the atrium and the
catheter body flexes (Panel c). The catheter is pulled back further
(Panels d & e). In Panels f-g, the catheter is pushed once
again, the tip stays in the right atrium, and the catheter shaft
flexes. Note that the full length of the catheter can be easily
visualized.
[0105] FIG. 6 demonstrates the multifunctional operation of a
prototype catheter, in the form of intracardiac electrogram
recordings acquired concurrent with catheter tracking by MRI. Shown
are selected images from a 15-second catheter push acquired with a
7 frame/sec, real-time MRI sequence. Annotations indicate the left
ventricle (LV), right ventricle (RV), chest wall, and the superior
vena cava (SVC). The catheter is initially advanced from the
jugular vein down the superior vena cava toward the heart (Panels a
& b). Once the catheter arrives in the right atrium, the
catheter tip gets stuck and the catheter body begins to flex (Panel
c). The catheter is withdrawn several centimeters, the shaft is
torqued, and then advanced again (Panels d & e). As the tip is
now angled anteriorly, it slips through the right atrium and into
the right ventricle (Panels f & g). Note that once the catheter
arrives in the ventricle, a large bipolar spike is seen in the
intracardiac electrogram. Bipolar endocardial recordings typically
show only electrical activation of the myocardium (in the
ventricle, these spikes are concurrent with the QRS complex in the
body surface ECG). In Panel h, the catheter is positioned and
stable in the ventricle. Note that a strong bipolar signal is
recorded once the catheter tip arrives in the right ventricle
(lower amplitude signal is also seen from the right atrium).
[0106] MRI data can be used to construct 3D map or images of the
areas in the heart or other organs being treated, that have
undergone ablation, and the surrounding organ or tissue. Areas of
ablation can typically be marked by elevated MRI transverse
relaxation time values (T2), or decreased longitudinal relaxation
values (TI) during infusion of an MRI contrast agent wherein
contrast is enhanced by alteration of such relaxation times. A
composite 3D rendering of the organ being targeted can be updated
after each ablation and displayed with an appropriate rendering
technique. The guidance of the catheter tip to the next site of
ablation and/or bio-potential measurement, can be assisted by MRI
wherein the physician uses the images to manipulate and steer the
catheter, or automatic tracking and feedback could assist that
physician to steer the catheter. This feature is facilitated by the
current availability of MRI frame rates of 10 frames/s or more,
which enables real-time catheter placement, bio-potential
measurements and intervention. The lesions may be visualized using
standard imaging techniques including the use of contrast agents,
as discussed in U.S. patent application Ser. No. 99/25858, filed
Nov. 4, 1999, entitled "System and method for Magnetic Resonance
Guided Electrophysiologic and Ablation Procedures" of which this
application is a Continuation In Part.
[0107] Electrical activation timing information obtained from
bio-potential measurements with the catheter, when combined with
catheter localization information, enables accurate activation maps
that are most useful in determining, for example, the site of
origin of an atrial or ventricular tachycardia in the heart.
Activation maps can be superimposed and/or color rendered on
anatomically accurate reconstructions of cardiac structure.
Spatially accurate voltage data is available from knowledge of the
location of each electrode in contact with the myocardium in 3D, as
derived from MRI. Thus, electrical data originating from each known
electrode position allows generation of activation and voltage maps
on true anatomic structures. This provides significant advantages
beyond the capabilities of the non-fluoroscopic electroanatomic
mapping systems that do not provide accurate anatomic
information.
[0108] When the ablation/imaging catheter is used for the delivery
of ablative radio-frequency energy, the high-resolution image
obtained via the present catheter system enables visualization of
the ablation lesion and of lesion growth. Again, directional
orientation, as well as location, of the catheter tip can be
determined in 3D space, and the high-resolution image data can be
displayed in any plane, and in particular, in the plane orthogonal
to the catheter. Since the image is obtained with the same catheter
that is delivering the ablative energy, the orthogonal-plane image
displays the lesion at its maximal radius, reducing the chances of
underestimation as often occurs with ultrasound. High-resolution
visualization of ablative lesions by the multi-functional MRI
catheter or probe as disclosed herein allows for documentation of
whether or not RF application resulted in successful lesion
development and of where lesions have and have not yet been made.
This facilitates efficient catheter placement so that RF is applied
only to tissue not previously ablated.
[0109] The combination of the high-resolution visualization,
bio-potential measurements and/or RF ablation functionality in a
single catheter as discussed above makes high-resolution MRI
guidance ideal for visualization and verification of ablative
lesion lines, particularly in atrial tissue. This is useful for
ablation of the re-entrant circuit in typical atrial flutter and is
crucial for successful ablation of atrial fibrillation. It has been
shown that atrial fibrillation can be eliminated with multiple
lines of ablative lesions placed in the right and left atria to
emulate the surgical maze procedure. Failures of the `percutaneous
maze` procedure have resulted primarily from incomplete lesion
lines. MRI guidance should allow rapid confirmation of lesion line
continuity and avoidance of unnecessary repetition of RF
application where tissue has already been successfully ablated.
[0110] The MRI-guided catheter bio-potential/ablation system offers
advantages in ablation of ischemic and idiopathic ventricular
tachycardias, ectopic atrial tachycardias, atrial flutter, and
atrial fibrillation. Unlike AV node reentry and accessory pathway
mediated tachycardia, these other arrhythmias have lower ablation
success rates and longer ablation procedure durations, primarily
due to difficulties in accurate activation mapping or confirmation
of lesion development with conventional equipment. Procedure
durations and risk of complications should thus be reduced
substantially with the MRI-guided catheter ablation system.
[0111] While the disclosed systems and methods have been described
in connection with embodiments shown and described in detail,
various modifications and improvements thereon will become readily
apparent to those skilled in the art. Accordingly, the spirit and
scope of the present disclosure is limited only by the following
claims.
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