U.S. patent number RE44,736 [Application Number 13/027,489] was granted by the patent office on 2014-01-28 for magnetic resonance probes.
This patent grant is currently assigned to MRI Interventions, Inc.. The grantee listed for this patent is Parag Karmarkar, Ingmar Viohl. Invention is credited to Parag Karmarkar, Ingmar Viohl.
United States Patent |
RE44,736 |
Karmarkar , et al. |
January 28, 2014 |
**Please see images for:
( Certificate of Correction ) ** |
Magnetic resonance probes
Abstract
A magnetic resonance probe may include a plurality of center
conductors, at least some center conductors including a conductive
core and an insulator disposed at least partially about the core
along at least a portion of the core, a first dielectric layer
disposed at least partially about the plurality of center
conductors in a proximal portion of the probe, an outer conductive
layer at least partially disposed about the first dielectric layer,
and a plurality of electrodes, at least one electrode being coupled
to one of the center conductors and disposed at least partly on a
probe surface.
Inventors: |
Karmarkar; Parag (Columbia,
MD), Viohl; Ingmar (Milwaukee, WI) |
Applicant: |
Name |
City |
State |
Country |
Type |
Karmarkar; Parag
Viohl; Ingmar |
Columbia
Milwaukee |
MD
WI |
US
US |
|
|
Assignee: |
MRI Interventions, Inc.
(Memphis, TN)
|
Family
ID: |
29711953 |
Appl.
No.: |
13/027,489 |
Filed: |
February 15, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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11810679 |
Oct 18, 2011 |
Re. 42856 |
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60383828 |
May 29, 2002 |
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Reissue of: |
10448736 |
May 29, 2003 |
6904307 |
Jun 7, 2005 |
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Current U.S.
Class: |
600/423;
324/318 |
Current CPC
Class: |
G01R
33/287 (20130101) |
Current International
Class: |
A61B
5/05 (20060101); G01V 3/00 (20060101) |
Field of
Search: |
;324/318,322,309,307,300
;600/423,422,421,411,410 |
References Cited
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Primary Examiner: Arana; Louis
Attorney, Agent or Firm: Finnegan, Henderson, Farabow,
Garrett & Dunner L.L.P.
Parent Case Text
.Iadd.Notice: More than one reissue application has been filed for
the reissue of U.S. Pat. No. 6,904,307. The reissue applications
are U.S. application Ser. No. 11/810,679 filed Jun. 6, 2007 which
issued on Oct. 18, 2011 as U.S. Pat. No. Re. 42,856 E, and the
present application (U.S. application Ser. No. 13/027,489), which
is a divisional reissue of U.S application Ser. No.
11/810,679..Iaddend.
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of U.S provisional application
Ser. No 60/383,828, filed May 29, 2002, which is hereby
incorporated herein in its entirety by this reference.
Claims
We claim:
.[.1. A magnetic resonance probe, comprising: a plurality of center
conductors, at least some center conductors; including a conductive
core and an insulator disposed at least partially about the core
along at least a portion of the core; and forming a first pole of a
magnetic resonance dipole antenna; a first dielectric layer
disposed at least partially about the plurality of center
conductors in a proximal portion of the probe; an outer conductive
layer at least partially disposed about the first dielectric layer
and forming a second pole of the magnetic resonance dipole antenna;
and a plurality of electrodes, at least one electrode being coupled
to one of the center conductors and disposed at least partly on a
probe surface..].
.[.2. The probe of claim 1, further comprising a second dielectric
layer at least partially disposed about the outer conductive
layer..].
.[.3. The probe of claim 1, further comprising a lubricious coating
at least partially disposed about the outer conductive
layer..].
.[.4. The probe of claim 1, wherein the plurality of center
conductors are magnetic resonance-compatible..].
.[.5. The probe of claim 1, wherein at least one insulator has a
thickness equal to or less than about 100 microns..].
.[.6. The probe of claim 1, wherein at least some center conductors
comprise at least one of a magnetic resonance compatible material,
a super elastic material, copper, silver-copper, gold, silver,
platinum, iridium, MP35N, tantalum, titanium, Nitinol, L605,
gold-platinum-iridium, gold-copper-iridium, and
gold-platinum..].
.[.7. The probe of claim 1, further comprising a connection to a
high-pass filter through which the probe is coupleable to a
magnetic resonance scanner..].
.[.8. The probe of claim 1, further comprising a connection to a
low-pass filter through which the probe is coupleable to at least
one of an electrophysiological recording system, a tissue
stimulator, and an ablation energy source..].
.[.9. The probe of claim 1, further comprising: a ribbon disposed
in a distal portion of the probe; and a pull wire coupled to the
ribbon..].
.[.10. The probe of claim 9, wherein the pull wire is disposed in a
lumen in the probe..].
.[.11. The probe of claim 1, further comprising a coolant
lumen..].
.[.12. The probe of claim 1, further comprising a plurality of
radially expandable arms, wherein at least one electrode is at
least partly disposed on one arm..].
.[.13. The probe of claim 12, further comprising a tubing that is
slideably displaceable between at least two positions to transition
the expandable arms between a retracted position and an expanded
position..].
.[.14. The probe of claim 12, further comprising a tubing that is
slideably displaceable between at least two positions to transition
the expandable arms between a retracted position and an expanded
position..].
.[.15. The probe of claim 1, further comprising an ablation
electrode disposed at a distal tip of the probe..].
.[.16. The probe of claim 1, further comprising an interface
circuit coupled to the probe, the interface circuit including: a
signal splitter that directs a signal received from the probe to a
magnetic resonance pathway and an electrophysiology pathway; a
high-pass filter disposed in the magnetic resonance pathway; a
low-pass filter disposed in the electrophysiology pathway; a
connector disposed in the magnetic resonance pathway for connecting
to a magnetic resonance scanner; and a connector disposed in the
electrophysiology pathway for connecting to at least one of a
tissue stimulator, a biopotential recording system, and an ablation
energy source..].
.[.17. The probe of claim 1, wherein the probe has an outer
diameter of less than about 15 French..].
.[.18. The probe of claim 1, wherein the probe has an outer
diameter of less than about 4 French..].
.[.19. The probe of claim 1, further comprising a connector portion
disposed at a proximal end of the probe, the connector portion
including: an outer conductor contact coupled to the outer
conductive layer; extended sections of at least some center
conductors extending proximally beyond the outer conductor contact,
at least one extended section having a center conductor contact
coupled to one center conductor; and an insulated area interposed
between the outer conductor contact and the at least one center
conductor contact..].
.[.20. The probe of claim 1, defining at least one lumen..].
.[.21. The probe of claim 20, further comprising a pull wire
disposed in the lumen, coupled to a distal portion of the probe,
and longitudinally displaceable..].
.[.22. The probe of claim 1, wherein at least one center conductor
is coupled to a distal portion of the probe and longitudinally
displaceable..].
.[.23. The probe of claim 1, wherein all of the center conductors
collectively form the first pole of the magnetic resonance dipole
antenna..].
.[.24. A method of performing a magnetic resonance-guided
procedure, comprising: placing a subject in a magnetic resonance
scanner; identifying a target site in the subject using data about
the subject obtained from the scanner; introducing into the patient
a magnetic resonance probe as defined by claim 1; advancing the
probe to the target site; and performing the procedure using the
magnetic resonance probe..].
.[.25. The method of claim 24, wherein the target site is located
in the subject's brain, and the probe is introduced by employing a
stereotactic frame..].
.[.26. The method of claim 24, wherein the target site comprises at
least one of the subject's thalamus, globus pallidum internus, and
subthalamic nucleus..].
.[.27. The method of claim 24, further comprising anchoring at
least one of the probe's electrodes in the subject..].
.[.28. The method of claim 24, further comprising electrically
connecting at least one of the probe's electrodes to a
pacemaker..].
.[.29. The method of claim 24, wherein the target site is located
in the subject's heart..].
.[.30. The method of claim 29, wherein at least one probe electrode
is an RF ablation electrode, and the method further comprises
ablating heart tissue..].
.[.31. The method of claim 30, wherein ablating comprises creating
a plurality of linear ablations in the subject's left and/or right
atrium..].
.[.32. A combined magnetic resonance imaging and electrophysiology
probe, comprising: a plurality of center conductors, at least some
center conductors including a conductive core and an insulator
disposed at least partially about the core along at least a portion
of the core, the insulator having a thickness equal to or less than
about 100 microns; a first dielectric layer disposed at least
partially about the plurality of center conductors in a proximal
portion of the probe; an outer conductive layer at least partially
disposed about the first dielectric layer; a second dielectric
layer disposed at least partially about the outer conductive layer;
and a plurality of electrodes, at least one electrode coupled to
one of the center conductors and disposed at least partly on the
probe surface..].
.[.33. A system for magnetic resonance imaging, comprising: a
magnetic resonance probe, including: a plurality of center
conductors, at least some center conductors: including a conductive
core and an insulator disposed at least partially about the core
along at least a portion of the core; and forming a first pole of a
magnetic resonance dipole antenna; a first dielectric layer
disposed at least partially about the plurality of center
conductors in a proximal portion of the probe; an outer conductive
layer disposed at least partially about the first dielectric layer
and forming a second pole of the magnetic resonance dipole antenna;
and a plurality of electrodes, at least one electrode coupled to
one of the center conductors and disposed at least partly on the
probe surface; and a interface electrically coupled to the probe,
the interface including: a signal splitter that directs a signal
received from the probe to a magnetic resonance pathway and an
electrophysiology pathway; a high-pass filter disposed in the
magnetic resonance pathway; a low-pass filter disposed in the
electrophysiology pathway; a connector disposed in the magnetic
resonance pathway for connecting to a magnetic resonance scanner;
and a connector disposed in the electrophysiology pathway for
connecting to at least one of a tissue stimulator, a
electrophysiological recording system, and an ablation energy
source..].
.Iadd.34. A magnetic resonance interventional probe assembly,
comprising: a magnetic resonance probe, comprising a steerable
distal section attached to a stiff proximal section with a
plurality of electrodes held on the distal section, the distal
section including a plurality of center conductors which form a
first pole of a magnetic resonance dipole antenna and an outer
conductive layer which forms a second pole of the antenna, each
center conductor including a conductive core and an insulator
disposed at least partially about the core, the insulator being
configured to electrically couple the plurality of center
conductors together when exposed to high frequency radiation and
electrically decouple the plurality of center conductors from each
other when exposed to low frequency radiation; and at least one
pull wire connected to a distal tip portion of the probe to allow
the distal tip to flexibly bend in response to pulling of the pull
wire, wherein the probe cooperates with the at least one pull wire
to define a loopless magnetic resonance antenna which allows MRI
visualization of the probe tip..Iaddend.
.Iadd.35. A magnetic resonance interventional probe assembly
according to claim 34, wherein the plurality of insulated center
conductors are held in a lumen defined by a first dielectric
material..Iaddend.
.Iadd.36. A magnetic resonance interventional probe assembly
according to claim 34, further comprising a decoupling circuit
comprising a PIN diode, wherein during a receive phase of an MRI
scanner, a negative voltage can reverse bias the diode thereby
rendering the diode non-conductive, and wherein during a transmit
phase of the MRI scanner, a positive voltage cause the PIN diode to
be conductive..Iaddend.
Description
BACKGROUND
Leads (catheters) for a wide variety of medical procedures, such as
Deep Brain Stimulation (DBS) and cardiac interventions, are
typically placed into the body of a subject under stereotactic
guidance, fluoroscopy, or other methods. Stereotactic guidance is a
static method based on high resolution images taken prior to the
procedure and does not take into account displacement of the brain
caused by the loss of cerebral spinal fluid (CSF), blood or simple
brain tissue displacement by the surgical tool. It is therefore
often necessary to perform a real time physiological localization
of the target area to augment and verify the previously obtained
stereotactic data by observing the patients response to stimulation
through the DBS electrodes or by recording and displaying (visual
or audible) the action potentials of individual neurons along the
path way to the target zone using microelectrodes. These additional
steps are time consuming; resulting in procedures between 6-8 hours
with a failure rate still remaining between 20-30%.
Cardiac procedures are mainly performed using X-ray fluoroscopy.
Because 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 whether the catheter has penetrated
the wall of the heart. Furthermore, lesions are invisible under
x-ray fluoroscopy. Thus, it is very difficult to discern whether
tissue has been adequately ablated.
SUMMARY
The systems and methods disclosed herein may simplify the
manufacturing process for magnetic resonance probes, increase
patient safety, reduce if not eliminate tissue heating, and
facilitate the performance of multiple functions during MRI
interventional procedures such as Deep Brain Stimulation,
Electrophysiological Mapping, and/or RF Ablation.
In an embodiment, a magnetic resonance probe may include a
plurality of center conductors, at least some center conductors
including a conductive core and an insulator disposed at least
partially about the core along at least a portion of the core. A
first dielectric layer may be disposed at least partially about the
plurality of center conductors in a proximal portion of the probe.
An outer conductive layer may be at least partially disposed about
the first dielectric layer. A plurality of electrodes may be
included, at least one electrode being coupled to one of the center
conductors and disposed at least partly on a probe surface.
In an embodiment, a probe may include a second dielectric layer at
least partially disposed about the outer conductor. In an
embodiment, the plurality of center conductors may be magnetic
resonance-compatible. In an embodiment, at least one insulator may
have a thickness up to about 100 microns. In an embodiment, at
least some center conductors may form a first pole of a dipole
antenna, and the outer conductive layer may form a second pole of
the dipole antenna. In an embodiment, a probe can include a
plurality of radially expandable arms. In an embodiment, at least
one electrode may be at least partly disposed on an arm.
In an embodiment, an interface circuit may be electrically coupled
to the probe, the interface circuit including a signal splitter
that directs a signal received from the probe to a magnetic
resonance pathway and an electrophysiology pathway, a high-pass
filter disposed in the magnetic resonance pathway, a low-pass
filter disposed in the electrophysiology pathway, a connector
disposed in the magnetic resonance pathway for connecting to a
magnetic resonance scanner, and a connector disposed in the
electrophysiology pathway for connecting to at least one of a
tissue stimulator, a biopotential recording system, and an ablation
energy source.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the disclosed systems and methods will be apparent
from the following more particular description of exemplary
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, nor are
individual elements necessarily in relative proportion to other
elements, emphasis instead being placed upon illustrating
principles of the disclosed systems and methods.
FIG. 1 depicts an exemplary embodiment of a magnetic resonance
probe having four center conductors and four electrodes.
FIGS. 2A-C depict an exemplary embodiment of a magnetic resonance
probe having four center conductors and four electrodes. FIG. 2A
depicts a side view. FIG. 2B depicts a cross section in a distal
portion of the probe. FIG. 2C depicts a cross section in a proximal
portion of the probe.
FIGS. 3A-E depict exemplary embodiments of an interface circuit.
FIGS. 3A and 3B depict exemplary electrical schematics; FIGS. 3C-3E
depict exemplary physical layouts.
FIGS. 4A-4C depict an exemplary embodiment of a steerable magnetic
resonance probe.
FIGS. 5A-5C depict an exemplary embodiment of a magnetic resonance
probe having cooling lumens.
FIGS. 6A-6D depicts an exemplary embodiment of a magnetic resonance
probe having expandable arms. FIG. 6A depicts a side view of the
exemplary probe. FIG. 6B depicts a long axis view of an arm. FIG.
6C depicts a cross section of expanded arms. FIG. 6D depicts a
cross section in a proximal portion of the exemplary probe.
FIGS. 7A-B show heating profiles of tissue surrounding an exemplary
magnetic resonance probe in the transmit mode that is decoupled
(FIG. 7A) or not decoupled (FIG. 7B).
FIGS. 8A-C depict an exemplary embodiment of a bidirectionally
steerable magnetic resonance probe having wires that are both pull
wires and center conductors.
FIGS. 9A-C depict an exemplary embodiment of a unidirectionally
steerable magnetic resonance probe having an offset wire that is
both a pull wire and center conductor.
FIGS. 10A-C depict an exemplary embodiment of a unidirectionally
steerable magnetic resonance probe having a centered wire that is
both a pull wire and center conductor.
DETAILED DESCRIPTION
The disclosed systems and methods relate to the guidance and
visualization of diagnostic and therapeutic procedures performed
under Magnetic Resonance Imaging (MRI). Such procedures in general
benefit from the excellent soft tissue contrast obtainable with
MRI. Examples of such applications are Deep Brain Stimulation (DBS)
for the treatment of movement disorders (Parkinson's disease,
Essential tremor, etc.) and other neurological disorders benefiting
from electrical stimulations of section of the brain, as well as
the diagnosis and treatment of cardiac arrhythmias including but
not limited to atrial fibrillation and ventricular tachycardia.
Real time Magnetic Resonance Imaging can overcome both the
inaccuracies of stereotactic planning and the lack of soft tissue
contrast as found in X-ray fluoroscopy. The use of Magnetic
Resonance Imaging guided interventions can therefore result in
shortened procedure times and increased success rates.
Some conditions that may benefit from MRI-guided DBS include
Parkinson's disease, essential tremor, and multiple sclerosis.
Parkinson's disease is a progressive neurological disorder in
regions of the midbrain containing a cluster of neurons known as
the "substantia nigra." These neurons produce the chemical
dopamine, a neurotransmitter (messenger) responsible for
transmitting signals between the substantia nigra and several
clusters of neurons that comprise the basal ganglia and is vital
for normal movement. When dopamine levels drop below 80%, symptoms
of Parkinson's disease begin to emerge causing nerve cells of the
basal ganglia to fire out of control; resulting in tremor, muscle
stiffness or rigidity, slowness of movement (bradykinesia) and loss
of balance. Although medication masks some symptoms for a limited
period, generally four to eight years in most patients, they begin
causing dose-limiting side effects. Eventually the medications lose
their effectiveness, leaving the patient unable to move, speak or
swallow. Several preventive and restorative strategies such as
neural cell transplantation, neural growth factors, gene therapy
techniques and surgical therapies (including DBS), have shown
promise in animal studies and human clinical trials. Important
links to the cause (including genetic susceptibility and the role
of toxic agents) are becoming established. Leading scientists
describe Parkinson's as the neurological disorder most likely to
produce a breakthrough therapy and/or cure within this decade.
Parkinson's disease afflicts approximately 1 million Americans,
nearly 40 percent of whom are under the age of 60. Roughly 60,000
cases of PD are diagnosed each year. It is estimated that
Parkinson's disease costs society $25 billion or more annually.
Essential tremor (ET) is considered the most common neurological
movement disorder affecting nearly 10 million people in the United
States. ET is a chronic condition characterized by involuntary,
rhythmic tremor of a body part, most typically the hands and arms,
often the head and voice, but rarely the legs. ET is generally
considered a slowly progressive disorder, although many individuals
may have a mild form of ET throughout life that never requires
treatment. The most common form of ET affects the arms and hands,
usually bilaterally, and is most prominent with the arms held
against gravity (postural tremor) or in action (kinetic tremor)
such as when writing or drinking from a cup. Unlike patients with
Parkinson's disease, patients with ET rarely exhibit a tremor when
the arm is at rest. Pharmacological treatment for ET includes a
class of drugs called Beta-adrenergic blocking agents (such as
propranolol), benefiting about 50 to 60 percent of patients.
Primidone (MYSOLINE) is commonly regarded as the most effective
drug. Side effects of these drugs include: bradycardia (slow heart
rate), hypotension (low blood pressure), dizziness, fatigue,
depression, diarrhea, nausea and/or sexual dysfunction. Surgical
treatment of ET has for years involved placing a lesion in certain
cluster of cells called the thalamus. This procedure, called
stereotaxic thalamotomy has been quite effective in substantially
reducing tremor intensity, although there is a finite risk of
stroke or other surgical complications and bilateral thalamotomies
increase the risk of speech impairment (dysarthria). The recent
development of high frequency stimulation of the thalamus (deep
brain stimulation) has provided a safer and more effective surgical
strategy for treating ET. This procedure involves the placement of
an electrode in a region of the thalamus (Ventral Intermediate
Nucleus or VIM).
Multiple sclerosis (MS) tends to begin in young adulthood and
affects about 500,000 people in the United States. Worldwide, the
incidence rate is approximately 0.01% with Northern Europe and the
northern US having the highest prevalence with more than 30 cases
per 100,000 people. MS is a chronic, progressive, degenerative
disorder that affects nerve fibers in the brain and spinal cord. A
fatty substance (called myelin) surrounds and insulates nerve
fibers and facilitates the conduction of nerve impulse
transmissions. MS is characterized by intermittent damage to myelin
(called demyelination) caused by the destruction of specialized
cells (oligodendrocytes) that form the substance. Demyelination
causes scarring and hardening (sclerosis, plague) of nerve fibers
usually in the spinal cord, brain stem, and optic nerves, which
slows nerve impulses and results in weakness, numbness, pain, and
vision loss. MS can affect any part of the central nervous system.
When it affects the cerebellum or the cerebellum's connections to
other parts of the brain, severe tremor can result. Since the sub
cortical gray matter also contains myelinated nerve fibers, plaques
can also be found in the striatum, pallidum and thalamus. This may
be the pathological basis for the other movement disorders seen in
a small proportion of patients with MS. Because different nerves
are affected at different times, MS symptoms often exacerbate
(worsen), improve, and develop in different areas of the body.
Early symptoms of the disorder may include vision changes (e.g.,
blurred vision, blind spots) and muscle weakness. MS can progress
steadily or cause acute attacks (exacerbations) followed by partial
or complete reduction in symptoms (remission). Most patients with
the disease have a normal lifespan.
In a typical DBS procedure, a stereotactic frame, e.g. an Ieksell
frame, is attached (bolted) to the patient prior to any portion of
the surgical intervention. This is often done in a separate small
operating room, either under sedation (Midazolam, Fentanyl,
Propofol) and/or local anesthesia (Lidocaine). After the frame is
attached, the patient is transferred to the table of the imaging
system (CT or MR) and the patient's head is immobilized. A box
containing fiduciary markers is fitted on to the frame. These
markers will show up in subsequent images in precisely known
locations, allowing an accurate mapping between the frame
coordinates and brain structures. Based on these detailed images
and coordinate mappings, the trajectory for the surgery using a
planning software program.
Typical targets for the procedure include regions in the Thalamus,
the Globus Pallidum Internus (Gpi) and the Subthalamic Nucleus
(SNT). The target selection strongly depends on the disease and
symptoms treated. DBS in the GPi seems to be very effective for
drug-induced dyskinesia and helps control tremor and bradykinesia.
DBS in the SNT seem to be most effective as measured by ability of
patients to reduce their medications, however, there is a potential
for increasing dyskinesia. The Thalamus is not necessarily a good
target for patients with Parkinson's disease but has been found to
improve conditions for patients with Essential Tremor and movement
disorder caused by Multiple Sclerosis.
Once the target has been effectively localized and noted to be in a
safe location, effort must be placed on a safe entry and trajectory
to the target. MRI surface images of the cerebral cortex in
combination with the DBS planning scans can be useful to avoid
injuries to cerebral arteries or veins at the initial drill holes
and due to passage of the DBS electrode, resulting in a
catastrophic hemorrhage. With the stereotactic software system,
trajectory slices are possible so that every stage of the
trajectory can be visualized in terms of its potential harm as an
electrode is passed toward the target. Fine adjustments to the
entry point can be made to avoid these critical structures or avoid
passage through the ventricular system in the patient with large
ventricles.
Entry point coordinates are not directly utilized during operative
planning but are used by the computer system in creating the
trajectory itself. An estimate of accuracy can then be obtained and
is usually accurate within several hundred microns and always less
than 0.5 cm accuracy so that the results from imaging and planning
can be used effectively during the surgical procedure.
Once the planning process is completed, the patient is transferred
to the operating room and a hole is drilled into the patient's
skull (0.5'' to 1.0''). At this point, most surgery centers will
perform a real time physiological localization of the target area
to augment and verify the previously obtained stereotactic data by
observing the patients response to stimulation through the DBS
electrodes or by recording and displaying (visual or audible) the
action potentials of individual neurons along the path way to the
target zone using microelectrodes. The additional step is
considered necessary because the shape of the brain and the
position of anatomical structures can change during neuro-surgical
procedures. Such changes can be due to differences between the
patient's position in acquisition and during surgery, reduction in
volume due to tissue resection or cyst drainage, tissue
displacement by the instruments used, changes in blood and extra
cellular fluid volumes, or loss of cerebrospinal fluid when the
skull is opened. The amount of brain shift can in a severe case be
a centimeter or more and is in most cases between 1 and 2 mm.
In addition to the brain shift phenomenon, some subsection of
specific nuclei cannot yet be identified by anatomic means, again
requiring a physiological determination of the target area. Given
these "uncertainties," several target runs may be required before
the desired results are achieved. Throughout the procedure,
responses from the patient are necessary to determine if the target
area has been reached and if there are any unwanted site effects.
Once the target area has been correctly identified, the
microelectrode is removed and replaced with the DBS electrode.
Stimulation voltage levels are determined by observing the patient
and the physiological response. Once all parameters have been
correctly adjusted, the DBS electrode is anchored in the skull, a
pacemaker is implanted subcutaneously in the subclavicular region
and the lead is tunneled under the scalp up the back of the neck to
the top of the head.
One of the major shortcomings with stereotactic DBS is the
requirement of sub millimeter accuracy in electrode placement for
the electrical stimulation of target areas deep inside the brain.
As pointed out, brain shifts of 1 to 2 mm can routinely occur
between the acquisition of images for the stereotactic surgery and
the surgery itself and is either caused by patient transport
(misregistration, image distortion), loss of fluid (blood, CSF) or
simple tissue displacement by the instruments used. A long
recognized solution to these issues has been to perform real time
MRI guided surgery. To this end a variety of MRI systems have been
developed. "Open MRI" systems which are typically operated at field
strength ranging from 0.12 T (Odin) to 1.0 T (Philips) offer a
clear advantage in patient access over the closed bore systems
ranging in field strength from 1.0 T to 3.0 T. However, these high
field short bore systems outperform the low field systems in
Signal-to-Noise Ratio since the SNR depends linearly on field
strength. Higher SNR translates directly into resolution and/or
imaging speed. Efforts have been undertaken to increase the field
strength of these open systems (Philips 1.0 T), however, it is not
clear that much higher magnetic fields are desirable or achievable
due to considerable mechanical challenges of stabilizing the
separated pole faces of these magnets and the fact that these
magnets are not easily shielded and have a larger fringe field than
comparable "closed bore" systems. Furthermore, significant progress
has been made to increase the patient access in high field systems
as well. Traditionally, whole body 3 T MRI systems have had a
length in access of 2 m. Over the past few years dedicated head
scanners (Allegra, Siemens) have been developed and have reduced
the system length to 1.25 m, allowing relatively easy access to the
patient's head. Similar progress has been made in whole body
scanners at 1.5 T. Since the actual magnet is significantly shorter
(68 to 80 cm) than the overall system further improvements in
patient access can be expected. Image quality, speed and patient
access are now at a point where true interventional MRI is
feasible. All major OEM's have recognized the need for a fully
integrated MRI operating room and have made significant progress
towards this goal. Siemens has introduced the "BrainSuite", a fully
integrated MRI suite for neuro-surgery. Philips, Siemens and GE
have also introduced XMRI systems, combining 1.5 T or 3 T whole
body systems with an X-Ray fluoroscopy with a patient table/carrier
linking both systems.
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.
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
atrioventricular (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.
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.
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.
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.
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.
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.
The major technology for guiding placement of a catheter is x-ray
fluoroscopy. For electrophysiologic 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. The intracardiac electrogram may be used
to guide the catheters to the proper cardiac tissue.
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.
Neither x-ray fluoroscopy nor intracardiac ultrasound have the
ability to accurately and reproducibly identify areas of the heart
that have been ablated.
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 3D-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.
Embodiments of fixed, steerable, cooled and Multi Electrode Array
probes are described that may incorporate multiple functions, such
as the recording of MRI imaging signals, bio potentials
(electrophysiological, neurological) and cooling. The probes can
significantly reduce heating-induced injury in materials
surrounding them and can be easily visualized under MRI or X-ray.
Disclosed embodiments are illustrative and not meant to be
limiting. Drawings illustrate exemplary embodiments and design
principles; absolute or relative dimensions are not to be inferred
therefrom as necessarily pertaining to a particular embodiment.
FIG. 1 shows schematically an exemplary embodiment of a magnetic
resonance probe 100. The probe 100 may have a distal portion 7 and
a proximal portion 8. The distal portion may include a plurality of
electrodes, such as electrodes 3, 4, 5, 6. As shown, the electrodes
may be disposed at least partly on a surface of the probe 100. An
electrode can be disposed so that the electrode is disposed on the
surface around the circumference of the probe 100 (as shown for
electrodes 4, 5, and 6), disposed at the tip of the probe 100 (as
shown for electrode 3), or so that the electrode is disposed at the
surface around one or more portions of the circumference. The probe
100 shown in FIG. 1 has four electrodes, but other numbers of
electrodes may be provided, such as few as one electrode. Probe 100
may include a plurality of center conductors, such as center
conductors 101, 102, 103, 104. Other numbers of center conductors
may be provided. As shown in this exemplary embodiment, center
conductors 101, 102, 103, 104 may be coupled to corresponding
electrodes 3, 4, 5, 6. The center conductors may extend through the
probe 100 and terminate in a connector 9 at the proximal end of the
probe 100. One or more additional layers, described in greater
detail below, may be disposed at least partially about the center
conductors in the proximal portion 8 of the probe 100.
FIGS. 2A-C depict additional features of an exemplary embodiment of
a probe 100. As shown in FIG. 2A, a junction J may define the
transition between the distal portion 7 and the proximal portion 8
of the probe 100. The position of the junction J may be selected to
provide the probe 100 with preferred electrical properties,
discussed in greater detail below. In an embodiment, the junction J
may be positioned so that the distal portion 7 of the probe 100 has
a length approximately equal to one quarter the wavelength of an MR
signal in the surrounding medium. For a medium such as blood or
tissue, the preferred length for the distal portion 7 can be in the
range of about 3 cm to about 15 cm. The center conductors 2
(referenced collectively) may be coiled to reduce the physical
length of the distal portion 7 while maintaining the "quarter wave"
electrical length. As shown in cross section FIG. 2B, the distal
portion 7 of probe 100 may include a plurality of center conductors
2 and a lubricious coating 1 disposed the plurality of center
conductors. Exemplary lubricious coatings include
polyvinylpyrrolidone, polyacrylic acid, hydrophilic substance,
silicone, and combinations of these, among others.
With continued reference to FIGS. 2A and 2C, the proximal portion 8
of the probe 100 may include one or more additional layers disposed
at least partially about the plurality of center conductors 2. For
example, a first dielectric layer 31 may be disposed at least
partially about the plurality of center conductors 2. The first
dielectric layer 31 may define a lumen 13 in which the plurality of
center conductors 2 may be disposed. An outer conductive layer 12
may be at least partially disposed about the first dielectric
layer. The outer conductive layer 12 may include a braiding. The
outer conductive layer 12 may extend through the probe 100 and
terminate at the connector 9. A second dielectric layer 10 may be
at least partially disposed about the outer conductive layer 12. A
lubricious coating 1 may be at least partially disposed about the
outer conductive layer 12 and/or the second dielectric layer 10 in
the proximal portion 8 of the probe 100.
As described above, a plurality of center conductors may be
provided. A center conductor may include a conductive core. A
center conductor may include an insulator disposed at least
partially about the core along at least a portion of the core. The
insulator may be disposed about the core to prevent contact between
various cores. The insulator may be disposed along the entire
length of the core or along one or portions thereof. In an
embodiment, an insulator may be disposed about substantially the
entire length of a core except for a distal portion for coupling to
an electrode. Insulator may be selectively disposed about core,
such as discontinuously or on only a selected aspect of a core,
such as an aspect that faces another core. Thus, insulator may be
disposed about one or more cores so that one or more center
conductors may be touching but cores are not in contact.
The insulator can facilitate positioning a center conductors in
close proximity to another center conductor. For example, two
center conductors may touch but not have the respective cores be in
contact. Such close arrangement of center conductors can permit
electrical coupling between the center conductors of high-frequency
energy, such as magnetic resonance energy, while preventing
coupling of low-frequency energy between the center conductors.
Coupling the center conductors for high-frequency energy
facilitates receiving magnetic resonance signals with the center
conductors because the center conductors so coupled can act as a
single electrical entity with respect to the high-frequency energy.
Thus, the electrical length of the distal portion 7 of the probe
100 can be preserved, because magnetic resonance energy can be
conducted straight through the plurality of center conductors,
without allowing the magnetic resonance energy to pass separately
through various conductors, thereby creating interference, or
causing the high-frequency energy to move through a longer path,
thereby unbalancing a magnetic resonance antenna. In contrast, a
thin insulating layer can be sufficient to prevent coupling between
conductors of the low-frequency signals that may be conducted along
selected center conductors. For example, low-frequency coupling may
not be desirable when the probe 100 is being operated to measure an
electrical potential between two electrodes contacting various
tissue regions. If the center conductors were permitted to couple
this low-frequency energy, then the potential measurement could be
distorted, lost in excessive noise, or attenuated entirely.
Similarly, ablation energy delivered along the probe 100 could be
shorted between center conductors if the center conductors were
permitted to couple low frequency energy.
Thus, the wire insulation is preferably sufficiently thin so that
the center conductors are electrically coupled through the
insulator at high frequency (e.g., above 10 MHz) but are isolated
at frequencies below 0.5 MHz.
Accordingly, insulator properties may be selected to facilitate
coupling of high-frequency energy between center conductors, while
lessening or inhibiting coupling of low-frequency energy.
Properties include the material or materials from which the
insulator is made, the thickness of the insulator, the number of
layers of insulator, the strength of the magnetic field in which
the probe 100 may be immersed, among others.
Because the insulator can prevent coupling of low-frequency energy
between the center conductors, the center conductors can be brought
into very close proximity to one another, also termed "tightly
coupled" to one another. The center conductors may be tightly
coupled, for example, by twisted around one another. Twisting or
otherwise tight-coupling the center conductors facilitates keeping
the center conductors in close proximity in the distal portion 7 of
the probe 100, where there may be no, e.g., first dielectric layer
to keep the center conductors closely apposed. In addition, because
reactive elements need not be interposed between the center
conductors to decouple low-frequency energy, manufacture of the
probe is simplified. Furthermore, the absence of reactive elements
can permit the achievement of small probe diameters. For example, a
probe having an outer of diameter of about 15 French or less,
suitable for, among other uses, cardiac catheterization,
observation, and/or ablation, can be readily constructed using
systems and methods disclosed herein. Moreover, deep brain
stimulation with a magnetic resonance probe is facilitated, because
the diameter can be reduced to, for example, 4 French or less, 3
French or less, 2 French or less, 1.3 French or less, 1 French or
less, 0.5 French or less, or even 0.1 French or less. The outer
diameter can be affected by the thickness of the center conductor
core, thickness of insulator, and thicknesses of other layers that
may be included. In an embodiment, wire may be used having a
thickness of 56 AWG to 16 AWG as well as thinner and/or thicker
wire.
A preferred insulator thickness may be determined as follows. The
inductance L and capacitance C between a twisted pair of wires per
unit length is given by the equations:
.mu..pi..times..times..times..function..intg..times..times..times.d.times-
..pi..times..times..function..intg..times..times.d ##EQU00001##
where .epsilon..sub.0=8.854 pF/m, d is the bare wire diameter in
meters, D is the insulated wire diameter in meters, and
.epsilon..sub.r is the relative dielectric constant of the
insulating material. In one illustrative embodiment, a 33 AWG
magnet wire was used, the wire having a nominal bare wire diameter
of 0.0071'' (0.00018034 m) and an insulated diameter of 0.0078''
(0.00019812 m) and an approximate dielectric constant of
.epsilon..sub.r=2. Thus, the insulator thickness was about 17.78
microns, or about 8.89 microns on a side. In this exemplary case
the estimated capacitance per unit length is 89 pF/m. This
corresponds to a capacitive impedance Z.sub.c=1/(2*.pi.*f) of about
28 .OMEGA./m at 63.86 MHz and giving a good coupling at the high
frequency range. Because the impedance scales inversely with
frequency, the low frequency impedance at 100 kHz is estimated to
be 14 k.OMEGA./m. An impedance of 10 k.OMEGA./m or greater is
sufficient in most applications to provide sufficient decoupling.
The high frequency impedance is preferably kept below 100
.OMEGA./m.
The impedance can also be controlled by the choice of dielectric
material. Typical materials include polyurethane resins, polyvinyl
acetal resins, polyurethane resins with a polyimide (nylon)
overcoat, THEIC modified polyester, THEIC modified polyester with a
polyamideimide (AI) overcoat, THEIC modified polyester, oxide-based
shield coat and a polyamideimide (AI) overcoat, aromatic polyimide
resin, bondable thermoplastic phenoxy overcoat, glass fiber, All
Wood Insulating Crepe Paper, Thermally Upgraded Electrical Grade
Crepe Kraft Paper, High Temperature Aramid Insulating Paper, and
combinations of these. The length of the proximal portion can be
modified by selecting dielectric materials for the first dielectric
layer and/or second dielectric layers. For example, a material with
a high dielectric constant can be incorporated in one or more
dielectric layers, thereby decreasing the electrical length of the
proximal portion and facilitating use of a probe in a relatively
shallow anatomic location. Examples of materials with appropriate
dielectric constants include ceramics.
An insulator disposed at least partially about a center conductor
core may have a thickness in a range up to about 2,000 microns,
preferably up to about 500 microns, more preferably up to about 200
microns, still more preferably up to about 100 microns, yet more
preferably in a range between about 1 micron and about 100 microns.
An insulator may have a thickness in the range of about 5 microns
to about 80 microns. An insulator may a thickness in the range of
about 8 microns to about 25 microns. An insulator may a thickness
in the range of about 10 microns to about 20 microns.
A core may have an insulator disposed about it by dipping the core
in insulator. A core may have an insulator disposed about it by
extruding an insulator over the core. A core may have an insulator
disposed about it by sliding the core into an insulator or sliding
an insulator over a core. A core may have an insulator disposed
about it by spraying.
A core may be formed of wire. The wire is preferably thin, to
promote small probe size, and may in one embodiment be thin
insulated copper wires (33 AWG), at times silver coated. In
preferred embodiments, the center conductors are formed of
magnetic-resonance compatible material. Preferably, the materials
are highly conducting, such as silver clad copper. The outer
conductive layer may also be formed of wire, such as braided wire.
Other preferred materials include a super elastic material, copper,
gold, silver, platinum, iridium, MP35N, tantalum, titanium,
Nitinol, L605, gold-platinum-iridium, gold-copper-iridium, and
gold-platinum.
As mentioned previously, the plurality of center conductors 2 in
the distal portion 7 of the probe 100 may form a first pole of a
dipole (loopless) magnetic resonance antenna, while the outer
conductive layer 12 in the proximal portion 8 of the probe 100 can
form the second pole. As discussed above, the length of the distal
portion, or first pole, is preferably approximately the
"quarter-wave" length, typically about 3 cm to about 15 cm. The
proximal portion or second pole can be of the same length, so that
the dipole antenna is balanced. A balanced dipole antenna can
provide slightly improved signal quality compared to an unbalanced
dipole antenna. However, a proximal portion of approximately even
15 cm may be impractical, because a user might want to introduce a
magnetic resonance probe into body structures deeper than 15 cm. In
practice, it has been found, fortuitously, that lengthening the
proximal portion or second pole, while unbalancing the antenna and
slightly degrading image quality, permits visualization of a
substantial length of the antenna, which facilitates tracking and
localization of the antenna. A significant complication of
unbalancing the antenna, namely heating effects during the
transmission mode, can be avoided by decoupling the antenna with,
for example, a PIN diode, as described below. FIGS. 7A-B depict the
effects of decoupling an unbalanced antenna. FIG. 7A shows a
heating profile of a decoupled antenna, which causes minimal
heating to surrounding tissue (typically less than 0.5 degrees
Celsius), while FIG. 7B shows a heating profile of a non-decoupled
antenna, which can cause gravely injurious and possible fatal
tissue heating of over 20 degrees Celsius in a matter of seconds.
Adjustments can typically be made to matching, tuning, and/or
decoupling circuits, examples of which are shown in FIGS. 3A-E.
The circuits shown in FIGS. 3A-E may have multiple functions and
can best described by examining four particular situations, the
transmit phase of the MRI system, the receive phase of the MRI
system, the recording of electrophysiological signals and the
stimulation or deliver of energy of or to the organ or tissue of
interest.
The MRI system typically alternates between a transmit and receive
state during the acquisition of an image. During the transmit phase
relatively large amounts of RF energy at the operating frequency of
the system, such as about 63.86 MHz, are transmitted into the body.
This energy could potentially harm the sensitive receiver
electronics and more importantly, the patient, if the imaging
antenna, in this case the probe, would be allowed to pick up this
RF energy. The antenna function of the probe therefore is
preferably turned off so that the probe becomes incapable of
receiving RF energy at the MRI system operating frequency. During
the receive phase, in contrast, the body emits the RF energy
absorbed during the transmit phase at the same frequency, i.e.,
63.86 MHz. A significant amount of the transmitted energy is
typically lost due to inefficiencies of the transmitter or has been
converted into heat by the body. The RF signal emitted by the body
containing the image information is typically therefore many orders
of magnitude smaller than the original signal send out by the
transmitter. In order to receive this small signal, the antenna
function of the probe is preferably turned on so that the probe
becomes a highly efficient receiver for RF signals at the MRI
systems operating frequency. The alternating state of the probe
from being a poor RF antenna (receiver) during the transmit phase
to being a good RF antenna (receiver) during the receive phase is
called T/R (Transmit/Receive) switching and may be facilitated via
a control signal send by the MRI system on the center conductor of
connector 15 in FIG. 3A. In an embodiment, this signal may be a
small positive voltage (5 to 15 Volts) during the transmit phase,
and a small negative voltage (-5 to -20 Volts) during the receive
phase. During the image acquisition, the system typically
alternates between the transmit and receive phase within
milliseconds, i.e., at about a kHz frequency.
During the transmit phase, the positive voltage on the center
conductor of connector 15 with respect to the system ground 14 may
cause the PIN diode 21 to be conductive and can therefore short the
top end of capacitors 23 to ground. The capacitors 23 in
combination with the proximal length of the probe form a
transmission line; thus, the impedance at the top of the capacitor
23 can be transformed via this transmission line to an impedance
Z.sub.J at the junction J connecting the poles of the electric
dipole antenna in FIG. 2A. A high impedance at this junction is
preferable to disable the reception of RF energy. To achieve a high
impedance at the junction J with shorted capacitors 23, the
transmission line should have an electrical length equivalent to a
quarter wavelength for RF propagation inside the transmission line.
The capacitance values for capacitors 23 may be selected to
fine-tune the effective electrical length of the transmission line
using routine experimentation. Typical values for capacitors can
fall in the range of 1-10,000 pF. The precise values of individual
capacitors 23 may vary slightly because each center conductor may
have a slightly different length (because center conductors may be
coupled to electrodes disposed at various positions along the
probe). In an embodiment, high Q capacitors such as ATC 100 A or B
are preferred. The wavelength may be determined by the diameter of
the center conductor bundle, the dielectric constant of the
dielectric material, and the inner diameter of the outer conductive
layer. In a typical exemplary embodiment, the physical length of
the proximal section of the probe forming the transmission line may
be 90 cm. Disabling the antenna function of the probe by presenting
a high impedance at the junction J is known as "decoupling."
With continued reference to FIGS. 3A-E, during the receive phase, a
negative voltage on the center conductor of connector 15 with
respect to the system ground 14 can "reverse bias" the diode 21,
thereby rendering it non-conductive. The antenna impedance seen by
the MRI system is preferably near 50 .OMEGA. for optimal
performance. Typically, the impedance of the electric dipole
antenna and the capacitors 23 is transformed to present the
appropriate impedance to the systems. This transformation may be
achieved via selection of appropriate inductor 19 and capacitor 17.
Preferably, the values for elements 19 and 17 may be chosen to pass
low frequency current, such as a switched DC signal to diode
21.
The T/R switching voltages are preferably not passed onto the probe
since the switching voltage, which can have a frequency around 1
kHz, may cause unwanted stimulation of the organ or tissue under
examination. To combat this, capacitors 23, providing a high-pass
filter function, can block propagation of the T/R switching voltage
into the probe.
With further reference to FIGS. 3A-E, because the antenna function
of the probe is enabled during the receive phase, the antenna will
pick up RF (63.86 MHz) signals emitted from the body. As shown in
FIG. 3A, the RF signal may be routed through the capacitors 23 to
the MRI system connector 15 and is processed by the MRI system. As
described above, capacitors 23 may function as high-pass filters so
that the high-frequency MRI signal is passed to the MRI system, but
lower frequency signal, such as the switching signal,
electrophysiological stimulation signal, biopotential measuring
signal, and/or ablative energy signal are blocked. The
lower-frequency signals may instead be routed through another
circuit, depicted in FIG. 3B. The signal at contacts 24 may be
split into two sets of leads, one set conveying the high-frequency
magnetic resonance signal to the magnetic resonance signal pathway
that may include capacitors 23 (FIGS. 3A and C), and the other set
conveying lower frequency signals to the electrophysiology pathway
that may include inductors 22 (FIGS. 3B and 3D). The inductors 22
can be chosen to block the high-frequency MRI signal (typically
around 64 MHz for a 1.5 Tesla field strength) but to pass lower
frequency signals such as the electrophysiological signals from the
brain, the heart, etc. Capacitors 20 can be provided to shunt to
ground MRI signal "leaking" through inductors 22. Thus, inductors
22 and capacitors 20 may form a low pass filter. Exemplary values
to filter high frequency MR signal at about 63.86 MHz can be about
10,000 pF for capacitors 20 and 5.6 .mu.H for inductors 22.
Electrophysiological (EP) signals may be measured independently of
the Transmit/Receive state of the MRI system because these signals
are typically in a frequency range far below the MRI signal
frequency and are separate from the MRI signal via a filter, such
as the signal split and low-pass filter depicted in FIG. 3B and
effected by inductors 22 and capacitors 20. The EP signals may pass
through this low pass filter to the connector 16 and can be routed
to the EP recording system, tissue stimulator, ablation energy
source, or the like. Similarly, tissue stimulation and/or tissue
ablation can be done independently of the Transmit/Receive state of
the MRI system because energy sent through the connector 16 from
either an ablation energy source, a cardiac stimulator, a
neurostimulator, etc. is at sufficiently low enough frequencies,
typically less than 500 kHz, that it will pass through the low pass
filter network shown in FIG. 3B and be conveyed into the probe to
one or more electrodes 3, 4, 5, 6, but will be blocked from
entering the MRI system by the high pass filter formed by
capacitors 23 in FIG. 3A. Examples of low voltage signals include
those for the treatment of Parkinson's disease as part of Deep
brain stimulation and RF energy at several hundred kilohertz that
may cause, among other effects, ablation of heart tissue. In the
latter case, the stimulus may be provided to only one electrode,
e.g., electrode 3, which may be located at the tip of the probe, to
facilitate precise delivery of heat therapy and to provide in some
embodiments a large contact area.
As depicted in FIGS. 3C-E, the magnetic resonance pathway can be
disposed on one substrate 26, and the electrophysiology pathway can
be disposed on another substrate 28. The substrates may be coupled
to a ground plane 29. The signal split at contacts 24 may be
provided through holes in substrate 26 to permit a connection to
contacts 27 for the electrophysiology pathway.
With further reference to FIGS. 2A-C and 3A-E, contacts 24 can mate
with the appropriate pins in the connector 9. The outer conductive
layer connector in connector 9 (ground) can mate with ground pin
25. During the transmit phase of the MR system, the pin diode 21
can be activated, as described above, and can thereby create a
short between the plurality of center conductors 2 and the outer
conductive layer 12. As described above, the electrical length of
the outer conductive layer 12 and capacitors 23 may be chosen so
that the short at diode 21 transfers down the transmission line
into an open at junction J at which the outer conductive layer
terminates.
FIGS. 4A-C depict an embodiment in which a probe is constructed to
be steerable (bi-directional). Many of the features are as
discussed for the embodiments shown in FIGS. 1 and 2A-C. The probe
100 may include a ribbon 36 disposed in the distal portion 7 of the
probe 100. In an embodiment, the ribbon 36 can extend to the tip of
the probe 100. The ribbon 36 can be bonded to the tip. The probe
100 can further include a pull wire 46. The pull wire 46 can be
coupled to the ribbon 36 so that the ribbon 36 may flex when the
pull wire 46 is manipulated. The pull wire 46 may be disposed in a
lumen 30 in the probe 30. The pull wire 46 may be coupled to a, for
example, a steering disc 33, which may be disposed in a handle 34
for the user's convenience. The plurality of center conductors 2
may be radially centered; they may be offset; they may be disposed
in a multi-lumen polymeric tubing; they may run along the length of
the probe. A second and/or additional lumens 30 can be provided. A
second and/or additional pull wires 46 can be provided. In the
distal portion 7, the steering assembly may be housed in a thin
walled flexible polymeric tubing to prevent direct electrical
contact with the center conductors 2 and/or electrodes 3, 4, 5, 6.
In the distal section the conductors may or may not be centered,
may be straight or coiled (around the steering mechanism assembly),
and/or may be connected to the electrodes electrically. The
steering mechanism if modified into a loop coil can have a
different matching-tuning and a decoupling circuit. The matching
tuning and decoupling circuitry for a steering mechanism acting as
a loopless antenna can be combined with that of the conductors
connecting to the electrodes. Materials used for the pull wires 46
may include non-metallic materials e.g. carbon fiber, composites,
nylon, etc to prevent the pull wires interacting with the center
conductors 2. The pull wires 46 can also be made from conducting
materials and turned either into loop or loopless coils based
described elsewhere.
FIGS. 5A-C depict a similar embodiment to the one shown in FIGS.
4A-C, with a coolant lumen 38 that may be provided to allow the
flow of coolants. Exemplary coolants include saline solution,
cooled gases, such as nitrogen, and water, among others.
Probes disclosed herein can facilitate three dimensional
electro-anatomical imaging. As depicted in FIGS. 6A-D, a probe can
be modified to a multi electrode array probe. The multi electrode
arrays (MEA) can be arranged on an expandable basket type probe.
This MEA probe can be used, for example, for non-contact or contact
endocardial mapping. The probe 100 may include a plurality of
expandable arms. The probe 100 may include a first dielectric layer
43. The probe 100 may include an outer conductive layer 42. The
probe 100 may include a second dielectric layer 41. The probe 100
may include a shaft 44 to push the basket and expand it. The probe
100 may include a bundle 45 of 8 insulated tightly coupled
conductors, resembling the center conductors described above, but
in this embodiment with more conductors in the bundle and multiple
bundles.
An electrode can be disposed on an arm. An electrode may be affixed
to an arm. An electrode may be glued or bonded to an arm. An arm
may include more than one electrode. A basket probe with, e.g., 8
expandable ribs and each carrying, e.g., 8 electrodes is depicted.
FIG. 6B depicts a long-axis view of an expandable arm 39, showing 8
electrodes disposed on the arm. During insertion into the body the
basket array probe may be collapsed to form a low profile probe,
once inside the desired anatomic space to be mapped, such as a
cardiac chamber, the basket may be expanded. The basket may be
expanded, for example, by coupling a pull wire to one or more arms,
or by forcing expansion with hydraulic force. The basket can expand
to a variety of sizes, such as space-limited by contacting the
walls of the anatomic site, or to a fixed diameter, dimension,
and/or shape, such that the arms of the basket expand in a
controlled manner, e.g. a cylinder. Mapping may then be carried out
by non-contact mapping. The electrical potentials measured at the
electrodes may be translated to the potentials on the endocardium.
The arms can be formed of materials similar to those used for
center conductors, as described above. The basket can be opened and
closed by advancing and retracting a sliding inner tubing. The
proximal shaft may include a sliding tubing centered in the outer
assembly which houses the conductors, dielectric/insulator,
shielding and an outer tubing. This assembly can act like a
loopless antenna, the shielding/braiding in the proximal shaft acts
as the ground, and the conductors connecting to the individual
electrodes act as the whip of the antenna. This assembly can be
matched-tuned and/or decoupled using systems and methods described
above. The probe can be provided with a curved tip for, e.g.,
maneuvering. An ablation electrode can be incorporated as described
above, such as at the distal tip. Steering systems as described
above can be provided. A steerable ablation multielectrode array
can facilitate mapping and treating tissue simultaneously. In an
embodiment, a non-contact EP map can be superimposed on a 3-D MR
image of the endocardium by using techniques described in, e.g.,
U.S. Pat. No. 5,662,108, hereby incorporated herein by reference.
In an embodiment, miniature loop coils may be placed adjacent one
or more electrodes to track the position of the one or more
electrodes and the distance from the electrode to the tissue
wall.
FIGS. 8A-C depict schematic diagrams of an exemplary embodiment of
a bi-directional steerable probe. In an embodiment, a steerable
probe may have two sections, a stiff proximal section and a
steerable distal section. In an embodiment, a steerable distal
section can have a length in the range of about 1 cm to about 15
cm. The steering can be achieved by including a fixed ribbon wire
in the distal section of the probe. The proximal section of the
flat ribbon wire can be anchored in the transition between the
stiff and flexible sections. The transition may include a joint,
such as a weld or a spot adhesive. The distal end of the flat wire
can be bonded to the distal tip of the probe. The pull
wires/steering mechanism wires may run along the length of the
probe. The proximal end of the pull wires can be attached to the
steering mechanism. The distal end of the pull wires/steering
mechanism may be attached to the distal end of the flat ribbon,
which is then bonded to the distal tip of the probe. In operation,
pulling or releasing the pull wire can bend or steer the distal tip
in the direction of the pull. The extent of the bending typically
depends on at least one of the inner diameter (ID) of the outer
tubing (distal section), the overall stiffness of the
tubing/assembly, and on other properties of the assembly. Steerable
probes may be modified so that they work like a RF loop antenna
coil, so that they may be actively tracked under MR. This helps the
operating clinician to know the exact position of the probe in the
anatomy.
Steerable probes may be modified for MR compatibility by using
non-magnetic materials. Steerable probes may be modified for MR
compatibility by using materials which create few or no
susceptibility artifacts. Appropriate materials include, e.g.,
polymers/plastics, metals--Nitinol, copper, silver or gold, gold
platinum alloy, MP35N alloy, etc. An exemplary design of the probes
is shown in FIGS. 8A-C. The proximal shaft of the probe may include
a multi-lumen tubing with at least 2 lumens parallel to each other.
These lumens can house a number of pull wires, such as 2 pull
wires. The pull wires may be connected to the steering handle at
the proximal end, and at the distal end they may be connected to
the distal end of the flat ribbon wire assembly, the proximal end
of which may be anchored in the transition. The two parallel pull
wires connected to the flat steering ribbon at the distal end can
form a loop antenna which can then be matched-tuned and/or
decoupled by the circuitry in the proximal handle. This creates an
MR compatible, MR safe bi-directional steering probe whose position
can be tracked under MRI.
Alternatively, as shown in FIGS. 8A-C, a bi-directional steerable
probe may include a loopless antenna. In this exemplary embodiment,
the outer proximal tubing has a braid under it or in the wall of
the outer tubing. This assembly acts like a loopless antenna, with
the pull wires and the flat ribbon assembly as the whip and the
braiding in or under the outer tubing as the ground forming a
loopless antenna. The matching-tuning and decoupling circuits may
be built proximal to the probe, e.g. in the steering handle. This
design enables the probe to be tracked under MR and capable of
acquiring high resolution images in the vicinity of the probe.
FIGS. 9A-C and 10A-C depict exemplary embodiments of unidirectional
steerable probes. These embodiments may be similar to in design to
the loopless bi-directional steerable probe, except that there is a
single pull wire. This design can be used to image under MRI and
also to be tracked under MR. The proximal shaft/section can have a
braiding in the wall or under the outer tubing. The pull wire may
run radially in the center of the tubing thus creating a structure
similar to a coaxial cable (FIGS. 10A-C) or can be radially offset
from the center (FIGS. 9A-C). The matching-tuning and decoupling
circuit can be built in the proximal section of the probe, making
it function similar to a loopless antenna, and/or enabling it to be
tracked under MRI. It can also be used to acquire high-resolution
images of the anatomy around the probe.
Additional teachings regarding construction of magnetic resonance
probes, selection of materials, preferable dimensions of
components, and electrical properties of probes are provided, e.g.,
in U.S. Pat. Nos. 5,928,145, 6,263,229, 6,549,800, and in U.S.
patent application Publication Ser. Nos. US 2002/0,045,816 A1, US
2002/0,161,421 A1, US 2003/0,028,095 A1, and US 2003/0,050,557 A1,
all of which patents and patent application publications are hereby
incorporated herein in their entireties by this reference.
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.
* * * * *