U.S. patent application number 13/104347 was filed with the patent office on 2011-09-01 for stimulation lead, stimulation system, and method for limiting mri-induced current in a stimulation lead.
Invention is credited to Brian Blischak.
Application Number | 20110213445 13/104347 |
Document ID | / |
Family ID | 44121963 |
Filed Date | 2011-09-01 |
United States Patent
Application |
20110213445 |
Kind Code |
A1 |
Blischak; Brian |
September 1, 2011 |
STIMULATION LEAD, STIMULATION SYSTEM, AND METHOD FOR LIMITING
MRI-INDUCED CURRENT IN A STIMULATION LEAD
Abstract
In one embodiment, a stimulation lead for delivering electrical
pulses from a pulse generator to tissue of a patient, comprises: a
plurality of electrodes; a plurality of terminals; a plurality of
conductors electrically coupling the plurality of electrodes with
the plurality of terminals; a lead body of insulative material for
enclosing the plurality of conductors; and at least one
magnetic-field actuated switch for limiting MRI-induced current
between the plurality of electrodes and the plurality of terminals,
wherein the magnetic-field actuated switch is actuated by
magnetostrictive material.
Inventors: |
Blischak; Brian; (Allen,
TX) |
Family ID: |
44121963 |
Appl. No.: |
13/104347 |
Filed: |
May 10, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12026078 |
Feb 5, 2008 |
7962224 |
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13104347 |
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60888184 |
Feb 5, 2007 |
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Current U.S.
Class: |
607/72 ;
607/116 |
Current CPC
Class: |
A61N 1/0553 20130101;
A61N 1/086 20170801 |
Class at
Publication: |
607/72 ;
607/116 |
International
Class: |
A61N 1/36 20060101
A61N001/36; A61N 1/05 20060101 A61N001/05 |
Claims
1. An implantable pulse generator for generating electrical pulses
to stimulate tissue of a patient, comprising: pulse generating
circuitry for generating electrical pulses; output circuitry for
coupling electrical pulses from the pulse generating circuitry to
output contacts adapted for electrical coupling to terminal
contacts of one or more stimulation leads; and at least one
magnetic-field actuated switch for modifying one or more circuit
paths between the pulse generating circuitry and the output
contacts to limit MRI-induced current between the pulse generating
circuitry and the output contacts, wherein the magnetic-field
actuated switch is actuated by magnetostrictive material.
2. The implantable pulse generator of claim 1 wherein the
magnetic-field actuated switch controls a resistance, inductance,
or capacitance within the circuit path.
3. The implantable pulse generator of claim 1 wherein the
magnetic-field actuated switch creates an open-circuit condition
for the circuit path.
4. The implantable pulse generator of claim 1 wherein the
magnetic-field actuated switch is disposed within a header of the
implantable pulse generator and mechanically separates the output
contacts electrical coupling with the output circuitry.
5. The implantable pulse generator of claim 1 wherein the
magnetostrictive material is a terbium, iron, and dysprosium
alloy.
6. The implantable pulse generator of claim 1 wherein the
magnetostrictive material deforms a piezoelectric material to
generate a voltage that in turn controls an electronic circuit, and
the magnetic-field actuated switch is controlled in response to the
voltage from the piezoelectric element.
7. An implantable extension connector for connecting an implantable
stimulation lead to an implantable extension lead, comprising: an
extension body adapted to receive the stimulation lead and the
implantable extension lead; a first set of electrical contacts
within the extension body for electrically coupling to terminals of
the stimulation lead; a second set of electrical contacts within
the extension body for electrically coupling to electrodes of the
extension lead; at least one magnetic-field actuated switch for
limiting MRI-induced current between the first and second sets of
electrical contacts, wherein the magnetic-field actuated switch is
actuated by magnetostrictive material.
Description
RELATED APPLICATION
[0001] This application is a divisional of U.S. application Ser.
No. 12/026,078, filed Feb. 5, 2008, pending, which claims the
benefit of U.S. Provisional Application No. 60/888,184, filed Feb.
5, 2007, the disclosure of which is incorporated herein by
reference.
TECHNICAL FIELD
[0002] The present application is generally related to limiting the
amount of MRI-induced current that flows between the surrounding
tissue and a stimulation lead such as a neurostimulation lead, a
cardiac stimulation lead, and/or the like.
BACKGROUND
[0003] Neurostimulation systems are devices that generate
electrical pulses and deliver the pulses to nerve tissue to treat a
variety of disorders. Spinal cord stimulation (SCS) is an example
of neurostimulation in which electrical pulses are delivered to
nerve tissue in the spine for the purpose of chronic pain control.
Other examples include cardiac rhythm management, deep brain
stimulation, cortical stimulation, cochlear nerve stimulation,
peripheral nerve stimulation, vagal nerve stimulation, sacral nerve
stimulation, optical nerve stimulation, functional electrical
stimulation, etc. While a precise understanding of the interaction
between the applied electrical energy and the nervous tissue is not
fully appreciated, it is known that application of an electrical
field to spinal nervous tissue can effectively mask certain types
of pain transmitted from regions of the body associated with the
stimulated nerve tissue. Specifically, applying electrical energy
to the spinal cord associated with regions of the body afflicted
with chronic pain can induce "paresthesia" (a subjective sensation
of numbness or tingling) in the afflicted bodily regions. Thereby,
paresthesia can effectively mask the transmission of non-acute pain
sensations to the brain.
[0004] Neurostimulation systems generally include a pulse generator
and one or several leads. The pulse generator is typically
implemented using a metallic housing that encloses circuitry for
generating the electrical pulses. The pulse generator is usually
implanted within a subcutaneous pocket created under the skin by a
physician. The leads are used to conduct the electrical pulses from
the implant site of the pulse generator to the targeted nerve
tissue. The leads typically include a lead body of an insulative
polymer material with embedded wire conductors extending through
the lead body. Electrodes on a distal end of the lead body are
coupled to the conductors to deliver the electrical pulses to the
nerve tissue.
[0005] There are concerns related to the compatibility of
neurostimulation systems with magnetic resonance imaging (MRI). MRI
generates cross-sectional images of the human body by using nuclear
magnetic resonance (NMR). The MRI process begins with positioning
the patient in a strong, uniform magnetic field. The uniform
magnetic field polarizes the nuclear magnetic moments of atomic
nuclei by forcing their spins into one of two possible
orientations. Then an appropriately polarized pulsed RF field,
applied at a resonant frequency, forces spin transitions between
the two orientations. Energy is imparted into the nuclei during the
spin transitions. The imparted energy is radiated from the nuclei
as the nuclei "relax" to their previous magnetic state. The
radiated energy is received by a receiving coil and processed to
determine the characteristics of the tissue from which the radiated
energy originated to generate the intra-body images.
[0006] Currently, most neurostimulation systems are designated as
being contraindicated for MRI, because the time-varying magnetic RF
field causes the induction of current which, in turn, can cause
significant heating of patient tissue due to the presence of metal
in various system components. The induced current can be "eddy
current" and/or current caused by the "antenna effect." As used
herein, the phrase "MRI-induced current" refers to eddy current
and/or current caused by the antenna effect.
[0007] "Eddy current" refers to current caused by the change in
magnetic flux due to the time-varying RF magnetic field across an
area bounding conductive material (i.e., patient tissue). The
time-varying magnetic RF field induces current within the tissue of
a patient that flows in closed-paths. When a conventional pulse
generator and a conventional implantable lead are placed within
tissue in which eddy currents are present, the implantable lead and
the pulse generator provide a low impedance path for the flow of
current. Electrodes of the lead provide conductive surfaces that
are adjacent to current paths within the tissue of the patient. The
electrodes are coupled to the pulse generator through a wire
conductor within the implantable lead. The metallic housing (the
"can") of the pulse generator provides a conductive surface in the
tissue in which eddy currents are present. Thus, current can flow
from the tissue through the electrodes and out the metallic housing
of the pulse generator- or vice versa. Because of the low impedance
path and the relatively small surface area of each electrode, the
current density in the patient tissue adjacent to the electrodes
can be relatively high. Accordingly, resistive heating of the
tissue adjacent to the electrodes can be high and can cause
significant, irreversible tissue damage.
[0008] Also, the "antenna effect" can cause current to be induced
which can result in undesired heating of tissue. Specifically,
depending upon the length of the stimulation lead and its
orientation relative to the time-varying magnetic RF field, the
wire conductors of the stimulation lead can each function as an
antenna and a resonant standing wave can be developed in each wire.
A relatively large potential difference can result from the
standing wave thereby causing relatively high current density and,
hence, heating of tissue adjacent to the electrodes of the
stimulation lead.
SUMMARY
[0009] In one embodiment, a stimulation lead for delivering
electrical pulses from a pulse generator to tissue of a patient,
comprises: a plurality of electrodes; a plurality of terminals; a
plurality of conductors electrically coupling the plurality of
electrodes with the plurality of terminals; a lead body of
insulative material for enclosing the plurality of conductors; and
at least one magnetic-field actuated switch for limiting the amount
of MRI-induced current that flows between the surrounding tissue
and the lead, wherein the magnetic-field actuated switch is
actuated by magnetostrictive material.
[0010] In one embodiment, an implantable pulse generator for
generating electrical pulses to stimulate tissue of a patient,
comprises: pulse generating circuitry for generating electrical
pulses; output circuitry for coupling electrical pulses from the
pulse generating circuitry to output contacts adapted for
electrical coupling to terminal contacts of one or more stimulation
leads; and at least one magnetic-field actuated switch for
modifying a circuit path between the pulse generating circuitry and
the output contacts to limit MRI-induced current between the pulse
generating circuitry and the output contacts, wherein the
magnetic-field actuated switch is actuated by magnetostrictive
material.
[0011] The foregoing has outlined rather broadly certain features
and/or technical advantages in order that the detailed description
that follows may be better understood. Additional features and/or
advantages will be described hereinafter which form the subject of
the claims. It should be appreciated by those skilled in the art
that the conception and specific embodiment disclosed may be
readily utilized as a basis for modifying or designing other
structures for carrying out the same purposes. It should also be
realized by those skilled in the art that such equivalent
constructions do not depart from the spirit or scope of the
appended claims. The novel features, both as to organization and
method of operation, together with further objects and advantages
will be better understood from the following description when
considered in connection with the accompanying figures. It is to be
expressly understood, however, that each of the figures is provided
for the purpose of illustration and description only and is not
intended as a definition of the limits of the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 depicts a paddle lead according to one representative
embodiment.
[0013] FIGS. 2A-2E depict a circuit connection that is controllably
interrupted in response to the detection of a sufficiently strong
magnetic field according to one representative embodiment.
[0014] FIG. 3 depicts a connection circuit that is controllably
established in response to the detection of a sufficiently strong
magnetic field according to one representative embodiment.
[0015] FIG. 4 depicts a circuit containing a piezo element
mechanically contacted by magnetostrictive material according to
one representative embodiment.
[0016] FIGS. 5A-5B depict implantable pulse generators according to
some representative embodiments.
[0017] FIG. 6 depicts an extension connector according to one
representative embodiment.
[0018] FIG. 7 depicts a current limiting circuit controlled
according to one representative embodiment.
[0019] FIG. 8 depicts a percutaneous lead according to one
representative embodiment.
DETAILED DESCRIPTION
[0020] FIG. 1 depicts paddle lead 100 for limiting MRI-induced
current according to one representative embodiment. Paddle lead 100
is preferably adapted for spinal cord stimulation. However, paddle
lead 100 may be utilized for any suitable stimulation application
such as cortical stimulation, peripheral nerve stimulation,
functional electrical stimulation as examples. Also, although some
embodiments are discussed in the context of various types of
neurostimulation, alternative embodiments can be employed in any
implantable medical device or system that applies electrical pulses
to patient tissue and/or measures physiological signals such as
cardiac pacing systems, defibrillators, gastric pacing systems,
etc.
[0021] Paddle lead 100 comprises a plurality of electrodes 101
implemented on paddle structure 102 at the distal end of lead 100.
In the specific embodiment shown in FIG. 1, two columns of
electrodes 101 are employed with four electrodes in each column.
The two columns of electrodes 101 are staggered with respect to
each other. Any suitable number of electrodes and any suitable
electrode pattern can be employed for paddle 100 according to other
representative embodiments. Also, paddle lead 102 may optionally
comprise indifferent electrode 107 to allow MRI-induced current to
be more safely distributed over a greater surface area.
[0022] Paddle structure 102 may be fabricated using any suitable
material as known to those skilled in the art. Such materials may
include polyetheretherketone (PEEK) and polyether-ketone ketone
(PEKK), and silicone rubber adapted to be disposed within the human
body. Further structural elements can be included within paddle
structure 102 such as a fiberglass or metal mesh (not shown) to
provide structural rigidity to paddle structure 102 and/or to
provide a desired shape to paddle structure 102. For example, the
side of paddle structure 102 opposite to the side where electrodes
101 are disposed may possess a curve corresponding to the posterior
shape of the epidural space 17. The side of paddle structure 102
where the electrodes are disposed may be shaped to allow electrodes
101 to be positioned substantially tangential to the dura. "Wing"
structures (not shown) extending above the surface may be provided
to assist in retaining paddle structure 102 within an optimal
position within the epidural space.
[0023] The proximal end of paddle lead 100 couples to an
implantable pulse generator, intermediate wiring, or other
stimulation device (not shown) as known to those skilled in the
art. The stimulation pulses produced by the implantable pulse
generator are received by terminals 106 and are conducted through
lead body 105 via wire conductors (not shown). The wire conductors
may take the form of solid conductors, drawn-filled-tube (DFT),
drawn-brazed-strand (DBS), stranded conductors or cables, ribbon
conductors, or other forms known or recognized to those skilled in
the art. The composition of the conductors may include aluminum,
stainless steel, MP35N, platinum, gold, silver, copper, vanadium,
alloys, or other conductive materials or metals known to those of
ordinary skill in the art. An insulative coating can be optionally
applied over the metal material of conductors.
[0024] Lead body 105 is typically fabricated by winding the wire
conductors around a mandrel (with or without one or more insulative
layers). One or more outer layers of insulative material are
applied over the wound conductive wires. The insulative material
may include silicone, polyurethane, polyethylene, polyimide,
polyvinylchloride, PTFT, EFTE, or other suitable materials known to
those skilled in the art. The insulative material and conductive
wires may be subjected to heat and pressure (e.g., using shrink
wrap tubing) to cause the insulative material to flow and fuse.
Such techniques for fabricating lead body 105 are discussed in
greater detail in U.S. patent application Ser. No. 10/630,233,
entitled "SYSTEM AND METHOD FOR PROVIDING A MEDICAL LEAD BODY
HAVING DUAL CONDUCTOR LAYERS," which is incorporated herein by
reference, although any other suitable technique for fabrication of
lead body 105 may be employed. The wire conductors of lead body 105
terminate at the distal end of lead body and are electrically
coupled to an initial set of wires 104 within paddle structure
102.
[0025] The initial set of wire conductors 104 are electrically
coupled through switch structure 110 to a second set of wire
conductors 103 which are, in turn, coupled to the respective
electrodes 101. Switch structure 110 is operable to selectively
control one or several electrical connections to limit MRI-induced
current from flowing through electrodes 103 to the wire conductors
of lead body 105. Switch structure 110 is actuated or controlled in
response to the detection of a magnetic field. In one embodiment,
the magnetic material of switch structure 110 is adapted to
automatically to rotate to align with the magnetic field. Such
rotation can be accomplished by attaching the magnetic material to
a gimble with a magnet on it so that an external magnet will spin
the gimble.
[0026] Switch structure 110 is preferably actuated or controlled
using magnetostrictive material. A preferred magnetostrictive
material is the Terfenol-D material (a terbium, iron, and
dysprosium alloy) which is commercially available. The
magnetostrictive effect is molecular in origin and is caused by the
rotation of small magnetic domains that result in internal strains
within the material. These strains cause a positive expansion of
the material in parallel to the magnetic field. As the field
strength increases, more domains within the material rotate and
become aligned until magnetic saturation is reached. If the field
is reversed, the direction of the domains is also reversed, but the
strain within the material still results in a positive expansion in
the field direction. The mechanical response is relatively fast
(e.g., microseconds) and magnetostrictive material conserves volume
(e.g., the diameter decreases as the length grows).
[0027] Switch structure 110 may be utilized to control, mitigate,
or otherwise limit MRI-induced current in a number of ways. Switch
110 may be implemented using any suitable mechanical, electrical,
or electro-mechanical design as examples. For example, switch
structure 110 could simply be implemented to allow each wire 104 to
electrically contact a corresponding wire 103 in the absence of a
magnetic field and to separate each wire 104 from its corresponding
wire 103 in the presence of a sufficiently strong magnetic field
due to the expansion of the magnetostrictive material. Effectively,
an open circuit would be created for each electrode 103 in the
presence of a sufficiently strong magnetic field thereby preventing
the conduction of MRI-induced current.
[0028] In another embodiment, switch structure 110 is operable to
change an impedance in the current path between each wire 104 and a
corresponding wire 103 in response to the detection of a
sufficiently strong magnetic field. A change in resistance,
inductance, or capacitance could be affected under the controler of
switch structure 110. Alternatively, switch structure 110 could be
implemented to control MRI-induced current by shunting the
MRI-induced current to an indifferent electrode. Preferably, the
indifferent electrode possesses a surface area greater than the
surface area of electrodes 103. By utilizing a greater surface
area, the current density is reduced and the amount of resistive
heating is also reduced. Switch structure 110 could be employed to
allow the wire conductors within lead body 105 to be coupled to
suitable circuitry to "de-tune" the wire conductors to the MRI
frequency so that the antenna effect captures less energy.
Specifically, switch structure 110 could be employed somewhere
within the middle of the lead to separate the wire conductors
thereby effectively shortening electrical length of the wire
conductors.
[0029] FIGS. 2A-2E depict circuit connection 200 that may be
included within switch structure 110 according to one
representative embodiment. Specifically, a respective circuit
connection 200 can be included within switch structure 110 between
each wire 103 and a corresponding wire 104 to control the
conduction of MRI-induced current. In some embodiments, circuit
connection 200 is preferably fabricated as part of a
micro-electrical-mechanical (MEMs) device.
[0030] FIG. 2A depicts circuit connection 200 that is controllably
interrupted in response to the detection of a sufficiently strong
magnetic field according to one representative embodiment. Circuit
connection 200 is controlled by magnetostrictive elements 201a and
201b. FIG. 2A depicts circuit connection 200 in the absence of a
magnetic field. As shown in FIG. 2A, wire 103 is in electrical and
mechanical contact with conductor 202a which is mechanically
coupled to magnetostrictive element 201a. Also, wire 104 is in
electrical and mechanical contact with conductor 202b which is
mechanically coupled to magnetostrictive element 201b. Intermediate
conductor 203 is in electrical and mechanical contact with
conductors 202a and 202b. Accordingly, current is able to flow from
wire 103 through conductor 202a, intermediate conductor 203,
conductor 202b, and wire 104. Magnetostrictive elements 201a and
201b are disposed in an orthogonal relationship to each other.
[0031] FIG. 2B depicts the "upper" portion of circuit connection
200 in the presence of a sufficiently strong magnetic field in the
direction parallel to magnetostrictive element 201a. The magnetic
field (e.g., the static magnetic field of a commercially available
MRI system) causes magnetostrictive element 201a to expand in the
direction of the magnetic field. Conductor 202a is translated by
the expansion and is no longer in mechanical and electrical contact
with wire 103 and intermediate conductor 203. Accordingly, current
can no longer flow through circuit connection 200 in the presence
of the magnetic field. If the magnetic field is in the direction
opposite the direction shown in FIG. 2A, magnetostrictive element
201a would expand while conductor 202a would be translated in the
opposite direction. Nonetheless, conductor 202a would no longer be
in electrical and mechanical contact with wire 103 and intermediate
conductor 203.
[0032] FIG. 2C depicts the "lower" portion of circuit connection
200 in the presence of a sufficiently strong magnetic field in the
direction parallel to magnetostrictive element 201b. The magnetic
field causes magnetostrictive element 201b to expand in the
direction of the magnetic field. Conductor 202b is translated by
the expansion and is no longer in mechanical and electrical contact
with wire 104 and intermediate conductor 203. Accordingly, current
can no longer flow through circuit connection 200 in the presence
of the magnetic field.
[0033] FIG. 2D depicts the "upper" portion of circuit connection
200 in the presence of a sufficiently strong magnetic field in the
direction orthogonal to both magnetostrictive elements 201a and
201b. As shown in FIG. 2D, magnetostrictive element 201a expands
"up" in the direction of the magnetic field. Conductor 202a is
translated by the expansion and is no longer in mechanical and
electrical contact with wire 103 and intermediate conductor 203.
Accordingly, current can no longer flow through circuit connection
200 in the presence of the magnetic field.
[0034] FIG. 2E depicts the "lower" portion of circuit connection
200 in the presence of a sufficiently strong magnetic field in the
direction opposite to the field shown in FIG. 2D. As shown in FIG.
2E, magnetostrictive element 201b expands "down" in the direction
of the magnetic field. Conductor 202b is translated by the
expansion and is no longer in mechanical and electrical contact
with wire 104 and intermediate conductor 203. Accordingly, current
can no longer flow through circuit connection 200 in the presence
of the magnetic field.
[0035] By utilizing the arrangement of circuit connection 200,
switch structure 110 is able to prevent the conduction of
MRI-induced current due to the presence of the strong static
magnetic field of the MRI system without regard to the specific
direction of the magnetic field relative to the orientation of
paddle structure 102.
[0036] FIG. 3 depicts connection circuit 300 according to another
representative embodiment. A plurality of connection circuits 300
could be implemented within switching structure 110 for each
circuit path electrically coupled to the plurality of electrodes
101. Connection circuit 300 is adapted such that an electrical
connection is not maintained in the absence of a magnetic field.
Instead, when a sufficiently strong magnetic field is present,
connection circuit 300 establishes an electrical connection to
shunt current from line 301 (which could be a respective wire 103
or wire 104) to an indifferent electrode. That is, connection
circuit 300 functions as a switching element to selectively switch
MRI-induced current to the indifferent electrode.
[0037] As shown in FIG. 3, magnetostrictive element 301a is
mechanically coupled to three conductors 302a, 302b, and 302c.
Conductors 302a and 303b are adapted such that a magnetic field
parallel to magnetostrictive element 301 causes one of these
conductors to establish an electrical connection. Magnetostrictive
element 301b is co-planar and orthogonal to magnetostrictive
element 301a. Magnetostrictive element 301b is mechanically coupled
to three conductors 302d, 302e, and 302f. Conductors 302d and 303f
are adapted such that a magnetic field parallel to magnetostrictive
element 301 causes one of these conductors to establish an
electrical connection. Although not visible in the view shown in
FIG. 3, conductors 302b and 302e are preferably offset in the
vertical direction such that conductors 302b and 302e do not make
an electrical connection in the absence of a magnetic field, i.e.,
one is offset "above" and the other is offset "below." When a
sufficiently strong magnetic field in the out-of-plane direction is
present, one of conductors 302b and 302e make an electrical
connection thereby allowing current flow to the indifferent
electrode.
[0038] Circuits 200 and 300 are provided by way of example. Any
suitable circuit design in which a circuit connection is
controllably made or disconnected in response to detection of a
magnetic field may be utilized according to representative
embodiments. Levers or other mechanical components can be utilized
to scale the dimensional changes of the magnetostrictive material
if deemed appropriate for a particular lead or system design.
Furthermore, the actuation or control by the magnetostrictive
material may be utilized to control multiple poles of a respective
switch.
[0039] Although switching structure 110 has been discussed as being
implemented at the proximal end of paddle structure 102, switching
structure 110 (or parts thereof) could be implemented at any
suitable location within lead 100. For example, respective
connection circuits could be employed anywhere along the length of
lead body 105. The connection circuits could be utilized to divide
the lead 100 into small lengths so that the wire conductors within
lead body act as smaller antennas which do not resonate at the MRI
frequency.
[0040] Also, although an embodiment has been discussed in regard to
a paddle lead, suitable MRI limiting functionality could be
implemented within a percutaneous lead, a cortical lead, or a deep
brain stimulation (DBS) lead according to some alternative
embodiments. For example, FIG. 8 depicts percutaneous lead 800
adapted according to one representative embodiment. Percutaneous
lead 800 comprises lead body 801 with a plurality of terminals 802
disposed at the proximal end of percutaneous lead 800. At the
distal end of lead 800, a plurality of electrodes 803 are provided.
Each electrode 803 is preferably selectively coupled to a
conductive wire (not shown) of the lead body through a
magnetostrictive switch element 804. When a sufficiently strong
magnetic field is detected by the magnetostrictive material, MRI
induced current is prevented from being conducted through a given
electrode 803 into its corresponding wire conductor of lead body
801.
[0041] In an alternative embodiment, magnetostrictive material may
be utilized to generate a control signal that controls switching
operations to limit MRI-induced current. For example, in circuit
400 as shown in FIG. 4, magnetostrictive material 401 may be
utilized to effect piezoelectric element 401. The presence of a
magnetic field causes deformation of magnetostrictive material 401.
The deformation places a strain on piezoelectric element 401 which
generates a voltage in response to the strain. The voltage can then
be communicated along sensor signal line 403 for detection
utilizing suitable circuitry (e.g., an analog-to-digital converter
and a microprocessor, or simple voltage-triggered switch).
[0042] Alternatively, the voltage can be provided (possibly in
amplified and filtered form) to control a current limiting device.
For example, as shown in FIG. 7, the sensor signal is preferably
amplified and rectified by processing circuitry 702 and is provided
to the gate of field effect transistor (FET) 701. As the magnetic
field increases, the output from processing circuitry 702 increases
and the channel impedance of FET 701 is increased. Preferably at
high magnetic field strengths, the impedance of the channel of FET
701 is sufficiently high that relatively little current flows
between the source and drain. In this manner, FET 701 essentially
functions as a choke to control the current in response to the
detection of a sufficiently strong magnetic field. Although FET 701
is shown between lines 103 and 104 in FIG. 7, such current control
could be employed at any suitable location with a stimulation
system or device.
[0043] FIG. 5A depicts pulse generator 500 that utilizes such a
sensor signal according to one representative embodiment. FIG. 5A
depicts pulse generation circuitry 501 which generates electrical
pulses for stimulation of patient tissue. The electrical pulses are
switched through connection circuitry 502 (e.g., a switch matrix)
for coupling to one or multiple electrodes through the electrical
connections of header structure 503. Connection circuitry 502 is
controlled, in part, by circuit 400. Specifically, if magnetic
field is detected, connection circuitry 502 is controlled by
controller 504 to create an open circuit condition utilizing its
switches thereby preventing MRI-induced current from being
conducted by the "can" of pulse generator 500.
[0044] FIG. 5B depicts pulse generator 550 according to an
alternative embodiment. Pulse generator 550 comprises header 551
modified to limit MRI-induced current. Specifically, instead of
connecting feedthrough wire(s) 553 directly to the electrical
contacts 552 that connect to the terminals of a stimulation lead,
the feedthrough wire(s) 553 are connected to contacts 552 through
switch 110. Accordingly, if a sufficiently strong magnetic field is
present, the stimulation lead will no longer be in electrical
contact with pulse generating circuitry 501 and, hence, current
cannot flow from the lead to the "can" of the pulse generator 550.
Thus, eddy current is limited by the operation of switch 110.
[0045] In an alternative embodiment, an "extension" connector may
be adapted according to one representative embodiment to limit
MRI-induced current. As shown in FIG. 6, extension connector 600 is
adapted to electrically couple to the electrodes of an "extension"
lead using contacts 601. Extension connector 600 preferably
comprises an outer housing or body of biocompatible, biostable
polymer that houses electrical contacts for a stimulation lead and
an extension lead. Extension connector 600 is adapted to
electrically couple to the terminals of a lead using contacts 602.
Switching structure 110 is disposed between contacts 601 and 602 to
control the electrical coupling between contacts 601 and 602 in
response to the presence of a magnetic field. The detection of the
electrical field may be utilized by the switching structure to
disconnect the electrical couplings or shunt the MRI-induced
current to an indifferent electrode (not shown) on the body of
connector 600 as examples.
[0046] The use of magnetostrictive elements to limit or control
MRI-induced current may be advantageous for a number of reasons.
For example, the magnetostrictive effect is an intrinsic property
at the molecular level. When the magnetostrictive material reaches
its maximum dimension (i.e., all magnetic poles aligned), then
further increases in the magnetic field have no effect. The
magnetostrictive effect allows a single switch to work for a
variety of commercially available MRI systems. Also, the use of
magnetostrictive elements is advantageous because the particular RF
frequency of an MRI system does not effect the operation of the
switch. Additionally, certain embodiments that utilize
magnetostrictive elements do not require electrical power to
function. Magnetostrictive elements are robust mechanically. In
contrast, conventional "reed switches" have been used to detect
magnetic fields to control the operations of pulse generator
systems. Reed switches utilize ferromagnetic material. The
magnetization of the ferromagnetic material can be permanently
changed in the large magnetic field of MRI systems and, thereby,
the reed switches can become non-functional. Reed switches tend to
be the least reliable components in a pulse generator and are
susceptible to "sticking" in a strong magnetic field. Also, reed
switches are relatively large thereby complicating the design of
implantable medical devices.
[0047] Although certain representative embodiments and advantages
have been described in detail, it should be understood that various
changes, substitutions and alterations can be made herein without
departing from the spirit and scope of the appended claims.
Moreover, the scope of the present application is not intended to
be limited to the particular embodiments of the process, machine,
manufacture, composition of matter, means, methods and steps
described in the specification. As one of ordinary skill in the art
will readily appreciate when reading the present application, other
processes, machines, manufacture, compositions of matter, means,
methods, or steps, presently existing or later to be developed that
perform substantially the same function or achieve substantially
the same result as the described embodiments may be utilized.
Accordingly, the appended claims are intended to include within
their scope such processes, machines, manufacture, compositions of
matter, means, methods, or steps.
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