U.S. patent application number 10/796239 was filed with the patent office on 2004-11-18 for multipolar electrode system for volumetric radiofrequency ablation.
Invention is credited to Haemmerich, Dieter, Johnson, Chris D., Lee, Fred T., Mahvi, David M., Webster, John G., Wright, Andrew S..
Application Number | 20040230187 10/796239 |
Document ID | / |
Family ID | 26863382 |
Filed Date | 2004-11-18 |
United States Patent
Application |
20040230187 |
Kind Code |
A1 |
Lee, Fred T. ; et
al. |
November 18, 2004 |
Multipolar electrode system for volumetric radiofrequency
ablation
Abstract
In radiofrequency ablation, larger lesion volumes are obtained
for a given energy delivery by energizing at least two electrodes
on either side of the tumor so that current is focused between them
rather than dispersed radially to a large area ground plate.
Modified standard umbrella probes may be used or a specialized dual
electrode array may be fabricated for simplified use. Differential
impedance between tumor and non-tumor tissues at certain
frequencies is exploited to further improve lesion shape and
size.
Inventors: |
Lee, Fred T.; (Madison,
WI) ; Haemmerich, Dieter; (Madison, WI) ;
Webster, John G.; (Madison, WI) ; Wright, Andrew
S.; (Madison, WI) ; Johnson, Chris D.;
(Madison, WI) ; Mahvi, David M.; (Middleton,
WI) |
Correspondence
Address: |
QUARLES & BRADY LLP
411 E. WISCONSIN AVENUE, SUITE 2040
MILWAUKEE
WI
53202-4497
US
|
Family ID: |
26863382 |
Appl. No.: |
10/796239 |
Filed: |
March 9, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10796239 |
Mar 9, 2004 |
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10167681 |
Jun 10, 2002 |
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10796239 |
Mar 9, 2004 |
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09873541 |
Jun 4, 2001 |
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60315383 |
Aug 28, 2001 |
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60210103 |
Jun 7, 2000 |
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Current U.S.
Class: |
606/32 ;
606/41 |
Current CPC
Class: |
A61B 2018/0075 20130101;
A61B 2018/00755 20130101; A61B 2018/00791 20130101; A61B 2018/00875
20130101; A61B 2018/00827 20130101; A61B 18/1206 20130101; A61B
2018/124 20130101; A61B 2018/00726 20130101; A61B 2018/1432
20130101; A61B 2018/143 20130101; A61B 18/1477 20130101; A61B
2018/1467 20130101; A61B 2018/00654 20130101; A61B 2018/00797
20130101 |
Class at
Publication: |
606/032 ;
606/041 |
International
Class: |
A61B 018/18 |
Claims
We claim:
1. A method for ablating a volume of tissue in a patient comprising
the steps of: (a) radially extending a first plurality of electrode
wires at a first position adjacent the volume of tissue to radial
points defining a first plane; (b) radially extending a second
plurality of electrode wires from a second opposing position
adjacent the volume of tissue to radial points defining a second
plane, offset from the first plane; and (c) connecting a power
supply between the first plurality and second plurality of
electrode wires to induce a current flow between them through the
tumor volume.
2. The method of claim 1 wherein the first and second plurality of
electrode wires are umbrella electrode sets having at least three
radially extending electrode wires.
3. The method of claim 2 wherein the three radially extending
electrode wires in the first set of electrodes are aligned with the
corresponding radially extending electrode wires in the second set
of electrodes.
4. The method of claim 3 wherein the oscillating electrical voltage
has an energy spectrum substantially concentrated in frequencies
below 100 kHz.
5. The method of claim 1 wherein each of the first and second sets
of electrode wires are selectively extendable from a shaft.
6. The method of claim 1, further comprising the step of monitoring
a temperature level at each of the first and second pluralities of
electrode wires.
7. The method of claim 1, wherein the steps of radially extending
the first and second electrode sets comprises radially extending
the wires of the first and second electrode sets at radial points
separated by substantially equivalent angles.
8. The method of claim 1, wherein the first and second electrode
sets are tripartite, and the steps of radially extending the first
and second electrode sets comprise radially extending the
tripartite electrode such that each of the wires in the tripartite
electrode is offset from another of the wires in the tripartite
electrode by substantially one hundred and twenty degrees, and the
tripartite electrode of the first electrode set is substantially
aligned with the tripartite electrode in the second electrode
set.
9. The method of claim 6, further comprising the step of
controlling a voltage applied between the first and second sets of
electrodes to maintain the temperature within a predetermined
temperature range.
10. A method for ablation of a tumor volume in a patient comprising
the steps of: (a) inserting a first electrode having a first
support shaft and a first umbrella electrode set percutaneously at
a tumor volume so that the first umbrella electrode set is at a
first location adjacent to the tumor volume and offset from a
center of the tumor volume; (b) inserting a second electrode having
a second support shaft and a second umbrella electrode set
percutaneously at a tumor volume so that the second umbrella
electrode set is at a second location opposed and at a
predetermined separation from the first location and about the
tumor volume; (c) extending the first and second umbrella
electrodes sets radially from the first and second shafts to an
extension radius wherein the electrode wires of the first umbrella
electrode set are provided at radial points defining a first plane
and the electrode wires of the second umbrella electrode are
provided at radial points defining a second plane; and (d)
connecting a power supply between the first and second electrode
umbrella sets to induce a current flow between them through the
tumor volume whereby current induced heating is concentrated in the
tumor volume defined between the first and second plane.
11. The method of claim 10 wherein each of the first and second
umbrella electrode sets each include at least three electrode wires
extending radially from the support shaft.
12. The method of claim 10 wherein the power supply provides an
oscillating electrical voltage with an energy spectrum
substantially concentrated in frequencies below 100 kHz.
13. The method of claim 10 wherein the oscillating electrical
voltage has an energy spectrum substantially concentrated in
frequencies below 10 kHz.
14. The method of claim 10 further comprising the step of aligning
the first and second umbrella electrode sets.
15. The method of claim 11, further comprising the step of
extending the at least three electrode wires at substantially
equivalent angles.
16. An electrode assembly for ablating tumors in a patient
comprising: (a) a support shaft sized for percutaneous placement in
the patient; (b) first and second wire electrode sets extensible
radially from the shaft to an extension radius, the first wire
electrode set being positioned adjacent to a tumor volume and
offset from the tumor volume and offset axially along the support
shaft from the second wire electrode set positionable at a second
location opposed from the first location about the tumor volume,
the wires of each of the first and second wire electrode sets being
positioned at radial points around the support shaft to define a
plane, wherein the first electrode set defines a first plane and
the second electrode set defines a second plane axially offset from
the first plane; and (c) a power supply connected between the first
and second electrode sets to induce a current flow between the
first and second electrode sets, wherein the first wire electrode
set is positionable adjacent to a tumor volume and offset from a
center of the tumor volume and the second wire electrode set is
positionable at a second location opposed from the first location
about the tumor volume such that the current flow is through the
tumor volume.
17. The electrode assembly of claim 16, wherein each of the
electrode sets comprises at least three electrode wires.
18. The electrode assembly of claim 16, further comprising at least
one temperature sensor coupled to each of the first and second
electrode sets.
19. The electrode assembly of claim 17, further comprising a
controller connected to the temperature sensor to receive
temperature level signals from each of the first and second
electrode sets and to the first and second electrode sets to
control the applied voltage level as a function of the temperature
level.
20. The electrode assembly of claim 19, wherein the electrode wires
in each of the first and second electrode sets are electrically
isolated, a temperature sensor is coupled to each of the wires in
the electrode wire sets, and the controller monitors the
temperature at each of the electrode wires and individually
controls the voltage applied to the electrode wires.
21. The electrode assembly of claim 20, wherein the electrode wires
in the first electrode set are axially aligned with the electrode
wires in the second electrode set.
22. The electrode assembly of claim 20, wherein each of the
electrode wires in the electrode set are offset at substantially
equivalent angles around the support shaft.
23-25
26. A kit, comprising: at least two electrode assemblies, each of
the electrode assemblies comprising: a support shaft; and a first
electrode and a second electrode set, retractably coupled to the
support shaft, the first and second electrode sets being separated
along the support shaft an axial distance and radially extendible
to a radial distance from the support shaft; wherein the axial
distance and the radial distance of each electrode assembly
provided in the kit is selected for ablating a tumor of a selected
volume.
27. The kit as defined in claim 26, wherein the radial distance of
each electrode assembly is less than four times the axial
distance.
28. The kit as defined in claim 26, wherein each of the electrode
assemblies is adapted to be connected to a power supply.
29. The kit as defined in claim 26, wherein the electrode wires in
the first electrode set are aligned axially with the corresponding
electrode wires in the second electrode set.
30. The kit as defined in claim 26, wherein the electrode wires in
each of the first and second electrode sets are offset from
adjacent electrode wires by a substantially equivalent angle.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of provisional
application Serial No. 60/315,383 filed Aug. 28, 2001, entitled "A
Device to Allow Simultaneous Multiple Probe Use During Application
of Radio Therapy"; hereby incorporated by reference, a
continuation-in-part of U.S. application Ser. No. 09/873,541 filed
Jun. 4, 2001 claiming the benefit of provisional application Serial
No. 60/210,103 filed Jun. 7, 2000 entitled "Multipolar Electrode
System for Radio-frequency Ablation"; and further a continuation of
U.S. application Ser. No. 10/167,681 filed Jun. 10, 2002 entitled
"Radio-Frequency Ablation system Using Multiple Electrodes".
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] Not applicable.
BACKGROUND OF THE INVENTION
[0003] The present invention relates to electrodes for
radiofrequency ablation of tumors and the like, and in particular
to a multipolar electrode system suitable for the ablation of
volumetric liver tumors.
[0004] Ablation of tumors, such as liver (hepatic) tumors, uses
heat or cold to kill tumor cells. In cryosurgical ablation, for
example, cold is used to kill the tumor by inserting a probe during
an open laparotomy and freezing the tumor. In radiofrequency
ablation (RFA), on the other hand, an electrode is inserted into
the tumor and current passing from the electrode into the patient
(to an electrical return typically being a large area plate on the
patient's skin) destroys the tumor cells through resistive heating.
A major advantage of RFA, particularly in comparison to
cryosurgical ablation, is that treatment may be delivered
percutaneously, without an incision, and thus with less trauma to
the patient. In some cases, RFA is the only treatment the patient
can withstand. Further, RFA can be completed while the patient is
undergoing a CAT scan.
[0005] Due to the advantages associated with RFA ablation, a number
of RFA electrodes for providing tumor ablation procedures have been
developed. In one prior art method, a conductive needle having an
uninsulated tip is placed within the tumor, and the needle is
energized with respect to a large area contact plate on the
patient's skin by an oscillating electrical signal of approximately
460 kHz. Current flowing radially from the tip of the needle
produces a spherical or ellipsoidal zone of heating (depending on
the length of the exposed needle tip) and ultimately a lesion
within a portion of the zone having sufficient temperature to kill
the tumor cells. While certain advantages are gained from using a
single needle, particularly in limiting the invasiveness of the
procedure, this "monopolar" method is limited in the size of the
tumor which can be treated due to fall-off in current density away
from the electrode, loss of heat to the surrounding tissue, and
limits on the amount of energy transferred to the tissue from the
electrode.
[0006] Because of the limited treatment size and other known
limitations associated with monopolar RFA ablation, RFA ablation
methods which provide current between two or more needles have also
been developed. In these "bipolar" methods, two needles are
provided on a shaft. In one known method, for example, the needles
are spaced along the length of the shaft, and one needle is
positioned, for example, on each side of a tumor. Current can then
be passed either through each of the needles, with reference to a
ground plane, as described above, or in bipolar mode with the two
needles. In this way, the amount of tissue which can be treated at
one time is increased. However, when using this method, the
treatment area is substantially limited to the area defined between
the two needles. To provide ablation for a volume, therefore, the
shaft must be rotated, thereby increasing the invasiveness of the
procedure and, due to the changing locations of the needles,
limiting the ability to adequately monitor and control the heating
process.
[0007] In a second known method, the two or more needles are
extended outwardly from and parallel to an end of a shaft. Here,
again, the needles can be provided on opposing sides of a tumor. As
in the example above, the amount of tissue which can be treated at
a single time is therefore increased as compared to monopolar
operation, but the treatment is again limited to the area between
the needles. A three dimensional volume of tissue, therefore,
cannot be adequately treated unless the probe is rotated, or a
third needle is added and treatment multiplexed among the three
needles. Even when the treatment is multiplexed, however, treatment
is limited to successive single dimensional planes between pairs of
needles. Therefore, the treatment period is long, and the treatment
is complicated and difficult to both monitor and control.
[0008] Because of these difficulties, known RFA methods often fail
to kill all of the tumor cells in a selected volume and, as a
result, tumor recurrence rates of as high as 40% have been
reported.
SUMMARY OF THE INVENTION
[0009] The present inventors have developed a method for treating a
tumor volume with improved efficiency while limiting the
invasiveness of the treatment and improving treatment control. The
method overcomes the limitations of current electrode designs by
adopting a multipolar electrode that increases the treatable tumor
size. Energy is focused on the tumor volume between two or more
sets of electrodes, thereby simultaneously treating a larger volume
of tissue than was possible in the prior art. By using axially
displaced umbrella electrodes supported by outwardly non-conductive
shafts, a large volume lesion area is created between two planes,
and the entire volume between the planes is treatable
simultaneously. This method therefore provides an improvement over
prior art monopolar and bipolar treatment methods, both in
decreasing treatment time, and simplifying control of the
treatment.
[0010] Specifically, the present invention provides a method for
ablating a volume of tissue in a patient in which a first plurality
of electrode wires are radially extended at a first position
adjacent the volume of tissue to radial points defining a first
plane, and a second plurality of electrode wires are radially
extended at a second opposing position adjacent the volume of
tissue to radial points defining a second plane, and offset from
the first plane. A power supply is connected between the first
plurality and second plurality of electrode wires to induce a
current flow between them through the tumor volume. Therefore, a
volume of tissue provided between the first and second planes can
be ablated simultaneously.
[0011] In another aspect of the invention, the first and second
plurality of electrode wires are provided in umbrella electrode
sets, each including at least three radially extending electrode
wires.
[0012] In another aspect of the invention, the electrode wires of
the first and second electrode sets are provided at radial points
separated by substantially equivalent angles around a defined
center point. The first and second electrode sets can, for example,
be tripartite. Here, each of the wires in the tripartite electrode
is offset from another of the wires in the tripartite electrode by
substantially one hundred and twenty degrees. The electrode wires
in each tripartite electrode set can also be aligned.
[0013] In another aspect of the invention the method includes
monitoring a temperature level at each of the first and second
pluralities of electrode wires. A voltage applied between the first
and second sets of electrodes can also be controlled to maintain
the temperature within a predetermined temperature range.
Particularly, a temperature sensor can be provided at each
electrode wire in the electrode set, thereby allowing for
monitoring the temperature of the tissue at a plurality of
locations in the volume, and controlling the energy dispersion as a
function of temperature throughout the volume. Additionally, the
electrode wires in each electrode set can be isolated and
controlled separately, thereby maintaining temperature control
throughout the volume.
[0014] Thus, in one aspect of the invention, multi-electrode
systems can be used to define arbitrary volumes and accurately
control temperature within those volumes for complete tumor
ablation. By maintaining individual control over the current flow
through the electrodes provided in disparate portions of the tumor,
adjustments can be made to account for inhomogeneities in the
tissue such as, for example, nearby blood vessels which carry heat
away from nearby tissue. By further using a conductive plate to
augment current flow in one electrode, energy delivery at that
electrode may be increased without changing the energy delivery at
the other electrode, thereby providing the ability to vary the heat
delivery significantly in various portions of the tumor.
[0015] The present invention also provides a method for ablation of
a tumor volume in a patient comprising the steps of inserting a
first electrode having a first support shaft and a first umbrella
electrode set percutaneously at a tumor volume so that the first
umbrella electrode set is at a first location adjacent to the tumor
volume and offset from a center of the tumor volume, inserting a
second electrode having a second support shaft and a second
umbrella electrode set percutaneously at a tumor volume so that the
second umbrella electrode set is at a second location opposed and
at a predetermined separation from the first location and about the
tumor volume, and extending the first and second umbrella
electrodes sets radially from the first and second shafts to an
extension radius wherein the electrode wires of the first umbrella
electrode set are provided at radial points defining a first plane
and the electrode wires of the second umbrella electrode are
provided at radial points defining a second plane. A power supply
is connected between the first and second electrode umbrella sets
to induce a current flow between them through the tumor volume
whereby current induced heating is concentrated in the tumor volume
defined between the first and second plane.
[0016] In another aspect, the present invention provides an
electrode assembly for ablating tumors in a patient. The electrode
assembly includes a support shaft sized for percutaneous placement
in the patient, first and second wire electrode sets extensible
radially from the shaft to an extension radius, each wire of each
wire electrode set being offset from the other wires in the wire
electrode set at radial points defining a plane, and a power supply
connected between the first and second electrode sets to induce a
current flow between the first and second electrode sets. The first
wire electrode set is positionable adjacent to a tumor volume and
offset from a center of the tumor volume and the second retractable
electrode set is positionable at a second location opposed from the
first location about the tumor volume such that the current flow is
through the tumor volume.
[0017] In another aspect of the invention, each of the electrode
sets in the electrode assembly can comprise a tripartite electrode,
and at least one temperature sensor can be coupled to each of the
first and second electrode sets. A controller can also be connected
to the temperature sensor to receive temperature level signals from
each of the first and second electrode sets and to the first and
second electrode sets to control the applied voltage level as a
function of the temperature level. The electrode wires in each
electrode set can be electrically isolated from the other electrode
wires and controlled separately to provide improved control of the
energy delivery to the vehicle.
[0018] In yet another aspect of the invention, a plurality of
electrode assemblies are provided in a kit. Each of the assemblies
in the kit includes first and second electrode sets which are
offset an axial distance along a shaft and in which the electrode
sets are radially extendible to a radial distance. The axial
distance and radial distance of each electrode assembly in the kit
is selected for a selected tumor size, thereby providing a series
of electrode assemblies suitable for use in ablating tumors of
various sizes, thereby providing a series of electrode assemblies
suitable for use in ablating tumors of various sizes. Preferably,
the axial distance is less than four times the radial distance.
[0019] The foregoing and other objects and advantages of the
invention will appear from the following description. In this
description, reference is made to the accompanying drawings, which
form a part hereof, and in which there is shown by way of
illustration, a preferred embodiment of the invention. Such
embodiment and its particular objects and advantages do not define
the scope of the invention, however, and reference must be made
therefore to the claims for interpreting the scope of the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a perspective view of two umbrella electrode
assemblies providing first and second electrode wires deployed per
the present invention at opposite edges of a tumor to create a
lesion encompassing the tumor by a passing current between the
electrodes;
[0021] FIG. 2 is a schematic representation of the electrodes of
FIG. 1 as connected to a voltage controlled oscillator and showing
temperature sensors on the electrode wires for feedback control of
oscillator voltage;
[0022] FIG. 3 is a fragmentary cross-sectional view of a tip of a
combined electrode assembly providing for the first and second
electrode wires of FIG. 1 extending from a unitary shaft arranging
the wires of the first and second electrodes in concentric tubes
and showing an insulation of the entire outer surface of the tubes
and of the tips of the electrode wires;
[0023] FIG. 4 is a simplified elevational cross-section of a tumor
showing the first and second electrode positions and comparing the
lesion volume obtained from two electrodes operating per the
present invention, compared to the lesion volume obtained from two
electrodes operating in a monopolar fashion;
[0024] FIG. 5 is a figure similar to that of FIG. 2 showing
electrical connection of the electrodes of FIG. 1 or FIG. 3 to
effect a more complex control strategy employing temperature
sensing from each of the first and second electrodes and showing
the use of a third skin contact plate held in voltage between the
two electrodes so as to provide independent current control for
each of the two electrodes;
[0025] FIG. 6 is a graph plotting resistivity in ohms-centimeters
vs. frequency in Hz for tumorous and normal liver tissue, showing
their separation in resistivity for frequencies below approximately
100 kHz;
[0026] FIG. 7 is a figure similar to that of FIGS. 2 and 5 showing
yet another embodiment in which wires of each of the first and
second electrodes are electrically isolated so that independent
voltages or currents or phases of either can be applied to each
wire to precisely tailor the current flow between that wire and the
other electrodes; and
[0027] FIG. 8 is a flow chart of a program as may be executed by
the controller of FIG. 7 in utilizing its multi-electrode
control.
DETAILED DESCRIPTION OF THE INVENTION
[0028] Referring now to FIG. 1, a liver 10 may include a tumor 12
about which a lesion 14 will be created by the present invention
using two umbrella-type electrode assemblies 16a and 16b having a
slight modification as will be disclosed below. Each electrode
assembly 16a and 16b has a thin tubular metallic shaft 18a and 18b
sized to be inserted percutaneously into the liver 10. The shafts
18a and 18b terminate, respectively, at shaft tips 20a and 20b from
which project trifurcated electrodes 22a and 22b are formed of
wires 32. The wires 32 are extended by means of a plunger 24
remaining outside the body once the shafts 18a and 18b are properly
located within the liver 10 and when extended, project by an
extension radius separated by substantially equal angles around the
shaft tips 20a and 20b. The exposed ends of the wires 32 are
preformed into arcuate form so that when they are extended from the
shafts 18a and 18b they naturally splay outward in a radial
fashion.
[0029] Umbrella electrode assemblies 16a and 16b of this type are
well known in the art, but may be modified, in one embodiment of
the invention, by providing electrical insulation to all outer
surfaces of the shafts 18a and 18b, in contrast to prior art
umbrella electrode assemblies which leave the shaft tips 20a and
20b uninsulated, and by insulating the tips of the exposed portions
of the wires 32. The purpose and effect of these modifications will
be described further below.
[0030] Per the present invention, the first electrode 22a is
positioned at one edge of the tumor 12 and the other electrode 22b
positioned opposite the first electrode 22a across the tumor 12
center. The term "edge" as used herein refers generally to
locations near the periphery of the tumor 12 and is not intended to
be limited to positions either in or out of the tumor 12, whose
boundaries in practice, may be irregular and not well known. Of
significance to the invention is that a part of the tumor 12 is
contained between the electrodes 22a and 22b.
[0031] Referring now to FIGS. 1 and 2, electrode 22a may be
attached to a voltage controlled power oscillator 28 of a type well
known in the art providing a settable frequency of alternating
current power whose voltage amplitude (or current output) is
controlled by an external signal. The return of the power
oscillator 28 is connected to electrodes 22b also designated as a
ground reference. When energized, power oscillator 28 induces a
voltage between electrodes 22a and 22b causing current flow
therebetween.
[0032] Referring now to FIG. 4, prior art operation of each
electrode 22a and 22b being referenced to a skin contract plate
(not shown) would be expected to produce lesions 14a and 14b,
respectively, per the prior art. By connecting the electrodes as
shown in FIG. 2, however, with current flow therebetween, a
substantially larger lesion 14c is created. Lesion 14c also has
improved symmetry along the axis of separation of the electrodes
22a and 22b. Generally, it has been found preferable that the
electrodes 22a and 22b are separated by 2.5 to 3 cm for typical
umbrella electrodes or by less than four times their extension
radius.
[0033] Referring again to FIG. 2, temperature sensors 30, such as
thermocouples, resistive or solid-state-type detectors, may be
positioned at the distal ends of each of the exposed wires 32 of
the tripartite electrodes 22a and 22b. For this purpose, the wires
32 may be small tubes holding small conductors and the temperature
sensors 30 as described above. Commercially available umbrella-type
electrode assemblies 16a and 16b currently include such sensors and
wires connecting each sensor to a connector (not shown) in the
plunger 24.
[0034] In a first embodiment, the temperature sensors 30 in
electrode 22a are connected to a maximum determining circuit 34
selecting for output that signal, of the three temperature sensors
30 of electrode 22, that has the maximum value. The maximum
determining circuit 34 may be discrete circuitry, such as may
provide precision rectifiers joined to pass only the largest
signal, or may be implemented in software by first converting the
signals from the temperature sensors 30 to digital values and
determining the maximum by means of an executed program on a
microcontroller or the like.
[0035] The maximum value of temperature from the temperature
sensors 30 is passed by a comparator 36 (which also may be
implemented in discrete circuitry or in software) which compares
the maximum temperature to a predetermined desired temperature
signal 38 such as may come from a potentiometer or the like. The
desired temperature signal is typically set just below the point at
which tissue boiling, vaporization or charring will occur.
[0036] The output from the comparator 36 may be amplified and
filtered according to well known control techniques to provide an
amplitude input 39 to the power oscillator 28. Thus it will be
understood that the current between 22a and 22b will be limited to
a point where the temperature at any one temperature sensors 30
approaches the predetermined desired temperature signal 38.
[0037] While the power oscillator 28 as described provides voltage
amplitude control, it will be understood that current amplitude
control may instead also be used. Accordingly, henceforth the terms
voltage and current control as used herein should be considered
interchangeable, being related by the impedance of the tissue
between the electrodes 22b and 22a.
[0038] In an alternative embodiment, current flowing between the
electrodes 22a and 22b, measured as it flows from the power
oscillator 28 through a current sensor 29, may be used as part of
the feedback loop to limit current from the power oscillator 28
with or without the temperature control described above.
[0039] In yet a further embodiment, not shown, the temperature
sensors 30 of electrode 22b may also be provided to the maximum
determining circuit 34 for more complete temperature monitoring.
Other control methodologies may also be adopted including those
provided for weighted averages of temperature readings or those
anticipating temperature readings based on their trends according
to techniques known to those of ordinary skill in the art.
[0040] Referring now to FIG. 3, the difficulty of positioning two
separate electrode assemblies 16a and 16b per FIG. 1 may be reduced
through the use of a unitary electrode 40 having a center tubular
shaft 18c holding within its lumen, the wires 32 of first electrode
22a and a second concentric tubular shaft 42 positioned about shaft
18c and holding between its walls and shaft 18c wires 44 of the
second electrode 22b. Wires 44 may be tempered and formed into a
shape similar to that of wires 32 described above. Shaft 18c and 42
are typically metallic and thus are coated with insulating coatings
45 and 46, respectively, to ensure that any current flow is between
the exposed wires 32 rather than the shafts 18c and 42.
[0041] As mentioned above, this insulating coating 46 is also
applied to the tips of the shafts 18a and 18b of the electrode
assemblies 16a and 16b of FIG. 1 to likewise ensure that current
does not concentrate in a short circuit between the shafts 18a and
18b but in fact flows from the wires 32 of the wires of electrodes
22a and 22b.
[0042] Other similar shaft configurations for a unitary electrode
40 may be obtained including those having side-by-side shafts 18a
and 18b attached by welding or the like.
[0043] Kits of unitary electrode 40 each having different
separations between first electrode 22a and second electrode 22a
may be offered suitable for different tumor sizes and different
tissue types.
[0044] As mentioned briefly above, in either of the embodiments of
FIGS. 1 and 3 the wires 32 may include insulating coating 46 on
their distal ends removed from shafts 18c and 42 so as to reduce
high current densities associated with the ends of the wires
32.
[0045] In a preferred embodiment, the wires of the first and second
electrodes 22a and 22b are angularly staggered (unlike as shown in
FIG. 2) so that an axial view of the electrode assembly reveals
equally spaced non-overlapping wires 32. Such a configuration is
also desired in the embodiment of FIG. 2, although harder to
maintain with two electrode assemblies 16a and 16b.
[0046] The frequency of the power oscillator 28 may be
preferentially set to a value much below the 450 kHz value used in
the prior art. Referring to FIG. 6, at less than 100 kHz and being
most pronounced and frequencies below 10 kHz, the impedance of
normal tissue increases to significantly greater than the impedance
of tumor tissue. This difference in impedance is believed to be the
result of differences in interstitial material between tumor and
regular cell tissues although the present inventors do not wish to
be bound by a particular theory. In any case, it is currently
believed that the lower impedance of the tumorous tissue may be
exploited to preferentially deposit energy in that tissue by
setting the frequency of the power oscillator 28 at values near 10
kHz. Nevertheless, this frequency setting is not required in all
embodiments of the present invention.
[0047] Importantly, although such frequencies may excite nerve
tissue, such as the heart, such excitation is limited by the
present bi-polar design.
[0048] Referring now to FIG. 5, the local environment of the
electrodes 22a and 22b may differ by the presence of a blood vessel
or the like in the vicinity of one electrode such as substantially
reduces the heating of the lesion 14 in that area. Accordingly, it
may be desired to increase the current density around one electrode
22a and 22b without changing the current density around the other
electrode 22a and 22b. This may be accomplished by use of a skin
contact plate 50 of a type used in the prior art yet employed in a
different manner in the present invention. As used herein, the term
contact plate 50 may refer generally to any large area conductor
intended but not necessarily limited to contact over a broad area
at the patient's skin.
[0049] In the embodiment of FIG. 5, the contact plate 50 may be
referenced through a variable resistance 52 to either of the output
of power oscillator 28 or ground per switch 53 depending on the
temperature of the electrodes 22a and 22b. Generally, switch 53
will connect the free end of variable resistance 52 to the output
of the power oscillator 28 when the temperature sensors 30 indicate
a higher temperature on electrode 22b than electrode 22a.
Conversely, switch 53 will connect the free end of variable
resistance 52 to ground when the temperature sensors 30 indicate a
lower temperature on electrode 22b than electrode 22a. The
comparison of the temperatures of the electrodes 22a and 22b may be
done via maximum determining circuits 34a and 34b, similar to that
described above with respect to FIG. 2. The switch 53 may be a
comparator driven solid-state switch of a type well known in the
art.
[0050] The output of the maximum determining circuits 34a and 34b
each connected respectively to the temperature sensors 30 of
electrodes 22a and 22b may also be used to control the setting of
the potentiometer 52. When the switch 53 connects the resistance 52
to the output of the power oscillator 28, the maximum determining
circuits 34a and 34b serve to reduce the resistance of resistance
52 as electrode 22b gets relatively hotter. Conversely, when the
switch 53 connects the resistance 52 to ground, the maximum
determining circuits 34a and 34b serve to reduce the resistance of
resistance 52 as electrode 22a gets relatively hotter. The action
of the switch 53 and switch 52 is thus generally to try to equalize
the temperature of the electrodes 22a and 22b.
[0051] If electrode 22a is close to a heat sink such as a blood
vessel when electrode 22b is not, the temperature sensors 30 of
electrode 22a will register a smaller value and thus the output of
maximum determining circuit 34a will be lower than the output of
maximum determining circuit 34b.
[0052] The resistance 52 may be implemented as a solid state
devices according to techniques known in the art where the relative
values of the outputs of maximum determining circuits 34a and 34b
control the bias and hence resistance of a solid state device or a
duty cycle modulation of a switching element or a current
controlled voltage source providing the equalization described
above.
[0053] Referring now to FIG. 7, these principles may be applied to
a system in which each wire 32 of electrodes 22a and 22b is
electrically isolated within the electrode assemblies 16a and 16b
and driven by separate feeds 53 through variable resistances 54
connected either to the power oscillator 28 or its return.
Electrically isolated means in this context that there is not a
conductive path between the electrodes 22a and 22b except through
tissue prior to connection to the power supply or control
electronics. As noted before, a phase difference can also be
employed between separate feeds 53 to further control the path of
current flow between electrode wires 32. This phase difference
could be created, e.g. by complex resistances that create a phase
shift or by specialized waveform generators operating according to
a computer program to produce an arbitrary switching pattern. The
values of the resistances 54 are changed as will be described by a
program operating on a controller 56. For this purpose, the
variable resistances 54 may be implemented using solid-state
devices such as MOSFET according to techniques known in the
art.
[0054] Likewise, similar variable resistances 54 also controlled by
a controller 56 may drive the contact plate 50.
[0055] For the purpose of control, the controller 56 may receive
the inputs from the temperature sensors 30 (described above) of
each wire 32 as lines 58. This separate control of the voltages on
the wires 32 allows additional control of current flows throughout
the tumor 12 to be responsive to heat sinking blood vessels or the
like near any one wire.
[0056] Referring to FIG. 8, one possible control algorithm scans
the temperature sensors 30 as shown by process block 60. For each
temperature sensor 30, if the temperature at that wire 32 is above
a "ceiling value" below a tissue charring point, then the voltage
at that wire is reduced. This "hammering down" process is repeated
until all temperatures of all wires are below the ceiling
value.
[0057] Next at process block 62, the average temperature of the
wires on each electrode 22a and 22b is determined and the voltage
of the contact plate 50 is adjusted to incrementally equalize these
average values. The voltage of the contact plate 50 is moved toward
the voltage of the electrode 22 having the higher average.
[0058] Next at process block 64 the hammering down process of
process block 60 is repeated to ensure that no wire has risen above
its ceiling value.
[0059] Next at process block 66 one wire in sequence at each
occurrence of process block 66 is examined and if its temperature
is below a "floor value" below the ceiling value but sufficiently
high to provide the desired power to the tumor, the voltage at that
wire 32 is moved incrementally away from the voltage of the wires
of the other electrode 22. Conversely, if the wire 32 is above the
floor value, no action is taken.
[0060] Incrementally, each wire 32 will have its temperature
adjusted to be within the floor and ceiling range by separate
voltage control.
[0061] As shown in FIG. 7, this process may be extended to an
arbitrary number of electrodes 22 including a third electrode set
22c whose connections are not shown for clarity.
[0062] While this present invention has been described with respect
to umbrella probes, it will be understood that most of its
principles can be exploited using standard needle probes energized
in a bipolar configuration. Further it will be understood that the
present invention is not limited to two electrode sets, but may be
used with multiple electrode sets where current flow is
predominantly between sets of the electrodes. The number of wires
of the umbrella electrodes is likewise not limited to three and
commercially available probes suitable for use with the present
invention include a 10 wire version. Further although the maximum
temperatures of the electrodes were used for control in the
above-described examples, it will be understood that the invention
is equally amenable to control strategies that use average
temperature or that also evaluate minimum temperatures.
[0063] It is specifically intended that the present invention not
be limited to the embodiments and illustrations contained herein,
but modified forms of those embodiments including portions of the
embodiments and combinations of elements of different embodiments
as come within the scope of the following claims.
* * * * *