Note: Descriptions are shown in the official language in which they were submitted.
38-20OF
~Z~ i3
--1--
The present invention relates to electrodes
used in the transfer of radio frequency energy to a
substance and more particularly to electrodes used in
the medical treatment of living tissue in animals by
hyperthermia techniques.
It has been well known for some time that many
substances, includin~ animal tissue, respond by heating
~o the passing of radio frequency (RF) energy there-
through. In particular, th~is has found application in
the selective destruction of certain tissues within
living animals including humans. More specifically, it
has been ~ound that because tumors (both malignant and
; benign) have a reduced capacity for the passage of blood~
therethrough with attendant cooling thereby, the appli-
cation of RF energy to the tumor and the surrounding tis-
sue will cause the tumDr to be elevated to a higher
tamperature than the surrounding tissue. It is possibla,
by careful control of the RF energy applied, to raise the
temperature of the tumor above the point were necrosis of
the tumor will occur while maintaining the temperature of
the health~ surrounding tissue below the destructive
temperature level. By such a technique, the tumor is des-
troyed while the surrounding healthy tissue is undamaged.
It has been common practice in the prior art
employing such techniques to connect a pair of paddle
type el~ctrodes to a source of RF energy. The electrodes
are then disposed on either side of the tissue containing
the tumor in the manner shown in Figure 1. The RF energy
passes between the electrodes 10 with the tissue contai~-
ing ~he tumor behaving in a manner of a die~ectric in a
capacitor. Ordinarily, the tissue placed between the
electrodes is a complex substance with varying resistances
such that those components with the greatest resistance
to.gO skin 12 and subcutaneous tissue 14) are preferen-
tially heated, while those components with lower resis-
! tance and those more deeply located (e.g. muscle 16 and
.,' ~
!
38~200F
~LZl~
--2--
the tumor 18) are least heated. In practice, this gener-
ally results in the inability to heat deeply, or in super-
ficial heat absorption which results in burning. Since
tumors and various disease states potentially treatable
by heat are often located deep within the body, a method
and apparatus for allowing deep heating without skin and
subcutaneous tissue injury would be desirable. To date,
with the apparatus of the prior art, the only method for
treatment of deeply located tissues by hyperthermia was
through the surgical implantation of tha electrodes.
m at is, the tissue to be treated is surgically exposed
so that the paddle type electrodes can be disposed on
either side and close adjacent to ,th~ a,rea of treatment.
A spiral, pancake type coil has also been used
commercially. It produces a localized magnetic field
which attenuates very rapidly with spacing from the body.
It also has a large voltage build-up between turns thus
creating a related voltage gradient across the skin sur-
face in that region. The result is superficial heating
in the skin and subcutaneous ~at layer. Such an electrode
is, therefore, of little value in deep medical treatment
by hyperthermia.
~ herefore, it is the object of the present in-
~ention to provide an e~ectrode for use in medical treat-
ment by hyperthermia which allows deep haating by the
trans~er of radio frequency energy without the attendant
hazards to healthy tissues described above wherein the
deep heating of animal tissue is accomplished by placing
the animal tissue in non-contacting relationship within
a substantially annular region defined by t,he electrode;
and, the electrode is thereafter excited with radio fre
~uency energy to establish a magnetic field comprising a
plurality of concentric electros,tatic field lines of
similar magnitude within the annular region whereby the
tissue is heated throughout. In a~complishing the objec-
tive, the electrode employed comprises electrically con-
ductive m,aterial disposed to form a col-~ single
'
~L2~4~3
turn loop of substantially almular shape self-resonant at medically assigned
radio frequencies ancl having means for coupling a supply of radio frequency
energy into the electrode disposed in operable relationship with the material.
In the preferred embodiment, the material is a strip having the ends thereof
overlapped in non-contacting spaced relationship and with a dielectric
material disposed between the overlapped ends. The dielectric material, the
distance of the spacing between the overlapped ends and the area defined by
the overlapping of the ends are such in relationship to the diameter of the
annular shape that the capacitive reactance of the overlapped ends is equal
to the inductive reactance of the loop.
Figure 1 is a simplified drawing of two paddle type electrodes
according to the prior art as used in hyperthermia techniques.
Figure 2 is a simplified drawing of the operation of the electrode
of the present invention.
Figure 3 is a simplified drawing of the electrode of the present
invention as used in the deep heating of an animal extremity.
Figure 4 is a drawing of the equivalent circuit of the electrode
of the present invention.
Figure 5 is a graph of the power level across the annular region
enclosed by a 20-inch diameter electrode according to the present invention.
Figure 6 is a graph of the temperatures reached in the various
tissue components of a human thigh by using a ten inch diameter electrode
according to the present invention.
Figure 7 is a more detailed drawing of the construction of the pre-
ferred embodiment of the present invention.
Figure 8 is a simplified drawing of the construction of a twenty-
inch diameter electrode according to the present invention used in the treat-
ment of a human torso.
--3--
~l~LZ~L~6~
Figure 9, on the second sheet of drawings, is a graph of tempera-
tures reached in treating an intra-abdominal tumor with the electrode of
Figure 8.
Figure 10, on the second sheet of drawings, is a graph o tempera-
tures reached in treating a liver metastases with the electrode of Figure 8.
Figure 11, on the first sheet of drawings, is a simplified drawing
of an alternate embodiment of the electrode of the present invention.
Figure 12, on the first sheet o drawings, is a drawing of the
equivalent circuit of the electrode of Figure 11.
The basic principle of the present invention is shown with reference
to Figures 2, 3 and 40 The electrode 30 comprises a strip of conductive
material formed into a cylinder having the ends thereof in overlapping, non-
contacting relationship. While an air dielectric could be employed, it is
preferred that a material such as polytetrafluoroethylene be disposed between
the overlapping ends of electrode 30 such as that labeled 32 in Figure 2.
The first tested embodiment of the present invention was configured such as
the electrode 30 of Figure 2 having a diameter of about 25 cm such that an
extremity of an animal 34 could be disposed within the annular region 36
defined by electrode 30 in the manner of Figure 3. In simplified form, the
animal extremity 34 would appear as shown in Figure 2 as comprising skin 38,
subcutaneous tissue 40, muscle 42, and bone 44. A tumor 46 to be treated is
shown as being disposed therein.
Electrode 30 is provided with means for coupling a supply of RF
energy into the electrode. One method employed was the providing of taps 48
to which a connector 50 providing the RF energy can be connected. When
electrode 30 is connected to a source of RF energy, a plurality of concentric
electrostatic field lines 52 are formed within annular region 360 The
equlvalent circuit is shown in
~3~
: . ~
:, ~, , ' ., ~
3 8--2 0 0 F
--5--
Figure 4. The inductance and capacitance are attributable
to the single turn loop of electrode 30 and the overlapped
ends having dielectric material therebetween respectively.
The resistance is virtually entirely due to the animal
extremity 34 within annular region 36. Thus, it can be
seen, that if the capacitive reactance o the overlapped
ends is made equal to the inductive reactance of the
single turn loop by adjusting the amount of overlap ~and
thereby the total area of overlap), the spacing between
the overlapped surfaces, and the dielectric material dis-
posed therebetween, the circuit is placed in self re-
sonance whereby a resistive load is present~d to the RF
ourcs so that maximum power is tr~nsferred to the-re-~
sistive load, e.g. the tis~ue of the animal extremity 34.
Referring briefly to Figure 5, actual power
measurements taken with an approximataly 50 cm diameter
tested embodiment of the present invention as a function
of the distance from the center of the electrode are
sh~wn. It can be seen from Figure 5 that the power level
from the center out to approximately 4+ inches is sub-
stantially constant. If an extremity is disposed co-
axi~lly with electrode 30 of Figure 2 within an annular
region extending from the center axis and less than a
`' radius of about 13 cm, the entire extremi y from the sur-
~ace to the center thereof will be subject to a substan-
tially constant electrostatic field of power. Note that
contrary to the prior art configuration of Figure 1, the
field li~es 52 do not have to pass ~hrough the skin 38
and subcutaneous tissue 40 to reach the tumor 46 with at-
tendant losses. Rather, each layer within the extremit~
34 i~ suhjected to its own electrostatic ield lines 52
; w~ich a~e affecting only that layer.
The 25 cm diameter electrode was designed to - - -
encircle a human limb. Figure 5 illustrates the the~mal
characteristics achie~ed on a human thigh. Power (250
watts) wa~s applied in 3 minute increments with 1 minute
~8-200~
-
63
--6--
off intervals for temperature measurements. Temperature
of skin, subcutaneous fat, and deep muscle were recorded
by insertion of a needle thermistor prohe during the off
time. The important significance of this data is that
the deep muscle tissue is ~miformly heated to the highest
temperature, while maintaining lower subcutaneous fat
temperatures and very low ~kin temperatures. No cooling
of any kind was used. The principle of creating concen-
tric electrostatic field lines within the resonant cy-
19 lindrical electrode and within the thigh was thus es-
tablished. Further test results employing the electrode
to treat a tumor located deep within the ~ody, below the
` `fat layers, are discussed~hereinafter.
Referring now to Figure 7, further details of
the actual construction of electrode 30 shown previously
in simplified form in Figure 2 are shown. The complete
electrode indicated generally as 54 comprises a 13 cm
wiaa strip of conductive material 30' having the ends
thereof overlapped with dielectric material 32-disposed
therebatween. In the actual embodiment, dielectric
material 32 consists of a .8 mm thick polytetrafluoro-
ethylene sheet. The metallic cylinder comprising con-
; ~uctive material 30' and dielectric material 32 is dis-
posed within an outer plastic cylinder 56 and an inner
plastic cylinder 58 providing an insulation so that direct
patient contact with ~he electrode is prevented. In the
preferred embodiment, the overlapped ends of conductive
' ~ ma~erial 30' comprising the capacitor plates and the di-
elsctric material 32 are held together by insulated screws
60 passi~g through the outer plastic cylinder 66. While
tapped coupling as shown in the simplified drawing of
Figure 2 was successfully employed, the praferred method
- ~- of coup~.ng the RF energ~-to the ele~trode is through
inductiv~ coupling employing a close-spaced half turn of
i number l;' wire 62 located at one end of the cylinder.
! Provisio~ are made to vary the coupling by pxoviding in-
' termediat:e taps 64 on the half turn coupling wire 62.
- : :
38-200F
..
~IZ~;3
--7--
A 50 cm diameter cylindrical electrode was de-
signed to encompass the human torso for the purpose of
heating a large part of the body. A cylinder 25 cm long
was chosen to meet the physical needs and to optimize the
electrical characteristics, The cylinder sections overlap
at two places as shown in the simplified drawing of Fi-
gure 8~ This double overlapping forms two capacitors
which have a total series reactance equal to the induc-
tive reactance. ~hus, each capacitor must have a value
twice that required if only one capacitor were used to
resonant the cylinder. The use of multiple overlapping
is done because the voltage build-up at the capacitors can
- be minimi~ed by the use of-multiple-capacitors in series
around the circumference, i.e. minimi7ing the undesired
capacitive coupling. With the cylinder mounted over the
human torso, it was found that excellent coupling to the
body is achieved producing a loaded Q of approximately 20.
The equivalent resonant circuit resistance is then con-
. veniently transformed to a 50 ohm input by ~apping across
one of the capacitors, as shown in Figure 8.
The temperature profile of a patient produced by
employing the 50 cm diameter electrode just described is
shown in Figure 9. The patient had a large intra-abdomin-
al sarcoma, i.e. a primary tumor. Sufficient power was
applied to raise the tumor temperature to approximately
50C while maintaining healthy tissue temperatures at 43C
and below. Because of the poor blood flow in the tumor,
previously described, it is possible to raise the tumor
temperature to lethal levels while not causing damage to
healthy tissue as shown. In this case, power was gradu-
ally increased from 250 watts to 750 watts. As may be
seen, from Figure 9, the tumor temperature is maintained
above 49C for 35 minutes.
~ -~ Another patient with a large secondary tumor,
-~ liver metastases, was treated using the 50 cm diameter
. .
,
38-20OF
~ 3L4~3
electrode. The temperature profile results from this
treatment are shown in Figure 10. In this ~ase, tem-
perature measurements were made of the liver metastases,
normal liver, subcutaneous fat and skin. The healthy
tissue, includin~ the liver, remained at acceptable tem-
perature l~vels while the tumor again achieved lethal
temperatures. Biopsy after three weekly similar treat-
ments showed that complete necrosis has occurred.
An alternate embodiment of the elec~rode of the
present invention is shown in Figure 11 with its equiva-
lence circuit shown in Figure 12. The electrode of Figure
11 generally indicated at 66 is in the form of an approx.i-
` mately`38 cm diameter loop fabricated ~rom 13 mm thin wall
coppr tubingO The two semi-annular copper conduits 68
are connected on one end thereof into a metallic shield
box 70 to form the 38 cm diameter loop. The opposite ends
of the copper conduit 68 not fastened to the shield
box 70 are disposed in close adjacent spaced relationship.
A paix of electrical cond~lctors (in this case, #12 poly-
tetrafluoroethylene covered wire) 72 is passed one each
through each of the copper conduits 68. On the ends
emerging from the copper conduits 68 away from shield box
70, each wire 72 is connected to ~he opposite conduit 68
in the manner shown of Figure 11. The other end of one
wire 72 emerging from one of the copper conduits 68 is
connected to a first variable capacitor 74 which is then
connected to shield box 70. The other wire 72 in the
o~her copper conduit 68 is connected on its other end to
a second variable capacitor 76 which in turn is connected
to the center connector of a coaxial connector 78 which
i~ turn is connected on its opposite side to shield box
~0. With a coaxial cable 80 connected to coaxial connec-
tor 78, the equivalent circuit appears as in Figure L2.
When employing the electrode of Figure 11, the loop in-
auctance is brought into resonance by capacitor 74.
By subseq[uent proper adjustment of both capacitors 74
a~d 76, the circuit impedance can be matched to the t~pi -
; cal 50 ohm power source.
38-200F
1~L;2146;~
_g
The design parameters described heretofore are
for th~ medical frequency 13.56 MHz. The same principles
apply to both 27.12 and 40.68 M~z medical frequencies ~
well, by scaling the inductance and capacitive parameters.
It is to be understood that while the present
electrode is primarily directed to and the tested embodi-
ments and test results described are in relation to the
medical treatment of tumors in human subjects, the elec-
trode can be equally well used in any application wherein
deep heating of a substance, be it animal tissue or other-
WiSQ, iS desired.
.
