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The present invention relates generally to the field of apparatus for
irradiating human and animal tissue with electromagnetic radiation for
medical hyperthermia purposes, and more particularly to body insertable
radiation applicators associated with such hyperthermia apparatus.
Hyperthermia or induced high body temperature has, for many years, been
considered beneficial in treating various human diseases, very importantly
including many types of cancer. For example, some types of malignant cells
can reportedly be destroyed by heating to temperatures slightly below that
injurious to most normal cells. Furthermore, some malignant cell masses
having poorer heat dissipation characteristics than normal tissue,
presumably due to abnormally low blood circulation, are subject to
preferential hyperthermia treatment. As a result, such malignant cell
masses can often be heated to temperatures substantially higher than that
of surrounding healthy cells, to enable hyperthermia treatment, even when
both types of cells are heated simultaneously. This characteristic not
only enables hyperthermia treatment of some types of malignancies which
are no more temperature sensitive than normal cells, but usually permits
much shorter hyperthermia treatment times, even of thermally sensitive
malignancies, as is important for various medical reasons.
More specifically, various types of malignant growths are considered by
many researchers to have a relatively narrow hyperthermia treatment
temperature range. Below a threshold temperature of about 41.5.degree. C.
(106.7.degree. F.), thermal destruction of these malignancies is not
believed to occur; for hyperthermia temperatures below this threshhold,
growth of some of these malignancies may tend to be stimulated. In
contrast, at temperatures above a range of about 43.degree. C. to
45.degree. C. (109.4.degree. F. to 113.degree. F.) thermal damage even to
most normal cells occurs, the exposure duration at any elevated
temperature being also a significant factor. Accordingly, if large or
critical parts of human body are heated into, or above, the 43.degree. C.
to 45.degree. C. range for even relatively short durations, serious
permanent injury or death is possible.
Some types of skin cancers are known to respond to direct application of
surface heat. However, deeply located malignant growths, due to normal
blood flow body heat transfer properties of the body, can rarely be heated
to a destructive temperature in such manner without overlying healthy
tissue being excessively damaged.
As a consequence, a promising alternative technique for inducing
hyperthermia is electromagnetic radiation (EMR) heating. In the late
nineteenth century, alternating electric currents at frequencies above
about 10 KHz were discovered to cause heating in human tissue. Such (then)
high frequency currents were subsequently used for thermally treating
various bodily disorders such as infected tissue and injured muscles. In
the early twentieth century, the term "diathermia" was introduced to
describe this tissue heating caused by conversion of high frequency
electric currents into heat.
Treatment of malignant growths by high frequency EMR induced hyperthermia
was described as early as 1933 by Dr. Schereschewsky, in his article
"Biological Effects of Very High Frequency Electromagnetic Radiation"
which appeared in RADIOLOGY in April of that year. Experimental EMR
induced hyperthermia treatment of tumors in mice at frequencies up to 300
MHz was reported in the article and a survey of research activity in the
field was given. More currently in 1974, Guy, Lehman and Stonebridge
presented a historical background of high frequency EMR hyperthermia
research and discussed recent experimental activity in the field in their
article "Therapeutic Applications of Electromagnetic Power", (PROCEEDINGS
OF THE IEEE, Volume 62, No. 1, January, 1974).
In spite of encouraging results in using EMR induced hyperthermia for
treatment of malignant growths, a persistent, serious problem has been
experienced with thermally destroying malignant cells without, at the same
time, causing excessive amounts of thermal damage to adjacent or overlying
healthy cells. Such thermal damage to healthy cells may, for example, be a
result of excessive EMR power density, improperly selected EMR frequencies
affecting depth of penetration, or by heat concentrations or "hot spots"
of standing energy waves caused by uncontrolled EMR energy reflections at
boundaries between different types of body tissue layers, such as adjacent
layers of fat and muscle.
Still requiring further definition and investigation are also the
potentially injurious nonthermal effects of low level electromagnetic
radiation. These nonthermal effects, apparently caused by electromagnetic
forces acting on cell molecules, include realignment of cell molecules
into undesirable, chain-like formations, coagulation of cell molecules,
other damage to normal cells which may actually lead to cancer and a
myriad of other undesirable physiological effects.
As an example of these physiological side effects, low EMR levels have been
observed to cause such anomalies of the central nervous and cardiovascular
systems as descreased arterial pressure and reduced heart rate. The
Soviets have reported these adverse side effects at EMR densities as low
as 10 milliwatts per square centimeter. A more detailed discussion of
these nonthermal effects is found, for example, in a fairly recent article
by Johnson and Guy, entitled "Nonionizing Electromagnetic Wave Effects in
Biological Materials and Systems" (PROCEEDINGS OF THE IEEE, Volume 60, No.
6, June 1972).
Because of these potentially harmful, and generally still not completely
understood, nonthermal EMR effects on healthy body tissue, the United
States has established a maximum power density for prolonged EMR exposure
at 10 milliwatts per square centimeter, the Soviets having established the
much lower maximum of 0.01 milliwatts per square centimeter. Since EMR
hyperthermia (and diathermy) apparatus commonly radiate power densities of
as high as one watt per square centimeter during treatment, considerable
research into nonthermal EMR effects on healthy cells is still needed, to
enable development of improved EMR hyperthermia treatment technique and
investigation of nonthermal efforts.
Broad band EMR hyperthermia apparatus, particularly adapted for research,
have been disclosed, for example, in my copending United States patent
applications, Ser. Nos. 002,583 and 002,584, both filed on Jan. 11, 1979.
In general, however, electromagnetic radiation applicators of these and
other disclosed EMR hyperthermia have been configured for irradiating
living tissue, or tissue simulating matter often used in research from
outside the body. EMR heating of subsurface growths from an exterior
surface is ordinarily enabled by configuration and placement of one or
more applicators and by appropriate selection of EMR frequency, phase and
intensity.
As can be appreciated, however, many malignant growths positioned deep
within a body, particularly those located inside of, or in close proximity
to, heat sensitive tissue or organs, are much more effectively and safely
heated by EMR irradiating applicators positioned within the body as
closely as possible to the growth requiring treatment.
Advantages of positioning the EMR applicators relatively close to the
growth to be heated by radiation include improved heating control, more
localized heating and consequently less possibility of overheating
adjacent healthy tissue and elimination of standing wave "hot spots"
caused by wave reflections at tissue layer boundaries.
Such close applicator access to certain types of malignant growths may be
possible by surgical procedures in which overlaying layers of tissue are
cut. When surgical access techniques are practical, small EMR applicators,
usually of the same type used for surface radiation, are placed over or in
the incision to provide more direct irradiation of the growth.
However, for many common occurring, deeply located tumors, surgical access
for the applicator may be impossible or impractical for many reasons,
including the reason that a patient may be unable to withstand the rigors
of a major operation.
For those relatively common malignancies which are located close to or
along a naturally occuring body passage, such as cancer of the esophagus,
larynx, prostrate gland and colon, applicator access is readily provided
by the passages, through the associated bodily orifice. An illustrative
type of a corresponding body passage insertable EMR applicator is
described in the U.S. Pat. No. 2,407,690 of Southworth.
However, special and difficult problems associated with radiation heating
of many prevalent types of malignancies found along body passages are
caused by tendency of the growths to spread around and along the passage,
often in a relatively thin layer. Typically, the malignant layer may be
less than a centimeter thick and may extend as far as 6-10 centimeters
along the passage.
Relatively uniform irradiation heating of the entire malignancy is
necessary to prevent excessively high energy levels possibly causing
thermal damage to surrounding healthy tissue, from being applied to some
malignant regions, while low irradiation levels, possibly causing only
growth stimulating temperatures, are applied to other malignant regions.
Thus, the applied EMR field should provide an elongated heating pattern
which is generally cylindrically uniform in configuration.
Heretofore, however, body passage insertable EMR applicators, such as the
type disclosed in the Southworth patent, have been configured in a manner
causing a heating pattern that tends to be concentrated at the radiating
tip of the applicator and which decreases at a usually exponential rate
from such tip towards the radiation source.
Applicant has, however, invented a body passage or cavity insertable EMR
applicator principally adapted for medical hyperthermia purposes, which
provides the generally cylindrical or longitudinally uniform EMR heating
pattern necessary to enable substantially uniform heating of malignant
growths or other tissue diseases associated with body passages or
cavities.
To achieve such longitudinally uniform, electric tissue heating field,
applicant's body insertable, electromagnetic radiation applicator
apparatus for irradiating living tissue and the like, comprises an
elongate, generally cylindrical applicator adapted for inserting into a
body passage or cavity through a natural body orifice or an incision. The
applicator, which has a length substantially greater than an outer
diameter thereof, is formed of concentric inner and outer conductors
separated by a dielectric media, the outer conductor being longitudinally
split to form generally symmetrical, spaced apart first and second,
arcuate outer conductor segments.
Included in the applicator apparatus, is an electrical conductor which
interconnects, at a termination end of the applicator, the inner conductor
with one of the first and second outer conductor segments. Electromagnetic
radiation transmission means, for example, a coaxial cable, are connected
to the applicator inner and outer conductors at radiation receiving ends
thereof, for transmitting electromagnetic radiation energy thereto from an
electromagnetic radiation source. Means are also included for causing an
external electric tissue heating field radiated by said applicator to be
substantially uniform at substantially all transverse cross sections along
an applicator to thereby provide substantially uniform tissue heating
along the applicator.
More specifically, to force the electric field outside the applicator for
radiation heating of surrounding tissue, spacings between adjacent pairs
of outer conductor segment edges preferably having defining angles of
90.degree. , thereby also causing both outer conductor segments to be
quarters of a cylindrical surface.
The means for causing the external, electric tissue heating field to be
uniform along the length of the applicator preferably comprises a
dielectric sheath covering the outside of the applicator which increases
in radial thickness at a selected exponential rate towards an applicator
termination end. Covering the termination end, thickness of the dielectric
sheath is at least about one half as great as the outer diameter of the
applicator.
Increasing radial thickness of the dielectric sheath in this manner
increases sheath impedance towards the applicator termination end. This
increased dielectric sheath impedance in turn reduces the radiated
electric tissue heating field towards the termination end at a rate off
setting or compensating for the characteristic increase of external
electric field towards the termination end associated with the applicator
configuration. As a direct result, the electric tissue heating field
radiated by the applicator is longitudinally uniform to enable
longitudinally uniform tissue heating. Although, it is to be appreciated
that malignant growth regions in the body passage along the inserted
applicator may be heated to higher temperatures than normal tissue
elsewhere along the applicator, due to poorer heat dissipation properties
of the malignant growths.
Dielectric sheath thickness increase towards the termination end may be
accomplished with either of two configurations. In a first configuration,
sheath thickness is increased while maintaining constant outer conductor
diameter, thereby causing the applicator apparatus outer diameter to
increase towards the termination end. Where this may be undesirable, for
example, for insertion reasons, the outer conductor diameter may be
decreased consistant with radial sheath thickness increase to maintain a
constant applicator apparatus diameter.
In one variation, edge spacing between the outer conductor segments
decreases towards the applicator termination at a rate causing the
radiated electric field to be constant along the applicator, the
dielectric sheath being formed having uniform radial thickness.
In another applicator variation, which employs the same means for providing
the longitudinally uniform electric field, the outer conductor segments
may be formed in a helical configuration to enhance applicator
flexibility, as may be desired for some uses.
Still another applicator variation has the outer conductor thereof split
into four, rather than two, equally spaced apart conductor segments. With
such configuration, the external electric tissue heating field provided
may be slightly more circumferentially uniform, as may be desirable where
existing tissue "thermal smearing" properties, caused by flood flow, are
poor.
Cooling means may be provided with any of the applicators to enable surface
cooling of body tissue when the applicator is inserted in a body passage
or cavity for deep irradiation heating thereof. Included in the cooling
means is a flexible, tissue shape conforming bladder which is disposed, in
fluid sealing relationship, around the applicator. Fluid inlet and outlet
lines communicate from an inside to an outside of the bladder to enable
flow of cooling fluid from a source therethrough.
To enhance electric field coupling into tissue being irradiated by the
applicator apparatus, the cooling fluid comprises a dielectric media (such
as distilled water) having a dielectric constant substantially equal to
that of the tissue. The cooling fluid dielectric in the bladder between
the applicator and the surrounding tissue eliminates electric field
anomalies which would otherwise be caused by air voids in such regions.
A better understanding of the present invention may be had from a
consideration of the following detailed description, taken in conjunction
with the accompanying drawings, in which;
FIG. 1 is a schematic diagram, showing in partial block diagram and in
partial cutaway form, an electromagnetic radiation (EMR) system which
incorporates a body passage or cavity insertable EMR applicator in
accordance with the present invention;
FIG. 2 is a partially cutaway perspective drawing of the EMR applicator of
FIG. 1, showing longitudinal splitting of an outer conductor of a coaxial
pair of conductors into first and second outer conductor segments and
connecting between the center conductor and one of such segments at a
termination end;
FIG. 3 is a longitudinal cross section view, taken along line 3--3 of FIG.
2, showing internal features of the EMR applicator;
FIG. 4 is a cross sectional view, taken along line 4--4 of FIG. 2, showing
a transverse section of the EMR applicator at a termination end thereof;
FIG. 5 is a cross sectional view, taken along line 5--5 of FIG. 2, showing
a transverse section of a coaxial EMR transmission line connected to an
EMR input end of the applicator;
FIG. 6 is a pictorial diagram depicting general configuration of transverse
electric tissue heating fields radiated by the applicator, FIG. 6(a)
depicting the transverse electric field associated with a transverse cross
section of the applicator relatively adjacent to the termination end
thereof; FIG. 6(b) depicting the transverse electric field associated with
the transverse cross section of the applicator relatively adjacent the
center thereof and FIG. 6(c) depicting the transverse electric field
associated with the transverse cross section through a coaxial EMR
transmitting cable connected to the applicator and adjacent to such
connection;
FIG. 7 is a partially cutaway perspective drawing of a first applicator
variation in which outside diameter thereof is maintained constant, while
dielectric sheath thickness increases towards the emitting end to attain a
longitudinally uniform, radiated electric heating field;
FIG. 8 is a partially cutaway perspective drawing of a second applicator
variation in which the longitudinally uniform, radiated electric heating
field is attained by reducing edge spacing between first and second outer
conductor segments towards a termination end while maintaining constant
radial thickness of a dielectric sheath covering the applicator;
FIG. 9 is a termination end view of the second applicator variation of FIG.
10, showing closer edge spacing between first and second outer conductor
segments at the termination end;
FIG. 10 is a partially cutaway perspective drawing of a third applicator
variation in which the outer conductor is longitudinally split into first,
second, third and fourth conductor segments in a symmetrical manner;
FIG. 11 is a transverse cross sectional view, taken along line 11--11 of
FIG. 10 showing mutual circumferential spacing of the four outer conductor
segments of the third application variation, and also showing a typical
transverse electric heating field pattern radiated thereby.
FIG. 12 is a partially cutaway, perspective drawing of a fourth applicator
variation in which the first and second outer conductor segments are
configured in helical form to enhance applicator flexibility;
FIG. 13 is a partially cutaway perspective drawing of a fifth applicator
variation having tissue surface cooling means; and
FIG. 14 is a transverse cross section view, taken along line 14--14 of FIG.
13, showing a cooling means disc fixed to the applicator at an EMR input
end of the applicator and having cooling fluid inlet and outlet tubes
mounted therethrough.
Shown in FIG. 1, in generally block diagram form, in an exemplary
electromagnetic radiation (EMR) hyperthermia system 10, which incorporates
a body passage or cavity insertable applicator apparatus 12, in accordance
with the present invention. The system 10 is particularly configured and
adapted for heating, by electromagnetic radiation in the radio frequency
or microwave frequency spectrum, of body tissue for medical hypothermia
treatment of thermally controllable diseases, including many types of
malignant growths. In a research environment, the system 10, including the
applicator apparatus 12, may be used to heat, by high frequency EMR,
animal body tissue or tissue simulating matter.
Included as part of the system 10 are a conventional EMR source 14
connected to the applicator apparatus 12 by a transmission line 16, which
may comprise a conventional coaxial cable. Conventional transmission line
tuning means 18, for protecting the source 14 from reflected EMR waves and
for improving system efficiency, may be connected to the line 16 by an
additional transmission line 20. In the exemplary system 10, the EMR
source 14 is shown controlled by conventional thermal control means 26
which preferably includes a plurality of thermal probes or thermocouples
28. Such thermocouples 28 are adapted for monitoring tissue temperature
when inserted into a body 30, in a body passage or cavity region 32
thereof, which is to be EMR heated by the applicator apparatus 12.
Connected between the source 14 and the thermocouples 28 is a conventional
thermal control unit 34 programmable to control operation of the EMR
source in a manner providing a preselected time/temperature tissue heating
profile, as monitored by the thermocouples.
Operatively associated with the applicator apparatus 12, and thus also
forming part of the system 10, are tissue surface cooling means 40.
Included in the cooling means is a conventional cooling fluid supply and
reservoir 42 which is connected to a flexible fluid cooling portion 44 of
the applicator apparatus 12 by fluid supply and return lines 46 and 48,
respectively. The cooling means 40 and the applicator cooling portion 44
cooperate, as described below, to cool surface and near-surface layers of
the body region 32 being irradiation heated by the applicator apparatus
12, thereby enabling deep heating without excessive surface tissue
heating. As indicated by the broken line 50, operation of the cooling
supply and reservoir unit may be controlled by the thermal control unit 34
in a conventional manner.
Also as described below, cooling fluid pumped by the fluid supply and
reservoir 42 through the applicator cooling portion 44 additionally
functions as a void-filling dielectric to improve electric field coupling
between the applicator apparatus 12 and the tissue region 32 being heated.
This dielectric function is particularly important when surfaces of the
tissue passage region 32 are irregular, as is often the case when
malignant growths are present, or when the passage is larger than the
applicator.
Other than as specifically described below, most portions of the system 10
form no part of the present invention, which is principally directed to
the applicator apparatus 12 (and variations thereof) and the cooling means
40. However, general aspects of the entire system 10 are shown and
described to illustrate a manner in which the applicator apparatus 12 can
be used to advantage and to provide a background for facilitating
description and understanding of the apparatus.
More particularly, and as better seen in FIGS. 2 through 5, the applicator
apparatus 12, (shown in these Figures with the cooling portion 44 and
associated cooling means 40 omitted for clarity) comprises a generally
cylindrical, elongated applicator 54, having a length substantially
greater than an outside diameter thereof. An EMR input or receiving end 56
of the applicator 54 is electrically connected to the coaxial transmission
line 16. The applicator 54 includes (concentric) inner and outer
conductors or radiating elements 58 and 60, respectively, which extend an
entire length "1" of the applicator, from the upstream end 56 to a
termination end 62. Preferably the inner conductor 58 is formed as a
continuation or extension of a corresponding inner conductor 70 of the
coaxial transmission line 16.
Filling the region between the inner and outer conductors 58 and 60 is a
dielectric media 72 which is preferrably a continuation of a corresponding
dielectric media 74 of the transmission line 16. The dielectric media 72
may, for example, be teflon and have a dielectric constant, e, of
approximately 2.5 for the apparatus operating frequency mentioned below.
To attain the desired radiated electric field characteristics, opposite
sides of the outer conductor 60 are longitudinally split along the entire
applicator length "1" (FIG. 3), in a manner dividing the outer conductor
into longitudinally symmetrical, arcuate first and second outer conductor
(radiating elements) segments 76 and 78, respectively, both adjacent pairs
of outer conductor segment edges 80, 82 and 84, 86 being thereby, spaced
apart a uniform distance "w" for forcing an electric heating field outside
of the applicator 54, the opposite angles "A" (FIG. 4) which define the
edge spacings between the outer conductor segments 76 and 78 are
preferably equal to about 90.degree., the two outer conductor segments 76
and 78 thereby each defining longitudinal quarters of a cylindrical
surface.
At the applicator input end 56 (FIG. 3), ends of the outer conductor
segments 76 and 78 are electrically connected, as by soldering, to a
corresponding outer conductor 96 of the coaxial transmission line 16, to
provide an electrical extension thereof. Assuming the line 16 comprises a
conventional coaxial cable, the outer conductor 96 thereof is of braided
wire configuration; in contrast, the outer conductor segments 76 and 78 of
the applicator 54 are each formed of a single sheet of a conductive
material such as copper.
To enable the desired outwardly radiated electric heating field along the
applicator 54 (FIGS. 2 & 3), the applicator inner conductor 58 is
connected, at the termination end 62, to either one of the outer conductor
segments 76 or 78 by a quarter circular conductive plate 98. Such
connection between the inner conductor 58 and the first outer conductor
segment 76 is illustrated.
Covering the entire outer surface of the applicator 54, to form an outer
layer thereof, is a dielectric sheath or covering 100 which, at the EMR
input end 56 of the applicator joins a corresponding outer dielectric
sheath 102 of the transmission line 16, Preferrably the dielectric
constant, e, of the two sheaths 100 and 102 are the same, and may, for
example, be equal to approximately 2.5.
When operatively connected by the transmission line 16 to the EMR source 14
(FIG. 1) and when configured in the above described manner the applicator
54 outwardly radiates an electric tissue heating field along the length
"1", as well as outwardly from the termination end 62. In this regard, it
should be noted that although both electric and magnetic field are
radiated from the applicator, as is inferred from the term
"electromagnetic radiation", tissue heating is principally caused by the
electric field, since the body is substantially "transparent" to magnetic
fields. Hence, only the external electric field characteristics are
considered herein. Assuming a sufficient length, 1, of the applicator 54
and proper positioning of the applicator in a diseased body passage or
cavity, all portions of a malignant growth extending around and along the
passage or cavity can thus be simultaneously irradiated for heating, as is
normally required.
However, because of the end connection, by the plate 98, between the inner
conductor 58 and the conductor segment 76, which is required to cause an
external electric field along the applicator 54, the radiated electric
heating field is stronger at the termination end 62 than elsewhere along
the applicator length "1". Measurements during operation indicate that the
radiated electric field decreases at an exponential rate away from the
termination end 62 towards the EMR input end 56. That is, the external
electric field increases exponentially as the termination end 62 is
approached from the input end 56.
As a result, without suitable compensation, even assuming natural tissue
cooling by blood flow which tends to cause relatively uniform tissue
heating or "thermal smearing", the exponentially emitted electric field
which varies along the applicator 54 would result in greater tissue
heating towards the termination end 62. As previously mentioned, this is
undesirable, since healthy tissue near the applicator termination end 62
may be overheated and thermally damaged, while malignant cells in regions
remote from the applicator termination end may be heated only to growth
stimulating temperatures.
Accordingly, means are provided for causing the tissue heating electric
fields to be substantially uniform at all transverse cross sections along
substantially the entire applicator length "1". Although, as described
below, the external electric field is not completely uniform around the
applicator 54, uniform tissue heating is generally provided by the thermal
smearing properties of the tissue. This thermal smearing, when
insufficient, may be augmented by periodic, partial axial rotation of the
applicator, or by other means described below.
Thermal monitoring of tissue irradiation by the applicator 54 has
established that electric field uniformity along the applicator length "1"
can be achieved by appropriately varying the thickness of the dielectric
sheath 100 (FIGS. 2 & 3).
External radiation from the applicator termination end 62, which
contributes to greater tissue heating at such end, is substantially
reduced by making a thickness "t" (FIG. 3) of a transverse sheath portion
104 covering the termination end equal to at least one half an outer
diameter "D" of the outer conductor 60 (FIG. 3).
Substantially uniform tissue heating along substantially the entire
applicator length "1" is further attained by increasing a radial thickness
"t.sub.x " of the sheath 100, covering the outer conductor segments 76 and
78, at a selected exponential or increasing rate from the input end 56
towards the termination end 62. Since the intensity of the external
electric field radiated by the conductors segments 76 and 78 is determined
by impedance of the dielectric sheath 100, and hence depends upon sheath
thickness "t.sub.x ", increasing thickness of the sheath towards the
termination end 62 results in reduced external electric field, as is
required to reduce the tissue heating at such end.
Stated otherwise, increasing the dielectric sheath thickness, "t.sub.x ",
towards the termination end at the same exponential rate the external
electric field would otherwise increase towards the termination end,
eliminates what may be considered "excess" external electric field all
along the applicator 54, thereby resulting in a longitudinally uniform
external electric field. It is to be appreciated that there may still be
some external electric field nonuniformities at the applicator ends, but
because of tissue "thermal smearing" characteristics, actual tissue
heating is substantially unaffected by these field nonuniformities.
Typical electric field patterns for the applicator 54 (FIG. 2) and adjacent
regions of the transmission line 16 are depicted in FIG. 6(a) through (c)
taken along lines 6a, 6b and 6c of FIG. 3. Internal non-tissue heating
electric field lines are identified by reference numbers 106 while
external, tissue heating electric field lines are identified by reference
numbers 108. This external, tissue heating electric field is caused by
reflection, at the applicator termination end 62, of EMR energy propogated
down the transmission line 16 and applicator 54 from the EMR source 14.
There is only a small external leakage field along a conventional coaxial
transmission line.
As seen in FIG. 6(a), internal electric field lines 106 associated with the
transmitted or forward EMR waves in the transmission line (coaxial cable)
16, assuming excitation of the fundamental or lowest order, TEM mode, are
directed inwardly from the outer conductor 96 to the inner conductor 70.
These field lines are typical of coaxial transmission lines, as can be
seen in any microwave text.
However, at the applicator termination end 62 (FIG. 6c), the internal
electric field lines 106 associated with the transmitted wave, radiate
from the second outer conductor segment 78 towards both the inner
conductor 58 and the connecting plate 98. The external electric field
lines 108, associated with the reflected electric field, extend outwardly
around the applicator 54, from the second outer conductor segment 78, to
the first outer conductor segment 76. As before mentioned, the dielectric
sheath 100 functions as an impedance element limiting the external
electric field beyond the sheath.
In intermediate applicator regions, depicted in FIG. 6(b) the forward wave
electric field lines 106 extend inwardly from the outer conductor segments
76 and 78 to the inner conductor 58. The external electric field lines 108
extend around the applicator 54 from the second outer conductor segment 78
to the first outer conductor segment 76. Because of reduced dielectric
sheath thickness tx in intermediate applicator regions, compared to
thickness at the termination end 62, the electric field lines 108
externally of the sheath 100 are substantially identical to those at the
termination end.
From FIGS. 6(b) and (c), it is seen that strength, and hence tissue heating
characteristics, of the external electric field, represented generally by
spacing and concentration of the field lines 108, is greater adjacent an
outer surface 114 of the sheath 100 at opposing regions of the outer
conductor segments 76 and 78, along a center line through such segments
and the inner conductor 70. Consequently slightly greater tissue heating
may be expected in these two opposing regions. However, as mentioned
above, thermal smearing characteristics are ordinarily sufficient to cause
actual tissue heating to be uniform all around the applicator 54. If
necessary, the applicator apparatus 12 (FIG. 1) can, however, be
periodically rotated to provide added heating uniformity.
The required variation in dielectric sheath thickness, t.sub.x, for any
particular configuration of the applicator 54, is readily determined
either by use of a conventional field strength measuring apparatus or by
monitoring tissue temperature during irradiation. An appropriate variation
in thickness, t.sub.x, can be achieved, for example, by applying
successively shorter layers of shrink-type plastic tubing over the
applicator 54, more layers being applied towards the termination end 62.
When the correct sheath thickness variation has been determined in this
manner for the particular applicator configuration being used, production
applicators are preferably constructed (as shown) having a continuously
varying sheath thickness, t.sub.x, as is preferable for sanitary and other
reasons.
As an illustrative example, with no limitation intended or implied, the
applicator 54 (FIG. 3) may be formed having a length "1" of 4 inches so
that the applicator is sufficiently long to span most malignant growths
expected to be found along natural body passages with which the apparatus
12 may be used. For ease of insertion through either corresponding natural
body orifices or other surgical incisions, and to be compatible with
typical body passage or cavity size, the outer diameter of the applicator
54, including the sheath 100 at the EMR input end 56 is 0.20 inches.
Thus, the applicator 54 is compatible with standard sized coaxial cables.
For use at an applied EMR frequency of 433 MHz, sheath thickness t.sub.x
increases exponentially so that at the termination end 62, applicator
thickness (including the sheath 100) is 0.25 inches.
When the outer diameter, length and operating frequencies of the applicator
54 are varied, external electric field characteristics may also vary and
accommodations may therefore be required. For example, for higher
operating frequencies, for the described applicator dimensions, the tissue
heating electric field pattern tends to shift towards the termination end
62, whereas, for lower operating frequencies, the heating pattern tends to
shift towards the input end 56.
By way of explanation, at higher frequencies, for example, at the standard
United States diathermy frequency of 915 MHz, the conductance of body
tissue increases from 1.4 mhos/meter (at 433 MHz) to 1.6 mhos/meter, due
to greater tissue conductance. Thus, typical body tissue has greater
thermal conductivity at higher frequencies and tends to dissipate heat
faster. At the same time, capacitive impedance, Z.sub.c, of the dielectric
sheath 100 is inversely proportional to frequency. Since capacitive
inductive decreases with increased frequency at a faster rate than body
conductance increases, the net result is that tissue heating shifts toward
the termination end. The reverse is true for lower operating frequencies.
Accordingly, for the same applicator configuration at higher applied
frequencies, thickness t.sub.x of the dielectric sheath 100 must
ordinarily be increased over that described towards the termination end 62
to maintain a uniform external electric heating field along the entire
applicator length. For lower frequencies, dielectric sheath thickness,
t.sub.x, must ordinarily be decreased over that described towards the
termination end 62.
To achieve and maintain substantially uniform tissue heating along
substantially the entire applicator length "1", relationships ex | | |
