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FIELD OF THE INVENTION
This invention relates to electrotherapeutic apparatus, and more
particularly to direct-contact applicators for use with mocrowave
diathermy machines.
BACKGROUND OF THE INVENTION
According to existing practice of diathermy therapy at the 2.45 GHz band,
only spaced external applicators are used for radiating the tissue under
treatment. Since the applicator is spaced from the patient's tissue, the
radiated energy is not confined to the prescribed tissue area but also
irradiates uprescribed tissue of the patient, as well as the operator and
other persons in the vicinity of the diathermy machine, causing the
possibility of exposing the operator and other persons from exposure to
the hazardous levels of energy fields.
SUMMARY OF THE INVENTION
Accordingly, a main object of the present invention is to overcome the
disadvantages of spaced applicators, namely, to avoid the possibilities of
unintended overexposures by eliminating the scatter of energy by employing
applicators which can be placed in direct contact with the tissue to be
treated and which confines the radiation substantially to the intended
tissue area.
A further object of the invention is to provide direct-contact microwave
diathermy applicators which provide a more uniform heating pattern than do
the presently available spaced applicators. Such a heating pattern is for
example needed for microwave induced hyperthermia treatments of cancer
because there is a critical temperature above which both cancer and
healthy tissues are killed. In a temperature region below this critical
temperature, cancer tissue responds to treatment while healthy tissue is
not affected.
A still further object of the invention is to provide an improved
direct-contact microwave diathermy applicator which employs an
inhomogeneously filled wave guide, which overcomes the disadvantages of
spaced applicators, and which eliminates the scatter of energy, while
providing a more uniform heating pattern than that obtained from a spaced
applicator of the type presently available, the improved applicator being
of relatively simple construction, being easy to use, and preventing
unintended radiation exposure of external tissue areas and to the operator
and other persons in the vicinity of the associated diathermy machine.
A still further object of the invention is to provide an improved
direct-contact microwave diathermy applicator for use in the 2.45 GHz
diathermy irradiation band which employs a loading waveguide containing
Teflon slabs which fit tightly in the waveguide and which are arranged to
provide a substantially uniform temperature distribution at the center of
the resultant heating pattern, the applicator operating with minimum
scatter radiation and being physically easy to manipulate.
BRIEF DESCRIPTION OF THE DRAWINGS
Further objects and advantages of the invention will become apparent from
the following description and claims, and from the accompanying drawings,
wherein:
FIG. 1 is a perspective view of a typical improved microwave diathermy
applicator assembly according to the present invention, shown with the
main applicator portion partly received its associated waveguide-coaxial
cable adaptor.
FIG. 2 is a perspective view of another form of microwave diathermy
applicator assembly according to the present invention, shown fully
assembled.
FIG. 3 is an enlarged vertical cross-sectional view taken substantially on
line 3--3 of FIG. 1.
FIG. 4 is an enlarged vertical cross-sectional view taken substantially on
line 4--4 of FIG. 2.
FIG. 5 is a perspective view showing how a typical direct-contact microwave
diathermy applicator according to the present invention may be tested,
using a fat-skin planar phantom of simulated tissue.
FIG. 6 is a perspective view showing how a thermographic camera may be
employed to obtain a scanning readout of the temperature distribution at
the center of the heating pattern in the midplane of a simulated-tissue
phantom such as that employed in FIG. 5.
DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the drawings, and particularly to FIG. 1, 11 generally
designates a typical microwave diathermy applicator assembly according to
the present invention, for direct application at the 2.45 diathermy
irradiation band. The main portion of the applicator 11 comprises a
rectangular waveguide member 12 provided with a front peripheral securing
flange 13. Tightly engaged in the opposite side portions of waveguide
member 12 are respective polytetrafluoroethylene (Teflon) slabs 14, 14,
whose front ends are substantially flush with the plane of flange 13. A
specified width of slab 14 is necessary to sustain a uniform
electromagnetic wave, the transverse electromagnetic mode (TEM), in the
air space between the two slabs as noted below.
In a typical small size applicator according to FIG. 1, the waveguide
section 12 is 3 inches long and has an inside cross-section 4.3 inches
wide and 2.15 inches high. The Teflon slabs 14, 14 are 3 inches long and
1.26 inches wide. For a medium size applicator, the waveguide section 12
is also 3 inches long, and has an inside cross-section 5.1 inches wide and
2.55 inches high, the Teflon slabs 14, 14 being also 3 inches long and
1.26 inches wide. In each case the slabs are of a height sufficient to fit
tightly in the opposite side portions of the waveguide cavity.
The waveguide section 12 is closely receivable in the rectangular body 15
of a waveguide-coaxial adaptor 16 provided with a connection terminal 17
for connecting the applicator to the end of a coaxial cable 18 (see FIG.
5) leading to the output of a 2.45 GHz microwave diathermy machine. The
above connection terminal 17 must be placed symmetrically with respect to
the vertical walls of the rectangular body 15 to prevent excitation of the
first order asymmetric Longitudinal Section Electric (LSE) mode which
would induce a less uniform heating pattern. The front end of body 15 is
provided with a peripheral flange 19. The flanges 13 and 19 have
registering apertures for receiving fastening screws or bolts to secure
the flanges in abutting relationship.
The spaced Teflon slabs 14, 14 serve as loading and absorption elements to
modify the heating action of the applicator, applied directly to a tissue
area to be treated, so as to provide a desirable heating pattern, for
example, a pattern wherein the temperature is substantially uniform at its
central portion. The slabs 14 must be spaced sufficiently close to each
other, as is this case in this disclosure, to prevent the excitation of
the second higher order LSE mode in the air space between them which would
induce a less uniform heating pattern.
FIG. 2 shows a large size direct-contact microwave diathermy applicator
according to the present invention, designated generally at 11'. In this
embodiment, the main portion of the applicator comprises a rectangular
waveguide member 12' provided with a front peripheral flange 13'. Tightly
fitted in the intermediate portion of member 12', coextensive in length
therewith, are the spaced longitudinally extending Teflon slabs 14', 14'
whose front ends are substantially flush with the plane of flange 13'. A
specified width of slabs 14' is necessary to sustain a TEM mode in the air
space between the two slabs as noted below. The waveguide section 12' is
closely received in the rectangular body 15' of a waveguide-coaxial cable
adaptor 16' provided with a terminal 17' for connecting the applicator to
a coaxial feed cable 18 leading from the output of a 2.45 GHz microwave
diathermy machine. The above connection terminal 17' must be placed
symmetrically with respect to the vertical walls of the rectangular body
15' to prevent excitation of the first order symmetric LSE mode. The front
end of body 15' is provided with a peripheral flange 19' which is secured
to flange 13' by suitable screws or bolts 20, as shown in FIG. 2.
In a typical embodiment, following the showing in FIG. 2, the waveguide
section 12' is 3 inches long and has an inside cross-section 6.5 inches
wide and 3.25 inches high. The Teflon slabs 14', 14' are 3 inches long and
1.26 inches wide. The spacing between the slabs 14', 14' at the midportion
of the waveguide cavity is 1/2 inch. The slabs 14', 14' are of a height to
fit tightly in the intermediate portion of the waveguide cavity. The slabs
14' must be spaced sufficiently close to each other, as is the case in
this disclosure, to prevent the excitation of the second higher order LSE
mode in the air space between them which would induce a less uniform
heating pattern.
As in the embodiment of FIG. 1, the Teflon slabs 14', 14' serve as loading
and absorption elements to modify the heating action of the
directly-applied applicator so as to provide a desirable heating pattern,
namely, wherein the temperature is substantially uniform over the central
portion due to the TEM excitation in the air space between the two slabs
14'.
In testing the performance of an applicator 11 or 11', a fat-skin planar
phantom 21 (see FIG. 5) may be used to stimulate tissue to be treated. The
phantom 21 may comprise a pair of symmetrically-mating blocks 22, 22,
abutting at a vertical midplane, as shown at 23 in FIG. 5. Each block has
an outer layer 24 of a material having microwave absorption or dielectric
characteristics similar to fat and an inside layer similar to muscle, such
as described in A. W. Guy, J. F. Lehmann, J. A. McGougal and C. C.
Sorensen, "Studies on Therapeutic Heating by Electromagnetic Energy", page
31, "Thermal Problems in Biotechnology", American Society of Mechanical
Engineers, N.Y., 1965; the fat material consists of 84.81% Laminac
Polyester Resin, 0.45% Catylist (Methyl Ethyl Ketone Peroxide "60%"),
0.24% Acetylene black and 14.5% aluminum powder; the muscle material
consists of 15.2% Powdered Polyethylene, 76.4% Saline Solution (12 gms
salt/liter) and 8.4% Silly "Stuff" (Silly Stuff from Whamo Co.
California.) Each block also has an inner matrix 25 simulating muscle. The
testing procedure comprises first heating the abutting blocks 22, 22,
arranged as in FIG. 5, with a direct contact applicator 11 or 11' placed
thereon over the abutment midplane at 23. The power output of the
microwave diathermy machine, operating at 2.45 GHz, is about 130 watts,
lasting for 5 seconds. The resultant temperature distribution at the
center of the heating pattern in the midplane of the phantom is measured
by using a thermographic camera 26 (see FIG. 6). The camera scanning line
is set parallel to the fatmuscle interface of the planar phantom. For the
small direct contact applicator of FIG. 1, in a typical test, the central
portion of the heating pattern, about 1.6 inches in length, showed an
average temperature rise of 2.7.degree. C, with limits of .+-. 0.2.degree.
C; for the medium size direct-contact applicator 11 above described, the
central portion, about 2.9 inches in length, showed an average temperature
rise of 1.2.degree. C, with limits of .+-. 0.2.degree. C, and for the
large direct-contact applicator 11' of FIG. 2, the central portion, about
1.85 inches in length, showed an average temperature rise of 0.8.degree.
C, with limits of .+-. 0.2.degree. C.
A choice of uniform heating patterns of different sizes with different
temperature distributions is for example needed in microwave induced
hyperthermia treatment of cancer because cancer therapy requires heating
of the entire diseased treatment area above a particular elevated
temperature to prevent the spread of cancer to other tissue.
While certain specific embodiments of improved direct-contact microwave
diathermy applicators have been disclosed in the foregoing description, it
will be understood that various modifications within the scope of the
invention may occur to those skilled in the art. Therefore it is intended
that adaptions and modifications should and are intended to be
comprehended within the meaning and range of equivalents of the disclosed
embodiments. For example, materials which act in a manner equivalent to
polytetrafluoroethylene (Teflon) in the present environment may be used in
its place.
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