A microwave applicator for applying microwave energy to living tissue for providing uniform heating without hot spots. The applicator includes a first electrical conductor and a second electrical conductor substantially shielding the first conductor in a transmission line configuration capable of propagating microwave energy in a frequency band suitable for heating living tissue. The first conductor has an unshielded portion extending a distance beyond the second conductor and there is additionally provided a coil as a third electrical conductor surrounding the extending portion of the first conductor and connected between the ends of the first and second conductors. The applicator is preferably configured for insertion through an opening into the body and includes a substantially smooth dielectric sleeve covering the coil of the third conductor.

INTRODUCTION 
This invention relates in general to methods and means for hypothermal 
medical treatment. More particularly the invention discloses an applicator 
for applying microwave energy to living tissue within a human or animal 
body for uniformly heating such tissue without "hot spots", and a novel 
method for achieving such uniform heating, to any desired temperature in a 
range including temperatures which will destroy tumorous tissue while 
being safe for viable tissue. 
Prior known applicators for this purpose are in the configuration of a 
simple coaxial monopole (illustrated at FIG. 2(a) of the accompanying 
drawings), which is characterized by intense heating in a region where the 
inner and outer conductors are close together; FIG. 2(a) illustrates the 
isothermal field lines of that kind of applicator. 
The following prior art is noted: 
Kraus "ANTENNAS", McGraw-Hill 1950, chapter 7, Sec. 7-16, pages 213-215; 
U.S. Pat. No. 3,014,791--Dec. 26, 1961--Benzing, et al. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the invention to provide an applicator 
sufficiently small, especially sufficiently thin, so that it may be 
inserted into the body for hypothermal medical treatment purposes. Such 
purposes include measurement of local temperature differences by 
radiometry, and heating of living tissue by application of RF energy. 
Another object of the present invention is to provide an applicator which 
heats the active zone of tissue uniformly, avoiding the creation of "hot 
spots" which could burn tissue and cause pain. 
In accordance with the present invention, an applicator is made of a first 
electrical conductor and a second electrical conductor substantially 
shielding said first conductor in a transmission line configuration 
capable of propagating microwave energy in a frequency band suitable for 
heating living tissue; the first conductor has an unshielded portion 
extending a distance beyond said second conductor; and a coil of a third 
electrical conductor surrounding the extending portion of the first 
conductor is connected between the ends of the first and second 
conductors. This arrangement is capable of providing a pattern of 
microwave radiation into living tissue which pattern is characterized by 
substantially uniform non-burning intensity distribution over a prescribed 
spatial distribution within said tissue. The applicator is configured for 
insertion through an opening into said body, and it includes a 
substantially smooth dielectric sleeve covering the coil of the third 
conductor.

DETAILED DESCRIPTION 
The applicator shown in FIG. 1 is the heating tip 10 of a unit intended to 
be inserted into a body cavity or duct for heating living tissue within 
the body with radio-frequency energy in a microwave frequency band, fixed 
to an end of a coaxial line 12. The coaxial line comprises the usual 
center conductor 14, outer conductor 16 and dielectric 18 between them. 
This is the RF input to the heating tip 10. The outer conductor 16 is 
removed to expose an end portion 15 of the inner conductor 14. A coil of a 
third conductor 17 surrounds the end portion 15 and dielectric 18 which 
envelopes it; the third conductor is, by conductive connection, connected 
in series between the end 19 of the outer conductor and the end 21 (radial 
conductor) of the end portion 15. A smooth insulating dielectric sleeve 22 
surrounds the heating tip 10 and the immediately--adjacent portion of the 
outer conductor 16. 
When the applicator of FIG. 1 is inserted into living tissue and 
microwave-frequency energy is applied to the coaxial line 12, local RF 
heating of the surrounding tissue will occur, the temperature reached in 
the tissue depending on many known factors such as power, inverse-square 
law radiation decrements, etc. FIGS. 2(a) and (b) respectively illustrate 
the approximate field pattern of a prior art applicator as compared with 
an applicator constructed in accordance with the present invention. In 
FIG. 2(b) reference characters similar to those in FIG. 1 are used; in 
FIG. 2(a) the coaxial line section 12 is coupled at the center conductor 
14 to a monopole 30, which is free of the outer conductor 16. A 
high-voltage gradient exists between the monopole 30 and the outer 
conductor 16 where they are closest together; i.e.: between a circular 
locus A at the end of the outer conductor 16 and a circular locus B on the 
monopole 30 which is nearest to it. The radiation field is strongest 
between these two loci, and tapers off in strength between regions of the 
outer conductor 16 and monopole 30 which are progressively further apart. 
Thus, the isothermal lines T1, T2--Tn, shown in FIG. 2(a) show that the 
temperature achieved in tissue in contact with the applicator varies along 
the applicator, with the potential for an excessively hot ring in the 
annular region between locus A and locus B. This puts a limit on how much 
power can be applied to the RF input for heating tissue more remote from 
the applicator. 
Referring to FIG. 2(b), the isothermal lines T1 and T2 indicate that 
heating of tissue surrounding the applicator is more nearly uniform along 
the applicator. The coil 17 is electrically connected at its ends to the 
inner and outer conductors, respectively, and there is no region axially 
along the heating tip 10 where the RF field is substantially stronger than 
in any other region. Thus the isothermal line T1 representing the highest 
temperature nearest to the applicator, is nearly flat throughout the axial 
extent of the heating tip. Direct electrical connections at the ends of 
the third conductor 17 eliminate any field build-up at the heating tip. 
Thus, with an applicator of the invention, RF power can be increased 
without causing a hot spot, or a hot ring. The power can be raised 
substantially entirely in accordance with what temperature the user 
desires to achieve in surrounding tissue for hypothermal medical purposes. 
Alternatively, an applicator according to the invention is a superior 
detector of heat being radiated from within the surrounding tissue, in 
that the predictably uniform radiation field indicated in FIG. 2(b) 
enables more reliable location of a source of heat within the tissue. 
In brief, applicators according to the invention can provide a more uniform 
heating or detection measurement of temperature over a wider zone of the 
human body than has heretofore been possible. Such improved capability is 
thought to be useful for detection and possible heat treatment of cancer 
sites within living tissue. 
In FIG. 3 the first coaxial line section 12 of FIG. 1 is surrounded by a 
second coaxial line section 42, the outer conductor 16 of the first 
section 12 being the inner conductor of the second section 42. The 
conductor 46 of the second section is a fourth conductor of the assembly. 
A second coil of a fifth conductor 47 surrounds the first coil of the 
third conductor 17, the fifth conductor being connected between the first 
conductor 14 and the fourth conductor 46. This is a dual-frequency 
applicator/probe, the second coil of conductor 47 and outer coaxial line 
section 42 being intended for use at a lower frequency than the first coil 
of conductor 17 and inner coaxial line section 12. The coils are similarly 
connected between the free end 21 of the first inner conductor 14 and the 
ends of the respective outer conductors 16 and 46. 
The coils of conductors 17 and 47 may be helical, in which event FIG. 3 
includes the case of two concentric helices.