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Description  |
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BACKGROUND OF THE INVENTION
1. Field of Invention
This invention relates to temperature measurement and more particularly to
providing an indication of a temperature within living tissue.
2. Description of the Prior Art
When a patient has a cancerous tumor, a surgical procedure is often
employed to remove the tumor. However, the surgical procedure may cause a
spread of cancer throughout the body of the patient. Therefore, a surgical
procedure may be undesirable.
Since ancient times, medical practitioners have known that the patient may
be successfully treated by raising the temperature of a tumor, a treatment
generally referred to as hyperthermia. One form of hyperthermia is the
elevation of the temperature of a large portion of the patient's body
where the tumor is located. Accordingly, in the late 1960's, medical
practitioners treated patients with tumors in their arms and legs by a
profusion therethrough of hot blood. Additionally, patients with tumors in
their bladders were treated by flushing the bladders with hot fluid. In
1974 Scottish medical practitioners immersed patients in hot wax. Other
medical practitioners have elevated the body temperature of patients by
infecting them with malaria. The tumor either shrinks or disappears at a
temperature of 43.degree. C. However, elevation of the temperature of a
large portion of the body to 42.5.degree. C. may cause death (37.degree.
C. is usually referred to as a normal body temperature). Therefore, the
elevation of the temperature of a large portion of the body has been only
moderately successful.
Although the elevation of a large portion of the human body above a
temperature of 42.5.degree. C. may cause death, local tissue of the human
body including the tumor may safely withstand temperatures above
43.degree. C. Accordingly, another form of hyperthermia is the elevation
of the temperature of the local tissue including the tumor, thereby
raising the temperature of the tumor. An early practitioner, Hippocrates,
(around 600 B.C.) attempted to raise the temperature of the tumor via
conduction by applying red hot irons to the skin. Hippocrates found, and
modern medical practitioners have learned, that when the tumor is two
centimeters beneath the skin, for example, heating the tumor to about
43.degree. C. via conduction may cause either severe burns or death.
American medical practitioners have used microwave radiation to elevate the
temperature of the tissue including the tumor. The microwave radiation may
be controlled to rapidly elevate the temperature of a known volume of
tissue that extends from the surface of the skin to a known depth beneath
the skin. While microwave radiation is used, it is desirable to prevent an
over heating that may damage the tissue including the tumor. Therefore, it
is correspondingly desirable to provide an indication of the temperature
of the tissue including the tumor. Heretofore, suitable apparatus for
providing an indication of a temperature of a tissue beneath the skin has
not been available in the art.
SUMMARY OF THE INVENTION
According to the present invention, a reflector that reflects a microwave
signal as a function of temperature is adapted for implantation within
living tissue. In one specific embodiment, a microwave signal is
transmitted to the reflector through the tissue via an applicator that is
in contact with the tissue. A portion of the signal is reflected from the
reflector through the tissue, via the applicator, to a meter that provides
an indication of temperature of the tissue near the reflector.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic block diagram of a preferred embodiment of the
present invention;
FIG. 2 is a front elevation of a reflector in the embodiment of FIG. 1;
FIG. 2A is a graph of reflectivity as a function of temperature of the
reflector of FIG. 1;
FIG. 3 is a side elevation of an applicator in the embodiment of FIG. 1;
FIG. 4 is a plan view of the applicator of FIG. 3;
FIG. 5 is an end view of the applicator of FIG. 3; and
FIG. 6 is a plan view of a coupling section of a bidirectional coupler in
the embodiment of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1 and FIG. 2, a surgical procedure is used to implant a
reflector 10 within tissue 12 of a patient. The reflector 10 is implanted
near a tumor 14 in the tissue 12 whereby the temperature of the reflector
10 is substantially the same as the temperature of the tumor 14. As
explained hereinafter, a meter provides an indication of the temperature
of the tissue in the vicinity of the reflector 10 as the temperature of
the tissue 12 is elevated to (and maintained at) a desired temperature of
43.degree. C. It should be understood that the tissue 12 is normally at a
temperature of 37.degree. C.
The reflector 10 is a square wafer of a ferrite material, approximately two
and one half millimeters on a side. As known to those skilled in the art,
when the temperature of a ferrite material is within a range extending
from what is known as a Curie temperature to about 20.degree. C. below the
Curie temperature, the level of a microwave signal reflected from the
ferrite has a substantial variation as a function of temperature.
Accordingly, as will be apparent to those skilled in this art, the changes
in the level of the reflected microwave signal are caused by the change in
impedance of the ferrite in response to the environmental temperature
changes of the tissue in which it is positioned.
Referring to FIG. 2A, in this embodiment, the reflector 10 is comprised of
a type of ferrite material, known as lithium-aluminum ferrite, that has a
Curie temperature of 50.degree. C. Therefore, the normal temperature
(37.degree. C.) and the desired temperature (43.degree. C.) are both
within a range 10R that extends from the Curie temperature to 13.degree.
C. below the Curie temperature. Accordingly, the normal temperature and
the desired temperature are both within a range where there is a
substantial variation of the level of the reflected signal as a function
of temperature. In one alternative embodiment, the reflector 10 is
comprised of a garnet having a compensation temperature of 50.degree. C.
In other alternative embodiments, the reflector 10 may be a P-N
semiconductor junction or a thermistor.
A flat surface 16 (FIG. 1 and FIG. 2) of the reflector 10 is oriented with
respect to a skin surface 18 of the tissue 12 to provide maximum exposure
of the reflector 10 to microwave signals applied at the skin surface in
the direction of the reflector 10. As explained hereinafter, a microwave
signal is transmitted through the skin 18 to heat the tumor 14 to the
desired temperature. A portion of the transmitted signal is reflected from
the surface 16, whereby a reflected microwave signal passes from the
tissue 12 through the skin 18.
The transmitted signal is provided by a microwave transmitter 20 similar to
transmitters that are used in microwave ovens and diathermy machines. The
transmitted signal penetrates the tissue 12 to a depth beneath the skin 18
that is a function of the frequency of the transmitted signal.
Accordingly, the transmitter 20 includes a frequency control knob 22 which
is operable to select a transmitted signal frequency in a range of 900 to
3000 megahertz. The frequency range corresponds to a range of depths of
penetration of approximately one to three centimeters.
The temperature of the tumor 14 is raised in direct relation to the output
power of the transmitter 20. Accordingly, the output power is manually
controlled by an output power control knob 23 of the transmitter 20
whereby the temperature of the tumor 14 is manually controlled. The output
of the transmitter 20 is connected to an applicator 24 through a
circulator 71 and a bidirectional coupler 26, all of which are described
hereinafter.
Referring to FIGS. 3-5, the applicator 24 includes a short section 28 made
from WR42 waveguide with an inside width of 1.067 centimeters, an inside
height of 0.4018 centimeters and a length of 1.661 centimeters. The
section 28 has one end comprised of a conductive plate 30 and another end
32 that is open, whereby electromagnetic waves are reflected from the
plate 30 to the end 32. The applicator 24 additionally includes a tapered
waveguide section 36 that is 2.99 centimeters long. The section 36 is
integrally connected to the section 28 at the end 32 whereby the end 32 is
a proximal end of the section 36. A distal end 38 of the section 36 has an
inside width of 1.212 centimeters and an inside height of 0.826
centimeters. In this embodiment, the end 38 is maintained in contact with
the skin 18.
The applicator 24 is filled with a dielectric material 40 having a
dielectric constant that substantially equals the dielectric constant of
the tissue 12. Typically, the dielectric constant of the material 40 (and
the tissue 12) is in a range of 35 to 85. The section 28 is connected to
the bidirectional coupler 26 through a 50 ohm coaxial connector 42 of any
suitable type. The applicator 24 substantially matches the impedance of
the connector 42 to the impedance of the tissue 12. The applicator 24 can
be similar to an applicator that is the subject of a U.S. Pat.
application, Ser. No. 671,554, filed on Mar. 29, 1976, and assigned to RCA
Corporation, the assignee of the instant application, now abandoned. This
application is hereby incorporated herein and made a part hereof. In an
alternative embodiment, the applicator 24 can be a rectangular waveguide
filled with the dielectric material 40.
The circulator 71 is a microstrip circulator of the type described in U.S.
Pat. No. 3,456,213 which is incorporated herein and made a part hereof.
The circulator 71 has ports 75,77 coupled to the transmitter 20 and the
bidirectional coupler 26, respectively, via transmission lines 50,73, .
Additionally, a port 79 of the circulator 71 is connected through a load
resistor 81 to ground. The circulator 71 couples the transmitted signal to
the bidirectional coupler 26; a signal from the bidirectional coupler 26
is coupled to the resistor 81.
The bidirectional coupler 26 (FIG. 1) includes a transmission line 44 that
extends from an input coupling terminal 46 of the bidirectional coupler 26
to a coupling terminal 48 thereof. The input terminal 46 is connected to a
circulator 71 through the transmission line 73 referred to hereinbefore.
Accordingly, a signal from the transmitter 20 is transmitted to the
reflector 10 via the lines 44,50,73, the circulator 71, the connector 42,
the applicator 24 and the tissue 12. Similarly, the reflected signal is
received at the terminal 48 from the reflector 10 via the tissue 12, the
applicator 24 and the connector 42.
A coupling section 52 of the bidirectional coupler 26 couples a portion of
the reflected signal to a meter assembly 53 as explained hereinafter.
Referring to FIG. 6, the coupling section 52 can be comprised of similar
copper conductors 54,56, about two mils thick, spaced from a ground
conductor by an alumina substrate 58. It should be understood that the
conductor 54 is included in the line 44 referred to hereinbefore (FIG. 1).
The conductors 54,56 include similar sections 60,62, respectively, that
have a length of about one quarter of the wavelength of the transmitted
signal. Additionally, the sections 60,62, have a width of approximately 8
mils and a separation therebetween of about 1 mil; all other portions of
the conductors 54,56, have a width of about 25 mils.
Referring to FIG. 1, one end of the conductor 56 is connected through a
terminating resistor 64 to ground; the other end is connected to the meter
assembly 53 to ground. An inherent property of the coupling section 52 is
that a portion of the reflected signal is coupled to the meter assembly
53, the remainder of the reflected signal being coupled to the resistor 81
whereby the reflected signal is absorbed by the meter assembly 53 and the
resistor 81; substantially none of the reflected signal is coupled to the
resistor 64. Correspondingly, a small portion of the transmitted signal is
coupled to the resistor 64, most of the transmitted signal being
transmitted to the tissue 12 as described hereinbefore; substantially none
of the transmitted signal is coupled to the meter assembly 53. Hence, the
bidirectional coupler 26 couples the transmitted signal to the tissue 12
and not to the meter assembly 53 whereas the reflected signal is coupled
to the meter assembly 53. Various forms of the bidirectional coupler 26
suitable for use in the instant application are available in the art and
the coupler itself forms no part of the instant invention.
The meter assembly 53 is comprised of a meter 66 connected in series with a
detector 68. The detector 68 rectifies the portion of the reflected signal
coupled to the meter assembly 53 whereby a d.c. current passes through the
meter 66. The d.c. current is proportional to the level of the reflected
signal thereby causing the meter 66 to provide an indication of the level
of the reflected signal.
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