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FIELD OF THE INVENTION
The present invention relates to an improved thermometic system, operating
in a microwave field, for measuring the temperature of a dielectric object
exposed to the field and, in particular, for measuring the temperature of
human tissue, e.g., brain tissue, exposed to microwave radiation.
BACKGROUND OF THE INVENTION
The heat produced when biological systems are exposed to microwave
radiation can contain important information regarding the response of the
biological systems to such radiation. For example, the measurement of
brain temperature during moderate to high level exposure to microwave
radiation is of considerable importance in studying microwave-induced
central nervous system pathophysiology.
A discussion of brain temperature, and previous measurements, in vivo, of
the microwave heating of brain tissue is contained in the article "A
Microwave Decoupled Brain - Temperature Transducer", by Larsen et al, IEEE
Transaction on Microwave Theory and Techniques, Vol. MTT-22, No. 4, April
1974. This article, which is co-authored by one of the inventors here,
also describes a transducer system based on a hybrid microwave integrated
circuit (MIC) construction used in making temperature measurements. In
general, the MIC transducer comprises a thick film thermistor mounted on
contact pads located at the distal end of a gold microline formed by
conventional metallization and photolithographic techniques on a sapphire
needle. Separate series resistors for suppression of dipole currents are
also employed. Although the MIC transducer construction disclosed in the
Larsen et al article provides advantages over prior art transducers used
for the purposes in question, there are problems with the MIC transducer
with respect to the effects of different temperature coefficients and
excessive heating in the carbon loaded polytetraflouroethylene (PTFE)
transmission line interposed between the transducer sub-assembly and the
resistance measurement instrumentation.
More generally, it has been found that a serious problem associated with
transducer probes or electrodes is that these electrodes tend to act
either as a heat source or a heat sink and that the heat added to or
substated from the tissue due to the electrode will distort the
temperature measurement that is being made. To explain, where the
transducer probe or electrode is more lossy than the tissue in the
microwave environment, the electrode will act as a source of heat, and
heat from the probe will be transferred to the tissue, thereby raising the
temperature of the tissue and thus disturbing the measurement to be made.
On the other hand, it also is possible for the electrode to act as a heat
sink so that heat flows from the tissue whose temperature is to be
measured, thereby lowering this temperature and distorting the results.
Ideally, an electrode or probe would have a loss that matches the
equivalent volume of the tissue displaced thereby. Strictly speaking, this
is not possible due to the fact that the loss tangent of the tissue is not
static. In fact, the microwave properties of the tissue constantly change
due to such factors as regional blood flow and physiological responses to
regional flow. However, as will become clear, a very important aspect of
the present invention is to provide an electrode which, in general, acts
neither as a heat source or as a heat sink.
One problem with the MIC transducer construction discussed above, as well
as with further developments thereof, concerns the heating provided by the
overall transducer system and, in particular, by the PTFE transmission
line. For example, it was necessary to steadily increase the resistance of
the line in order to reduce heating and, for the required operation in
air, this could only be achieved by reducing the carbon density. The
result was that where the lineal resistance of the line was increased to
on the order of 100 to 150 Kohms, serious problems were encountered in
making reliable connections to the line. As a consequence, the junction
impedance increased over time and the transducer probe or electrode acted
more as a mechanical transducer than a temperature transducer. Another
problem concerns the presence of standing waves on the line where the line
was operated, in air, parallel to the polarization of the electric field.
Further, experiments with the transducer sub-assembly without the
transmission line demonstrated that heat sinking could become a problem if
the line was to be adequately decoupled.
Finally, the thermal conductivity of the transducer electrode is another
matter of importance where the electrode must traverse regions wherein
temperature gradients exist. As pointed out in the Larsen et al article,
there is a 0.5.degree. C. gradient between the cortex of the brain and the
brain stem due to circulatory patterns. Thus, an electrode which is
suitable for use for purposes outlined above must have the lowest possible
thermal conductivity as well as provide the best possible loss matching.
SUMMARY OF THE INVENTION
Generally speaking, the present invention concerns a thermometric
transducer system for use in tissue during microwave exposure which
provides electrothermal matching of the heat produced in an equivalent
volume of the tissue by providing a varying impedance in the connecting
line which varies between a relatively high value, for operation in air,
and a relatively low value, for operation tissue. As a result, the
transducer electrode does not distort the induced field (i.e., is
microwave transparent) and there is no heat sourcing or heat sinking as is
provided with prior art devices. In addition, the thermal conductivity of
the transducer electrode is extremely low and thus the two prerequisites
of a successful thermometric transducer device discussed above, i.e., good
loss matching and low thermal conductivity, are met. Further, and more
generally, the thermometric transducer device or system of the invention
provides high stability and high accuracy and is of an extremely small
size.
In accordance with a preferred embodiment, the transducer system of the
invention includes a transducer electrode subassembly comprising an
elongate substrate, a sensor means, mounted at one end of the substrate,
for sensing the temperature of a dielectric object and for converting the
temperature sensed into a corresponding electrical signal, and conductor
means mounted on the substrate and extending along the length thereof for
connecting the sensor means to a transmission line for transmitting the
electrical signal to appropriate instrumentation, the conductor means
including at least one conductor whose impedance varies along the length
thereof so as to provide electrothermal matching of the transducer
electrode sub-assembly to the dielectric object and the surrounding medium
such that the transducer electrode sub-assembly acts neither as a heat
source nor a heat sink with respect to the dielectric object during
exposure of the dielectric object to microwave radiation.
In accordance with a preferred embodiment of the transmission line, an
ultrathin thin film transmission line is provided whose thickness relative
to the skin depth of the microwave fields is such that the conductor is
poor medium for microwave conduction and thus is microwave decoupled. In
addition, this ultrathin conductor prevents standing waves from forming at
the interface between the transmission line and the transducer substrate.
In a preferred embodiment, the thickness of the conductor is 10-4 (1/100
of 1%) of the skin depth at 3 GHz for the nichrome conductor employed.
The transducer electrode sub-assembly preferably comprises a cylindrical or
tubular insulating substrate on which the conductor is mounted. The sensor
device comprises a thermistor which is advantageously encapsulated in
glass in order to provide an extremely stable microenvironment for the
thermistor. The thermistor includes four terminals, and four thick film
conductors are equally spaced about the circumference of the tubular
substrate. The conductors are tapered to provide a relatively high
resistance at the proximal end of the electrode sub-assembly and a
relatively lower resistance at the distal end. The distal ends of the
conductors are shorted together.
The transmission lines each preferably comprise an extremely thin metallic
film, e.g., nichrome, disposed on an insulating substrate such as Mylar
and include metallic pads at ends thereof for connecting the line into the
system.
In an important application thereof, the invention enables high accuracy
measurement of temperature during microwave exposure in order to estimate
local energy deposition and physiological thermoregulatory responses for
microwave hazards analysis. For example, the device of the invention can
be used in medical applications to measure the temperature of a tumor
during therapeutic microwave exposure or during microwave exposure used to
accelerate the healing of a wound.
Other features and advantages of the invention will be set forth in, or
apparent from, the detailed description of a preferred embodiment found
hereinbelow.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a thermometric transducer device
constructed in accordance with a preferred embodiment of the invention;
FIG. 2 is a perspective view, drawn to an enlarged scale, of the transducer
electrode sub-assembly shown in FIG. 1, with the electrode inserted into
human tissue;
FIG. 3 is an end sectional view of the transducer electrode sub-assembly
shown in FIG. 2; and
FIG. 4 is an exploded view of line 22 of the trassmission lines shown in
FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a preferred embodiment of the thermometric transducer
system of the invention is illustrated. The system basically comprises a
transducer sub-assembly 10 and a transmission line 12 for connecting the
transducer sub-assembly to conventional measurement instrumentation, as
illustrated. It is noted that this instrumentation can, for example, take
the form described in the Larsen et al article referred to above and forms
no part of the present invention.
The transducer sub-assembly 10 is illustrated in more detail in FIGS. 2 and
3 and, as shown, includes a hollow tubular insulating substrate 14 on
which four tapered, thick film conductors 16 are located. A sensor in the
form of four terminals, to a glass encapsulated thermistor 18 is located
at the distal end of the transducer sub-assembly 10 and conductors 16
provide an electrical connection between the thermistor 18 and the
transmission line 12. As can be best seen in FIG. 3, the four thick film
conductors 16 are equally spaced about the circumference of tubular
substrate 14 and, as discussed below, the thick film conductors 16 are
tapered so to provide electrothermal matching, the conductors providing a
relatively low impedance at the distal end of transducer 10 (which is
shown as inserted into tissue) and a relatively high impedance at the
proximal end adjacent to transmission line 12. As shown in FIG. 3, and as
is also described in more detail hereinbelow, the conductors 16 are
connected to the terminals of thermistor 18 by connections indicated
diagrammatically at 16a, and two pairs of the conductors 16 are shorted
together by connections indicated diagrammatically at 16 b. Further, an
encapsulating medium is indicated at 20 in FIG. 3.
Referring again to FIG. 2, the conductors 16 are connected to individual
lines 22 of the transmission line 12 in a manner to be described
hereinafter. Lines 22 are themselves enclosed in a protective tubing 24. A
specific embodiment of one of the transmission lines 22 is shown in FIG.
4. Line 22 comprises an ultra-thin film conductor 26 mounted on a
substrate 28. Conductive pads 30, used in connecting the thin film
conductor 26 into the system, are provided at the ends of conductor 26, as
illustrated.
The improved thermometric transducer system of the invention is preferably
constructed by a method which combines thick film and thin film
techniques, thick film techniques being used for the four terminal
transducer sub-assembly 10, and thin film techniques being used for the
transmission line 12.
Considering the invention in more detail, in an exemplary embodiment, the
thick film construction of the transducer sub-assembly 10 begins with
hollow cylindrical substrate 14 which, in a preferred embodiment, is
fabricated of porcelain, is approximately 2 inches long, and has a 20 mil
outer diameter and a 3 to 5 mil wall thickness. A gold ruthenium palladium
(GRP) paste used for the thick film conductors 16 has a specific
resistance of 50 ohms per square. The paste is applied over a screen with
linearly tapering cross section in order to taper the linear resistance
from approximately 200 Kohm per inch at the proximal portion of the
transducer sub-assembly 10 to a value of approximately 50 Kohm per inch at
the distal end, (these values being measured in approximately half inch
segments from each end of the line). The gold portion of the paste
provides the relatively "low" resistance component, and rethenium added to
the paste aids in attachment to the substrate 14. Powered silicon added to
the paste provides passivation when the composite is fired. The result is
a glass passivated line which is virtually immune to scratching and
degradation in physiological or hydrocarbon media.
The glass encapsulated thermistor 18 is attached after shorting the four
conductors 16 into two pairs at the distal end of substrate 14 with minute
amounts of conductive epoxy corresponding to connections 16b shown in FIG.
3. In this way, the conductors 16 are attached and the linear resistance
verified without the presence of a temperature sensitive resistance. The
thermistor sensor 18 (VECO 52A19) is conventionally supplied with two
short stubs of 2 mil wire (PtIr) which were removed. The result is two
"pads" (not shown) which are flush with the glass of the tubular substrate
14. These pads are used for connection to the shorted pairs of the thick
film conductors 16, which is the last step in the fabrication of the
mounting for thermistor 18.
Each thin film line 22 was fabricated by vacuum deposition of nichrome to
form thin film conductor 26 on substrate 28, the latter being fabricated
of 2 mil thick polished mylar. The vacuum deposition took place through a
mask with a constant 10 mil width. Resistance should be monitored during
the nichrome deposition as a means to reduce variability. The deposition
was controlled to result in a thin film line having a lineal resistance of
200 Kohms per inch. Each line 22 is fabricated as a six inch strip. The
width of conductor 26 is approximately 1 mil while the thickness of
conductor 26 is approximately 30 Angstrom units (30A.degree.) thick which,
as noted, is approximately 10-4 of the skin depth of the conductor at
3GHz. The ends of conductor 26 are then processed through a second
deposition to produce pads 30, which, in the specific embodiment being
discussed, are 1 mil thick pads of aluminum and extend over the terminal
0.200 inch of the nichrome conductor 26. A final deposition provides a
uniform SiO.sub.2 passivation.
The four finished lines 22 are attached to at their distal ends (the
proximal end of the transducer sub-assembly 10) by minute amounts of
conductive epoxy. All junctions are strain relieved with clear
nonconductive epoxy. Because of problems with minute fractures in the thin
film at the region near the A1 pads 30, presumably caused by different
coefficients of expansion for the thick A1 and the tin nichrome, room
temperature setting epoxies are used.
The four thin film lines 22 are covered in Tygon tubing of 40 mil ID and 60
mil OD, and corresponding to tubing 24 of FIG. 2. The proximal ends of the
four thin film lines 22 are then attached to a 4-pin connector (not shown)
with an intermediate piece of substrate (not shown) and point-to-point 1
mil gold wire. The entire interior of the connector shell (not shown) and
the tubing 24 are then potted and strained relieved with nonconductive
epoxy.
In prior testing of the devices, power densities were always 246
mW/cm.sup.2 incident, and the duration varied between 8 and 30 sec. in
most cases. Short exposures are necessary in order to reduce thermal
diffusion. The methods for field perturbation testing were essentially the
same before-and-after design as described in the Larsen et al article. In
brief, a 3 cm sphere of brain tissue phantom was exposed to far field 2450
MHz at a predetermined power density and exposure duration, after which
thermographic scans mapped the thermal analog of the induced field
distribution. The electrode was then inserted, either after cooling of the
original target, or in another identical target, and the exposure regimen
was repeated with thermographic scans. To the extent that the electrode
affected field induction in the brain phantom, the two heating patterns
would be different.
Thermographic testing of the thin film line 12 took place in air. Short
segments (2 inches) were used with thermographic line scans at the center
of the line segment. This arrangement obviated the need for two
dimensional scans to find the location of peaks in the standing wave
pattern. It is noted that the latter is a very difficult procedure for
quantitative testing due to the rapid dissipation of heat which gives
erroneously low values because of the 4 second frame time for high
resolution two dimensional imagery. The short segments could be analyzed
by a single line scan in the geometric center of the line segment. Under
these conditions, the line segment was a short dipole in which a
triangular current distribution would produce peak heating in the
geometric center. Standing waves are detectable in the two dimensional
scans in the phantom since all of these scans involved complete electrodes
including the connector and shell referred to above.
The transducer electrode 10 was located in a brain phantom, at a distance
of 10 mm from the leading edge of the sphere, and at a depth such that the
200 Kohm per inch thick film conductor 16 was operated in air with 5 mm of
exposed length beyond the brain. The thin film line 12, of course,
operated totally in air.
It is noted that the thermometric evaluation of the electrodes
necessitated, and resulted in, the development of a temperature encoding
electronic package. This package (which is not illustrated) consists of an
IC constant current generator operating at 7Hz to drive the thermistor 18
with 100 microamps over one unshorted pair of lines. A voltage detector is
provided which comprises two high input impedence (10.sup.9 ohms) buffers
followed by an IC instrumentation amplifier buffer with a high common mode
rejection ratio (90 dB) and a frequency shaped gain provided by a feedback
network. The lines for voltage detection are, of course, the remaining
unshorted pair of lines. The output voltage is measured by a Hewlett
Packard 3340A Digital Multi-Meter (DMM). Chart records were made on a
Hewlett Packard 7100B after processing by a Hewlett Packard 580A
digital-to-analog converter (DAC). The system was tested in a Leeds &
Northrup 8401 oil bath with better than millidegree regulation. The bath
temperature was monitored with a Hewlett Packard 2801A quartz thermometer.
The electrode was first tested for stability over a four day period. After
this the bath was slewed over a temperature range of 35.degree. to
40.degree. C. over an eight hour period for RT calibration and hysteresis
testing. This procedure was repeated four times over a seven day period.
Considering the results of the tests, the thermographic evaluation of the
electrode of the invention showed that thermal analogs of field
aberrations could be either in the direction of heat sourcing or in the
direction of heat sinking. As a result, the concept of electrothermal
matching places an upper limit on electrode decoupling to the extent that
the thermal conductivity of the electrode allows heat sinking.
Conversely, an inadequately decoupled electrode may serve as a heat source
as described above where heating due to the line may be seen to diffuse
down the substrate into the brain phantom.
As noted above, the vastly different loss of air and brain is reconciled in
accordance with the invention by linearly tapering the thick film
conductors 16 from the highest value at the proximal end of the substrate
14 which operates in air to the distal end which is to be embedded in the
brain. The results with this configuration indicates only a slight heat
sinking is detectable in the line scan, and no heat sourcing is evident.
Line scans through the thin film conductors 22 in air have been compared to
similar scans through PTFE conductors of the same width. Width matching
appears to be necessary in order that the effects of the modulation
transfer function of the infrared imaging system are constant. Although
standing waves were detectable with wide transmission lines (i.e. for
widths of about 100 mil), such standing waves could not be detected with
the 10 mil line, in contrast with PTFE lines as discussed above.
A thermometric evaulation disclosed environmental temperature sensitivity
in the temperature encoding electronics such that a 1.degree. C. change in
room temperature produced a 20 millidegree apparent change in the
measurand, and of the opposite direction. Stability tests in temperature
controlled conditions established the resolution of the system to be
determined by a noise equivalent temperature of about 7 millidegrees. Long
term stability proved to be largely determined by variations in room
temperature. Over a single 8 hour hyteresis test, the ascending and
descending voltage-temperature (VT) curves were largely superimposed
except for a 10 millidegree discrepancy at the end of the run. On two
successive days, the VT curves tracked within 25 millidegrees.
Although the invention has been described relative to an exemplary
embodiment thereof, it will be understood that other variations and
modifications can be effected in this embodiment without departing from
the scope and spirit of the invention.
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