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BACKGROUND OF THE INVENTION
The subject invention generally relates to the measurement of temperatures
within a body, and more particularly to a system of temperature
measurement by monitoring the pressure fluctuations due to thermal
vibrations in the body as a function of frequency.
There are many situations in medical diagnosis and treatment, industrial
processing, geophysical exploration, and other fields where the
temperature inside a material body is desirable to measure, but it is not
practical to insert a probe beyond the surface of the body. In medical
diagnosis, the usefulness of temperature measurement at the few places
available for probe insertion is well established. In recent years,
thermograms produced by infrared camera equipment and other surface
temperature measurement have shown promise as a means of detecting breast
cancer lesions. A technique which extends temperature measurement to all
soft-tissue parts of the body offers promise as a powerful new diagnostic
tool.
In medical therapy, a non-invasive temperature monitoring technique would
be useful in almost any procedure involving heating or cooling of the soft
tissues of the body. For example, hyperthermia has been found to be a
promising technique, either alone or in combination with other modalities,
for the treatment of cancer. However, its effectiveness is very sensitive
to the temperature which is reached, becoming more effective as one
approaches 45.degree. C., but tissue necrosis becomes a serious problem if
the temperature goes above 45.degree. C. Therefore, a non-invasive method
of monitoring temperature profiles is important if hyperthermia is to have
wider potential.
In industrial processing, a suitable temperature distribution inside a
large, hot body is often important during the heat treatment and cooling
process. For example, the casting of large thicknesses of glass and other
brittle materials is costly, partly because of a high failure rate which
might be alleviated by a non-invasive temperature monitoring system. Data
needed for geophysical exploration and monitoring would be more readily
obtained if non-invasive accoustic-radiometry could provide temperature
profiles as a function of depth for distances of several meters into
surface rocks or into the region around a bore hole.
SUMMARY OF THE INVENTION
It is, therefore, an object of this invention to make a non-invasive
temperature measurement of a body from acoustic thermal noise spectra.
It is another object of the invention to provide a passive remote
temperature sensor system for non-invasive temperature measurement of the
interior of a body.
It is a further object of the present invention to provide a method of
passive, non-invasive temperature measurement of the interior of a body
using the acoustic thermal noise spectra radiated from within the body.
The foregoing and other objects of the invention are accomplished by
receiving the acoustic thermal noise spectra along one or more well
defined paths within the interior of the body. This is conveniently done
by coupling one or more acoustic transducers with the surface of the body
to intercept the acoustical noise signal from within the interior of the
body along said paths. The acoustic transducers convert the received
acoustic thermal noise spectra into a corresponding electrical signal.
This electrical signal is then analyzed by means of a power spectrum
analyzer to develop an output representing the temperature-depth
distribution along said paths. The output of the power spectrum analyzer
can be displayed and/or recorded or subjected to further signal processing
.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects, aspects and advantages of the invention
will become better understood from the following detailed description of
the invention which makes reference to the accompanying drawing, in which:
FIG. 1 is a block diagram illustrating the practice of the invention using
a single transducer coupled to the surface of a body, the interior
temperature of which is to be measured;
FIGS. 2 and 3 are block diagrams representing modifications of the basic
system shown in FIG. 1 incliuding means for focussing the acoustical noise
signal on the transducer; and
FIG. 4 is a block diagram of a system according to the invention employing
a plurality of acoustic transducers.
DETAILED DESCRIPTION OF THE INVENTION
Acoustic-radiometric temperature sensing is based upon an analogy between
electro-magnetic and acoustic (i.e., elastic wave) radiation. It should be
understood, however, that electro-magnetic and acoustic waves are entirely
different physical phenomena, one being a phenomenum of electric and
magnetic fields propagating in empty space, the other being a property of
the mechanical motions of material media (solids, liquids, gases). There
are many similarities in the mathematics of electro-magnetic and acoustic
waves which result in some analogies of behavior.
In the case of electro-magnetic radiation, it is well known that any
surface at any absolute temperature T>0 emits "black-body" radiation. A
broad band of frequencies is emitted from 0 to an upper limit determined
by the temperature T. For example, for objects at temperatures in the
neighborhood of room temperature, the frequencies extend through radio and
microwave frequencies into infrared frequencies. Many temperature
measuring systems determine temperature of the surface of a "black-body"
by measuring the intensity of all or some portion of the "black-body"
spectrum. For example, pyrometers make use of the visible "black-body"
radiation, comparing the apparent color of the radiation from the surface
of unknown temperature to that of a surface of known temperature. Infrared
cameras measure surface temperature by the intensity of infrared
emissions. Less well known are microwave radiometers, which determine
temperature by measuring the intensity of "black-body" radiation in the
microwave frequency region. When only the microwave frequencies are
measured, the total intensity is directly proportional to the absolute
temperature T of the "black-body" surface. It is possible to measure
temperature down to within a few degrees from absolute zero with a
microwave radiometer. In fact, the most publicized application of this
technique has been the measurement of "cosmic" or extraterrestrial
3.degree. K. "black-body" radiation which is attributed to the remnants of
the "big bang" in cosmological theory.
If the surface is "black," e.g. perfectly absorbing, the intensity of the
emitted radiation depends only upon the temperature T. The power received
at the detecting device depends of course, on the properties of the
detector and its geometric arrangement and distance relative to the
"black-body" surface. It will be assumed that these properties are known,
so that received-power measurements can be interpreted as temperatures. In
fact, this is especially simple if the receiving device has a highly
directional response in a "beam" region, and if the "black-body" surface
extends across the entire beam. When these conditions can be arranged, the
received power is unaffected by the relative distances between detector
and "black-body" surface or by the orientation of the "black-body" surface
(provided only that it intercept the entire beam); hence, the received
power can be directly interpreted as a "black-body" temperature. In the
case of a microwave radiometer, the directional beam response is achieved
by a suitable antenna, often a parabola followed by a conical horn
section. In a pyrometer, the optical system provides the directionally
characteristic.
Suppose the body which is at temperature T does not appear "black," that
is, it does not completely absorb incident radiation. If the fraction
absorbed is A, then the intensity of the emitted "black-body" radiation
must be the same fraction A of the amount expected for a perfect
"black-body." This can be deduced from very general arguments based upon
the conditions which must prevail when thermodynamic equilibrium is
established. The factor A is less than unity if some incident radiation is
reflected, but applications where reflection is negligible or a separate
reflection correction is computed, will be specifically considered herein.
Of prime interest in this discussion is the situation where the factor A
is less than unity because some of the radiation passes through to reach
subsequent bodies or layers in the same body. Consider a body with a large
number of identical layers, each of which absorbs one-half the radiation
intensity incident at its depth. Then the amount absorbed by layers 1, 2,
. . . , n, . . . would be
1/2, 1/4, 1/8, . . . , 1/2n, . . .
because each layer receives one-half as much radiation as the preceding
layer. Now, when emission is considered, each layer emits equally at
one-half the "black-body" rate corresponding to temperature T. However, as
viewed from the outside, the first layer contributes fully, but one-half
of the radiation from layer 2 is absorbed in passing through layer 1 for a
net contribution of 1/4. The relative amount contributed to emission to
the outside by layers 1, 2, 3, . . . , n, . . . would be
1/2, 1/4, 1/8, . . . , 1/2n, . . .
When all these fractions, which represent emission relative to a perfect
"black-body" are summed, the total is the same as for a normal
"black-body." Thus, for a surface in which radiation is gradually absorbed
with increasing depth, the emission is still that of an ideal "black-body"
at temperature T provided that all absorbing layers are at the same
temperature T. When temperature is measured with pyrometer or infrared
cameras, the depth in which the radiation is absorbed is generally very
small, so the assumption of uniform temperature is very good. The
microwave radiometer, however, has found application to situations where
the temperature cannot be assumed uniform throughout the range of depth
where the radiation is absorbed.
Suppose, in the example of the preceding paragraph, that the absolute
temperatures of layers 1, 2, . . . were T.sub.1, T.sub.2, . . . As was
already mentioned, the microwave radiation intensity is proportional to
the absolute temperature, so the contribution from the nth layer must be
proportional to T.sub.n, and the relative emission from layers 1, 2, 3, .
. . , n, . . . is
T.sub.1 /2, T.sub.2 /4, T.sub.3 /8, . . . , T.sub.n /n, . . .
The sum of the above series would predict a total emission corresponding to
an apparent "black-body" temperature T.sub.a, where
T.sub.a =1/2T.sub.1 +1/4T.sub.2 +1/8T.sub.3 + . . . +1/2.sub.n T.sub.n + .
. .
The temperature T.sub.a is a weighted average of the temperature of the
various layers, the first layer having a weight of one-half, the second
one-fourth, and so forth. Suppose, by utilizing a different band of
frequencies, the relative absorption in (and emission from) the layers is
altered, say one-fourth is absorbed in each layer. Then the total emitted
radiation corresponds to an apparent temperature given by
T.sub.b =1/4T.sub.1 +3/4(1/4)T.sub.2 +3/4(3/4.multidot.1/4)T.sub.3 + . . .
+(3/4).sup.n-1 .multidot.1/4T.sub.n + . . .
T.sub.b =1/4T.sub.1 +3/16T.sub.2 +9/64T.sub.3 + . . .
Notice that the relative weights of successive layers are different; deep
layers contribute more heavily in T.sub.b than T.sub.a. In an article
entitled "Atmospheric Absorption Measurements With a Microwave
Radiometer", Physical Review, Volume 70, Nos. 5 and 6, Sept. 1 and 15,
1946, pages 340 to 347, R. H. Dicke and collaborators reported a
demonstration of this effect using a microwave radiometer arrangement
directed toward the atmosphere. The temperature as a function of altitude
was independently determined with balloon-borne radiosondes. Three
frequency bands were employed which suffered different amounts of
absorption due to the presence of water vapor. They demonstrated that the
observations taken at several zenith angles were in accord with the above
analysis of a partially-absorbing body. Later workers pointed out that if
one had a series of apparent temperatures T.sub.a, T.sub.b, . . .
corresponding to different relative absorption in the atmosphere, the
temperature T.sub.1, T.sub.2, . . . of successive layers could be
obtained. See, for example, Louis D. Kaplan, "Inference of Atmospheric
Structure From Remote Radiation Measurements," Journal of the Optical
Society of America, Volume 49, No. 10, October 1959, pages 1004 to 1007;
A. H. Barrett and V. K. Chung, "A Method for the Determination of High
Altitude Water Vapor Abundance From Ground-Based Microwave Observations,"
Journal of Geophysical Research, Volume 67, 1962, pages 4259 et seq.; and
M. L. Meeks and A. E. Lilley, "The Microwave Spectrum of Oxygen in the
Earth's Atmosphere", Journal of Geophysical Research, Volume 68, No. 6,
Mar. 15, 1963, pages 1683 to 1703. This topic has been of considerable
interest for remote sensing of the atmosphere, and mathematical techniques
for going from the directly measured T.sub.a, T.sub.b, . . . to the
inferred temperature distribution T.sub.1, T.sub.2, . . . as a function of
depth have been extensively devloped. See, for example, S. Twomey,
Introduction to the Mathematics of Inversion in Remote Sensing and
Indirect Measurements (Elsevier, Amsterdam, 1977).
In the field of acoustic or elastic waves, the analog of "black-body"
radiation is generally called thermal noise or mechanical Nyquist noise.
The mathematical development of the theory is almost identical to the
electromagnetic radiation case. The electromagnetic theory begins with
consideration of radiation in an empty cavity inside a body of temperature
T. The emphasis tends to be with calculation of the radiation emitted and
absorbed at the wall; they must exactly balance if there is thermal
equilibrium. The acoustic theory generally considers a volume of fluid of
finite volume in a container at temperature T. In analogy with
electromagnetic waves in the "black-body" cavity, the acoustic waves are
generally visualized as travelling across the volume without attenuation.
Since the basic results, such as the radiation energy passing through a
surface, are independent of attenuation, provided the temperature is
everywhere the same, it is simplest to analyze the case of no attenuation.
If attenuation is present in an acoustic medium, then the apparent
temperature derived from measuring the thermal noise intensity would be a
weighted average of the contributions from various depths just as
described above for the microwave radiometer. The detection device of the
acoustic radiometer should be designed with a directional beam-like
sensitivity pattern so that the received noise power would be directly
proportional to the apparent temperature, just as it was for the microwave
radiometer. Such a directional acoustic receiver may consist of focussing
mirrors and/or lenses in combination with a suitable geometric shape for
the transducer device which converts changes of acoustic pressure and
velocity into electrical voltages and currents for subsequent
amplification and measurement.
The unique capabilities of an acoustic radiometer temperature sensing
system result from the acoustic attenuation versus frequency
characteristics of a wide variety of materials, and from the short wave
length of the acoustic radiation in the useful range of frequencies. For
all materials, the attenuation is small at low frequencies, and increases
with frequency; thus, any desired amount of attenuation can be selected by
choosing the appropriate frequency. Many liquids exhibit "classical"
absorption which is proportional to frequency squared. Over limited ranges
of frequencies, many materials have an acoustic attenuation directly
proportional to the frequency. This is true, for example, for most
polycrystaline metals and for most soft tissues of the human body. One
convenient way to characterize attenuation is to specify the distance in
which the wave amplitude is attenuated by some specified fraction (usually
1/e). Since the intensity is proportional to the amplitude squared, the
intensity would be reduced by 1/e.sup.2 in one attenuation length.
As a specific numerical example, consider the case already discussed: the
attenuation length of the radiation determining T.sub.a is 2.9 layers and
for T.sub.b, 7.0 layers. Clearly, if information is needed in the range of
1 to 10 layers, the most useful frequencies at which measurements should
be taken for T.sub.a, T.sub.b . . . are those with attenuation lengths
ranging from 1 to 10 layers. If the attenuation length is very short, only
the surface temperature is measured; if very long, depths much beyond the
layers of interest contribute to the result.
Consider the specific example of medical applications of the invention. It
is well established that attenuation of ultrasound in body tissue is
proportional to frequency, so that one can write the contribution of
thermal noise power by the layer between x and dx as kbf.multidot.T(x)
exp(-bfx)dx, where k is the Boltzmann constant, b is a constant
characteristic of tissue (a typical value for soft tissue is
b=0.2(cm-MHz).sup.-1), f is the frequency, and T(x) is the temperature at
depth x. From this expression, one can show that this layer makes its
maximum contribution to the received noise power at a frequency
f(max)=1/(bx). At a lesser depth, the maximum is located at a higher
frequency. This is the basis of the connection between the temperature
distribution with depth x and the power spectrum with frequency f. The
important range of frequencies is given by the expression for f(max).
Suppose the temperature distribution is desired to a depth of 16 cm. in
soft tissue. Then the lowest frequency f.sub.1 approximately equals
1/(0.2)(16)=0.3 MHz. The smallest distance is determined by the fact that
large thermal gradients are not expected in tissues due to limits on the
magnitudes of possible heat sources and sinks: 0.5 cm. might be reasonable
corresponding to the highest frequency f.sub.2 =1/(0.2)(0.5)=10 MHz. Since
the wavelength .lambda.=c/f, where c is the velocity of sound
(c.congruent.0.15 cm/.mu. sec in soft tissue), .lambda..sub.1
.congruent.0.5 cm. and .lambda..sub.2 .congruent.0.015 cm. This is very
convenient since any device to create a directional beam sensitivity
pattern must have dimensions large compared to a wavelength. This being
the case, a diameter d.congruent.5 cm. of the acoustic transducer might be
conveniently selected. The frequencies involved are also conveniently high
so that the measurement time is not too long. When measuring a noise
spectrum with an acoustic radiometer, the fractional error in measuring
absolute temperature T is 1/[.tau..DELTA.f].sup.1/2, where .tau. is the
measuring time and .DELTA. f is the frequency bandwidth of the radiometer.
The largest possible value for .DELTA.f is .DELTA.f=f. If .tau.=100
seconds, .DELTA.f=1 MHz, then the fractional error is 10.sup.-4. If
T=310.degree. K. (body temperature), then the temperature error would be
0.031.degree. K. in the apparent temperature.
Consider now why electromagnetic "black-body" radiation could not be used
for the same purpose. In order to be useful for body temperature
measurements, the diameter of the beam pattern must be no more than, say,
5 cm. The directional receiving antenna must be this size, or smaller.
Since the wavelength must be less than the dimensions of the directional
antenna, .lambda.<<5 cm. For such electromagnetic radiation, the
attenuation length in soft tissue is too short.
For materials in which attenuation is proportional to frequency, the
attenuation length is a fixed multiple of the wavelength. For example,
using the numbers given above for tissue, this ratio R=67. In many
materials of geophysical interest, R.apprxeq.100 to 500. It can be seen
from the manner in which R was calculated that it corresponds to the
number of cycles of oscillation for the wave motion to decrease to 1/e of
its original amplitude. Whatever the value of R, if large attenuation
lengths are desired in order to sense temperature to great depth, one must
utilize frequencies which correspond to long wavelengths (.lambda.=(depth
of interest)/R). Since f=c/.lambda., where c is the acoustic or elastic
wave velocity in the medium of interest, this means going to low
frequencies. At least two difficulties may arise:
(1) The time required to obtain a given temperature precision will increase
as 1/f.
(2) The noise background from other sources (wind, surf, seismic activity,
human activity) may increase.
These considerations will limit the depths to which temperature can be
sensed by acoustic radiometry in the natural environment.
The invention may be embodied in acoustic radiometer systems of varying
degrees of complexity, depending on the application. The simplest as shown
in FIG. 1 is a single probe system. More specifically, a transducer 10
such as a piezoelectric disc is adapted to be coupled to the body 11 under
observation by means of an acoustic coupling 12. If the acoustic impedance
of body 11 is Z, then the acoustic coupling 12 which may be aliquid or
other suitable medium is chosen to have a matching acoustic impedance of
approximately Z. Further, the transducer is provided with a backing 13.
The thickness of the transducer 10 and the transducer backing 13 are
chosen either for broad-band response with the lowest resonant frequency
of the transducer system above 10 MHz for the specific example described
above, or for more efficient coupling by means of band-pass response with
one or more resonant frequencies within one or more pass bands in the
0.3-10 MHz region for the specific example described above. If the
diameter of the transducer is 5 cm., then an approximately parallel "beam"
having the 5 cm. diameter describes a region 14 of tissue in which the
temperature would be measured. If the transducer face were curved or an
acoustic lens were inserted, the "beam" could be focussed at any specified
depth and the diameter of the temperature monitored region would vary with
depth x. A variation of the basic system is shown in FIG. 2 wherein an
acoustic lens is interposed between the transducer 10 and the acoustic
coupling media at 12. The acoustic lens comprises a high-velocity acoustic
media 15 in contact with the transducer face. The high-velocity acoustic
media 15 has a generally plano-convex shape. Another high-velocity
acoustic media 16 contacts the acoustic coupling media 12. This
high-velocity acoustic media 16 has the general shape of a negative
miniscus lens. The two high-velocity acoustic media 15 and 16 are
connected by a low-velocity acoustic media 17 having a generally
frusto-conical shape. This acoustic lens assembly produces a parallel wave
front of diameter D having a magnification m=D/d, where d is the diameter
of the transducer 10. Obviously, acoustic lens systems can be designed to
produce converging as well as collimated beams. A further variation of the
basic system shown in FIG. 1 is illustrated in FIG. 3. In this case, the
transducer 10 is located at the focal point of a parabolic reflector 18.
The parabolic reflector 18 may be filled with an acoustic coupling fluid
and thereby perform the function of the acoustic coupling medium 12 as
well as a focussing function. Again, the arrangement shown in FIG. 3
produces a parallel wave front having a diameter D in the body with a
magnification m=D/d.
The output of transducer 10 is connected to a preamplifier 19 to provide an
amplified electrical noise spectrum signal to a power spectrum analyzer
20. In the simplest case, only a single frequency band would be analyzed.
This would provide a single weighted-average temperature measurement. Such
a system might be suitable for some process control applications where the
shape of the temperature distribution is known fom other considerations,
but a temperature which is related to internal temperature is needed. On
the other hand, the spectrum analyzer 20 would analyze a broader spectrum
of frequencies to give temperature-depth profiles in which the depth
resolution limit increases (becomes poorer) in direct proportion to the
depth.
The basic system can be made more complex by adding an additional acoustic
transducer as shown in FIG. 4. In this system, the two acoustic
transducers 21 and 22 are placed on opposite surfaces of the same body,
facing each other. This arrangement greatly improves the depth resolution
and the precision of the temperature measurements. Carrying the system
shown in FIG. 4 one step further, a plurality of acoustic transducers are
arranged about the peripheral surface of the body to view the same planar
section of the body. Each would be placed at a different position around
the perimeter of the section, and the data from all of the acoustic
transducers and frequencies could then be combined to provide a
two-dimensional temperature profile map of the section. For purposes of
simplifying the description, however, the two acoustic transducer systems
shown in FIG. 4 will be understood to represent, in principle, a typical
multi-transducer system. The outputs of the acoustic transducers 21 and 22
are connected to respective preamplifiers 23 and 24 which amplify the
outputs of the transducers. It is important to appreciate that the
electrical "noise" voltage generated by the acoustic transducers must be
amplified without introducing a significant amount of additional noise to
the power level suitable for typical commercially-available spectrum
analyzers (about -100 dbm into 50 ohms). The amount of additional
amplifier noise which can be tolerated is thus dictated in part by the
precision required in the system and the available electronics.
Alternatively, the effect of amplifier noise can be subtracted by
alternately switching the input between the acoustic transducer and a
known noise source, such as an electrical wire-wound resistance at a known
temperature. This approach is illustrated in FIG. 4 wherein a noise source
25 has an output supplied to a preamplifier 26. The outputs of each of the
preamplifiers 23, 24 and 26 are connected to a switching device 27 such as
multiplexer. While switching device 27 is illustrated as a mechanical
switch following the preamplifiers 23, 24 and 26, those skilled in the art
will understand that the switching device may be conveniently made of
solid state switches using, for example, MOS technology and that the
switch may be inserted before the preamplification. The switching device
27 can be operated to selectively connect the outputs of preamplifiers 23,
24 and 26 to the power spectrum analyzer 28. The order in which the
outputs of the preamplifiers 23, 24 and 26 are connected to the power
spectrum analyzer 28 is not necessarily sequential. Besides providing a
compensation for the effects of amplifier noise, the reference noise
source 25 can also make possible a relative temperature measurement as
opposed to an absolute temperature measurement. It will, of course, be
appreciated that the use of a reference noise source 25 is not limited to
a multi-transducer system, but may also be used to equal advantage in a
single transducer system such as that shown in FIG. 1.
The output of the power spectrum analyzer 28 may be subjected to further
signal processing. For example, the output of the power spectrum analyzer
28 could be supplied to a microprocessor 29 programmed with a mathematical
inversion algorithm of the type described by S. Twomey referenced above.
In a multi-transducer system, the data from each transducer can be
assembled and processed in a manner analogous to axial tomography
presently used in X-ray imaging. The output of either the power spectrum
analyzer 28 or the microprocessor 29 can be displayed by a display 30,
such as a cathode ray tube, and/or recorded by recorder 31, such as a
printer or magnetic medium recorder. In many cases where the temperature
profile might indicate abnormal or diseased tissue, comparison spectra
could be taken at a symmetrical location on the same individual.
Soft tissue has been mentioned in the above discussion because its acoustic
characteristics are fairly uniform, and very little acoustic energy is
reflected at boundaries between different layers or types of tissue. Even
where bone or air, with very different acoustic characteristics, is
located within the transducer "beam, " it is still true that a uniform
temperature must result in a flat spectrum. However, when temperature is
not uniform, reflections at large acoustic impedance discontinuities can
effect the apparent temperature distribution inferred from the noise power
spectrum. The presence of any such large impedance discontinuities could
be detected by utilizing the acoustic transducer in a conventional
pulse-echo diagnostic ultrasound mode. The temperature profiles also could
be displayed by all the techniques employed for conventional diagnostic
ultrasound. Superposition of both types of information would aid in
compensating for the effects of inhomogenities and would give more
diagnostic information than either display alone.
The diagnostic usefulness of temperature measurements by probe inserted in
body cavities is well known. This invention extends diagnostic temperature
measurements to all parts of the body consisting primarily of soft tissue.
The invention has therapeutic applications to monitor temperature changes
in almost any procedure involving heating or cooling of body tissues. If
heat energy is introduced to a small region of the body by focussed
ultrasound or other means, monitoring the subsequent change of temperature
with time by means of this invention would measure the perfusion, another
diagnostically useful quantity.
While the invention has been described with specific reference to preferred
embodiments, those skilled in the art will understand that these
embodiments illustrate the principles of the invention, and the invention
is not limited in its practice to these particular embodiments. For
example, while a piezoelectric transducer was specifically described,
other mechanical vibrations to electrical signal transducers may be used
with equal effect. These would include recently developed transducers
employing fiber optics and lasers to first convert the mechanical
vibrations to an optical signal or modulation which, in turn, is converted
to an electrical signal. See, for example, J. H. Cole et al., "Fiber Optic
Detection of Sound", Journal of the Acoustical Society of America, Vol.
62, 1977, pp. 1302 to 1304; and B. Culshaw et al., "Acoustic Sensitivity
of Optical Fiber Waveguide," Electronic Letters, Vol. 13, 1977, pp. 760
and 761. Broadly, what the invention provides is a way to non-invasively
measure temperature inside the living as well as inanimate bodies by means
of detecting ultrasonic noise. By measuring the noise power spectrum over
a suitable frequency range, the temperature-depth distribution along a
well-defined path can be determined. The invention provides an absolute
temperature measurement, as well as a means for relative temperature
comparison.
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