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Description  |
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FIELD OF THE INVENTION
The invention concerns a method of measuring the temperature of a body by a
special method based on microwave radiometry. To be more precise, the
invention concerns a method of measurement of the temperature by a
measurement of the thermal noise in the microwave range emitted by a
material which makes it possible to have control of the volume of material
under investigation and to obtain a better spatial resolution, as well as
a device for carrying out the method. The method and apparatus of the
invention are, for example, applicable to measuring subsurface temperature
of living tissue to detect local deviation of temperature which may
indicate the presence of a tumor or other abnormality.
BACKGROUND OF THE INVENTION
The existing methods of measuring the temperature of a body in microwaves
consist of placing a probe in the vicinity of the surface of the body, or
in contact with this surface, for the purpose of picking up the signals of
thermal noise emitted by a volume of the body being investigated by the
probe. The thermal noise signals are then amplified, processed if
necessary, in such a manner as to output a signal which is a function of
the temperature which prevails in the volume under investigation.
It must be noted that in such methods the volume under investigation
depends on the nature of the body and on the characteristics of the probe
used. Hence, for the same body, the volume under investigation remains
approximately similar to itself, especially when the probe is displaced in
relation to the surface of the body.
Hence, when the purpose of a device for carrying out one of the existing
methods is to detect a possible local deviation of temperature, under the
surface of a body, for example, of a living tissue, it will in fact be
possible to locate this anomaly when it is included in the volume which is
being investigated by the probe. However, it will be difficult, even
impossible, to locate this anomaly within the volume under investigation.
Hence, diagrammatically, the spatial resolution of the existing devices is
limited to the volume under investigation associated with the probe. In
order to improve this spatial resolution, it is now possible to change the
probe, in such a manner as to change likewise the volume associated with
it. However, such a change requires manipulations and adjustments for
adapting the chain of measurements to the new probe.
On the other hand, it must be noted that for a volume under investigation
associated with a probe, the signals emitted by elementary volumes closest
to the probe present, at the level of the latter, a greater weight than
the signals emitted by elementary volumes which are further away. In other
words, the signals emitted by elementary volumes close to the surface of
the body mask those emitted by the rest of the volume under investigation.
SUMMARY OF THE INVENTION
One of the aims of the present invention is to provide a method of
measuring temperature of a selected sub-surface volume of a body by
measuring thermal noise signals in the microwave range emitted by the
body, as well as a device for implementing the method, which make it
possible to have control of the volume under investigation, without
requiring a change of probe.
Another aim of the present invention is to provide a method and a device
which make it possible to measure the temperature prevailing in a volume
which forms part of volumes investigated by two probes, but which is
reduced in relation to these, and to improve the spatial resolution.
Another aim of the present invention is to provide a method and a device
which make it possible to vary the spatial resolution.
Other aims and advantages of the present invention will appear during the
course of the description which follows, which is, however, only given as
an indication, and does not aim to limit it.
The method of measuring the temperature of a selected sub-surface volume of
a body according to the invention comprises placing at least two probes A
and B near or in contact with a surface of the body to pick up thermal
noise signals in the microwave range emitted by a volume of the body
associated respectively with each probe. The probes are positioned ajacent
one another so that the volumes of the body from which the respective
probes receive thermal noise signals in the microwave range overlap one
another to provide a common volume part Vi from which both of the probles
receive thermal noise signals in the microwave range and remaining volume
parts Va and Vb from which only the individual probes A and B respectively
receive thermal noise signals in the microwave range. The signals received
by the two probes are amplified and are correlated so that thermal noise
signals in the microwave range emitted by the common volume part Vi are
correlated while thermal noise signals in the microwave range emitted by
the remaining volume parts Va and Vb are decorrelated so that thermal
noise signals in the microwave range emitted by the common volume Vi are
made preponderant by correlation with respect to the thermal noise signals
in the microwave range emitted by the remaining volume parts Va and Vb
from which the probes A and B respectively receive thermal noise signals
in the microwave range.
Apparatus in accordance with the invention for measuring the temperature of
a selected sub-surface volume of a body comprises at least two probes A
and B adapted to be placed on or near the surface of such body to pick up
thermal noise signals in the microwave range emitted by the body. The two
probes are positioned adjacent one another so that the volumes of the body
from which the respective probes receive thermal noise signals in the
microwave range overlap one another to provide a common volume part Vi
from which both of the probes receive thermal noise signals in the
microwave range and remaining volume parts Va and Vb from which only
probes A and B respectively receive thermal noise signals in the microwave
range. The apparatus further comprises means for amplifying and
correlating signals from the probes to correlate thermal noise signals in
the microwave range received from the common volume part Vi while
decorrelating thermal noise signals in the microwave range received from
the remaining volume parts Va and Vb so as to accentuate the thermal noise
signals in the microwave range emitted by the common volume part Vi with
respect to the thermal noise signals in the microwave range emitted by the
remaining volume parts Va and Vb from which the probes receive thermal
noise signals in the microwave range.
The invention will be better understood if the description given below is
referred to together with the drawings which form an integral part of it.
FIG. 1. shows in diagrammatic form a conventional method of measuring
temperature in microwaves.
FIG. 2. is a diagram showing the principle of the invention.
FIG. 3. gives a diagrammatic representation of the method according to the
invention, in one method of implementation.
FIG. 4. is a variant of the method relating to FIG. 3.
FIG. 5. is a diagram of another method of implementing the invention.
FIG. 6. is a variant of the diagram of FIG. 5.
FIG. 7. is a diagram of a variant of the means of cyclic phase shifting of
180.degree..
FIG. 8. further illustrates the method according to FIGS. 5 and 6.
FIGS. 9. & 10 are diagrams of the modes of variation of the spatial
resolution.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1. a conventional method of measuring temperature in microwaves is
shown diagrammatically. In this method a probe 1 is placed in the vicinity
of the surface 2 of a body, or in contact with this surface. The probe
picks up the thermal noise signals emitted by a volume V' of the body,
situated under its surface 2, which is known as the volume under
investigation.
The volume V' is shown diagrammatically in FIG. 1. by the dotted line 3. It
depends on the nature of the body, on the one hand, and on the
characteristics of the probe 1 on the other hand.
The probe 1 is connected to an amplifier receiver 4, which amplifies the
signals picked up by the probe, and emits a signal S which is a function
of the temperature prevailing in the volume under investigation V'.
FIG. 2. relates to the diagram of the principle of the present invention.
This implements at least two probes 5 and 6, which will also be designated
as probe a and probe b. Each probe, 5 and 6, or a and b has its own
sub-surface volume of investigation, 7 and 8 or V'.sub.a and V'.sub.b.
In the same way as the preceding case, the volumes under investigation
V'.sub.a and V'.sub.b depend on the nature of the body and the
characteristics of the probes a and b.
According to the invention, the two probes a and b, or, more generally, the
various probes, are arranged in such a manner that their respective
volumes under investigation overlap so as to present an intersection which
is not zero. Hence, in the case of FIG. 2 the volumes V'.sub.a and
V'.sub.b present a common volume part V.sub.i.
If the theory of sets is referred to, the volumes V'.sub.a, V'.sub.b,
V'.sub.i are in accordance with the following equations:
V.sub.i =V'.sub.a V'.sub.b (1)
V'.sub.a =V.sub.a V.sub.i (2)
V'.sub.b =V.sub.b V.sub.i (3)
In other words, for the volume under investigation V'.sub.a by the probe a,
the signals proceeding from the various elementary volumes .DELTA.v.sub.a
of the volume V.sub.a will be picked up solely by the probe a. The same
applies to signals proceeding from the different elementary volumes
.DELTA.v.sub.b of the volume V.sub.b which will be picked up solely by the
probe b.
On the other hand, the signals emitted by the elementary volumes
.DELTA.v.sub.i of the common volume V'.sub.i of the volumes under
investigation, will be picked up by both the probe a and the probe b.
According to the invention the signals S.sub.a and S.sub.b picked up by the
probes a and b respectively are processed in parallel by a correlator 9
also called a correlation-type receiver. At the level of this device, the
signals emitted by the volumes V.sub.1 and V.sub.b are "decorrelated",
that is, they do not have any relationship to one another. On the other
hand, the signals of thermal noises emitted by the common volume V.sub.i
are "correlated", because they proceed from the same volume via different
probes. The signals emitted by the common volume part are thereby made
preponderant with respect to the signals emitted by the remainder of the
volumes from which the probes receive thermal noise signals.
At the output of the correlator 9, the signals are processed by an
amplifier receiver 10. If necessary, the correlator and amplifier receiver
can be combined.
It must also be noted that the signals of thermal noises taken into
consideration are situated in the area of microwaves, or hyperfrequencies,
that is, frequencies approximately between 0.1 and 20 GHz inclusive and
are of a random nature.
The present invention will be illustrated with two different variants of
execution. In these two methods, the wave lengths and the phase shifting
set forth are relative to the central frequency of the frequency band in
which the thermal noise signals are processed and amplified. In fact,
strictly speaking it would be necessary to extend the present argument to
all the frequencies situated in the frequency band of the device.
FIGS. 3 and 4 relate to a mode of carrying out the method according to the
invention. For these figures, two probes a, b, designated respectively 12,
13, or more are arranged on the surface 2 of a body, in such a manner that
their respective volumes under investigation V'.sub.a and V'.sub.b have a
common volume V.sub.i. The probes a and b are electrically connected at
14. If necessary other probes are likewise connected at 14. The electrical
paths 15 and 16 separating the probes from the connecting point 14 are
approximately identical. The connection point 14 is electrically connected
to an amplifier receiver 10 of a known type, which emits a signal S.
The inventors have noticed that when two probes are connected electrically
in parallel, without their volumes under investigation presenting a common
part, the power of the signals is of the same order as the power of the
signals which would be picked up by a single probe. Thus, the power of the
signals picked up by each of the probes does not add up arithmetically,
but is approximately the half-total of the powers picked up by each of the
probes, which is transmitted to the amplifier receiver in the case of
identical probes in the presence of an approximately uniform ambient
temperature.
Hence, the contribution of an elementary volume, under investigation by one
of the probes is approximately divided into two, when two probes are used,
compared with the case in which a single probe is connected to the
amplifier receiver.
The inventors think that this phenomenon originates from the electrical
communication between the probes, and from the losses of power at the
level of the connector 14.
In the case of FIG. 3, the thermal noise signals proceeding from the volume
V.sub.a are picked up solely by the probe a. The same applies to the
volume V.sub.b with the probe b. On the other hand, the elementary volumes
.DELTA.v.sub.i of the common volume V.sub.i are picked up both by the
probe a and probe b. Hence, approximately the elementary volumes
.DELTA.v.sub.i of the volume V.sub.i can present a contribution which is
double those of the elementary volumes of the volumes V.sub.a and V.sub.b.
The signals proceeding from V.sub.a and V.sub.b are in some measure
effaced with regard to the signals proceeding from the volume V.sub.i. The
greater the number of probes, the more this phenomenon is accentuated, in
view of the fact that the contribution of each of the probes is
approximately divided by the number of probes, compared with the case in
which a single probe is connected to the amplifier receiver.
This is only approximate, for the thermal noise signals emitted by an
elementary volume .DELTA.v.sub.i of the common volume V.sub.i in the
direction of the probes a, b, do not add up arithmetically at the level of
the point of electrical connection 14. In fact, a phase shift exists
between the two signals, owing to the difference in distance between the
volume .DELTA.v.sub.i and the probes a and b.
Beyond the connection point 14, the power S' of the signal is approximately
in accordance with the following equation:
S'=.SIGMA.V.sub.a k.sub.a .multidot.A.sub.a.sup.2 .multidot.T.sub.a
+.SIGMA.V.sub.b k.sub.b .multidot.B.sub.b.sup.2 .multidot.T.sub.b
+.SIGMA.V.sub.i k.sub.i A.sub.i B.sub.i T.sub.i .multidot.cos
.theta..sub.i (4)
In this equation k.sub.a, k.sub.b, k.sub.i designated constants. A.sub.b,
B.sub.b, A.sub.i and B.sub.i designate respectively the coefficients of
transmission of the electrical field between the elementary volumes
.DELTA.v.sub.a, .DELTA.v.sub.b, .DELTA.v.sub.i and their associated probes
a and b. T.sub.a, T.sub.b, T.sub.i designate respectively the temperatures
prevailing in the elementary volumes .DELTA.v.sub.a, .DELTA.v.sub.b,
.DELTA.v.sub.i.
The first term of the equation (4) is the contribution of the volume
V.sub.a, the second is that of the volume V.sub.b and the third that of
the volume V.sub.i. In this third term .theta..sub.i designates the
dephasing between the signals emitted respectively towards the probes a
and b for the average frequency of measurement. This dephasing is
expressed by
.theta..sub.i =2.pi..multidot.f.multidot..tau..sub.i (5)
where f designates the frequency and .tau..sub.i the delay between the two
signals.
Hence, in the signal S', the first two terms relating to the volumes
V.sub.a and V.sub.b are independent, the signals of thermal noises
proceeding from these two volumes are decorrelated. On the other hand, in
the third term, the signals proceeding from the volume V.sub.i are
correlated, and a phase shift .theta..sub.i intervenes.
FIG. 4 is a variant of FIG. 3, according to which a phase shifter 17 has
been introduced into one of the electrical lines, for example line 15, on
this side of the connection point 14. This dephaser introduces a phase
shift .DELTA..theta. into the signal of the probe.
In equation (4) of the signal S', the first term remains overall unchanged,
and the phase shift .DELTA..theta. takes place at the level of the third
term which is now written.
.SIGMA.V.sub.i k.sub.i .multidot.A.multidot.B.sub.i .multidot.T.sub.i cos
(.theta..sub.i +.DELTA..theta.) (6)
Thus the phase shift .DELTA..theta. makes it possible to vary the
correlation between these signals proceeding from the volume V.sub.i. In
other words, the phase shift .DELTA..theta. varies in particular the zones
of the common volume V.sub.i for which the cosine is approximately zero
and those for which the cosine is close to 1.
Thus the electrical coupling of probes in parallel achieves a correlation
between the signals emitted by the volume common to the different volumes
being investigated by the probes. Owing to this fact the contribution of
the common volume to the final signal can be more important than that of
the volumes such as the volumes V.sub.a and V.sub.b. The specialist will
easily extrapolate the present reasoning to the case in which more than
two probes are electrically connected.
FIGS. 5-8 are concerned with another implementation of the invention.
According to this implementation, the signals of thermal noises proceeding
from the above volumes V.sub.a and V.sub.b have an approximately zero
contribution to the final signal, and only the thermal noise signals
originating from the common volume V.sub.i intervene in this signal.
The diagram of the principle is given in FIG. 5, for the case in which two
probes are used.
Two probes a and b, designated 18 and 19 respectively are arranged in the
vicinity of the surface 2, or in contact therewith. They present a volume
under investigation V'.sub.a and V'.sub.b, 20 and 21 respectively which
have a common part V.sub.i, 42.
One of the probes, for example, the probe a, is connected to means of
summation 22, such as a T. The probe b is connected to the summation
means, via cyclic phase shifting means 23. These cyclic phase shifting
means are controlled by a pulse generator 24, which imparts to them a
cyclic ratio equal to 1/2. The phase shifting means 23 effects a phase
shift of 180.degree. and is thus a phase inverter.
At the output of the phase shifting means 23, the signal S.sub.b,
proceeding from the probe b, is phase shifted by 0.degree. on a
half-period of the pulse generator and by 180.degree. on the following
half period.
At the output of the summating means 22, the signal is processed by means
25, which consist for example, of a large gain and low noise square-law
detector. The square-law detector achieves an elevation to the square of
the input signal of the processing means 25.
Moreover, the pulse generator 24 likewise controls at the level of the
processing means 25, a detection which is synchronous to the cycle of the
cyclic phasing shifting means 23 to output a signal approximately
proportional to the difference in amplitude of the signal corresponding to
the two half periods of the phase shifting cycle.
It can be shown that at the output of the processing means, the signal S is
presented in the form:
S.sub.1 =.SIGMA.V.sub.a k.sub.a .multidot.A.sub.a.sup.2 .multidot.T.sub.a
+.SIGMA.V.sub.b k.sub.b .multidot.B.sub.b.sup.2 .multidot.T.sub.b
+.SIGMA.V.sub.i k.sub.i .multidot.[A.sub.i.sup.2 +B.sub.i.sup.2
+2.multidot.A.sub.i .multidot.B.sub.i cos .theta..sub.i ].multidot.T.sub.i
(7)
on the half-period where the signal originating from the probe b is shifted
by 0 degrees by the phase shifting means 23, and
S.sub.2 =.SIGMA.V.sub.a k.sub.a .multidot.A.sub.a.sup.2 .multidot.T.sub.a
+.SIGMA.V.sub.b k.sub.b .multidot.B.sub.b.sup.2 .multidot.T.sub.b
+.SIGMA.V.sub.i k.sub.i [A.sub.i.sup.2 +B.sub.i.sup.2 +2.multidot.A.sub.i
.multidot.B.sub.i .multidot.cos (.theta..sub.i +180.degree.)]T.sub.i (8)
on the half-period in which the signal which proceeds from the probe
S.sub.b is shifted by 180.degree. by the phase shifting means 23.
The above equation (8) can be transformed. It becomes
S.sub.2 =.SIGMA.V.sub.a k.sub.a .multidot.A.sub.a.sup.2 .multidot.T.sub.a
+.SIGMA.V.sub.b k.sub.b .multidot.B.sub.b.sup.2 .multidot.T.sub.b
+.SIGMA.V.sub.i k.sub.i [A.sub.i.sup.2 +B.sub.i.sup.2 -2.multidot.A.sub.i
.multidot.B.sub.i .multidot.cos .theta..sub.i ].multidot.T.sub.i (9)
In the equation of S.sub.2 the shift of 180.degree. does not take place at
the level of the second term, for the signal which proceeds from the probe
b, phase shifted by 180.degree., is raised to the square by the square law
detector.
In the above formulae, as in relation to the first method of
implementation, k.sub.a, k.sub.b, k.sub.i are constants which depend on
the nature of the body, on the characteristics of the probes and on other
parameters. A.sub.a, B.sub.b, A.sub.i and B.sub.i are coefficients of
transmission of the electrical field, respectively of the elementary
volume .DELTA.v.sub.a towards the probe a, of .DELTA.v.sub.b towards the
probe b, and .DELTA.v.sub.i towards the probe a and the probe b as shown
diagrammatically in FIG. 8. T.sub.a, T.sub.b and T.sub.i are the
temperatures prevailing in the elementary volumes .DELTA.v.sub.a,
.DELTA.v.sub.b and .DELTA.v.sub.i respectively.
The variable .theta..sub.i designates the phase shift between the signals
originating from the volume .DELTA.v.sub.i via the probe a on the one hand
and via the probe b on the other hand. In the equation (8), this phase
shift is increased by 180.degree. owing to the phase shifting means 23.
The signals S.sub.1 and S.sub.2 are processed by any suitable means, and
for example by a synchronous detection or by a numerical filtering, in
such a manner as to output a signal S which is approximately proportional
to the difference in the amplitude of the signal corresponding to two half
periods of the phase shifting cycle and thus approximately equal to the
distance between the signals S.sub.1 and S.sub.2. This signal S hence
assumes approximately the following form:
S=.SIGMA.V.sub.i k'.sub.i .multidot.A.sub.i .multidot.B.sub.i .multidot.cos
.theta..sub.i .multidot.T.sub.i, (10)
where k'.sub.i is equal to k.sub.i at approximately a multiplicative
constant. Thus the means of processing have considerably eliminated the
signals proceeding from the volumes V.sub.a and V.sub.b, and the signal S
only depends on the signals emitted by the volume V.sub.i which is common
to the two volumes which are being investigated.
It must be noted that the above equations take into account the random
nature of the signals emitted by the different elementary volumes. They
take into account equally the first that the signals emitted by the
volumes V.sub.a and V.sub.b are independent or decorrelated and that the
signals emitted by the common volume V.sub.i in the direction of the
probes a and b are correlated.
Moreover, preferably the electrical paths of the two probes a and b at the
summation means 22, when the phase shifting means 23 output a signal with
a zero phase shift, are approximately equal.
FIG. 6. illustrates diagrammatically a variant of FIG. 5 according to which
the adjustable phase shifting means 26 are introduced into the electrical
circuit of one of the probes, for example, of the probe a. These means
introduce a variable phase shift .DELTA..theta., which has repercussions
on the final dephasing, which becomes:
S=.SIGMA.V.sub.i k'.sub.i .multidot.A.sub.i .multidot.B.sub.i .multidot.cos
(.theta..sub.i +.DELTA..theta.).multidot.T.sub.i (11)
the phase shift .DELTA..theta. modifies the weight of the different
elementary volumes of the volume V.sub.i in the final signal. It thus
modifies the correlation relation.
The phase shifting means are represented in FIGS. 5 and 6 in the form of a
coupler of 180.degree., which has two outputs. On one of these outputs,
marked 0.degree., the phase of the signal is not shifted. On the other
output, marked 180 degrees, the signal is in phase opposition in relation
to the input signal. A microwave switch 27 connects the two outputs of the
180.degree. coupler cyclically to one of the inputs of the summating means
22. This switch 27 is controlled by the pulse generator 24 which is shown
diagrammatically by the dot-dash line 28. The pulse generator 24 also
controls the square-law detector 25 so that the square-law detector is
synchronized with the cyclical phase inventor 27.
Other phase shifting means 23 are shown in FIG. 7. They consist of a
circulator 29 which is placed in the connection of the probe to the
summation means 22 and which diverts the signal in the direction of a
reflective modulator 30. This modulator has two states, and is controlled
by the pulse generator 24 as shown diagrammatically by the dot-dash line
31.
In one of these states the modulator 30 operates in short circuit and sends
back the signal in the direction of the circulator, which sets off again
in the direction of the summation means. This corresponds to a zero phase
shift.
In the other state, the modulator 30 sends a signal on a line 32 which ends
in a short circuit 33. The line 32 presents a length which is equal to a
quarter of the wavelength corresponding to the central measuring
frequency. Thus the signal passes along the lines 32, is reflected by the
short circuit 33, returns in the direction of the modulator 30 then in the
direction of the circulator 29. Thus, in relation to the previous state,
it has passed over, a distance which is approximately equal to a half
wavelength, which phase shifts it by 180.degree..
The present mode of implementing the invention has been described with two
probes. This mode of implementation can be extrapolated and more than two
probes could be connected to the means of correlation which have just been
described. In this case the supplementary | | |
