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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a measuring method and apparatus for
measuring a non-linear parameter and/or its distribution in an acoustic
medium. It also relates to the application of the method to measurements
such as temperature measurement of human tissue for an ultrasonic
diagnosis, hyperthermia, etc. The measuring apparatus of the present
invention especially relates to a device for measuring a non-linear
parameter of an acoustic medium such as biomedical tissue, measuring the
space distribution of the non-linear parameter of the acoustic medium and
displaying an image of the space distribution on a display unit. The
measuring method and apparatus of the present invention is useful in many
applications and primarily intends to provide special apparatus to measure
the temperature of internal tissue noninvasively.
2. Discussion of the Prior Art
Usually, an ultrasonic apparatus such as an ultrasonic tomograph, is
designed while assuming a pressure independent sound speed. In the first
approximation, sound speed is regarded as a pressure independent constant
and the density of the acoustic medium is also regarded as pressure
independent for a liquid and a solid medium like tissue. However, more
accurately, sound speed relates to the density (it is called dynamic
density) at each infinitesimal portion of the acoustic medium through
which the sound wave propagates, and because a sound wave is a
compressional wave, the density of each part of the acoustic medium is
varied by an increase in sound pressure.
The relationship between sound speed and density of a medium through which
the sound wave propagates can be expressed as
##EQU1##
This results in a nonlinearity relationship between sound speed and
pressure expressed as
##EQU2##
where, .DELTA..rho.: dynamic density change caused by .DELTA.p;
.DELTA.P: dynamic sound pressure, that is a variation of the pressure
caused by the sound wave propagating in the acoustic medium;
C: sound speed resulting under dynamic sound pressure .DELTA.p;
C.sub.0 : sound speed (phase velocity) resulting under static pressure,
that is sound speed of infinitesimaly small amplitude propagating through
the medium;
.rho..sub.0 : static density, density of the acoustic medium under static
pressure; and
B/A: a non-linear parameter of the acoustic medium.
Equation (1) is applicable only under a second order approximation where
the higher order terms have been neglected or eliminated from the
equation. If the second order term is neglected, equation (1) keeps only
first order term resulting linear acoustic equation; that is, sound speed
is pressure independent as mentioned above as a "first approximation".
More details about the non-linear acoustics are explained, for example, by
T. G. MUIR, E. L. CARSTENSEN in 1979 (Prediction of Nonlinear Acoustic
Effects at Biomedical Frequencies and Intensities; Ultrasound in Med. &
Biol., Vol.6; Pergamon Press Ltd.).
Recently, the non-linear parameter B/A has become worthy of notice in the
field of ultrasonic technology, because it includes novel information such
as: the structure, the coupling state, the visco-elasticity, and the
temperature of biomedical tissue; the chemical state of a body fluid; the
metabolic activity; and so on. However, the quantity B/A is so small that
it has been difficult to put it into practical use. To overcome this
difficulty, an equivalent non-linear parameter (B/A).sub.e was introduced
by N. Ichida, T. Sato, O. Ikeda and M. Linzer (Ultrasonic Imaging of the
Non-linear Parameter of Tissue Using Scanned Low Frequency Pumping Waves
and High Frequency Probe Waves) at the 7th International Symposium On
Ultrasonic Imaging And Tissue Characterization, held on Jan. 6-9, 1982,
sponsored by NBS, and in Japanese Patent Application No. TOKUGANSHO
57-167036, in 1982 (Ultrasonic Diagnostic Processing System) by the
inventors. This new measuring method introduced a second sound wave
(pumping wave) into the acoustic medium to be measured, to detect the
second order term in equation (1). Therefore, two kinds of sound waves are
superimposed in the acoustic medium. One is a "probing wave" to measure
the non-linear parameter of the medium, and the other is "pumping wave" to
generate the .DELTA.p. The amplitude of the pumping wave is made
sufficiently higher than the probing wave, so that the non-linear
parameter is enhanced to make it easy to measure. The non-linear parameter
measured in this way is called "equivalent non-linear parameter
(B/A).sub.e ", and equation (1) is modified as follows
##EQU3##
where, (B/A).sub.e is an equivalent non-linear parameter;
P is sound pressure of the pumping wave; and
C, C.sub.0, and .rho..sub.0 are the same as defined in equation (1),
respectively.
At first a measuring method of the parameter (B/A).sub.e by the prior
Patent Application will be described briefly. A pumping wave consists of a
continuous wave (CW), whose wave length is varied. A probing wave is also
CW and is applied to the medium by a pair of ultrasonic transducers; one
of which transmits the probing wave and the other receives the probing
wave. The phase of the transmitted probing wave is modulated by the
pumping wave as it crosses the probing wave beam through the medium. The
parameter (B/A).sub.e along the medium can be obtained by a Fourier
transformation of the phase deviations of the received probing wave caused
by the pumping wave corresponding to the various wavelengths of the
pumping wave.
The above-mentioned prior art method of measuring the parameter
(B/A).sub.e, however, had the following problems in attaining a good
result rapidly.
(1) The frequency of the pumping wave had to be varied very widely,
however, it is not normal for a transducer to have the necessary frequency
band-width capable of transmitting the pumping wave effectively.
(2) The pumping wave had to be varied though many frequencies to get a high
degree of resolution, so the apparatus received plenty of time to
accumulate the data, and to perform the Fourier transformation.
SUMMARY OF THE INVENTION
The object of the present invention, therefore, is to provide a means to
measure an "equivalent" non-linear parameter (B/A).sub.e of the acoustic
medium effectively and rapidly.
Another object of the present invention is to provide a device to measure a
conventionally defined (not equivalent) non-linear parameter (B/A) of the
acoustic medium with a more practical convenience of application.
A further object of the present invention is to provide two/three
dimensional image(s) of the acoustic non-linear parameter of the acoustic
equivalent non-linear parameter.
Still another object of the present invention is to providing a device to
measure the temperature of internal tissue noninvasively by applying the
above-mentioned devices.
For the measurement of the equivalent non-linear parameter (B/A).sub.e or
the non-linear parameter (B/A) the present invention provides two methods.
The first method measures the parameter (B/A).sub.e by applying a pulsed
pumping wave instead of prior art continuous pumping wave, and the pulsed
pumping wave propagates and intersects the probing beam. Applying the
pulsed pumping wave is equivalent to using many frequency for the pumping
wave, and it is not necessary to vary the CW frequency and waste time.
This method is performed by using a pair of transducers for the probing
wave, and a single transducer for the pumping wave. The former are
arranged face to face with each other across the acoustic medium to be
measured, transmitting and receiving the probing wave (probing beam). The
latter produces a wide beam of sound covering the measured region, and
supplies pumping sound pressure to the medium uniformly and
simultaneously.
The second method measures the parameter (B/A) by providing a pumping beam
in which the pulsed pumping wave propagates in a direction counter to the
probing wave propagation direction. This method is performed by using a
pair of transducers for the probing wave, and the single pumping
transducer is located near the receiving probing transducer. An array type
transducer is applicable for the above transducer.
A space distribution image of the parameter (B/A).sub.e or (B/A) can be
obtained by using the probing beam which scans the acoustic medium in one
or two dimensions.
A device which measures the internal temperature of an acoustic medium can
be provided based on a referential data for the parameter (B/A).sub.e or
(B/A) measured previously as a function of temperature in the acoustic
medium such as biomedical tissue. Because the parameter (B/A).sub.e or
(B/A) is sensitive to temperature. An image of the internal medium
temperature distribution can be obtained by the intermediation of
(B/A).sub.e or (B/A) along the probing beam, in two or three dimensions in
the same way as described above.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram illustrating the principle of the first measuring
method of the present invention having a pumping wave which propagates
perpendicular to a probing beam.
FIG. 2, including FIG. 2(A)-2(D), is a perspective view illustrating a
phase relation between the probing wave and the pumping wave of the
measuring method of FIG. 1.
FIG. 3 is a block diagram of a measuring apparatus of the present invention
having a pumping wave which propagates perpendicular to the probing beam.
FIG. 4 shows the waveforms of the signals at the points indicated in FIG.
3.
FIGS. 5(A)-5(D) are diagrams illustrating the pressure and phase
relationship for a spherical pumping wave onto Z axis of the measuring
method at various times.
FIG. 6 is a diagram illustrating the waveforms of a signal and noise at an
input and an output of the adding circuit of the measuring method
according to the present invention having a pumping wave which propagates
perpendicular to the probing beam.
FIG. 7 is another block diagram of a measuring apparatus embodying the
present invention which improves the S/N ratio of the apparatus.
FIG. 8 is still another block diagram of a measuring apparatus embodying
the present invention having a pumping wave propagating perpendicular to
the probing beam, a scanning mechanism and a display unit to obtain a two
or three dimensional distribution of the equivalent non-linear parameter
in the acoustic medium.
FIG. 9 is a diagram illustrating the principle of the second measuring
method of the present invention, having a pumping wave which propagates
along the probing beam in a reverse direction.
FIG. 10 illustrates a relationship between the pumping wave and the probing
wave of the second measuring method.
FIG. 11(A) is a block diagram of the measuring apparatus of the second
measuring method.
FIG. 11(B) and FIG. 11(C) are a perspective view and a cut-away view of a
transducer, combining the pumping transducer and the receiving probing
transducer for the second measuring method.
FIG. 12 is a block diagram of a Time Gain Control Amplifier (TGC) provided
at the output circuit a phase detector of the measuring apparatus.
FIG. 13 is a block diagram embodying the measuring apparatus of the second
measuring method, comprising a scanning mechanism and a display unit to
obtain a two or three dimensional distribution of the equivalent
non-linear parameter in the acoustic medium.
FIGS. 14(A)-14(D) are perspective diagrams of various transducer
constructions for the second measuring method of the present invention.
FIGS. 15(A)-15(D) are perspective diagrams of various array transducer
constructions for the second measuring method of the present invention.
FIGS. 16(A)-16(B) are perspective diagrams of the transducer construction
for the pumping wave radiation and probing wave reception using an array
transducer, for the second measuring method, where FIG. 16(A) shows an
array construction having one array in which a probing transducer element
and a pumping element are arranged alternately; and FIG. 16(B) shows an
array construction having one array in which probing transducer elements
are arranged in every third place.
FIG. 17 is a graph of the frequency characteristics of a transducer which
functions as both the probing and pumping transducer, for the second
measuring method of the present invention, where FIG. 17(A) shows a total
serviceable sound frequency band of the transducer, FIG. 17(B) shows a
sound frequency band for pumping wave radiation and FIG. 17(C) shows a
sound frequency band for probing wave reception.
FIG. 18 is a perspective view of a transducer which has a layer structure
where the front layer is for the receiving transducer and back layer is
for the pumping transducer.
FIG. 19 is a block diagram of the temperature measuring system, which is an
embodiment of the apparatus of the present invention.
FIG. 20 shows a block diagram of a memory unit 12, used in the circuit of
FIG. 19.
FIG. 21 shows a conversion curve between apparent (B/A).sub.a and .DELTA.T,
contained in unit 20 of the block diagram of FIG. 19.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Further details and advantages of the present invention will be understood
with respect to preferred embodiments discussed herein and drawings
attached hereto and made a part hereof.
As previously noted, the present invention involves an improved measuring
method and apparatus for measuring the equivalent non-linear parameter and
the non-linear parameter of the acoustic medium using the pulse pumping
wave.
FIG. 1 is a diagram illustrating the first measuring method of the present
invention. Usually, measurement is carried on in water, though it is not
shown in the figure, by immersing the medium to be measured and the
instruments in the water. The method uses a pumping wave W.sub.p which is
generated pulsively, and has a planar wave front, which propagates in the
medium to be measured perpendicularly to the probing wave propagation
path. In FIG. 1, 5 is a medium placed in water between the transducers
X.sub.A and X.sub.B which form a probing beam in the direction of the Z
axis. X.sub.A transmits a beam-shaped probing wave W.sub.m into the medium
5, X.sub.B receives W.sub.m which is modulated by a sound pressure of
pumping wave W.sub.p, while W.sub.m passes through the medium 5. W.sub.p
is radiated by a pumping wave transducer X.sub.p in the direction of the X
axis. X, Y and Z are the axes of a rectangular coordinate respectively.
Under the above conditions, the equivalent non-linear parameter (B/A).sub.e
is calculated from equation (2) as
##EQU4##
When the medium is not uniform, the parameter (B/A).sub.e varies at each
point corresponding the heterogeneity of the medium at that point. The
suffix i and j indicate directions which are perpendicular to each other,
and suffix s means "under an isoentropic condition", that is under a
quasistatic process, or in a process of adiabatic compression or
expansion. A conventionally defined non-linear parameter B/A in an
acoustic field is determined when i=j, by
##EQU5##
If the medium is isotropic, the following relationship can be obtained,
##EQU6##
where, .nu. is Poison's ratio whose value is about 0.5 for liquid or
biomedical tissue, so it can be said that (B/A).sub.e is almost equal to
B/A if the acoustic medium is assumed to be something such as biomedical
tissue. Therefore, .DELTA.C a variation of sound speed due to the pressure
P of the pumping wave can be determined by
##EQU7##
In FIG. 1, for simplicity, it can be assumed that sound pressure of W.sub.m
is sufficiently lower than sound pressure of W.sub.p so that P of equation
(2) or (6) only depends on the sound pressure of W.sub.p.
In a period when the pumping wave W.sub.p (which is a pulsed wave)
intersects the probing beam, the sound speed change .DELTA.C of probing
wave W.sub.m occurs. The value of .DELTA.C varies at each position on the
Z axis, so equation (6) can be expressed by
##EQU8##
where, z is a coordinate of the Z axis. Equation (7) indicates that the
waveform of W.sub.m at each position along the Z axis suffers a phase
change proportional to .DELTA.C(z). Then, (B/A).sub.e (z) can be estimated
from phase information included in the output signal of the receiving
transducer X.sub.B. The value of P in equation (7) can be measured
previously in an equivalent liquid. Detail concerning the above
considerations are as follows.
FIG. 2 shows diagrammatically perspective views of the probing wave W.sub.m
which is affected by sound pressure of the pumping wave W.sub.p while
W.sub.m propagates through the acoustic medium. The medium is assumed to
be homogeneous. The sound of W.sub.p is a function of time, therefore, P
can be described as P(t), where t is time. At z=z.sub.0 and when t=0,
W.sub.m is affected by pressure P(O) (see FIG. 2 (A)). After a short time
period both the probing wave and pumping wave shift their position, so
that at t=.DELTA.t, z=z.sub.0 +.DELTA.z=z.sub.0 +C.sub.0 .DELTA.t and the
sound pressure becomes P(t+.DELTA.t) (see FIG. 2(B)). In the same manner,
W.sub.m is affected by sound pressure P such as shown in FIG. 2(C) when
t=2.DELTA.t, FIG. 2(D) when t=3.DELTA.t, respectively.
From the above explanation, it can be said generally that a probing wave
W.sub.m positioned at z.sub.0 at t=0, will receive following pressure
change (that is a change of sound velocity) at z
##EQU9##
however, value of .DELTA.C(z) is less than C.sub.0, .DELTA.t is a time
interval in which W.sub.m propagates from z.sub.0 to z and is given by the
following approximate equation
##EQU10##
Therefore, when probing wave W.sub.m propagates from z.sub.0 to z, a phase
change of W.sub.m can be expressed as
##EQU11##
where, K: proportional constant, and
.DELTA..psi..sub.z.sbsb.o (z): phase change of W.sub.m as a function of z.
The total of the phase change of W.sub.m while the probing wave propagates
in the acoustic medium from transmitting transducer X.sub.A to receiving
transducer X.sub.B is given by
##EQU12##
substituting following function g into the equation (11),
##EQU13##
the following equation is obtained,
##EQU14##
Equation (13) is so-called convolution integral. If Fourier transformation
is applied to equation (13), following equation can be obtained.
##EQU15##
where; .PSI.(.omega.), (B/A).sub.e (.omega.), G(.omega.) are respectively
the Fourier transform of .phi.(z), (B/A).sub.e (z), and g(z); and a
variable .omega. is a space frequency along the coordinate z. From
equation (14), following relationships are obtained,
##EQU16##
where F.sup.-1 is the inverse Fourier transformation. From these
equations, (B/A).sub.e (z) can be obtained.
Equation (15) shows that (B/A).sub.e can be obtained by filtering
.PHI.(.omega.) using a filter having frequency characteristics such as
1/(KG(.omega.)) which is the inverse characteristics of the pumping wave
frequency characteristics. Therefore, the inverse Fourier transformation
like equation (16) is not necessary to obtain (B/A).sub.e (z). It can be
obtained simply by applying the above filter. G(.omega.) in equation (15)
or (16) can be obtained from P(t) in equation (12).
The space distribution of the non-linear parameter B/A(z) obtained by the
above means can be displayed as an image, namely the image of the space
distribution of B/A(z). FIG. 3 is a block diagram of an apparatus for
realizing this image.
In FIG. 3, reference numeral 1 designates a timing controller which
controls the emission of the pumping wave W.sub.p, 2 is an oscillator for
the probing wave W.sub.m, 3 is a driver to drive a transducer X.sub.A, 4
is a transducer X.sub.A which transmits a probing wave W.sub.m, 5 is an
acoustic medium to be measured, 6 is a transducer X.sub.B which receives
W.sub.m modulated by W.sub.p, 7 is a receiving amplifier, 8 is a phase
detector, 9 is a filter whose characteristics are defined by
(1/K)(1/G(.omega.)) in equation (15), 10 is a driver for the pumping wave
W.sub.p, and 11 is a transducer X.sub.p which radiates pumping wave
W.sub.p. A distance from X.sub.p to the probing beam is shown by x.sub.1,
and z.sub.2 is a range of actual measurement in the medium 5.
FIG. 4 shows some waveforms of signals at several points in the circuit of
FIG. 3. The same reference symbols are used as in corresponding parts of
FIG. 3. Symbol DV is a driving pulse for the pumping transducer X.sub.p,
V.sub.R is an output signal of receiving amplifier 7, .phi.(t) is an
output signal of phase detector 8, and (B/A)(t) is an output signal of
filter 9. In FIG. 4, t.sub.1 is a sum of two time intervals (x.sub.1
/C.sub.0) and (z.sub.1 /C.sub.0). The former is a time interval between
the moment the pumping wave W.sub.p is emitted and the moment W.sub.p
arrives at the probing wave W.sub.m, and the latter is a time interval
between the moment W.sub.m is modulated by W.sub.p and the moment the
front edge of the modulated part of W.sub.m is received by the transducer
X.sub.B. The received signal of probing wave W.sub.m begins to be phase
modulated after passing the time interval t.sub.1 from the moment W.sub.p
had been transmitted and produces a phase modulated output V.sub.R during
a time duration of t.sub.2 which is the time interval during which the
modulated W.sub.m passes through the length z.sub.2, so it can be
expressed as t.sub.2 =z.sub.2 /C.sub.0.
Phase detector 8 detects a phase shift of the received signal V.sub.R by
comparing it with a phase of an output signal of oscillator 2 which
produces a reference signal, and provides an output .phi.(t) as a function
of time. However (t) can be considered as a function of position z, as
mentioned before with respect to equation (9), since there is a linear
relation between t and z. This output signal is filtered by filter 9, and
the equivalent non-linear parameter (B/A).sub.e can be obtained as a
function of time, (B/A).sub.e (t), that is a function of z, (B/A).sub.e
(z).
As disclosed above, by applying the measuring method of the present
invention which uses a pulsed pumping wave, the space distribution of the
equivalent non-linear parameter (B/A).sub.e (z) can be rapidly measured by
a simple construction an apparatus without providing various CW pumping
waves and a complex Fourier transformation.
The following consideration may be helpful for the understanding of the
above and for following description. Since, the pulse of the pumping wave
will pass through the Z axis in an instant, because it propagates in
orthogonal direction to the Z axis. At that instant, the probing wave
(that is a continuous wave) is propagating in a queue (beam) on the Z
axis. At the instant when the pumping wave passes over the probing wave,
each portion of the probing wave will be affected by the pumping wave, and
each portion of the probing wave will be phase shifted corresponding to
(B/A).sub.e at its respective point. The wave train of probing wave is
stamped with the phase the value of (B/A).sub.e at each point by the
pumping pulse. Therefore, the pumping pulse may be called as stamping
pulse. After the wave train is stamped, the queue (beam) of the probing
wave successively reaches the receiving transducer. So, the time sequence
of the phase change of the probing wave is the information regarding the
distribution of (B/A).sub.e along the Z axis.
The output signal .phi.(t) of phase detector 8 is described on a time axis
in FIG. 4, however the phase shift of each time corresponds to the phase
shift of the probing wave affected by the pumping wave at each point on
the Z axis.
Therefore, if V.sub.R in FIG. 4 is obtained, it contains the information
regarding (B/A).sub.e on the Z axis. The phase of V.sub.R must have some
relationship to (B/A).sub.e. The filter which extracts (B/A).sub.e from
the wave train is what is designated by reference numeral 9 in FIG. 3. If
the data comes out successively from filter 9 and is plotted successively
along Z direction, it shows the profile of (B/A).sub.e along the Z axis.
In the above explanation, the pumping wave is assumed to be a plane wave,
however, the plane wave is hard to realize in a practical apparatus
because if a perfect plane wave is to be realized, an aperture of a
pumping wave transducer should be a plane spreading infinitely. Therefore,
in the actual apparatus of the present invention, a part of a spherical
wave having a large radius of curvature is used as a pumping wave, thus
assuming that it approximates a plane wave. It is practical to do this
when considering the power needed for the generation of the pumping wave.
However, in this case, the energy of the wave tends to decrease at the
edge of the pumping wave. Therefore, the influence caused by a spherical
wave must be compensated.
In this case, the sound pressure of the pumping wave is not only a function
of t but also of position z. Therefore, the deviation of the actual
pumping wave from an ideal plane wave must be compensated for, and this
compensation can be performed as follows.
FIG. 5 shows schematically the above-mentioned situations. FIG. 5(A) shows
the distribution of the sound pressure of the pumping wave at a time t=0,
when the wave front of a spherical pumping has arrived at the z axis
(position of the probing beam). Two broken lines indicate the front and
rear edge of the pumping wave pulse respectively, propagating in the X
direction. FIG. 5(B) shows the sound pressure distribution of an ideal
plane pumping wave, by which a probing wave will be affected while the
probing wave propagates on the Z axis. The probing wave propagates from
z=z.sub.0 (at t=0) to z=z (at t=t), during that time the pumping wave
passes over the Z axis, and affects the probing wave. As can be seen in
the figure, the time t or time axis can be replaced by the z or Z axis.
FIG. 5(C) shows the sound pressure distribution of a spherical pumping
wave on the Z axis, from which the probing wave will be affected, while
the probing wave propagates on the Z axis. In the figure, the probing wave
was at z=z.sub.r at the instant t=0, when the front edge of pumping wave
arrived at the Z axis. The deformation of the sound pressure curve,
compared with that of (B) will be explained as follows. For the plane
pumping wave, sound pressure is uniform on the Z axis, that is it varies
only with time. So, if sound pressure P varies in a triangular shape as
shown in the FIG. 5(A), the sound pressure which will affect to the
probing wave, will be as shown in FIG. 5(B). For a spherical pumping wave,
on the other hand, the pumping wave diverges when it propagates, and it
decreases in sound energy (pressure). So, sound pressure is the highest at
the center of the wave front (at z=z.sub.r in the Figure). As the probing
wave propagates from z.sub.r, the sound pressure of the pumping wave
decreases, and it decreases even more when it passes the edge of the
pumping wave. This tendency is enhanced by the sound loss of the medium,
because, the portion of the pumping wave apart from the center of the
pumping beam must propagate a longer distance to reach to the Z axis,
compared with the portion in center part. FIG. 5(D) shows another sound
pressure distribution of a spherical pumping wave, which the probing wave
will encounter on the Z axis. In this case the probing wave was at z=0,
when the front edge of pumping wave arrived at the Z axis (t=0). Using
similar reasoning, it will be understood that the sound pressure varies as
shown, contrary to the manner of FIG. 5(C).
Therefore, the weighting function which is equivalent to g(z) in equation
(12), and the filter corresponding to 1/G(.omega.) in equation (15) must
have different values along z. So, the characteristics of the filter must
be varied at each point on the Z axis to obtain (B/A).sub.e (z). However,
sound pressure which affects the probing wave, at each point on the Z
axis, as shown in FIG. 5, can be measured previously. Therefore, the
filter characteristics for each value of z.sub.0 can be calculated
beforehand.
Therefore, it is possible to prepare several of filters, each which are
applicable to a specified portion of a specimen to be measured (Z axis).
The number of filters to be prepared depends on the resolution and
accuracy of the desired measurement. Usually only few filters are enough,
because the resolution is also defined by many other factors.
In the actual apparatus the distribution of B/A(z) can be obtained in the
same way as that of the plane pumping wave by applying various filters to
the output of phase detector 8 in FIG. 3. The output of these filters are
switched one by one corresponding to the portion to be measured. It is
also possible to design a filter having variable characteristics.
For the filter which sequentially varies its characteristics, a digital
filtering technique can be applied to the output of the phase detector
after an A/D (analog-digital) conversion.
Another method available to reduce the effect of the spherical pumping wave
is to separate the transducer for pumping wave generation as far as
possible from the medium to be measured. Using this method the
approximation of spherical wave to plane wave becomes better, however,
this methods needs a high power pumping wave and as a result, a practical
balance between the resolution and the required power is necessary.
Next, a method to increase the S/N (signal to noise) ratio of a measurement
will be discussed. When the phase deviation of the probing wave because
the equivalent non-linear parameter is small or sound pressure of pumping
wave is small, noise produced by the circuits increases so high that
detection of the phase deviation .phi.(z) is impossible, and (B/A).sub.e
(z) obtained from .phi.(z) also includes much noise as shown in FIG. 6. In
the FIG. 6, curve (A) shows the output of phase detector (designated 8 in
FIG. 3) without noise. Curve (B) indicates schematically a form of noise.
Curves (C) show a practical output of the phase detector .phi.(z).
To extract the curve (A) from curve (C), an adding circuit is provided
which adds the output of the phase detector such as S.sub.1
.about.S.sub.k, K times at the same position along the probing beam. As a
result, the signal amplitude increases as much as K times, but the noise
amplitude increases as much as only .sqroot.k times, because noise is
irregular and added as a power summation. Consequently, S/N ratio of the
phase detector output can be improved .sqroot.K times as shown in FIG.
6(D). This adding circuit is also called a synchronized adding circuit.
FIG. 7 is a block diagram of an apparatus embodying the present invention
comprising the above device. In FIG. 7 like reference numerals are used
for the same or corresponding parts as in FIG. 3. In the FIG. 7, 12 is the
adding circuit which adds the signal of the phase detector output, and 13
is a delay circuit which delays the output signal of the adding circuit
12. Delay circuit 13 delays the output signal of 12 as much as a period T,
the repetition time of the pumping pulse. These circuits are well known in
the art, and they can be performed on a digital signal by converting the
output signal of the phase detector into a digital signal using an A/D
converter.
Applying the present invention, it is possible to realize an apparatus to
display the distribution of parameter (B/A).sub.e in two-dimensions or
three-dimensions. As mentioned above, the distribution of the parameter
can be measured along the probing beam, this is analogous to a line
scanning measurement. So, if the transmitting and receiving transducer
pair is kept in a fixed relative position, and the pair moves in the X or
Y direction, the pair provides a plane scanning, and a two-dimensional
display can be obtained. Further, if the couple moves in the X and Y
directions, three-dimensional scanning occurs, and a three dimensional
display can be provided.
FIG. 8 shows a block diagram of the apparatus described above. The figure
uses like numerals designating like or similar parts as in FIG. 7. In FIG.
8, 14 is a driving mechanical unit which shifts a pair of the transducers
4 and 6 using a device such as a stepping motor driven by a control pulse
from timing controller 1. Position detector 15 detects the position of the
pair of transducers, using a position finder such as a rotary encoder
joined to a shaft of the stepping motor. Memory controller 16 generates
the memory address corresponding the output of position detector 15, based
on control signals such as a synchronous signal and clock signal for the
pumping wave from timing controller 1. Memory 17 stores the measured data
for the parameter (B/A).sub.e (z) after converting an analog signal from
filter 9 into a digital signal.
The memory content are processed by control and address signals from
write/read from memory controller 16. Display 18 reads the data from
memory 17 and displays the distribution of the parameter (B/A).sub.e.
Disclosed above is a first method and its application according to the
present invention for measuring an equivalent non-linear acoustic
parameter. Many application and modification may occur for the one skilled
in the art. For example, the scanning method can be modified to electronic
scanning. In this case, there is no need to use the moving mechanism. An
afterglow characteristic of a display tube can be applied without using
memory 17 for the two-dimension display of (B/A).sub.e (z). These changes
are all within the scope of the present invention.
Next, a second method for measuring the acoustic non-linear parameter will
be discussed. This method is a modification of the first method, however,
it provides additional merits in a practical application. The significant
feature of the second measuring method is the application of the pumping
wave along the same axis of the probing beam.
As explained with respect to the compensation for a spherical pumping wave.
It can be understood that, the pumping wave in FIG. 1 is not always
necessarily perpendicular to the probing beam. Therefore, as shown in FIG.
9, an axis of the pumping wave is rotated in the counter clockwise by
90.degree. degree, and it travels in the opposite direction to the probing
beam. In FIG. 9 the probing beam are shown by W.sub.m, and transducers for
it is shown as X.sub.A and X.sub.B. The probing wave propagates from left
to right and the pumping wave W.sub.p propagates from right to left
through the probing beam. The transducer generating the pumping wave is
not shown in the figure.
FIG. 10 shows an arrangement of transducers for the second method. The
probing beam and the transducer for the probing wave are the same as those
of FIG. 1. A pair of transmitting transducer X.sub.A and a receiving
transducer X.sub.B are used, but the pumping transducer X.sub.P is placed
around X.sub.B as shown in FIG. 10. The pumping wave W.sub.P is shown with
a broken line since it propagates along the probing wave beam, but in the
opposite direction to the probing wave W.sub.p.
Distribution of the conventionally defined non-linear parameter can be
obtained in a way quite similar to the way described previously. The wave
train of the probing wave (a continuous wave) is stamped on a phase change
by a pulse of the pumping wave. The stamped information is detected by the
phase detector, and the information related to the non-linear acoustic
parameter is extracted from it.
A block diagram for a preferred embodiment of the second measuring method
is shown in FIG. 11(A). The figure uses similar reference numerals and
symbols for the same or corresponding parts as in FIG. 3. A pumping
transducer 11 (X.sub.P) is arranged around the receiving transducer
X.sub.B for the probing wave. A shadowed portion in medium 5 is a portion
in which the non-linear parameter can be measured and its length is
indicated by z.sub.2. FIG. 11(B) is a diagram illustrating a structure of
the transducer 11. X.sub.B is a probing transducer for receiving the
probing wave W.sub.m. X.sub.P is a pumping transducer shaped as a ring
surrounding the receiving transducer X.sub.B. FIG. 11(C) shows a cut-away
view of the transducer 11 in FIG. 11(B).
In FIG. 11(A), receiving amplifier 7 provides a signal output V.sub.R
corresponding to the modulated probing wave, phase detector 8 detects the
phase change in V.sub.R and produces an output .phi.(t). This output is a
train of the phase changes sequentially received, having been stamped with
the non-linear parameter (B/A) of the respective position on the probing
wave, where the probing wave encountered the pumping wave pulse. As
mentioned b | | |
