Acoustic microscope

An acoustic microscope of the type of mechanical scanning in which a single transducer works to generate an acoustic beam as well as to detect echoes reflected from the specimen. An echo reflected from an interface between an acoustic lens and an acoustic propagation medium is detected, and the detected intensity is used as a reference to display the distribution of attenuation factors of the specimen in a two-dimensional manner. Among the signals representing the distribution of attenuation factors of the specimen, furthermore, only those signals having intensities that lie within a predetermined range are displayed to obtain a picture of the specimen that represents the distribution of attenuation factors of a predetermined range only.

BACKGROUND OF THE INVENTION 
The present invention relates to an imaging device which works utilizing 
acoustic energy, and particularly to an acoustic microscope. 
In recent years, attention has been given in the medical world to 
ultrasonic waves that can be effectively utilized for observing the 
internal structure of human bodies. Namely, ultrasonic waves have a 
property to penetrate through materials that may be optically opaque to 
light or electron rays. The higher the frequency, the more finely the 
objects can be described. Furthermore, the data obtained with ultrasonic 
waves reflect dynamic properties of the objects, such as elasticity, 
density, viscosity, and the like, and make it possible to learn the 
internal structure that could not be obtained with light or electron rays. 
Study has been forwarded concerning the acoustic microscope which makes the 
most of ultrasonic waves by utilizing ultra-high frequency sound waves of 
as high as 1 GHz, i.e., having a sound wavelength of about 1 .mu.m in the 
water (literature entitled "A Scanning Acoustic Microscope" by R. A. 
(Lemons) and C. F. (Quate), IEEE Cat. No. 73 CH 14829 SU, pp. 423-426). 
The principle of an acoustic microscope consists of mechanically scanning 
the surface of a specimen in a two-dimensional manner with an acoustic 
beam which is focused to as narrow as about 1 .mu.m, collecting the 
disturbed sound waves such as those scattered and reflected by the 
specimen or those attenuated as they travel through the specimen, 
converting the collected sound waves into electric signals, and displaying 
the electric signals on a cathode-ray tube in a two-dimensional manner in 
synchronism with the mechanical scanning, thereby to obtain a microscope 
image. 
If the sound waves which have transmitted through the specimen are detected 
and displayed on the acoustic microscope, the obtained image reflects the 
distribution of acoustic attenuation constant (hereinafter simply referred 
to as attenuation constant) of the specimen. In the practically used 
apparatus, the intensity of RF pulses for oscillating the sound waves is 
fixed, and the amplification factor of an amplifier which amplifies sound 
wave detection signals is suitably adjusted such that the image is 
displayed on the cathode-ray tube with a suitable brightness. According to 
the conventional apparatus, therefore, there exists no definite relation 
between the attenuation constants of the specimen and the brightness of 
signal on the cathode-ray tube. Namely, it is not allowed to use density 
informations of the obtained sound wave image as measured data of 
attenuation constant of the specimen. 
If mentioned in further detail, even if the amplification factor of the 
amplifier is displayed, it is difficult to correctly measure the 
attenuation constant of the specimen. This is because, the transmitting 
efficiency of a transducer which generates sound waves varies depending 
upon the frequency. Besides, even if a fixed frequency is used, the 
abovementioned efficiency varies with aging. Accordingly, to presume the 
intensity of sound waves incident upon the specimen relying upon the 
intensity of RF pulses, involves incorrect factors. Another reason is that 
the sensitivity of a receiving transducer for detecting sound waves that 
have transmitted through the specimen, also varies depending upon the 
frequency and aging. Therefore, to presume the intensity of sound waves 
that have transmitted through the specimen relying the amplification 
factor of an amplifier which amplifies detection signals or relying upon 
the brightness of a picture on the cathode-ray tube, also involves 
incorrect factors. 
The present invention deals with an acoustic microscope of the reflection 
type which obtains a picture that reflects the distribution of attenuation 
factors of a specimen by detecting echoes reflected from the back surface 
of the specimen. The acoustic microscope of this type has been disclosed 
in Japanese Patent Application No. 35828/ 1983 filed on March 7, 1983 that 
is earlier than the filing date of the present application. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an acoustic microscope 
which is capable of displaying an attenuation constant inherent in a 
specimen at all times without being affected by the operating conditions. 
Another object of the present invention is to provide an acoustic 
microscope which is capable of producing a display with good contrast by 
converting the change in two-dimensional distribution of an attenuation 
constant into an natural scale. 
A further object of the present is to provide an acoustic microscope which 
is capable of displaying, in a sliced manner, only arbitrary selected 
ranges of attenuation constants depending upon the distribution of 
attenuation constants of a specimen being observed, so that the picture 
can be produced with clear and increased contrast. 
A feature of the present invention resides in that an echo reflected from 
an interface between a lens and an acoustic propagation medium, that had 
hitherto been regarded as useless, is used as a reference signal, and a 
detection signal of the echo reflected from the back surface of the 
specimen is compared with the above reference signal, to display 
attenuation constants of the specimen. Like the intensity of echo from the 
specimen, the intensity of the echo from the interface of the lens varies 
depending upon the intensity of RF pulses that are applied, 
transmitting/receiving sensitivity of the transducer, and gain of the 
receive amplifier. However, the ratio of these two echoes intensity 
remains constant irrespective of these quantities. Relying upon the above 
fact, attenuation constants of the specimen can be displayed definitly 
without being affected by these quantities. 
Another feature of the present invention resides in the provision of a 
level selection circuit which introduces receiving signals from an 
acoustic transducer that displays a two-dimensional distribution of 
attenuation constants of the specimen, which permits said receiving 
signals to pass through only when they have intensities that lie within a 
predetermined range, and which inhibits the passage of said receiving 
signals when they have intensities that lie outside said range.

DETAILED DESCRIPTION 
Prior to describing an embodiment of the present invention, the fundamental 
setup of an acoustic microscope of the reflection type to which the 
present invention will be adapted, is described below with reference to 
FIG. 1. 
A transducer which generates and detects ultrasonic waves consists chiefly 
of a piezoelectric thin film 20 and an acoustic lens 40. That is, a lens 
crystal 40 (e.g., a cylindrical crystal of sapphire, quartz glass or the 
like) has a flat surface at its one end 41 that is optically polished, and 
a semispherical hole 42 of a very small radius of curvature (for example, 
0.1 to 1 mm) at the other end. If electric signals produced by an RF pulse 
generator 100 are applied across the upper and lower electrodes 
constructed in the form of layers consisting of an upper electrode 10, 
piezoelectric thin film 20 and lower electrode 11, which are metallized on 
the end surface 41, plane ultrasonic waves 80 of RF pulses are emitted 
into the lens crystal 40 due to the piezoelectric effect of the 
piezoelectric thin film. The plane acoustic waves are focused on a 
specimen 60 placed on the surface of a predetermined focal point owing to 
a positive acoustic spherical lens formed by the interface between the 
semi-spherical hole 42 and a medium 50 (which usually is pure water). 
The ultrasonic waves reflected by the specimen 60 are collected by the 
acoustic lens, converted into plane ultrasonic waves, propagate through 
the lens crystal 40, and are finally converted into RF pulse electric 
signals due to the inverse piezoelectric effect of the piezoelectric thin 
film. These RF pulse electric signals are amplified and detected by an RF 
receiver 110, converted into video signals (1 to 10 MHz), and are used as 
brightness signals (Z inputs) for the cathode-ray tube 130. 
In the above-mentioned construction, the specimen 60 stuck onto the 
specimen stage 70 is two-dimensionally vibrated by a two-dimensional 
scanning means 120 on the x-y plane, and the video signals are displayed 
on the cathode-ray tube 130 in synchronism with the scanning thereof. A 
microscopic picture is thus obtained. 
FIG. 2 illustrates this condition in terms of video frequency regions, in 
which the abscissa represents the time, and the ordinate represents the 
intensity of signals. In FIG. 2, symbol A denotes a signal passed through 
the transmitter, B denotes an echo from the lens interface 42, and C 
denotes an echo reflected by the specimen. They are repeated at a 
repeating time t.sub.R, to constitute every picture element. The reflected 
echo C changes depending upon the acoustic properties of the specimen or 
scanning of the specimen. Therefore, if the intensity of the reflected 
echo C is sampled in synchronism with the repetitive period, and the 
signals are displayed on the cathode-ray tube in synchronism with the 
mechanical scanning of the specimen, then an acoustic image is obtained. 
Here, when a piece of tissue of a living thing is observed relying upon the 
setup which is shown in FIG. 3, the intensity of the reflected echo C 
serves as the data that reflects an attenuation constant of a specimen of 
the living thing. In FIG. 3, the specimen 143 of living thing is backed 
with a specimen plate 144 which is composed of a glass or a metal having 
an acoustic impedance which is greater than an acoustic impedance of the 
specimen. If the upper surface of the specimen 143 of living thing is 
denoted by l.sub.1, the lower surface thereof is denoted by l.sub.2, and 
the thickness thereof is denoted by d, the ultrasonic wave beam 141 which 
is incident upon the specimen from the upper direction is, first, partly 
reflected by the interface l.sub.1. However, most of the beam 141 is 
transmitted into the specimen 143. Here, the amount of reflection is very 
small since the acoustic impedance of the specimen 143 of a living thing 
is close to the acoustic impedance of the medium 142. The sound waves 
which have propagated through the specimen 143 are reflected by the 
interface l.sub.2, propagate through the specimen upwardly in the drawing, 
enter into the water 142 via the interface l.sub.1, and are detected by a 
probe system 140 as reflected sound waves. If the backing material 144 has 
an acoustic impedance which is sufficiently greater than that of the 
specimen of the living thing, it can be regarded that the sound waves are 
completely reflected by the interface l.sub.2. With the setup shown in 
FIG. 3, it can be said that the reflected signals are virtually determined 
by the signals reflected by the interface l.sub.2. 
If acoustic impedances of the specimen of living thing, water and backing 
material are denoted by Z.sub.S, Z.sub.W, Z.sub.B, respectively, a 
reflection constant R is given by, 
##EQU1## 
where .theta.=2(k-j.alpha.s)d, and k denotes a wave number. 
Therefore, since Z.sub.B &gt;Z.sub.S, Z.sub.W and Z.sub.S =Z.sub.W +.DELTA.Z, 
and .DELTA.Z/Z.sub.W &lt;1, the reflection constant R is given by, 
EQU R=2.sup.-2.alpha. sd (2) 
where .alpha..sub.s denotes an attenuation constant of the specimen. 
That is, with the above-mentioned setup, the reflected signal is equivalent 
to a transmission signal which has propagated through the specimen of the 
living thing twice. Therefore, excellent contrast is obtained owing to a 
relation of square power, to reflect the attenuation constant 
.alpha..sub.s of the specimen. 
So far, the electric signals (reflected ultrasonic waves) proportional to 
the reflection factor R had been displayed in the form of brightness on 
the cathode-ray tube. The above-mentioned situation, however, indicates 
the fact that the attenuation factors of the specimen, or the 
two-dimensional distribution of physical quantities inherent in the 
specimen, can be found by examining the intensity of the reflected echoes 
C. Based upon this viewpoint, the inventors of the present invention have 
found that there exists substantial difficulty if the intensity of 
reflection is simply displayed in the form of brightness in a customary 
manner. First, the conventional method of processing the echoes C will be 
described below. 
With reference to FIG. 4, an output pulse (intensity E) produced by an RF 
pulse oscillator 150 is applied, via a directional coupler 151, to a 
transducer 153 which consists of a lens and a piezoelectric thin film. The 
RF electric signal containing an ultrasonic wave signal (such as the one 
shown in FIG. 2) reflected by the specimen passes through the directional 
coupler 151, amplified through a variable RF amplifier 154 (having an 
amplification degree G), detected by a diode in a video detector 155 (the 
waveform in FIG. 2 corresponds to the waveform of this output), taken out 
by a sampling circuit 156 in the form of the intensity of echo C only, and 
is used as a brightness signal for the cathode-ray tube. 
According to the conventional method, as described above, the intensity E 
of the applied RF pulses is fixed, and the gain of the amplifier 154 is 
manually adjusted such that the intensity of the reflected echo C will 
assume a level to suitably brighten the cathode-ray tube. 
In the conventional method, therefore, there is no definite relation 
between the attenuation constants of the specimen and the brightness 
signals on the cathode-ray tube, and it is not allowed to use displayed 
data of the obtained sound wave image as measured data. 
According to the present invention, on the other hand, use is made, as a 
reference signal, of an echo B reflected from the interface of a lens and 
a medium, that had hitherto been regarded as useless. Like the intensity 
of echo C from the specimen, the intensity of echo B from the interface of 
lens also varies in proportion to the above-mentioned three quantities, 
i.e., varies in proportion to the intensity E of the applied RF pulses, 
transmitting/receiving sensitivity T of the transucer, and gain G of the 
variable amplifier. It was, however, discovered that the ratio of these 
two echoes remains constant irrespective of these quantities. 
This situation will be qualitatively described below with reference to FIG. 
5. In FIG. 5, the echo B from the interface of lens is provided by a 
phenomenon in which an ultrasonic wave pulse generated in the lens 220 by 
a piezoelectric thin film 210 is reflected by the interface 230 of lens. 
Therefore, the intensity V.sub.B of echo B is given by, 
##EQU2## 
where Z.sub.L denotes an acoustic impedance of the lens. 
Here, (Z.sub.W -Z.sub.L)(Z.sub.W +Z.sub.L) represents a reflection factor 
of the interface of the lens. 
With regard to the echo C reflected by the specimen, on the other hand, the 
ultrasonic wave pulse which has reached the interface 230 of lens further 
propagates through the medium 240 while being attenuated, reflected by the 
specimen 250, and is collected again by the lens. Therefore, the intensity 
V.sub.C of echo C is given by, 
##EQU3## 
where .alpha..sub.W denotes an attenuation factor per unit propagation 
distance in the medium, and d denotes a distance between the lens and the 
specimen. Here, the term 4Z.sub.L Z.sub.W /(Z.sub.L +Z.sub.W).sup.2 
denotes a transmission factor of when the echo passes through the 
interface of lens twice, and e.sup.-2.alpha. wd denotes an attenuation 
factor for the echo that reciprocates by 2d in the medium. 
Therefore, if the intensity of echo B from the interface of lens is based 
upon as a reference, then the intensity of echo C reflected by the 
specimen is given by a ratio, 
##EQU4## 
Thus, it is possible to set an absolute level for the reflected echo C 
irrespective of the abovementioned three variable quantities E, T, G. 
Even when the quantities E, T and G are changed to obtain an optimum 
picture, therefore, the quantity of reflected echo C, i.e., the intensity 
of reflected echo C, remains unchanged with the intensity of echo B from 
the interface of lens as a reference. Namely, even when the gain is so set 
as will be adapted for observing the picture, the two-dimensional 
distribution of attenuation constant of the specimen can be measured 
independently and qualitatively. 
In the embodiment of the present invention that will be described later, a 
novel technique is employed; i.e., among the signals of various 
attenuation factors of the specimen obtained as described above, signals 
having attenuation factors that lie within a predetermined range only are 
displayed on the cathode-ray tube to describe the picture. For instance, 
when a specimen of a living thing is observed, the attenuation constant 
may be about -20 to -30 dB for the connective tissue, and may be about -40 
to -60 dB for a cancered region of tissue. Thus, the magnitudes of 
attenuation constant are localized depending upon the object to be 
observed. Therefore, if the distribution of attenuation constant that lies 
within a predetermined range only is taken out and displayed, the picture 
can be displayed with more clear contrast based upon the two-dimensional 
distribution of attenuation constants. For this purpose, the attenuation 
factors of degrees that lie within a given range (e.g., from -20 dB to -30 
dB) only need be displayed. Here, this range should be allowed to be 
changed arbitrarily. 
An embodiment of the present invention will now be described below with 
reference to FIG. 6 and FIG. 7 which is a time chart illustrating the 
operation of the embodiment. 
An RF continuous wave oscillator 300 produces RF continuous wave electric 
signals of a frequency of, for example, 1 GHz. An analog switch 310 is 
controlled by a control signal 345 (shown in FIG. 7(b)) from a control 
circuit 340. An RF signal which has passed through the analog switch 310 
is transformed into an RF pulse signal of a duration time t.sub.d (for 
example, 100 ns), and is applied to a transducer 330 via a directional 
coupler 320. The transducer 330 is the same as the transducer of FIG. 1. A 
reflection detection signal obtained from the transducer passes through 
the directional coupler 320, amplified through an AGC amplifier 350 and an 
RF variable amplifier 360, converted into a video signal (band of up to 10 
MHz) through an RF detector 370, and is applied to an analog gate circuit 
380 which extracts only an echo C reflected by the specimen among the 
reflected echoes A, B and C shown in FIG. 7(a) responsive to a control 
signal 347 shown in FIG. 7(d), thereby to form a sampling signal of 
attenuation factor of the specimen. 
The output of the AGC amplifier 350 (waveform of FIG. 7(a)) is converted on 
a video band through an RF detector 390, and is applied to an analog gate 
circuit 400 which extracts the echo B reflected from the interface of lens 
responsive to a control signal 349 shown in FIG. 7(c) and supplies it to 
one input terminal of a comparator 410. The comparator 410 detects a 
difference between a voltage of the reflected echo B and a preset 
reference voltage Vref. An AGC loop has been formed to control the gain of 
the AGC amplifier 350, so that the output of the comparator 410 will 
become zero. 
According to the above-mentioned setup, the gain of the AGC amplifier 350 
(the gain of the AGC amplifier at this moment is denoted by Go) is so 
adjusted that the intensity of echo B from the interface of the lens is 
brought into agreement with a preset reference voltage, irrespective of 
the intensity E of the applied RF pulses or the transmitting/receiving 
sensitivity T of the transducer. Therefore, the echo reflected by the 
specimen is automatically amplified by Go times. By utilizing the 
amplification factor G.sub.1 of the variable amplifier 360, therefore, the 
data resulting from attenuation factors only can be taken out of the data 
carried by the echoes reflected by the specimen, based upon a relation. 
EQU V.sub.C =G.sub.1 .multidot.V.sub.B (6) 
with the intensity of echo B from the interface of the lens as a reference. 
According to this embodiment, the attenuation factor sampling output is 
applied to a level selector circuit 600 via a logarithmic compress circuit 
500, and the output thereof is used as a brightness signal for the 
cathode-ray tube. 
FIG. 8 shows an embodiment of the level selector circuit, in which an 
output of digital quantity (consisting, for example, of 8 bits) indicating 
the attenuation factor is converted into an analog signal through a DA 
converter 610, and is used as a brightness signal for the cathode-ray tube 
via an analog switch 630. The analog signal is input to a window 
comparator 620. When the analog signal has an amplitude that lies between 
a lower limit level x.sub.1 and an upper limit level x.sub.2 of 
attenuation factor designated by the operator, the analog switch 630 
permits the passage of output of the comparator 620. Then, the signals 
within a range x=x.sub.1 to x.sub.2 are directly displayed on the 
cathode-ray tube. Other signals, however, are not permitted to pass 
through the analog switch 630, and are not displayed. The function of 
level selection is thus produced. 
Although an analog comparator was used as a level selector circuit in this 
embodiment, it is also allowable to control the arithmetic operation in a 
digital manner by using a microcomputer. 
Finally, to calibrate 0 dB which is an absolute value for the attenuation 
factor, the apparatus should be calibrated based upon the signals 
reflected by a mirror that serves as backing material when there is no 
specimen of a living thing.