JPH0229985B2 - - Google Patents

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an ultrasound microscope.

[Conventional technology]

In recent years, the generation of high frequency sound waves up to 1 GHz,
Since detection became possible, it became possible to realize sound wavelengths of approximately 1 μm underwater, and as a result, it became possible to obtain high-resolution sonic imaging devices. That is, a concave lens is used to create a focused ultrasound beam, a sample is inserted into this ultrasound beam, and the reflected sound waves from the sample are detected to obtain information reflecting the elastic properties of the sample, or the sample is machined. By scanning and creating images, high resolution of 1 μm level is achieved. (For example, described in JP-A-50-116058).

A conventional example of obtaining such a sound wave image will be explained with reference to FIGS. 1 and 2.

FIG. 1 is a diagram showing a schematic configuration of a probe system for obtaining reflected signals from a sample. In the figure, an acoustic lens 20 made of a cylindrical crystal such as sapphire or quartz glass has one end surface having an optically polished flat surface, and the other end surface having a lens surface 30 formed by a spherical hole. An RF pulsed sound wave of a plane wave is radiated into the acoustic lens 20 by the RF pulsed electrical signal applied to the piezoelectric thin film 10 . This plane sound wave is focused onto a sample 50 placed at a predetermined focus by a positive lens formed at the interface of the spherical hole 30 with a medium (generally water) 40. The sound waves reflected from the sample 50 are collected by the same lens surface, converted into plane waves, propagated within the acoustic lens 20, and finally converted into electrical signals by the piezoelectric thin film 10. If you look at this situation in the video area, it will look like Figure 2. In FIG. 2, the horizontal axis represents the time axis, and the vertical axis represents the signal strength of the received reflected wave. A is the launch echo, B is the reflected echo from the lens interface 30, and C is the reflected echo from the lens interface 30.
is the reflected echo from the sample. These are repeated with a repetition time _tR . Since the reflected echo C changes over time depending on the acoustic properties of the sample and the scanning of the sample, this reflected echo C is sampled in synchronization with the repetition period and only its echo intensity is extracted.
A sound wave image can be obtained by mechanically scanning the sample and displaying it on a cathode ray tube in synchronization with this.

By the way, the intensity of reflected ultrasound is determined by the acoustic impedance of the medium and the sample, and is proportional to the following reflectance R.

R=Z _s −Z _w /Z _s +Z _w ≡X−1/X+1 ………(1) Here, Z _s : Acoustic sympedance of the sample Z _w : Acoustic impedance of the medium X≡Z _s /Z _w : Acoustic impedance of the sample normalized by the acoustic impedance of the medium (hereinafter referred to as relative impedance). Conventionally, an electrical signal proportional to the sound pressure of the reflected echo was used.
Since the brightness was displayed on the CRT, the reflectance of the sample was displayed.

[Problem that the invention seeks to solve]

The above circumstances indicate that by examining the height of the reflected echo C, it is possible to obtain a two-dimensional distribution of a physical quantity unique to the sample, called the relative impedance of the sample. This conventional method of simply displaying the reflected intensity as a luminance has an essential drawback. First, a conventional echo C processing method will be explained.

As shown in FIG.
0 to the transducer 130 made of the above-mentioned lens or piezoelectric thin film. The RF electric signal including the reflected ultrasound signal from the sample is sent to the matching box 1.
20 and the directional coupler 110, the variable RF
After amplification by the amplifier 140, diode detection is performed by the video detector 150 to obtain an output waveform as shown in FIG. This output waveform is extracted as the height of the echo C by the sampling circuit 160, and is used as the brightness signal of the CRT.

In this way, in the conventional method, the intensity of the applied RF pulse emitted by the RF pulse oscillator is fixed, and the gain of the amplifier 140 is adjusted so that the height of the reflected echo C is at a level that makes the CRT light up appropriately. Adjusted manually.

Therefore, in the conventional method, there is no established relationship between the relative impedance of the sample and the brightness signal on the CRT, and it has been impossible to use the gradation information of the obtained sound wave image as measurement data. In view of this, one object of the present invention is to provide a means for quantitatively displaying the magnitude of reflectance of a sample.

The second difficulty of the conventional method is that when the acoustic impedance of a sample is large, the conventional display method cannot clearly display subtle differences in acoustic impedance. In other words, the acoustic impedance of the target sample for an ultrasound microscope is as small as that of a biological cell (Z _s
1.6×10 ⁶ Kg/m ² /s) to tungsten (Z _s
However, the relationship between ^{the relative} impedance and the reflectance is not linearly proportional. Figure 4 shows this situation, with the relative impedance on the horizontal axis and the reflectance R as the display signal on the vertical axis.
The relationship between the two is shown by a solid line, and the relative impedance of a typical material is shown by an arrow. As can be seen from this figure, the magnitude of reflectance is not proportional to the magnitude of relative impedance, and where the relative impedance is large, the brightness on the CRT is almost negligible even if the relative impedance differs depending on the material. There is no difference, and no clear contrast occurs in samples made of materials with high relative impedance such as Al, Si, Cu, and Au, such as ICs and LSIs.

This situation cannot be solved simply by nonlinear processing of image signals such as compression and expansion for display purposes. A second object of the present invention is to provide a means for displaying the relative impedance of a sample, which is a material-specific quantity, instead of displaying the reflectance as in the conventional method.
A third purpose is to provide a means for displaying differences in relative impedance of samples with clearer contrast.

[Means for solving the purpose and their effects]

Regarding the first problem, as a result of examining conventional circuits, we found that the proportionality constant between the conventionally displayed luminance signal and the reflectance R of the sample is mainly due to the strength of the RF pulse applied to the transducer 130. It is expressed as the product of E, the transducer's wave transmitting/receiving sensitivity T, and the amplification degree G of the receiving system including the video area.

Therefore, in order to quantitatively determine the relative impedance of the sample, it is sufficient to control the three quantities E, T, and G mentioned above. Of course, as in the conventional method, the intensity E of the applied RF pulse is fixed, and the amplification degree G when adjusted so that the reflected echo reaches a certain set height is taken as the absolute value of the intensity of the reflected ultrasound. That doesn't suit this purpose. The reason for this is that if the ultrasonic frequency used or the sensor itself is changed, the transmitting and receiving sensitivity of the transducer changes, so the above amplification degree G is used as the absolute standard for the intensity of reflected ultrasonic waves. In order to do this, it is necessary to create a calibration table for each sensor and for each ultrasonic wave used, which is too complicated to be practical.

The feature of the present invention is that the echo B from the interface between the lens and the medium, which was treated as useless in the past, is
is used as a reference signal.

Of course, the height of the echo B from this lens interface, like the height of the echo C from the sample, depends on the three quantities mentioned above: the strength E of the applied RF pulse, the transducer sensitivity T, and the variable amplifier. Although it is proportional to the gain G, it has been found that the ratio of the height of the echo from the sample to the height of the echo from the lens interface does not depend on these quantities.

This situation will be quantitatively explained below with reference to FIG. Echo B from the lens interface 30 is an ultrasonic pulse generated from the piezoelectric thin film 10
The height _VB of the echo B is given by the following equation.

V _B = ETGZ _w − Z _L / Z _w + Z _L ………(2) Here, Z _L : Acoustic impedance of lens material In equation (2), (Z _w − Z _L ) / (Z _w + Z _L ) is the reflectance of the lens interface.

On the other hand, the reflected echo C from the sample is a process in which the ultrasonic pulse that has reached the lens 30 further propagates through the medium 40 while being attenuated, is reflected by the sample 50, and is again collected by the lens interface. The height Vc of echo C is given by the following equation.

Vc=ETG4Z _L Z _w / (Z _L + Z _w ) ² e ^-2awd R……(3) Here, a _w : Attenuation rate per unit propagation distance in the medium d : Distance between the lens and the sample (3 ), the term 4Z _L Z _w / (Z _L + Z _w ) ² is the transmittance when passing through the lens interface twice, and e ^-2awd is the attenuation rate when going back and forth a distance d in the medium. It represents.

Therefore, if the height of the echo C reflected from the sample is expressed using the height of the echo B from the lens interface as a reference, the following equation is obtained.

Vc／V _B ＝4Z _L Z _w )／(Z _L + Z _w ) (Z _w − Z _L ) e ^-awd R...
...(4) Equation (4) shows that the absolute level of the reflected echo C can be set regardless of the above three variations E, T, and G. In other words, since the height of the reflected echo C expressed with respect to the height of the echo B from the lens interface remains unchanged, in order to obtain the optimal image,
Even if the settings of E, T, and G are changed, the reflectance of the sample is
The measured value of the dimensional distribution can be obtained quantitatively without affecting E, T, and G.

The second problem can be solved if a signal proportional to the relative impedance of the sample can be obtained with good reproducibility. Since the relative impedance and reflectance R have the relationship shown in equation (1), converting equation (1) gives the following equation (5).

X=1+R/1-R (5) Using this equation (5), the signal processed by the calculation of equation (5) can be displayed from the reflectance R, that is, the intensity of the reflected ultrasonic wave. In this way, the reflected ultrasonic waves can be converted into a signal representing relative impedance in proportion to the reflectance and displayed. FIG. 5 shows this calculation process, where the horizontal axis shows the intensity of the input signal, that is, the reflected ultrasound signal, and the vertical axis shows the output signal to be displayed on the CRT. The relationship between the display signal and the relative impedance after such arithmetic processing is completely proportional to the display signal and the relative impedance, as shown by the broken line in FIG.

The third problem of the present invention was born from the usage experience that when displaying the relative impedance X of the sample obtained in the above manner on a cathode ray tube, it is not always necessary to display the entire range of X = 1 to 100. It is something that In other words, when observing a biological sample, X = about 1 to 4, and when observing an integrated circuit, etc., X = 10 to 40. Localized in size. Therefore, if only the area near the localized relative impedance is extracted and displayed, it is expected that the contrast of the image based on the two-dimensional distribution of relative impedance will become clearer.
This problem can be solved by displaying only a certain range of relative impedance (for example, X=10 to 20) and changing this range so that it can be set arbitrarily.

[Embodiments of the invention]

FIG. 6 is a diagram showing an embodiment of the present invention, in which RF
For example, 1 GHz RF generated by continuous wave oscillator 300
Convert the continuous wave electrical signal to an RF pulse signal with a duration _td of, for example, 100 ns using an analog switch 310,
(control signal in FIG. 7b) is applied to the transducer 330 via the directional coupler 320. After the reflected detection signal is amplified by the AGC receiving amplifier 350 and the RF variable amplifier 360 via the directional coupler 320, it is converted to a video signal with a band of about 10 MHz by the diode detector 370, and then the time gate sampling circuit 380 is used to sample the reflected signal C from the sample, which is a desired signal (control signal in FIG. 7d), and use it as an imaging signal. Furthermore, AGC receiving amplifier 350
The output (waveform a in Figure 7) is the diode detector 3.
After conversion to the video band at step 90, the height of the echo B from the lens interface is detected by the time gate sampling circuit 400 (control waveform shown in FIG. 7c), and the difference between this and a preset reference voltage is detected by the comparator 410.
is detected, and the gain of the AGC receiver 350 is controlled so as to make the output of the comparator 410 zero. In addition, the control circuit 3
40 is a circuit that generates the control signals shown in FIGS. 7b to 7d at a repetition period _tR .

According to this configuration, the height of the echo B from the lens interface always matches the preset reference voltage value, regardless of the strength E of the applied RF pulse and the transducer sensitivity T of the transducer. AGC
The gain of amplifier 350 is adjusted. at this time
When the gain of the AGC amplifier is G ₀ , the reflected echo from the sample is automatically amplified by G ₀ times, so when the amplification rate of the variable amplifier 360 is G ₁ , the following
The relationship is as shown in equation (b).

Vc=G ₁・R・V _B ………(6) V _B is always adjusted to a constant value and G ₁ is known, so if you measure Vc, R can be found in proportion to Vc. It will be done. That is, of the information contained in the reflected echo C from the sample, only the information based on the reflectance can be extracted, always using the height of the echo B from the lens interface as a reference.

In this embodiment, this sampled output is passed through a reflectance-relative impedance converter 500 and a relative impedance level selection circuit 600, so that only signals carrying a relative impedance in a specific range are used as CRT luminance signals. The reflectance-relative impedance converter 500 is a means for embodying the second object of the present invention, and the relative impedance level selection circuit 600 is a means for embodying the third object of the present invention. This will be explained below.

Figure 8 shows the reflectance-relative impedance converter 5.
00 is shown. That is, the output of the sampling circuit 380 is received by the buffer amplifier 510 and digitized by the AD converter 530. Since this digital quantity represents an input signal proportional to the reflectance mentioned above, this digital quantity is
If the input/output characteristics shown in FIG. 5 or the conversion data according to equation (5) are written in advance as the address of the ROM 540, the digital output of the ROM 540 will be the amount converted to relative impedance.

FIG. 9 shows an embodiment of a relative impedance level selection circuit 600, in which a digital amount (for example, 8 bits) representing the above-mentioned relative impedance is used.
The output is converted to an analog signal by the DA converter 610, and this is sent via the analog switch 630.
It is used as a brightness signal for CRT. The analog signal is input to a window comparator 620. The lower limit level of relative impedance is set by the operator.
If X ₁ and upper limit level X ₂ are input to the window comparator 620 as specified values, the output of the window comparator 620 will be based on the intensity of the signal level between X ₁ and X ₂ of the output signal of the DA converter 610. Turn analog switch 630 only when it is a signal.
Turn on. Therefore, signals in the range _{of X} = Ru.

In the above configuration, a read-only memory (ROM) is used to perform the "reflectance-relative impedance conversion" which is the gist of the present invention, but it may also be assembled using random logic or arithmetic control by a microcomputer. good.

Note that the absolute value of the relative impedance may be calibrated by a commonly used method using a material whose acoustic impedance value is known.

〔Effect of the invention〕

As described above, according to the present invention, the reflectance,
Not only can relative impedance, a quantity specific to a sample, be displayed at all times regardless of operating conditions, but changes in acoustic impedance can be converted to a straightforward scale and displayed with good contrast, making it possible to display any relative impedance. It is possible to extract and display only the range, and the contrast of the ultrasound microscope image can be clarified.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the schematic configuration of the probe system of an ultrasound microscope, Fig. 2 is an explanatory diagram of the received echo, and Fig. 3 is a block diagram explaining the conventional processing method of reflected waves. FIG. 4 is a characteristic diagram showing the relationship between relative impedance and reflectance, FIG. 5 is a diagram showing the relationship between the magnitude of the reflected ultrasound signal and the magnitude of the output signal to be displayed, and FIG. 6 is a characteristic diagram showing the relationship between relative impedance and reflectance. FIG. 7 is an explanatory diagram of the signal operation, and FIG.
9 and 9 are diagrams showing the configuration of an embodiment of the main part of the present invention.

Claims (1)

[Scope of Claims] 1. An acoustic lens having a piezoelectric film on one end face and a concave spherical lens interface on the other end face, the ultrasonic wave emitted from the piezoelectric film is converted into a convergent beam by the acoustic lens, and the In an ultrasound microscope, the reflected wave of the convergent beam from the sample is received by the piezoelectric film via the acoustic lens, and the received signal is processed and displayed as brightness on a CRT. Means for comparing the height of the reflected echo from the lens interface with a predetermined standard value and outputting a deviation value, and using this deviation value signal, the height of the reflected echo from the lens interface is kept at a constant value. An AGC circuit that adjusts the amplification factor and a means for amplifying the reflected echo from the sample by the adjusted amplification factor and outputting it, thereby obtaining a signal with an intensity proportional to the reflectance of the sample. An ultrasonic microscope featuring: 2. An acoustic lens having a piezoelectric film on one end face and a concave spherical lens interface on the other end face is provided, and the ultrasound emitted from the piezoelectric film is converted into a focused beam by the acoustic lens, and the focused beam is emitted from the sample. In an ultrasound microscope in which reflected waves are received by the piezoelectric film via the acoustic lens, and the received signals are processed and displayed as brightness on a CRT, the reflected echo from the lens interface of the ultrasound emitted from the piezoelectric film means to compare the height of the lens with a predetermined standard value and output a deviation value, and use this deviation value signal to adjust the amplification rate so that the height of the reflected echo from the lens interface becomes a constant value. an AGC circuit for amplifying the reflected echo from the sample at the adjusted amplification rate and outputting the amplified signal; An ultrasonic microscope characterized by being equipped with reflectance-relative impedance conversion means that outputs. 3 An acoustic lens having a piezoelectric film on one end face and a concave spherical lens interface on the other end face is provided, and the ultrasound emitted from the piezoelectric film is converted into a convergent beam by the acoustic lens, and the convergent beam is reflected from the sample. In an ultrasound microscope in which waves are received by the piezoelectric film via the acoustic lens, and the received signals are processed and displayed as brightness on a CRT, the echoes of the ultrasound waves emitted from the piezoelectric film are reflected from the lens interface. A means for comparing the height with a predetermined standard value and outputting a deviation value, and an AGC circuit that uses this deviation value signal to adjust the amplification rate so that the height of the reflected echo from the lens interface becomes a constant value. a means for amplifying the reflected echo from the sample at the adjusted amplification rate and outputting the amplified signal; and a means for correcting the output signal to a predetermined signal intensity according to the intensity of the signal and outputting the same 1. An ultrasonic microscope comprising: a reflectance-relative impedance conversion circuit, and means for selectively outputting only output signals having an intensity within an arbitrarily specified range among the output signals of the circuit.