Substance characteristic measuring method and substance characteristic measuring instrument

#The present invention relates to a method and apparatus for measuring material characteristics using pulse waves, and more particularly, to a method and apparatus for measuring material characteristics of characteristic values corresponding to hardness of tissue materials in a living body using ultrasonic pulses.

#One conventional method for measuring the hardness of a tissue material of a living body is a method in which a probe is pressed against the material to be measured, vibration is applied thereto, mechanical vibration response of the tissue material to the input vibration is detected using a sensor, and characteristic values corresponding to the hardness are obtained based on variations in the frequency, phase, etc. Examples of this method are disclosed in, for example, “Measurements of the Hardness of a Soft Material with a Piezoelectric Vibrometer and Their Analysis” (Sadao Omata, Medical Electronics and Bioengineering, Vol. 28, No. 1, 1990, pp1-8) and “New tactile sensor like the human hand and its applications” (S. Omata et al, Sensors and Actuators A, 35 (1992) pp9-15). FIG. 7 shows an apparatus for measuring the hardness of tissue of a living body disclosed in Japanese Patent Laid-Open Publication No. Hei 9-145691. FIG. 7 shows a probe unit 2 pressed against the material 1 to be measured of a living body, for example, the tissue of the skin or the tissue of the viscus, such as a liver tissue exposed when the abdomen is opened. The probe unit 2 includes a vibrator 3 and a strain detection sensor 4, and is connected to an external control unit 5. The control unit 5 has a self-excited oscillation circuit 6 and the self-excited oscillation circuit 6 has an amplifier 7 and a gain variation correction circuit 8. Furthermore, the self-excited oscillation circuit 6 is connected to a frequency measuring circuit 9 to measure its frequency and a voltage measuring circuit 10 to measure its amplitude.

#The operation of this conventional example will be explained below. When the probe unit 2 is pressed against the material 1, the vibrator 3 inside the probe unit 2 generates self-excited vibration as an electric signal is converted to mechanical vibration by a mechanical/electrical vibration system of the vibrator 3 and the self-excited oscillation circuit 6 in the control unit 5, and the vibration is input from the end of the probe unit 2 to the material 1. The material 1 responds to this input vibration according to its mechanical vibration transmission characteristic. The strain detection sensor 4 detects this output vibration (strain) and converts it to an electric signal. The vibrator 3 and the strain detection sensor 4 can be implemented by, for example, a piezoelectric vibration element and a piezoelectric sensor. The vibrator 3 and the strain detection sensor 4 together with the amplifier 7 form a feedback loop and then oscillation is self-excited. Here, as a result of the material 1 responding to the input signal from the vibrator 3, generally the frequency changes, a phase difference is generated and the amplitude is reduced. But the gain variation correction circuit 8 has a function of correcting the amplitude gain of the output signal of the strain detection sensor 4. Furthermore, because the gain variation correction circuit 8 is formed in the feedback loop of the self-excited oscillation circuit 6, feedback is provided in such a way that the phase difference generated becomes to zero while the amplitude gain is being corrected. When the phase difference is fed back to zero, the resonance frequency of the mechanical/electrical vibration system, which includes the material 1, the vibrator 3, the self-excited oscillation circuit 6, and the strain detection sensor 4, can be obtained by the frequency measuring circuit 9.

#FIG. 1 is a block diagram of a method for measuring characteristics corresponding to the hardness of a material according to an embodiment of the present invention;

#FIG. 2 illustrates pulse waves of an incident wave and a reflected wave according to the embodiment of the present invention using the horizontal axis as the time axis;

#FIG. 3(a) and FIG. 3(b) schematically illustrate examples of a result of a frequency component analysis of the incident wave and reflected wave respectively, according to the embodiment of the present invention;

#With reference now to the attached drawings, a preferred embodiment of the present invention will be explained in detail below. FIG. 1 shows a case where a material 1 to be measured is inside organism tissue in a living body 11. The part denoted as a sensor 12 is an ultrasonic probe which contains an ultrasonic wave input section, a transmitter, that introduces an ultrasonic pulse into an external material and an ultrasonic wave reception section, a receiver, that receives the reflected wave which is reflected from the external material introduced the incident pulse. This sensor 12 is coupled with an amplifier 13, a frequency component analysis section 14, a frequency/phase difference specification section 15 and a phase difference compensating operation section 16 in that order. An output section A is extracted from the result of the frequency/phase difference specification section 15 and an output section B is extracted from the result of the phase difference compensating operation section 16.

#An example operation of such a configuration will be explained. When the sensor 12, which is an ultrasonic probe, is pressed against the tissue of the living body 11, an ultrasonic pulse wave is input from an ultrasonic wave input section inside the sensor 12 to the living body 11, propagates inside the tissue, reaches the material 1, is reflected, propagates all the way back through the tissue of the living body 11, its reflected pulse wave is detected by the ultrasonic wave reception section inside the sensor 12 and converted to an electric signal. Various ultrasonic probes are commercially available as the sensor 12, such as one having both functions of the incident section and reception section on a single element or one provided with the incident section and reception section separately. FIG. 2 schematically shows an incident wave and a reflected wave with the horizontal axis indicating the time axis and the vertical axis indicating the amplitude of electric signals of the pulse waves. Generally, the amplitude of the reflected wave is reduced compared to that of the incident wave, and the reflected wave shows changes in the spectra of frequency components, phase difference, etc. These changes reflect the characteristic of the material 1, which may be, for example, a liver. Furthermore, when the incident wave passes through the tissue of the living body 11, is reflected by the liver as the material 1, and returned, the characteristic of the propagation path when the pulse wave is reflected from the surface of the liver, etc., is the same as the characteristic of the incident propagation path to the liver, etc., and therefore the above-described change reflects only the characteristic of the liver, etc., which is the material 1.

#The signals of these incident wave and reflected wave are amplified by the amplifier 13 to an appropriate signal level, the frequency component analysis section 14, then performs a frequency component analysis of the incident wave and reflected wave and obtains frequencies of the respective sine wave components and a spectral distribution from the sine wave components and the cosine wave components at those frequencies. FIG. 3 schematically shows an example result of such a frequency component analysis. FIG. 3(a) shows the frequency component of the incident wave and FIG. 3(b) shows the frequency component of the reflected wave with the horizontal axis indicating the frequency and the vertical axis indicating the gain and the phase of each frequency component expressed as the length of a solid line in the height direction and the position of the height marked with a circle, respectively. Here, a technique generally called “Fourier analysis” can be used for the frequency component analysis. As is well known, a periodic function of an electric signal can be expanded into a Fourier series, and therefore a pulse wave can be expressed as a set of many sine wave components with different frequencies and amplitudes. Here, the Fourier series is divided into a sine wave component and a cosine wave component at the same frequency and these components are combined and expressed as a single sine wave component having a phase difference at that frequency. Therefore, the phase difference here refers to a phase difference calculated from the sine wave component and cosine wave component at that frequency.