機械インピーダンスを用いた生体の硬さ計測

Abstract

The measurement of stiffness on the body surface is of great importance for palpation, percussion and pressation such is used in a detection of an edema in clinics. From the other standpoint of an industrial measurement, it is also significant for getting hold of a soft object such as the hardness sensor of a manipulator. In order to estimate biomechanical properties of the body surface by means of biomechanical impedance, we have developed a measurement system for biomechanical impedance. The system consists of a measuring device, a measuring probe and a personal computer. A vibrator, an impedance head and a load-cell are included in the measuring probe. A random vibration is applied to the body surface through the probe, and force and acceleration at the driving point are detected by the impedance head. The measuring probe is of useful size, and almost all parts of the body surface are measured by the probe. The random vibration method makes a short measurement time possible. The preload (contact force) of probe to the body surface during a measurement is kept constant by means of computer programming. This allowed satisfactory accuracy and measurement repeatability. The mechanical impedance of the body surface is significantly influenced by the measuring conditions (diameter of the vibrating tip of the probe and the preload). It also depends on stratified body structures, such as a bone under the skin surface. The applied vibration is reflected at the layer according to its form and thickness. Consequently there are three typical patterns (soft, intermediate and hard) in the biomechanical impedance spectra. We have proposed a new index SI (Stiffness Index), which is obtained from the mechanical impedance spectrum. The SI has [N/m] dimensionally and indicates a kind of extended spring constant into a visco-elastic medium. The stiffness distribution on the back of the hand is illustrated as a SI mapping. From the mechanical impedance spectrum with a soft pattern, we are able to obtain a visco-elastic constant of the body surface. When the obtained visco-elasticity includes the influences of the stratified structure of the body, it should be termed 'apparent' visco-elasticity. In order to obtain the visco-elasticity of the skin and subcutaneous tissue independent of body structure, we have proposed a correction method for the measured results. This method was applied to the measured result for the chest and the corrected visco-elasticity of the tissue in situ was obtained.