Sound field analysis for biological acoustic impedance microscope for its precise calibration

Acoustic impedance microscopy for biological soft tissues was proposed. A target is placed on a plastic substrate. Ultrasonic beam, which is focused on the target, is transmitted and the reflection is received by the same transducer. The reflection is normalized by using pure water, and interpreted into acoustic impedance. As the beam is focused, oblique incident analysis is required to acquire a precise interpretation. Sound potential is calculated at a particular plane, and decomposed into plane wave components with different wave numbers using Fourier Transform. Both pressure and shear waves are generated and taken into account, when oblique incident impinging the substrate. As a pulsed wave is propagating, pressure and shear waves can be separated in time domain. Reflection signal is calculated for each plane wave component, and the integral through the k-space represents the received signal. As the substrate has higher acoustic impedance than target, the normalized reflection intensity reduces with the increase in acoustic impedance of the target. The experimental plots were acquired by using different contents of saline solutions. They agreed with the calculation results by sound field analysis. Frequency dependence is negligible in the region of 30 - 100 MHz. By scanning the transducer, an acoustic impedance microimage was acquired and calibrated based on the above analysis.