Change in acoustic impedance and sound speed of HIFU-exposed chicken breast muscle

Abstract

High-intensity focused ultrasound (HIFU) is used as a therapeutic method to treat non-superficial-tumors by inducing high temperature to coagulate them. In this study, we measured the acoustic impedance distribution and sound speed of the HIFU-exposed chicken breast muscle. HIFU exposure was at about 800 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for 60 s. Ultrasound B-mode images were taken at before and after the HIFU exposure for focal plane. The HIFU-exposed specimen was sliced as thickness of 3 mm to measure sound speed. Acoustic impedance was mapped for five specimens using an ultrasound impedance microscope at room temperature. Mean acoustic impedance of non-denatured region and thermally denatured region was (1.65 ± 0.021) and (1.61 ± 0.023) [×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> kg·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ], respectively. Sound speed of the specimen was measured for four specimens by a pulse-echo method at 26.5 ± 0.5°C in a thermostat bath. The mean sound speed of thermally denatured and non-denatured specimens was (1603.8 ± 8.0) and (1569.0 ± 12.8) [m/s], respectively. The acoustic impedance and sound speed of the coagulated specimen was similarly lower than that of the non-coagulated specimen. The decrease ratios of those were quantitatively consistent. A B-mode image of the HIFU-exposed specimen shows a bright region at the HIFU focal point, corresponding to the thermally denatured region. In a B-mode image from the acoustic impedance map, a similar bright region was observed.