Reciprocity Calibration of Hydrophones in High Intensity Focused Ultrasound Field

Abstract

The primary problem of safety and efficiency for the high intensity therapeutic ultrasound (HITU) is the acoustic measure and dose control. The key technique is the pressure and intensity in the acoustic field especial in the focal region using the small calibrated hydrophone. The calibration accuracy of the used hydrophone is very important for HITU. Although the small hydrophone calibration has realized but there was no report of the hydrophone calibration in high pressure field. In this paper, our objective is to develop an absolute calibration method for the measurement of free field voltage sensitivity of hydrophone for high intensity focused ultrasound. First the acoustic pressure at the focal point by the self-reciprocity method of spherically curved auxiliary transducer is calibrated, then the free field voltage sensitivity of hydrophone at the geometric focal point of the calibrated pressure is obtained. The spatial average effect of acoustic pressure on hydrophone surface at the focal point is theoretically modified, and the expression and value table of correction coefficient of spatial average effect of hydrophone are given. The maximum acoustic pressure measured at the focal point was up to 5.58MPa (1.02kW/cm2) and used to calibrate a hydrophone from 0.95 MHz to 1.10 MHz with maximum local distortion parameter 0.72. The results show the rationality and feasibility of the measurement principle and method.