Voltage sensitivity response of ultrasonic hydrophones in the frequency range 0.25-2.5 MHz

Frequency responses of different PVDF polymer hydrophones, including membrane and needle designs, were measured and are presented in terms of end-of-cable voltage sensitivity vs. frequency over a wide, 4.5-octave bandwidth ranging from 0.25-2.5 MHz. The experimental data indicate that the membrane PVDF hydrophones can exhibit uniform, to within +/- 0.75 dB, responses. However, a widely used bilaminar membrane hydrophone-preamplifier combination may display sensitivity variations of +/- 2 dB. Also, even well-designed needle-type hydrophones show a more distinct sensitivity variation below 1 MHz that is on the order of 3-4 dB. The overall uncertainty of the calibration technique was estimated to be better than +/- 2 dB in the frequency range considered. The technique, which uses a combination of swept frequency chirp and reciprocity so that both the relative and absolute plots of sensitivity vs. frequency can be obtained, is also briefly described. The results of this work are important to implement procedures for adequate determination of the mechanical index of ultrasound (US) imaging devices. Mechanical index is widely accepted as a predictor of potential bioeffects associated with cavitation phenomena. Also, absolute calibration data are essential in development of therapeutic procedures based on the use of high-intensity focused ultrasound (HIFU), and in characterization of conventional therapeutic US applicators operating at frequencies below 1 MHz.