Practical aspects of ultrasound dosimetry: Hydrophone performance

Abstract

Abstract Wideband piezoelectric polymer (PVDF) hydrophones are now available with well documented performance characteristics; these probes are especially well suited for the measurement and comprehensive characterisation of both pulsed and continuous wave acoustic fields generated by ultrasound diagnostic devices. The hydrophones feature flat (to within ± 1.5 dB) frequency response, voltage (pressure) sensitivity uniform well beyond 15 MHz, well-behaved, predictable directivity patterns and excellent linearity in the pressure response up to 10 8 Pa (kidney stone crushers). These characteristics make the polymer probes uniquely applicable to measurements of the acoustic output parameters of ultrasound devices, including peak instantaneous pressure amplitude, as specified in the AIUM/NEMA Standard, IEC Draft document and FDA 510(K) requirements. The hydrophones are absolutely calibrated in terms of the end-of-cable voltage sensitivity (in units V/Pa or their derivatives) with a specified terminating load. Most common designs of ultrasonic polymer probes are the needle and membrane type which when properly designed provide reference hydrophones of good spatial and temporal resolution. Absolute, (end-of-cable) voltage sensitivity calibration has been carried out in the range of intensities from 10 mW/cm 2 to 100 W/cm 2 (SPTP) using both the conventional two transducer reciprocity calibration technique and the Planar Scanning Technique. Calibration techniques indicate that overall uncertainty of typically ±1.5 dB can be obtained. The area where uncertainties mostly occur are in plane wave approximation due to transmitter and receiver size and excitation frequency, and phase variation. Combining one of the absolute calibration techniques (e.g. reciprocity and a swept frequency measurement procedure) allows hydrophones to be calibrated as a continuous function of frequency. Moreover both amplitude and phase response may be determined. The spectral analysis technique, combined with swept frequency system (TDS), constitutes a useful tool for assessment of both the amplitude only and the complex frequency response of acoustic radiators. Transducer impulse response can be determined, based on far field measurements. Transducer artifacts can be removed by inverse filtering (deconvolution) procedures for quantitative ultrasound measurements. The effect of spatial averaging was determined experimentally and it was found that it is not significant at frequencies up to 5 MHz, providing 0.5–0.6 mm diameter probes (the smallest currently available) are used. For frequencies above 5 MHz the experimental data indicate that in the “worst case” the estimated “true” spatial peak value in the measured waveform exceeded the measured value by a factor of 2, depending on frequency and the diameter of the polymer probe used. Work is in progress to estimate more accurately the effect of spatial averaging due to the finite aperture of the hydrophone probes.