Improvement of hydrophone measurements on diagnostic ultrasound machines using broadband complex-valued calibration data

Abstract

Non-ideal hydrophone frequency responses may impede correct acoustic outputmeasurements on medical ultrasound equipment, in particular when high frequency ornonlinearly distorted waveforms have to be detected. It is shown that correct pressurewaveforms p(t) and the associated standard pulse parameters such as positive peak pressure p+and rarefactional peak pressure p- can be obtained by impulse deconvolution if the non-idealfrequency response of the hydrophone M(f) is provided with high frequency resolution both inamplitude and phase in a broad frequency range. The complex-valued calibration data requiredcan be obtained by a secondary hydrophone calibration technique which was recentlydeveloped and uses broadband, nonlinearly distorted, focused ultrasound pulses and an opticalmultilayer hydrophone as reference. The results obtained for a membrane and a needle-typehydrophone are applied to improve exposure measurements on a commercial diagnosticultrasound machine. This is shown for different operation modes and parameter settings of thediagnostic machine by comparison of the pressure pulse waveforms and pulse parametersobtained by the commonly applied evaluation method using the voltage-to-pressure transferfactor at the acoustic working frequency M(fawf) with those obtained by pulse deconvolutionusing the complete broadband complex-valued transfer function M(f).