Improved exposimetry on diagnostic ultrasound equipment using broadband complex-valued hydrophone frequency responses

Abstract

Nonideal hydrophone frequency responses may impede correct acoustic output measurements on medical ultrasound equipment, in particular when high-frequency or nonlinearly distorted waveforms have to be detected. However, correct pressure waveforms and the associated standard pulse parameters can be obtained by impulse deconvolution if the nonideal frequency response of the hydrophone M(f) is provided with high frequency resolution both in amplitude and phase in a broad frequency range. The complex-valued calibration data required can be obtained by a novel secondary hydrophone calibration technique using broadband, nonlinearly distorted, focused ultrasound pulses and a very broadband optical multilayer hydrophone as reference receiver. The calibration data obtained for a bilaminar membrane hydrophone are applied to improve exposure measurements on a commercial diagnostic ultrasound machine. In comparison to the results obtained by the proposed pulse deconvolution method using the complete broadband complex-valued transfer function M(f), an overestimation of the positive peak pressure by up to 50%, an underestimation of the rarefactional peak pressure by up to 11%, and an overestimation of the pulse intensity integral by up to 28% are found if the common evaluation method using the voltage-to-pressure transfer factor at the acoustic working frequency M(fawf) is applied.