Gerald R. Harris
Jeffrey M. Porter
Paul M. Gammell
Christopher A. Patton
FDA/CDRH, 12721 Twinbrook Pkwy., HFZ-132, Rockville, MD 20852
Recent calibration efforts for miniature hydrophones used to measure medical diagnostic ultrasound fields have been devoted to increasing the upper frequency range of calibration (>10--15 MHz). However, a bandwidth extending to at least 10 times below the diagnostic pulse center frequency is needed for accurate (error (approximately equal to)5%) measurement of the peak rarefactional pressure and mechanical index, both important quantities. Since at present no commercial hydrophones for medical ultrasound use provide sensitivity information below 1 MHz, a study was undertaken to determine these low frequency sensitivities. The technique uses broadband, plane-wave pressure pulses generated by electrical shock excitation of a thick piezoceramic disk. The hydrophone response is calculated from measurements of the source transducer and hydrophone voltage waveforms. The frequency responses of both needle and membrane polymer hydrophones were measured using this technique. The membrane hydrophones studied had bandwidths extending below (approximately equal to)0.2 MHz, but one of the needle probes began rolling off above 0.5 MHz. Therefore, given the above criterion regarding diagnostic pulse center frequency, sensitivity to 0.1--0.2 MHz should be established for diagnostic use hydrophones, because a uniform response below 1 MHz cannot be assumed.