Pressure pulse distortion by hydrophones due to diminished low frequency response

In characterizing the bandwidth of measurement devices used in ultrasound exposimetry, attention has been focused on the high frequency response. However, current diagnostic ultrasound measurement standards have low frequency specifications for hydrophones and associated amplifiers, and the response below 1 MHz can be especially significant when measuring lithotripsy pulses. To model the effects of diminished low frequency response, simulated diagnostic and lithotripsy pulses were filtered with a single-pole high-pass filter for a range of -3 dB cutoff frequencies (denoted f/sub a/). For lithotripsy pulses, it was found that the pulse quantities peak rarefactional pressure (p/sub r/) and pulse width (t/sub w/) were most sensitive to f/sub a/, and to keep errors in p/sub r/, and t/sub w/ below 10%, f/sub a/ should be in the 10-60 kHz range for the pulses examined. For the diagnostic case, p/sub r/ was the quantity most significantly affected, and for an f/sub a/ value approximately one-half the center frequency, p/sub r/ was decreased by more than 30% for a pulse modeled to show the effects of finite amplitude distortion typical of diagnostic pulses measured in water. Given this latter result, current hydrophone and amplifier low frequency specifications may need to be reconsidered.