The speed of mechanical heart-valve leaflets is known to be an important quantity for predicting cavitation, yet no simple computational means exists for predicting the leaflet speed. In this study, a model for simulating the motion of heart-valve leaflets in rigid test systems is presented. The input for the simulations is the ventricular pressure trace, readily measured in heart-valve tests. The model is based upon an impulsive-motion approximation, wherein the motion within the system is produced by rapid acceleration at the boundary, e.g., by a moving piston. A set of quasisteady, linear equations for the pressure field that are decoupled from the leaflet equation of motion is derived. The pressure field and leaflet moment are computed without the need to treat moving boundaries. Model predictions of closing time compared favorably with those measured in a 1994 cavitation study. Computed values of leaflet tip speed were also compared with those of a previous study, at the same value of average pressure slope. The model values were in agreement with measured speeds, given the limitations of using the average pressure slope as a metric for comparison.
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 ≊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 ≊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.
The low-frequency sensitivity of piezoelectric receivers usually is assumed to be represented accurately by the −3-dB cutoff frequency fco=(2πRiCi)−1, where Ri is the loading (e.g., amplifier) resistance and Ci is the total capacitance. For typical needle-type hydrophones such as used in medical ultrasound exposimetry, fco is less than 50 kHz. However, theoretical studies have shown that diffraction effects at the needle tip can cause a low-frequency rolloff in sensitivity at frequencies much higher than that predicted by this simple electrical model. To examine this effect, broadband frequency response measurements of several needle-type hydrophones were made in the frequency range 0.2–2 MHz. The active sensor material was polyvinylidene fluoride, and needle diameters ranged from a few tenths of a millimeter to approximately 1 mm. In all cases the sensitivity decreased with decreasing frequency, with the −3 dB points all lying above 400 kHz. Such behavior calls the use of these devices into question when accurate knowledge of the pressure waveform is required, particularly with regard to measuring the peak rarefactional pressure in pulsed waveforms displaying significant finite amplitude distortion.
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