Application to Therapeutic Ultrasound

Studies of pressure fields generated by commercial lithotripsy devices [49-56] have demonstrated the benefits of PVDF sensors in terms of measured pulse fidelity over conventional quartz or tourmaline ballistics pressure transducers. These latter transducers, while robust, have resonant frequencies less than 1 MHz and element dimensions of several millimeters [57, 58]. 

For the work described in [49,50,52], bilaminar membrane hydrophones having a total thickness of 50 pm (2 x 25 pm) and active diameters of 1 mm or less were used. These hydrophones provided generally faithful reproductions of the liihotripsy pressure pulses. However, three potential problems were identified. In [52] an artifact was observed in the hydrophone waveform due to reflections at the hydrophone housing. In most cases this delayed disturbance would not interfere with the analysis of the main pulse detected by the 

Two other possible problems were discussed in [50]. First, the thickness resonance of 24 MHz has the potential of causing errors in the measmement of rise time and peak compressional pressure, a situation dealt with theoretically in [63]. However, a comparison check with an 18 pm thick bilaminar hydrophone showed no significant differences for a rise time of approximately 30 ns and peak pressure of 40 MPa. The second problem was the aforementioned issue of hydrophone damage in the form of pitting of the metal electrodes and leads, attributed to the action of cavitation in the water measmement medium. While the performance of the hydrophone was not affected during the comse of the study, it was concluded that eventually the hydrophone would have failed. Regarding cavitation, it has been noted that measurement of the negative pressme portion of the lithotripsy pulse may be affected by bubble formation at the smface of the hydrophone [62, 64]. 

Several modifications to the basic spot poled membrane design have been developed to try to circumvent the damage problem. In one approach, unmetallized PVDF film was spot poled and mounted in an enclosed chamber with an acoustic window. The chamber was filled with either a dielectric fluid [65,66] or low resistivity electrolyte [67,68]. Electrodes were located in the chamber away from the film, close enough to collect the pressure-induced charge, via capacitive coupling in the dielectric fluid case, but far enough away to minimize damage from the shock waves. 

For in vivo measurements of lithotripsy pulses, both needle and membrane designs have been used. In one study, pressure waveforms in pigs were obtained using the needle probe described in [59, 60] and a commercial lithotripsy system [78]. Measurements were made at various axial distances and the results were compared to in vitro data. Two modifications of the single sheet, spot poled membrane design also have been reported. In one, previously metallized and spot poled PVDF film was formed 

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