THE CAVITATION THRESHOLD OF HUMAN TISSUE EXPOSED TO 0.2-MHZ PULSED ULTRASOUND - PRELIMINARY MEASUREMENTS BASED ON A STUDY OF CLINICAL LITHOTRIPSY

Evidence of acoustic cavitation was identified in the form of transien t echoes in ultrasound B-scan images of patients receiving extracorpor eal shock-wave lithotripsy treatment on a Storz Modulith SL20. This li thotripter generates 10-mu s duration pulses with a centre frequency o f 0.2 MHz at a pulse repetition frequency of 1 Hz. The visual appearan ce of B-scan images was examined in a total of 30 patients and a quant itative analysis of echogenicity changes was carried out in six cases involving lithotripsy treatment of stones in the renal pelvis. In thes e patients new echoes were identified in images unaffected by movement artefacts and were found to occur in perinephric fat and adjacent mus cle and kidney tissue at positions close to the axis of the shock-wave field between 1 and 2 cm in advance of the indicated beam focus of th e lithotripter. The echogenicity within each region increased signific antly above the background level when the output of the lithotripter w as increased above a threshold value. The acoustic pressures correspon ding to this threshold were measured in water using a calibrated PVDF membrane hydrophone. After correction for attenuation in tissue the ca vitation thresholds, in terms of the temporal peak negative pressure, are found to lie between 1.5 MPa and 3.5 MPa in all six cases. Interpr etation of the measured values in terms of the likely threshold at the higher frequencies used in diagnostic ultrasound is considered using a theoretical model.