THE INTENSITY DEPENDENCE OF THE SITE OF MAXIMAL ENERGY DEPOSITION IN FOCUSED ULTRASOUND SURGERY

The relationship between spatial peak intensity and the position of ul trasound induced tissue damage was studied in in vitro tissue models, using a 1.69 MHz spherical bowl transducer, The models corresponded to the transabdominal route to the bladder and prostate, which are poten tial target sites for focused ultrasound surgery, The results confirm that there is a relationship between lesion position and intensity, wi th lesions forming, under some exposure conditions, ahead of the geome tric focus, Forward growth of lesions appears to be due to changes in the absorption characteristics of the tissue in the beam path, Using a computer model, we have demonstrated that the absorption coefficient of the tissue must increase significantly in front of the focus to ena ble lesions to form ahead of the predicted position, A possible mechan ism for this is bubble formation as a result of acoustic cavitation, T he effect of nonlinear propagation in the tissue, at the intensities s tudied, is shown to be relatively small.