The intensity dependence of lesion position shift during focused ultrasound surgery

Knowledge of the spatial distribution of intensity loss from an ultrasonic beam is critical for predicting lesion formation in focused ultrasound (US) surgery (FUS). To date, most models have used linear propagation models to predict intensity profiles required to compute the temporally varying temp erature distributions used to compute thermal dose contours. These are used to predict the extent of thermal damage. However, these simulations fail t o describe adequately the abnormal lesion formation behaviour observed duri ng ex vivo experiments in cases for which the transducer drive levels are v aried over a wide range, In such experiments, the extent of thermal damage has been observed to move significantly closer to the transducer with incre ased transducer drive levels than would be predicted using linear-propagati on models. The first set of simulations described herein use the KZK (Khokh lov-Zabolotskaya-Kuznetsov) nonlinear propagation model with the parabolic approximation for highly focused US waves to demonstrate that both the peak intensity and the lesion positions do, indeed, move closer to the transduc er, This illustrates that, for accurate modelling of heating during FUS, no nlinear effects should be considered. Additionally, a first order approxima tion has been employed that attempts to account for the abnormal heat depos ition distributions that accompany high transducer drive level FUS exposure s where cavitation and boiling may be present, The results of these simulat ions are presented. It is suggested that this type of approach may be a use ful tool in understanding thermal damage mechanisms, (C) 2000 World Federat ion for Ultrasound in Medicine & Biology.