LESION DEVELOPMENT IN FOCUSED ULTRASOUND SURGERY - A GENERAL-MODEL

An analytical model has been constructed for the process of formation of thermal lesions in tissue, resulting from exposure to intense, high ly focused ultrasound beams such as may be used in minimally invasive surgery. The model assumes a Gaussian approximation to beam shape in t he focal region and predicts, for any such focal beam, the time delay to initiation of a lesion and the subsequent time course of growth of that lesion in lateral and axial dimensions, taking into account the e ffects of thermal diffusion and blood perfusion. The necessary approxi mations and assumptions of the model are considered. Comparison of pre dictions with experimentally measured data on excised pig liver indica te generally good agreement. Comparisons are also made of this theory with previously published data on exposure-time dependence of lesionin g threshold intensity. Deficiencies are identified in existing practic e for measuring and reporting acoustic exposures for focused ultrasoun d surgery, and the proposal is therefore made that a quantity that wou ld be more satisfactory, from the viewpoints both of metrology and bio physical relevance, is the intensity spatially averaged over the area enclosed by the half-pressure-maximum contour in the focal plane, as d etermined under linear conditions, provisionally denoted as I(SAL)