BLOOD-FLOW COOLING AND ULTRASONIC LESION FORMATION

This article examines lesion formation using focused ultrasound and de monstrates how blood flow may affect lesion dimensions using a theoret ical model. The effects of blood flow on temperature distributions dur ing ultrasonic lesioning are examined for both regional cooling by the microvasculature and localized cooling due to thermally significant v essels. Regional cooling was critically assessed using two models: the Pennes bioheat transfer equation and the scalar effective thermal con ductivity equation. Localized cooling was modeled by adding an advecti ve term in the heat diffusion equation in regions enclosed by thermall y significant vessels. A finite difference approach was used to solve the basic equations of heat transfer in perfused tissues in cylindrica l coordinates. The extent of the lesioned tissue was determined by the accumulated thermal dose at each location. The size of the lesion was then calculated from the boundaries of the thermal isodose curves gen erated by the simulations. The results were compared to published in v ivo lesion data in rat liver. It was shown that even for short ultraso und exposure times (similar to 8 s), blood flow may play an important role in the thermal dose distribution. (C) 1996 American Association o f Physicists in Medicine.