A nonlinear finite-element program was developed to simulate the dynamic evolution of coagulation in tissue considering temperature and thermal-dose dependence of the ultrasound attenuation and blood perfusion rate. The effects of these dynamic parameters on the lesion formation were investigated in the particular case of ultrasound hepatic ablation with bi-focus intensity pattern. The results of simulations were compared that incorporate dynamic changes of ultrasound attenuation and perfusion and results that neglect these effects. The result shows that thermal-dose-dependent ultrasound attenuation is the dominating factor in the full dynamic model. If the dynamic ultrasound attenuation is ignored, a relatively significant underestimation of the temperature rise appears in the focal plane and the region next to the focal plane, resulting in an underestimation in predicting diameter of coagulation. Higher heating intensity leads to greater underestimation.
A new method for targeted heating of deep tissue was developed by using an ultrasound phased-arraysystem which can generate various multiple foci patterns by electronically changing its amplitude or phasepattern.This method involves using a technique of combining switching and rotating of multiple foci pat-terns to create a uniform temperature over tissue volumes in various size.Using this method,the targettissue deep in the body can be heated to a specified temperature,which gives conditions for thermo-sensi-tire liposomes release.A simulation study for a 108-element,spherically sectioned array was performed todetermine an optimal heating scheme from a set of multiple focus fields which were produced by inputtingdifferent combinations of phases and amplitudes.Comparisons of a static multiple foci field,the switchedfields and the switched-rotated fields indicated that the technique of combining switching and rotating ofmultiple foci patterns has advantages of both lowering the peak temperature and evening the temperaturedistribution.The simulation results also show that the therapeutic heating zones in various size(φ5mm～φ40mm)with uniform temperature distributions can be obtained employing the combined method.Theseresults offer significant data for designing thermotherapy equipment for tumor-specific drug release withthermo-sensitive hposomes.
