A 3-D finite-element model for computation of temperature profiles and regions of thermal damage during focused ultrasound surgery exposures

Although there have been numerous models implemented for modeling thermal d iffusion effects during focused ultrasound surgery (FUS), most have limited themselves to representing simple situations for which analytical solution s and the use of cylindrical geometries sufficed. For modeling single lesio n formation and the heating patterns from a single exposure, good results w ere achieved in comparison with experimental results for predicting lesion size, shape and location. However, these types of approaches are insufficie nt when considering the heating of multiple sites,vith FUS exposures when t he time interval between exposures is short. In such cases, the heat dissip ation patterns from initial exposures in the lesion array formation can pla y a significant role in the heating patterns for later exposures. Understan ding the effects of adjacent lesion formation, such as this, requires a thr ee-dimensional (3-D) representation of the bioheat equation. Thus, we have developed a 3-D finite-element representation for modeling the thermal diff usion effects during FUS exposures in clinically relevant tissue volumes. T he strength of this approach over past methods is its ability to represent arbitrarily shaped 3-D situations. Initial simulations have allowed calcula tion of the temperature distribution as a function of time for adjacent FUS exposures in excised bovine liver,,vith the individually computed point te mperatures comparing favorably with published measurements. In addition to modeling these temperature distributions, the model was implemented in conj unction with an algorithm for calculating the thermal dose as a way of pred icting lesion shape. Although used extensively in conventional hyperthermia applications, this thermal dose criterion has only been applied in a limit ed number of simulations in FUS for comparison,vith experimental measuremen ts. In this study, simulations were run for focal depths 2 and 3 cm below t he surface of pig's liver, using multiple intensity levels and exposure tim es. The results also compare favorably to published in vitro experimental m easurements, which bodes well for future application to more complex proble ms, such as the modeling of multiple lesion arrays within complex anatomica l geometries, (C) 1998 World Federation for Ultrasound in Medicine & Biolog y.