Numerical study of the effects of large blood vessels on three-dimensional tissue temperature profiles during cryosurgery

摘要

Large blood vessels can produce steep temperature gradients in frozen tissues, resulting in inadequate cooling temperatures during cryosurgery. In addition, blocking of blood vessels and/or bleeding due to ruptures of large blood vessels by the iceball during the cryoablation procedure may cause undesired damage to healthy tissues or organs. However, such important issues have received little attention up to now. In this article, several typical vascular models, which have been widely used in simulation of tissue temperature during tumor hyperthermia, are applied to study the effects of large blood vessels on the transient tissue temperature distributions during cryosurgery treatment. The thermal model combines the Pennes bioheat transfer equation describing perfused tissues and the energy equation for single or countercurrent large blood vessels with a constant Nusselt number. A finite-difference algorithm based on the effective heat capacity method is applied to solve these complex heat transfer problems with phase change in biological tissues embedded with large blood vessels. In the algorithm, the tissues are treated as nonideal materials, freezing over a temperature range, and the effects of blood perfusion and metabolic heat generation in the unfrozen tissues are also included. Numerical analyses are then performed to test the influence of the blood vessels on the temperature distributions of tissues. The results indicate that different vascular models produce significantly different temperature transients for a given freezing pattern. Therefore, without careful treatment planning on some specific tumors close to or with large vessels transmitting through, the final cryosurgery may turn out to fail. In other words, insufficient cooling of the targets due to heating of large and warm blood vessels may lead to the regeneration of tumor cells. This study has raised quite a few important issues in modeling the cryosurgical phase-change behavior of living tissues embedded with large blood vessels.