AN INTEGRATED MODEL OF HEAT TRANSFER AND TISSUE FREEZING FOR CRYOSURGERY USING CRYO-SPRAY OR CRYOPROBE

Cryosurgery is a novel surgical technique using low temperature and tissue freezing to destroy undesired tissue, especially cancerous tissue. A numerical model of cryosurgery is highly desirable in clinic practice to predict target tissue thermal history, assess treatment outcome and optimize cryosurgical protocol. The present dissertation dedicates to develop an integrated model addressing multiple complex physics involved in cryosurgery, including fluid flow, bioheat transfer and tissue freezing. Extensive and in-depth numerical studies have been conducted for two typical yet distinct cryosurgeries: cutaneous cryosurgery (using liquid nitrogen spray) and hepatic cryosurgery (using liquid nitrogen cryoprobe). The modeling of cutaneous cryosurgery starts from treating target skin tissue as a homogeneous media. The anatomic structure of human skin tissue is then incorporated by considering three sub-layers: epidermis, dermis and subcutaneous fat. Bioheat transfer during the cryosurgery is described by classic Pennes Equation, while critical modifications are made on the thermal effects of blood perfusion and metabolism according to the physiological characteristics of each sub-layer. The model is then applied to study a general cutaneous cryosurgery and a specific clinic case: the cryosurgery on nodular basal cell carcinoma (BCC). A methodology for quantitatively determining the extent of intracellular ice formation (IIF) based on the tissue temperature and cooling rate (CR) is proposed, followed by systematic parametric study to investigate the effect of various protocol parameters on the final IIF zone. These results contribute to support dermatologists to estimate the treatment outcome in cryosurgery pretreatment planning. The modeling of hepatic cryosurgery emphasis the heat interaction between the fluid flow of LN2 within a cryoprobe and the surrounding tissue. Laminar flow model and turbulent flow model are all validated and applied to study the effect of LN2 flow on the heat transfer characteristic of the cryoprobe. A conjugate model is eventually developed, which simultaneously addresses the turbulent flow of LN2 within the cryoprobe, bioheat transfer in surrounding hepatic tissue and subsequent tissue freezing. Parametric study is also conducted to investigate the impact of LN2 flow on tissue freezing.