M.Sc Thesis

M.Sc StudentMagalov Zaur
SubjectInvestigation of the Temperature Field Developed by
Simultaneously Operating Cryosurgical Probes
Embedded in a Phase-changing Medium
DepartmentDepartment of Mechanical Engineering
Supervisors PROFESSOR EMERITUS Avraham Shitzer


   Cryosurgery is a common medical technique involving the application of sub-zero temperatures to destroy abnormal tissues. The instrument used for this purpose is a cryoprobe, which is a metal tube with a conical tip, which facilitates insertion into the patient's body. Cryoprobe cooling is produced either by boiling of liquid nitrogen (liquid to gaseous state) or by adiabatic expansion of a compressed gas through a restricting orifice (Joule-Thomson effect).

   A numerical solution of the heat transfer problem in a gel simulating a biological tissue, is obtained by using a commercial finite elements program, ANSYS7.0. The numerical model provided by ANSYS was tested against two problems for which there are known analytical solutions. The first problem refers to half-space solidification and the other to axi-symmetric heat conduction in a hollow cylinder. Both numerical solutions were found to be in good agreement with the analytical solutions.

   The transient temperature field developed by simultaneously operating up to 3 cryosurgical probes embedded in a phase-changing gel was studied numerically. The 1.5 mm OD probes were operated by a high pressure Argon gas and were placed at 5x5 mm configurations. Temperature variations at one location on the surface of the active part of the cryoprobe tips were monitored by thermocouples and were used as the boundary condition in the analysis. Frozen front propagation was found to be fastest in the radial direction. In all cases the -40ºC isotherm, which is considered in the literature to be the lethal temperature for prostate cell destruction, was found to reach its maximal volume rather quickly. This volume, however, was relatively small in comparison to that of the 0ºC isothermal surface. In the one-, two- and three-probe applications the -40ºC isotherm attained, after 30 minutes, about 3%, 3% - 6% and 3% - 8 %, respectively, of the total frozen volume, depending on probe placement configurations.

   Temperature fields around the cryoprobe were experimentally measured (in a separate study) by 2 mm spaced thermocouples in three directions: radial, downward axial and upward axial. Agreement between calculated and measured results in the radial direction at points close to the probe's surface was generally good. In the axial directions the agreement between measured and calculated results was not so good. The temperature at the probe's surface was experimentally measured at one point by a soldered thermocouple but the exact temperature distribution on the cryoprobe's surface is unknown.

   Results of this study should be useful in the design of probe placement and operation strategies and in understanding the limitations of the freezing-ablation process.