M.Sc Thesis

M.Sc StudentCohen Joseph
SubjectAnalysis of the 3 Dimensional Temperature Field Around
Cryoprobes Inserted into a Malignant Tumor
DepartmentDepartment of Mechanical Engineering
Supervisor PROFESSOR EMERITUS Avraham Shitzer
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


This study was carried out as an extension of studies into cryo-surgery conducted over recent years. The main purpose of cryo-surgery is the total eradication of malignant and benign tumor. No less important is the minimization of damage to the surrounding healthy tissues. Destruction may be achieved by either processes: immediate or delayed. The immediate process is dominated by the formation of intracellular ice generated at high cooling rates which involves direct destruction of cells. The delayed process occurs at low cooling rates, which invoke the appearance of extracellular ice due to osmotic drying of the cells.

Cryo-surgery is applied by inserting a needle-like cryoprobe into the tumor. The cryoprobe is cooled by either the boiling of liquid nitrogen or by the expansion of a compressed gas, e.g., Argon thus inducing a sharp drop in temperature in the tumor and surrounding tissues.

Precise information of temperature changes in the tumor is a critical aspect during the cryosurgical procedure. Contemporary medical practices combine imaging tools with computerized models which can map the thermal variations occurring inside the tumor (CT, MRI, US). The first two are expensive and cumbersome for online use in cryo-surgery. Hence Ultrasound (US) is employed for online tumor mapping.  Its advantages are mobility, short operation times, and low-cost operation. The disadvantage of US is its inability to map the thermal field inside the frozen tissue, while reflecting only the contour of the frozen tissue. This problem hinders the surgeon from ascertaining the total eradication of the tumor, since cell destruction temperature (around -40°C) is considerably below the tissue's freezing temperature (around 0°C). In this study models were developed by three-dimensional CAD software, and analyzed by finite elements software (Ansys).

The calculations were based on the Pennes bioheat equation, which is applicable to tissue regions with dense capillary networks and low blood perfusion rates.

The first stage of the analysis involved comparison of results derived with conditions borrowed from a previous study yielding good conformities. Subsequently, analyses with different thermal effects of blood perfusion rates were conducted. Results indicated that its effects on lower than -20°C isotherms are negligible. For blood flowrates lower than 1% of nominal its effects are negligible for all generated isotherms. Another test of effects related to different temperature forcing functions along the cryoprobe. No significant differences were observed, mainly because the final temperatures on the cryoprobe were identical among all tested curves.

The second stage examined the interaction among two and three cryoprobes operated uniformly placed at different distance configurations (horizontal and vertical). Additionally, a non-uniform activation of cryoprobes was analyzed for the three probes configurations. It was observed in all studies with two cryoprobes that the freezing front expanded and merged as far as the horizontal distances were increased. Longer times were required for the lethal temperature isotherms to be generated and to merge. Nevertheless, for different insertion depths there seems to exist an optimal placing configuration which combines the horizontal distance and the relative insertion depth.