טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentRybko Noga
SubjectExperimental Simulation of a Thermally Significant Blood
Vessel in a Tissue Undergoing Freezing
DepartmentDepartment of Mechanical Engineering
Supervisors Professor Emeritus Avraham Shitzer
Professor Emeritus David Degani
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

This study relates to cryosurgery, which is a medical technique used for destroying malignant tissues by the application of localized freezing. The treatment is performed by a cryoprobe, which may be placed either in contact with the tissue surface, or inserted into the targeted volume.

Thermal perturbations inside the treated region may distort the temperature field. Thus, understanding the thermal behavior of the freezing medium, in which a thermally significant blood vessel is located, is essential.

The purpose of this study is to investigate experimentally the influence of a thermally significant blood vessel on a tissue phantom, while it was being frozen. A 0.2% mass concentration Agar-Bacto/water solution was used to simulate the tissue thermal behavior. Freezing was performed by a surface cryoprobe. Tracking the temperature field was performed by 33 Chromel-Alumel thermocouples, placed inside the medium in four cross-sections. The blood vessel was simulated by a 6.0/5.4mm OD/ID brass tube, with 7 additional thermocouples inserted inside the tube.

The experiments were conducted under several water flow rates in the tube. The cooling rates under the cryoprobe varied from -4°C/min to -10°C/min. Each experiment was performed at the same initial and boundary conditions, which were achieved by circulating water at a uniform temperature prior to the initiation of the experiment.

The experimental results show the effects of the tube on the propagating frozen region beneath the cryoprobe. These effects are noticed as a hot region beneath the tube, in areas that are far from the cryoprobe. The area that reaches temperatures lower than -40°C in the main cross-section, in the experiment without flow in the tube, occupied some 30% of the total frozen area. This area shrank to only 20% of the frozen area in experiments with flow in the tube. At a flow rate of 30 ml/min, partial freezing occurred inside the tube. In the experiment without flow in the tube, the tendency of the frozen region to form a spherical-like ice ball was observed, and the stagnant water in the tube froze completely. At a cooling rate of -9°C/min, the area with temperatures lower than -40°C was 20% of the frozen area, and decreased to 15% when the cooling rate decreased.

In future studies, the use of a tube with thermo-physical properties similar to those of biological tissues, and a fluid which better simulates the properties of blood, are recommended.