Pilot study on cryogenic heat transfer in biological tissues embedded with large blood vessels

摘要

Blood flow through large vessel plays an important role in affecting the temperature profiles of the living tissues under cryosurgery. Besides, arresting of blood vessels due to freezing may possibly cause danger to the patient, which needs to be considered when operating the cryoprobe. However, such important issues received few attentions in the bioheat field even up to date. In this paper, pilot studies were performed to investigate the cryogenic heat transfer behaviors in biological tissues embedded with large blood vessels. First, a simple however intuitive theoretical model was established and then analytically solved. Parametric studies were performed to test the influences of the blood vessel entrance temperature, the vessel diameter, the blood flow velocity and the vessel length etc. to the whole region's temperature distribution. The critical tissue surface temperature to freeze the blood vessel was theoretically predicted. Second, to reveal the role of the countercurrent artery-vein blood flows to the transient phase change in living tissues subject to freezing, qualitative simulating experiments on phantom gel were performed. A 3 cm-diameter, 14 cm-length cylindrical copper block pre-frozen by liquid nitrogen was applied to freeze the gel embedded with two parallel countercurrent 1.1 mm OD/0.8 mm ID Teflon tubes with 30℃ warm water flowing through at the velocity of 0.1 m/s. Temperatures were measured at the selected positions on the tube wall in a step of 2 cm. As a comparison, experiments were also conducted on the same gel without running warm water. It was demonstrated that the countercurrent water flow has significant effect on the freezing progress of the phantom gel in comparison with that of non-flow gel. This study raised an important issue to study the phase change heat transfer of blood vessels to the living tissues subject to cryosurgery, which may have significant clinical applications. The present method can also possibly be extended to wider fields such as heat transfer in buried pipes and collectors.