An investigation of the flow dependence of temperature gradients near large vessels during steady state and transient tissue heating

Temperature distributions measured during thermal therapy are a major progn ostic factor of the efficacy and success of the procedure. Thermal models a re used to predict the temperature elevation of tissues during heating. The oretical work has shown that blood flow through large blood vessels plays a n important role in determining temperature profiles of heated tissues. In this paper, an experimental investigation of the effects of large vessels o n the temperature distribution of heated tissue is performed. The blood Bow dependence of steady state and transient temperature profiles created by a cylindrical conductive heat source and an ultrasound transducer were exami ned using a fixed porcine kidney as a flow model. In the transient experime nts, a 20 s pulse of hot water, 30 degrees C above ambient, heated the tiss ues. Temperatures were measured at selected locations in steps of 0.1 mm. I t was observed that vessels could either heat or cool tissues depending on the orientation of the vascular geometry with respect to the heat source an d that these effects are a function of how rate through the vessels. Temper ature gradients of 6 degrees C mm(-1) close to large vessels were routinely measured. Furthermore, it was observed that the temperature gradients caus ed by large vessels depended on whether the heating source was highly local ized (i.e. a hot needle) or more distributed (i.e. external ultrasound). Th e gradients measured near large vessels during localized heating were betwe en two and three times greater than the gradients measured during ultrasoun d heating at the same location, for comparable hows. Moreover, these gradie nts were more sensitive to Bow variations for the localized needle heating. X-ray computed tomography data of the kidney vasculature were in good spat ial agreement with the locations of all of the temperature variations measu red. The three-dimensional vessel path observed could account for the compl ex features of the temperature profiles. The Bow dependences of the transie nt temperature profiles near large vessels during the pulsed experiments we re consistent with the temperature distributions measured in the steady sta te experiments and provided unique insights into the process of convective heat transfer in tissues. Finally, it was shown that even for very short tr eatment times (3-20 s), large vessels had significant effects on the tissue temperature distributions.