Experimental verification of bioheat transfer theories: measurement of temperature profiles around large artificial vessels in perfused tissue

Abstract

The verification of thermal models for use in hyperthermia treatment planning is essential. We investigated the heat transfer between a single vessel and the surrounding vascularised tissue, comparing the conventional bioheat transfer theory and the recently developed keff model using analytical and numerical methods. A plastic tube inserted into the tissue of an isolated perfused organ served as an artificial vessel. This enabled us to vary the blood flow in the vessel and in the tissue independently. The organ used was a bovine kidney, turned into a perfused tissue phantom using an alcohol fixation technique. The temperature profile within the tissue was mapped with constantan-manganin thermocouple wire sensors with a total diameter of 50 microns. The temperature profile relative to the temperature difference between the vessel and organ was measured; increased perfusion caused a reduction of the vessel wall temperature but did not affect the width of the profile. Studying the transient tissue temperature after a step-wise change of the blood temperature in the vessel revealed a faster diffusion of heat at higher perfusion rates. These facts are in accordance with the keff model, but not with the conventional heat-sink theory.

Tissue thermal conductivity by magnetic resonance thermometry and focused ultrasound heating

“…Consistent in vivo estimation of conductivity is demonstrated in the measurements made in the rabbit thigh muscle. The results show small variations in tissue conductivity, with and without perfusion, and are in stark contrast to previous studies (18), where a maximum sixfold increase in apparent conductivity was attributed to perfusion.…”

“…In the case where perfusion w b ϭ 0 and the heat source Q is an impulse in either 3D or 2D and in time, it can be shown that the resulting temperature on the plane orthogonal to the axis of the heat source is a 2D Gaussian distribution expanding at a rate proportional to k t , the conductivity. This notion has been used previously (12,18) to characterize the effects of perfusion. Specifically, it was assumed that both conductivity and perfusion influenced the expansion rate, and the notion of an effective conductivity measure, k eff , which captured information about both k t and w b , was coined:…”

“…In the work of Crezee and Lagendijk (18), k eff was derived for varying perfusion levels applied in excised bovine kidney, from which a coefficient of ␤ ϭ 720 (g/cm 3 / second) -1 was obtained (assuming 1.0 g/cm 3 for tissue and blood densities). By this method, k eff ϭ k t for zero perfusion, and one can expect k eff to increase 2.2-fold due to microcirculatory perfusion in resting muscle (w b ϳ 0.00167 g/cm 3 /second (23)) and over 50-fold in kidney (w b ϳ 0.07 g/cm 3 /second (24)).…”

“…In order to estimate perfusion based on the BHT, the conductivity is assumed to be either negligible or known (4,5,8,9,(15)(16)(17). Another approach is to lump conductivity and perfusion into a single mea-sure, known as the "effective conductivity," from which the estimation of perfusion is attempted (12,18). In all these cases, accurate perfusion estimation is hampered by inaccurate representation of conduction effects due to coarse spatial sampling.…”

Local heating of human skin by millimeter waves: Effect of blood flow

“…Song et al [1997] experimentally showed a quadratic dependence of k eff on f b in muscle confirming that the WJE for k eff was valid. However, our skin measurements were in agreement with other literature data showing a linear relationship between k eff and f b [Crezee and Lagendijk, 1990;Ducharme and Tikuisis, 1991;Crezee et al, 1994].…”