A transient heating technique for the measurement of thermal properties of perfused biological tissue

Knowledge of tissue thermal transport properties is imperative for any therapeutic medical tool which employs the localized application of heat to perfused biological tissue. In this study, several techniques are proposed to measure local tissue thermal diffusion by heating with a focused ultrasound field. Transient as well as near steady-state heat inputs are discussed and examined for their suitability as a measurement technique for either tissue thermal diffusivity or perfusion rate. It is shown that steady-state methods are better suited for the measurement of perfusion; however the uncertainty in the perfusion measurement is directly related to knowledge of the tissue's intrinsic thermal diffusivity. Results are presented for a transient thermal pulse technique for the measurement of the thermal diffusivity of perfused and nonperfused tissues, in vitro and in vivo. Measurements conducted in plexiglas, animal muscle, kidney and brain concur with tabulated values and show a scatter from 5-15 percent from the mean; measurements made in perfused muscle and brain compare well with the nonperfused values. An estimate of the error introduced by the effect of perfusion shows that except for highly perfused kidney tissue the effect of perfusion is less than the experimental scatter. This validation of the tissue heat transfer model will allow its eventual extension to the simultaneous measurement of local tissue thermal diffusivity and perfusion.