A thermal washout technique is capable of measuring perfusion from the tissue surface. A cold applicator is held on the tissue for 60 seconds. During this time the tissue surface is cooled. The applicator is removed, and a thermal camera is used to monitor the rewarming of the tissue surface. Since blood flow significantly affects local heat transfer, this technique is quite senstive. Tissue with a high perfusion can sink more heat, hence will have a smaller initial temperature change and a faster recovery back to baseline.
Heat transfer analysis has been performed using the finite element method. It is critical that the boundary and initial conditions match the actual experimental coonditions. The numerical studies were used to determine the relationship betwen perfusion and the measured temperature.
A thermistor based thermometer system was used in the preliminary in vitro experiments. The water-cooled copper-plated cold applicator includes a microthermistor which measures the surface temperature during cooling. Experiments in agar-gelled water were performed to optimize the applicator design and to validate the numerical model.
Preliminary in vivo experiments were performed on rat livers. Spatial surface temperatures were measureed with a calibrated thermal camera. A few of the disturbing factors include: Uncertainity of the temperature measurement; uncertainity of the time meaurement; baseline temperature gradients; and perfusion gradients.
J. W. Valvano, S. A. Prahl, J. C. Chan, and J. A. Pearce. Thermal camera imaging to measure tissue blood flow. In Sixth Southern Biomedical Engineering Conference, Dallas, TX, 1987.