The thermal camera has been successfully applied in a wide range of biomedical thermal studies. It is possible to use these cameras for both qualitative and quantitative purposes. The feasibility of quantitatively mapping perfusion using thermal imaging is under current investigation. The high spatial and temporal resolution of the thermal camera make it ideal for studying epicardial perfusion. It is possible to determine the amount of systolic epicardial perfusion which originates from deeper tissues relative to the amount from the epicardial coronaries.
A cooled disk of temperature T0 is applied to the epicardial surface initially at Ti for t0 seconds. This results in a reduction of myocardial temperature. The disk is then removed and the epicardial temperature is monitored for 70 seconds using a thermal camera. When the disk is removed, the tissue warms through both conductive and convective (perfusion) processes. An analytic model which treats the tissue as a semi-infinite medium with constant perfusion has been developed to predict perfusion w, from thermal camera measurements of the epicardium. In the absence of perfusion, the surface temperature is
which is independent of thermal diffusivity $\alpha$, the thermal conductivity k, and the specific heat c. In the presence of perfusion, the surface temperature is
Perfusion can be predicted by calculating: T(t)=T(t;w)-T(t;w=0). The figure below plots T versus time for various perfusion rates with t0=60 sec, Ti=37°C, and T0=27°C.
A limitation of the analytical model is that perfusion was assumed constant both in space an time. In reality, perfusion varies through the cardiac cycle, varies from the epicardium to the endocardium, and could vary spatially over the surface of the myocardium. Thermal conduction will limit the spatial and temporal resolution of perfusion measurements. Another challenging aspect is that the analytical solutions show that the perfusion signal is on the order of 0.2°C, which is approximately the noise level of the thermal camera. Thus this technique will rely on temporal as well as spatial data averaging. With such processing, the recovered temperature histories, calculated from sequential thermal images, gives a two-dimensional map of the epicardial perfusion field.
This research was funded in part by the Whitaker Foundation.
A. F. Badeau, S. A. Prahl, and J. W. Valvano. Thermal camera imaging to measure perfusion of the epicardium. Los Angeles, 1987.