Self-heated thermistor measurements of perfusion.

A microcomputer-based control system applies a combination of steady state and sinusoidal power to a thermistor probe which is inserted into the tissue of interest. The steady-state temperature response is an indication of the effective thermal conductivity (keff), which includes a component due to intrinsic conduction plus a convective component due to the tissue blood flow near the probe. By careful choice of the excitation frequency, the sinusoidal temperature response can be used to measure intrinsic thermal conductivity (km) in the presence of blood flow. Optimal sinusoidal heating frequency depends on the thermistor size. Experimental results in the alcohol-fixed canine kidney cortex show that perfusion is linearly related to the difference keff minus km. The instrument can measure tissue thermal conductivity with an accuracy of 2%. The instrument can resolve changes in perfusion of 10 mL/100g-min with a Thermometrics P60DA102M thermistor. The maximum error in measured perfusion is about 30%. When tissue trauma due to probe insertion is minimized, the self-heated thermistor method gives a reliable indication of local tissue blood flow.