Self-heated thermistor measurements of perfusion

Abstract

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.

A Small Artery Heat Transfer Model for Self-Heated Thermistor Measurements of Perfusion in the Kidney Cortex

“…Published experimental data agree with this prediction (Anderson 1987(Anderson , 1989a(Anderson , 1989b(Anderson , 1992, although the extent of the upswing in k ss slope varies greatly between different measurements. One explanation for this might be the distance between the thermistor and the arteries and veins will vary depending on where the thermistor is placed in the kidney.…”

“…Analytical and experimental studies show that k ss depends on tissue thermal conductivity and perfusion (Balasubramaniam 1977, Valvano 1984, Patel 1987, Anderson 1992. In these experiments, k ss is either linearly related to the perfusion rate or has an increasing slope with increasing perfusion rate.…”

“…The experimental data was measured in an alcohol-fixed canine kidney with a P60 thermistor (Anderson 1989a(Anderson , 1989b(Anderson , 1992. where ∆ is arbitrarily chosen to fit the experimental data.…”

“…Experimental results in alcoholfixed canine kidneys have shown that k sin will be independent of perfusion rate if the sinusoidal heating frequency is fast enough (Anderson 1988(Anderson , 1989a(Anderson , 1989b(Anderson , 1992. k sin measured with thermistors of 0.25 and 0.076 cm diameters heated with a sinusoidal period of 20 seconds is independent of perfusion rate.…”

“…Self-heated thermistor techniques to measure perfusion have been investigated in recent years (Chato 1968, Balasubramaniam 1977, Holmes 1980, Valvano 1984, Patel 1987, Anderson 1992 , Holmes 1980, and Xu 1991), a KEFF model , Roemer 1987, Patel 1987 and the Weinbaum-Jiji model (Weinbaum 1985, Song 1987, Valvano 1990, Charny 1990, Xu 1991, Charny 1992.…”