A Self-Heated Thermistor Technique to Measure Effective Thermal Properties From the Tissue Surface

Abstract

A microcomputer based instrument to measure effective thermal conductivity and diffusivity at the surface of a tissue has been developed. Self-heated spherical thermistors, partially embedded in an insulator, are used to simultaneously heat tissue and measure the resulting temperature rise. The temperature increase of the thermistor for a given applied power is a function of the combined thermal properties of the insulator, the thermistor, and the tissue. Once the probe is calibrated, the instrument accurately measures the thermal properties of tissue. Conductivity measurements are accurate to 2 percent and diffusivity measurements are accurate to 4 percent. A simplified bioheat equation is used which assumes the effective tissue thermal conductivity is a linear function of perfusion. Since tissue blood flow strongly affects heat transfer, the surface thermistor probe is quite sensitive to perfusion.

Multimodal imaging of cutaneous wound tissue

“…17,18 Thermography has been used to identify latent inflammation in diabetic feet and pressure ulcers where the higher temperature in the wound site may imply the presence of inflammation, critical colonization, or other factors which disturb the wound healing. 18,19 Although various thermological techniques have been used to detect temperature distribution and thermal properties of biologic tissue, 20,21 it is still difficult to quantitatively correlate wound tissue temperature change with inflammation because other pathophysiologic conditions, such as perfusion, infection, and metabolism, also contribute to tissue temperature fluctuations. In summary, oxygen saturation, perfusion, and inflammation are important tissue parameters associated with successful wound healing.…”

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

“…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.…”

“…Some experimental data can be best described by the Pennes model (Valvano 1984), while other data supports the KEFF model (Patel 1987, Anderson 1988, Anderson 1989a). This paper examines the specific case of perfusion measured in the canine kidney cortex.…”

“…Patel (1987) and Roemer (1989) proposed an effective conductivity models to describe the heat transfer from a thermistor due to perfusion. In Patel's model the perfusion term is approximated by:…”

“…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.…”