A technique for measuring the thermal conductivity and evaluating the "apparent conductivity" concept in biomaterials

Noninvasive method for simultaneously measuring the thermophysical properties and blood perfusion in cylindrically shaped living tissues

An easy-to-use noninvasive method was developed to simultaneously measure the thermophysical parameters and blood perfusion in cylindrically shaped living tissues. This method is based on a two-dimensional mathematical model which requires temperature measurements at only three separate points along the axial direction on the cylinder surface. A sensitivity analysis has shown that the key thermophysical parameters, such as the thermal conductivity, volumetric heat capacity, and blood perfusion can be estimated simultaneously with high accuracy. Genetic algorithm (GA) selection, crossover, and mutation operators were developed to solve this multi-parameter optimization problem. This three-point method was validated by measuring the properties of a dynamic tissue-equivalent phantom with known thermal parameters. The method has also been applied to measure the thermophysical parameters and blood perfusion in human forearms with measured results agreeing well with the literature values.

Sinusoidal heating method to noninvasively measure tissue perfusion

Noninvasively measuring the tissue blood perfusion has been an important however difficult problem in the biomedical engineering field. Based on the newly developed phase-shift principle, an improved sinusoidal heating method to estimate the perfusion was proposed in this paper to replace the original heating algorithm. The phase shift between the sinusoidal heat flux and the surface temperature response was both theoretically and experimentally revealed to be a time-dependent value which however will approach a constant value after a sufficiently long time. Only using this constant phase shift can the perfusion be properly estimated. Following the theory, an instrument consisting of low-frequency sinusoidal signal generator, power amplifier, heating plate, temperature and heat flux monitoring unit, as well as the data-acquisition system was carefully constructed. It allows generating a high-quality sinusoidal heat flux whose frequency and magnitude can be easily regulated. An auxiliary heat-conducting plate was introduced to simultaneously measure the surface temperature and the heat flux, which are hard to do otherwise. Experiments on human bodies were performed and the forearm perfusion was estimated and then validated through a constant surface heating experiment. Several issues related to the instrument integration and perfusion measurement were discussed. The instrument was also tested through experiments on nonperfused materials and good results were obtained. These efforts will help to build a compact device for noninvasively measuring the human perfusion, which may have significant applications in future clinics.

Estimation of blood perfusion using phase shift in temperature response to sinusoidal heating at the skin surface

A closed form analytical solution of the Pennes' bio-heat equation was obtained for temperature distributions in the skin tissue subject to the sinusoidal heat flux. Phase shifts in the surface temperature response were revealed to be related to local blood perfusion rate and heating frequency. The influence of the thermal contact resistance on the perfusion estimation was investigated. It has been proved that this influence is relatively small because of the phase shift based estimation and can be effectively eliminated by application of highly conductive grease. This analysis provides the theoretical foundation for a new noninvasive modality of blood perfusion estimation based on the surface temperature measurement which can have significant applications in future clinical practices.