Thermal diffusion probes for tissue blood flow measurements

Abstract

A minimally invasive instrumentation to perform real-time measurements of temperature, thermal conductivity and tissue blood flow, has been designed for experimental researches and clinical uses. The essence of the probes is a thermistor located at the tip of mounting shafts (glass or steel needles, or catheters; 0.8 mm o.d.). The transient method is used with a self-heated thermistor at constant temperature increment. Tissue blood flow and thermal conductivity are determined by use of a coupled tissue-probe thermal model. Effects of baseline temperature shifts are minimized by an analog compensation. Very short heating and cooling periods ( 3 12 s) provide near-continuous measurement (4/min). Calibration experiments performed in media of known thermal conductivity exhibit a linear response with thermal conductivity (average error ±1% for glass and catheter probes, ±3% for steel probes). Isolated perfused dog liver experiments demonstrate tissue blood flow estimation better than ±3%. Long-term measurements (several months) performed in cat brain demonstrate the high biocompatibility of the probes. During hyperthermia, human tumor experiments show the efficiency of the baseline temperature shift compensation and the non-influence of spatial temperature gradients on the measurement. During hypothermic storage, pig heart experiments show the ability of this instrumentation to carry out measurements of water content.