The possible use of combined electrical impedance and ultrasound velocity measurements for the non-invasive measurement of temperature during mild hyperthermia.

This paper explores the possibility of using combined measurements of electrical impedance and changes in ultrasound time of flight for determining deep body temperature during mild hyperthermia. Simultaneous electrical impedance spectra (1 kHz-1024 kHz) and ultrasound time-of-flight measurements were made on layered sheep liver and fat tissue samples as the temperature was increased from 30-50 degrees C. The change in propagation velocity for 100% fat and 100% liver samples was found to vary linearly with temperature and the temperature coefficient of the time-of-flight was shown to vary linearly with the % fat in the sample (0.009% degrees C-1%(-1)). Tetrapolar impedance measurements normalized to 8 kHz were shown to have a small sensitivity to temperature for both liver (0.001% degrees C-1 <= 45 degrees C) and fat (0.002% degrees C-1 <= 512 kHz) and the best linear correlation between the normalized impedance and the % fat in the sample was found at 256 kHz (gradient 0.026%(-1), r(2) = 0.65). A bootstrap analysis on 15 layered tissue samples evaluated using the normalized impedance at 256 kHz to determine the % fat in the sample and the temperature coefficient of the time of flight to determine the temperature. The results showed differences (including some large differences) between the predicted and measured temperatures and an error evaluation identified the possible origins of these.