The effect of hyperthermia-induced tissue conductivity changes on electrical impedance temperature mapping.

Changes in tissue electrical conductivity in the low radiofrequency range due to tissue temperature coefficients (TCS), approximately 2% degrees C-1, have been investigated by others as a non-invasive means of determining tissue temperature changes during the application of therapeutic hyperthermia. However, the occurrence of additional changes in conductivity due to non-TC effects, for example, from heat-induced oedema, or changes in cellular volume or membrane characteristics, can result in incorrect temperature determination based solely on the TC. This paper (i) presents estimates of the errors that will occur in temperature mapping using electrical impedance measurements if non-TC effects are ignored, using examples of excised and in vivo EMT6 tumours, (ii) presents a method of differentiating the onset of non-TC effects from TC effects using conductance measurements, thereby allowing for use of the TC method until the onset of non-TC effects, and (iii) suggests a means of using conductance measurements to monitor the course of hyperthermia treatments, without the use of temperature information, based on hyperthermia-induced cellular changes.