Problems involved in temperature measurements using EIT

In cancer therapy, hyperthermic treatment by microwaves requires a non-invasive and reliable method for measuring the temperature distribution inside the body. EIT seems to be able to evaluate the temperature-dependent tissue impedance for delivering the temperature profile in a cross-section of the body. Assuming a temperature coefficient of the resistivity of an electrolyte of about -2% degrees C-1 and temperature measurement to an accuracy of 0.5 degree C, the error in impedance measurement must be lower than 1%. Irrespective of the accuracy of the tomographic measuring system itself, a problem arises from the fact that the fluid content in the tissue as well as the fluid distribution between the extracellular and the intracellular compartment change with temperature. Measurements of the impedance spectra of skeletal muscle and tumours of rats during hyperthermic treatment deliver very different temperature coefficients of the resistivity from -1.3% degree C-1 to -3% degree C-1, thus questioning the feasibility of the EIT as a temperature measuring method. However, changes in the tissue caused by hyperthermia (e.g., fluid shifts, development of oedema and membrane disintegration) can be detected.