A thermistor device for the continuous recording of mass transport velocity in tissue based on the heat clearance principle.

A method for the recording of mass transport velocity in tissue, related to the relative local mass flow, has been developed. It is based on the heat clearance principle, applied in the continuous mode. Thermistors are used as temperature sensors; heating is performed by a high frequency square wave, thus avoiding an additional heating coil. The cooling of the heated thermistor, with as a reference a non-heated one, is related to the local mass transport velocity. A test equipment has been built for the calibration of the device and a set of results have been obtained in test fluids with a thermal conductivity approaching the one of brain tissue (glycerin and water). A mathematical model has been set up which gave results consistent with the recorded data. From this model significant parameters could be derived which, introduced in a compensation circuit, may be used in order to linearize the relation between the measuring value and the velocity variable. In vivo results in the brain cortex of the rabbit have been obtained.