Temperature field estimation using electrical impedance profiling methods. II. Experimental system description and phantom results.

An electrical impedance tomography system has been developed and tested for the purpose of thermal imaging. Since impedance changes with temperature, images of impedance subtracted from normothermic baselines will provide a map of temperature data. A system was designed to be operational at 10-50 kHz and to utilize 16 external electrodes around the periphery of a tissue-equivalent phantom encompassing the region of interest. These electrodes serve as current sources for the 5 mA constant-current inputs and are also used for reading differential voltages. Hyperthermia treatments for cancer require that internal thermometry probes be inserted into the tumour volume. Linear arrays of electrodes with thermometry tracks for micro-dimension thermometry serve this function, as well as providing localized voltage measurements in the region of interest. The embedded temperature sensors provide a quality assurance and calibration standard for the linear arrays in reconstruction of impedance profiles. Results of transient heating experiments with conductive and ultrasound heating are shown where image reconstruction is performed using a finite element model. Temperature predictions in these studies were accurate to better than 1 degree C on average when using information from surface electrodes combined with internal linear arrays. Maximum temperature errors, however, was found to be > 5 degrees C which suggests that further noise reduction during data acquisition and improvements in the reconstructions algorithms are needed.