INITIAL IN-VIVO EXPERIENCE WITH EIT AS A THERMAL ESTIMATOR DURING HYPERTHERMIA

Thermal imaging experiments using electrical impedance tomography (EIT ) have been conducted during hyperthermia treatments delivered to two human patients and one animal subject. Coplanar and circumferential ar rays of 16 and 32 tin-plated copper electrodes etched on a 0.005 '' po lyimide sheet were used to inject 12.5 KHz current patterns of increas ing sinusoidal spatial frequencies and subsequent potential distributi ons were recorded at each electrode site. Image reconstruction was ach ieved with a finite element method and difference images of conductivi ty changes during the course of treatment were formed. An assumed line ar relationship (2%/degrees C increase) between tissue impedance chang e and temperature change was used to produce thermal images of the tre atment field in patients whereas an empirically measured nonlinear rel ationship obtained from excised tissue samples was applied retrospecti vely in the animal subject case. Reconstructed conductivity changes ar e shown to be possible given electrical data measured in vivo during h yperthermia delivery with conventional equipment (spiral microstrip ap plicator at 433 MHz). These correlated well with direct temperature me asurements and demonstrated quantitative levels of agreeement to the e xtent that estimated temperature accuracies were approximately 1.5 deg rees C; although large errors (> 5 degrees C) did exist. This work sug gests that EIT is a potentially useful tool for hyperthermia treatment monitoring and assessment. The relationship between tissue impedance and temperature is complex and confounds the ability to make simple co rrelations between conductivity and temperature changes. Further, stud y is required to discern whether this will ultimately limit EIT as a t hermal estimator or whether it will lead to more fundamental uses of i mpedance as an indicator of thermal effect.