Electrical impedance imaging for temperature field estimation during hyperthermia treatment of cancer

The implementation of Electrical Impedance Tomography (EIT) is described for quasi-invasive thermal imaging in conjunction with hyperthermia treatments. Finite Element algorithms were used to implement the Newton-Raphson method for non-linear image reconstructions in 2D and 3D. These are the first reported EIT studies incorporating a systematic set of internal measurements in conjunction with thermal simulation and phantom experiments. Theoretical simulations show that this strategy improves the image in the target region when either internal measurement locations are added to a given number of external recording sites or some external measurement locations are replaced by internal recording positions. This is corroborated in the laboratory where temperature changes induced in phantom material were implemented by means of conductive heating and with clinical ultrasound techniques. A factor of 2 improvement in accuracy was found with the incorporation of internal measurement sites in the reconstructed EIT images. The 3D work explores the potential of EIT to reconstruct 3D images from both laboratory and clinical data, which is unique to the present state of the electrical impedance imaging field. The in vivo thermal studies which are presented with both animal model and clinical treatment data also represent new contributions in the field. EIT studies are presented with several different clinical hyperthermia treatments of superficial scalp lesions utilizing a spiral microstrip antenna. Analysis of the measured laboratory and clinical data suggests that impedance changes can be measured during hyperthermia delivery and temperature estimates based on these observed changes are possible in the clinical setting. These findings also point to the complex, yet critical nature of the impedance versus temperature relationship for tissue in vivo. The reconstructed thermal images may provide complementary information about the overall thermal damage imposed during heating. Based on these experiences, it is concluded that EIT may have good potential as a viable clinical aid in imaging the temperature changes imposed during hyperthermia.