There is increasing evidence that alterations in the electrical property spectrum of tissues below 10 MHz is diagnostic for tissue pathology and/or pathophysiology. Yet, the complexity associated with constructing a high-fidelity multichannel, multifrequency data acquisition instrument has limited widespread development of spectroscopic electrical impedance imaging concepts. To contribute to the relatively sparse experience with multichannel spectroscopy systems this paper reports on the design, realization and evaluation of a prototype 32-channel instrument. The salient features of the system include a continuously selectable driving frequency up to 1 MHz, either voltage or current source modes of operation and simultaneous measurement of both voltage and current on each channel in either of these driving configurations. Comparisons of performance with recently reported fixed-frequency systems is favorable. Volts dc (VDC) signal-to-noise ratios of 75-80 dB are achieved and the noise floor for ac signals is near 100 dB below the signal strength of interest at 10 kHz and 60 dB down at 1 MHz. The added benefit of being able to record multispectral information on source and sense signal amplitudes and phases has also been realized. Phase-sensitive detection schemes and multiperiod undersampling techniques have been deployed to ensure measurement fidelity over the full bandwidth of system operation.

Electrical properties of tissues in the 10KHz to 10MHz range are known to be temperature sensitive making the monitoring and assessment of thermal insult delivered for therapeutic purposes possible through imaging schemes which spatially resolve these changes. We have been developing electrical impedance imaging technology from both the hardware data acquisition and software image reconstruction perspectives in order to realize the capability of spectroscopically examining the electrical property response of tissues undergoing hyperthermia therapy. Results from simulations, in vitro phantom experiments and in vivo studies including in human patients are presented. Specifically, a new prototype multi-frequency data acquisition system which is functional to 1MHz in both voltage and current modes is described. In addition, recent advances in image reconstruction methods which include the enhancement techniques of total variation minimization, dual meshing and spatial filtering are discussed. It is also clear that the electrical impedance spectrum of tissue has the potential to monitor other types of treatment-induced injury. Preliminary in vivo electrical impedance measurements in a rat leg model suggest that the tissue damage from radiation therapy can be tracked with this technique. Both dose and time-dependent responses have been observed in the electrical impedance data when compared to measurements recorded in an untreated control. Correlations with histological examination have also been performed and indicate that electrical impedance spectroscopy may provide unique information regarding tissue functional status and cellular morphology. Representative results from these studies are reported.