Faculty Bibliography

We have recently built and tested a 32 channel, multi-frequency (1 kHz to 1 MHz) voltage mode system to investigate electrical impedance spectroscopy (EIS) imaging. We completed a series of phantom experiments to define the baseline imaging performance of our system. Our phantom consisted of a plastic circular tank (20 cm diameter) filled with 0.9% aqueous NaCl solution. Conductors and nonconductors of decreasing width (W5: 3.4 cm, W4: 2.54 cm, W3: 0.95 cm, W2: 0.64 cm and WI: 0.32 cm) were positioned at various distances from the tank edge (1 cm, 2 cm, 4 cm and 8 cm). The results suggest that the detection of objects less than 1 cm in width is limited to the first 1 to 2 cm from the tank edge for absolute images, but this depth can extend to 8 cm in difference images. Larger 3.4 cm wide objects can be detected in absolute images at depths up to 8 cm from the tank edge. Generally, conductor images were clearer than their nonconductor counterparts. Not only did electrode artefacts lessen as the frequency increased, but the system's maximum resolution was attained at the highest operating frequencies. Although the system recovered the value of the electrical conductivity at the correct order of magnitude, it tended to smooth out large property discontinuities. The calculated electrical permittivity in these phantom studies was inconclusive due to the presence of electrode artefacts

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

This study evaluates the potential of electrical impedance spectroscopy (EIS) as a noninvasive technique for tracking the progression of radiation-induced damage in normal muscle tissue. Male Sprague-Dawley rats were irradiated locally to the gastrocnemius and biceps femoris muscle. Single doses were administered using a procedure that spares skin and bone. Complex impedance spectral measurements (taken at 50 frequency points between 1 kHz and 1 MHz) were made at monthly intervals using recessed disk electrodes applied to the skin. A histological scoring scheme was developed for evaluation of injury. A strong dose-dependent progression of injury evident in both spectral measurements and histological scoring has been observed. Latent time also appears to be dependent on dose with changes induced by 70 Gy evident by 2 months, changes induced by 90 Gy observed by 1 month, and dramatic changes found within 3 weeks at 150 Gy. Injury was morphologically comparable to the type of damage that occurs in response to small, fractionated doses, but on a much shorter time scale. Increased spectral shift was a consistent indicator of the extent of tissue injury at the time of measurement. The use of a large single dose resulted in an excellent model in terms of inducing a significant progression in tissue injury over a short post-treatment follow-up period in the muscle mass while also providing a consistent location for in vivo electrical impedance measurements. The results show that EIS can follow radiation-induced tissue change, suggesting that EIS has the potential to monitor the types of injury observed in late radiation damage of muscle tissue noninvasively

We have applied a number of modeling schemes to previously reported in vivo electrical impedance measurements on irradiated and normal muscle in the hind legs of rats. Specifically, seven-parameter parallel pathways and embedded membrane circuit models have been fit to group averages of impedance spectra measured at different doses and time points. Correlations between histologically scored tissue sections and model parameters have also been determined. The results show that both models produce good fits to the experimental observations, especially in the case of the irradiated tissues. The correlations between histology scores and circuit parameters were, however, higher with the embedded model. Trends in the spectra and the model parameters were found to agree with the expected changes in tissue pathophysiology associated with the progression of tissue injury from radiation exposure. Quantitative correlations with specific histological criteria were less conclusive, suggesting that more information may be needed to refine the model architecture if model parameters are to be explicitly related to the types and extent of tissue damage induced by radiation treatments