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M J Moskowitz
Publications and source records attributed to M J Moskowitz.
Temperature field estimation using electrical impedance profiling methods. I. Reconstruction algorithm and simulated results.
Algorithmic methods for estimating complete temperature fields during hyperthermia treatments based on surface and internal electrical measurements are presented. The techniques utilized draw upon impedance imaging concepts, but rather than limit the measurements to positions on the body surface, internal impedance recording sites are allowed. Theoretical simulations show that this strategy improves the reconstructed 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. The algorithms developed are tested on a set of problems with increasing levels of complexity. The culmination of these investigations is a complete simulation of a hyperthermia treatment and reconstruction of a thermal image for a body cross-section of an actual cancer patient. The results of this work suggest that the surface plus internal measurement approach holds some promise as a method for estimating temperature distributions during hyperthermia treatments. However, the simulations while promising are idealizations in that they are two-dimensional with modest levels of additive noise. In a companion paper, we explore the viability of this approach in several laboratory phantom experiments which include both static and heat-induced transient electrical property profiles.
Algorithms↗
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.
Electric Impedance↗
Clinical implementation of electrical impedance tomography with hyperthermia.
We describe the use of electrical impedance tomography (EIT) for non-invasive thermal imaging in conjunction with a clinical treatment of a superficial scalp lesion utilizing a spiral microstrip antenna. This is our first reported use of EIT with a clinical hyperthermia treatment and perhaps the first world-wide. The thermal measurements recorded during treatment compare favourably with the images reconstructed from impedance data gathered during heating. A linear relation, measured in phantom material, between the change in temperature with the change in reconstructed impedance was assumed. The average discrepancy between the measured temperature changes with the temperatures reconstructed from the impedance changes was 1.4 degrees C, with the maximum being 8.9 degrees C. These preliminary data suggest 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 this initial clinical experience we feel that EIT has great potential as a viable clinical aid in imaging the temperature changes imposed during hyperthermia.
Electric Impedance↗
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 arrays of 16 and 32 tin-plated copper electrodes etched on a 0.005" polyimide sheet were used to inject 12.5 KHz current patterns of increasing sinusoidal spatial frequencies and subsequent potential distributions were recorded at each electrode site. Image reconstruction was achieved with a finite element method and difference images of conductivity changes during the course of treatment were formed. An assumed linear relationship (2%/degree C increase) between tissue impedance change and temperature change was used to produce thermal images of the treatment field in patients whereas an empirically measured nonlinear relationship obtained from excised tissue samples was applied retrospectively in the animal subject case. Reconstructed conductivity changes are shown to be possible given electrical data measured in vivo during hyperthermia delivery with conventional equipment (spiral microstrip applicator at 433 MHz). These correlated well with direct temperature measurements and demonstrated quantitative levels of agreement to the extent that estimated temperature accuracies were approximately 1.5 degrees C; although large errors (> 5 degrees C) did exist. This work suggests 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 correlations between conductivity and temperature changes. Further, study is required to discern whether this will ultimately limit EIT as a thermal estimator or whether it will lead to more fundamental uses of impedance as an indicator of thermal effect.
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