Experimental evaluation of two iterative reconstruction methods for induced current electrical impedance tomography

The Newton-Raphson (N-R) with two different regularization methods: the Levenberg-Marquardt (N-R-LM) and the Hachtel´s Augmented Matrix (N-R-HAM), were used to reconstruct images of conductivity changes in a cylindrical medium by Induced Current Electrical Impedance Tomography (ic-EIT). Experimental data were obtained from an 8-cm high, 19.2-cm diameter tank with 16 electrodes on the boundary surface and surrounded by eight 50-cm diameter coils. The coils were angularly displaced by 45° and offset 12.4 cm from the center of the tank. They were driven by a 150-mA (peak) 20-kHz sine wave. Potential differences between adjacent electrodes were measured with phase-sensitive demodulators. The scalar potential field in the electrode plane of the conducting medium, resulting from eddy currents generated by each coil, was computed by the Finite Element Method. Image reconstruction by the N-R-HAM method was found to provide higher resolution and better noise immunity than the N-R-LM method. Two 2.2-cm diameter nonconducting rods located 3.9 cm from the center of the tank, 180° from each other, were clearly resolved. Spatial resolution is estimated at 15% of the tank diameter and is comparable to the resolution obtained by conventional EIT using the Sheffield protocol. Higher resolution could be achieved with more coils and/or electrodes. A 16-coil system should present no construction problems. However, voltages induced by stray magnetic flux through the electrode leads and measurement circuits are significant and may limit the ability of ic-EIT to perform static imaging of conductivity distributions