A HIGH-RESOLUTION MICROSCOPIC ELECTRICAL IMPEDANCE IMAGING MODALITY: SCANNING IMPEDANCE IMAGING by

A HIGH-RESOLUTION MICROSCOPIC ELECTRICAL IMPEDANCE IMAGING MODALITY: SCANNING IMPEDANCE IMAGING Hongze Liu Department of Electrical and Computer Engineering Doctor of Philosophy Electrical impedance imaging is an imaging technique which has the capability of revealing the spatial distribution of the electrical impedance inside biological tissues. Classical electrical impedance imaging including Electrical Impedance Tomography (EIT) typically has low resolution. Advances in electrical impedance imaging typically involve methods that either increase image resolution or image contrast. This study investigates the possibility of the resolution improvement for electrical impedance imaging using motion, and presents a novel high-resolution and calibrated impedance imaging method called Scanning electrical Impedance Imaging (SII). SII uses an electrical probe held at a known voltage and scanned over a thin sample immersed in a conductive medium on a grounded conducting plane to obtain high-resolution calibrated impedance images of samples. For system improvement and image reconstruction, a numerical model is developed to describe the SII system. This model simulates the measurement process by solving a 3-D electrostatic field at each scanning position using a modified approach of the finite difference method (FDM). The simulation consists of a quasi-statics problem involving inhomogeneous media with a complicated boundary condition. This 3-D model is used to optimize both the probe height and the shield-spacing for probe fabrication and also to evaluate system parameters including the frequency and the resistor in the peripheral circuit. Based on this model, an approach is also developed to quantifying conductivity values using the SII system. However, a large computational cost due to the motion involved in SII leads to challenges for a fast and accurate image reconstruction based on this 3-D model. Alternative fast models are derived as a replacement of the 3-D model for quick image reconstruction. In particular, the Modified Linear Approximation (MLA) involving two conductivity-weighted convolutions based on the reciprocity principle, explains the function of the special shield design introduced in the SII system reasonably well. Based on the MLA a nonlinear inverse method using total variation regularization and the Polak-Ribiére variant of the nonlinear conjugate-gradient method is developed for fast image reconstruction of the SII system. The inverse method is accelerated using convolution which eliminates the requirement of a numerical solver for the 3-D electrostatic field. 2-D images of small biological tissues and cells are measured using the SII system. The corresponding conductivity images are reconstructed using the MLA method. The successful improvement of resolution shown in both simulation and experimental results demonstrates that the idea of this approach can potentially be expanded to other imaging modalities for resolution improvement using motion.