Microwave image reconstruction utilizing log-magnitude and unwrapped phaseto improve high-contrast object recovery

Reconstructing images of large high-contrast objects with microwave methods has proved difficult. Successful images have generally been obtained by us ing a priori information to constrain the image reconstruction to recover t he correct electromagnetic property distribution, In these situations, the measured electric field phases as a function of receiver position around th e periphery of the imaging field-of-view vary rapidly often undergoing chan ges of greater than pi radians especially when the object contrast and illu mination frequency increase. In this paper, we introduce a modified form of a Maxwell equation model-based image reconstruction algorithm which direct ly incorporates log-magnitude and phase of the measured electric field data . By doing so, measured phase variation can be unwrapped and distributed ov er more than one Rieman sheet in the complex plane. Simulation studies and microwave imaging experiments demonstrate that significant image quality en hancements occur with this approach for large high-contrast objects. Simple strategies for visualizing and unwrapping phase values as a function of th e transmitter and receiver positions within our microwave imaging array are described. Metrics of the degree of phase variation expressed in terms of the amount and extent of phase wrapping are defined and found to be figures -of-merit which estimate when it is critical to deploy the new image recons truction approach, In these cases, the new algorithm recovers high-quality images without resorting to the use of a priori information on object contr ast and/or size as previously required.