Resolution Enhancement of UWB Time-Reversal Microwave Imaging in Dispersive Environments

Abstract

Time Reversal (TR) techniques allow optimal refocusing of ultrawideband (UWB) electromagnetic waves in complex propagation environments, due to the invariance of the wave equations. Propagation of electromagnetic waves through dispersive or lossy media breaks this invariance. Consequently, such media degrade the refocusing ability of TR techniques. In this work, we propose a novel algorithm for the enhancement of the UWB TR microwave imaging resolution in dispersive environments. The presented algorithm takes into account the frequency-dependent complex permittivity of the propagation medium across the entire bandwidth of the UWB pulse. Using this complex permittivity, it models the medium-, time-, and frequency- dependent attenuation in the wavelet domain to create inverse filters which compensate for the effects of the attenuation. This is the first algorithm that constructs inverse filters in the wavelet domain using the complex permittivity of the dispersive propagation medium across the entire bandwidth, rather than at a center frequency, of the excitation pulse. We also introduce a smart scaling concept to minimize undesired noise amplification. While our proposed approach is agnostic to the application scenario and thus could be utilized in various disciplines, we apply it in a biomedical imaging scenario and achieve enhancement in the resolution of UWB microwave TR imaging of a simulated brain tumor inside the digital human phantom (DHP).