Microwave Measurement System for Breast Cancer Imaging: An Experimental Prototype Towards Time-Domain Inverse Scattering.

Abstract

Detection of malignant breast tumors at their earliest stage, when they are less than 5mm in diameter, remains a challenge. Microwave imaging at frequencies of 1-4GHz seeks to address the limitations of the existing imaging modalities. Current 'gold' standard techniques - X-ray mammography, magnetic resonance imaging (MRI), and ultrasound - are advancing towards early stage tumor detection, but have shortcomings. The widely used X-ray mammography uses ionizing energy sources. MRI has high operating costs. Ultrasound images carry signal processing artifacts. A hopeful contender is non-ionizing, lower-cost microwave imaging, applied at power levels on the order of a few milliwatts. Microwave imaging holds the promise of distinguishing between malignant and benign lesions, based on ex-vivo studies which reported distinct permittivity contrasts between malignant and benign tissues. The challenge of imaging at microwave frequencies is resolving tumors when they are 5mm or smaller. A microwave imaging algorithm recently developed at the University of Michigan shows the potential to achieve this resolution with a time-domain inverse scattering technique. This thesis research seeks for the first time to validate several key components of the experimental system to support this imaging approach, including the system analytic design, experimental implementation, and data acquisition. The specific goal is the proof-of-concept for a high-fidelity measurement of the scattered waves due to a transmitted ultra-wideband microwave signal, traveling through a 'microwave tissue-mimicking' environment including a matching medium and tumor-like phantoms. The measurement system is designed to have low dispersive behavior as required by the time-domain super-resolution inverse scattering algorithm. This end-to-end hardware experimental set-up includes an array of ultra-wideband tapered planar elliptical dipole antennas, immersed in an imaging space filled with an empirically designed microwave coupling medium, and tumor-like phantoms suspended in the coupling medium. A vector network analyzer provides signal source and receive functions. The measured ultra-wideband signal has been shown to exhibit low-dispersion at the receiving antenna, showing that it is indeed possible to transmit and receive high-fidelity time-domain pulses for the super-resolution inverse scattering imaging algorithm.