Microwave imaging for the detection and localization of breast tissue malignancies using time reversal beamforming

University of Technology Sydney. Faculty of Engineering and Information Technology.Breast cancer is the most common cancer suffered by Australian women. Early detection of cancer provides the best chance of survival to the victims. Microwave imaging has shown the potential to be a complimentary imaging modality to the existing breast cancer imaging techniques such as mammography, MRI and ultrasound. Microwave imaging can overcome the drawbacks of conventional imaging techniques such as patient discomfort and ionizing radiation hazard. The principle of microwave imaging for breast cancer detection is based on the dielectric property contrast between healthy breast tissues and malignant tissues. However, in dense breasts that have high amounts of dense fibro-glandular tissue content, the dielectric property contrast between tumor and surrounding healthy glandular tissues can be quite low. To overcome the problems arising from imaging in low contrast scenario, contrast enhancing agents and hybrid imaging modalities have been proposed in the literature. But, such complex modalities not only complicate the screening process but also add to patients discomfort and cost. Moreover, such techniques may still fail to detect multiple tumors unambiguously in highly dense breasts. In this thesis, we investigate the use of computational time reversal imaging techniques for breast cancer detection and localization using anatomically realistic numerical breast phantoms. Both radar imaging and tomography imaging techniques have been applied for breast cancer detection. Microwave tomography cannot detect abrupt change in dielectric properties when contrast is low. On the other hand radar imaging can reveal the target location information even under low contrast scenario but suffers from clutter and noise in the medium. Time reversal microwave imaging can be considered to be a variant of radar imaging. Time reversal uses medium heterogeneity as an advantage and is highly suitable for imaging in heterogeneous medium. However, the performance of time reversal can also be affected by low dielectric property contrast between target and surrounding tissue clutter. To overcome the effects of clutter interference on target detection and localization, in this thesis, we propose novel beamforming techniques for time reversal microwave imaging. Firstly, we extend beamspace processing for time reversal imaging technique with an aim to reduce clutter effects and improve robustness of imaging. However, when we use ultra-wideband microwave pulses for imaging, a coherent approach is necessary to overcome problems due to random phase variations arising in each frequency bin. We propose two different novel coherent beamspace time reversal imaging techniques for breast cancer screening. The focusing matrix based coherent signal subspace processing is found to be more suitable for subspace and maximum likelihood based time reversal imaging techniques whereas the focusing matrix based on wavefield modelling method is found more suitable for time reversal minimum variance imaging. We propose to combine coherent focusing with beamspace processing (C-B) to obtain superior imaging localization performance. We have also derived Cramer Rao Lower Bound (CRLB) for beamspace time reversal imaging. We have proposed Coherent beamspace DORT (C-B-DORT), C-B-TR-MUSIC, C-B-TR-RCB, C-B time reversal maximum likelihood (C-B-TR-ML) methods to detect small single and multiple tumors in highly dense breasts where conventional techniques are prone to fail. Our investigations have revealed that C-B-TR-ML imaging has superior performance compared to other techniques in detecting three small sized tumors embedded in a highly dense breast phantom

Computational polarimetric microwave imaging

We propose a polarimetric microwave imaging technique that exploits recent advances in computational imaging. We utilize a frequency-diverse cavity-backed metasurface, allowing us to demonstrate high-resolution polarimetric imaging using a single transceiver and frequency sweep over the operational microwave bandwidth. The frequency-diverse metasurface imager greatly simplifies the system architecture compared with active arrays and other conventional microwave imaging approaches. We further develop the theoretical framework for computational polarimetric imaging and validate the approach experimentally using a multi-modal leaky cavity. The scalar approximation for the interaction between the radiated waves and the target---often applied in microwave computational imaging schemes---is thus extended to retrieve the susceptibility tensors, and hence providing additional information about the targets. Computational polarimetry has relevance for existing systems in the field that extract polarimetric imagery, and particular for ground observation. A growing number of short-range microwave imaging applications can also notably benefit from computational polarimetry, particularly for imaging objects that are difficult to reconstruct when assuming scalar estimations.Comment: 17 pages, 15 figure

On Time-Reversal Imaging by Statistical Testing

This letter is focused on the design and analysis of computational wideband time-reversal imaging algorithms, designed to be adaptive with respect to the noise levels pertaining to the frequencies being employed for scene probing. These algorithms are based on the concept of cell-by-cell processing and are obtained as theoretically-founded decision statistics for testing the hypothesis of single-scatterer presence (absence) at a specific location. These statistics are also validated in comparison with the maximal invariant statistic for the proposed problem.Comment: Reduced form accepted in IEEE Signal Processing Letter

Deep-tissue focal fluorescence imaging with digitally time-reversed ultrasound-encoded light.

Fluorescence imaging is one of the most important research tools in biomedical sciences. However, scattering of light severely impedes imaging of thick biological samples beyond the ballistic regime. Here we directly show focusing and high-resolution fluorescence imaging deep inside biological tissues by digitally time-reversing ultrasound-tagged light with high optical gain (~5×10(5)). We confirm the presence of a time-reversed optical focus along with a diffuse background-a corollary of partial phase conjugation-and develop an approach for dynamic background cancellation. To illustrate the potential of our method, we image complex fluorescent objects and tumour microtissues at an unprecedented depth of 2.5 mm in biological tissues at a lateral resolution of 36 μm×52 μm and an axial resolution of 657 μm. Our results set the stage for a range of deep-tissue imaging applications in biomedical research and medical diagnostics

Breast cancer detection in highly dense numerical breast phantoms using time reversal

In this paper we investigate the detection of breast cancer using two-dimensional slices of realistic numerical phantoms employing time reversal microwave imaging. We used maximum-likelihood estimation coupled with time reversal technique to detect and estimate the location of tumor using FDTD based breast phantoms that contain dense fibroglandular tissue clutter. We show that time reversal maximum-likelihood estimation can detect and accurately localize tumors even in highly dense breasts where the dielectric contrast between healthy dense breast tissue and cancerous lesions is quite low without requiring any contrast enhancing agents. © 2013 IEEE

A comparative study of coherent time reversal minimum variance beamformers for breast cancer detection

© 2015 EurAAP. In this study we consider coherent processing for time reversal microwave imaging for breast cancer detection. We derive coherent time reversal standard Capon beamformer (C-TR-SCB) and coherent time reversal robust Capon beamformer (C-TR-RCB) and compare their imaging performances for breast cancer detection in anatomically realistic heterogeneous 3-D breast phantoms

An optimized ultrasound detector for photoacoustic breast tomography

Photoacoustic imaging has proven to be able to detect vascularization-driven optical absorption contrast associated with tumors. In order to detect breast tumors located a few centimeter deep in tissue, a sensitive ultrasound detector is of crucial importance for photoacoustic mammography. Further, because the expected photoacoustic frequency bandwidth (a few MHz to tens of kHz) is inversely proportional to the dimensions of light absorbing structures (0.5 to 10+ mm), proper choices of materials and their geometries, and proper considerations in design have to be made for optimal photoacoustic detectors. In this study, we design and evaluate a specialized ultrasound detector for photoacoustic mammography. Based on the required detector sensitivity and its frequency response, a selection of active material and matching layers and their geometries is made leading to a functional detector models. By iteration between simulation of detector performances, fabrication and experimental characterization of functional models an optimized implementation is made and evaluated. The experimental results of the designed first and second functional detectors matched with the simulations. In subsequent bare piezoelectric samples the effect of lateral resonances was addressed and their influence minimized by sub-dicing the samples. Consequently, using simulations, the final optimized detector could be designed, with a center frequency of 1 MHz and a -6 dB bandwidth of ~80%. The minimum detectable pressure was measured to be 0.5 Pa, which will facilitate deeper imaging compared to the currrent systems. The detector should be capable of detecting vascularized tumors with resolution of 1-2 mm. Further improvements by proper electrical grounding and shielding and implementation of this design into an arrayed detector will pave the way for clinical applications of photoacoustic mammography.Comment: Accepted for publication in Medical Physics (American Association of Physicists in Medicine