IEEE TRANSACTIONS ONCOMPUTATIONAL IMAGING, VOL. 6, 2020 167 Microwave Breast Imaging Using a Dry Setup João M. Felício , Member, IEEE, José M. Bioucas-Dias , Fellow, IEEE, Jorge R. Costa , Senior Member, IEEE, and Carlos A. Fernandes , Senior Member, IEEE Abstract—This article demonstrates for the first time, both numerically and experimentally, the feasibility of radar-based microwave imaging of anthropomorphic heterogeneously dense breasts in prone position, requiring no immersion liquid. The dry, contactless approach greatly simplifies the setup, favors patient comfort, and further avoids lengthy sanitation procedures after each exam. We use a radar-type technique with the antennas distributed in cylindrical configuration around the breast phan- tom. The reflectivity map is reconstructed using a wave-migration algorithm in the frequency domain. This article presents new developed strategies to cope with the challenges of a dry setup, namely increased skin artifact due to the concomitant absence of matching liquid and nonuniform breast shape. We propose an iterative and adaptive algorithm based on singular value decom- position that effectively removes the skin backscattering under the abovementioned conditions. It is compatible with automatic processing, and computationally fast. One of its inputs is the breast three-dimensional surface information, and its distance to the antennas, all obtained automatically from a proposed low-cost procedure based on a webcam. The imaging method is reasonably resilient to the presence of fibroglandular tissues, and to uncer- tainties of tissue permittivity. Another tackled challenge is the miniaturization of the antenna in air, which is achieved with an optimized balanced antipodal Vivaldi of the same size as coun- terparts used in dense immersion liquids. Finally, all the building blocks are combined to demonstrate experimentally the overall dry system performance, with very good detection of the tumor at three different positions in the breast, even in low-contrast scenarios. Index Terms—Artifact removal, balanced antipodal Vivaldi antenna (BAVA), breast surface estimation, broadband antenna, dry imaging setup, heterogeneous breast imaging, inverse prob- lem, medical microwave imaging (MWI), phantom, singular value decomposition (SVD), skin backscattering, cascade transmission line, wave-migration. Manuscript received April 6, 2019; revised June 24, 2019; accepted July 6, 2019. Date of publication July 26, 2019; date of current version January 13, 2020. This work was supported in part by Fundação para a Ciência e Tecnologia under Project PTDC/EEI-TEL/30323/2017 and Grant SFRH/BD/115671/2016, in part by Instituto de Telecomunicações, and Universidade de Lisboa, in part by the FCT/MEC through national funds, and in part by the FEDER—PT2020 partnership agreement under Project UID/EEA/50008/2019. This work has been developed in the framework of COST Action TD1301 (MiMed). The associate editor coordinating the review of this manuscript and approving it for publication was Dr. Francesco Soldovieri. (Corresponding author: João M. Felício.) J. M. Felício and J. R. Costa are with Instituto de Telecomunicações, Instituto Superior Técnico, Universidade de Lisboa, Lisbon 1049-001, Portugal, and also with Departamento de Ciências e Tecnologias de Informação, Instituto Universitário de Lisboa (ISCTE-IUL), Lisbon 1649-026, Portugal (e-mail: joao.felicio@lx.it.pt; jorge.costa@iscte-iul.pt). J. M. Bioucas-Dias and C. A. Fernandes are with Instituto de Telecomuni- cações, Instituto Superior Técnico, Universidade de Lisboa, Lisbon 1049-001, Portugal (e-mail: bioucas@lx.it.pt; carlos.fernandes@lx.it.pt). Digital Object Identifier 10.1109/TCI.2019.2931079 I. INTRODUCTION O VER the last two decades, Microwave Imaging (MWI) has been investigated as an alternative imaging modality for breast cancer screening. De facto techniques, such as mammog- raphy (X-rays) and Magnetic Resonance Imaging (MRI), have well known limitations [1], which contributed to the emergence of new technologies. Active MWI systems illuminate the breast with electromag- netic (EM) energy usually comprised in the frequency spectrum between 1 GHz and 10 GHz. The microwave energy is radiated by antennas distributed around the breast. Given the permittivity contrast of different tissues, the EM waves are scattered and picked up by the same or separate antennas [1]. In the specific case of breast cancer detection, MWI benefits from high contrast between healthy and malignant tissues [1]. It is common to categorize MWI methods as quantitative or qualitative. The first type aims at spatially mapping the dielectric properties of media in the frequency-domain, for instance by means of the distorted Born’s or Rytov’s linear approximation [2], [3]. Quantitative methods involve time-consuming iterative calculations to find the solutions of ill-posed inverse problem, which depends greatly on the accuracy of the forward model and on good initializations [2], [4]. In contrast, qualitative imaging aims at reconstructing the reflectivity of a given volume [5] and is compatible with real-time examinations. In the present work we focus on qualitative techniques based on radar-approach. Due to the restricted bandwidth and relatively large wave- lengths, the attainable resolution of microwaves is of a few cen- timeters in free-space, which contrasts with the fine resolution of X-rays. Nevertheless, unlike mammography, microwaves do not pose any health risk and are compatible with contactless imaging setup. Moreover, medical MWI is intended as a primary screening examination method and not as the ultimate and single-validation examination. In case of such detection, patients would be forwarded to other exams, such as MRI or ultra- sound. We highlight that this is already standard procedure with mammography. The attainable resolution using microwaves is addressed in more detail ahead in this article. Given the high permittivity contrast between skin and air, the received signals are dominated by this early-time reflec- tion [6]. This artifact is easily hundreds of times larger in magnitude than the tumor response, which can potentially mask the tumor, thus precluding its detection. Consequently, before addressing the image reconstruction, it is necessary to eliminate (or at least significantly reduce) the early-time response from the skin. This processing step is known as artifact removal. 2333-9403 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.