Experimental Validation on Tissue-Mimicking Phantoms of Millimeter-Wave Imaging for Breast Cancer Detection  
[PERSON]  
1 Department of Electrical, Computer and Biomedical Engineering, University of Pavia, Via Ferrata 5, 27100 Pavia, Italy; [EMAIL_ADDRESS] (G.M.); [EMAIL_ADDRESS] (M.P.)1 Correspondence: [EMAIL_ADDRESS]; Tel.: +39-0382-98522311004321114321143211432114  
Generally, microwave medical imaging systems are based on the detection of the dielectric contrast between healthy and neoplastic tissues, and several experimental campaigns demonstrated that this contrast exists up to 50 GHz, especially when the volume percentage of adipose tissue is comparatively high [5; 6; 7; 8; 9; 10].  
Microwave imaging systems rely on tomographic or radar approaches. In both cases, several prototypes have been proposed over the past decade [11; 12; 13; 14; 15; 16; 17], and some of them reached a high level of maturity, being tested on up to 400 women in pre-clinical stage [11]. Most recent works on microwave imaging have been published in 2019 and 2020 [18; 19; 20; 21; 22].  
However, the central working frequency of all these systems does not exceed15 GHz. This, for radar systems, provides a good penetration depth for virtually all breasts, but resolutions are not always optimal for an early diagnosis, especially for breasts with a high fat content (hence, low dielectric permittivity, which in turn means a comparatively large wavelength). On the other hand, for tomographic approaches, which in principle could offer optimal resolution in breasts with a high fat content, problems like high computational costs, long processing times, and the non-univocal solution of a usually ill-posed inverse problem, leave the search for better solutions still open.  
To address this problem, higher frequencies could be used, even if the high values for propagation losses within biological tissues have been historically considered as the main motivation for them being employed only in the biomedical imaging of shallower regions [23].  
Nevertheless, the idea of using millimeter-wave (mm-wave) frequencies has been recently explored, aiming at a possible use on breasts with a high fat content, by working on the dielectric characterization of breast tissues, the fabrication of breast phantoms suited for mm-wave operations, and the validation of the working principle on simulated and preliminary setups [7; 8; 9; 10; 24; 25; 26; 27].  
In this work, for the first time, several experimental tests of a mm-wave imaging prototype on tissue-mimicking breast phantoms (with dielectric losses up to around 40 dB/cm) are shown, demonstrating the possibility to reach a penetration depth of several centimeters (up to 3 cm), while achieving a good resolution. In particular, the linear synthetic array prototype used for these tests was based on two double-ridge waveguides (with mono-modal working bandwidth from 18 GHz to 40 GHz) and two linear actuators to move the antennas in the desired positions, emulating an array of 24 radiators in 28 positions. Different types of breast phantoms with increasing values for dielectric losses were used as imaging scenario. After the calibration of the measured data, the Delay and Sum (DAS) algorithm was employed for image formation.  
The paper is organized as follows. In Section 2, the experimental setup is described, together with both the complete procedure for system calibration and the characterization of the system Point Spread Function (PSF). In Section 3, the algorithm for image reconstruction is described. In Section 4, the imaging results are shown and discussed. Finally, some conclusions are given in Section 5.