## 4 Experimental Evaluation  
This section describes the experimental measurement system and acquisition hardware. Additionally, the four breast phantoms and five tumour models used are presented. A flexible microstrip antenna was used to collect backscattered data. A 16-element antenna array has previously been used in pilot clinical trials [3; 91]. In this work, experimental data were collected using a 24-element hemispherical conformal antenna array, meaning that 276 independent, multistatic channels were available for imaging. This is an increase from the 120 independent multistatic channelsused previously [3]. 24 antennas were chosen based on the available space in the radome, as the antenna footprint is approximately 4-5 cm\({}^{2}\). The antennas were designed to be in contact with skin of \(\varepsilon_{r}=30\). In this system, the antennas were placed in direct contact with the skin layer of the breast phantoms. Fused deposition modelling (FDM) was used to fabricate a hemispherical radome to house the antennas. The radome, printed using polylactic acid (PLA) with an Ultimater 2+ Extended (Ultimaker, Geldermasen, The Netherlands), was designed with 24 holes to house the antennas. The antenna array is equally spaced across the hemisphere, as shown in Figure 2a. Modular polyurethane breast phantoms and tumour models were fabricated according to [92; 93; 94]. A 2 mm skin layer was modelled with relative permittivity of \(\varepsilon_{r}=30\), an interior fatty background was modelled with relative permittivity of \(\varepsilon_{r}=6\), conical internal glandular structures were modelled with relative permittivity of between \(40\leq\varepsilon_{r}\leq 50\) and tumours of \(60\leq\varepsilon_{r}\leq 74\). All relative permittivity values are quoted at 3 GHz. Modelled tissue dielectric properties were chosen in accordance with [20; 21], with the target tumour values representing the maximum values reported. The tumour targets are shown in Figure 2b. Tumours of different levels of spiculation were fabricated and used to evaluate algorithm performance. The smallest spherical tumour (maximum diameter of 5 mm; labelled \(T_{1}\) in Figure 2b) was used to evaluate the point spread function of this experimental system. A ZNB40 2-port VNA and ZN-Z84 24-port switching matrix (Rohde and Schwartz GmbH, Munich, Germany) were used to collect all multistatic signals at 201 linearly spaced frequency points between 0.5 GHz and 8.5 GHz. The response for each channel was shaped by a Gaussian pulse modulated with a sine wave in the frequency-domain with centre frequency of 3 GHz and a bandwidth of 3 GHz. Figure 2: The acquisition system, example phantom and targets are shown here. (**a**) shows the 2-port VNA connected to the 24-port switching matrix. The antennas are shown housed in the 3D printed radome; (**b**) shows the five spherical and smooth tumour models used for evaluation of the FQMs; (**c**) shows the interior of the phantom with 10% glandular content. Three other similar phantoms with 0%, 20% and 30% glandular content were used in this study. All dimensions are in mm. The shaped response was then transformed to the time-domain using the inverse Chirp-Z Transform and sampled at 80 GHz. Rotational subtraction was used for artefact removal [95], to isolate the tumour response from the signal and reduce unwanted reflections. This technique has been successfully used in the largest microwave imaging clinical trial to date [1; 2].