## 2 Materials and Methods  
The propagation-based PCT experiments reported in this article were performed in hutch 3B with a source-to-sample distance of 137 m and a sample-to-detector distance of 0.19 m (this is the minimum practically achievable distance for absorption contrast) and 6 m (phase contrast). Figure 1 illustrates the schematic diagram of the experimental setup. The large distance between the sample and detector allows the propagation-based X-ray phase contrast to be observed [5; 24; 41]. The experiments were performed at a wiggler magnetic field of 3 Tesla with the following in-vacuum filters: C(0.45), C[hd](5), C[hd](10), Al(1)-Al(1), and Al(1)-Al(1) (all but the C(0.45) filter are at 45\({}^{\circ}\)). The experiments were conducted with monochromatic X-rays at the energies of 26, 28, 32, and 34 keV. Unlike the polychromatic X-rays in the clinical instruments that contain a broad spectrum with the disadvantage that the lower-energy component contributes more to the patient dose, monochromatic X-rays help to minimize the patient dose. To flatten the beam in the vertical direction, the DCLM was set at an appropriate bending configuration. First crystal benders were set to 2.01 and second crystal benders were set to 1.84. These values refer to the ratio of the source to crystal distance and the bending radius. Due to the low dose requirement for the breast tissue samples, detuning of the DCLM was applied in order to move away from the Bragg peak position. The beam size in the sample plane was 20 cm (horizontally) \(\times\) 4 cm (vertically). The samples were fresh full-breast mastectomies that were obtained from breast cancer surgery operations and were imaged at the IMBL on the same day in a complete, intact, and unfixed state. No additional sample preparation was applied prior to scanning. However, before and after scanning, samples were handled inside a biosafety cabinet (Level 1), and the appropriate biosafety procedures were followed when the samples were placed inside or removed from the sealed 11 cm diameter plastic cylinders (used for sample storage) for  
Figure 1: Schematic diagram of the experimental setup for absorption-contrast CT (detector at 0.19 m) and propagation-based PCT (detector at 6 m). the PCT scans. The sample was positioned vertically inside the plastic cylinder along the direction from the nipple (top) to the pectoralis major (bottom). Most samples contained different types and grades of breast cancer lesions, but as a control, we also received and imaged cancer-free samples from prophylactic surgery. The imaging experiments were conducted under a Human Ethics Certificate of Approval from Monash University (Project ID 26399) and with written consent from the patients to image their clinical specimens. For evaluation and quantification of the detector resolution, we used a resolution test phantom target (QRM-D100-HCR, QRM GmbH, Mohrendorf, Germany). The phantom was composed of materials designed to mimic soft tissue and had an overall diameter of 10 cm and incorporated line patterns of different sizes. The samples were exposed to X-rays under clinically relevant mean glandular doses (MGD) of 2, 4, and 8 mGy (the 8 mGy MGD was above the dose used under actual clinical conditions but was included for comparison purposes). The MGD was determined for a breast composed of 30% glandular and 70% adipose tissue. Before the experiment, an ion chamber was installed in the X-ray beam to calibrate the detected flux at the detector by adjusting the rocking angle \(\Delta\theta\) of the monochromator to control the incident photon fluence. To this end, the ion chamber readings and the corresponding detector readings were collected for two sample-to-detector distances, 0.19 and 6 m. Ion chamber readings were used to estimate the corresponding MGD using Monte Carlo simulations for a numeric breast phantom with a diameter of 11 cm [5,32,34].