## 3 Results
### Detector Comparison  
This section presents a comparison of the performance of the two detectors used at the IMBL (Hamamatsu and Xineos) in the context of PCT imaging. Both the Hamamatsu and the Xineos detectors have high quantum efficiency and a very low noise level, which is important for this type of imaging, i.e., low-dose PCT scans with short exposure times. The images produced by the Hamamatsu and the Xineos detectors are shown in Figure 5. Both detectors produced similar-quality images. Figure 4: Comparison of PCT images of a breast tissue sample from a sagittal slice obtained with different MGD values: (a) 2, (b) 4, and (c) 8 mGy. Figure 3: Comparison of PCT images of a breast tissue sample from a sagittal slice obtained using four different X-ray energies: (a) 26, (b) 28, (c) 32, and (d) 34 keV. For the dense glandular component (white color), the result from 34 keV shows the best quality, while 32 keV produces the best quality for the adipose component (gray color). Image Quality Assessment  
The objective image quality shown in the PCT reconstructions of the breast tissue (see Figure 5, top row) using both detectors was evaluated with several metrics, including the intrinsic spatial resolution of the reconstructed slice, SNR, and the ratio of the SNR to the average spatial resolution (SNR/res) [40]. These measurements were carried out using the X-TRACT software within small uniform regions selected in the images. The intrinsic spatial resolution is estimated in X-TRACT by calculating the noise power spectrum within these uniform regions and evaluating the inverse of the square root of the second moment of that spectrum. Further details of the resolution calculations using X-TRACT can be found in [PERSON] et al. [5] and [PERSON] et al. [48]. These measurements were carried out at every tenth slice from the 3D reconstruction stacks with 280 slices in total yielding ten resultant values from the central region of the CT stack. The intrinsic spatial resolution was systematically measured as \(x_{res}\) and \(y_{res}\) in two orthogonal directions of the reconstructed slices. This measurement was performed in the adipose and glandular regions separately by averaging over the values from the ten slices from the CT stack. The results are presented in Table 2. In the case of the Hamamatsu detector, a spatial resolution of 150 \(\upmu\)m \(\times\) 174 \(\upmu\)m was determined predominantly by the detector's point-spread function. Similar spatial resolution anisotropy was also observed in the Xineos detector: 167 \(\upmu\)m \(\times\) 185 \(\upmu\)m. Furthermore, we saw a typical trade-off that the Hamamatsu detector produced slightly better spatial resolution than the Xineos detector; however, the Xineos detector produced slightly better values of SNR/res than the Hamamatsu detector. Figure 5: Comparison between images produced by the Hamamatsu (**left** column) and the Xineos (**right** column) detectors at 32 keV (6 m distance, 4 mGy) of a breast tissue sample inside an 11 cm diameter plastic container (**top** row) and a resolution test phantom (**bottom** row). To further compare the image quality, the visibility of the images produced by the two detectors was evaluated. Visibility was defined as the ratio of the difference between the maximum (_max_) and minimum (_min_) values in the reconstructed image divided by their sum, as first formulated by [PERSON] [49], using the following equation:  
\[V=\frac{max-min}{max+min} \tag{4}\]  
The line pattern visibility was calculated for the PCT images collected with the resolution test phantom as shown in Figure 5 (bottom row) by averaging over five repeated measurement visibility values. The measurement error of \(\pm\)0.02 was calculated from the standard deviation of these five values. The line pattern visibility plot is presented in Figure 6 as a function of spatial frequency (line pairs per centimeter). The visibility performance of both detectors was comparable, with the Xineos detector producing slightly better image visibility in the range of 10 to 20 lp/cm.