## 4 Discussion  
The characteristics of the imaging devices, reconstruction algorithms, and corrections used for the MPI require a thorough image quality evaluation as part of the comprehensive characterization of imaging systems. Such image quality analysis consists of an evaluation through physical image quality descriptors to validate the performance of the _MPI systems_. In this investigation, the image quality of different _MPI systems_ was studied by using specific image quality descriptors related to contrast, self-attenuation characteristics, and perfusion detection capability. The contrast defined during the preparation of the phantom was \(C_{LV/LVIC,phantom}=0.91\). The differences in the \(C_{LV/LVIC}\) values between SPECT-IRR-SC and SPECT-IRR were statistically significant (63.8 \(\pm\) 1.4% vs. 57.4 \(\pm\) 1.4%). IQ-IRR-SC-AC was found to have a considerably higher \(C_{LV/LVIC}\) value (70.3 \(\pm\) 1.2%) than any of the other _MPI systems_ investigated (Figure 2). This can also be seen in Figure 3c, where the \(LV\) inner chamber has the same signal intensity as the background, unlike in other _MPI systems_. If corrections are not used in IQ SPECT, \(C_{LV/LVIC}\) decreases by more than 15%. The same tendency was found by [PERSON] et al. [10] who reported a higher contrast in IQ-IRR-SC-AC compared with IQ-IRR. However, they reported considerably higher values (\(\sim\)20%). The contrast values obtained by IQ-IRR and SPECT-IRR are not significantly different, which is in line with findings reported by [PERSON] et al. (2012) [2]. This result shows the superiority of IRR with SC and AC in combination with Smart Zoom collimators with respect to other _MPI systems_ in terms of \(C_{LV/LVIC}\). Figure 8: Short-axis images of the pathological heart obtained using IQ-IRR-SC-AC. Different \(PD\) positions are shown: (**a**) anterior; (**b**) lateral; (**c**) posterior; (**d**) septal. Figure 7: Net contrast values for different positions of the perfusion defect: anterior, lateral, posterior, septal. Values represent least-square average values; vertical bars represent 95% confidence intervals. The differences between _MPI systems_ could be visually evaluated by comparing short-axis images (Figure 3). When using IQ-IRR-SC-AC (Figure 3c), the _LV_ inner chamber can be easily recognized and _LV_ walls appear thinner and with sharper borders. If corrections are not used for IQ systems (Figure 3d), the quality of the image data decreases in both contrast and spatial resolution. Between SPECT-IRR (Figure 3a) and SPECT-IRR-SC (Figure 3b), an improvement in the image quality due to applied correction for the scatter component can be observed. A quantitative analysis of the self-attenuation characteristics of the _MPI system_ was performed through the evaluation of the intrinsic contrast. The results were used to correct the measured contrast between the _PD_ and the _LV_ wall to obtain reference values of the maximum contrast for each _MPI system_ and different _PD_ positions. As far as the intrinsic contrast is concerned, an overestimation of the signal intensity (_IC_ < 0) in the anterior position and an underestimation (_IC_ > 0) of the signal intensity in the lateral and posterior position was observed for the majority of the investigated _MPI systems_, except for IQ-IRR-SC-AC (Figure 4). The non-uniformity of the signal intensity is more pronounced for SPECT-IRR-SC and IQ-IRR in the lateral and anterior position, respectively. The observed decrease in the signal intensity in the lateral and posterior positions for the majority of the _MPI systems_ could be related to the farthest location of this area from the detector and correlated to the attenuation characteristics. On the other hand, the observed overestimation of the signal intensity in the anterior position for IQ-IRR underlines the importance of using corrections for scatter and attenuation for this cardio-centric system.