## 4 Discussion
In IQ-IRR-SC-AC, uniform signal intensity was obtained in the anterior, lateral, and posterior positions, resulting in _IC_ values close to zero, but an unexpected underestimation of the signal intensity in the septal position was detected. When multifocal collimators are used, attenuation artifacts are position-dependent and are expected to be more pronounced than with LEHR collimators [13]. Nevertheless, the non-uniformity of the signal intensity in the _LV_ wall could be substantially corrected if the AC and SC are applied on image data obtained by the IQ SPECT device. The slight septal non-uniformity of the signal intensity could be a result of the application of the AC that seems to overcorrect values in the septal position. The _MPI system_ and _PD_ position were both found to be the main effects on _NC_. Lesion position has a dominant effect on the _NC_ variability. IQ-IRR-SC-AC and SPECT-IRR-SC were found to have the largest _NC_ mean values overall, with the differences not being statistically significant. SPECT-IRR had lower _NC_ mean values than IQ-IRR-SC-AC and SPECT-IRR-SC. The differences were not statistically significant, mainly due to the relatively large variation in the results determined with SPECT-IRR. SPECT-IRR also performed similarly to IQ-IRR, with differences that were not statistically significant. Therefore, if least-squares average values are considered, IQ-IRR performs similar to SPECT-IRR (Figure 5). Nevertheless, substantial differences in the _NC_ values at different _PD_ positions were noted between IQ-IRR and SPECT-IRR (Figure 6). For IQ-IRR, the _NC_ values were uniform for anterior, lateral, and septal positions, while a decrease of up to 15% for the posterior position was observed due to a loss of sensitivity with multifocal collimators in the area farthest from the detector. It is also interesting to note that in the anterior and lateral positions, there was no difference between IQ-IRR-SC-AC and IQ-IRR in terms of _NC_ values. On the contrary, in the posterior and septal positions, corrections made a difference, enabling an improved perfusion detection capability of the cardio-centric system in these areas highly influenced by attenuation and scatter contribution. Attenuation artifacts that are more pronounced for multifocal collimators could be corrected if AC is applied. In SPECT-IRR-SC, an increase in the mean _NC_ values for the lateral, posterior, and septal positions is visible compared to those for SPECT-IRR. Concerning the effect of lesion position on _NC_, the posterior position was found to produce the lowest values with statistically significant differences between the posterior and all other lesion positions (Figure 7). Even though the correction due to the intrinsic contrast was considered in the calculation of the _NC_ (Equation (7)), the perfusion detection capability of _MPI systems_ in the posterior part of the heart is still the lowest. The results for _NC_ obtained in this study can be used as reference values of the maximum contrast between the _PD_ and _LV_ wall. In this investigation, a set of physical descriptors related to image contrast were used for the characterization of different imaging device-reconstruction algorithm-correction combinations. Once image quality is evaluated, advanced features can be implemented to optimize SPECT procedures with the use of convolutional neural networks [22; 23; 24; 25]. IQ. SPECT technology with the IRR algorithm may produce better spatial resolution, contrast, and sensitivity. Consequently, such _MPI systems_ provide the possibility for the reduction of acquisition time or administered activity. This calls for a revision of practice and national diagnostic reference levels for respective nuclear medicine diagnostic procedures [26].