Simultaneous PET-MR offers the possibility to use high-resolution MR images for motion correction of PET data. Several studies have shown that this approach leads to a strong improvement in PET image quality. Here we present a numerical simulation study analysing the impact of temporal and spatial resolution of MR motion fields on the final image quality of myocardial perfusion PET scans in order to find the most efficient motion field parameters yielding accurate cardiac motion compensation in the shortest possible scan time.

A numerical representation of a human torso and parameterised models of beating heart motion are obtained using a commercially available software (XCAT⁽¹⁾). XCAT generates emission maps, where each voxel represents a tissue dependant standardized uptake value (SUV), and attenuation maps for each motion state, that are then used as input for PET simulations. Reconstructed PET images were assessed based on metrics used for clinical diagnosis including detectability of damaged segments in standard polar plots computed from the left ventricular myocardium⁽²⁾, and extent and degree of transmurality (i.e. fraction of the wall affected) of the lesion.

Four myocardial lesions with different transmurality (100%, 50% and 25%), extension and location (anterior and lateral wall) were simulated using realistic 18F-FDG uptake values⁽³⁾ in XCAT (Fig.1). Cardiac motion was simulated based on a normal cardiac cycle with a duration of 1s. Analytical PET simulations were performed in STIR⁽⁴⁾ using the geometry of the Siemens Biograph mMR scanner (60cm bore size, 8 rings of LSO crystal detectors, 258mm axial FOV). Motion fields of five temporal (20, 10, 5, 2 and 2* cardiac phases) and four spatial (2, 4, 5.68 and 9.84mm³ isotropic voxel size) resolutions were studied. The 2* non-uniform division of the cardiac cycle groups two thirds of the cardiac cycle around maximum relaxation in one cardiac phase and the remaining third as the second phase. Image reconstruction was performed using a motion compensated algorithm⁽⁵⁾ (21 subsets, 3 iterations) with a voxel size of 2.03×2.08×2.08mm³, a matrix size of 127×285×285 and a 4mm isotropic Gaussian post-filtering. Additionally, a motion-free image was simulated and reconstructed for reference purposes, and an uncorrected reconstruction was performed for further comparison.

For both transmural and non-transmural perfusion defects, results showed that although image quality degrades when reducing temporal and spatial resolution of the motion fields, non-viable myocardium can be detected even without motion correction (Fig.1). For defects 1 (transmural) and 2 (50% transmural), a resolution of 10mm and 2* phases allowed to detect more of 90% of the non-viable segments. For defects 3 (25% transmural) and 4 (50% transmural), a resolution of 2mm and 2* phases is required to detect more than 90% of the non-viable segments correctly. For the transmurality analysis both (2mm, 20 phases) and (2mm, 2* phases) lead to accurate results; considering defects 2, 3 and 4, an average error of 7.17 ± 3.44% and 5.58 ± 1.53% compared to the motion-free reference was obtained respectively. For defect 4 (50% transmural), located in an area of large cardiac motion amplitude, when 2 phases are used or no motion correction is performed, the lesion is erroneously detected as transmural (Fig. 3)We have presented a simulation study to assess the impact of spatial and temporal resolution of motion fields for MR-based beating heart motion corrected cardiac perfusion PET imaging. Results show that non-viable myocardium can be detected even without motion correction however motion correction is essential for accurate estimation of the size and degree of transmurality of perfusion defects. The non-uniform two-phase division of the cardiac cycle allowed for achieving less than 10% error in the estimation of extent and transmurality of all simulated defects using a spatial resolution of 2mm. Thus, temporal resolution of the motion fields can be significantly reduced without losing diagnostic accuracy. The subsequent reduction of time allocated for MR-based motion estimation would potentially allow for acquiring diagnostic MR information simultaneously with PET, reducing total exam time in PET-MR clinical routine.