It has been well established that PET and SPECT scans of human myocardium are subject to partial volume related effects, which can cause artifactual regional variations in activity around the myocardium. This study investigated the sources and magnitude of such artifactual inhomogeneity in subjects with normal cardiac function.
Method: Using multi-slice, gated MRI scans from 9 normal subjects, we examined separately the influences on measured activity of wall motion, axial resolution and the relationship between wall thickness and in-plane resolution.
Results: Two patterns of artifactual inhomogeneity were found: a depression in activity at the antero-apex and an elevation in activity in the free wall compared with the septum. Thus, in ungated PET images the true apical/septal ratio was artifactually reduced by a factor of 0.89 (0.92 for SPECT), while the true free wall/septal ratio was enhanced by a factor of 1.12 (1.19 for SPECT). Gating improved uniformity in end-systolic (ES) images but degraded uniformity in end-diastolic (ED) images. With gating, the true PET apical/septal ratio was artifactually reduced by only 0.97 at ES, and 0.82 at ED. Similar behavior was found for SPECT. Improvements in axial resolution were found to have little effect on artifactual variations.
Conclusion: We find that the relationship between in-plane resolution and wall thickness, but not axial resolution, is of prime importance in determining the degree of artifactual inhomogeneity in ungated scans of normal human myocardium. Gating improved ES but degraded ED homogeneity.