Misalignment between emission scan and X-ray CT derived attenuation maps causes artificial defects in attenuation corrected myocardial perfusion SPECT

Attenuation correction increases the specificity of myocardial perfusion SPECT. However, we noticed an unusually high number of scans with defects only visible in the attenuation-corrected images. We suspected these to be false positive readings. Visual inspection using the supplied software suggested mismatch in the ventrodorsal (Y-) direction between SPECT images and transmission maps as an explanation. As the fusion tool only allows for a coarse grading, we wrote software to quantify the mismatch. A phantom study was done to verify that the observed mismatch can cause the defect patterns visible in the attenuation-corrected images. 25 patients who showed the most pronounced artifact were chosen for re-alignment and re-evaluation. Overall, the defects in the attenuation-corrected images got less intense (15/25) or even vanished (6/25) after re-aligning emissions images and transmission maps. In response to our complaints, the vendor replaced the support rollers which should prevent the bed from deflecting with a re-engineered, more robust version. No more clinically relevant artefacts were observed after this modification. Evaluation of another 28 probably-normal patients showed the mean mismatch between emission and transmission scan to be significantly reduced. We conclude that great care has to be taken to ensure correct alignment of the scans even in a dual-modality imaging device. Bed deflection can be a major source for misalignment and artifacts.