Single-Phase CT Aligned to Gated PET for Respiratory Motion Correction in Cardiac PET/CT

Respiratory motion can induce artifacts in cardiac PET/CT because of the misregistration of the CT attenuation map and emission data. Some solutions to the respiratory motion problem use 4-dimensional CT, but this increases patient radiation exposure. Realignment of 3-dimensional CT and PET images can remove apparent uptake defects caused by mispositioning of the PET emission data into the lung regions on the CT scan. This realignment is typically done as part of regular clinical quality assurance. We evaluated a method to improve on this standard approach, without increasing the radiation exposure to the patient, by acquiring a respiration-gated PET scan and separately aligning the 3-dimensional CT scan to each phase of the PET study. Methods: Three hundred ten clinical PET perfusion scans (82Rb [n = 187] and 13N-ammonia [n = 123]) were retrospectively assessed. Studies were respiration-gated, and motion was measured between inspiration and expiration phases. Those studies with motion ≥ 8 mm were evaluated for significant differences between inspiration and expiration. Studies with significant differences were reprocessed with the phase-alignment approach. The observed motion with 82Rb and 13N-ammonia for rest and stress imaging was also compared. Results: Twenty-three scans (7.41%) had motion ≥ 8 mm, and 9 of these had significant differences between inspiration and expiration, suggesting the presence of respiratory artifacts. Phase-aligned respiratory motion compensation reduced this difference in 8 of 9 cases (89%). No significant differences were observed between 82Rb and 13N-ammonia, and motion during stress imaging was correlated with motion at rest (r = 0.61, P < 0.001). Conclusion: Phase-aligned correction improves the consistency of PET/CT perfusion images by reducing discrepancies caused by respiratory motion. This new approach to CT-based attenuation correction has no additional patient radiation exposure and may improve the specificity of PET perfusion imaging.