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

Abstract

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 ( 82 Rb [n 5 187] and 13 N-ammonia [n 5 123]) 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 82 Rb and 13 N-ammonia for rest and stress imaging was also compared. Results: Twentythree 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 82 Rb and 13 N-ammonia, and motion during stress imaging was correlated with motion at rest (r 5 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. Accurat e attenuation correction (AC) is a major clinical advantage of PET, compared with SPECT, and has been well accepted when the attenuation map is formed using data from radioactive transmission sources. However, respiratory motion can induce artifacts in cardiac PET/CT because of misregistration of the CT attenuation map and emission data. Misregistration has been shown to occur in up to 40% of clinical cardiac PET/CT studies (1). The most common artifact is an apparent decrease in uptake in the anterior or lateral wall due to a mispositioning of the activity from the heart into the low-attenuating tissues of the lungs. Many studies have examined the effect of this misalignment and found that motion of 6-8 mm produces mild to moderate errors, and motion .8 mm leads to severe errors that may affect clinical interpretation (1,2). Several approaches have been suggested to reduce the impact of this problem including simple realignment of the CT scan and the PET data (2,3), slow CT scans (4), and methods that estimate the motion from r...

Interpolated average CT for cardiac PET/CT attenuation correction

“…29,30 Different PET respiratory phases can also be transformed and registered to a reference CT phase for further reconstruction. 31 However, respiratory gating was still not a clinical routine procedure probably due to implementation complexities and the increased noise level in each respiratory bin. While ACT was proven to be an effective AC method for respiration-ungated cardiac PET, 32 similar radiation concern arises as 4D CT.…”

Current state of hybrid imaging: attenuation correction and fusion

“…Examples of these techniques include aligning a single-phase CT to discrete respiratory phases of an emission study, 42 or collection of phase-matched CT and emission data. [43][44][45] Several groups have developed software algorithms to derive motion transformations from respiratory-gated emission images and to transform the emission data to a single image.…”

Attenuation Correction Strategies for Positron Emission Tomography/Computed Tomography and 4-Dimensional Positron Emission Tomography/Computed Tomography

“…Many potential solutions have been suggested to accommodate differences between breathing patterns including retrospective AC using free breathing CT, 48 the use of optimal CT acquisition protocols, 49,50 respiratory averaged CT, 42,[51][52][53]59 interpolated average CT, 54,55 phased CT acquisitions, [56][57][58] cine CT acquisition, 59 respiratory correlated acquisitions, [60][61][62] deep-inspiration breath-hold acquisition, 44,[63][64][65] and the use of respiratory-gated PET/CT acquisitions. [66][67][68][69][70][71][72][73][74][75] One approach to improve registration between the CT and the PET data is to bring the temporal resolution of the CT images to that of the PET data.…”