Investigation of motion induced errors in scatter correction for the HRRT brain scanner

Patient motion during PET scans introduces errors in the attenuation correction and image blurring leading to false changes in regional radioactivity concentrations. However, the potential effect that motion has on simulation-based scatter correction is not fully appreciated. Specifically for tracers with high uptake close to the edge of head (e.g. scalp and nose) as observed with [11C]Verapamil, mismatches between transmission and emission data can lead to significant quantification errors and image artefacts due to over scatter correction. These errors are linked with unusually high values in the scatter scaling factors (SSF) returned during the single scatter simulation process implemented in the HRRT image reconstruction. Reconstruction of µ-map with TXTV (an alternative µ-map reconstruction using non-linear filtering rather than brain segmentation and scatter correction of the transmission data) was found to improve the scatter simulation results for [11C]Verapamil and [18F]FDG. The errors from patient motion were characterised and quantified through simulations by applying realistic transformations to the attenuation map (µ-map). This generated inconsistencies between the emission and transmission data, and introduced large over-corrections of scatter similar to some cases observed with [11C]Verapamil. Automated Image Registration (AIR) based motion correction was also implemented, and found to remove the artifact and recover quantification in dynamic studies after aligning all the PET images to a common reference space.