A method of motion tracking during CT for motion correction

Patient motion is a significant problem in pediatric PET/CT, and motion correction techniques could potentially remove the need for anaesthesia or sedation that are normally required for very young patients. Effective methods exist for motion correction of neurological PET images, and although there is a scarcity of equivalent methods for CT, a potential method of correcting for rigid head motion during the CT scan has recently been proposed. In this study, we describe a motion tracking method for CT using an optical motion tracking system. Since pose is reported in tracker coordinates, and motion correction requires motion in CT scanner coordinates, a calibration is required to determine the transformation needed to convert between the coordinate systems. We describe such a calibration method, and evaluate it by acquiring two CT scans of a Hoffman 3D brain phantom, with a tracker target attached, in different poses. The applied motion between two scans was calculated from the change in target pose measured by the tracker, and converted to scanner coordinates. This motion was applied to one of the reconstructed image volumes, which was then compared with the other image volume. The mean registration error of the two volumes was estimated from landmark analysis to be less than 0.3 mm on average in all directions, which agreed well with a calculated maximum uncertainty of 0.7 mm. Errors of this magnitude could be acceptable if the CT scans were only used for attenuation correction, but may need to be further reduced for motion correction applications. We anticipate that this will be achievable with improvements to our technique, and intend in future work to use motion data to attempt motion correction in spiral CT studies.