Chair for Computer Aided Medical Procedures & Augmented Reality
Lehrstuhl für Informatikanwendungen in der Medizin & Augmented Reality

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A. Martinez-Möller, M-J. Martínez, S. I. Ziegler, N. Navab, M. Schwaiger, S. Nekolla
Emission Driven Motion Correction in PET/CT Cardiac Imaging
Journal of Nuclear Medicine (2005) 46 (suppl. 2), p163. Proceedings of the Annual Meeting of the Society of Nuclear Medicine (SNM), held in Toronto, Canada, July 2005 (bib)

In cardiovascular PET/CT, misalignment of transmission and emission data originating from voluntary and involuntary patient motion can cause considerable artifacts due to over- or undercorrection for attenuation. In such a case, where parts of the heart do not coincide with CT data, the emission signal is incorrectly considered to arise from e.g. the lungs. The purpose of this work is to implement and validate an automatic correction method which modifies the attenuation map in order to compensate for motion. Methods: Using PET/CT N-13 NH3 rest and stress exams, a study population of 10 subjects was analyzed. In 13 scans, the PET and CT images had no visible misalignment. To simulate patient motion, the attenuation data obtained from the CT was horizontally shifted and used for reconstruction. A polar map using volumetric sampling was computed to delineate regional tracer uptake, which was used to quantify the results. The following motion correction algorithm was applied: 1) Segmentation of the left ventricle from the PET image using automatic localization of the heart and histogram thresholding 2) For each voxel belonging to the left ventricle, the attenuation factor is replaced by an average heart tissue value if it is found to be lower than the average e.g. due to motional blurring 3) Reconstruction of the image using the corrected attenuation map In presence of motion, the algorithm will effectively enlarge the heart in the attenuation data, thus recovering the lost activity due to the misaligned attenuation correction. This approach was also applied to the remaining 7 scans where misalignment was visible and the recovered activity was determined. Results: The simulated misalignment caused a significant decrease of the activity in all polar map segments as shown in Tab. 1. A displacement of 2 pixels (1.1cm) reduced the mean lateral activity by 20% and was fully recovered by the algorithm. In case of a 2.2 cm displacement, a reduction of 39% was found and a recovery up to 16% was achieved. In the scans where initial PET/CT misalignment was appreciable, a mean increase of 17, 5, 4, 11, 6 % in the Ant/Sep/Inf/Lat/Apx wall segments was obtained. Conclusions: PET/CT misalignment has an important impact on the measured cardiac tracer uptake. The proposed, non-interactive method utilizing emission driven motion correction reduced its effect significantly.
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