Comparison of template-based versus CT-based attenuation correction for hybrid MR/PET scanners

Attenuation correction (AC) of cerebral PET data acquired in hybrid MR/PET scanners is still a challenge. To overcome this problem we previously proposed a correction method by obtaining template-based attenuation maps (AM) using MR and ECAT EXACT HR+ transmission scans. In the present study we investigated (a) the basic difference between template-based and CT-based AC methods and (b) their influence on reconstructed PET images. The data of 11 subjects undergoing ¹⁸FDG imaging in the Siemens 3T MR-BrainPET scanner were used. Additionally, from all participants a CT scan of the whole head was acquired at the same day. These CT images were transformed to CT-based AMs. They were filtered by a 3D Gaussian kernel with 3 mm (BrainPET resolution) filter width, which was considered as reference. Comparisons between both AMs (CT-based and template-based) were performed by estimating the Dice coefficients D and calculating the numbers of true positive, true negative and false negative voxels. The BrainPET emission data were reconstructed with both AMs (AM_CT3mm and AM_Template). All reconstructed PET images were scaled to standardized uptake values (SUVs) and normalized to the MNI brain for using the AAL-VOI Atlas analysis. Correlation plots with regression equation, coefficients of determination R² and relative differences (RD) between AM_CT3mm and the AM_Template were derived. The fraction of the overall true positive voxels averaged over the 11 subjects was 80.4 ± 7.5% for AM_Template compared to AM_CT3mm (D_bone = 0.63 ± 0.08; D_soft-tissue = 0.85 ± 0.08; D_air = 0.79 ± 0.04). A misclassification of bone as soft tissue and vice versa was evident in the comparison. The correlation plot of all VOIs considered (1,276 values) showed a mean R² of 0.964 and a slope of 1.02. A mean RD of 1.33 ± 0.95% (min = -0.12%, max = 2.85%) was found. The template-based AC method proposed by our group shows considerable differences in comparison to the higher resolution CT-based AM with respect to the Dice coefficients, in particular in the classification of bone and soft tissue. However, this has no major influence on the reconstructed ¹⁸FDG PET data.