Methods for the computation of templates from quantitative magnetic susceptibility maps (QSM): Toward improved atlas- and voxel-based analyses (VBA).

Purpose: To develop and assess a method for the creation of templates for voxel-based analysis (VBA) and atlas-based approaches using quantitative magnetic susceptibility mapping (QSM). Materials and Methods: We studied four strategies for the creation of magnetic susceptibility brain templates, derived as successive extensions of the conventional template generation (CONV) based on only T1-weighted (T1w) images. One method that used only T1w images involved a minor improvement of CONV (U-CONV). One method used only magnetic susceptibility maps as input for template generation (DIRECT), and the other two used a linear combination of susceptibility and T1w images (HYBRID) and an algorithm that directly used both image modalities (MULTI), respectively. The strategies were evaluated in a group of N510 healthy human subjects and semiquantitatively assessed by three experienced raters. Template quality was compared statistically via worth estimates (WEs) obtained with a log-linear Bradley-Terry model. Results: The overall quality of the templates was better for strategies including both susceptibility and T1w contrast (MULTI: WE50.62; HYBRID: WE50.21), but the best method depended on the anatomical region of interest. While methods using only one modality resulted in lower WEs, lowest overall WEs were obtained when only T1w images were used (DIRECT: WE50.12; U-CONV: WE50.05). Conclusion: Template generation strategies that employ only magnetic susceptibility contrast or both magnetic susceptibility and T1w contrast produce templates with the highest quality. The optimal approach depends on the anatomical structures of interest. The established approach of using only T1w images (CONV) results in reduced image quality compared to all other approaches studied. Level of Evidence: 2 Technical Efficacy: Stage 1