The impact of coregistration of gradient recalled echo images on quantitative susceptibility and R2* mapping at 7T

Introduction: While image coregistration is inevitable and various quantitative parametric maps, such as R2* and QSM, are increasingly used in multi-parametric studies of the brain, there is a lack of investigations on the reliability of the calculated quantitative metrics after image coregistration. The purposes of this study were 1) to evaluate the reliability of R2* and QSM at 7T and 2) to assess the statistical agreement in the quantitative metrics obtained by two different coregistration approaches. Methods: We compared the reliability of R2* and quantitative susceptibility maps obtained from brains of eight healthy participants by two coregistration approaches: 1) transformation of pre-processed quantitative maps from native space to a reference space and 2) processing quantitative maps after transformation of source images to a reference space using pixel- and ROI-based analyses. Two-sample Kolmogorov-Smirnov (KS-test), Mann-Whitney U (U-test), Paired T, Intraclass-correlation (ICC) tests were performed appropriately. Results: R2* remained invariant after either GRE data coregistration (p > 0.095) or direct R2* map coregistration (p > 0.077). However, magnetic susceptibility was significantly altered (58/64 KS-tests and 58/64 U-tests with p-value 0.091) in all KS-tests and pixel values (PVs) were only different in 2 out of 64 U-tests for direct QSM coregistration. Paired t-test revealed that ROI-based group-mean R2* was invariant (p-value at least 0.154) to both approaches, while group-mean susceptibility was invariant (p at least 0.325) to direct coregistration but differed (p 0.9). ICCs for magnetic susceptibility were excellent (> 0.9) when processed in its native space while the ICCs were lower than 0.9 or poor (< 0.5) when processed in transformed space. Further analysis revealed that the choice of interpolation approach affected the resultant. Conclusion: Our study shows that R2* could be safely processed in a transformed space, whereas QSM was less reliable when processed in the transformed space. Hence, caution is advised when using QSM in a multi-parametric study, and it is strongly recommended to process QSM in its native space prior to any coregistration or spatial transformation.