Effects of MP2RAGE B1+sensitivity on inter-site T1reproducibility and morphometry at 7T

Most neuroanatomical studies are based on MR images, whose intensity profiles are not solely determined by the tissue’s longitudinal relaxation times (T 1 ) but also affected by varying non-T 1 contributions, hampering data reproducibility. In contrast, quantitative imaging using the MP2RAGE sequence, for example, allows direct characterization of the brain based on the tissue property of interest. Combined with 7 Tesla (7T) MRI, this offers unique opportunities to obtain robust high-resolution brain data characterized by a high reproducibility, sensitivity and specificity. However, specific MP2RAGE parameters choices – e.g., to emphasize intracortical myelin-dependent contrast variations – can substantially impact image quality and cortical analyses through remnants of B 1 + -related intensity variations, as illustrated in our previous work. To follow up on this: we (1) validate this protocol effect using a dataset acquired with a particularly B 1 + insensitive set of MP2RAGE parameters combined with parallel transmission excitation; and (2) extend our analyses to evaluate the effects on hippocampal and subcortical morphometry. The latter remained unexplored initially but will provide important insights related to generalizability and reproducibility of neurodegenerative research using 7T MRI. We confirm that B 1 + inhomogeneities have a considerably variable effect on cortical T 1 and thickness estimates, as well as on hippocampal and subcortical morphometry depending on MP2RAGE setup. While T 1 differed substantially across datasets initially, we show inter-site T 1 comparability improves after correcting for the spatially varying B 1 + field using a separately acquired Sa2RAGE B 1 + map. Finally, as for cortical thickness, removal of B 1 + residuals affects hippocampal and subcortical volumetry and boundary definitions, particularly near structures characterized by strong intensity changes (e.g. cerebral spinal fluid and arteries). Taken together, we show that the choice of MP2RAGE parameters can impact T 1 comparability across sites and present evidence that hippocampal and subcortical segmentation results are modulated by B 1 + inhomogeneities. This calls for careful (1) consideration of sequence parameters when setting acquisition protocols; as well as (2) interpretation of results focused on neuroanatomical changes due to disease. Highlights Previously observed effects of B 1 + inhomogeneities on cortical T 1 and thickness depend strongly on MP2RAGE parameters Inter-site comparability of cortical T 1 and thickness greatly improves after removal of B 1 + residuals Post-hoc MP2RAGE B 1 + correction affects hippocampal (and subcortical) size and shape analyses Neuroradiological research would benefit from careful examination of imaging protocols and their impact on results, especially when B 1 + maps are not acquired