Quantitative T1 and Effective Proton Density (PD*) mapping in children and adults at 7T from an MP2RAGE sequence optimised for uniform T1-weighted (UNI) and FLuid And White matter Suppression (FLAWS) contrasts

Introduction Quantitative MRI is important for non-invasive tissue characterisation. In previous work we developed a clinically feasible multi-contrast protocol for T1-weighted imaging based on the MP2RAGE sequence that was optimised for both children and adults. It was demonstrated that a range of Fluid And White Matter Suppression (FLAWS) related contrasts could be produced while maintaining T1-weighted uniform image (UNI) quality, a challenge at higher field strengths. Here we introduce an approach to use these images to calculate effective proton density (PD*) and quantitative T1 relaxation maps especially for shorter repetition times (TRMP2RAGE) than those typically used previously. Methods T1 and PD* were estimated from the analytical equations of the MP2RAGE signal derived for partial Fourier acquisitions. The sensitivity of the fitting results was evaluated with respect to the TRMP2RAGE and B1+ effects on both excitation flip angles and inversion efficiency and compared to vendor T1 maps which do not use B1+ information. Data acquired for a range of individuals (aged 10-54 years) at the shortest TRMP2RAGE (4000ms) were compared across white matter (WM), cortical grey matter, and deep grey matter regions. Results The T1 values were insensitive to the choice of different TRMP2RAGE. The results were similar to the vendor T1 maps if the B1+ effects on the excitation flip angle and inversion efficiency were not included in the fits. T1 values varied over development into adulthood, especially for the deep grey matter regions whereas only a very small difference was observed for WM T1. Effective PD maps were produced which did not show a significant difference between children and adults for the age range included. Conclusion We produced PD* maps and improved the accuracy of T1 maps from an MP2RAGE protocol that is optimised for UNI and FLAWS-related contrasts in a single scan at 7T by incorporating the excitation flip angle and inversion efficiency related effects of B1+ in the fitting. This multi-parametric protocol made it possible to acquire high resolution images (0.65mm iso) in children and adults within a clinically feasible duration (7:18 min:s). The combination of analytical equations utilizing B1+ maps led to T1 fits that were consistent at different TRMP2RAGE values. Average WM T1 values of adults and children were very similar (1092ms vs 1117ms) while expected reductions in T1 with age were found for GM especially for deep GM. ### Competing Interest Statement Raphael Tomi-Tricot is an employee at Siemens Healthineers. ### Funding Statement This research was supported by GOSHCC Sparks Grant V4419, King's Health Partners, in part by the Medical Research Council (UK) (grants MR/ K006355/1 and MR/LO11530/1) and Medical Research Council Center for Neurodevelopmental Disorders, King's College London (MR/N026063/1), and by core funding from the Wellcome EPSRC Centre for Medical Engineering at King's College London [WT203148/Z/16/Z]. J.O.M, K.V, and C.C were funded by a Sir Henry Dale Fellowship jointly by the Wellcome Trust and the Royal Society (206675/Z/17/Z). C.C was also funded by a grant from GOSHCC (VC1421). M.E was funded by Action Medical Research (GN2835) and the British Paediatric Neurology Association. This research was funded in whole, or in part, by the Wellcome Trust [WT203148/Z/16/Z and 206675/Z/17/Z] and by the National Institute for Health Research (NIHR) Biomedical Research Centre based at Guy's and St Thomas' NHS Foundation Trust and King's College London and/or the NIHR Clinical Research Facility. The views expressed are those of the author(s) and not necessarily those of the NHS, the NIHR or the Department of Health and Social Care. For the purpose of open access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission. The authors would like to acknowledge David Leitão, Ronald Mooiweer, and Oral Ersoy Dokumacı for valuable discussions. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Institutional ethics committee of King's College London gave ethical approval for this work (REMAS 8700 and 18/LO/1766). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes The datasets that support the findings of this study are available upon request for any reasonable scientific purposes.