Multi-centre, multi-vendor reproducibility of 7T QSM and R* in the human brain: Results from the UK7T study.

Introduction: We present the reliability of ultra-high field T* MRI at 7T, as part of the UK7T Network's "Travelling Heads" study. T*-weighted MRI images can be processed to produce quantitative susceptibility maps (QSM) and R* maps. These reflect iron and myelin concentrations, which are altered in many pathophysiological processes. The relaxation parameters of human brain tissue are such that R* mapping and QSM show particularly strong gains in contrast-to-noise ratio at ultra-high field (7T) vs clinical field strengths (1.5-3T). We aimed to determine the inter-subject and inter-site reproducibility of QSM and R* mapping at 7T, in readiness for future multi-site clinical studies.

Methods: Ten healthy volunteers were scanned with harmonised single- and multi-echo T*-weighted gradient echo pulse sequences. Participants were scanned five times at each "home" site and once at each of four other sites. The five sites had 1× Philips, 2× Siemens Magnetom, and 2× Siemens Terra scanners. QSM and R* maps were computed with the Multi-Scale Dipole Inversion (MSDI) algorithm (https://github.com/fil-physics/Publication-Code). Results were assessed in relevant subcortical and cortical regions of interest (ROIs) defined manually or by the MNI152 standard space.

Results And Discussion: Mean susceptibility (χ) and R* values agreed broadly with literature values in all ROIs. The inter-site within-subject standard deviation was 0.001-0.005 ppm (χ) and 0.0005-0.001 ms (R*). For χ this is 2.1-4.8 fold better than 3T reports, and 1.1-3.4 fold better for R*. The median ICC from within- and cross-site R* data was 0.98 and 0.91, respectively. Multi-echo QSM had greater variability vs single-echo QSM especially in areas with large B inhomogeneity such as the inferior frontal cortex. Across sites, R* values were more consistent than QSM in subcortical structures due to differences in B-shimming. On a between-subject level, our measured χ and R* cross-site variance is comparable to within-site variance in the literature, suggesting that it is reasonable to pool data across sites using our harmonised protocol.

Conclusion: The harmonized UK7T protocol and pipeline delivers on average a 3-fold improvement in the coefficient of reproducibility for QSM and R* at 7T compared to previous reports of multi-site reproducibility at 3T. These protocols are ready for use in multi-site clinical studies at 7T.