Multi-centre, multi-vendor reproducibility of 7T QSM and R2* in the human brain: results from the UK7T study

Abstract:

We present the reliability of ultra-high field T 2 * MRI at 7T, as part of the UK7T Network’s “Travelling Heads” study. T 2 *-weighted MRI images can be processed to produce quantitative susceptibility maps (QSM) and R 2 * 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 2 * 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 2 * mapping at 7T, in readiness for future multi-site clinical studies. Methods Ten healthy volunteers were scanned with harmonised single- and multi-echo T 2 *-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 1x Philips, 2x Siemens Magnetom, and 2x Siemens Terra scanners. QSM and R 2 * 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 2 * 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 -1 (R 2 *). For χ this is 2.1-4.8 fold better than 3T reports, and 1.1-3.4 fold better for R 2 *. The median ICC from within- and cross-site R 2 * data was 0.98 and 0.91, respectively. Multi-echo QSM had greater variability vs single-echo QSM especially in areas with large B 0 inhomogeneity such as the inferior frontal cortex. Across sites, R 2 * values were more consistent than QSM in subcortical structures due to differences in B 0 -shimming. On a between-subject level, our measured χ and R 2 * 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 2 * 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.