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

Abstract

Introduction 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 1× Philips, 2× Siemens Magnetom, and 2× 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 i...

Extra-basal ganglia iron content and non-motor symptoms in drug-naïve, early Parkinson’s disease

“…Second, QSM is known to be affected by the subjects’ head orientation with respect to the main magnetic field. In a recent study [ 39 ], such dependence was reported to be about 0.01 ppm per 5° in the deep brain region. Since the head orientation was not controlled in our study, such orientation dependence could have added to the overall variability of our QSM results (Supplementary Material 3).…”

“…Since the head orientation was not controlled in our study, such orientation dependence could have added to the overall variability of our QSM results (Supplementary Material 3). Lastly, while R2* was calculated with a relatively robust method of voxel-wise fitting of the magnitude data, QSM was primarily based on the image phase, which is more prone to errors due to physiological motion and streaking artifacts [ 38 , 39 ]. Along with the small sample size, such increased variability may have rendered QSM weak in its statistical power to distinguish between different subject groups.…”

Multiparametric MRI for Characterization of the Basal Ganglia and the Midbrain

“…Moreover, no dedicated B 0 and B 1 correction or correction for gradient delays was performed. Susceptibility mapping has been shown not to depend on B 1 inhomogeneties even in multi-center ultra-high field studies QSM ( Rua et al, 2020 ; Voelker et al, 2021 ), but the correction of dynamic B 0 fluctuations improves image quality in susceptibility-based methods and could thus enable a better characterization for multi-contrast approaches including QSM segmentation ( Jorge et al, 2020 ). However, MTR measurements are more sensitive to B 1 inhomogeneites and especially for multi-site studies, B 1 correction would be advisable ( Barker et al, 2005 ).…”

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“…Alternative sequences have been used to observe changes in iron in the healthy brain and in disease (19,20) by measuring the T2* transverse relaxation time (or its inverse rate, R2*) or the local tissue magnetic susceptibly χ using quantitative susceptibility mapping (QSM) (21)(22)(23). Susceptibility weighted imaging (SWI) uses post-processing of T2*-w images, with the magnitude and filtered phase information providing image contrast from iron storage (24).…”