Quantitative susceptibility mapping (QSM) and R₂* in the human brain at 3 T. Evaluation of intra-scanner repeatability

Quantitative susceptibility mapping (QSM) and the effective transverse relaxation rate (R₂*) can be used to monitor iron and myelin content in brain tissue, which are both subject to changes in many neurological diseases but also during healthy aging. In this study, we quantitatively assessed the repeatability of QSM and R₂* by applying four independent scans in eight young healthy, female subjects on a 3 T MRI scanner. Since QSM does not yield absolute values for bulk magnetic susceptibilities, we additionally investigated the influence of the choice of a reference brain region for susceptibility by computing susceptibility differences with respect to five different brain structures (whole brain, frontal white matter (fWM), internal capsule (IC), cerebrospinal fluid (CSF) in the lateral ventricle, cortical gray matter (cGM)). The intra-class correlation coefficient (ICC), variance ratio (VR) and repeatability coefficient (RC) were used to evaluate the repeatability of the calculated susceptibility differences and the R₂* values in six different subcortical brain structures. Linear regression was used to analyze the correlation between susceptibility differences and R₂*. We found that the susceptibility differences with respect to each investigated reference region (0.868 ≤ mean ICC ≤ 0.914) and the R₂* values (mean ICC = 0.923) were highly repeatable across the four times repeated scans. With consistently higher ICC, higher VR and lower RC, whole brain and cGM appeared to be the two most suitable reference regions for QSM with respect to repeatability.