Effect of motion, cortical orientation and spatial resolution on quantitative imaging of cortical R₂* and magnetic susceptibility at 0.3 mm in-plane resolution at 7 T

MR images of the effective relaxation rate R₂* and magnetic susceptibility χ derived from multi-echo T₂*-weighted (T₂*w) MRI can provide insight into iron and myelin distributions in the brain, with the potential of providing biomarkers for neurological disorders. Quantification of R₂* and χ at submillimeter resolution in the cortex in vivo has been difficult because of challenges such as head motion, limited signal to noise ratio, long scan time, and motion related magnetic field fluctuations. This work aimed to improve the robustness for quantifying intracortical R₂* and χ and analyze the effects from motion, spatial resolution, and cortical orientation. T₂*w data was acquired with a spatial resolution of 0.3 × 0.3 × 0.4 mm³ at 7 T and downsampled to various lower resolutions. A combined correction for motion and B₀ changes was deployed using volumetric navigators. Such correction improved the T₂*w image quality rated by experienced image readers and test-retest reliability of R₂* and χ quantification with reduced median inter-scan differences up to 10 s⁻¹ and 5 ppb, respectively. R₂* and χ near the line of Gennari, a cortical layer high in iron and myelin, were as much as 10 s⁻¹ and 10 ppb higher than the region at adjacent cortical depth. In addition, a significant effect due to the cortical orientation relative to the static field (B₀) was observed in χ with a peak-to-peak amplitude of about 17 ppb. In retrospectively downsampled data, the capability to distinguish different cortical depth regions based on R₂* or χ contrast remained up to isotropic 0.5 mm resolution. This study highlights the unique characteristics of R₂* and χ along the cortical depth at submillimeter resolution and the need for motion and B₀ corrections for their robust quantification in vivo.