Reliability of quantitative magnetic susceptibility imaging metrics for cerebral cortex and major subcortical structures

Background and purposeSusceptibility estimates derived from quantitative susceptibility mapping (QSM) images for the cerebral cortex and major subcortical structures are variably reported in brain magnetic resonance imaging (MRI) studies, as average of all (mu all${{{{\mu}}}_{{\mathrm{all}}}}$), absolute (mu abs${{{{\mu}}}_{{\mathrm{abs}}}}$), or positive- (mu p${{{{\mu}}}_{\mathrm{p}}}$) and negative-only (mu n${{{{\mu}}}_{\mathrm{n}}}$) susceptibility values using a region of interest (ROI) approach. This pilot study presents a reliability analysis of currently used ROI-QSM metrics and an alternative ROI-based approach to obtain voxel-weighted ROI-QSM metrics (mu wp${{{{\mu}}}_{{\mathrm{wp}}}}$ and mu wn${{{{\mu}}}_{{\mathrm{wn}}}}$).Background and purposeSusceptibility estimates derived from quantitative susceptibility mapping (QSM) images for the cerebral cortex and major subcortical structures are variably reported in brain magnetic resonance imaging (MRI) studies, as average of all (mu all${{{{\mu}}}_{{\mathrm{all}}}}$), absolute (mu abs${{{{\mu}}}_{{\mathrm{abs}}}}$), or positive- (mu p${{{{\mu}}}_{\mathrm{p}}}$) and negative-only (mu n${{{{\mu}}}_{\mathrm{n}}}$) susceptibility values using a region of interest (ROI) approach. This pilot study presents a reliability analysis of currently used ROI-QSM metrics and an alternative ROI-based approach to obtain voxel-weighted ROI-QSM metrics (mu wp${{{{\mu}}}_{{\mathrm{wp}}}}$ and mu wn${{{{\mu}}}_{{\mathrm{wn}}}}$).MethodsTen healthy subjects underwent repeated (test-retest) 3-dimensional multi-echo gradient-echo (3DMEGE) 3 Tesla MRI measurements. Complex-valued 3DMEGE images were acquired and reconstructed with slice thicknesses of 1 and 2 mm (3DMEGE1, 3DMEGE2) along with 3DT1-weighted isometric (voxel 1 mm3) images for independent registration and ROI segmentation. Agreement, consistency, and reproducibility of ROI-QSM metrics were assessed through Bland-Altman analysis, intraclass correlation coefficient, and interscan and intersubject coefficient of variation (CoV).MethodsTen healthy subjects underwent repeated (test-retest) 3-dimensional multi-echo gradient-echo (3DMEGE) 3 Tesla MRI measurements. Complex-valued 3DMEGE images were acquired and reconstructed with slice thicknesses of 1 and 2 mm (3DMEGE1, 3DMEGE2) along with 3DT1-weighted isometric (voxel 1 mm3) images for independent registration and ROI segmentation. Agreement, consistency, and reproducibility of ROI-QSM metrics were assessed through Bland-Altman analysis, intraclass correlation coefficient, and interscan and intersubject coefficient of variation (CoV).ResultsAll ROI-QSM metrics exhibited good to excellent consistency and test-retest agreement with no proportional bias. Interscan CoV was higher for mu all${{{{\mu}}}_{{\mathrm{all}}}}$ in comparison to the other metrics where it was below 15%, in both 3DMEGE1 and 3DMEGE2 datasets. Intersubject CoV for mu all${{{{\mu}}}_{{\mathrm{all}}}}$ and mu abs${{{{\mu}}}_{{\mathrm{abs}}}}$ exceeded 50% in all ROIs.ConclusionsAmong the evaluated ROI-QSM metrics, mu all${{{{\mu}}}_{{\mathrm{all}}}}$ and mu abs${{{{\mu}}}_{{\mathrm{abs}}}}$ estimates were less reliable, whereas separating positive and negative values (using mu p,mu n,mu wp,mu wn${{{{\mu}}}_{\mathrm{p}}},\ {{{{\mu}}}_{\mathrm{n}}},\ {{{{\mu}}}_{{\mathrm{wp}}}},\ {{{{\mu}}}_{{\mathrm{wn}}}}$) improved the reproducibility within, and the comparability between, subjects, even when reducing the slice thickness.These preliminary findings may offer valuable insights toward standardizing ROI-QSM metrics across different patient cohorts and imaging settings in future clinical MRI studies.