Investigation of intrascanner reproducibility of quantitative susceptibility mapping (QSM) and R2* at 3T

Xiang Feng^{1}, Andreas Deistung^{1}, Marianne Cleve^{1}, Ferdinand Schweser^{2,3}, and Juergen Reichenbach^{1}

**Acquisition**: Eight young subjects (female, age 24.1±2.4 years) were scanned 4 times using the same measurement protocol on a 3T MRI scanner (Tim Trio, Siemens Healthcare, Erlangen, Germany). The time periods between two consecutive scans in a single 4-scan-series were variable and ranged from 3 (smallest) to 38 (largest) days. The acquisition protocol included whole brain T1-weighted imaging (MP-RAGE sequence) with isotropic spatial resolution of 1mm and whole brain multi-echo GRE imaging (TE_{1}-TE_{6}/ΔTE/TR=3.07ms-27.72ms/4.93ms/32ms, FA=20°, voxel size = 0.57mm×0.57mm×2.00mm).

**Analysis**: The multi-echo GRE phase images were processed with sophisticated harmonic artifact reduction for phase data (SHARP) [8] and homogeneity enabled incremental dipole inversion (HEIDI) [9] to calculate susceptibility maps. R2* maps were calculated by mono-exponential fitting of the signal decay of GRE magnitude images. Six subcortical structures, including putamen, globus pallidus, caudate nucleus, accumbens, hippocampus, and thalamus, were segmented using FIRST [10] incorporating a dedicated hybrid contrast and nonlinear registration (HC-nlFIRST framework) [11]. Cerebrospinal fluid (CSF) in the ventricles was segmented using FMRIB’s Automated Segmentation Tool (FAST) on T1-weighted images, and then transformed to the GRE image space. Additionally, the region of interest (ROI) for frontal WM (fWM) was manually defined in MNI space and then warped to the GRE image space for each subject using the affine transformation matrix and inverse warping field generated from the HC-nlFIRST framework.

**ROI-based analysis of reproducibility**: Referencing of susceptibility values was compared with respect to fWM and CSF, where the global offset for QSM was determined by the mean value of the reference region. Mean and standard deviations (SD) of susceptibility values and R2* of the subcortical structures determined from all repetitions were statistically compared using analysis of variances (ANOVA) and intra-class correlation (ICC).

Fig.1: Axial susceptibility and R2* maps obtained in 4 scan sessions from the same subject demonstrating good visual similarity in the depiction of subcortical structures. (a) – (d) Susceptibility maps, where ‘DX’ represents the scanning date. (e) – (f) Corresponding R2* maps.

Fig.2: Automatically segmented ROIs (b,d) of subcortical structures with HC-nlFIRST. Comparison of these segmentations with the corresponding susceptibility (a) and R2* map (c) reveals accurate alignment of the anatomical subcortical structure boundary (blue: globus pallidus, purple: putamen, yellow: hippocampus, green: thalamus, cyan-blue: caudate nucleus).

Fig.3: Quantitative analysis of normalization of QSM. (a-b) The histograms of susceptibility values in the CSF and fWM are shown from 2 random selected subjects in 4 scan series. Mean susceptibility values in globus pallidus (GP) referenced to (c) fWM and (d) CSF are presented as boxplots for all subjects.

Table 1: Summary of mean values, inter-subject standard deviation of susceptibility values, R2* and volumes in the 6 subcortical structures.

Table 2: Summary of p-value of the ANOVA and ICC values from 4 scan-series in the 6 subcortical structures.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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