Quantitative susceptibility mapping in multiple sclerosis

Abstract

Purpose: To apply quantitative susceptibility mapping (QSM) in the basal ganglia of patients with multiple sclerosis (MS) and relate the findings to R2* mapping with regard to the sensitivity for clinical and morphologic measures of disease severity.

Materials and methods: The local ethics committee approved this study, and all subjects gave written informed consent. Sixty-eight patients (26 with clinically isolated syndrome, 42 with relapsing-remitting MS) and 23 control subjects underwent 3-T magnetic resonance (MR) imaging. Susceptibility and R2* maps were reconstructed from the same three-dimensional multiecho spoiled gradient-echo sequence. Mean susceptibilities and R2* rates were measured in the basal ganglia and were compared between different phenotypes of the disease (clinically isolated syndrome, MS) and the control subjects by using analysis of variance, and regressing analysis was used to identify independent predictors.

Results: Compared with control subjects, patients with MS and clinically isolated syndrome had increased (more paramagnetic) magnetic susceptibilities in the basal ganglia. R2* mapping proved less sensitive than QSM regarding group differences. The strongest predictor of magnetic susceptibility was age. Susceptibilities were higher with increasing neurologic deficits (r = 0.34, P < .01) and lower with normalized volumes of gray matter (r = -0.35, P < .005) and the cortex (r = -0.35, P < .005).

Conclusion: QSM provides superior sensitivity over R2* mapping in the detection of MS-related tissue changes in the basal ganglia. With QSM but not with R2* mapping, changes were already observed in patients with clinically isolated syndrome, which suggests that QSM can serve as a sensitive measure at the earliest stage of the disease.

Figure 1:

Overview of the image processing steps for QSM. A morphology mask (B) is calculated from the multiecho magnitude data (A,), which enables inclusion of morphologic information in the reconstruction process. The raw phase image (C) is unwrapped (D) and is then used to estimate the local field distribution by using projection onto dipole field (E). When solving the inverse problem from the local field (E), the morphologic information (B) is included in the morphology-enabled dipole inversion (MEDI), resulting in a quantitative susceptibility map (F).

Figure 2:

Representative R2* maps (top row) and quantitative susceptibility maps (bottom row) of two 29-year-old individuals, a healthy control subject and an MS patient. Note increased (more paramagnetic) susceptibility in the basal ganglia in the MS patient. Differences are most evident in the putamen (arrow, 0.049 vs 0.092 ppm). Image window settings were identical: R2* mapping, from 0 (black) to 40 sec⁻¹(white); QSM, from −0.1 (black) to 0.25 ppm (white).

Figure 3:

Box plots show mean regional R2* relaxation rates and magnetic susceptibilities in components of the basal ganglia (BG) and their average in CIS and MS patients and in control subjects (HC). Horizontal bars = significant differences. * = P < .05, ** = P < .01, *** = P < .001.