Imaging of axonal damage in multiple sclerosis: Spatial distribution of magnetic resonance imaging lesions

We performed magnetic resonance imaging and magnetic resonance spectroscopic imaging on 28 patients with multiple sclerosis stratified for disability and clinical course (relapsing with at least partial remissions or secondary progressive disease). Lesions were segmented on the conventional proton density and T2‐weighted magnetic resonance images, and lesion distribution images were generated for each patient. The conventional magnetic resonance and spectroscopic images were transformed into a standard brain‐based stereotaxic coordinate space, allowing comparison of images from different patients on a voxel‐by‐voxel basis. The spatial distribution of lesions in the transformed magnetic resonance images did not differ significantly between the relapsing and the progressive disease groups. We then generated from the individual data sets, group lesion probability distribution images for the relapsing and the progressive disease groups. The spatial distribution of metabolites was characterized with respect to lesion distribution using the magnetic resonance spectroscopic images transformed into stereotaxic space and averaged. The neuronal marker N‐acetylaspartate was diffusely lower in the multiple sclerosis patients than in normal control subjects. Comparison of the averaged metabolite and T2‐weighted lesion probability images confirmed loss of N‐acetylaspartate in regions of both high and low lesion probability. This suggests that diffuse axonal volume loss or dysfunction extends beyond the inflammatory lesions of multiple sclerosis, perhaps due to microscopic disease or wallerian degeneration along projection pathways of axons traversing the lesions.