Magnetization transfer imaging of the spinal cord and the optic nerve in patients with multiple sclerosis

Magnetization transfer (MT) imaging has been successfully applied to patients with multiple sclerosis (MS), showing lesion heterogeneity, subtle changes in the normal-appearing white matter, and a better correlation with disability, in comparison with conventional magnetic resonance imaging. MT imaging is a fairly simple technique, which allows a quantitative analysis with high spatial resolution to delicate structures like the optic nerve and spinal cord. In the spinal cord, MT imaging can be applied as a contrast augmentation technique. Using the MT ratio (MTR), two studies have reported a mild, but significant, reduction in MT ratio in the cervical spinal cord, compared with healthy controls. In one study, clinical disability correlated independently of cord atrophy with MTR, which may relate to preliminary findings of a correlation between axonal loss and MTR in the spinal cord. In the optic nerve, two studies reported strongly decreased MTR in affected nerves, even in the absence of lesions on conventional imaging; unaffected nerves showed values similar to white matter in the brain. In one study, MTR was significantly correlated with electrophysiological parameters, but not with vision. In conclusion, MT imaging provides a quantifiable parameter that can be applied with high spatial resolution to delicate structures, such as the spinal cord and the optic nerve. Further work is needed to correlate MTR measurements with pathology and, most importantly, with the functional status. Such relationships being established, a quantitative technique such as MTR could be useful in monitoring disease progression in MS.