Magnetization Transfer Contrast - Is it Sensitive Enough to Differentiate Demyelination from Axonal Loss?

Magnetization transfer contrast imaging (MTC) exploits the principle of exchange of energy between the bound and free protons and was shown to be pathologically informative. There is, however, controversy as to whether it correlates with axonal loss (AL), demyelination (DM), or both. This study addresses the pathophysiological process that underlies the white matter injury using the metric derivative of MTC, magnetization transfer ratio (MTR), and defines the role of MTR in identifying the different stages of inflammation, that is, edema, DM, and AL, using optic nerve as the model.

One hundred and forty-two patients with a single, unilateral episode of optic neuritis (ON) were included in the study. Patients were divided into three groups - those with AL, those with DM, and those who were clinically optic neurites but without any electrophysiological changes suggestive of either AL or DM. MTR and electrophysiological studies were performed in the post-acute stage of ON and the results were compared to those obtained from the unaffected optic nerve.

MTR was significantly reduced in the optic nerves of both DM and AL groups when compared to that in normal optic nerves (P < 0.001). The difference in MTR between the AL and DM groups did not reach statistical significance. Patient group with acute ON did not show any change in the MTR values compared to the normal controls.

MTR is a sensitive technique to identify neuronal injury, whether it is DM or AL. It, however, cannot differentiate these two pathological processes. MTR is not sensitive to identify acute ON.