Is the magnetization transfer ratio a marker for myelin in multiple sclerosis?

PURPOSE: To investigate the correlation between water content (WC) and magnetization transfer ratio (MTR) in normal and multiple sclerosis (MS) brain. The MTR has been proposed as a marker for myelin in central nervous system tissue. However, changes in WC due to inflammation and edema may also affect the MTR. MATERIALS AND METHODS: Seven MS subjects with active disease and seven age- and gender-matched controls were scanned using quantitative magnetic resonance techniques. WC, myelin water content, T(1) relaxation time, and MTR were calculated from areas of lesion (divided into new lesions less than 2 months old, isointense T(1) lesions, and hypointense T(1) lesions), contralateral normal-appearing white matter (NAWM), and location-matched normal white matter (NWM) in controls. Linear regression was used to determine the correlation between WC and MTR. RESULTS: A significant correlation was found between WC and MTR across all tissue (R = -0.65, P < 0.0005). CONCLUSION: MTR was correlated with WC in MS tissue, indicating that inflammation and edema influence MTR. Therefore, caution should be used when associating MTR exclusively with myelin content.