Magnetization transfer ratios of multiple sclerosis lesions with variable durations of enhancement

We evaluated whether new multiple sclerosis (MS) lesions with variable durations of enhancement have different magnetization transfer ratios (MTR) at the time of their appearance. We scanned ten patients with relapsing- remitting MS every four weeks on four occasions. During each of the monthly sessions, we obtained dual echo, MT and T₁-weighted scans. Five minutes after gadolinium-DTPA (Gd) injection, another T₁-weighted scan was obtained. We measured the MTR of new enhancing lesions on co-registered quantitative MTR images. During the three-month follow up, 54 new enhancing lesions were seen with a mean MTR value of 33.6% (SD=7.8%). The mean MTR values were 35.3% (SD=7.0%) for lesions enhancing on only one scan and 29.3% (SD=8.6%) for those enhancing on at least two consecutive scans (P=0.01). These results suggest that enhancing lesions in MS have heterogeneous pathological substrates, which may be associated with different durations of the enhancing phase.