MAGNETIZATION-TRANSFER RATIOS IN MULTIPLE-SCLEROSIS LESIONS ENHANCINGAFTER DIFFERENT DOSES OF GADOLINIUM

We performed serial monthly magnetization transfer (MT) imaging to eva luate whether MS lesions that enhance only after the injection of a tr iple dose (TD) of gadolinium-DTPA (Gd) have different pathologic chara cteristics and evolution than those that enhance after the injection o f a standard dose (SD). Every 4 weeks far 3 months and in two separate sessions, we obtained T1-weighted scans from 10 patients with relapsi ng-remitting MS, 5 minutes after SD (0.1 mmol/kg) or TD (0.3 mmol/kg) Gd injection. During each of the first monthly sessions, we obtained M T images and dual-echo scans before Gd injection. We measured the MT r atio (MTR) of newly enhancing lesions on co-registered quantitative MT R images. During the 3-month follow-up, 81 newly enhancing lesions wer e seen on SD scans. An additional 46 lesions enhanced only on TD scans . The mean (+/- standard deviation) MTR values were 31.4% +/- 8.4% for lesions enhancing after SD and 38.2% +/- 6.0% for lesions enhancing o nly after TD injection (p < 0.0001). The mean MTR values of lesions se en with both SD (p < 0.00001) and TD (p = 0.002) increased significant ly during the follow-up. At each time point during the follow-up, the MTR values of TD lesions were significantly higher than the SD lesions . These results indicate that the enhancing lesion population in MS is heterogeneous and that the tissue damage occurring within lesions enh ancing only after TD injection is less severe than in lesions enhancin g after the injection of an SD.