PATTERNS OF LESION DEVELOPMENT IN MULTIPLE-SCLEROSIS - LONGITUDINAL OBSERVATIONS WITH T1-WEIGHTED SPIN-ECHO AND MAGNETIZATION-TRANSFER MR

PURPOSE: We evaluated the appearance of enhancing multiple sclerosis ( MS) lesions on unenhanced T1-weighted MR images and the natural course of enhancing MS lesions on serial unenhanced TI-weighted and magnetiz ation transfer (MT) MR images, METHODS: One hundred twenty-six enhanci ng lesions were followed up monthly for 6 to 12 months to determine th eir signal intensity on unenhanced T1-weighted and MT MR images. At th e time of initial enhancement, the size of the lesion and the contrast ratio of enhancement were calculated for each enhancing lesion. Durin g follow-up, the contrast ratio on the corresponding unenhanced T1-wei ghted image was measured, and an MT ratio (MTR) nas calculated. RESULT S: Twenty-five enhancing lesions (20%) appeared isointense and 101 les ions (80%) appeared hypointense relative to normal-appearing white mat ter on unenhanced T1-weighted images. During 6 months of follow-up, fo ur MR patterns of active lesions were detected: initially isointense l esions remained isointense (15%); initially isointense lesions became hypointense (5%, most of which reenhanced); initially hypointense lesi ons became isointense (44%); and initially hypointense lesions remaine d hypointense (36%). MTR was significantly lower for hypointense lesio ns as compared with isointense lesions at the time of initial enhancem ent. For lesions that changed from hypointense to isointense, MTR incr eased significantly during 6 months of follow-up, Multiple regression analysis showed that strongly decreased MTR at the time of initial enh ancement and enhancement duration of more than one scan were predictiv e of a hypointense appearance on unenhanced T1-weighted images at 6 mo nths' follow-up. Ring enhancement was found to be the only (weak) pred ictor of persistently hypointense signal intensity. CONCLUSION: Most e nhancing lesions appear slightly to significantly hypointense on unenh anced T1-weighted images. Although most hypointensities are reversible , only those lesions that fail to recover on unenhanced T1-weighted an d MT images may have considerable irreversible structural changes.