ABSTRACT
Although conventional T2-weighted magnetic resonance imaging (MRI) is very sensitive for the detection of multiple sclerosis (MS) lesions, it is not without relevant limitations. First, MRI lacks specificity with regard to the heterogeneous pathological substrates of individual lesions[1] and, as a consequence, does not allow the characterization and quantification of tissue damage-specifically, edema, inflammation, demyelination, remyelination, gliosis, and axonal loss, all lead to a similar appearance of hyperintensity on T2-weighted images.[2] Secondly, T2-weighted images do not delineate tissue damage in the gray matter and in the normal-appearing white matter (NAWM), which usually represents a large portion of the brain tissue from MS patients and which is known to be damaged in MS.[3]
These limitations are only partially overcome by the use of postcontrast T1-weighted scans. Gadolinium-enhanced T1-weighted images allow active lesions to be distinguished from inactive lesions,[4,5] since enhancement occurs as a result of increased blood-brain barrier permeability[6] and corresponds to areas with ongoing inflammation.[7] However, the activity of the lesions as demonstrated on postcontrast T1-weighted imaging still does not provide information on tissue damage. Chronically hypointense areas on T1-weighted images correspond to areas where severe tissue disruption has occurred,[8] and their extent is correlated with the clinical severity of the disease and its evolution over time.[9,10] Even so, the extent of T1-hypointense lesions may not correspond to the severity of intrinsic pathology in the lesions and provides no information about damage to the NAWM and the gray matter. Finally, the definition of what constitutes a hypointense area is highly subjective.
