T2-weighted and fluid-attenuated inversion-recovery (FLAIR) images are sensitive for depicting focal lesions in patients with multiple sclerosis (MS), but lack histopathologic specificity. Other lesions such as inflammation, edema, demyelination, remyelination, reactive gliosis and axonal loss have an MR appearance similar to MS lesions and can often not be distinguished from MS . Hypointense T1 lesions in MS are usually caused by matrix destruction and loss of axons . These lesions, which are hypointense on T1-weighted images and have a low magnetization transfer ratio (MTR), correlate better with clinical disability than proton density/T2 lesions. A low MTR in hypointense lesions on T1-weighted images has in one study been related to clinically more severe MS . Decreased magnetization transfer ratio is, however, also observed in normal-appearing white matter in MS patients .
Increased apparent diffusion coefficient (ADC) values and decreased fractional anisotropy can be seen in normal-appearing white matter of patients with MS. This is clearly different from healthy control subjects, where these abnormalities are not seen [5, 6]. The ADC and fractional abnormalities may repre sent occult small MS plaques, gliosis or wallerian degeneration.
Multiple sclerosis plaques usually show hyper- or isointensity on diffusion-weighted (DW) images, with increased ADC, in both contrast-enhancing active plaques (Fig. 9.1) and chronic plaques (Fig. 9.2). MS plaques are reported to have decreased anisotropy [6,7]. The increased ADC and decreased anisotropy in MS plaques are thought to be related to an increase in the extracellular space due to demyeli-nation, perivascular inflammation with vasogenic edema, and gliosis. An enhancing portion of MS plaques has slightly increased ADC, histologically representing prominent inflammation with mild de-myelination, while the non-enhancing portions tend to have more increased ADC, representing scarring with mild inflammation and myelin loss . ADC values of MS plaques seem to be related to the severity of MS. The ADC values in secondary-progressive MS are higher than those in relapsing-remitting MS .
In the acute phase of MS, decreased ADC can also be observed in plaques, although it is rare  (Figs. 9.3 and 9.4). Decreased ADC of plaques is presumably caused by intramyelinic edema, which may be located in the periphery of a plaque. Intramyelinic edema occurs in the myelin sheath itself and/or in the in-tramyelinic cleft. Some of the intramyelinic edema is reversible, probably because the edema is mainly located in the intramyelinic cleft.
Multiple sclerosis in a 28-year-old man presenting with visual problems. a T2-weighted image shows a hyperintense lesion in the right frontotemporal region (arrow). b Gadolinium T1-weighted image shows mild enhancement of this lesion, representing an active plaque.c,d DW image (c) shows a hyperintense lesion associated with increased ADC (d), T2 shine-through (arrow)
Multiple sclerosis in a 59-year-old man with a long history of recurrent seizures. T2-weighted (a) and FLAIR (b) images show multiple periventricular hyperintense lesions with ventricular dilatation. c On gadolinium Tl-weighted image with magnetization transfer contrast,there is no enhancement of these lesions, representing relatively chronic plaques.d DW image shows a right frontal lesion as mildly hyperintense (arrow). e ADC is increased (T2 shine-through) and the other periventricular lesions are isointense with increased ADC (T2 washout)
Multiple sclerosis in a 36-year-old woman presenting with subacute onset of progressive aphasia. a T2-weighted image shows a hyperintense lesion in the left periventricular white matter (arrow). b Gadolinium T1-weighted image with magnetization transfer contrast shows rim enhancement of this lesion.c DW image shows combination of a moderately hyperintense (arrow) and a significantly hyperintense lesion (arrowheads).d The moderately hyperintense lesion on DW image with increased ADC may represent demyelination (arrows), and the very hyperintense lesion on DW image with decreased ADC may represent intramyelinic edema (arrowheads). e Histopathology of another case shows that in-tramyelinic edema (arrows) is located in the periphery of a plaque (PL) (Luxol fast blue PAS stain, original magnification x40). (From )
Multiple sclerosis in a 13-year-old female presented with acute-onset right-sided weakness and dysarthria. a T2-weight-ed image shows multiple hyperintense lesions in the bilateral centrum semiovale (arrows). b DW image shows some lesions as hyperintense (arrows). c ADC is decreased representing acute cytotoxic plaques (arrows)
Acute disseminated encephalomyelitis in a 15-year-old male presenting with right hemiparesis. a T2-weighted image shows multiple hyperintense lesions in the left frontal lobe (arrows) b Gadolinium Tl-weighted image with magnetization transfer contrast shows inhomogeneous enhancement of these lesions. c DW image shows left frontal lesions as hyperintense due to T2 shine-through. d ADC is increased
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