White matter diseases can be divided into demyelinating diseases, in which the white matter is normally formed and then pathologically destroyed, and dysmyelinating diseases, in which there is usually a genetically determined enzymatic disorder that interferes with the normal production or maintenance of myelin (86). The enzymatic disturbances are relatively rare; therefore, their imaging characteristics will not be described.
The most common of the demyelinating disorders is multiple sclerosis (MS). The demyelinating plaques of MS are better visualized by MRI than by CT. In fact, MRI has become the primary complementary test to confirm a clinical diagnosis of MS. It also provides a quantitative means of evaluating the present state of a patient’s disease and a mode of following its progress (87). Although the T1-weighted MR images are usually normal, the FLAIR and T2-weighted images demonstrate MS plaques as high–signal-intensity areas. These are most frequently seen in the periventricular white matter, especially around the atrium and the tips of the anterior and posterior horns of the lateral ventricles (Fig. 6-94A,B). The high–signal-intensity plaques also can be seen in other white matter areas of the cerebral hemispheres, the brain stem, and even the upper spinal cord. When these lesions are seen in patients younger than 40 years of age, they tend to be relatively specific for MS (86). In patients more than 50 years of age, the MRI findings of MS are similar to findings in some aging brains, and correlation with the clinical findings helps establish the diagnosis. Recent MS plaques that involve damage to the blood-brain barrier frequently enhance with the use of IV gadolinium-DTPA (Fig. 6-95).
FIGURE 6-94. T2-weighted (A) and FLAIR (B) MRI demonstrates the periventricular demyelinating plaques of MS as hyper- intense areas (arrows ) adjacent to the anterior horns and atria of the lateral ventricles.
FIGURE 6-95. Active MS. T1 gadolinium enhanced MR image shows periventricular enhancing MS plaques (arrows).
CT demonstrates MS plaques with less reliability than does MRI. On CT, these plaques appear as areas of hypodensity. Recent plaques in the acute phase of an exacerbation of the disease will have damage to the blood-brain barrier, and IV contrast will then enhance the periphery of the lesion. In the chronic plaque, no contrast enhancement occurs on CT or MRI. Other demyelinating diseases, although numerous, are of relatively low incidence and therefore are not described.