Typical MS lesions are ovoid and >3 mm in diameter and are easily identified on T2W (A), proton density weighted (B), and FLAIR (C) images They can occur anywhere in the brain, spinal cord, or optic nerves, and a periventricular distribution is common (A–C) Lesions in the corpus callosum (D) as well as flame-shaped perivenular lesions (D) are characteristic of MS In general, T2/PD sequences are optimal for posterior fossa lesion detection (E) when compared with FLAIR (F); however, FLAIR is better for periventricular (C) and juxtacortical lesions that are in contact with the cortex (G) T2/PD and FLAIR will detect the majority of MS plaques but are not specific to lesion age or severity (H) The corresponding Gd-enhanced T1 images will detect newly active lesions due to transient breakdown of the blood-brain barrier (I, open arrow) T1 hypointense lesions that not enhance and persist for or more months are permanent or chronic black holes and represent lesions with the greatest tissue damage (I, closed arrow) Diffusely abnormal white matter (DAWM) is present in 17% of RRMS patients visible on T2W images as large, diffuse lesions with poorly defined boundaries, usually located around the ventricles (J, open arrow) DAWM corresponds with extensive loss of myelin phospholipids and variable degrees of axonal loss DAWM spares the subcortical U fibers MR imaging is highly sensitive for detecting MS lesions but pathologically nonspecific Many white matter diseases can mimic the appearance of MS Examples included here are primary vasculitis of the central nervous system (A), cerebral autosomal dominant angiopathy with subcortical infarcts and leukoencephalopathy (B), lymphoma (C, D), acute disseminated encephalomyelitis (E), chronic hypertension (F), nonspecific unidentified bright objects (G), and enlarged perivascular spaces (H) MR imaging lesions are dynamic over time Most new lesions enhance transiently with Gd Often, this enhancement is accompanied by the formation of a new T2 lesion Although the Gd enhancement will disappear as the inflammation resolves, a new permanent T2 lesion remains Gd enhancement may also occur in pre-existing T2 lesions and is a sign of reactivation Persistent Gd enhancement for more than months would be extremely unusual for MS lesions In many cases, the only evidence of recent disease activity is the detection of a new T2 lesion, especially when MR imaging studies are preformed infrequently (A) Baseline PD image showing multiple T2 lesions, none of which enhance on the corresponding T1 postcontrast image (B) One month later, a new enhancing lesion is detected (D, open arrow) along with a corresponding new T2 lesion (C, open arrow) There is an additional new T2 lesion (arrow) that did not enhance (closed arrow) Tumefactive demyelinating lesions are an uncommon presentation occurring in both children and adults and can be confused with highgrade tumors or abscess, especially if the lesion is solitary Mass effect can be minimal or sometimes moderate (A, sagittal FLAIR), and an open ring enhancement pattern is common and favors demyelination (B, axial FLAIR; C, axial postcontrast T1) These unusual lesion types can also be seen in neuromyelitis optica (D–G) Over time, the lesions tend to decrease in size consistent with demyelination (E, baseline FLAIR; F, month; and G, month follow-up MR image) ... fibers MR imaging is highly sensitive for detecting MS lesions but pathologically nonspecific Many white matter diseases can mimic the appearance of MS Examples included here are primary vasculitis... in the corpus callosum (D) as well as flame-shaped perivenular lesions (D) are characteristic of MS In general, T2/PD sequences are optimal for posterior fossa lesion detection (E) when compared... juxtacortical lesions that are in contact with the cortex (G) T2/PD and FLAIR will detect the majority of MS plaques but are not specific to lesion age or severity (H) The corresponding Gd-enhanced T1 images