Specific Imaging Findings

Imaging features indicative of Central Nervous System Vasculitis (CNSV) are multiple infarcts of various ages in more than one vascular territory. CT images may be negative in the hyperacute phase and multifocal low-density lesions may be depicted in the acute–subacute phase. Localized subarachnoid hemorrhage is occasionally seen. On MRI, CNSV is characterized by scattered areas of high T2 and low T1 signal intensity in the deep gray matter, cortical and subcortical regions, which may be quite nonspecific. In the acute–subacute phase dark foci of cytotoxic edema are present on ADC maps. Increase in ADC values in the otherwise normal-appearing white matter may represent vasogenic edema, chronic destruction, and/or Wallerian degeneration. Microhemorrhages are frequently present on T2*-weighted images, subarachnoid blood is sometimes seen, usually along the cortical sulci. Scattered areas of leptomeningeal enhancement with underlying brain signal abnormalities are highly characteristic for vasculitis, but are seen in only about a third of cases. Wall thickening and enhancement of large intracranial vessels is a diagnostic sign of CNSV, present in most patients. Classic angiographic findings are diffuse segmental arterial wall narrowing and associated poststenotic dilation, referred to as “beading” or “string-of-beads”. Multi-vessel involvement increases the suspicion for CNSV. When more than two stenoses in at least two separate vascular distributions are depicted on MRA, DSA is not needed.

Pertinent Clinical Information

CNSV can affect any age group, but is more common in individuals 40–60 years old. Clinical manifestations are nonspecific, including headache, encephalopathy and, less frequently, focal neurological deficits. Diagnosis is often very difficult. Cerebral catheter angiography has been the standard for presumed vasculitis, with sensitivity in the range of about 80%, while MRA (and CTA) are slightly less sensitive. Cerebral ormeningeal biopsy is the gold standard but also has limited sensitivity. A combination of steroids, immunosuppressants, and immunomodulating agents is used for treatment.

Specific Imaging Findings

Cranial nerve (CN) schwannoma (neuroma, neurinoma, Schwann cell tumor, neurolemmoma, neurilemmoma) is typically welldelineated, hypodense on CT and T1 hypointense to isointense to the pons on pre-contrast images and with avid post-contrast enhancement. Schwannomas can arise along any cranial nerve,most commonly the vestibular nerve in the internal auditory canal (IAC), followed by the trigeminal nerve (including Meckel's cave), facial nerve, glossopharyngeal, vagus, and spinal accessory nerve. Schwannomas involving oculomotor, trochlear, abducens, and hypoglossal nerves are rare. Enhancement may be homogenous or heterogenous, based on the absence or presence of necrosis and/or intramural cysts. Calcifications are not present. Smaller tumors are usually round, ovoid-shaped or less frequently poly-lobular, T2 iso- to hypointense. Larger tumors are typically of high T2 signal and heterogenous with internal cystic areas. Schwannomas exhibit increased diffusion and are hyperintense on ADC maps. Eighty-five percent of vestibular schwannomas are centered at the IAC meatus. Larger tumors show the typical “ice creamon cone” appearance and usually lead to smooth widening of the internal auditory canal. Arachnoid cysts/trapped CSF are found around the tumor in a minority of cases, while increased signal within the ipsilateral inner ear structures is frequently present on FLAIR images.

Specific Imaging Findings

Cerebral cavernous malformations (CCM, cavernous angiomas/hemangiomas, cavernomas) show characteristically mild-to-moderate increased density on unenhanced CT reflecting the presence of calcium and blood at various stages of degradation with some cavernomas disclosing clear internal calcifications. MR imaging of CCM demonstrates typical popcorn-like multilobulated lesion with smooth margins and multiple internal septations. There is characteristically high T2 signal centrally, frequently with associated T1 hyperintensities, and a T2 dark peripheral hemosiderin ring. Smaller cavernomas may appear as foci of just low T2 signal. On T2* imaging the characteristic signal loss from hemosiderin due to susceptibility effects reveals cavernomas missed on other sequences. T2* images are very helpful in patients with CCM, as they frequently show multiple lesions, especially in familial cases. Surrounding edema and mass effect may be seen in acute hemorrhage. T1 hyperintensity in the perilesional edema appears to be a characteristic feature of CCMs with a recent bleed. Contrast enhancement is usually absent, while peripheral enhancement may surround an acute hematoma. There is a commonly observed association with a developmental venous anomaly (DVA), which does enhance. CCM are otherwise occult at DSA, CTA, and MRA.

Pertinent Clinical Information

CCM are common vascular malformations of the CNS that may present with seizures, headache, or focal neurological deficits, usually from hemorrhage. They may also be asymptomatic and incidentally discovered. CCM can occur everywhere in the brain, including the extra-axial locations. Compared with adults, pediatric patients with brainstem cavernomas tend to have larger lesions and higher recurrence rate.

Specific Imaging Findings

Increased density in the occluded sinus leading to a “cord sign” is the classic imaging finding of dural venous sinus thrombosis (DVST) on unenhanced CT images. However, a high variability in the degree of thrombus density is responsible for a low sensitivity of this sign. Thus, evaluation with CT angiogram, MR and MRV may be required to confirm the diagnosis. The “empty delta” sign consisting of a triangular area of enhancement with a relatively low-density center is seen in 25-30% of cases on contrast-enhanced CT scans. On MRI, acute thrombus is T1 isointense, T2 and T2* hypoin-tense. Of note, this T2 hypointensity may mimic normal flow-void. Peripheral enhancement is seen around the acute hypointense clot corresponding to the empty delta CT sign. Subacute thrombus becomes T1 and T2 hyperintense. Chronic thrombus is most commonly T1 isointense and T2 hyperin-tense. DWI/ADC signal of the thrombus is variable, as is the degree of enhancement in organized thrombus. Visible serpiginous intrathrombus flow-voids on T2WI, corresponding areas of flow signal on TOF-MRV, and brightly enhancing channels on post-contrast MRV are present in most cases of chronic partial recanalization. Thrombosis shows no flow-related signal on phase contrast MRV, and absent to diminished enhancement on post contrast MRV and CTV. Engorged collateral veins may be present, primarily in the chronic phase. TOF-MRV of a subacute T1 bright clot may potentially misrepresent sinus patency.

Specific Imaging Findings

In Wernicke encephalopathy (WE, Wernicke-Korsakoff syndrome) symmetric signal intensity alterations in the mammillary bodies, medial thalami, periventricular regions of the third ventricle, tectal plate, and periaqueductal gray matter are typical MRI findings. Selective involvement of the cranial nerve nuclei, cerebellum, dentate nuclei, fornix, splenium of the corpus callosum, cerebral cortex, and basal ganglia characterize nonalcoholic WE. Lesions in the basal ganglia mainly affect the putamen and are most frequently observed in children. The lesions are iso to hypointense to the gray matter on T1-weighted images and T2 hyperintense. DWI also shows high signal intensity in the acute phase with variable ADC values. Enhancement of the mammil-lary bodies is most frequently observed in the alcholic population and may be the only imaging finding. Degree of enhancement is quite variable. Hemorrhagic transformation is rare.

Brain atrophy develops in chronic WE, in particular of the fornices and mammillary bodies, while T2 hyperintensity becomes less obvious.

Pertinent Clinical Information

WE is an acute neurologic disorder resulting from thiamine (vitamin B1) deficiency and its incidence is underestimated in both adult and pediatric patients. Clinical presentation is characterized by changes in consciousness, ocular dysfunction, and ataxia. However, this triad is not present in many patients. The most common presenting symptom is nonspecific mental status changes. Untreated patients can progress to irreversible brain damage leading to Korsakoff syndrome and, eventually, to death.

Specific Imaging Findings

Developmental venous anomalies (DVAs, venous angiomas, venous malformations) consist of dilated medullary veins draining centripetally and radially into a transcerebral collector that ultimately converges into the deep or superficial venous system in an area in which there is an absence of normal draining veins. The caput medusae (head of the Medusa) represents the typical morphological appearance of DVAs: contrast enhancement of the network of feeding veins converging into the single draining vein is caused by the slow flow on both CT and MR imaging. Size of DVAs is quite variable, and large DVAs draining an entire hemisphere may be occasionally observed. The draining vein of large DVAs can demonstrate flow-void (high-velocity signal loss best seen on T2WI), whereas medullary veins and smaller collecting veins are frequently seen as T2 hyperintensities, or they may remain imperceptible on non-contrasted MR images. If the vessel is obliquely oriented, a “yin–yang” symbol appearance may occur because of the characteristic spatial misregistration artifacts associated with venous flow. DWI/ADC findings are usually unremarkable. Brain parenchyma adjacent to DVAs is most commonly normal on standard images however, a minority of DVAs are associated with T2 hyperintensity in the drainage territory, usually in a periventricular location. DVAs are usually not visible on unenhanced CT.

Pertinent Clinical Information

DVAs are most commonly incidentally found on MR imaging and are estimated to occur in around 3% of individuals. There is an association with sporadic cavernous malformations but not with familial cavernomas. De-novo cavernomas have been described to occur adjacent to pre-existing DVAs. DVA is generally considered a benign finding. Notwithstanding, a variety of symptoms including headache, non-communicating hydrocephalus, tinnitus, and cranial nerve dysfunction have been described in association with DVAs. As with other venous structures, thrombosis may occur in DVAs. Hemorrhage in association with DVAs has been described, especially in the cerebellum.