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subacute, late subacute, and chronic.

Nontraumatic Subarachnoid Hemorrhage

Nontraumatic subarachnoid hemorrhage is typically related to

an aneurysm (75%) or a vascular malformation, such as an

arteriovenous malformation or cavernous angioma.

Nonaneurysmal "perimesencephalic" subarachnoid

hemorrhage is uncommon.

Venous Infarction

Dural sinus or cerebral vein occlusion is rare, representing <

1% of strokes. Venous thrombosis risk factors include

pregnancy, trauma, dehydration, infection, oral

contraceptives, coagulopathies, malignancies, collagen

vascular diseases, and protein C and S deficiencies. Venous

infarcts occur in only ~ 50% of venous thrombosis cases and

can be differentiated from arterial infarcts by the location of

the ischemia. Superior sagittal sinus thrombosis typically

results in T2/FLAIR hyperintense parasagittal lesions, whereas

thrombosis of the transverse sinus often results in T2/FLAIR

hyperintensities in the posterior temporal lobe. Additionally,

venous infarcts more commonly present with associated

hemorrhage. CECT is useful to identify the "empty delta" sign

representing nonenhancing thrombus within a major dural

sinus, typically the superior sagittal or transverse sinus.

Approach to Stroke Imaging

Cerebral ischemia results from significantly decreased blood

flow to selected areas or the entire brain. Stroke progresses in

stages from ischemia to actual infarction. In the most common

situation, MCA occlusion, there is a densely ischemic central

core and a less densely ischemic "penumbra." The central core

is usually irreversibly damaged unless reperfusion is quickly

established, whereas the cells within the penumbra may

remain viable but at risk for several hours. Current stroke

therapies attempt to rescue the "at-risk" cells.

Currently, acute stroke protocols vary among different

institutions. The exact protocol is often based on the

availability of CT vs. MR, technology/software, time of stroke,

physician expertise, and the possibility of neurointervention.

Typically, stroke neurologists work with neuroradiologists to

devise a plan that best serves the patient's needs.

Most stroke protocols begin with a noncontrast head CT to

evaluate for hemorrhage or mass, which directly affects

treatment decisions. Additionally, > 1/3 of MCA territory

hypodensity at presentation is considered by most to be a

contraindication to thrombolysis, as it is associated with a

greater risk of fatal hemorrhage. CTA is useful to evaluate for

large-vessel occlusion. When available, CT perfusion is an

excellent way to evaluate for large=vessel ischemia.

MR with DWI is particularly useful for acute ischemia when CT

perfusion is negative and the clinical suspicion for stroke

remains. MR is also the primary imaging tool when the clinical

question includes a posterior fossa or brainstem lesion. MR

with PWI has been found extremely helpful in guiding therapy

when available.

Most stroke protocols use 3-hour and 6-hour windows for

treatment of nonhemorrhagic ischemic stroke. If the patient

presents within 6 hours after the initial onset of symptoms, an

unenhanced CT is typically the initial study of choice to

exclude a mass or hemorrhage. If there is a hemorrhage or

mass, no thrombolytic therapy is initiated. If there is no

hemorrhage or mass and the patient is within 3 hours after

onset of symptoms, the patient is eligible for intravenous (IV)

thrombolysis. If the patient is between 3 and 6 hours of onset,

either a CTA with CT perfusion or an MR with DWI and PWI is

performed to determine whether they are eligible for

treatment. If the patient has an intracranial thrombus with a

penumbra, intraarterial (IA) thrombolysis or IA thrombectomy

is recommended. If there is no penumbra, IA therapy may not

benefit the patient, so each case is evaluated individually.

Trauma, and Stroke

Brain: Pathology-Based Diagnoses: Malformations,

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