John M. Barkley
L. Anne Hayman
Pedro J. Diaz-Marchan
The subarachnoid space is the CSF containing space between the arachnoid membrane and the pia that covers the brain. CSF originates in the choroids plexus and flows from the ventricular system to the basilar cisterns, then finally to the surface of the brain. CSF is then absorbed by the small arachnoid granulations within the dural venous sinuses. CSF bathes the brain surface and extends into the sulci.
Subarachnoid hemorrhage (SAH) most commonly (90 percent) occurs secondary to a ruptured aneurysm involving the Circle of Willis. Usually subarachnoid hemorrhage secondary to a ruptured aneurysm is larger than bleeding caused by trauma. The locations are different as well. A ruptured aneurysm typically has most of the blood near the site of rupture. For example, if the aneurysm is located in the Circle of Willis, the basal cisterns usually fill with blood. Traumatic subarachnoid hemorrhage is much more variable in location. SAH presents on CT as hyperdense blood within the sulci (Figure 11-12) or basal cisterns. CT is very sensitive for the detection of acute SAH (2). MRI is less sensitive for the detection of acute SAH, however is very good for imaging chronic blood products or hemosiderin.
Besides a ruptured aneurysm or trauma, other causes of subarachnoid hemorrhage include arteriovenous malformations and coagulopathies (3). There are also many complications that may occur with SAH and follow-up imaging is recommended. These include vasospasm, stroke, communicating hydrocephalus, and arachnoiditis.
MRI imaging in patients with SAH may show blood products in varying stages within the sulci or cisterns. Fluid attenuated inversion recovery (FLAIR) sequences employ a longer T1 and a 180-degree inversion pulse to reduce the signal level of CSF and detect abnormalities in the sulci such as SAH with a greater degree of certainty (1). As described earlier, chronic SAH may present on gradient echo or magnetic susceptibility sequences as leptomeningeal hemosiderosis—hypointense (dark) meninges on T2-weighting (3).
Original: Brain Injury Medicine. Principles and Practice
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