BrainTF17-Subarachnoid Hemorrhage

Clinical Profile:

C/O sudden onset of headaches and vomiting

Findings:

There are hyperintense areas on the FLAIR images within the quadrigeminal cistern and superior cerebellar cistern (subarachnoid hemorrhage). The small, well circumscribed lesions in the region of the tip of the basilar artery and distal aspect of the left posterior cerebral artery were seen to represent aneurysms (proved on DSA).

Discussion: 

Etiology:

• Aneurysms

• Trauma

• Vascular malformations

• Tumors, e.g. spinal ependymomas

On MRI:

Subarachnoid hemorrhage (SAH) requires immediate diagnosis and therapy. It is not as well imaged on MRI in the acute stage. This is due to the oxygen content of the subarachnoid space. The conversion of oxyhemoglobin to deoxyhemoglobin (and subsequently to methemoglobin) requires a relatively narrow range of oxygen tension. This is rapidly achieved in isolated intracranial hematomas, resulting in characteristic marked hypointensity on long TR/TE images in the acute stage and it is also seen in isolated clots in the subarachnoid space. However, in acute diffuse subarachnoid hemorrhage, the ambient oxygen tension of the subarachnoid CSF is too high for expected evolutionary changes to occur. SAH is usually seen with MRI only when there are large focal clots. It may be seen more optimally with either T1 Weighted or T2 Weighted scans, depending on the age of the hemorrhage and is usually detected more easily in the subacute phase as areas of T1 shortening in the subarachnoid space. After recurrent or chronic SAH, subpial deposition of hemosiderin may result in superficial hemosiderosis (superficial siderosis). This may be incidental, but patients can demonstrate cranial nerve palsies, hearing loss, and cerebellar ataxia.

Saccular aneurysms usually occur at vessel bifurcations, typically on the convexity of a curve in the parent vessel, and point in the direction that flow would have continued had the curve not been present. Larger arteries in the region of the circle of Willis are most frequently involved. More than 90% of saccular aneurysms originate at one of the following five locations: the junction of the anterior cerebral and the anterior communicating artery, the internal carotid artery (ICA) at the origin of the posterior communicating artery, the bifurcation of the middle cerebral artery (MCA), the tip of the basilar artery and the bifurcation of the ICA. 20% to 25% of aneurysms are multiple. Aneurysms may be diagnosed on MRI, mainly based on the identification of regions of flow void in an area morphologically consistent with a saccular aneurysm. In selected cases, MRI may also provide information regarding the bleeding site in patients with multiple aneurysms by the identification of hemorrhage adjacent to the aneurysm. In the investigation with MRI of patients suspected of harboring intracranial aneurysms, it is absolutely essential to use high resolution 3D imaging with thin slices and generate projection images from post-processing of the data. Catheter angiography remains the modality of choice for it's diagnosis. The cardinal diagnostic sign of ruptured aneurysm is a severe headache of sudden onset, and CT scan remains the most sensitive imaging method for the detection of acute subarachnoid blood.

1. Intracranial Hemorrhage. In: Atlas SW: Magnetic Resonance Imaging of the Brain and Spine, Lippincot-Raven, pp:811-818, Volume 1, Third Edition.
2. Intracranial Vascular Malformations and Aneurysms. In: Atlas SW: Magnetic Resonance Imaging of the Brain and Spine, Lippincot-Raven, pp:891-912, Volume 1, Third Edition.