Info

hypodense areas

Acute

Hours to 1 week

Hyperdense

Subacute

1-6 weeks

Isodense

Chronic

>6 weeks

Hypodense

seen in hemorrhagic tumors but also in any of the many causes of cerebral hemorrhage.

Low-density extra-axial collections are not necessarily hematomas. Empyemas are low density with mass effect and, when subdural, may expand the interhemispheric fissure (Fig. 2.2). Often the patient is more clinically unwell than would be suggested by the size of the collection. Intravenous contrast typically causes enhancement of a surrounding membrane, which may aid diagnosis. Contrast medium is used routinely in CT scanning, particularly if there is a possibility of infection, tumor or vascular lesion. Unnecessary use should be avoided, e.g. in acute trauma or hydrocephalus, as there is a risk of serious reaction in 1 in 2,500 injections [6].

When subarachnoid hemorrhage (SAH) mixes with CSF, the attenuation of the hema-

Fig. 2.2. CT scan post i.v. contrast. Low-density subdural collection with mass effect. Note fluid in the interhemispheric fissure, which suggests a subdural empyema. This is complicated by a sagittal sinus thrombosis (arrow points to the delta sign).

toma is reduced, becoming the same as brain. Therefore smaller SAHs will be subtle with apparent effacement of sulci and cisterns - an appearance that should not be mistaken for brain swelling. CT will detect 95% of SAHs within 24 hours of the ictus. A reliable indirect sign is the presence of mild hydrocephalus. After 1 week, CT is much less reliable as the density of the hematoma is significantly reduced.

Current CT scanners use multislice techniques of acquisition, allowing greater coverage in shorter times. This reduces any movement artifact and facilitates CT angiography as a significant length of vessel can be scanned while the bolus of contrast medium passes through. Reformation of the scans can be performed in several ways, such as multi-planar reconstructions, maximum intensity projections, volume-rendered images or even "fly through" endoluminal views. CT angiography (CTA) has been used to demonstrate carotid artery disease, intracranial vascular anatomy, including arteriovenous malformations (AVMs), and aneurysms [7] and is now frequently used as a first investigation of SAH. The three-dimensional images obtained can be used to decide between an endovascular or open neurosurgical operation on an aneurysm. It is useful in the emergency situation when a large hematoma has been demonstrated and when there is a suspicion of an underlying vascular lesion. CTA in this situation may demonstrate an underlying aneurysm, enabling the patient to proceed rapidly to evacuation of the clot and clipping of the aneurysm without the delay of organizing a formal angiogram.

CTA requires the administration of iodinated contrast media and also for the patient to lie very still during the scan time of approximately 30 seconds.

A large aneurysm may be apparent on the initial CT scan as blood within the aneurysm is less dense than the surrounding hematoma (Fig. 2.3).

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