Owing to its speed and availability unenhanced computed tomography (CT) is the mainstay of acute neuroimaging in ICU. The main advantage of unenhanced CT is in the detection of acute hemorrhage, hemorrhagic transformation of an ischemic infarct, or a new intraaxial or extraaxial hemorrhage. Even inexperienced observers readily appreciate a large bleed. Small intracerebral hemorrhages in the posterior fossa and adjacent to the skull may be difficult to detect because of partial volume averaging and beam-hardening artifacts from adjacent bone.

In addition to the detection of hemorrhage, CT allows the assessment of the ventricles and extraaxial fluid spaces, as well as of the effacement of cortical sulci and basal cisterns suggestive of diffuse cerebral edema and increased intracranial pressure. Although non-specific, prominence of extraaxial fluid spaces and ventricles in disproportion to the patient’s age, as well as cerebellar atrophy, may provide clues of possible chronic alcohol abuse as a seizure precipitant. CT allows assessment of vasogenic edema accompanying intracranial lesions, of the degree of exerted mass effect and midline shift or of uncal herniation. The usefulness of CT in the detection of acute cerebral ischemia presenting as subtle hypoattenuation and loss of grey-white matter differentiation is limited and is strongly dependent on the observer’s experience. Established ischemia is easier to appreciate but care should be taken not to misinterpret this as edema if the grey matter is spared. CT can depict advanced global hypoxic ischemic brain injury leading to effacement of cortical sulci and loss of grey-white matter differentiation.

Contrast-enhanced CT may be helpful in the monitoring of intracerebral abscesses, ventriculitis, or intracranial infection. CT angiography is useful in evaluating the patency of intracranial arteries, cerebral ischemia and vasospasm. CT venography has a role to play in the assessment of the patency of the dural venous sinuses.

Due to the practical limitations (see Chap. 4) Magnetic Resonance Imaging of ICU patients is usually reserved for cases in which CT fails to detect the cause of seizures, or if further characterization of abnormal CT findings is needed. The main advantages of MRI stem from its unsurpassed ability to detect changes in brain parenchyma, and to partially categorize them. MRI is more sensitive than CT in the detection of posterior fossa and pituitary abnormalities, as well as pachymeningeal and leptomeningeal enhancement. MR angiography and venography have similar sensitivity to their CT counterparts but are less practical for obvious reasons.

Cortical vein thrombosis is an uncommon condition presenting with seizures followed by a focal cortical deficit, secondary to cortical venous infarction. The isolated form of this condition (ICVT) is rare, although its frequency is probably underestimated due to the inherent difficulty in establishing the diagnosis. More often, cortical vein thrombosis occurs in combination (CCVT) with superior sagittal sinus thrombosis due to retrograde thrombus propagation and is accompanied by clinical features such as headache, increased intracranial pressure and papilledema. Predisposing factors include hypercoagulable states, head trauma, parameningeal or meningeal infections, pregnancy and the puerperium, dehydration, nephrotic syndrome, and malignancy.