Computed Tomography

In contrast to the indirect diagnostic information provided by ICP measurements, CT technology may noninvasively illustrate the volumetric changes and alterations in parenchymal density resulting from cerebral edema. Expansion of brain tissue due to most forms of edema may be detected on CT, although diffuse processes including fulminant hepatic failure or osmotic edema may be more difficult to discern. Diffuse swelling may be recognized by a decrease in ventricular size with compression or obliteration of the cisterns and cerebral sulci. Cellular swelling associated with cytotoxic and ischemic edema can manifest as subtle enlargement of tissue with obscuration of normal anatomic features, such as the differentiation between gray matter and white matter tracts. Vaso-genic edema may also cause tissue expansion, although the associated density changes may be more prominent. In contrast to the increased volume of brain tissue noted in most forms of edema, hydrocephalic edema may be suspected in cases in which ventricular expansion has occurred. Extensive volumetric changes and the associated pressure differentials resulting in herniation may be noted on CT as shifts in the location of various anatomic landmarks.

The increased water content associated with edema causes the density of brain parenchyma to decrease on CT. The attenuation effects of other tissue contents complicate precise correlation of water content with density on CT. Although slight decrements in tissue density result from cytotoxic and osmotic processes, more conspicuous areas of hypodensity result from the influx of fluid associated with disruption of the BBB in vasogenic edema. The injection of a contrast agent permits enhanced visualization of vascular structures and thereby further defines regions of extravasation where the BBB has been altered. Contrast CT improves the demonstration of infectious lesions and tumors that present with significant degrees of vaso-genic edema.

The differentiation of specific forms of edema is limited with CT, but this modality may provide sufficient information to guide therapeutic decisions in many situations. CT perfusion, a new dynamic imaging study, allows for the determination of cerebral blood volume based on serial density changes associated with a contrast bolus injection. CT may be inferior to MRI in the characterization of cerebral edema, but logistic constraints may preclude MRI in unstable trauma patients, uncooperative patients, and patients with contraindications due to the presence of metallic implants or pacemakers.

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