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What is the appearance of an acute subdural hematoma on a CT scan?

  1. Crescent-shaped peripheral high-density lesion

  2. Round hyperdense mass

  3. Linear hypodense area

  4. Patchy isodense region

The correct answer is: Crescent-shaped peripheral high-density lesion

An acute subdural hematoma typically appears as a crescent-shaped peripheral high-density lesion on a CT scan. This characteristic shape results from the accumulation of blood between the dura mater and the arachnoid layer, often following trauma. The high density is due to the presence of fresh blood, which typically appears hyperdense in the initial stages post-injury. The crescent shape reflects the contour of the brain and the dural surface, as the hematoma extends along the inner surface of the skull, contrasting sharply against the brain's lower density tissue. Other shapes observed in imaging may indicate different conditions. For instance, a round hyperdense mass would more likely suggest a different type of hematoma or lesion, such as an acute epidural hematoma or a tumor. Linear hypodense areas could indicate edema or other pathologies rather than a hematoma. Patchy isodense regions may be associated with chronic hematomas or other slower-developing conditions, where the density of the blood has equilibrated with surrounding structures. Thus, the crescent-shaped peripheral high-density lesion is a distinctive and defining feature of an acute subdural hematoma on CT imaging.