**Question:** Acute SDH is seen on CT scan as:

A. Diffuse signal intensity increase in the subdural space
B. Signal intensity decrease in the subdural space
C. Thickened dura mater
D. Calcification within the subdural space

**Core Concept:** Subdural hematoma (SDH) is a collection of blood between the dura mater and pia mater in the brain. Acute SDH can be caused by a traumatic brain injury or spontaneous rupture of a cerebral vessel. CT (computed tomography) scan is a crucial imaging tool to diagnose SDH.

**Why the Correct Answer is Right:** Acute subdural hematoma is characterized by a collection of fresh blood within the subdural space. On a CT scan, acute SDH demonstrates a diffuse signal intensity increase in the subdural space due to the presence of the blood, which is hyperdense compared to the surrounding brain tissue.

**Why Each Wrong Option is Incorrect:**

A. Signal intensity increase in the subdural space is incorrect because the correct answer demonstrates a diffuse increase, not a decrease.
B. Signal intensity decrease in the subdural space is incorrect because SDH causes an increase in signal intensity on CT scans, not a decrease.
C. Thickened dura mater is not a characteristic feature of acute SDH on CT scan; instead, it is an indirect sign of SDH and may be present in chronic SDH.
D. Calcification within the subdural space is a feature of chronic SDH, not acute SDH. Acute SDH typically demonstrates increased signal intensity on T1-weighted sequences, while calcification is seen in chronic SDH, which has undergone hematoma liquefaction.

**Clinical Pearl:** Acute SDH is a time-sensitive condition that requires prompt intervention, such as surgery, as it can lead to brain compression, cerebral herniation, and severe neurological deficits.

**Correct Answer:** Acute SDH is seen as a diffuse signal intensity increase in the subdural space on CT scan.