Final answer:
Subdural hematomas appear on non-contrast head CT as crescent-shaped, higher-density areas that may look white or bright, indicating the presence of blood between the dura mater and arachnoid mater typically from torn bridging veins. Their appearance can change over time due to the varying density of the clotted blood.
Step-by-step explanation:
On a non-contrast head CT, subdural hematomas appear as crescent-shaped regions that cross suture lines, often located along the surface of the brain, adjacent to the inner table of the skull. Because acute subdural hematomas consist of clotted blood, they manifest on CT scans with a higher density than the surrounding brain tissue, which looks white or bright compared to the grey appearance of the brain parenchyma. Over time, the blood density in a subdural hematoma will change and may become isodense (similar density) or even hypodense (lower density) compared to the brain tissue.
The appearance is due to the accumulation of blood between the dura mater and arachnoid mater, typically due to the tearing of bridging veins. Bridging veins are those that traverse the subdural space to drain into the dural venous sinuses such as the superior sagittal sinus, a dural sinus located along the top of the longitudinal fissure. If these veins are torn, usually because of head trauma or injury, they can lead to a subdural hematoma which, if left untreated, may increase intracranial pressure and have serious consequences.