The arachnoid mater has an outer layer that adheres to the inner cell layer of the dura. Thin strands of collagen connective tissue called arachnoid granulations extend down to merge with the pia mater. Because the arachnoid is attached to the dura it follows the general contour of the brain covering only the superifical surface 1.
The subdural space is a potential space. Cortical veins en-route to the dural sinuses can stretch or tear causing bleeding and dissection into this potential space. Unlike the dura which is firmly adherent to the skull, the arachnoid is easily separable from the dura allowing blood to dissect over a larger area than an epidural hemorrhage 2. This contributes to the classic crescentic shape of a subdural hemorrhage.
The classic CT appearance of acute subdural hemorrhage is a crescentic high density extra-axial collection spread diffusely over the affected hemisphere 3. If the arachnoid is disrupted there may be mixture of blood and CSF 3. Subdural hematomas are free to cross sutures but are limited by dural reflections such as the falx cerebri and tentorium cerebelli and can be seen layering along these structures.
The appearance of a subdural hemorrhage by CT varies with age.