The dura mater, also commonly referred to as the pachymeninx or pachymeninges (pleural), is the outermost meningeal layer and is itself composed of two layers. The outer layer is tightly adherent to the inner table of the skull serving as the periosteum of the skull. The inner layer is the actual meningeal dura. At dural reflections the inner layer folds in on itself as a sheet-like protrusion into the cranial cavity, principally the falx cerebri and tentorium cerebelli. There is no sharp histologic boundary between the two layers of the dura except at the edges of dural reflections where the two layers are separated to form channels called dural venous sinuses 1.
A potential space exists between the skull and the periosteal layer of the dura. The periosteal layer may be dissected away from the skull by a developing hemorrhage as in this case. The source of hemorrhage is most commonly the middle meningeal artery in 60-90%, and the middle meningeal vein, venous sinus or diploic veins in 10-40% 2.
The typical appearance of an epidural hemorrhage on CT is of a biconvex, high density extra-axial collection which does not cross sutures due to the firm adherence of the dura, but may cross midline. This is in contrast to a subdural hemorrhage which is confined by dural reflections and unable to cross midline. For illustration please see these diagrams of various intracranial hemorrhages.