Heterogeneous or decreased attenuation and a hyperdense membrane are characteristics of a chronic extradural hemorrhage (EDH) as a result of degradation of the blood products. Timing wise, chronic EDH are typically considered >21 days from injury. Chronic EDH may also show peripheral enhancement representing dura and membrane formation between the EDH and adjacent brain parenchyma.
The differential diagnosis is subacute EDH, which may be differentiated from chronic EDH by its characteristic homogeneously hyperdense attenuation, consisting of a solid blood clot. Timing wise, subacute EDH are typically considered 4-21 days from injury.
There is some discrepancy between the history and imaging features, and perhaps the estimation since time of injury is inaccurate.