There are 2 types of subdural haematomas :
Acute subdural - Chronic subdural
The blood is again of high attenuation, but may spread more widely in the subdural space, with a crescentic appearance and a more irregular inner margin.
While acute subdural haematomas have increased attenuation, this decreases with time, becoming isodense after a week or so, and hypodense thereafter. Consequently chronic subdurals are often hypodense crescentic collections, often with mass effect. The collection may be more complex with layering of more dense material posteriorly and a gradual transition. Expansion due to osmosis may tear further veins leading to recurrent bleeds; hyperdense red blood cells from fresh bleeding may layer posteriorly, and complex septated collections may develop.
Isodense collections may be better demonstrated after intravenous contrast as the density will then be less than that of the brain. However this is rarely a problem with more modern scanners.