Originally posted on Radiopaedia:
Extradural haemorrhage - a collection of blood between the inner surface of the skull and the outer layer of the dura. It is typically due to meningeal arterial bleeding and very often associated with a skull fracture. The lentiform shape of extradural haematoma is classically taught as the major method of distinguishing it from the typically crescent shaped subdural haematoma, however there are other helpful distinguishing signs. Unlike subdural blood, extradural blood is not able to cross skull sutures and cannot extend along dural reflections (falx and tentorium).