Attenuation values of chronic subdural haematoma and subdural effusion in CT scans.

When peripheral blood is serially diluted with saline solution, the bloody colour is determined by the derivative of haemoglobin. Solutions over 10(5)/mm3 red cells or 0.5 g/dl haemoglobin are represented as being sanguineous. This is the lower limit for a subdural haematoma, because the name itself requires at least a bloody appearance of its contents. Diluted blood solutions less 0.5 X 10(4) red cells or 15 mg/dl haemoglobin are macroscopically translucent or watery clear. Solutions between bloody and translucent are xanthochromic. If the contents of a subdural collection is watery clear or xanthochromic, it must be called an effusion or hygroma. The haemoglobin in haematomas is the most important factor determining the attenuation values in the CT scan. Protein, iron or calcium ions have only minimal concentration in haematomas and are negligible for attenuation values of haematomas in the CT scan. The lightest sanguineous solution, that is a haematoma, corresponds to 15 Hounsfield's units measured by the CT scan.