What is the low density change surrounding the lesion?
Oedema due to accumulation of fluid in the white matter.
What two main types of cerebral oedema are described and how to they differ on imaging?
Vasogenic and cytotoxic odema. Vasogenic is due to accumulation of extracellular fluid which tends to be confined to the white matter where the extracellular space is more easily distended (as in this case). Cytotoxic oedema is due to accumulation of intracellular fluid due to failure of the Na/K pump. This is typically seen in ischaemia and also affects the grey matter.
Following the administration of contrast the mass demonstrates relatively vivid enhancement. It remains an isolated abnormality.