Brain in stroke. Coloured 3D magnetic resonance imaging (MRI) angiogram of the brain of a patient showing a large hypodense area (blue) due to a cerebrovascular emergency, most likely caused by a transient ischaemic attack (TIA), or mini-stroke. Mini-strokes have the same underlying cause as strokes, which occur when the blood supply to an area of the brain is restricted, or cut off completely (ischaemia), resulting in cell death of the affected tissue.