False-colour scintigram (gamma camera scan) of a human heart showing ischaemia (reduced blood flow) following myocardial infarction, the death of a section of heart muscle following obstruction of a coronary artery - a heart attack. The image shows the uptake of a radio-isotope, Thallium-201, by the heart's muscle cells. Radioactive emissions recorded by the gamma camera are colour-coded to reveal a defect (cold spot"), represented by the break (at bottom) in the horseshoe shaped region of healthy heart muscle (in purple, red & yellow). A series of such scans taken at rest & during exercise can distinguish between ischaemic & infarcted tissue."