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 incomplete horseshoe shape (bottom left) - the red & white areas represent normal heart muscle. This form of "cold-spot" scanning is used to demonstrate ischaemia by comparing scans taken at rest & during exercise."