False-colour scintigram (gamma camera scan) of a human heart showing ischaemia (reduced blood flow) & myocardial infarction, the death of a section of heart muscle resulting from an obstruction in a coronary artery - a heart attack. The image displays the distribution of radioactivity emitted by a previously-injected radionuclide, Thallium- 201, according to the amount of light given off by a scintillator (a gamma camera) as it is struck by radiations of different intensities. The red areas represent heart muscle with an adequate blood supply; a healthy heart would appear as a red horseshoe. Here, the horseshoe is incomplete, with only an isolated red area on the left.