False-colour scintigram (gamma camera scan) of the human heart recorded during diastole, the period between two successive contractions when the chambers (atria & ventricles) fill with blood. The heart appears in centre - the two distinct blue areas at bottom represent blood filling the left & right ventricles (the left ventricle is at right). Colour-coding represents the amount of radiation from blood cells labelled with the radio-isotope, technetium-99m, injected intravenously into the subject prior to taking the scan.