False-colour scintigram (gamma camera scan) of a normal human heart. This is an example of a inscold spotins scan, recorded after the intravenous injection of a radio-isotope, thallium-201. The central horse-shoe shaped area represents a normal uptake of the radio-isotope by the myocardium (heart muscle) of the left ventricle. The concentration of thallium-201 in heart muscle depends on a normal blood flow and the integrity of the membranes surrounding the muscle cells of the heart. Comparison of scans made during exercise & at rest may reveal defects in blood supply (ischaemia) & infarction (death of heart muscle) resulting from a heart attack.