False-colour scintigram (gamma camera scan) of a normal human heart, obtained following the intravenous injection of the radio-isotope, thallium-201 (Tl- 201). The gamma camera maps the amount of radioactivity emitted by the tracer as it becomes concentrated in the heart's cells. The doughnut-shaped, pink & red feature represents normal Tl-201 uptake by healthy heart muscle. Concentration of Tl-201 in heart muscle depends on a normal blood flow and the integrity of the myocardial cell membranes. Comparison of scans made during exercise & at rest may reveal defects in blood supply (ischaemia) that may subsequently obstruct & cause a heart attack.