False-colour scintigram (gamma camera scan) of normal human knees, revealing the joint cartilages in red & white. The gamma scan is a map of radioactivity emitted from an isotope, technetium- 99m, that was injected intravenously into the subject prior to taking the scan. Technetium-99m (Tc-99m) is readily taken up by bone and cartilage & this technique is often used to diagnose secondary bone cancer (metastases). Cancerous bone concentrates the isotope more strongly than normal bone & this shows up as brighter, hot spots" on a scan."