The tracers used in medicine are complex organic molecules specifically chosen for their affinity for the organ under examination. These molecules are ‘marked’ by the presence of a radioactive nucleus, and are often injected intravenously. On these skeletal scans, we can see the areas where a diphosphate marked with technetium 99 has built up, indicating the possible presence of metastasis. This particular marker is very popular in gamma scintigraphy.