It is desirable sometimes to inject a patient with a substance containing a radioactive source so that some part of the body can be imaged. The amount of radioactive used should be as small as possible to minimise the exposure of the patient. The radioactive isotope used should have a short half life, but it should also be absorbed preferentially by the part of the body that is to be imaged. To ensure this is the case, we use radiopharmaceuticals When a bone scan is taken, Technetium-99 is combined with a phosphorus containing compound and the patient is injected with a dose containing 600MBq of radioactivity. This is taken up preferentially by bone tissue, and particularly cancerous bone tissue, which has a high metabolic rate. Substance used in this way are called tracers.