A medical tracer is a compound labelled with a radioactive nuclide. It is injected, swallowed or inhaled, and the compound is chosen so that the body carries it to the organ or tissue being studied. The radiation it emits is detected outside the body, showing where the tracer has gone and how quickly. This gives information about how organs are working, not just their shape.A good tracer should:• emit gamma rays, which pass out of the body to the detector. Alpha and beta particles would be absorbed in the body, causing ionisation damage without producing an image (the exception is a positron emitter, whose positrons produce gamma photons, used in PET)
• have a half-life of a few hours: long enough to prepare it and carry out the scan, short enough that the patient's dose is small
• have low chemical toxicity and be easy to bind to a suitable compound
• decay to a stable, or very long-lived, daughterThe body also removes the tracer by excretion, which further reduces the dose.Worked example: a patient is injected with 400 MBq of a tracer with a half-life of 6.0 h. Find the activity after 24 h.number of half-lives = 24 / 6.0 = 4
A = 400 / 24 = 400 / 16 = 25 MBq After one day the activity has fallen to one sixteenth of its starting value, even before excretion is taken into account.
• have a half-life of a few hours: long enough to prepare it and carry out the scan, short enough that the patient's dose is small
• have low chemical toxicity and be easy to bind to a suitable compound
• decay to a stable, or very long-lived, daughterThe body also removes the tracer by excretion, which further reduces the dose.Worked example: a patient is injected with 400 MBq of a tracer with a half-life of 6.0 h. Find the activity after 24 h.