6.34P Explain some of the uses of radioactive substances in diagnosis of…

The idea

Diagnostic use relies on detecting radiation that leaves the body, so the tracer must emit radiation that penetrates tissue and is detected externally, and it should have a short half-life to limit the patient's dose. A tracer is a radioactive substance taken into the body, often chemically bound to a compound the body uses, so it concentrates in the organ of interest; a detector outside the body maps where the radiation comes from. In a PET scanner a positron-emitting radionuclide decays, the positron annihilates with an electron and two gamma photons leave in opposite directions; detectors surrounding the patient register coincident pairs and a computer builds an image of metabolic activity. Choose the option that matches penetration, half-life and detection logic.

What earns the marks

• Identifies that a tracer emits radiation that penetrates tissue so it can be detected outside the body.
• Explains that a short half-life limits the radiation dose to the patient while still allowing the scan.
• States that a tracer is taken into the body and concentrates in the organ or tissue being investigated.
• Describes PET detection as registering pairs of gamma photons produced when a positron annihilates with an electron.
• Links the detected radiation to an image showing where the tracer has concentrated.

Common mistakes and how to put them right

• Selects an alpha emitter as a tracer.

  Put it right by choosing a gamma or positron emitter because alpha particles are stopped within the body.

• Chooses a very long half-life as an advantage.

  Put it right by explaining that a long half-life prolongs the patient's exposure.

• Assumes the scanner measures radiation transmitted through the body from an external source.

  Put it right by explaining that the detected radiation originates from the tracer inside the patient.

Exam tips

• Check each option against two tests: does the radiation escape the body, and is the half-life short enough to limit dose.
• Eliminate options that describe treating a condition rather than diagnosing it.
• Read the stem carefully to see whether it asks about tracers generally or PET specifically.