Medical Tracers (OCR A Level Physics)

Revision Note

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Katie M


Katie M



Medical Tracers

  • A radioactive tracer is defined as:

A radioactive substance that can be absorbed by tissue in order to study the structure and function of organs in the body

  • Radioactive isotopes, such as technetium-99m or fluorine-18, are suitable for this purpose because:
    • They both bind to organic molecules, such as glucose or water, which are readily available in the body
    • They both emit gamma (γ) radiation and decay into stable isotopes
    • Technetium-99m has a short half-life of 6 hours (it is a short-lived form of Technetium-99)
    • Fluorine-18 has an even shorter half-life of 110 minutes, so the patient is exposed to radiation for a shorter time

  • A common tracer used in PET scanning is a glucose molecule with radioactive fluorine attached called fluorodeoxyglucose
    • The fluorine nuclei undergoes β+ decay – emitting a positron (β+ particle)

  • The radioactive tracer is injected or swallowed into the patient and flows around the body
  • Once the tissues and organs have absorbed the tracer, then they appear on the screen as a bright area for a diagnosis
    • This allows doctors to determine the progress of a disease and how effective any treatments have been

  • Tracers are used not only for the diagnosis of cancer but also for the heart and detecting areas of decreased blood flow and brain injuries, including Alzheimer's and dementia

Worked example

Write a nuclear decay equation for the decay of fluorine-18 open parentheses straight F presubscript 9 presuperscript 18 close parentheses into an isotope of oxygen by β+ emission.

Step 1: Work out the reactants and products

    • Reactant:
      • Fluorine F presubscript 9 presuperscript 18
    • Products:
      • Beta-plus particle (positron) straight beta presubscript 1 presuperscript 0
      • Oxygen straight O presubscript 8 presuperscript 18
      • Gamma-ray γ

Step 2: Write the nuclear decay equation

straight F presubscript 9 presuperscript 18 space rightwards arrow space straight O presubscript 8 presuperscript 18 space plus space straight beta presubscript 1 presuperscript 0 space plus space straight gamma

Worked example

Discuss the advantages of using a gamma-emitting tracer in a patient rather than a beta-emitting tracer.

Step 1: Consider the properties of gamma and beta particles

    • Gamma particles are not (very) ionising and have a long range
    • Beta particles are very ionising and have a short range

Step 2: Compare the effects of the gamma and beta particles in relation to detection

    • Gamma radiation will pass through the patient and hence can be easily detected
    • Beta particles will be absorbed by the patient and hence cannot be detected

Step 3: Compare the effects of the gamma and beta particles in relation to patient safety

    • Gamma radiation is not very ionising, hence, it does little damage to cells
    • Beta particles is highly ionising, hence, it can cause a lot of damage to cells

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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