6.12.1 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

Step 1: Work out the reactants and products

• • Reactant:
    • Fluorine 
  • Products:
    • Beta-plus particle (positron) 
    • Oxygen 
    • Gamma-ray γ

Step 2: Write the nuclear decay equation

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