Radioactive Tracers (Cambridge International A-Level Physics)

Radioactive tracers

• A radioactive tracer is defined as:

A substance containing radioactive nuclei that is introduced to the body and 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

Using Tracers in PET Scanning

• Positron Emission Tomography (PET) is:

A type of nuclear medical procedure that images tissues and organs by measuring the metabolic activity of the cells of body tissues

• A common tracer used in PET scanning is a glucose molecule with radioactive fluorine attached called fluorodeoxyglucose

  • Fluorine nuclei undergo β+ decay i.e. they emit positrons (beta-plus particles)

• 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 Complete the nuclear equation for the decay of fluorine-18 to an isotope of oxygen by the emission of a β⁺ particle.

F _ 9 ^ 18 -> O

Answer:

Step 1: Work out what will be on the reactants and products side

• Reactant:

  • Fluorine F _ 9 ^ 18

• Products:

  • Oxygen, O _ 8 ^ 18
  • Beta-plus (positron), beta _ + 1 ^ 0
  • Electron neutrino, $ν_{e}$

Step 2: Complete the nuclear equation

F _ 9 ^ 18 -> O _ 8 ^ 18 + beta _ + 1 ^ 0 + nu _ e