Radioactive Tracers (AQA A Level Physics)

Radioactive 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

• Gamma emitters make good radioactive tracers, as the gamma can leave the body and doesn't ionise tissues as much as alpha or beta radiation

• To be suitable for medicine, the radioactive isotope must be able to be bonded to molecules

  • A molecule labelled with a radioactive isotope is known as a radiopharmaceutical product

  • To be a good tracer, this molecule must not affect the body's regular function, but gather in tissues

  • Different molecules can be used to make the tracer accumulate in specific organs or tissues

• Three common radioactive tracers which emit gamma radiation are:

  • Technetium-99m (the m refers to a metastable excited state of the nucleus)

  • Iodine-131

  • Indium-111

• 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 of a gamma camera as a bright area

  • Using tracer-labelled glucose, for example, highlights areas of higher respiration (e.g. tumours) which use more glucose

  • Labelling white blood cells can show the location of an infection in the body

  • Labelling red blood cells can highlight areas with decreased blood supply (e.g. regions in the brain, for a diagnosis of Alzheimer's disease)

Imaging Respiration using Radioactive Tracers

Tracers can show areas of increased respiration, if bonded to glucose.

Properties of Radioactive Tracers

• Each radioactive tracer has a unique set of properties, making them ideal for different medical uses

• The main properties of radioactive tracers are: 

  • The types of radiation it emits

  • The half-life of the isotope

  • The energy of the gamma radiation it emits

  • Any tissues or organs it has a specific affinity for

  • Whether it can be used to label specific cells and compounds

Technetium-99m

• This is the most commonly used radioisotope for diagnosis

• Tc-99m decays to Tc-99

  • The 'm' indicates the nucleus is in an excited, but somewhat stable, state (metastable)

• A gamma photon with 140 keV of energy is emitted in this decay

• The half-life is 6.0 hours

  • This is short enough to avoid prolonged irradiation of the body

• The chemical properties of Tc-99m allow it to be bonded to many molecules which have specific affinities for different organs and tissues

• This means it can be used to study:

  • The skeleton

  • The flow of blood

  • The heart

  • The brain

  • The thyroid

  • Tumours 

Iodine-131

• Unlike Tc-99m, I-131 emits both beta-minus particles and gamma photons when decaying

  • The gamma photons have energies of 360 keV, whereas the beta minus particles have energies of keV

• The half-life of this decay is 8.0 days

• Iodine can bond to thyroid hormones, so it is the most commonly used tracer for thyroid activity

  • I-131's beta emission can be used to destroy parts of an overactive thyroid or kill remaining thyroid cells after a thyroid gland is removed (usually because it contains cancerous cells)

• In recent years, I-131 as a thyroid tracer has been replaced with I-132, as it doesn't emit beta and has a shorter half-life

Indium-111

• In-111 emits gamma photons at energies of 170 keV and 250 keV

• The half-life of this decay is 68 hours (2.8 days)

• In-111 is particularly useful for 'labelling' certain cells, such as:

  • Red blood cells and platelets (to identify issues in the blood)

  • White blood cells, bone marrow and spinal fluid (to locate inflammation or infection)

  • Tumours (to allow precise cancer detection)

• This makes it useful for diagnosing blood diseases or rare cancers

Comparing Radioactive Tracers

• Different tracers are used for different purposes, but the essential properties of a tracer are:

  • It must be a gamma emitter so that the radiation can be detected outside the body

  • The gamma rays must be as low-energy as possible to reduce the risks of ionisation damage

  • It must have a half-life which is short enough to reduce the total dose given to the patient but produces a high enough gamma intensity to be detected