10.6.1 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.

• 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

	Technetium-99m	Iodine-131	Indium-111
types of radiation emitted	γ only	β−, γ	γ only
half-life	6 hours	8 days	68 hours
energy of gamma photons	140 keV	360 keV	170 keV & 250 keV
common uses	to investigate most organs, blood and tumours	to treat overactive thyroids or thyroid cancer	to label cells and diagnose blood disorders and rare cancers