Radiation - introduction & terminology

Radioactivity is a natural process in which ionising radiation is emitted from unstable atoms known as radioisotopes. Nuclear medicine makes use of this ionising radiation to produce an image showing the distribution of a radioisotope within a patient.

There are three main types of radiation that can be emitted in radioactive decay:

  • alpha particles
  • beta particles
  • gamma-rays
Gamma-ray emitting radioisotopes are suitable for use in imaging.

Alpha emitting radioisotopes are not used in nuclear medicine. Beta particles can only penetrate through a few millimetres of tissue, so their radiation will all be absorbed in the local organ. This makes beta emitters useful for therapy, but beta particles cannot be used for imaging because they can’t escape from the patient. Gamma-rays are much more penetrating and most will escape from inside a patient as long as their energy is above about 100 keV (100 thousand electron volts). This makes gamma-ray emitting radioisotopes suitable for use in imaging.

The strength of a radioactive source is measured by its activity in megabequerels (MBq). The activity will gradually decay away and the time taken for the activity to halve is known as the half-life. For most applications a half-life of a few hours is convenient for imaging without leaving activity in the patient for an unnecessary length of time. In nuclear medicine, the most commonly used radioisotope is 99mTc, which emits gamma-rays with an energy of 140 keV and has a half-life of 6 hours. 

Page last edited: 25 November 2010