Introduction to imaging: ionising radiation; in the first part of our new series about medical imaging, John Frank looks in detail at imaging techniques that use ionising radiation.(education)

What imaging techniques are there?

There are four basic methods of imaging the body, using x rays, gamma rays, sound waves, or magnetism. Each method has its own place; the techniques are complementary, not mutually exclusive. In the first part of this series, I concentrate on using ionising radiation.

What is ionising radiation?

Ionising radiation is part of the electromagnetic spectrum and is used in imaging techniques in the form of either x rays-for conventional radiography and computed tomography--or gamma rays for imaging in nuclear medicine. The energy of an x ray determines the penetrating power of the beam and is measured in kilovolts (kV). The energy can be varied by the radiographer to suit the specific examination (see box 1).

Nuclear medicine

Imaging with x rays and nuclear medicine compared

X rays provide anatomical information whereas nuclear medicine images provide physiological information. Although the images in nuclear medicine are vaguely anatomical--for example, in bone scans [FIGURE 1 OMITTED]--they usually show the metabolism of the organ and allow altered metabolic states to be seen. A less anatomical but more metabolic image is shown in [FIGURE 2 OMITTED]. These types of scan are extremely sensitive to disease but not specific to a particular disease. By using both the anatomical and physiological information obtained, we can understanding better the underlying pathology and are better placed to diagnose the problem and guide the clinician in further investigations and treatment.

How is the radiation used?

Both x ray imaging and the techniques of nuclear medicine use ionising radiation. X rays actually pass through the patient and are detected by the x ray film or the computed tomography scanner. In nuclear medicine studies, however, carrier molecules labelled with a radioactive tracer, usually metastable [sup.99.Tc], are injected into the patient. Because the patient is injected with the tracer, he or she becomes the source of …