6 Radionuclide imaging
Radionuclide imaging is a very valuable way of examining the function of an organ, as opposed to the more structural images obtained by other methods such as X-ray and CT.
The basic principles of radionuclide imaging are as follows:
a radioactive substance, usually combined with a biologically active compound, is injected into the patient;
this targets a particular organ or tissue type;
the radiation emitted is detected and used to form an image of, or the function of, that organ.
The most common radioactive substances used emit gamma rays (usually in the energy range 100–300 keV). More than 95 per cent of all gamma camera imaging techniques use technetium-99m (Tc-99m). This radionuclide is particularly useful because:
it produces only gamma rays (by isomeric transition);
the gamma rays have an appropriate energy of 140 keV;
the physical half-life of Tc-99m is about 6 hours, so injection and imaging can take place in a reasonable period of time but the patient does not remain radioactive for a long period;
it can readily be combined with biologically active substances to form a variety of radiopharmaceuticals.
Tc-99m is produced by the beta decay of molybdenum-99 by the reaction: