Nuclear medicine / Medical Applications

b) Nuclear medicine

For a diagnostic procedure in nuclear medicine, the patient is given a radionuclide in a carrying substance, such as a pharmaceutical, which is preferentially taken up by the tissue or organ under study. Administration may be by injection, ingestion, or inhalation. The radionuclide emits gamma rays. Most of the diagnostic procedures make use of the radionuclide technetium-99m. It has a half-life of 6 hours, gives off gamma rays with an energy of 0.14 MeV, can be conveniently prepared in the hospital, and readily labels a variety of carrying substances. A special detector called a gamma camera is used to observe how the organs or tissue behave or how quickly the radionuclide moves. Individual doses from technetium scans are comparable to those in diagnostic radiology. The collective dose from nuclear medicine is, however, lower by more than an order of magnitude, because the number of procedures is much lower.

Effective dose (mSv) Organ scan 7 Brain 4 Bone 1 Thyroid, lung 1 Liver, kidney Table (12) : Typical doses to patients from common investigations of organs in nuclear medicine ( UNSCEAR 2000 Report, Vol. 1, Annex D, Table 42 )

When radionuclides are used for treatment rather than diagnosis, much greater activities are given to the patient and much higher doses are given to the target tissues or organs. The treatment of an overactive thyroid gland - hyper­thyroidism - is probably the most common therapeutic procedure, the radionuclide used being iodine-131. Although the radionuclides used for these procedures have short half ­lives, medical staff need to take account of the fact that activity will remain in the body of a patient to whom a radionuclide has been given for some time after the procedure. This might need to be taken into account, especially after therapeutic procedures, in deciding when he or she can be discharged from hospital. Family and friends of the patient may also sometimes be advised by the hospital to take appropriate precautions against inadvertent exposure from this residual activity.