courtesy of http://www.kvi.nl/~shoekstra/webpage2.htm

Isotopes in Nuclear Medicine

In NUCLEAR MEDICINE a radio-isotope is administered to a patient either to aid the diagnosis of disease or for the treatment of disease.

The radioisotopes used in DIAGNOSTIC nuclear medicine are selected on the basis of their ability to provide useful clinical information (usually by providing an image of an internal structure in the human body or by visualising various stages in the function of an organ) while exposing the patient to only minimal radiation.

To ensure this, certain selection criteria are applied :-

For example the radioisotope should -

• possess a short half-life(hours) which is commensurate with the duration of the investigative procedure
• not emit alpha or beta radiation, because these particles would be trapped in the patient's tissues and not be detected externally
• emit gamma radiation of an energy which will allow its origin to be efficiently assessed
• be available in the highest possible specific activity, so that it will not invoke either a toxic or pharmacological response in the patient.

On the other hand, in THERAPEUTIC nuclear medicine, a different set of criteria apply :-

• the half life should not be the cause of an extended stay in hospital for the patient
• the radioisotope should emit particulate (alpha or beta) radiation of sufficient energy to penetrate to all parts of the lesion
• it should, in addition, emit gamma rays to facilitate the assessment that the appropriate region of the body has been targeted.

From a possible population of more than 2300, only a handful of radioisotopes come close to satisfying the selection criteria for use as a diagnostic agent. Of these, reactor-produced technetium-99m ( 99m Tc) is pre-eminent, being used in more than 80% of the estimated 100,000 patient studies that are performed world-wide each day.

After ( 99m Tc), a series of cyclotron produced radioisotopes, such as thallium-201( 201 Tl), gallium-67 ( 67 Ga), indium-111( 111 In) and iodine-123( 123 I), are the next most popular.

A different group of radioisotopes is used for therapeutic purposes.

Well-established examples are iodine-131 ( 131 I), phosphorus-32 ( 32 P) and yttrium-90 ( 90 Y) but several others are being investigated for possible application.

Examples of these are samarium-153 ( 153 Sm), rhenium-186 and rhenium-188 ( 186 Re, 188 Re), dysprosium-165 ( 165 Dy) and holmium-166 ( 166 Ho).

The various radioactive substances found naturally in terrestrial materials are very ancient remnants of the time when the Earth was formed. None of them satisfy the nuclear medicine selection rules and consequently are not used clinically.

information courtesy of http://www.ansto.gov.au/info/reports/radboyd.html#Nuc%20Med

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