Medical Physics Notes

Topics: Positron, Radioactive decay, Positron emission tomography Pages: 4 (1088 words) Published: June 11, 2012
Medical Physics
*Outline properties of radioactive isotopes and their half-lives that are used to obtain scans of organs. Radioactive isotopes are commonly used as tracers in medical diagnosis. In this procedure a compound is synthesised using a radioactive isotope and injected or ingested into the body. Because isotopes are chemically identical to stable atoms, the compound will follow the normal metabolic pathway for such compounds in the body. By measuring the radioactive emission that is emitted from the body, the path of the compound can be followed. Gamma ray emitting isotopes are commonly used as gamma rays readily pass through the body.

It is possible to study specific organs using radioactive isotopes that accumulate naturally in the organ. Radioactive iodine (131I) accumulates in the thyroid gland and is often used to diagnose and treat diseases of the thyroid. Full body bone scans can be produced using the radioactive strontium (87Sr). Strontium displaces calcium in the bones. Such scans are a little like X-Rays, though they are produced by the emission of gamma radiation rather then the absorption of X-Rays.

Some of the common radioisotopes used in medical diagnosis are described below and there relative half-lives shown.

Iodine 131- Widely used in treating thyroid cancer and in imaging the thyroid; also in diagnosis of abnormal liver function, renal (kidney) blood flow and urinary tract obstruction. A strong gamma emitter, but used for beta therapy.

Properties of iodine 131 – Half lives
Physical – 8.04 days
Biological – Thyroid- 120 days; rest of tissue 12 days

Lutetium-177 (half-life- 6.7 days): Lu-177 is increasingly important as it emits just enough gamma for imaging while the beta radiation does the therapy on small tumours. Its half-life is long enough to allow sophisticated preparation for use.

Iodine-125 (half-life- 60 days): Used in cancer brachytherapy (prostate and brain), also diagnostically to evaluate the filtration...
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