About Nuclear Medicine
I. HISTORY AND OVERVIEW
Nuclear medicine is a largely diagnostic medical specialty, which by broad definition includes all medical uses of unsealed radioactive materials. (The application of radiation(s) from sealed sources of radioactive materials is the province of radiation therapy). The use of unsealed radioactive materials for treatment is the province of nuclear medicine. Although sporadic uses of radioactive materials in humans date back to the 1920’s or before, modern nuclear medicine really began in the 1940’s when radioactive iodine was first used to study and treat patients with thyroid diseases.
II. RADIATION AND RADIOACTIVITY
Atoms of a given element with an excess of nuclear mass or energy are unstable, and lose their excess mass or energy through the process of RADIOACTIVE DECAY, and they are therefore called radioactive isotopes or RADIOISOTOPES. The excess energy or mass leaves the nucleus as RADIATION, which may be of two basic types; particulate or non-particulate radiation. Particulate radiations (ALPHA and BETA) are characterized by very short ranges in tissue, and as a result they deliver high radiation absorbed doses (RADS). Alpha and Beta particles are useful in radioisotope (radionuclide) therapy. Diagnostic nuclear medicine is based on non-particulate GAMMA radiation. “Gamma rays” are high energy photons of electromagnetic radiation, just like X-rays. The only difference between X-rays and Gamma rays is their origin – Gamma rays come from the energy losses in the nucleus of the atom, whereas X-rays come from energy losses of the orbital electrons..
III. DIAGNOSTIC NUCLEAR MEDICINE
In diagnostic nuclear medicine imaging, a radioisotope is administered which will localize in the organ or organ system we wish to study. An external detector produces images of internal distribution of the administered radioactivity, which depends on the biologic behavior of the isotope administered and the functional status of the organ under investigation. The studies may be called “tracer” studies, because the fate of a normal biochemical substrate may be traced with a “tracer” dose of a radioactive form of that substrate. Nuclear medicine images are thus physiologic maps of the distribution of function within the body or an organ. The prototype tracer is radioactive iodine (radioiodine) used to study the thyroid. The radioiodine behaves exactly as does normal stable iodide, and we can measure its uptake in the gland and image its distribution within the gland. Tc-99m is the “work horse” of nuclear medicine because of important physical characteristics (emits only gamma radiation, physical half life is only 6 hours, ideal photon energy). Tc-99m can be attached to a number of chelating agents to carry the radioactive isotope to many organ systems. The wide variety of tracers and imaging procedures possible therewith is beyond the scope of this brief introduction.
IV. RADIONUCLIDE THERAPY
Ionizing radiation – either high energy electromagnetic radiation (Gamma- or X- rays) – or particulate radiations may be used for therapy. Radionuclide therapy with unsealed sources – ie liquids for oral, intravenous, or intracavitary administered is the realm of Nuclear Medicine. Application of radiations from sealed sources (external beam or interstitial “seeds” or “needles” is the province of Radiation Therapy. Radionuclide therapy includes radioactive iodine (I-131) therapy for hyperthyroidism or thyroid cancer, and various agents for therapy of painful bony metastases and other malignant conditions.
V: Internet Resources
The Society of Nuclear Medicine and Molecular Imaging
SNM’s “What is Nuclear Medicine?” page
SNM’s patient oriented website
The Health Physics Society has a good site in general and this is a pretty good short list of FAQs
The Health Physics Society’s fact sheet on radiation exposure from diagnostic procedures
Fairly comprehensive “What is Nuclear Medicine?” page
American Society of Nuclear Cardiology’s “About Nuclear Cardiology” page
World-nuclear.org is a fascinating site and has a good page about medical uses of radioactive isotopes