Ionizing radiation occurs in four characteristic forms, a (alpha) decay, ß (Beta) decay, g (gamma) ray emission, and the emission of characteristic x-rays. Alpha radiation results from heavy elements such as uranium-238 that eject a particles from their nucleus in order to reach a more stable energy state. Ejection of the two proton-two neutron a-particle reduces both the atomic mass and atomic number of the element concerned. a-Particles travel negligible distances in tissue but are highly damaging because of the ionization they produce. Hence, they are unsuitable for diagnostic imaging. Beta radiation occurs when either a positively or negatively charged electron is ejected from the nucleus through an internal conversion process. When a neutron is converted to a proton, a ß particle (electron) is produced, then ejected from the nucleus. ß-particles travel only a few millimeters in soft tissue. Beta emitting isotopes are most useful for tissue-specific radiotherapy (eg, ¹³¹I treatment for hyperthyroidism), experimental tissue counting methods, and some radioimmunoassay procedures. The second form of ß-radiation (ß+ or positron emission) is more important for diagnostic imaging. When a proton is converted to a neutron in the nucleus a positron is ejected from the nucleus and then is annihilated when it encounters a nearby electron. The annihilation reaction produces two 511 keV photons of electromagnetic energy traveling in opposite directions. Detectors capable of registering the coincidental arrival of each photon pair form the basis of positron emission tomography (PET) scanning, a new tomographic imaging method.
Gamma (g) emission is another means by which unstable atomic nuclei lower their energy state, producing externally detectable energy in the form of electro-magnetic radiation (g-rays), rather than particles. These reductions in energy occur in single or multiple discrete steps, resulting in g-rays of one or more specific energies that are characteristic of the isotope in question. Gamma emission does not change the atomic number because no particles are emitted. If the decay process is slow, the decaying nuclide is termed metastable, designated by a small “m” following the mass number, (eg, technetium-99m).
Gamma emitting radionuclides form the basis of most nuclear imaging procedures. The g-emitting isotope ^99mTc accounts for approximately 90% of all veterinary and human scintigraphy procedures because its physical and chemical characteristics most closely approach the ideal scintigraphic agent. Technetium-99m’s abundant 140 keV gamma photon is ideal for imaging, the 6 hour half-life is suitable for most procedures performed within 24 hours of administration. Technetium is also chemically highly adaptable, able to be bound to numerous pharmaceuticals supplied in freeze-dried kits for specific organ studies (eg, phosphonates for bone scanning, DTPA for renal and aerosolized ventilation studies, macroaggregates of albumin (MAA) for perfusion scanning, and sulfur-colloid for liver and splenic imaging).
Characteristic x-rays differ from g-rays only because they arise from interactions between orbital electrons rather than from events within the nucleus. They occur in the decay scheme of some proton-rich nuclei when the nucleus converts one of its protons to a neutron by capturing an inner shell electron (“electron capture”). The energy change as an outer electron moves into the vacant inner shell is emitted as a characteristic x-ray an energy specific to the atom in question.