Particle radiation is made up of any subatomic particles, such as protons, neutrons, and high-speed electrons, capable of causing ionization. Alpha and beta particles are two of the more common types of particle radiation. They come from the nuclei of radioactive atoms through radioactive decay. Particle radiation has a mass component, may have a charge, and travels at varying speeds (slower than the speed of light). Electromagnetic radiation is an electric and magnetic disturbance traveling through space at the speed of light (2.998 × 108 m/s). It contains neither mass nor charge but travels in packets of radiant energy called photons, or quanta. Examples of EM radiation include radio waves and microwaves, as well as infrared, ultraviolet, gamma, and x-rays. Some sources of EM radiation include sources in the cosmos (e.g., the sun and stars), radioactive elements, and manufactured devices. EM exhibits a dual wave and particle nature. Electromagnetic radiation travels in a waveform at a constant speed. The wave characteristics of EM radiation are found in the
relationship of velocity to wavelength (the straight line distance of a single cycle) and frequency (cycles per second, or hertz, Hz), expressed in the formula Because the velocity is constant, any increase in frequency results in a subsequent decrease in wavelength. Therefore, wavelength and frequency are inversely proportional. All forms of EM radiation are grouped according to their wavelengths into an electromagnetic spectrum. The particle-like nature of EM radiation manifests in the interaction of ionizing photons with matter. The amount of energy (E) found in a photon is equal to its frequency (ν) times Planck's constant (h): Photon energy is directly proportional to photon frequency. Photon energy is measured in eV or keV (kilo-electron volts). The energy range for diagnostic x-rays is 40 to 150 keV. Gamma rays, x-rays, and some ultraviolet rays possess sufficient energy (>10 keV) to cause ionization. The energy of EM radiation determines its usefulness for diagnostic imaging. Because of their extremely short wavelengths, gamma rays and x-rays are capable of penetrating large body parts. Gamma rays are used in radionuclide imaging. X-rays are used for plain film and
computed tomography (CT) imaging. Visible light is applied to observe and interpret images. Magnetic resonance imaging (MRI) uses radiofrequency EM radiation as a transmission medium.
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