Radiation is energy in movement. Electromagnetic radiation (EMR) refers to energy that travels in the form of waves. Another way of thinking about EMR is as a matterless bundle of energy called a photon. The photon is the “package” of energy that carries the energy through space. The other type of radiation is particulate, in which small particles (electrons, alpha particles) carry the energy. Particulate radiation can emanate from radioactive materials and linear accelerators. Particulate radiation is not effective for imaging due to low penetration of matter.
X-rays are produced from electric energy in the form of electrons flowing into the x-ray tube. Electrons are minute particles that possess a negative electric charge. The potential energy of an electron depends on its position in an electric circuit or on its energy level in an atom (electron shell). Within an electric circuit, electrons act as a conduit to transport energy. At the “source” (eg, electric socket), electrons possess a high potential energy that is then lost as the energy is converted into different forms when the electrons reach the “load” (eg, the x-ray tube).
All forms of energy used in medical imaging, with the exception of ultrasound, are a type of radiation. Radiation interacts with the human body in a way that creates information. The trajectory of radiation is straight until it interacts with matter. Matter both absorbs and scatters radiation. A fundamental principle of physics is that energy can neither be created nor destroyed, only converted from one form to another. When x-rays are used to image the human body, electric energy from a source is converted into x-rays and subsequently transformed into light, heat, and chemical energy.
The four main types of imaging modalities are x-rays (including computed tomography and fluoroscopy), ultrasound, magnetic resonance imaging, and nuclear medicine (including positron emission tomography [PET]).
