Radiation can ionize matter caused by the high energy which displaces electrons during interactions with atoms. In the electromagnetic spectrum higher frequency ultraviolet radiation begins to have enough energy to ionize matter.
Examples of ionizing radiation include alpha particles, beta particles, gamma rays, x-rays, neutrons, high-speed electrons, high-speed protons, and other particles capable of producing ions by direct or secondary processes in passage through tissues.
Damage of living tissue results from the transfer of energy to atoms and molecules in the cellular structure. Ionized cells have to repair themselves to remain alive. Generally, healthy cells have a higher capability to repair themselves than cancer cells.

Biological effects of ionizing radiation exposure:
Ionizing radiation are used in a wide range of facilities, including health care, research institutions, nuclear reactors and their support facilities, and other manufacturing settings. These radiation sources can pose a serious hazard to affected people and environment if not properly controlled.

See also Radiation Safety, Controlled Area, Radiotoxicity and As Low As Reasonably Achievable.

Radiation safety concerns the safe use of ionizing radiation. The radiation exposure has to be controlled to protect people and the environment from unnecessary exposure and the damaging effect to the health. Legal regulations require that radiation exposure (individual radiation exposure as well as collective dose) must be kept as low as reasonably achievable.
The electromagnetic spectrum includes x-rays, gamma rays, ultraviolet radiation, visible light, infrared radiation, and radio waves. Additionally, there are several types of particulate radiation e.g., alpha and beta particles. All types of radiation are used in a wide range of medicine, industry, research and communication. Radiation risks can occur due to either long-term low level exposure or short-term high level exposure. A well-functioning dosimetry program is essential for a safe use and for compliance with federal and state regulations.

Three basic rules have to be observed for a safe use of ionizing radiation.

See also Inverse Square Law, Administrative Dose Guidelines and Annual Dose Limit.

Absorbers consist of material that stops ionizing radiation. For example, lead, steel and concrete attenuate x-rays. Alpha particles and most beta particles can be stopped or absorbed by a sheet of paper or thin metal.
The absorption depends on the atomic number, density, thickness, etc. of the used material.
The interactions between the radiation and the absorber are three major processes: photoelectric absorption, Compton scattering, and pair production.

See also Absorption.

(CT or CAT scan) Computed tomography is a diagnostic imaging technique, previously also known as computerized axial tomography (CAT), computer-assisted tomography (CAT), computerized tomographic imaging, and reconstructive tomography (RT).
A CT scan is based on the measurement of the amount of energy that a tissue absorbs as a beam of radiation passes through it from a source to a detector. As the patient table moves through the CT scanner, the CT tube rotates within the circular opening and the set of x-ray detectors rotate in synchrony. The narrow, fan-shaped x-ray beam has widths ranging from 1 to 20 mm. The large number of accurate measurements with precisely controlled geometry is transformed by mathematical procedures to image data. Corresponding to CT slices of a certain thickness, a series of two-dimensional cross-sectional images is created.
A CT is acquired in the axial plane, while coronal and sagittal images can be rendered by computer reconstruction. Although a conventional radiography provides higher resolution for bone x-rays, CT can generate much more detailed images of the soft tissues. Contrast agents are often used for enhanced delineation of anatomy and allow additional 3D reconstructions of arteries and veins.
CT scans use a relatively high amount of ionizing radiation compared to conventional x-ray imaging procedures. Due to widespread use of CT imaging in medicine, the exposure to radiation from CT scans is an important issue. To put this into perspective, the FDA considers the risk of absorbed x-rays from CT scans to be very small. Even so, the FDA recommends avoiding unnecessary exposure to radiation during diagnostic imaging procedures, especially for children.
CT is also used in other than medical fields, such as nondestructive testing of materials including rock, bone, ceramic, metal and soft tissue.

See also Contrast Enhanced Computed Tomography.

Dosage is an important factor in the use of ionization radiation as well as in application of contrast agents or radiopharmaceuticals and the dosage should be comply with the ALARA principle (As Low As Reasonably Achievable).
Ionizing radiation comes from natural and artificial sources. Radiation effects depend on the type of radiation, and various units are used for measurement of dosages including gray, sievert, radiation absorbed dose (RAD), roentgen equivalent in man (REM), and roentgen. The amount of radiopharmaceutical given to a patient is measured in becquerels (Bq).
The dosage of contrast media in radiographic or computer-tomographic procedures should be tailored according to the diagnostic indications, the iodine concentration, and the patient's body size and age.

See also Administrative Dose Guidelines.