Usually, magnification is the enlargement of an area by interpolation after the reconstruction of an image. Magnification does not provide more information, but allows a better view of certain object details. A zoom reconstruction is based on the raw data of the scan. Magnification software enlarges an image by mapping one pixel onto an n x n array of screen pixels (pixel stretching).
Other types of magnification include electron-optical, geometric, the product of geometric and the electron-optical magnification and enlargement by imaging procedures.
Electron-optical magnification is the ratio of the dimension of the detector input image and the size of the image on the screen. This ratio is determined by all electronic and optical imaging processes of the image chain, provided that one camera pixel is mapped onto accurately one monitor pixel.
Geometric magnification occurs in x-ray images when the focal spot is theoretically assumed to be a point and not an area. For nanofocus and microfocus radiographic systems, the focus-to-detector (film) distance and the focus-to-object (film) distance defines the geometric magnification.
The total magnification is the product of the electron-optical and geometric magnification. Possible magnifications are up to a factor of 26,000.
Magnification procedures in medical imaging are usually produced by extended distance between the subject and the image receptor.

Natural background radiation originates from radioactive elements in the environment, including food, water, soil and rock (also building materials), the atmosphere and cosmic rays. The level of natural exposure to radiation can vary greatly between different locations. In the US, the average annual exposure from natural sources to humans is about 3 mSv (millisievert) corresponding to 0.3 rem. Radon gas accounts for two-thirds of this exposure.
Background radiation may also interfere with measurements. Background radiation includes radioactive contamination of samples or incomplete absorption of radiation in a detector.

A calibration is a correction procedure that determines the relationship between the measured output of a system and the reference standard. Calibration procedures include scanning air or an appropriate test phantom.
The calibration of a CT system takes account of variations in beam intensity or detector efficiency in order to achieve best homogeneity within the field of view and the accuracy of CT numbers.

See also Calibration Factor and Acceptance Checking.

(CTE) Computed tomography enterography is an imaging procedure to evaluate diseases affecting the mucosa and bowel wall of the small intestine. CTE uses oral contrast agents to improve bowel wall visualization. Several studies established that small bowel distention using negative oral contrast agent increases diagnostic performance of some abdomen CT studies.
The multi-detector row CT (MDCT) improves temporal and spatial resolution and 3D imaging processes offer a full examination of the small bowel with surrounding structures, depicting the small bowel inflammation associated with Crohn's disease by displaying mural hyperenhancement, stratification, and thickening.
CT enterography versus capsule endoscopy provides a non invasive study with comparable sensitivity, high specificity and overall accuracy.

See also Colonoscopy and Virtual Colonoscopy.

(CA) Contrast agents are used to change the imaging characteristics, resulting in additional information about anatomy, morphology or physiology of the human body. Radiocontrast agents (also called photon-based imaging agents) are used to improve the visibility of internal body structures in x-ray and CT procedures. Contrast agents are also used to increase the contrast between different tissues in MRI (magnetic resonance imaging) and ultrasound imaging. The ideal imaging agent provides enhanced contrast with little biological interaction.
First investigations with radiopaque materials are done shortly after the discovery of x-rays. These positive contrast agents attenuate x-rays more than body soft tissues due to their high atomic weight. Iodine and barium have been identified as suitable materials with high radiodensity and are used until today in x-ray and CT contrast agents. Iodine-based contrast agents are water-soluble and the solutions are used nearly anywhere in the body. Iodinated contrast materials are most administered intravenous, but can also be introduced intraarterial, intrathecal, oral, rectal, intravesical, or installed in body cavities. Barium sulfate is only used for opacification of the gastrointestinal tract. Negative contrast agents attenuate x-rays less than body soft tissues, for example gas.

Iodinated contrast media are differentiated in;

Intravascular iodinated contrast agents are required for a large number of x-ray and CT studies to enhance vessels and organs dependent on the blood supply. Injectable contrast agents are diluted in the bloodstream and rapidly distributed throughout the extracellular fluid. The main route of excretion is through the kidneys, related to the poor binding of the agent to serum albumin. The liver (gall bladder) and small intestine provide alternate routes of elimination particularly in patients with severe renal impairment. The use of special biliary contrast agents is suitable for gallbladder CT and cholecystograms because they are concentrated by the liver to be detectable in the hepatic bile.
The introduction of fast multi-detector row CT technology, has led to the development of optimized contrast injection techniques. The amount of contrast enhancement depends on the contrast agent characteristics, such as iodine concentration, osmolality, viscosity, and the injection protocol, such as iodine flux and iodine dose. Adverse reactions are rare and have decreased with the introduction of nonionic contrast agents.
See also Contrast Enhanced Computed Tomography, Abdomen CT, Contrast Media Injector, Single-Head CT Power Injector, Multi-Head Contrast Media Injector, Syringeless CT Power Injector, CT Power Injector.