This term usually refers to the storage of patient data and images. Images are best archived in digital form (e.g., on optical disks, DVDs, PACS systems) and not only on films (hard copies, prints). Data compression via a reduction in matrix size, pixel depth or CT numbers, will result in a loss of spatial and contrast resolution. Digital images should be converted into a universal format such as DICOM. Raw data saving is necessary when additional image reconstructions are required.

See also Picture Archiving and Communication System, and Digital Imaging and Communications in Medicine.

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.

X-rays contain a range of energies (polychromatic photons), the higher energies pass through the patient, the lower energies are absorbed or scattered by the body. Ideally, the x-ray beam should be monochromatic or composed of photons having the same energy. Strong filtration of the beam results in more uniformity. The more uniform the beam, the more accurate the attenuation values or CT numbers are for the scanned anatomical region.
There are two types of filtration utilized in CT:
Inherent tube filtration and filters made of aluminum or Teflon are utilized to shape the beam intensity by filtering out the undesirable x-rays with low energy. Filtration of the x-ray beam is usually done by the manufacturer prior to installation. The half value layer provides information about the energy characteristics of the x-ray beam. Too much filtration produces a loss of contrast in the x-ray image.
A mathematical filter such as a bone or soft tissue algorithm is included into the CT reconstruction process to enhance resolution of a particular anatomical region of interest.

(H) The Hounsfield scale displays radiodensity in a linear scale of gray shades expressed in Hounsfield units (HU). The Hounsfield scale is a quantitative transformation of the attenuation coefficient.
The Hounsfield value -1000 is defined as the radiodensity of air, 0 H that of distilled water at standard pressure and temperature, and denser tissues like for example cranial bone can reach 2000 H. The radiation attenuation of dental fillings or artificial implants depends on atomic number of the elements used. Titanium usually has an amount of +1000 HU. Iron steel can have a density greater than the highest range (traditional 3095 H) covered by the standard Hounsfield scale of a CT scanner. Areas with attenuation coefficients that exceed the scale's maximum are white areas in which no detail is visible.
Some CT machines are relatively tolerant, precise representing regions with very high densities. Sometimes, an option is available to select an extended CT number scale.

(LCR) The low contrast resolution describes the ability to discriminate between tissues with slightly differences in attenuation properties. The LCR depends on the stochastic noise.
The low contrast resolution is usually expressed as the minimum detectable size of an image structure, for a fixed percentage difference in contrast relative to the adjacent background.
A strength of computed tomography (CT) is its ability to visualize structures of low contrast in an object, a task that is limited by noise and is closely associated with the radiation dose. For example, a reduction of the dose at constant spatial resolution affects the visibility of structures with low contrast (e.g. vessels in the liver), due to increased noise. The visibility of these low contrast structures can partly be improved by decreasing the spatial resolution, while keeping the dose constant.

See also CT Number, Image Quality and Low Contrast Detectability.