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.

The calibration factor helps to convert between the measured value of a parameter and the actual value of that parameter in a system.
This normalization is necessary in cases where detectors and sensors do not directly sample a parameter, for example the radiation energy is measured by a detector that produces an electrical pulse whose total charge is proportional to the supplied radiation.

(DQA) This procedure is used by system operators to verify an x-ray system, gamma camera, or CT scanner operation based on relevant image quality parameters like e.g., geometric distortion, x-ray yield, signal to noise ratio. The quality assurance should carry out according to instructions of the manufacturer.

See also Acceptance Checking, Calibration and Calibration Factor.

Raw data are the values of all measured detector signals during a scan. After calibration for fluctuations in tube output and beam hardening, the attenuation properties of each x-ray signal are accounted and correlated with the ray position. From these data the CT images are reconstructed including the use of mathematical procedures like convolution filtering and back projection.
Raw data can also be used for later reconstruction of additional planes and images by using a different convolution filter, zoom reconstruction, or an alternative CT number scale.

See also Magnification, Archiving, Number of Measurements and Convolution.