Siemens SOMATOM Spirit Operator's Manual page 69

Safety
Calibration
04/2012
If the CT system is not properly setup (for example, no check-up
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performed) CT scale displacements and inhomogeneities can
result. This may result just after switching on the unit, during
warm-up to operational temperature (calibration), or due to an
extended length of service of the X-ray tube. The same applies
to defects in the measurement system.
The daily quality checkup usually detects and reports such dis-
crepancies.
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Additional calibrations may also be necessary if large changes in
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temperature occur in the room.
Other system defects or calibration deviations cause streaks or
(partial) rings to appear in the CT image.
Information about Personal Safety
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