The geometric factors do not normally change with time and need only be measured once. Depending on their nature, the misalignment factors may either be fixed, or may need to be re-measured as components are replaced. The time window alignment factors should be re-measured whenever detector components are replaced. The detector efficiency and block profile components can change with time, as photo-multiplier tube gains drift, and should be re-measured routinely (usually monthly or quarterly, but possibly more often in a less stable environment such as that found in a mobile PET system). The rate-dependent component of the transaxial block profile can, if necessary, be determined for each individual scan using self-normalisation.
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