Siemens SOMATOM Manual page 46

Dose Information
For spiral scan protocols, the indicated mAs is the
effective mAs per image. The correlation between tube
current and effective mAs of spiral scans on a Multi-
slice CT scanner is expressed by the following formula:
Effective mAs = mA x RotTime
Pitch Factor = Feed per Rotation
nrow x Slice collimation
mA = effective mAs
RotTime
where Slice collimation refers to the collimation of one
detector row, and nrow is the number of used detector
rows.
46
Pitch Factor
x Pitch Factor