Scanner User Manual

30
Dose Information
Effective mAs
In sequential scanning, the dose (Dseq) applied to the
patient is the product of the tube current-time (mAs)
and the CTDI
w
per mAs:
In spiral scanning, however, the applied dose (Dspiral)
is influenced by the “classical“ mAs (mA x Rot Time)
and in addition by the Pitch Factor. For example, if a
Multislice CT scanner is used, the actual dose applied
to the patient in spiral scanning will be decreased
when the Pitch Factor is larger than 1, and increased
when the Pitch Factor is smaller than 1. Therefore, the
dose in spiral scanning has to be corrected by the Pitch
Factor:
To make it easier for the users, the concept of the
“effective“ mAs was introduced with the SOMATOM
Multislice scanners.
The effective mAs takes into account the influence of
pitch on both the image quality and dose:
To calculate the dose you simply have to multiply the
CTDIw per mAs with the effective mAs of the scan:
D
seq
= D
CTDIw
x mAs
D
spiral
= (D
CTDIw
x mA x Rot Time)/Pitch Factor
Effective mAs = mAs/Pitch Factor
D
spiral
= D
CTDIw
x effective mAs
C2-025.630.01.01.02_APPLICATIONGUIDE_SPIRIT.book Page 30 Friday, April 8, 2005 9:55 AM