Siemens SOMATOM SENSATION CARDIAC A60 - Manual (Page 12)

12
General
Effective mAs
In sequential scanning, the dose (D
seq
) applied to the
patient is the product of the tube current-time (mAs)
and the CTDI
w
per mAs:
D
seq
= D
CTDIw
x mAs
In spiral scanning, however, the applied dose (D
spiral
)
is influenced by the mAs (mA x Rot Time) and in addi-
tion 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:
D
spiral
= (D
CTDIw
x mA x Rot Time)/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:
Effective mAs = mAs/Pitch Factor
To calculate the dose on the SOMATOM Sensation
Cardiac, you simply have to multiply the CTDI
w
per mAs
with the effective mAs of the scan:
D
spiral
= D
CTDIw
x effective mAs
For spiral scan protocols, the indicated mAs is the
effective mAs per image. The correlation between tube
current mA and effective mAs of spiral scans on a Multi-
slice CT scanner is given by the following formula:
Effective mAs = mA x RotTime/Pitch Factor
Pitch Factor =
Feed/Rot
nrow x Slice collimation
mA =
effective mAs
x Pitch Factor
RotTime
where collimated Slice refers to the collimation of one
detector row, and nrow is the number of used detector
rows.