Effective Mas - Siemens SOMATOM Manual

Dose Information

Effective mAs

In sequential scanning, the dose (D
patient is the product of the tube current-time (mAs)
and the CTDI
w
D = D
seq CTDIw
In spiral scanning, however, the applied dose (D
influenced by the conventional mAs (mA x Rot Time)
and additionally 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 greater than 1, and increased
when the pitch factor is less than 1 (for constant mA).
Therefore, the dose in spiral scanning has to be cor-
rected by the pitch factor:
D = (D
spiral CTDIw
To simplify this task, the concept of the "effective" mAs
was introduced with the SOMATOM Multislice scan-
ners.
The effective mAs takes into account the influence of
pitch on both the image quality and dose:
Effective mAs =
To calculate the dose, you simply multiply the CTDI
per mAs with the effective mAs of the scan:
D = D
CTDI
spiral w
per mAs:
x mAs
x mA x Rot Time)
Pitch Factor
mAs
Pitch Factor
x effective mAs
) applied to the
seq
spiral
) is
w
45