Siemens SOMATOM, Manual, Page: 45 / 590

Dose Information
45
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:
In spiral scanning, however, the applied dose (D
spiral
) is
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:
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:
To calculate the dose, you simply multiply the CTDI
w
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
CTDI
w
x effective mAs