Dose Information
Effective mAs
In sequential scanning, the dose (Dseq) applied to the patient is the product of the tube
Dseq = DCTDIw x 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:
Dspiral = (DCTDIw 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 you simply have to multiply the CTDIw per mAs with the effective mAs of the scan:
Dspiral = DCTDIw x effective mAs
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