Siemens syngo CT 2005C - Manual (Page 29)

Dose Information
29
The CTDI
vol
value does not provide the entire informa-
tion of the radiation risk associated with CT examina-
tion. For the purpose, the concept of the “Effective
Dose“ was introduced by ICRP (International Commis-
sion on Radiation Protection). The effective dose is
expressed as a weighted sum of the dose applied not
only to the organs in the scanned range, but also to the
rest of the body. It could be measured in whole body
phantoms (Alderson phantom) or simulated with
Monte Carlo techniques.
The calculation of the effective dose is rather compli-
cated and has to be done by sophisticated programs.
These have to take into account the scan parameters,
the system design of individual scanner, such as x-ray
filtration and gantry geometry, the scan range, the
organs involved in the scanned range and the organs
affected by scattered radiation. For each organ, the
respective dose delivered during the CT scanning has
to be calculated and then multiplied by its radiation
risk factor. Finally, the weighted organ dose numbers
are added up to get the effective dose.
The concept of effective dose allows the comparison of
radiation risk associated with different CT or x-ray
exams, i.e. different exams associated with the same
effective dose would have the same radiation risk for
the patient. It also allows comparing the applied x-ray
exposure to the natural background radiation,
e.g., 2 – 3 mSv per year in Germany.
C2-025.630.01.01.02_APPLICATIONGUIDE_SPIRIT.book Page 29 Friday, April 8, 2005 9:55 AM