Siemens SOMATOM Emotion 6, Application Manual, Page: 16 / 194

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The CTDI
Vol
value does not provide the entire information
of the radiation risk associated with CT examination.
For this purpose, the concept of the “Effective Dose“ was
introduced by ICRP (International Commission 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 would allow the com-
parison 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.
General