Siemens EMOTION 16-Jun - Manual (Page 43)

Dose Information
43
Note: Previously the dose display on the user interface
was labeled “CTDI
W
“. This displayed CTDI
W
was also cor-
rected for the pitch and was therefore identical to the
current CTDI
Vol
.
The CTDI
w
value does not provide the entire informa-
tion of the radiation risk associated with CT examina-
tion. For this 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 performed by sophisticated pro-
grams. These have to take into account the scan
parameters, the system design of the individual scan-
ner, 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 a comparison of the applied
X-ray exposure to the natural background radiation,
for example, 2 – 3 mSv per year in Germany.