13
General
Dose Information
The dose as described by CTDIwis displayed on the
user interface for the selected scan parameters. The
CTDIwis measured in the dedicated plastic phantoms –
16 cm diameter for head and 32 cm diameter for
body (as defined in IEC 60601-2-44). This dose number
gives a good estimate for the average dose applied
in the scanned volume as long as the patient size is
similar to the size of the respective dose phantoms.
Since the body size can be smaller or larger than 32 cm,
the CTDI value displayed can deviate from the dose in
the scanned volume.
The CTDIwvalue does not provide the entire information
of the radiation risk associated with CT examination.
For the 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 com-
plicated 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.