The average dose in the scan plane is best described by the CTDIW for the selected scan parameters. The CTDIW is measured in dedicated plastic phantoms – 16 cm diameter for head and 32 cm diameter for body (as defined in IEC 60601 – 2 – 44). For scan modes with z- Sharp the CTDI100 is calculated using the single number of tomographic sections (not doubled by z-Sharp) to remain within the terms of IEC 60601-2-44. The zcoverage with and without z-Sharp is the same and so is the dose. This dose index gives a good estimation of 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 CTDIW value displayed can deviate from the dose in the scanned volume.
The CTDIW definition and measurement are based on single axial scan modes. For clinical scanning, i.e.scanning of entire volumes in patients, the average dose will also depend on the table feed between axial scans or the feed per rotation in spiral scanning. The dose, expressed as the CTDIW, must therefore be corrected by the pitch factor of the spiral scan or an axial scan series to describe the average dose in the scanned volume:
For this purpose the IEC defined the term “CTDIVol“ in September 2002:
This dose number is displayed on the user interface for the selected scan parameters.
Note: Previously the dose display on the user interface was labeled “CTDIW“. This displayed CTDIW was also corrected for the pitch and was therefore identical to the current CTDIVol.
The CTDIw 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 complicated and has to be performed by sophisticated programs. These have to take into account the scan parameters, the system design of the 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 a comparison of the applied X-ray exposure to the natural background radiation, for example, 2 – 3 mSv per year in Germany.
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