Establishment of scatter factors for use in shielding calculations and risk assessment forcomputed tomography facilities

Abstract

The specification of shielding for CT facilities in the UK and many other countries hasbeen based on isodose scatter curves supplied by the manufacturers combined with thescanner’s mAs workload. Shielding calculations for radiography and fluoroscopy are linkedto a dose measurement of radiation incident on the patient called the kerma–areaproduct (KAP), and a related quantity, the dose-length product (DLP), is nowemployed for assessment of CT patient doses. In this study the link betweenscatter air kerma and DLP has been investigated for CT scanners from differentmanufacturers. Scatter air kerma values have been measured and scatter factorsestablished that can be used to estimate air kerma levels within CT scanning rooms.Factors recommended to derive the scatter air kerma at 1 m from the isocentre are0.36 µGy (mGy cm)−1 for thebody and 0.14 µGy (mGy cm)−1 for head scans. The CT scanner gantries only transmit 10% of the scatter air kerma leveland this can also be taken into account when designing protection. The factors can be usedto predict scatter air kerma levels within a scanner room that might be used in riskassessments relating to personnel whose presence may be required during CT fluoroscopyprocedures.