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

The specification of shielding for CT facilities in the UK and many other countries has been based on isodose scatter curves supplied by the manufacturers combined with the scanner's mAs workload. Shielding calculations for radiography and fluoroscopy are linked to a dose measurement of radiation incident on the patient called the kerma-area product (KAP), and a related quantity, the dose-length product (DLP), is now employed for assessment of CT patient doses. In this study the link between scatter air kerma and DLP has been investigated for CT scanners from different manufacturers. Scatter air kerma values have been measured and scatter factors established 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 are 0.36 µGy (mGy cm)(-1) for the body and 0.14 µGy (mGy cm)(-1) for head scans. The CT scanner gantries only transmit 10% of the scatter air kerma level and this can also be taken into account when designing protection. The factors can be used to predict scatter air kerma levels within a scanner room that might be used in risk assessments relating to personnel whose presence may be required during CT fluoroscopy procedures.