A comparison of shielding calculation methods for multi-slice computed tomography (CT)systems

Abstract

Currently in the UK, shielding calculations for computed tomography (CT) systems arebased on the BIR–IPEM (British Institute of Radiology and Institute of Physics inEngineering in Medicine) working group publication from 2000. Concerns have been raisedinternationally regarding the accuracy of the dose plots on which this method depends andthe effect that new scanner technologies may have. Additionally, more recent shieldingmethods have been proposed by the NCRP (National Council on Radiation Protection)from the USA. Thermoluminescent detectors (TLDs) were placed in three CT scannerrooms at different positions for several weeks before being processed. Patient workload anddose data (DLP: the dose length product and mAs: the tube current–time product) werecollected for this period. Individual dose data were available for more than 95% ofpatients scanned and the remainder were estimated. The patient workload datawere used to calculate expected scatter radiation for each TLD location by boththe NCRP and BIR–IPEM methods. The results were then compared to themeasured scattered radiation. Calculated scattered air kerma and the minimumrequired lead shielding were found to be frequently overestimated compared to themeasured air kerma, on average almost five times the measured scattered air kerma.