Comparison of the CT scatter fractions provided in NCRP Report No. 147 to scanner-specific scatter fractions and the consequences for calculated barrier thickness.

The new NCRP Report No. 147 includes methodology to determine x-ray protective shielding for CT scanner rooms. This methodology assumes fixed values of the scatter fraction per centimeter (kappa) for the peripheral axis of the head and body CT phantoms. An investigation was performed to determine kappa for different makes and models of CT scanner and examine the consequences of the differences between these and the fixed NCRP values on a typical shielding calculation. kappa values were calculated using an equation for the scattered air kerma at 1 m from NCRP 147 (Kerma(scatter) = kappa x ScanLength x CTDI(100) x pitch(-1)) and using scattered air kerma data provided by the manufacturers and measured CTDI(100) (periphery) values. Typical barrier calculations, following NCRP 147 methodology, were performed for each CT scanner using the fixed kappa values and, separately, using the calculated scanner-specific values. Ten CT scanner models from three manufacturers were investigated. The calculated scanner-specific kappa values varied from the NCRP fixed values by as much as 82%. However, when these kappa values were used in typical barrier calculations, the final shielding requirements using the NCRP fixed values were 0.5 to 13% less than those using the scanner specific values. It is likely that such small underestimates in the shielding requirement due to using the NCRP fixed kappa values would be more than compensated by the conservative assumptions that are incorporated in a typical barrier calculation.