Purpose : In Japan, an authorized calculation method when designing shielding for an x ray room was recommended in Notification No. 188 of Pharmaceutical and Medical Safety Bureau, Ministry of Health, Labour and Welfare dated Mar. 12, 2001, referencing ICRP Publication 33. This method was applied to a limited cone beam CT unit with rotating field using approximations for the convenience of calculation. The accuracies of the approximations were evaluated by measurements of related radiation quantities.
Materials and Methods : The assumed x ray room was 2 m square using a 3DX Multi Image Micro CT (J. Morita MFG) with an 80kV tube potential. The lead equivalent thickness of the image detector, spatial distribution and equivalent tube potential of scattered radiation from the phantom and leakage radiation from the tube housing were measured. The effective dose due to the primary beam, scattered radiation from the patient and leakage radiation from the tube housing were calculated, where the air kerma rate of leakage radiation 1m from the source was assumed to be 1 mGy/h, the worst case value described in Notification No. 188 as well as the measured value. The required shielding thicknesses were determined for lead, concrete and iron.
Results : The contribution of the primary beam to the total effective dose was negligible, since the lead equivalent thicknesses of image detectors were more than 2mm. The maximum measured air kerma of the scattered radiation 1m from the rotation center was 3 μGy per scan, lower than the calculation by 30%. The equivalent tube potential of scattered radiation was about 60kV. This suggested that the transmission of scattered radiation would be reduced by a factor of 3 compared to the calculation using transmission data for 80kV x rays. The leakage radiation intensity was 55μGy/h, lower than the worst case by a factor of 18. In order to leave the space around the x ray room out of controlled area, the required thicknesses were 0.83mm lead, 4.9cm concrete and 4.2mm iron assuming the worst case leakage radiation from the tube housing, while these values were 0.19mm lead, 2.8cm concrete and 1.6mm iron using the measured value.
