Example Outside (Car Park and seating) Wall A 4.2m 1.3m Wall D Wall B Waiting Area (not attended) 1.1m Offices 3.8m Operator Area Wall C Distance to ceiling from table – 2m Offices on floor above Corridor not drawn to scale

Example – Exposure Details Only two investigations in this room AP Abdomen (patient supine on table) DAP – 6.5 Gy.cm2 30 examinations per day AP Pelvis (patient supine on table) DAP – 5.5 Gy.cm2 20 examinations per day All exposures at 100 kV Room in use 5 days per week, 52 weeks per year

Calculation required Thickness of lead required to shield walls adequately (specify as lead code). Thickness of concrete required to shield ceiling adequately. (note, this is a ground floor room with no basement) Assuming a dose constraint (Dc ) of 0.3mSv per annum

X-ray transmission through lead

X-ray transmission through concrete

Coefficients for empirical formula at 100 kV Material Lead 2.5 15.28 0.756 Concrete 0.0395 0.0844 0.519

Solution Total DAP per annum for Abdomen 48750 Gy.cm2 6.25 Gy.cm2 x 30 per week x 5 days per week x 52 weeks Total DAP per annum for Pelvis 28600 Gy.cm2 (5.5 x 20 x 5 x 52) Scatter factor at 100 kV [(0.031 x 100) + 2.5] = 5.6 mGy.(Gy.cm2)-1 Scatter in room at 1m 5.6 x (48750 + 28600) = 433160 mGy per annum ie 433.16 mGy per annum

Transmission Distance (m) Annual scatter (mGy) Kinc*52 Occupancy T (x10-3) B Wall A 1.3 256.3 0.1 11.7 Wall B 2.1 98.2 1 3.05 Wall C 2.5 69.3 0.2 21.6 Wall D 0.05 61.1 Operator area 1.1 358.0 0.84 Ceiling 2.0 108.3 2.77 inverse square law corrected

( ) ú û ù ê ë é + = a b ag 1 ln B x Shielding required - g Lead (mm) Lead code Concrete (mm) Wall A 0.84 3 66 Wall B 1.32 3/4 96 Wall C 0.65 54 Wall D 0.37 35 Operator area 1.81 5 126 Ceiling 1.35 4 98 ( ) ú û ù ê ë é + = - a b ag g 1 ln B x

Notes on the solution Derive the shielding requirements using the empirical formula and use graphs to check that thicknesses are reasonably accurate. You may disagree or prefer to be more conservative with the occupancy factors. If the wall requires lead, the minimum code easily available is Code 3 but if the amount of lead is very small it is usually worth seeing if the material and thickness of the existing wall would be sufficient. Wall B requires exactly 1.32mm Pb (Code 3). In this case it would be prudent to advise using the higher code (Code 4). Thicknesses of concrete should normally be rounded up to the nearest 50mm.