IAEA Training Material on Radiation Protection in Diagnostic

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY Part 12. 1 : Shielding and X-ray room design Practical exercise IAEA International Atomic Energy Agency

Overview / Objectives • Subject matter : design and shielding calculation of a diagnostic radiology department • Step by step procedure to be followed • Interpretation of results IAEA 12. 1 : Shielding and X-ray room design 2

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 12. 1 : Shielding and X-ray room design Design and shielding calculation of a diagnostic radiology department Practical exercise IAEA International Atomic Energy Agency

Radiation Shielding - Calculation • Based on NCRP 147 • Assumptions used are very pessimistic, so overshielding is the result • Various computer programs are available, giving shielding in thickness of various materials IAEA 12. 1 : Shielding and X-ray room design 4

Shielding Calculation - Principle • We need, at each calculation point, the dose per week per m. A-min, modified for U and T, and corrected for distance • The required attenuation is simply the ratio of the design dose to the actual dose • Tables or calculations can be used to estimate the shielding required IAEA 12. 1 : Shielding and X-ray room design 5

Shielding Calculation - Detail Dose per week - primary • Data being used for NCRP 147 suggests that for : • 100 k. Vp, dose/unit workload = 4. 72 m. Gy/m. Amin @ 1 meter • 125 k. Vp, dose/unit workload = 7. 17 m. Gy/m. Amin @ 1 meter IAEA 12. 1 : Shielding and X-ray room design 6

Shielding Calculation - Detail • Thus if the workload were 500 m. A-min/week @ 100 k. Vp, the primary dose would be : 500 x 4. 72 m. Gy/week @ 1 meter = 2360 m. Gy/ week IAEA 12. 1 : Shielding and X-ray room design 7

Sample Shielding Calculation • Using a typical x-ray room, we will calculate the total dose per week at one point Office Calculation Point 2. 5 m IAEA 12. 1 : Shielding and X-ray room design 8

Shielding Calculation - Primary If U = 0. 25, and T = 1 (an office) and the distance from the x-ray tube is 2. 5 m, then the actual primary dose per week is : (2360 x 0. 25 x 1)/2. 52 = 94. 4 m. Gy/week IAEA 12. 1 : Shielding and X-ray room design 9

Shielding Calculation - Scatter • Scatter can be assumed to be a certain fraction of the primary dose at the patient • We can use the primary dose from the previous calculation, but must modify it to the shorter distance from the tube to the patient (FSD, usually about 80 cm) • The “scatter fraction” depends on scattering angle and k. Vp, but is a maximum of about 0. 0025 (125 k. Vp @ 135 degrees) IAEA 12. 1 : Shielding and X-ray room design 10

Shielding Calculation - Scatter • Scatter also depends on the field size is simply related to a “standard” field size of 400 cm 2 - we will use 1000 cm 2 for our field • Thus the worst case scatter dose (modified only for distance and T) is : (2360 x 1 x 0. 0025 x 1000) ----------------= 3. 7 m. Gy (400 x 2. 52 x 0. 82) IAEA 12. 1 : Shielding and X-ray room design 11

Shielding Calculation - Leakage • Leakage can be assumed to be at the maximum allowable (1 m. Gy. hr-1 @ 1 meter) • We need to know how many hours per week the tube is used • This can be taken from the workload W, and the maximum continuous tube current • Leakage is also modified for T and distance IAEA 12. 1 : Shielding and X-ray room design 12

Shielding Calculation - Leakage • For example: if W = 300 m. A-min per week and the maximum continuous current is 2 m. A, the “tube on” time for leakage calculation = 300/(2 x 60) hours = 2. 5 hours • Thus the leakage IAEA = 2. 5 x 1 x 0. 25 / 2. 52 m. Gy = 0. 10 m. Gy 12. 1 : Shielding and X-ray room design 13

Shielding Calculation - Total Dose • Therefore the total dose at our calculation point: = (94. 4 + 3. 7 + 0. 1) = 99. 2 m. Gy / week • If the design dose = 0. 01 m. Gy / week then the required attenuation = 0. 01/99. 2 = 0. 0001 IAEA 12. 1 : Shielding and X-ray room design 14

Shielding Calculation - Lead Required • From tables or graphs of lead shielding, we can find that the necessary amount of lead is 2. 5 mm • There are tables or calculation formula for lead, concrete and steel at least • The process must now be repeated for every other calculation point and barrier IAEA 12. 1 : Shielding and X-ray room design 15

Shielding Calculation Reduction factor 105 50 75 k. V 100 150 200 k. V 250 104 300 k. V 103 102 10 IAEA Lead Required 1 2 3 4 5 6 7 12. 1 : Shielding and X-ray room design 8 mm 16

Radiation Shielding Parameters IAEA 12. 1 : Shielding and X-ray room design 17

Room Shielding - Multiple X-Ray Tubes • Some rooms will be fitted with more than one x-ray tube (maybe a ceiling-mounted tube, and a floor-mounted tube) • Shielding calculations MUST consider the TOTAL radiation dose from all tubes IAEA 12. 1 : Shielding and X-ray room design 18

CT room design • General criteria: • Large room with enough space for: • CT scanner • Auxiliary devices (contrast media injector, emergency bed and equipment, disposable material containers, etc) • 2 dressing-rooms • Other spaces required: • Console room with large window large enough to see the patient • • all the time Patient preparation room Patient waiting area Report room (with secondary imaging workstation) Film printer or laser film printer area IAEA 12. 1 : Shielding and X-ray room design 19

Room shielding • Workload • Protective barriers • Protective clothing 2. 5 Gy/1000 m. As-scan Typical scatter dose distribution around a CT scanner IAEA 12. 1 : Shielding and X-ray room design 20

Protective barriers • Workload (W): The weekly workload is usually expressed in milliampere minutes. • The workload for a CT is usually very high • Example: 6 working day/week, 40 patients/day, 40 slices/patient, 200 m. As/slice, 120 k. V W= 6. 40. 200 60 = 32000 m. Amin/week • Primary beam is fully intercepted by the detector assembly. Barriers are interested only by scattered radiation IAEA 12. 1 : Shielding and X-ray room design 21

Computation of secondary protective barriers Ku. X = Scattered radiation Typical maximum scatter radiation around a CT : Sct= 2. 5 Gy/m. Amin-Scan @ 1 meter and 120 k. V. This quantity may be adopted for the calculation of protective barriers The thickness S is otained from the attenuation curve for the appropriate attenuation material assuming scattered photons with the same penetrating capability of those of useful beam dsec P (dsec )2 WSct T Secondary barrier Example: 120 k. V; P = 0. 04 m. Sv/week, dsec= 3 m, W= 32000 m. Amin/week, T= 1 Requires 1. 2 mm of lead or 130 mm of concrete IAEA 12. 1 : Shielding and X-ray room design 22

Where to Get More Information • National Council on Radiation Protection and Measurements “Structural Shielding Design for Medical X Rays Imaging Facilities” 2004 (NCRP 147) IAEA 12. 1 : Shielding and X-ray room design 23