PERSONNEL PROTECTION Unit Objectives: Describe the factors available to occupationally exposed individuals to reduce radiation exposure Differentiate between Primary and Secondary barriers Identify sources of radiation dose to the Radiographer Explain the Inverse Square Law Define the terms controlled and uncontrolled areas in radiologic facilities. Means that it is the responsibility of all occupationally exposed personnel to keep their exposure well below the maximum limits discussed previously. This is possible without being irrationally cautious. 1
Scatter radiation is the largest source of exposure to the diagnostic radiographer. The largest source of scatter radiation is the PATIENT (from Compton radiation). Other sources of scatter radiation include the x ray table, upright stand and any walls from which scatter can ricochet. Photons that have scattered 90 degrees from the scattering object are only 1/1000 as intense as the primary beam 1 meter (approx. 3 feet) from the scattering object. 2
Therefore, distance and shielding (such as lead aprons) are your friends. Use them as needed to minimize scatter dose to you. This means on portables, too. Not only does collimation reduce the area of exposure to the patient, thereby reducing patient dose, but scatter PRODUCTION decreases as well, which reduces occupational dose. Although intended primarily to reduce patient skin dose, the removal of low energy photons from the primary beam also reduces the amount of scatter production that can take place. This reduces occupational dose. 3
Can continue to work, but are urged to inform their employer, receive safety counseling and wear a second dosimeter at the waist (under the lead apron when worn). Monthly dose is not to exceed.05 rem/month (or 0.5 msv/month). In addition to lead aprons worn by personnel, walls and other physical barriers are utilized to protect radiation workers. Two types of barriers are identified: primary and secondary barriers. These are designed to shield personnel and the public from the primary beam. In other words, the wall is positioned PERPENDICULAR TO THE PRIMARY BEAM. 4
Secondary barriers are devices used to shield personnel from scattered radiation and LEAKAGE RADIATION. In general, secondary barriers are positioned PARALLEL TO THE PRIMARY BEAM. In general, control booths employ secondary barriers. By design, x ray photons must scatter twice before entering the control booth (NCRP #105, Shielded Booths ). 5
Unless you are certain that the shielding surrounding the control booth is a primary barrier, make sure that the x ray tube does not point in the direction of the control booth. Determination of how much leadequivalent shielding each type of barrier should have is based on a number of variables such as the distance that the x ray tube is from a barrier, the average kvp used, and how often the room is used. Final determination of barrier thickness is based on whether the barrier is used in a controlled area or noncontrolled area. 6
FACTORS W = WORKLOAD IN ma MIN/WEEK U = % TIME THAT BEAM FACES BARRIER = USE FACTOR T = % TIME ROOM IS USED =OCCUPANCY FACTOR D = DISTANCE TUBE IS FROM BARRIER The space occupied by personnel which includes the x ray control booth. Regulations require that the exposure (dose rate) to that area not exceed 100 mrem/week or 5 rem per year. Patient waiting areas, front desk and offices are classified as noncontrol areas. The exposure (dose rate) to those areas are not allowed to exceed 2 mrem/week or 0.1 rem per year. 7
The housing should be able to limit the amount of LEAKAGE RADIATION from which the technologist can be exposed. Leakage is radiation emanating from the x ray tube in directions other than the primary beam. Regulations stipulate that leakage radiation must not exceed 100 mr/hour at 1 meter from the source when operating at maximum potential and current. 8
ACCESSORIES APRONS AND GLOVES BUCKY SLOT COVER PROTECTIVE CURTAIN FLUOROSCOPIC EXPOSURE MONITOR Lead aprons must be worn when the radiographer cannot stand behind a lead barrier. It should have a minimum lead content equivalent to 0.5 mm. Lead gloves are worn if hands may be placed in the fluoroscopic beam and require the same equivalent lead content. Those who do extensive fluoroscopy can use a thyroid shield consisting of 0.5 mm lead equivalent shielding. Also available are leaded glass eyewear which provide 0.35 mm lead equivalent protection. 9
Tableside fluoroscopy can prove hazardous if a bucky slot cover is not available. This prevents scatter radiation from exposing the fluoroscopist s gonadal area. The fluoro carriage is also equipped with a drape to block scatter from the patient. A device which can be worn at the waist or neck (outside of the apron) which assists the user to minimize exposure. It is not a protective device as it does not shield. Its effectiveness depends on the user. Therefore, distance and shielding (such as lead aprons) are your friends. Use them as needed to minimize scatter dose to you. This means on portables, too. 10
When assisting during tableside fluoroscopy, try to avoid standing at the headend or foot end of the table. Radiation dose is highest there. STATIONARY FLUOROSCOPY C ARM FLUOROSCOPY 11
Never, ever stand in the direct path of the x ray beam for any reason. If holding a patient for an exposure becomes unavoidable, do so without being in the path of the primary beam. It is preferred that nonoccupationally exposed personnel be utilized. OTHER PROTECTIVE MEASURES EXPOSURE SWITCHES PORTABLE EXPOSURE CORDS USE OF INVERSE SQUARE LAW Whether portable or stationary equipment is used, exposure switches are of the dead man type. On stationary units. The switch will be located in such a way that the operator cannot peer out from behind the barrier during an exposure. 12
The exposure cord is must be at least 6 feet long in order for the operator to stand at least that far away from the patient and x ray tube during the exposure. After shielding and distance considerations have been taken into account, it may be better to stand 90 degrees from the patient in order to reduce occupational dose. PORTABLE RADIOGRAPHY 13
A practical application of the inverse square law is how far to stand from an x ray source during a procedure. If you move from an initial position of 3 feet to 6 feet from a radiation source, your exposure should diminish by a factor of 4. Calculate the barrier thickness given the information that will be supplied on TEST #3. There will be 4 questions valued at 2 points per correct answer. You must fill in the entire table and base your responses to the calculations you make. You may use a calculator. The process uses the half value layer concept and can incorporate different shielding materials such as lead and concrete. In practice, combinations of materials are included in the design of barriers in order to economize on cost and weight. 14
Example: Assume that a primary barrier is to be designed to protect occupants in an adjoining office. The occupants are nonradiation workers. What do we need to know? BARRIER THICKNESS CALCULATION ENERGY OF THE BEAM WORKLOAD FACTOR USE FACTOR OCCUPANCY FACTOR DISTANCE BARRIER IS FROM RADIATION SOURCE W U d T = d 2 mrem / week 15
A lookup table will show the thickness of shielding material that will cut radiation dose in half based on the energy of the radiation. For example, if the highest energy used is 125 kvp, how much lead is required to cut radiation dose by ½? How much concrete? PEAK KILOVOLTAGE LEAD (mm) CONCRETE (in) 50 0.05 0.17 70 0.15 0.33 100 0.24 0.60 125 0.27 0.80 150 0.29 0.88 PEAK KILOVOLTAGE LEAD (mm) CONCRETE (in) 50 0.05 0.17 70 0.15 0.33 100 0.24 0.60 125 0.27 0.80 150 0.29 0.88 16
Create a table that shows the mrem value with each increment of barrier thickness in lead and concrete. How many half value layers are needed to shield non radiation workers in the adjoining room? What if the adjoining space was a controlled area? Assume a peak energy of 100 kv. HALF-VALUE DOSE RATE LEAD (mm) CONCRETE (in) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 HALF-VALUE DOSE RATE LEAD (mm) CONCRETE (in) 0 1876 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 17
HALF-VALUE DOSE RATE LEAD (mm) CONCRETE (in) 0 1876 0 0 1 938 0.24 0.60 2 3 4 5 6 7 8 9 10 11 12 13 14 15 HALF-VALUE DOSE RATE LEAD (mm) CONCRETE (in) 0 1876 0 0 1 938 0.24 0.60 2 469 0.48 1.20 3 4 5 6 7 8 9 10 11 12 13 14 15 HALF-VALUE DOSE RATE LEAD (mm) CONCRETE (in) 0 1876 0 0 1 938 0.24 0.60 2 469 0.48 1.20 3 234.5 0.72 1.80 4 5 6 7 8 9 10 11 12 13 14 15 18
HALF-VALUE DOSE RATE LEAD (mm) CONCRETE (in) 0 1876 0 0 1 938 0.24 0.60 2 469 0.48 1.20 3 234.5 0.72 1.80 4 117.25 0.96 2.40 5 58.63 1.20 3.00 6 29.31 1.44 3.60 7 14.66 1.68 4.20 8 7.32 1.92 4.80 9 3.66 2.16 5.40 10 1.83 2.40 6.00 11 0.92 2.64 6.60 12 0.46 2.88 7.20 13 0.23 3.12 7.80 14 0.11 3.36 8.40 15 0.06 3.60 9.00 HALF-VALUE DOSE RATE LEAD (mm) CONCRETE (in) 0 1876 0 0 1 938 0.24 0.60 2 469 0.48 1.20 3 234.5 0.72 1.80 4 117.25 0.96 2.40 5 58.63 1.20 3.00 6 29.31 1.44 3.60 7 14.66 1.68 4.20 8 7.32 1.92 4.80 9 3.66 2.16 5.40 10 1.83 2.40 6.00 11 0.92 2.64 6.60 12 0.46 2.88 7.20 13 0.23 3.12 7.80 14 0.11 3.36 8.40 15 0.06 3.60 9.00 HALF-VALUE DOSE RATE LEAD (mm) CONCRETE (in) 0 1876 0 0 1 938 0.24 0.60 2 469 0.48 1.20 3 234.5 0.72 1.80 4 117.25 0.96 2.40 5 58.63 1.20 3.00 6 29.31 1.44 3.60 7 14.66 168 1.68 420 4.20 8 7.32 1.92 4.80 9 3.66 2.16 5.40 10 1.83 2.40 6.00 11 0.92 2.64 6.60 12 0.46 2.88 7.20 13 0.23 3.12 7.80 14 0.11 3.36 8.40 15 0.06 3.60 9.00 19
Question: Is this an accurate way to calculate barrier thicknesses given the fact that we are dealing with a type of radiation that is polyenergetic? 20