Shielding Design Guidance
Many of the following guidance items are from the Ionizing Radiation Rules Governing the Use of Radiation Machines, reports of the National Council on Radiation Protection and Measurements (NCRP) or from the Suggested State Regulations for the Control of Radiation (SSRCR) published by the Conference of Radiation Control Program Directors (CRCPD):
- Rule 36(4) of the Rules requires protective barriers to be designed with a weekly radiation shielding design goal of 0.1 mSv for persons in controlled areas and 0.02 mSv for persons in uncontrolled areas.
- The distance to the occupied area of interest should be from the source to the nearest likely approach of the sensitive organs of a person. For a wall, this may be assumed to be not less than 0.3 m. For a source located above potentially occupied spaces, the distance can be assumed to be not more than 1.7 m above the lower floor. For ceiling transmission, a distance of at least 0.5 m above the floor of the room above is reasonable. (NCRP Report 147, Section 4.1.2)
- Operator barriers should be designed to be primary protective barriers, pursuant to the Rules, and to provide protection equivalent to at least 1/16-inch lead.
- Medical, dental and veterinary radiation facilities shielded in accordance with recommendations contained in NCRP Reports No. 145, No. 147, and No. 148 (NCRP, 2003a; 2004a; 2004b) (i.e., designed to not exceed an effective dose of 1 mSv per year to the maximally exposed individual in an uncontrolled area) will, according to the NCRP in Statement 10 of December 2004, provide adequate protection to employees and members of the public that access the uncontrolled areas. An effective dose for uncontrolled areas that does not exceed 1 mSv per year is justified, according to the NCRP in Statement 10, by the conservatively safe assumptions used in the recommended shielding design methodology.
- Based on ICRP (1991), NCRP (1993) and Rule 60(1) recommendations for the annual limit of effective dose to a member of the general public, shielding designs shall limit exposure of all individuals in uncontrolled areas to an effective dose that does not exceed 1 mSv per year. After a review of the application of the guidance in NCRP (1993) to medical radiation facilities, NCRP has concluded that a suitable source control for shielding individuals in uncontrolled areas in or near medical radiation facilities is an effective dose of 1 mSv in any year [see Section 1.4.2 of NCRP Report # 147]. According to the NCRP in Report 147, this recommendation can be achieved for the medical radiation facilities covered in and designed to meet Reports No. 145, No. 147, and No. 148 with a weekly shielding design goal of 0.02 mGy air kerma (i.e., an annual air-kerma value of 1 mGy) for uncontrolled areas.
- Vertical barriers must extend from the floor to a minimum height of 7 feet. However, if it is necessary to add additional shielding to the ceiling of the room, then the shielding in the walls must be extended above the 7-foot height to overlap the ceiling shielding. Also, if there is a multi-story building in close proximity of the x-ray room, and if that building has occupied space that could be exposed to scatter or secondary radiation that is not attenuated by the 7-foot-high wall shielding, then additional shielding may be necessary to protect that occupied space.
- Required primary protective barriers in a general purpose radiographic room floor or wall:
- The primary protective barrier must intercept the primary beam plus an additional area extending at least 30 centimeters (1 foot) beyond, as required by Rule 331(3) of the Rules.
- For general purpose radiographic x-ray equipment, preshielding by the patient, image receptor, or structures supporting the image receptor should not normally be factored into primary beam shielding recommendations. This especially applies in cases where either adjustable beam collimation or a non-fixed alignment with the image receptor assembly can permit a part of the primary beam to miss the patient and image receptor.
- Outside walls and, particularly, windows may require shielding to protect distant areas even if the nearest occupied area is not immediately adjacent.
- According to NCRP 147, Section 4.1.3, care must be taken when assigning a low occupancy factor to an uncontrolled area such as a corridor immediately adjacent to an x-ray room. The actual limitation for shielding design may be a more distant, fully occupied area, such as an office across the corridor. One needs to therefore also take a larger view of the facility in arriving at the appropriate limitations for shielding design, according to NCRP advice.
- The edge of the observation window should be at least 18 inches from the edge of the control partition or at least one square foot of the window should be centered no closer than 2 feet from the open edge of the partition.
- The control booth for a radiographic room shall have a window or viewing device that allows the operator to view the patient during all x-ray exposures performed in the room. The operator must be able to view the wall bucky and x-ray table, as well as patients confined to stretchers. When an observation window is used, the window and frame shall provide the necessary attenuation required to reduce the air kerma to the shielding design goal. According to NCRP 147, Section 2.2.1, the window(s) should be at least 45 × 45 cm (about 18" x18") and centered 1.5 m above the finished floor.
- Since shielding or design corrections or additions after facilities are completed can be expensive, it is important that structural shielding be properly designed and installed in the original construction process, per NCRP Report No. 147, Section 1.5. It is also advisable, according to the NCRP, that the planning include consideration of possible future needs of new equipment, higher radiation energies, and increased workloads.
- The final drawings and specifications should be submitted to and then reviewed and approved by the Radiation Safety Section before construction is begun.
- Rule 36(5) requires facilities to maintain for inspection by the department a scale drawing of the room where a stationary radiation machine is located. The drawing or accompanying attachments shall indicate the use of areas adjacent to the room and include an estimate of the occupancy in each room. In addition, the drawing or attachment shall include at least 1 of the following:
- The type and thickness of materials, or lead equivalency, of each protective barrier.
- The results of a survey for radiation protection at the operator's position and at pertinent points outside the room under specified test conditions. A copy of the approved shielding plan.
- The cost of increasing shielding beyond the minimum value often represents only a small increase in cost during the initial design and construction stage and may actually save the facility considerable expense in the long run by reducing or eliminating the potential need for future shielding evaluations and upgrades.
- It may not be sufficient merely to provide shielding between the source and the person to be protected. Additional shielding may be required to prevent radiation from scattering under vertical barriers from crawl spaces or unoccupied basements, as shown in Figure 1.
Figure 1: Typical unsatisfactory shielding arrangement. The lead barrier between the operator and the source does not necessarily provide adequate shielding against scattered radiation. Satisfactory protection would be achieved in the situation shown here if lead shielding were provided on the floor to keep the scattered radiation from the basement from reaching the occupied area.
Design Recommendations for an Operator's Booth (SSRCR)
- Viewing System Requirements
- Each booth shall have at least one viewing device which will:
- Be so placed that the operator can view the patient during any exposure; and
- Be so placed that the operator can have full view of any occupant of the room and should be so placed that the operator can view any entry into the room. If any door which allows access to the room cannot be seen from the booth, then outside that door there shall be an "x-ray on" warning sign that will be lighted anytime the rotor of the x-ray tube is activated. Alternatively, an interlock shall be present such that exposures are prevented unless the door is closed.
- When the viewing system is a window, the following requirements also apply:
- The window shall have a viewing area of at least 0.09 m2 (1 square foot);
- Regardless of size or shape, at least 0.09 m2 (1 square foot) of the window area must be centered no less than 0.6 m (2 feet) from the open edge of the booth and no less than 1.5 m (5.0 feet) from the floor; (Note: LARA believes that the SSRCR meant to say that at least 1 square foot of the viewing window must be centered horizontally no less than 0.6 m (2 feet) from the open edge of the booth and centered vertically 1.5 m (5 feet) above the floor.)
- The window shall have at least the same lead equivalence as that required in the booth's wall in which it is mounted.
- When the viewing system is by electronic means:
- The camera shall be so located as to accomplish the general requirements listed above; and
- There shall be an alternate viewing system as a backup for the primary system.
MIOSHA-RSS-852; Application for a Radiation Shielding Plan Review