Safety Code 35: Safety Procedures for the Installation, Use and Control of X-ray Equipment in Large Medical Radiological Facilities

Section B: Facility and Equipment Requirements

1.0 Facility Requirements

1.1 General Criteria

In the planning of any medical X-ray facility the main priority is to ensure that persons in the vicinity of the facility are not exposed to levels of radiations which surpass the current regulatory exposure limits. Appropriate steps must be taken to ensure adequate shielding is present to meet the following requirements:

1. the radiation levels in controlled areas that are occupied routinely by radiation workers must be such that no radiation worker is occupationally exposed to more than 20 mSv per year; and
2. the radiation levels in uncontrolled areas must be such that no person receives more than 1mSv per year.

Appendix I provides a detailed description of the regulatory dose limits. For medical X-ray imaging facilities, controlled areas are typically in the immediate areas where X-ray equipment is used such as the procedure room and X-ray control booths. The workers in these areas are primarily equipment operators such as radiologists and radiation technologists who are trained in the proper use of the equipment and in radiation protection. Uncontrolled areas are those occupied by individuals such as patients, visitors to the facility, and employees who do not work routinely with or around radiation sources (NCRP 2004).

In general, attention to the basic principles of distance, time and shielding are required to determine shielding needs.

1.2 Design and Plan of X-ray Facility

In the early stages of designing and planning a medical X-ray facility, three steps should be taken to ensure adequate shielding is in place to provide the necessary level of radiation protection:

1. preparation of facility plans;
2. considerations for room design and layout;
3. determination of parameters governing shielding requirements.

1.2.1 Preparation of Facility Plan

In order to determine the shielding requirements for an X-ray facility a floor plan must be prepared, clearly identifying the following components:

1. The dimensions and shape of the room where the X-ray equipment is operated and the physical orientation of the room (a mark indicating North).
2. The location where the X-ray equipment is planned to be placed and the range of movement of the X-ray tubes.
3. The location of the control booth, if applicable.
4. The location, use, occupancy level and accessibility of adjacent rooms, as well as rooms above and below the facility.
5. The designation of the adjacent rooms, whether to be designated as a controlled or uncontrolled area. Controlled areas, mainly occupied by radiation workers, are subject to the limit of 20 mSv per year, whereas uncontrolled areas, mainly occupied by non-radiation workers, are subject to the limit of 1 mSv per year. In uncontrolled areas, where radiation sensitive populations are present, such as paediatric wards, a constraint level of 0.30 mSv per year should be used.
6. The location where image processing is performed, i.e., location of darkrooms, film storage area, computer workstations.
7. The position of all windows, doors, louvers, etc., that may affect radiation protection requirements.
8. The planned and existing materials used to construct the walls, floor, ceiling, and the control booth, and their thicknesses including additional materials currently being used, or planned for use, as radiation shielding barriers.
9. The application of the protective barriers. Will the intervening shield between the X-ray tube and the occupied area act as a primary or as a secondary protective barrier, i.e., will the barrier be required to attenuate the direct X-ray beam or stray radiation only?

1.2.2 Considerations for Room Design and Layout

When designing the layout of the X-ray facility, the following general recommendations must be considered.

1. Radiology rooms, with stationary X-ray equipment, which can be accessed from public areas should be equipped with a self-closing door, and must be identified with warning signs incorporating the X-ray warning symbol and the words "Unauthorized Entry Prohibited". Acceptable forms of the X- ray warning symbol are given in Appendix VI, Radiation Emitting Devices Regulations for Diagnostic X-ray Equipment.
2. Mobile X-ray equipment used routinely in one location must be considered as a fixed installation and the shielding needs for the equipment and room must be determined accordingly.
3. The rooms containing the X-ray equipment should be designed to provide adequate working space to the equipment operator and to allow for ease of patient movement.
4. The X-ray equipment should be positioned in the room in such a way that, during an irradiation, no one can enter the room without the knowledge of the equipment operator.
5. The X-ray beam must always be directed toward adequately shielded areas. Particular attention must be paid to the adequacy of shielding for chest radiography using wall-mounted image receptors.
6. Whenever possible, the X-ray beam and scattered radiation must be absorbed as close as possible to the patient or scatterer.
7. A control booth must be provided for the protection of the operator, if applicable, for the type of equipment. The control booth, and the viewing window, must have shielding properties such that no operator is occupationally exposed to more than 0.4 mSv/week. The ALARA principle requires that additional shielding be specified in the design to further reduce operator exposure, wherever this can reasonably be done. Mobile protective screens must not be considered adequate as a control booth for radiological procedures.
8. The control booth should be located in an area, whenever possible, such that the radiation has to be scattered at least twice before entering the booth.
9. Shielding must be constructed to form an unbroken barrier and if lead is used, it should be adequately supported to prevent "creeping".

1.2.3 Determination of Parameters Governing Structural Shielding Requirements

The thickness of the shielding material, such as lead, concrete, or gypsum wallboard, required to reduce radiation levels to the recommended dose limits can be determined through calculations. In general, the radiation exposure to individuals depends primarily on the amount of radiation produced by the source, the distance between the exposed person and the source of the radiation, the amount of time that an individual spends in the irradiated area, and the amount of protective shielding between the individual and the radiation source.

The parameters listed below must be considered for the calculation of barrier thicknesses. Allowance should be made for possible future changes in anyone or all of these parameters, including increases in use and occupancy factors, in operating tube voltage and workload, as well as modifications in techniques that may require ancillary equipment.

• 1. The maximum X-ray Workload, (W) or the workload distribution.

The workload is a measure of the operational time or the amount of use of the X-ray equipment. A workload distribution indicates the workload across a range of operating voltages. The workload and workload spectrum can be determined by recording the operating voltage and current-time product of each irradiation taken in each X-ray suite over a set period of time (i.e., week). For irradiations made under Automatic Exposure Control, the operating voltage, procedure type and patient thickness should be recorded to be used later to estimate the current-time product. If actual workload values are not available, Table 5 presents estimated total workloads for various medical X-ray facilities (NCRP 2004).

Table 5: Typical Workloads (NCRP 2004)

	Total Workload per patient (mA min/patient)	Typical Number of Patients (per 40 hour week)		Total Workload per week (mA min/week)
		Average	Busy	Average	Busy
a. R and F is a room that contains equipment for both radiography and radioscopy (fluoroscopy).
Radiographic Room (chest)	0.6	120	160	75	100
Radiographic Room (other)	1.9	120	160	240	320
Dedicated Chest Room	0.22	200	400	50	100
R and Fa Room (radioscopic system)	13	20	30	260	400
R and Fa Room (radiographic system)	1.5	25	40	40	40
Angiography Room (cardiac)	160	20	30	3,200	4,800
Angiography Room (other vascular)	64	20	30	1,300	2,000

• 2. The occupancy factor (T)

The Occupancy factor is the fraction of time that the area under consideration is occupied by the individual (employee or public) who spends the most time at that location while the X-ray equipment is operating. The following table present recommended occupancy factors.

Table 6: Occupancy Factors

• 3. The use factor (U)

The use factor, is the fraction of the workload during which the X-ray beam is pointed in the direction under consideration. The following table present recommended occupancy factors.

Table 7: Use Factor for Primary Barrier

Primary Barrier
U=1	Floors of radiation rooms, walls containing a vertical image receptor; any other walls, doors or ceiling areas routinely exposed to the direct radiation beam.
U=1/4	Doors and wall areas of radiation rooms not routinely exposed to the direct radiation beam.
U= 1/16	Ceiling areas of radiation rooms not routinely exposed to the direct radiation beam.
Secondary barrier
U=1	The use factor for secondary protective barriers is always taken to be 1.

1.3 Shielding Calculations

Shielding calculations must be made for both primary and secondary protective barriers. Primary protective barriers provide shielding from the direct X-ray beam and therefore must be placed in such an orientation as to intersect the X-ray beam. Secondary protective barriers are required to provide shielding from scattered and leakage X-rays.

Comprehensive shielding calculations for large radiological facilities should only be performed by individuals with current knowledge of structural shielding design and the acceptable methods of performing these calculations. It is recommended that shielding calculations be performed using the methodology presented in the National Council on Radiation Protection and Measurements (NCRP) Report No. 147: Structural Shielding Design for Medical X-Ray Imaging Facilities (NCRP 2004). However, it must be noted that the shielding design goals specified in NCRP Report 147 are not adopted in this Safety Code. The shielding design goal values may be lower but must not exceed the limits set out in section B1.1 for controlled and uncontrolled areas. Due to the extensiveness of the information, the methodology of NCRP 147, including equations, tables and figures, is not provided in this Safety Code. Alternatively, the methodology presented in NCRP Report No. 49 (NCRP 1976) is also acceptable and presented in Appendix III.

Under the methodology used in NCRP Report 147, the following are assumptions made in the shielding calculation:

• The attenuation of the radiation beam by the patient is neglected.
• The incidence of the radiation beam is always perpendicular to the barrier being evaluated.
• The calculation does not take into account the presence of materials in the path of the radiation other than the specified shielding material.
• The leakage radiation from the X-ray equipment is assumed to be an air kerma of 0.876 mGy h^-1.
• The minimum distance to the occupied area from a shielded wall is assumed to be 0.3 m.

The information outlined in sections B1.1 and B1.2 along with the final plans of the installation must be submitted for reviewed by the appropriate responsible government agency. For installations under federal jurisdiction, the responsible agency is the Consumer and Clinical Radiation Protection Bureau, Health Canada, Ottawa, Ontario K1A 1C1. Radiological facilities that fall under provincial or territorial jurisdiction should contact the responsible agency in their respective province or territory listed in Appendix V.

1.3.1 Radiographic Films

Film storage containers must be adequately shielded to ensure that excessive exposure of film by X- rays does not occur. Sufficient film shielding must be in place to reduce the radiation level to stored film to less than 0.1 mGy over the storage period of the film. The values presented in Appendix IV are very conservative but will protect films from radiation exposure for most circumstances. Once films are loaded into cassettes, radiation exposure levels should be less than 0.5 µGy and the resulting increase in the base-plus-fog should be less than 0.05 O.D. Refer to Appendix IV for storage guides for radiographic film.

1.3.2 Radiographic X-Ray Equipment and Dedicated Chest Radiographic Equipment

Primary and secondary shielding must be provided for radiographic equipment where the tube can be manipulated in several directions. The walls and floor where the X-ray tube can be directed are considered primary barriers whereas the other walls and ceiling are secondary barriers. The primary barrier includes the wall behind the vertical image receptor, or "wall or chest bucky", and the floor under the radiographic table. For dedicated chest radiographic equipment, the wall behind the image receptor is considered a primary barrier.

The X-ray tube should never be directed towards the control booth. Therefore the walls of the control booth are calculated as secondary barriers. The information required for calculation of the shielding of radiographic X-ray equipment and dedicated chest radiographic equipment is found in Table AII.1 of Appendix II.

1.3.3 Radioscopic X-Ray Equipment and Angiographic X-Ray Equipment

The design of radioscopic X-ray equipment is such that only secondary shielding must be provided for these types of systems. However, in systems where an X-ray tube for radiography is also present, the shielding for this X-ray tube must be evaluated independently, as in Section B1.3.2. When equipment include more than one X-ray tube, such as in cardiac systems, the shielding calculation must take into account each X-ray tube independently. The information required for calculation of the shielding of radioscopic X-ray equipment is found in Table AII.2 of Appendix II.

1.3.4 Computed Tomography Equipment

The design of computer tomography equipment is such that only secondary shielding must be provided. The calculation of shielding for CT rooms should not rely on workload values as defined in Section B1.2.3 and therefore it is recommended that shielding requirements be calculated using the methodology of NCRP 147 for CT equipment. The information required for calculation of the shielding of CT equipment is found in Table AII.3 of Appendix II.