Radiation Protection in Mammography: Recommended Safety Procedures for the Use of Mammographic X-Ray Equipment - Safety Code 33

5. Equipment specifications

5.1 Newly purchased medical X-ray equipment

All new, used, and refurbished mammographic X-ray equipment, and accessories for such equipment, which is sold, imported or distributed in Canada, must conform to the requirements of the Radiation Emitting Devices Act and the Food and Drugs Act and their promulgated regulations. These are the Radiation Emitting Devices Regulations and the Medical Devices Regulations. The Radiation Emitting Devices Regulations specify standards of design, construc- tion and performance, with respect to radiation safety. The Medical Devices Regulations encompass all other safety considerations and the question of efficacy for all medical X-ray equipment sold in Canada. It is the responsibility of the manufacturer or distributor to ensure that the equipment conforms to the requirements of these regulations.

The Radiation Emitting Devices Regulations in effect and relevant for mammographic X-ray equipment, at the time of printing of this Safety Code, are reproduced in Appendix VII. These regulations may be amended, from time-to-time, to keep up-to-date with changing technology in the field. Information on the applicability and currency of the Radiation Emitting Devices Regulations may be obtained by contacting the Radiation Protection Bureau, Health Canada, Ottawa, Ontario, K1A 1C1.

In addition, mammographic X-ray equipment must meet any applicable requirements under provincial jurisdictions for such equipment.

5.2 Existing medical X-ray equipment

When possible, existing mammographic X-ray equipment must be upgraded to incorporate as many as possible of the safety and performance features required of new mammographic X-ray equipment. It should be noted that it is a requirement of the Radiation Emitting Devices Regulations that replacements for any component or subassembly of an X-ray machine, for which a design, construction or performance standard has been specified in the Regulations applicable to the class of X-ray equipment, must comply with the standards in effect at the time of replacement.

Only equipment designed specifically for mammography shall be used, and to ensure maximum protection for patients and staff, all existing mammographic X-ray equipment must at least meet certain basic requirements. These requirements are itemized in the remainder of this Section.

5.3 General requirements for mammographic X-ray equipment

The following requirements must be met by all mammographic X-ray equipment. Specific requirements for film-screen mammographic X-ray equipment are listed in Section 5.3.1, whereas requirements for Xeromammographic X-ray equipment are listed in Section 5.3.2.

1. Warning Signs - The X-ray control panel must bear a permanent and conspicuous sign warning that hazardous X-radiation is emitted when the equipment is in operation and prohibit unauthorized use.
   
   
2. Markings - All controls, meters, lights and other indicators relevant to the operation of the equipment must be readily discernible and clearly labelled or marked as to function.
   
   
3. Focal Spot Marking - The location of the focal spot must be clearly and accurately marked on the X-ray tube housing. In the case of dual focus X-ray tubes, the location of the mark should be midway between the centres of the two focal spots.
   
   
4. Indicator lights - There must be readily discernible, separate indicators on the control panel that indicate:
   1. when the control panel is energized and the machine is ready to produce X-rays, and
   2. when X-rays are being produced.
      
      
5. Indication of Loading Factors - For mammographic X-ray equipment having adjustable loading factors, the control panel must incorporate electrical meters or other indicators that enable determination of the X-ray tube voltage, X-ray tube current and time, or combinations of these. For equipment having nonadjustable loading factors, permanent marks or labels may be used to indicate these parameters. The loading factors should be displayed after the irradiation is complete.
   
   If the mammographic X-ray equipment is battery-powered,
   there must be a visual indicator that shows whether the battery is
   adequately charged for proper operation.
   
   
6. Irradiation Control - There must be an irradiation switch, or other device to initiate and terminate X-ray production. The irradiation switch must require continuous pressure by the operator to produce X-rays.
   
   
7. Timer - A timing device must be provided to automatically terminate the irradiation. The timer must be designed and constructed in such a way that it is not possible to energize the X-ray tube without automatic or manual resetting of the timer after each loading, that an irradiation cannot be initiated with the timer set at its zero or OFF position and the production of X-rays is automatically terminated after a preset time, preset milliampere-second value, or a preset exposure or air kerma value. The timing device must be accurate to 1/60 second or to 7 percent, whichever is greater.
   
   A backup timer must be provided to limit the product of the X-ray tube current and the irradiation time during an irradiation. The product of the X-ray tube current and the irradiation time must not exceed 2,000 milliampere-seconds per irradiation. When possible, the product of the X-ray tube current and the irradiation time should be set not to exceed 1,200 milliampere-seconds per irradiation.
   
   
8. X-ray Tube Shielding - The X-ray tube must be enclosed in a shielded housing. The shielding of the housing must be such that, at every rating specified by the manufacturer, the leakage radiation does not exceed 17.5 µGy (2 mR) per hour at 5 cmfrom any point on the external surface of the housing.
   
   
9. Beam Limiting Devices - Suitable beam limiting devices capable of restricting the radiation beam shall be provided and shall provide the same attenuation as the X-ray tube housing. The beam limiting device should be designed in such a way that, for any focal spot to image receptor distance, the radiation beam does not extend beyond the edge of the image receptor except at the edge adjacent to the chestwall where the X-ray field shall not extend beyond the edge by more than 2 percent of the focal spot to image receptor distance.
   
   If a light localizer designed to define the outline of the X-ray field is included, the misalignment, in the plane of the image receptor, of the light field with respect of the X-ray field along either length or width must not exceed 2 percent of the source to image receptor distance (SID).
   
   
10. Image Receptor Support Shielding - The image receptor support shall transmit less than 0.87 µGy (0.1 mR) per irradiation at all operating loading factors at the minimum source to image receptor distance.
    
    
11. Breast Compression Device - A device to maintain firm breast compression shall be provided on the mammographic X-ray equipment. This device must provide adjustable, uniform and constant compression of the breast during mammography. The X-ray beam attenuation of the compression plate should be less than that of 2.5 mm of polymethylmethacrylate (PMMA) equivalent. There is no known optimal value of compression. However, a minimum of 20 kg should be generated but compression forces of 25 kg or greater should be achievable only with manual control.
    
    The breast compression device should not produce inhomogeneities or artifacts which can degrade image quality.
    
    The deformation of the breast compression device at maximal compression force should not be greater than 15 mm at each corner between the surface of the image receptor system and the breast compression device when a piece of foam rubber is compressed.
    
    
12. Protective Barrier - A protective radiation barrier shall be provided. This barrier must allow the technologist to observe the patient during the entire procedure and should provide attenuation equal to or greater than 0.25 mm Pb equivalent at 50 kVp. The barrier should be at least 0.6mwide and 1.85mhigh and reach within 0.15 m above the floor.
    
    
13. Mechanical Stability - The X-ray tube must be securely fixed and correctly aligned within the tube housing. The X-ray tube housing must maintain its required position without drift or vibration during operation and must be balanced to provide smooth operation.
    
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5.3.1 Requirements for film-screen mammographic X-ray equipment

1. Target Material - Molybdenum (Mo) or Molybdenum-Tungsten (Mo-W) alloy target X-ray tubes shall be used for film-screen mammographic X-ray equipment. Alternate target material may be appropriate when used with alternate filter material provided that it produces comparable image quality at equal or reduced dose to the breast.
   
   
2. Focal Spot Size - The focal spots must be small enough not to create excessive geometric unsharpness. The focal spots should be measured using either a slit camera, the pinhole method, or other method where the nominal focal spot size can be determined. For film-screen mammographic X-ray equipment, the nominal focal spot size should be:
   
   equal to or less than 0.40 mm for contact or grid techniques
   at 65 cm SID,
   equal to or less than 0.30 mm for contact or grid techniques
   at 50 cm SID,
   equal to or less than 0.15 mm for 1.5 magnification,
   equal to or less than 0.10 mm for 2.0 magnification
   
   
3. X-ray Beam Filtration - A permanent filter of about 0.025 to 0.030 mm Mo shall be permanently installed. For magnification, a Tungsten target microfocal-spot X-ray tube may be used and this tube shall have at least 0.5 mm Al equivalent total filtration. Alternate filter material may be appropriate provided that it produces comparable image quality at equal or reduced dose to the breast.
   
   
4. Radiation Beam Quality -There must be radiation-absorbing filters that provide a degree of attenuation such that the first Half-Value Layer of aluminum is not less or greater than the values shown in Table 1 for a selected X-ray tube voltage. For other X-ray tube voltages, the Half-Value Layer of the radiation beam must be calculated by linear interpolation from that Table. Half-Value Layer measurements must include the attenuation of the breast compression device if the device is of uniform thickness and without holes.
   
   

   Table 1:Acceptable Half-Value Layer forMo or Mo-W alloy target X-ray tube

   X-ray Tube Voltage (kilovolt)	First Half-Value Layers (minimum - maximum) (millimetre of Al)
   24	0.24 - 0.34
   26	0.26 - 0.36
   28	0.28 - 0.38
   30	0.30 - 0.40
   35	0.35 - 0.45

    

5. X-ray Tube Voltage - The X-ray tube voltage should be adjustable in 1 kVp increments. The lowest selectable X-ray tube voltage shall be equal to or less than 24 kVp. The peak X-ray tube voltage should correspond to within 5 percent of the selected or indicated value. The X-ray tube voltage repro- ducibility should be within 2 percent.
   
   
6. X-ray Tube Radiation Output - The X-ray tube radiation output shall be high enough to minimize irradiation time to eliminate perceptible motion artifacts and reduce the dose to the patient resulting from reciprocity law failure of film-screen combination. The X-ray tube output should be at least 4.4 mGy/s (500 mR/s) at 28 kVp.
   
   For any combination of operating loading parameters, the coefficient of variation of any ten consecutive radiation exposure measurements, taken at the same source to detector distance within a time period of one hour, is no greater than 0.05, and each of the ten radiation exposure measurements is within 15 percent of the mean value of the ten measurements.
   
   
7. Automatic Exposure Control - An Automatic Exposure Control (A.E.C.) system must be provided. The system should be able to maintain a net film density of ±0.15 O.D. (Optical Density) units within a range of 1.0 to 1.5 O.D. for a film with an average gradient of 3.0 O.D., for the range of breast thicknesses examined, and for all types of technique (non-grid, grid, and magnification) and loading factors used by the facility. In addition, a film density control for the A.E.C. should be provided with each increment increasing or decreasing film-screen cassette dose by 20 percent.
   
   When manual irradiation control is used, the selectable control interval (time or mAs) shall be small enough to permit increments smaller than 25 percent.
   
   
8. Breast Support Table - The attenuation of the breast support table should not exceed 0.3 mm Al equivalent at 30 kVp.
   
   
9. Anti-scatter Grid and Bucky System - If an anti-scatter grid or Bucky system is used, it must be designed for mammographic purposes and must not produce inhomogeneities or artifacts which can degrade image quality. Grid lines should not be visible on mammograms.
   
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5.3.2 Requirements for Xeromammographic X-ray equipment

1. Target Material - For Xeromammographic X-ray equipment, a Tungsten (W) or Molybdenum-Tungsten (Mo-W) alloy target X-ray tube with Aluminum (Al) filtration shall be used. Alternate target material may be appropriate when used with alternate filter material provided that it produces comparable image quality at equal or reduced dose to the breast.
   
   
2. Focal Spot Size - The focal spots must be small enough not to create excessive geometric unsharpness. The focal spots should be measured using either a slit camera, the pinhole method, or other method where the nominal focal spot size can be determined. For Xeromammographic X-ray equipment, the nominal focal spot size should be:
   
   equal to or less than 0.6 mm for contact techniques at 80 cm SID, equal to or less than 0.5 mm for contact techniques at 65 cm SID, equal to or less than 0.20 mm for 1.5 × magnification, equal to or less than 0.15 mm for 2.0 × magnification.
   
   
3. X-ray Beam Filtration - For Tungsten target X-ray tube, the total filtration shall not be less than 2.0 mm Al equivalent at 50 kVp, and for Molybdenum-Tungsten target X-ray tube, the total filtra-tion shall not be less than 1.6 mm Al equivalent at 50 kVp. Alternate target material may be appropriate provided that it produces comparable image quality at equal or reduced dose to the breast.
   
   
4. Radiation Beam Quality - There must be radiation attenuating filters that provide a degree of attenuation such that the first Half-Value Layer of aluminum is not less or greater than the values shown in Table 2 for a selected X-ray tube voltage. For other X-ray tube voltages, the Half-Value Layer of the radiation beam must be calculated by linear interpolation from that Table. Half-Value Layer measurements must include the attenuation of the breast compression device.

    

   Table 2: Acceptable Half-Value Layer for W or Mo-W alloy target with Al filter

   X-ray Tube Voltage (kilovolt)	First Half-Value Layers (minimum - maximum) (millimetre of Al)
   Tungsten target
   40	0.40 - 1.50
   49	0.50 - 1.60
   Molybdenum-Tungsten target
   40	0.40 - 0.90
   49	0.50 - 1.00

    

5. X-ray Tube Voltage - The X-ray tube voltage should be adjustable in 2 kVp increments. The lowest selectable X-ray tube voltage shall be less than or equal to 40 kVp. The peak X-ray tube voltage should correspond to within 5 percent of the selected or indicated value. The X-ray tube voltage reproducibility should be within 2 percent.
   
   
6. X-ray Tube Radiation Output - The X-ray tube radiation output shall be high enough to minimize irradiation time to eliminate perceptible motion artifacts.
   
   For any combination of operating loading parameters, the coefficient of variation of any ten consecutive radiation exposure measurements, taken at the same source to detector distance within a time period of one hour, is no greater than 0.05, and each of the ten radiation exposure measurements is within 15 percent of the mean value of the ten measurements.
   
   
7. Automatic Exposure Control - An Automatic Exposure Control (A.E.C.) system should be provided. The system should be able to maintain the dose to the cassette to within 15 percent of the nominal value for the range of breast thicknesses examined and for all types of technique and loading factors used by the facility. In addition, a density control should be provided with each increment increasing or decreasing cassette dose by 25 percent.
   
   When manual irradiation control is used, the selectable control interval (time or mAs) shall be small enough to permit increments of 25 percent.