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

6. Image processing and handling

The irradiation necessary to produce a mammogram of satis-factory diagnostic quality with minimum radiation dose to the breast depends on the loading factors, film-screen employed, the handling and processing of the film, and on the conditions of viewing the image. It is essential that automated processing be used to develop mammographic films. It is also highly recommended that the image processor be dedicated for mammographic use only. Good image quality requires proper darkroom techniques, routine processor Quality Control monitoring, and careful adherence to film and processor manufacturers' instructions.

6.1 Film processing

Improper or careless processing of exposed mammographic films can result in poor diagnostic image quality and consequently increase the possibility of wrong diagnosis or the need for repeat mammographic X-ray examinations. To achieve full development, the film must be processed in chemically fresh developer, at the correct temperature and for sufficient time to ensure that the silver in exposed silver halide crystals in the film emulsion is completely reduced. If this is not done, the blackening of the film will not be optimum and the tendency will be to increase irradiation to achieve proper image density.

Other factors can also affect the quality of the processed film. These include cleanliness of the processing system, film immersion time, and the efficiency of the rinsing. The use of automatic film processing equipment will produce films of more uniform density with the possibility of lower patient dose than by using manual processing. Manual film processing shall not be used in mammography. To ensure proper processing of films certain basic procedures must be followed:

1. The only acceptable method to monitor the operation of an automated image processor is with the use of a sensitometer to produce repeatable light exposure of the film and with the use of a densitometer to monitor the processed sensitometric film. Processor monitoring must be done every morning when the processor is started and has stabilized, and at additional times after the processor has been cleaned, or a few hours after fresh chemical has been added to the replenisher tanks. The processor should be given sufficient time to stabilize, before mammographic procedures are performed on patients.
   
   
2. Manufacturers' recommendations with respect to strength of solution, temperature and time must be followed to ensure optimum development.
   
   
3. Developing solutions must be replenished as necessary and must be changed or recycled regularly, as required. This should be done often enough to avoid oxidation of the developing solutions. Even unused developer deteriorates with time. Processing chemicals must be protected from freezing. Manufacturers' recommendations should be followed in storing chemicals to avoid oxidation, any chemicals showing signs of oxidation or sedimentation must not be used.
   
   
4. Fixer must be adequately removed from processed films. Manufacturers' recommendations for film wash should be followed. Fixer retention tests should be done on a regular basis.
   
   
5. Cleanliness is extremely important for reducing film artifacts. The film transport mechanisms of film processors must be cleaned frequently. Abrasive cloths or cleaners should never be used on mammographic processors.
   
   
6. Film processors must be maintained regularly, in accordance with the manufacturers'instructions. The accuracy of the processor thermometer should be checked regularly with a nonmercury thermometer. A digital processor thermometer should be accurate to within 0.5° C.
   
   
7. When film processing volume is less than 50 films per day, it may not be possible to adequately control chemical concentrations. In this situation, it is preferable to use flood replenishment to better control chemical concentrations.
   
   
8. In some situations, it is possible to reduce patient dose by extending processing time. It is important to make sure that image quality is not reduced when using this processing method.

6.2 Darkroom

With the exception of daylight automatic image processors not requiring darkrooms, automatic film processors require properly designed darkrooms. While specific details may vary from installation to installation, all darkrooms must include certain basic features:

1. The room must be light-tight. Particular attention must be paid to the door seal and the mounting of the film processor if the film insertion to the processor is done through a wall. The darkroom should incorporate a lockable door, double doors or a blackened maze entrance to ensure light-tightness when undeveloped films are being handled. A film strip exposed to an optical density of 1.2 units must not show an increase in optical density greater than 0.05 units in two minutes exposure to the darkroom light environment.
   
   
2. If the darkroom is adjacent to a radiographic room, the film storage container must be adequately shielded to ensure that excessive exposure of film by X-rays does not occur. Film shielding is specified in Appendix III.
   
   
3. A warning light should be located outside the darkroom, at the entrance, to indicate when the room is in use. The warning light is not required if the door is locked when it is closed.
   
   
4. Safelights, fitted with bulbs of intensity not greater than 15watts, must be provided above thework areas inside the darkroom. The safelight must have filters appropriate to the specifications of the film used and must be positioned at distances greater than 1 metre from work areas to minimize film fogging.

6.2.1 Darkroom maintenance

Cleanliness in the darkroom and of the screens and cassettes is essential. It is important to maintain the cleanest environment possible in order to minimize any artifacts caused by dirt, dust, or improper handling of film. An ultraviolet light should be used to find dust areas around the darkroom. No one should eat, drink or smoke in the darkroom area. All working surfaces, tops of counters and the floor should be cleaned regularly, at least once a day. Tops to cabinets, vents, light fixtures and any other areas which can collect dust should also be cleaned on a regular basis. The ventilation system should be checked to make sure that no dust is carried from it to inside the darkroom; any filter should be changed on a regular basis. If possible, the darkroom should be under positive pressure so that chemical fumes and dust are not sucked into the room when the door is opened. The number of air changes should be high enough for the processor to operate properly and to not create a hazardous situation for personnel. Chemicals should not be mixed inside the darkroom since this operation can result in chemical splashes onto the equipment or working surfaces.

To avoid putting fingerprints on the film and to avoid dirtying the screens, it is important to wash the hands frequently with soap that does not leave any residue. Clutter which may collect dust should be eliminated. Corrugated cardboard boxes containing film boxes, chemicals, and other supplies should not be stored or opened inside the darkroom; opening corrugated cardboard boxes creates a lot of dust. The boxes should be opened outside the darkroom, and films and supplies carried inside. Any articles of clothing made of loose fibres or static-generating such as wool, silk, some cottons or cotton blend fabric should not beworn in the darkroom or should be covered with a laboratory coat.

6.3 Mammographic X-ray film

Mammographic X-ray films are sensitive to light, heat, humidity, chemical contamination, mechanical stress and X-radiation. Unexposed mammographic film must be stored in such manner that it is protected from stray radiation, chemical fumes and light. The level of optical density from the base material and film fog from all causes must not be greater than 0.2 units.

Storage should be provided so that no film receives more than 1.75 µGy (0.2 mR) of radiation before use. The amount of shielding required will depend on the storage time and on the workload of the facility. For the majority of facilities 1.0mmof lead shielding will be adequate.

Generally, mammographic X-ray films should be stored at temperatures in the range of 10°C to 21°C with humidity between 30 percent to 60 percent. It is advisable to follow film manufacturers' instructions. It is recommended that film be stored on edge in an area away from chemical fumes. Allow sealed film packages to reach room temperature before opening to prevent condensation on the films.

Loaded cassettes must be stored in an area shielded from exposure to radiation. This area is usually in or near the mammographic X-ray room. The location of exposed and unused cassettes must be clearly marked. The area should be large enough to accommodate the required supply of cassettes needed during the operation of the facility.

6.4 Viewbox

The condition of viewboxes should be checked regularly. The conditions under which radiologists and other health care professionals examine mammograms may influence diagnostic accuracy. Problems with improper illumination due to the non-uniformity of fluorescent tube manufacture, degradation and the discoloration of the viewing surface must be corrected. Illumination should be constant between all boxes used to read mammograms. The viewing conditions including ambient light level and film masking are as important as the actual luminance of the viewing box. Ambient and extraneous light should be reduced to a minimum since a very bright viewing box is necessary when the ambient light level is high. It is best to use only one type of fluorescent tube within a facility; these tubes should be changed when signs of aging develop. Care should be taken to clean the viewing surface of the viewbox such that no dirt could influence diagnostic accuracy.

There is no optimal level of illumination for viewboxes. However, the viewbox brightness should be at least 3000 nits (cd/m²). The light output from the viewboxes should be uniform to within 10 percent and the light output homogeneity between all viewboxes used for mammograms should be uniform to within 15 percent. The ambient light within the reading room should be less than 50 lux.

6.5 Cassette and screen

Cassettes or screens in poor condition will impair diagnostic quality. Problems are caused by dirty or damaged screens, warped cassettes, fatigue of foam compression material or closure mechanism, light leaks, and poor film-screen contact. Cassettes should be checked regularly for wear, and cleanliness. Manufacturer recommended screen cleaner should be used. To avoid artifacts caused by dirt and dust, the intensifying screens and cassettes should be cleaned at least weekly. The cassette holder tunnel should be checked regularly for dirt and dust. The intensifying screens should be inspected with an ultraviolet light to find dust particles. Cleaning tools include a screen cleaner with antistatic solution, lint-free cloths, compressed air, and a camel hair brush.

There must be no region of poor contact between the film and the screen greater than 1 cm² where image is present or greater than 2 mm wide 1 cm long at the chest wall. The film optical density must be within ±0.15 units for all cassettes used in the facility when tested with identical loading factors.

The proper amount of filtration placed in the X-ray beam between the X-ray tube and the patient reduces radiation exposure to the patient. However, any material placed between the patient and the mammographic image receptor has the effect of increasing patient exposure.With this in mind, only cassettes specifically designed for mammographic use must be used. Cassettes intended for general radiographic purpose must never be used in mammography.

Cassettes and screens should be numbered for identification and matching. The preferred location for marking is on the long side away from the chest wall.