Fig. 3.19. Graph illustrating percent film-speed change, film contrast, and film base-plus-fog values plotted versus developer temperature for the single-emulsion mammographic film [three-dimensional grains (-), cubic grains (. . . .)] and double emulsion [tabular grain film ( )] using film manufacturer's recommended processor and chemicals. The vertical line represents recommendation for a standard processing cycle (Haus, 1999a).

Mammography films with cubic grain emulsions are less sensitive to temperature change than are three-dimensional grain emulsions. As noted in Figure 3.19 an increase in film contrast occurs for three-dimensional grains but not for tubular or cubic grains with increased developer temperature (Haus, 1999a).

The ACR Mammography Quality Control Manual section for radiologic technologists (ACR, 1999), indicates that the developer temperature should be within ±0.3 °C of that recommended by the manufacturer for the specific film-developer combination being used. The measurement accuracy and precision of the thermometer used to monitor developer temperature is most important (ACR, 1993). In the radiology or medical imaging department, a variety of thermometers are used to measure developer temperature. These thermometers vary in accuracy, precision, ease of reading, and cost.

Clinical digital thermometers, which are available in pharmacies and supermarkets, are inexpensive, but accurate devices for measuring the temperature of the developer solution (Wilson et al., 1993) are not widely available and are more expensive. These thermometers have a temperature range of 32 to 42 °C and accuracy of approximately ±0.1 °C. It is also recommended that the thermometers used to measure developer temperature be evaluated against a thermometer that has a calibration traceable to the National Institute of Standards and Technology.

It is important to confirm that proper film contrast, film speed, and base-plus-fog values are being obtained for each film type used (according to the manufacturer's specifications and tolerances). This information is available from the film manufacturer (Kimme-Smith et al., 1992; Moore et al, 1993). To maintain consistent film contrast, film speed, and base-plus-fog values, it is important to implement a processor QC program (ACR, 1999). Chemicals. All film manufacturers recommend chemicals for processing their films. Many users consider chemicals from various manufacturers to be interchangeable. However, surveys have documented that film speed, film contrast, and base-plus-fog respond differently to various types of chemicals (Haus, 1999a) (Figure 3.20). These effects also depend on the type of film being processed (Kofler and Gray, 1991).

Chemical manufacturers distribute chemicals as concentrates. Solution service providers add water locally to complete the mixture. In some cases, chemicals are not mixed to the appropriate concentration in accordance with the manufacturer's recommendations. It is also important to avoid the use of chemicals beyond the manufacturers expiration date.

Processing chemical variability can occur in medical imaging due to a number of factors. Although most manufacturers use similar processing chemicals to achieve development and fixing, the concentration of these chemicals can vary, either initially or after being mixed by solution service providers. This concentration variation can result in changes in film response of differing magnitudes depending on the film type. In addition, variability can also result from improper replenishment (Section Either overdevelopment or underdevelopment can occur depending on the degree of replenishment or initial chemical concentration.

For the initial start-up or when fresh chemicals are used, it is important to follow the manufacturer's recommendations by: (1) using the proper chemicals, (2) mixing to the correct concentration, and (3) adding the appropriate amount of starter solution.

Fig. 3.20. Chart produced from film-processing survey data which shows film-processing variations due to use of different chemicals for single-emulsion mammographic film. The letter "K" indicates processing data (and expected values) using the film manufacturer's processor and chemicals. A horizontal line is drawn at the letter "K" data point. Letters A through H are data for different brands of chemicals. Data were obtained using film strips which were sensitometrically exposed to light that simulates the light spectrum from a mammographic screen. Film speed differences, film contrast (average gradient), and base-plus-fog values were determined from the sensitometry data (Haus, 1999b).

Fig. 3.20. Chart produced from film-processing survey data which shows film-processing variations due to use of different chemicals for single-emulsion mammographic film. The letter "K" indicates processing data (and expected values) using the film manufacturer's processor and chemicals. A horizontal line is drawn at the letter "K" data point. Letters A through H are data for different brands of chemicals. Data were obtained using film strips which were sensitometrically exposed to light that simulates the light spectrum from a mammographic screen. Film speed differences, film contrast (average gradient), and base-plus-fog values were determined from the sensitometry data (Haus, 1999b).

Adding starter solution begins the seasoning process. The developer solution becomes more completely seasoned as more films are processed. This additional seasoning may continue to cause slight changes in film speed and contrast; at some point, film speed and contrast will stabilize. Seasoning effects depend on film type, chemical formulation, and replenishment.

Since both the concentration and composition of chemicals used in film processing can have an effect on the contrast, speed, base-plus-fog, and long-term retention of films used in medical imaging, it is sometimes of interest to attempt to analyze the chemicals used. There are several approaches that are being used to accomplish this. They include: (1) pH measurement, (2) specific gravity measurement, (3) laboratory component analysis, and (4) process control sensitometry.

Determination of pH is a measure of the activity of process chemistry, because development activity generally decreases as pH decreases. This measurement, however, is not very accurate and is useful only for finding trends or large changes in developer concentration. Evaluation of pH is difficult to achieve in solutions (such as developers) containing high concentrations of salt, unless carefully calibrated electrodes are used.

Specific gravity measurements can also be used to determine relatively large changes in concentration. Measurement of specific gravity involves determining the ion and salt concentration of a solution. This same type of measurement is used to measure the acid content of a car battery and is not accurate or specific with respect to any particular chemical. Again, although information about large changes in concentration can be determined, it is not specific enough to determine whether critical components such as developer antifoggants are missing from the developer solution (ACR, 1993; Haus, 1999a; Haus and Jaskulski, 1997).

Analysis of samples of developer solution by an analytical laboratory is the most accurate and predictive approach. However, this approach is costly and time consuming.

The last approach, and probably the most widely used, is to do processor control sensitometry. By monitoring changes in sensito-metric response of a processor control film strip, changes due to process chemistry can be detected. If variations in processor control values (speed, contrast, and base-plus-fog) exceed operating tolerances, the chemicals should be changed to insure appropriate and consistent results. Although this approach does not identify the actual cause of sensitometry change, it is probably the most cost-and time-effective approach. Sensitometry must be carried out with the types of emulsion as processed in the film processor. The cost of changing chemistry is small compared to the total cost of doing medical radiography; the down time and investigative time required to identify the cause of a specific change in processor chemistry may not be justified (Haus and Jaskulski, 1997). Replenishment. Replenishment is important to maintain stable developer and fixer activity. Proper replenishment: (1) provides stable sensitometric results (film contrast, film speed, and base-plus-fog); (2) reduces or eliminates artifacts such as wet-pressure emulsion pick-off; and (3) enables long-term retention of the films. Replenishment rates are sometimes divided into groups based on daily film volumes. Low film use per day requires higher replenishment per sheet. Processors with very low film volume

(such as surgery rooms) are very difficult to stabilize and it is difficult to maintain consistency. Flooded replenishment is recommended under these conditions (Frank et al., 1980). Starter solution is added to the developer replenisher holding tank; the processor is replenished at specific time intervals independent of film volume, in addition to replenishment per sheet of film processed. Flooded replenishment provides a stable fresh process. High film use per day requires a lower replenishment per sheet.

Film throughput (film sheets per day) is the basis for determining replenishment volumes; however, since the typical film sizes for mammography are 18 x 24 cm and 24 x 30 cm, the actual area of the film is less than is used in general radiography. Consult with the manufacturer to correctly adjust and set up the film processor, and replenishment rates to obtain the desired results and to obtain consistency in those results. Agitation. Agitation maintains processing uniformity and temperature control. Film surface agitation is provided by roller contact, while tank solution agitation is provided by recirculation pumps. Drying. The adjustable range of drying temperatures is from 38 to 71 °C. Drying conditions depend on the environment. This may range from cool and dry to hot and humid. Many users tend to over-dry films, which may cause surface pattern artifacts on the film (e.g., water spotting, that may impact the radiologist's ability to read films). The dryer temperature should, therefore, be adjusted as low as possible, while still providing dry films exiting the processor. This will also result in energy savings for the processor operations.

3.2.5 Maintaining the Darkroom and the Processor

Optimal processing conditions are more imperative for mammo-graphic quality than for any other type of medical imaging because of the need to identify imaging subtleties, such as fine calcifications inherent in diagnosing breast cancer. The processor itself, the chemicals, the temperature, and the length of the processing time are all crucial elements as discussed previously.

Mammograms often reveal problems associated with film processing. The processor requires properties such as:

• Correct electrical current. To avoid problems associated with overloaded circuits and power surges, the processor should have its own electrical circuit.

• Correct water flow. A reduced water flow can allow algae to form. In a cold-water processor with metal tanks, too much water flow may lower the temperature in the developer and fixer.

• Darkroom air, ventilation and temperature. To make sure that the processor functions correctly, the darkroom needs a constant flow of fresh air. Filtered air should enter the darkroom through an air conditioner. If the processor is not adequately ventilated, streaking and mottling of the film emulsion will result. Without adequate ventilation, not enough air will flow across the rollers to prevent condensation and not enough air will flow into the dryer to dry the films correctly. The air exiting the processor should be adequately ventilated to prevent the buildup of fumes from the developer and fixer fluids because some technologists are sensitive to these vapors.

• Eliminating dust and artifacts. Due to the processor's ability to attract dirt which can spot, veil, and obliterate a mam-mographic image, scrupulous cleaning is essential. Before shutdown, the crossover racks from the developer to the fixer and from the fixer to the wash tank should be cleaned. The processor should be left open until next use. This prevents the chemicals from condensing and crystallizing on the rollers as the processor cools. To be sure that the transport rollers are clean, the technologist must always process the transport roller cleanup film before processing patient films.

Dust is one of the darkroom's greatest problems. Dust interposed between the screen and the film is more visible on single-emulsion films. Small amounts of dust do not hinder accurate assessment of glandular tissue, but the resulting noise can be distracting for the interpreter. The darkroom should not be carpeted because carpeting creates and harbors dust. Every day, the technologist should wipe the counters in the darkroom with a damp cloth and clean the feed tray of the processor with an antistatic solution. Every week, the air vents should be vacuumed and wiped and the darkroom floor should be vacuumed and mopped. Every month, the air-conditioner filter should be replaced.

• Humidity. Controlling the quality of the air is also a necessity. The technologist should check the darkroom's hygrometer at least once a day to be sure that the relative humidity remains at 50 to 55 percent. If the hygrometer registers above 60 percent, which indicates that the darkroom is too humid, the technologist should turn on the dehumidifier. When the hygrometer registers lower than 50 percent, the air is too dry and the technologist should turn on the humidifier not only to prevent static marks, but also to help avoid the electrostatic charging of the cassette that will attract dust.

• Safelight illumination. Safelight illumination is an important part of maintaining the darkroom. The adjective "safelight" is only a relative term. Given sufficient time, safelight emissions will expose any film. This exposure, which reduces contrast, is called "fog." It is necessary, therefore, to limit the time that the film is exposed to the safelight and minimize the intensity of the light.

To prevent film fogging from the safelight:

- The technologist should process exposed film immediately after removing it from the cassette.

- The safelight filters should be those recommended by the film manufacturer and must be installed correctly. The identifying marks on the filter should be legible when looking at the lamp. If the filter's orientation is mistakenly reversed, heat buildup inside the lamp's housing may crack the dye layer and cause it to leak "unsafe" light.

- The wattage of the bulb must be correct based on the film being used. A 110 to 120 volt, 60 Hz source requires no more than a 15 W frosted bulb. Higher wattage will produce excessive illumination and may damage the safelight filters. If the safelight must be placed <4 feet from the work area, the technologist should change the 15 W to a 7.5 W bulb.

- The position of the safelight should be no closer to the film during processing than the manufacturer recommends. The safelight lamp should be no closer than 4 feet (1.22 m) from the film during processing.

- Every six months, the technologist should test for fog and check to be sure that the safelight has remained within recommended limits, that the safelight filter has neither faded nor cracked, that it is the recommended filter for the film, and that nobody has inadvertently replaced the bulb with one of incorrect wattage. The technologist shall verify that the safelight is still located the correct distance from the film. A darkroom fog test is an MQSA (1992) requirement.

The semi-annual test for fog should include examining the darkroom for light leaking in from outside. The technologist should check for light leaks around doors, cracks in the walls, suspended ceilings, junctions between wall partitions, or seams between walls and ceilings. The vibration of an automated processor may disturb a seal or a gasket or the cover may be loose. A darkroom requires white incandescent lights because the afterglow from fluorescent lights can produce fogging. For a darkroom to reflect all the light available from the safelight and illuminate the darkroom better, its walls should be white or light-colored with a white ceiling.

• Film storage. Film storage conditions may be brand specific. Unopened boxes of film require a cool, dry spot for storage. Normally the temperature should be no higher than 21 °C and the relative humidity at 50 to 55 percent. The storage area for film should be shielded from chemicals, x rays, and other sources of radiation. Film needs gentle treatment, without any pressing, creasing or buckling. To avoid pressure marks, the technologist should store the boxes of film upright. Films should not be used after the manufacturers expiration date.

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