Section 12: Darkroom and Processing

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Introduction

In most of the modalities discussed in this manual, film is used and the final link in the diagnostic imaging chain, photographic processing, is very important and often the one that is overlooked. All of the efforts by the technical staff aimed at maintaining quality imaging equipment will be futile, if the photographic processor is not maintained under a good quality control program. The photographic process controls not only the image quality (eg. contrast, density, base plus fog) but also the patient exposure.

This chapter deals not only with the photographic processor and all of the associated aspects of film processing including darkroom, fog, daylight loading systems, darkroom conditions, film and chemical storage, but also with daily processor quality control.

Definition

Photographic quality control is a process that ensures the optimization of film processing in order to maintain high image quality and low radiation exposure to patients.

In a photographic quality assurance program the following information should be logged as appropriate:

1. Complete cleaning
2. Total chemistry change
3. Developer change
4. Fixer change
5. Replenishment rate increase or decrease
6. New developer mixed
7. New fixer mixed
8. Wash temperature adjusted
9. Mechanical repair
10. New control film exposed
11. Sensitometric analysis on a daily basis including week-ends if applicable. This is optional in dental facilities.

All of the above information should be included on the sensitometry charts in an effort to assess continuity in contrast and density.

 

Processing Techniques

Standards

Manual Processors

Sight developing must never be done. The time-temperature method of manual processing must be followed according to the chemical manufacturer's specification. The accuracy of the timer and thermometer should be checked monthly.

Manual developer containers must be opaque and must be covered to prevent deterioration or oxidation of the developer. A non-mercury thermometer and a minute timer are necessary for proper manual processing. A thermometer on the in-coming hot water line (mixing valve), or a thermometer external to the developer tank may not accurately measure the developer temperature.

Renewal of chemistry depends on workload, original volume, replenishment (if any), on the appearance of the radiographs before a need to increase patient exposure is in order to obtain a good film quality.

Quality control checks, ie. sensitometry, should be performed to ensure the activity level of the chemicals before processing clinical films.

Automatic Processing

Automatic processors should be cleaned regularly and the cross-over rollers should be cleaned each operational day. Digital readouts of developer temperature or temperature gauges that are part of the processor itself should not be used as the sole check on developer temperature as they have been seen to be inaccurate. A separate thermometer is required to monitor the developer's temperature.

Dental Processors

Not all processors are fully automatic. Some have automatic transportation systems, but use manual chemicals because they do not have a heater to warm the developer.

Processors designed for use at room temperature do not contain a heating element. 21 As a result, special attention should be given to the location of such processors. A draft from a window may cause a fluctuation in the temperature of the chemicals. Overheated, small enclosures with improper air circulation may make the developer too warm. Both of these situations can result in poor quality images and increase the potential for repeat radiographs or exposures.

21 Some automatic processors without heating elements use manual chemicals.

Evaluate as follows:

Automatic Processors

The developer temperature, processing chemistry, cycle time of the automatic processor, and radiographic exposure, are factors which influence the quality of the final image. In order to obtain high quality films, the manufacturer's recommendations concerning the operation of the processor should be closely followed.

When measuring developer temperature a separate thermometer must be uses as it is not acceptable to rely solely on the integrated digital readouts of developer temperature or of the temperature gauges that are part of the processor, as these can be out of calibration by as much as 5 degrees Celsius.

Equipment

• Non-mercury Thermometer 22

22 A non-mercury thermometer (i.e. alcohol or metal) is recommended, in order to prevent contamination of the processor or chemicals by mercury in the event of thermometer breakage.

• Sensitometric films

Procedure

1. Check the temperature of the developer solution daily. Corrective action should be taken if the temperature varies by more than +/- 2 degrees Fahrenheit from one day to the next.
2. Check the solution levels daily and replenish/replace solutions according to the manufacturer's instructions.
3. Develop a sensitometric film on a daily basis including week-ends if applicable. This is optional in a dental office. See next page for sensitometry.

In addition, it is important that you follow the manufacturer's recommendations for proper maintenance of your processor by:

4. Cleaning the processor regularly. Each manufacturer has specific cleaning instructions for their equipment. These directions should be followed explicitly.
5. Making sure the transport sections are properly aligned after cleaning by processing a test film.
6. Lubricating moving parts, such as rollers and gears, regularly with a suitable lubricant as specified by the manufacturer.
7. Checking the water filter for clogging.
8. Visually checking the electrical components for both proper insulation and protection from chemical/mechanical damage.

 

Manual Processing

The time/temperature method of processing films must be followed, according to the chemical manufacturer's directions. Sight development must NEVER be done as it promotes the use of higher radiation exposure to patients and often results in poor quality radiographs.23

23 Proper photographic chemical tanks (opaque) should be used.

The tanks should have a lid to cover the chemicals to minimize oxidation.

Solutions should be changed at regular intervals as recommended by the manufacturer.

Solutions should be topped up or replenished according to manufacturer's instructions.

Equipment

• Non-mercury thermometer
• Minute timer

Procedure

1. Post the chemical or film manufacturer's time/temperature development chart near the

1. processing tanks so that it is readily available for reference.
2. Check the temperature immediately before processing the film.
3. The film is then developed for the time specified for that temperature on the development chart. An alarm timer is necessary for accuracy.
4. After development, wash and fix according to the chart.

 

Sensitometry Analysis

Optional with dental equipment.

Sensitometry is performed on a daily basis to ensure that all processors are operating at the same levels and producing consistent, high-quality films.

Equipment

• Sensitometer
• Densitometer
• Digital thermometer or metal-stem thermometer
• Control film

Procedure

1. Follow the manufacturer's start-up procedures every day.
2. Allow sufficient time for the processor temperature to stabilize -- about 20 -30 minutes (or the manufacturer's recommendation). Assure that the wash water temperature is adjusted properly, where appropriate.
3. Check the following:
1. Solution temperatures
2. Replenishment rates
3. Water flow rates
4. Dryer temperature
4. Run several clean-up sheets and check them for roller marks and scratches. (Use exposed but unprocessed film for clean-up sheets.)
5. Expose enough sensi-strips for a single day's use. If the film is a dual emulsion, expose a strip on both sides if you are using a sensitometer that only exposes one side.
6. Process the strips with the thin end leading.
7. Zero and calibrate the densitometer.
8. Read the density patches and the base plus fog level from the strip from each processor.
9. Calculate and plot the base fog, contrast, speed and developer temperature.
10. Analyze the control charts:
1. are all points within the control limits?
2. are any trends apparent?
11. If any single point falls outside of the control limits, run another strip to verify that the readings were correct. The following limits are suggested as control points:
1. Base + Fog +/-10%
2. Speed +/- 15%
3. Contrast +/- 20%
4. Temperature +/-2 degrees F
12. If the reading still falls outside of the control limits take corrective action.

 

Dental Step Wedge Analysis (Sensitometry)

Frequency

Weekly 24

24 Each x-ray machine should yield films of similar density and contrast. However, for photographic quality assurance and machine output reproducibility assessment, only compare the films taken for each x-ray machine.

Equipment

• Step Wedge (instructions for assembly provided at the end of this section)
• Periapical films

Procedure

1. Place a periapical film on a counter top or other convenient surface and position the step wedge on top of the film.
2. Centre the cone of the x-ray tube head to the step wedge and position it 1/2 cm above the step wedge.
3. Expose the film using normal bitewing exposure factors.
4. Develop this film. The resultant film should show a degradation of densities. If not, you could be overexposing the film. Keep this film by the processor as it becomes your reference film "standard" to which all other films taken in the course of the week will be compared to until new chemistry is put in the developer.
5. Repeat the first three steps on eight to ten additional unexposed periapical films for future testing. Do not develop these films at this time. During the course of the week, each day develop one of the exposed film and compare this to film developed the first day in step 4 and note any changes. If the film densities do not match continue to step 6. Continue this process until new chemistry is in place at which time the whole process is started up again.
6. If you suspect the quality of your film is deteriorating, try to determine what is causing the problem and rectify before developing any films taken of patients.

Note:

A visible difference in corresponding step images indicates an output difference of approximately +/- 20 percent. It is possible that the patient may be receiving an overexposure.

Safelights

Standard

A safelight must have the correct filter for the film type being used and the bulb must be of the correct wattage. A direct safelight cannot exceed 15 watts. An indirect safelight cannot exceed 25 watts.

Safelights must be placed a minimum of 1 metre from the working area.

The filter must not be cracked or scratched or otherwise permit unfiltered light (white light) to reach the film. The filter must be of a type specific to the film being used.

Evaluation

1. Do not direct a dental office to purchase a safelight if they do not have one.
2. A facility which has a safelight must ensure that it is of the correct wattage and filter type for the film being used.

Dental automatic processors are frequently fitted with a light-tight compartment covered by a filter screen which enables the dentist to use the processor in a location other than a darkroom. The operator slips his/her hands through a set of sleeves and views the unwrapping of the film and the insertion of it into the processor by looking through the filter screen.

1. Check that the sleeves permitting entry to the compartment fit tightly around the wrists.
2. Check that the filter screen is not cracked and that light cannot enter around the edges.

 

Darkroom Integrity

Standard

There must be no white light shining in the darkroom when film are being transferred to and from their protective holders.

Evaluation

Close the door to the darkroom. Turn out the lights, allow your eyes to adjust to the dark and check the darkroom for light leakage. In particular, check around the vertical edge of the door frame, window blinds and pass boxes. On automatic processors, which have daylight loading, check entry sleeves for tears or cracks that may cause film fogging.

Also refer to darkroom testing in mammography section for more information.

Procedure

1. Test fogging with the fastest film normally handled in the darkroom.
2. Expose the test film for one minute in the darkroom with the safelight on.

Evaluate as follows

The exposure should produce less than a 0.05 increase in the mid-density portion of the film. Ideally, less than 0.05 increase should also be obtained with a two minute exposure to the darkroom safelight.

 

Unused Film Storage

Standard

If unused film is stored in the x-ray room, then it must be stored in a container which provides adequate protection against scatter radiation such as a lead container or something equivalent to 1.6 mm of lead. Film should be stored standing on its edge, not flat, as it is sensitive to pressure marks. Outdated film must not be used as it has an increased base fog.

Photographic materials should be stored at temperatures less than 24 degrees C, preferably in the range of 15 to 21 o C. Open packages of film should be stored in an area with humidity ranging between 40 and 60 per cent.

Many x-ray facilities provide lead shielding on the walls to the seven foot level. Care should be taken to ensure either that film is not stored above this level in darkrooms adjacent to exposure rooms or that the lead shielding is extended to the ceiling in such areas.

Evaluation

If in doubt about the safety of the film storage, take a scatter measurement at the storage position. Any reading will indicate the film is being exposed to scatter radiation. If there is no reading, but there is a possibility that the film could be fogged (tube near or pointed at film), a direction warranted.

Film can be stored in a freezer to extend shelf life. The expiry date on the box must be changed, so that the duration the film remains in the freezer is not included in the shelf life.

Table 12-1 Summary of Processor Maintenance

Procedure	Frequency	Comments
Process sensitometric strip and measure the temperature of the developer daily	At least once per day and including week-ends if applicable	Strips should also be run when the processor is cleaned or any maintenance is performed
Wash crossover assemblies	At least once per day	Clean when processor is turned on and turned off
Check replenishment rates	Daily
Remove and wash all racks	Weekly	Inspect racks for damage
Check recirculation pumps	Weekly	Developer and fixer
Check replenishment pumps	Weekly	Developer and fixer
Check fixer and wash temperatures	Weekly	Use a calibrated thermometer placed in the tank
Check processing time	Weekly	Dry to drop
Drain and clean tanks	Monthly	Remember to use starter
Check and replace filters	Monthly
Clean intensifying screens	Semi-annually	Or as required
Check screen-film contact	Semi-annually	Or as required
Check film speed	Annually
Check safelight	Semi-annually	Check safe handling time
Check darkroom	Semi-annually	Check for light leaks