Radiographic Quality Assurance and Infection Control

Compare Radiographs with Reference Film

One of the most common causes of poor radiographs is poor film processing in the darkroom, in particular the use of depleted solutions. A simple and effective means for constant monitoring of the quality of images produced in an office is to check daily films against a reference film. Soon after film-processing solutions are replaced, mount a patient film that has been properly exposed and processed with exact time-temperature technique on a corner of the viewbox. This image, with optimal density and contrast, serves as a reference for the radiographs made in the following days and weeks (Fig. 8-1). All subsequent images should be compared with this reference film.

Comparison of daily images with the reference film may reveal problems before they interfere with the diagnostic quality of the images. When a problem is identified, it is important to determine the probable source and to take corrective action. For instance, if the processing solutions have become depleted, the resultant radiographs are light and have reduced contrast. Both developer and fixer should be changed when degradation of the image quality is evident. Light images may also result from cold solutions or insufficient developing time. Dark images may be caused by excessive developing time, developer that is too warm, or light leaks.

There are two methods that are more accurate than a reference film but require additional equipment and more time to perform. These are sensitometry/densitometry and the use of a step wedge.

Make Step-Wedge Test of Processing System

The most accurate and rigorous method of testing film-processing solutions is to use a sensitometer and densitometer. A sensitometer exposes film to a calibrated light pattern. After processing, a densitometer is used to measure the optical density of each step in the test pattern of the film exposed by the sensitometer. A change in the density readings from day to day indicates a problem in the darkroom.

For most dental offices a variation of this method using a step-wedge test provides accurate monitoring of day-to-day processing conditions. This information is used to measure the speed of the imaging system and image contrast. Both are sensitive measures of the processing environment. A step wedge is readily made with the lead foil from film packets. Stack five sheets together and staple at one end (Fig. 8-2). Cut off four fifths of the top layer, three fifths of the second layer, two fifths of the third layer, and one fifth of the fourth layer to create a five-step wedge. Lay the wedge on top of a film packet and expose with the usual setting for an adult bitewing view. The resultant image should show five steps from dark to light. Save the first film after changing to fresh processing solution for comparison with images made on subsequent days. Monitor the processing solutions at the beginning of each day with a step-wedge image to ensure that the processing system is operational for patient care.

Enter Findings in Retake Log

Another simple and effective means of reducing the number of faulty radiographs is to keep a retake log. Record all errors for images that must be re-exposed.

Replenish Processing Solutions

At the beginning of each work day, check the levels of the processing solutions and replenish if necessary. Replenish the developer with fresh developing solution and the fixer with fresh fixing solution.

Check Temperature of Processing Solutions

At the beginning of each work day, check the temperature of the processing solutions. The solutions must reach the optimal temperature before use—68° F (20° C) for manual processing and 82° F (28° C) for heated automatic processors. The instructions accompanying the film and processor verify the optimal temperature. Unheated automatic processors should be located away from windows or heaters that may cause their temperature to vary during the day. Proper temperature regulation is required for accurate time-temperature processing.

Replace Processing Solutions

The replacement frequency of processing solutions depends primarily on the rate of use of the solutions but also on the size of tanks, whether a cover is used, and the temperature of the solutions. In most offices the solutions should be changed weekly or every other week. The results of the step-wedge test will help determine the proper frequency.

Clean Processing Equipment

Regular cleaning of the processing equipment is necessary for optimal operation. Clean the solution tanks of manual and automatic processing equipment when the solutions are changed. Clean the rollers of automatic film processors weekly according to the manufacturer’s instructions. After cleaning, rinse the tanks and rollers twice as long as the manufacturer recommends to prevent the cleaner from interfering with the action of the film-processing solutions.

Clean Viewboxes

Clean viewboxes weekly to remove any particles or defects that may interfere with film interpretation.

Review Retake Log

Review the retake record weekly and identify any recurring problems with film processing conditions or operator technique. Use this information to educate staff or to initiate corrective actions.

Check Darkroom Safelighting

Film becomes fogged in the darkroom because of inappropriate safelight filters, excessive exposure to safelights, and stray light from other sources. Such films are dark, show low contrast, and have a muddy gray appearance. Inspect the darkroom monthly to assess the integrity of the safelights (preferably GBX-2 filters with 15-watt bulbs). The glass filter should be intact, with no cracks. To check for light leaks in a darkroom, turn off all lights, allow vision to accommodate to the dark, and check for light leaks, especially around doors and vents. Mark light leaks with chalk or masking tape. Weather stripping is useful for sealing light leaks under doors.

The following simple penny test can be used monthly to evaluate for fogging caused by inappropriate safelighting conditions (Fig. 8-3):

Clean Intensifying Screens

Clean all intensifying screens in panoramic and cephalometric film cassettes monthly. The presence of scratches or debris results in recurring light areas on the resultant images. The foam supporting the screens must be intact and capable of holding both screens closely against the film. If close contact between the film and screens is not maintained, the image loses sharpness.

Rotate Film Stock

Dental x-ray film is quite stable when it is properly handled. Store it in a cool, dry facility away from a radiation source. Rotate stock when new film is received so that old film does not accumulate in storage. Always use the oldest film first, but never after its expiration date.

Check Exposure Charts

Each month inspect exposure tables listing the proper peak kilovoltage (kVp), milliamperes (mA), and exposure times for making radiographs of each region of the oral cavity that are posted by each x-ray machine (Fig. 8-4). Verify that the information is legible and accurate. These tables help ensure that all operators use the appropriate exposure factors. Typically the mA is fixed at its highest setting; the kVp is fixed, usually at 70 kVp; and the exposure time is varied to account for patient size and location of the area of interest in the mouth. Exposure times are initially determined empirically. Careful time-temperature processing (described in Chapter 6) must be used with fresh solutions during this initial determination of exposure times.

Check Leaded Aprons and Collars

Visually inspect leaded aprons and collars for evidence of cracking. A fluoroscopic examination performed by a qualified individual can confirm any cracks in the lead shielding. Replace as necessary. Such cracking is usually caused by folding the shields when not in use. It can be minimized by hanging the aprons from a hook or draping them over a handrail.

1. X-ray output. Use a radiation dosimeter to measure the intensity and reproducibility of radiation output (Fig. 8-5). Acceptable values are shown in Figure 3-3.

2. Collimation and beam alignment. The field diameter for dental intraoral x-ray machines should be no greater than 2¾ inches. The tip of the position-indicating device (PID), or aiming cylinder, should be closely aligned with the x-ray beam.

3. Beam energy. The kVp or half-value layer (HVL) of the beam should be measured to ensure that the beam has sufficient energy for film exposure without excessive soft tissue dosage. Measurement of kVp requires specialized equipment. It should be accurate within 5 kVp. Measurement of HVL requires a dosimeter. The HVL should be at least 1.5 mm aluminum (Al) at 70 kVp and 2.5 mm Al at 90 kVp.

4. Timer. Electric pulse counters count the number of pulses generated by an x-ray machine during a preset time interval. The timer should be accurate and reproducible.

5. mA. Verify the linearity of the mA control if two or more mA settings are available on the machine. Make an exposure using the usual adult bitewing setting. Then reduce the mA to the lower value and select another exposure time, ensuring that the product of the mA and time in seconds (impulses) is the same as for the adult bitewing. For example, if the machine has 10- and 15-mA settings, and 15 mA and 24 impulses are used for adult bitewings, select 15 mA and 24 impulses for the first exposure and measure the dose. Make a second exposure at 10 mA and 36 impulses and measure the dose. The dose at each exposure combination should be the same (15 × 24 = 10 × 36). A discrepancy implies nonlinearity in the mA control or a fault in the timer. The step wedge described previously may also be used in place of the dosimeter. In this case the density of each step of each image should be the same.

6. Tube head stability. The tube head should be stable when placed around the patient’s head, and it should not drift during the exposure. When the tube head is not stable, service is necessary to adjust the suspension mechanism.

7. Focal spot size. Measure the size of the focal spot because it may become enlarged with excessive heat buildup within an x-ray machine. An enlarged focal spot contributes to geometric fuzziness in the resultant image. A specialized piece of equipment is required for this test.

Wear Gloves During All Radiographic Procedures

The practitioner should always wear gloves when making radiographs or handling contaminated film packets or associated materials such as cotton rolls and film-holding instruments or when removing barrier protections from surfaces and radiographic equipment. After the patient is seated, the practitioner should wash his or her hands and put on disposable gloves in sight of the patient if the operatory arrangement permits. Dental assistants should wear eyewear or a mask or faceshield if exposure to bodily fluids is anticipated.

Charts should be kept away from sources of contamination and not handled during the radiographic examination. Chair adjustments should be made in advance, or adjustments should be made on control surfaces that are covered, such as the headrest control.

Disinfect and Cover X-Ray Machine, Working Surfaces, Chair, and Apron

The goal of preventing cross-contamination is addressed in part by disinfecting all surfaces and by using barriers to isolate equipment from direct contact. Although barriers greatly aid infection control, they do not replace the need for effective surface cleaning and disinfection. Experience has demonstrated that, during the daily activity of treatment, failure of mechanical barriers is common. It is advantageous and reassuring to the operator to know that whenever this happens, the surfaces that may become accidentally exposed are clean and disinfected. Any surface that may be contaminated should be surface disinfected. This includes the x-ray machine control panel, tube head, and beam alignment device, dental chair and headrest, surfaces on which film is placed, leaded apron and thyroid collar, and the doorknob of the operatory. Operators should avoid touching walls and other surfaces with contaminated gloves. Good surface disinfectants include iodophors, chlorines, and synthetic phenolic compounds. Although the American Dental Association does not recommend specific chemical disinfectants and sterilants, it does suggest that when dentists use a chemical agent for disinfection or sterilization, the agent should be an Environmental Protection Agency (EPA) registered hospital disinfectant of low to intermediate activity. The agent should also be tuberculocidal—an effective killer of tuberculosis—and capable of preventing other infectious diseases, including hepatitis B and HIV.

Barriers should cover working surfaces that were previously cleaned and disinfected. Barriers protect the underlying surface from becoming contaminated. An effective barrier for the countertops and x-ray control console is plastic wrap, which may be conveniently stored in a butcher’s paper dispenser mounted on a wall (Fig. 8-6). When covering the x-ray control console, the operator should be sure to include the exposure switch and the exposure time control if they are integral parts of the unit (Fig. 8-7). An x-ray exposure switch that is independent of the console should be covered with a sandwich bag or food storage bag or wrapped with plastic wrap.

The dental chair headrest, headrest adjustments, and chair back may be easily covered with a plastic bag (Fig. 8-8). The x-ray tube head, PID, and yoke should be covered while they are still wet with disinfectant with a barrier to stop any dripping (Fig. 8-9). The bag should be secured by tying a knot in the open end or by placing a heavy rubber band over the x-ray tube head just proximal to the swivel. Also, the leaded apron should be cleaned, disinfected, and covered between patients because it is frequently contaminated with saliva as the result of handling (readjusting its position) during a radiographic procedure. The apron should be suspended on a heavy coat hanger to permit turning front to back. It should be sprayed with a detergent-containing disinfectant and then wiped and covered with the same type of plastic garment bag used for the x-ray head and chair back (Fig. 8-10). The operatory is now prepared for radiography.

Panoramic and cephalometric equipment should receive the same maintenance for decontamination and disinfection as other equipment. Panoramic bite blocks, chin rest, and patient handgrips should be cleaned with detergent-iodine disinfectant and covered with a plastic bag. Disposable bite blocks may be used. The head-positioning guides, control panel, and exposure switch should be carefully wiped with a paper towel that is well moistened with disinfectant. The radiographer should wear disposable gloves while positioning and exposing the patient. The gloves should be removed before the cassette is removed from the machine for processing because the cassette and film remain extraoral and should not be handled with contaminated disposable gloves. Cephalostat ear posts, ear post brackets, and forehead support or nasion pointer should be cleaned and disinfected with iodine-detergent disinfectant. These may then also be covered in plastic.

After patient exposures are completed, the barriers should be removed, contaminated working surfaces (including those in the darkroom) and the apron sprayed with disinfectant and wiped as described previously. Then the barriers should be replaced in preparation for the next patient.

Sterilize Nondisposable Instruments

It is best to use film-holding instruments that are heat sterilizable. After sterilization, the instruments should be kept in bags for storage and subsequent transport to the radiography area. When the instruments are taken to the radiography area, it is good technique to remove them from the bag immediately before use. After use, instruments should be replaced in the bag to reinforce cleanliness in the area. The same sterilization bag should be used to transport the contaminated instruments back to the cleaning and sterilizing room.

Use Barrier-Protected Film (Sensor) or Disposable Container

Film should be obtained in advance from a central source. To prevent contamination of bulk supplies of film, they should be dispensed in procedure quantities. The required number of films for a full-mouth or interproximal series should be prepackaged in coin envelopes or paper cups in the central preparation room. These envelopes of films should be dispensed with the film-holding instruments. For unanticipated occasions in which an unusual number of films are required, a small container of films can be on hand in the central preparation and sterilizing room. No one wearing contaminated gloves should retrieve a film from this supply. Films should be dispensed only by staff members with clean hands or wearing clean gloves.

Film packets may be packaged in a plastic envelope (Fig. 8-11), which protects the film from contact with saliva and blood during exposure. Barrier-protected film fits in most film-holding instruments. An attractive feature of the protective envelopes is the ease with which they may be opened and the film extracted. For best results, the packet should be immersed in a disinfectant after the films have been exposed in the patient’s mouth. Then the packet should be dried and opened, allowing the film to drop out. The barrier envelopes can be conveniently opened in a lighted area, the film dropped onto a clean work area or into a clean paper or plastic cup, and the film transferred to the daylight loader or darkroom for processing.

If barrier-protected film is not used, the exposed film should be placed in a disposable container for later transport to the darkroom for processing. Paper film packets are exposed to saliva and possibly blood during exposure in the patient’s mouth. To prevent saliva from seeping into a paper film packet, a paper towel should be placed beside the container for exposed films. The practitioner should use this towel to wipe each film as it is removed from the patient’s mouth and before it is placed with the other exposed films. This problem may also be avoided by using film packaged in vinyl.

Sensors for digital imaging are not able to be sterilized; thus it is important to use a barrier to protect them from contamination when placed in the patient’s mouth (Fig. 8-12). Typically the manufacturers of these sensors recommend the use of plastic barrier sheaths. The supplemental use of latex finger cots provides significant added protection and is recommended for routine use when using digital sensors. Because such barriers may fail, the sensors should be cleaned and disinfected with an EPA-registered intermediate-level hospital disinfectant after every patient. The manufacturer of such equipment should be consulted for proper disinfectant.

Prevent Contamination of Processing Equipment

After all exposures are made, the operator should remove his or her gloves and take the container of contaminated films to the darkroom. The goal in the darkroom is to break the infection chain so that only clean films are placed into processing solutions. Two towels should be placed on the darkroom working surface. The container of contaminated films should be placed on one of these towels. After the exposed film is removed from its packet, it should be placed on the second towel. The film packaging is discarded on the first towel with the container.

Removing film from a packet without touching (contaminating) it is a relatively simple procedure. Figure 8-13 illustrates the method for opening a contaminated film packet while wearing contaminated gloves without touching the film. The practitioner dons a clean pair of gloves, picks up the film packet by the color-coded end, and pulls the tab upward and away from the packet to reveal the black paper tab wrapped over the end of the film. Now, holding the film over the second towel, he or she carefully grasps the black paper tab that wraps the film and pulls the film from the packet. When the film is pulled from the packet, it will fall from the paper wrapping onto the clean towel. The paper wrapper may need to be shaken lightly to cause the film to fall free. The packaging materials should be placed on the first paper towel. After all films are opened, the practitioner gathers the contaminated packaging and container and discards them along with the contaminated gloves. The clean films are processed in the usual manner. It is not necessary to wear gloves when handling processed films, film mounts, or patient charts.

An alternate procedure when exposing films in vinyl packaging is to place the exposed film, still in the protective plastic envelope, in an approved disinfecting solution when it is removed from the mouth and after wiping it with a paper towel. It should remain in the disinfectant after the exposure of the last film for the recommended time. Immersion for 30 seconds in a 5.25% solution of sodium hypochlorite is effective.

Automatic film processors with daylight loaders offer a special problem because of the risk for contaminating the sleeves with contaminated gloves or film packets. One approach is to clean the films by immersion in a disinfectant, with or without a plastic envelope, as previously described. With this method the operator cleans the films, puts on clean gloves, and then takes only cleaned film packets into the daylight loader. An alternate approach is to open the top of the loader, place a clean barrier on the bottom, and insert the cup of exposed film packets and a clean cup. The operator then closes the top, puts on clean gloves, pushes his or her hands through the sleeve, and opens the film packets, allowing the film to drop into the clean cup. After all film packets have been opened, the contaminated gloves are removed, the films are loaded into the developer, and hands are removed. Then the top of the loader may be removed and the contaminated materials removed.