Organization

There should be essentially two sides to the darkroom: a dry side and a wet side (Fig. 7-1).

Figure 7-1 A sample dark room layout showing a wet side and a dry side.

Dry side.

The dry side of the darkroom is where the cassettes are unloaded and reloaded. A countertop or tabletop large enough to accommodate the largest cassette in the open position should be available. The tabletop should be constructed of a material that allows frequent cleaning, which is necessary to reduce the source of darkroom artifacts that can potentially get on the film. It must be impossible for chemicals to splash into the dry side. At no time should anything “wet” be brought to the dry side. It is customary to store film under the dry table, either in a cupboard or in a film bin, to allow easy access for reloading cassettes (Fig. 7-2). Film hangers for each size of film should be hung above the table on the dry side on an appropriate bracket. Brackets can be purchased commerially or constructed inexpensively using large hooks found at any hardware store.

Figure 7-2 Film storage bin. When closed, the film is stored light-tight in a vertical position.

Film hangers are available in two designs: channel hangers and clip hangers (Fig. 7-3). Channel hangers tend to retain water and chemicals and need special cleaning and drying to prevent contamination of the dry side. Films must also be removed from the channel hangers to be dried. However, clip hangers are more fragile than the channel type. When the clips are used frequently over a period of time, they become weak and lose the ability to “stretch” the film. The clips also puncture the four corners of the film, which, when filed, can scratch other films in the same envelope. It is important to cut off the corners of films processed with clip hangers before filing to prevent this. When more than one film is processed at the same time in the tanks, the clips on the hangers can scratch neighboring films.

Figure 7-3 A clip film hanger (left) versus a channel film hanger (right).

Darkroom Lightproofing

As mentioned earlier, one criterion of a good darkroom is that it be lightproof. Light leaks in a darkroom can cause significant film fog; therefore taking appropriate measures to lightproof the darkroom is imperative. Lightproofing a room is more difficult than may be expected. The first step is locating the light leaks. Small light leaks may not be perceptible until the eyes have acclimated to the dark, and it may be necessary to spend 5 minutes waiting for the eyes to adapt. To achieve a truly lightproof room, a number of tasks may be necessary.

The entrance to the darkroom is a common site for light leaks. A double-door system or revolving door is preferred but not always practical in a veterinary practice (Fig. 7-4). The first step in lightproofing a standard door is to fit it tightly into its frame against strips of felt or rubber molding. Weather stripping is also useful around doors to prevent the entrance of light. Light entering from underneath the door can be prevented by a vapor seal designed specifically for the bottom of a door. A sliding bolt lock or doorknob lock prevents someone from accidentally entering the darkroom at an inopportune time. A suspended ceiling can be a radiographer’s nightmare. It may be necessary to place a large black sheet of plastic above the ceiling tiles to prevent light in adjacent rooms from entering through the seams.

Figure 7-4 A revolving door for the darkroom. This door is an effective means of entering and exiting the darkroom without risking exposure of the film to light.

It is a common fallacy that the walls of a darkroom should be “dark.” The opposite is true. The walls of the darkroom should be painted white or cream with a good-quality, washable paint. By painting the walls a light color, more reflection of the safelight is produced, providing a more visible work environment. If the quality and intensity of the light are “safe,” the illumination reflected from any surface also is “safe,” regardless of the color of that surface.

Darkroom safelight.

Correct safe lighting in the darkroom is crucial. A “safe” light means that the light produced will not affect the film. Radiographic film is sensitive to ultraviolet light. Safelights use a small-wattage bulb and a special filter to eliminate the light from the blue and green spectrum. The light bulb should be 15 watts or less. The filter varies by manufacturer. The most common types are a brown filter (Wratten 6B, Kodak) for blue-light-sensitive film and a dark-red filter (Wratten 6BR or GS-1) for green-light-sensitive film. The dark-red filter is recommended because of its versatility: both green-light- and blue-light-sensitive film can be used in this lighting.

Safelights should be positioned so that darkroom work can be performed without fumbling. There are two types of safe lighting: direct and indirect (Fig. 7-5). Direct lighting is a diffused light that shines directly over a work area such as the dry or wet side of the darkroom. Indirect lighting is a filtered light directed toward the ceiling and reflected over the entire room. Indirect lighting is often combined with direct lighting. At no time should the safelight be closer than 4 feet from a work area. A safelight that is too close, has a too-high wattage bulb, or has incorrect filtration may cause film fog.

Figure 7-5 Indirect (left) and direct (right) safe lighting for the darkroom.

The efficiency of a safelight can be tested; this is discussed in Chapter 10. Remember, no light is “safe” if the film is exposed to it for a prolonged period. Therefore the film bin should be open only when removing or replacing film. Fogging will result even with a safelight if the bin is left open or if film is left on the counter.

The Developer

The developer is a chemical solution that converts the latent image on a film to a visible image. The primary function of the developer is to convert the exposed silver halide crystals to black metallic silver.

The developing time usually is specified by the chemical manufacturer. Keep in mind that the developer temperature affects the developing time. Time-temperature developing is preferred over visual inspection when using the manual processing technique. This manual inspection is called “sight developing,” which consists of increasing or decreasing the time according to visual inspection of film density while the film is still in the tank. This technique requires attention and skill and is often subject to error. Sight developing should be avoided if at all possible.

The developer consists of developing agents, accelerators, preservatives, restrainers, hardeners, and a solvent.

The Rinse Bath

After a film has been in the developer, it retains a substantial amount of developer in the gelatin (approximately 60 mL on a 14- × 17-inch film). If the film were transferred directly into the fixer, the alkaline developer would neutralize the acid of the fixer. The rinse bath serves to stop the developing process, rinse the developer from the film, and prevent carryover contamination to the fixer.

Normally, the rinse bath consists of circulating water in which the film is rinsed for 30 seconds. A chemical solution such as acetic acid and water can be used as another method of stopping the development procedure. This chemical solution is called a stop bath. In automatic processing, a rinse or stop bath is not necessary because the rollers tend to remove excess developer from the film before it reaches the fix tank.

The Fixer

After a film has been properly developed and the exposed silver halide crystals have been converted to metallic silver, one other step involving the silver crystals remains. The unexposed silver halide crystals remaining on the film are unaffected by the developer solution and must be removed. If these silver crystals were to remain on the film, they would discolor and darken with exposure to light.

The fixer serves two basic purposes: (1) it clears the unexposed silver halide crystals from the film, and (2) it hardens the gelatin coating so that it can be dried without damaging the film surface. This process is known as fixation. The general guideline is that the film should be fixed for twice the development time to ensure maximum hardening of the emulsion. (Note: A radiograph can be viewed briefly after it has been in the fix for 1 minute and then returned after evaluation.)

The fix solution consists of clearing or fixing agents, preservatives, hardeners, acidifiers, buffers, and a solvent.

The Wash Bath

The wash portion of the development procedure is vital to a quality radiograph. Unfortunately, the value of the wash procedure is often underestimated and inadequately performed. The purpose of the wash is to remove the processing chemicals from the film surface. If a film is not washed properly for a long enough period, the image will eventually discolor and fade.

Films should be washed in circulating water so that both surfaces of the film receive fresh water continuously. In manual processing, the average suggested wash time is 20 to 30 minutes with periodic agitation or water circulation. In automatic processing, the water system of the processor keeps a constant flow of temperate water through and around the wash rack and film.

Wetting Agent

A common problem of drip-drying films is the possibility of water spots or other drying streaks. The drying process can be hastened and some artifacts avoided by using a wetting agent bath known as a surface-tension reducing agent (a detergent). These agents are commercially available.

Solution Replenisher

In manual processing, chemical depletion is a natural result of chemical carryover into adjacent tanks. As much as 60 mL of developer can be “carried” on a 14 × 17-inch piece of film into the rinse bath. Both the developer and the fixer need frequent replenishment to keep chemicals at a proper level and cover the entire film.

Replenishment solutions are available in powder and liquid concentrate form. The liquids are easier to work with because they eliminate the problem of powder settling on the countertops of the darkroom. Generally, the replenisher has a higher concentration than the original solution to maintain chemical potency.

Solution Replacement

Exhausted processing chemicals (or, more likely, oxidation/deterioration of chemicals in limited-use situations) are a primary cause of poor-quality radiographs. The developer and fixer solutions are often the last elements checked when a film has poor quality, yet exhausted chemicals are the most common cause. Chemicals that have lost their potency will produce radiographs that have increased film fog and decreased contrast and density.

Determining the need for developer and fixer replacement is based on a couple of clues. In general the developer solution turns from brown to green when it needs to be changed. The developer usually requires less changing than the fixer. Because each film brought into the fix tank brings with it a certain amount of water from the rinse tank, the fix tends to become diluted. The activity of the fix solution, however, cannot be determined by a change in color. The fixer needs to be changed when the “clearing time” is greater than 2 to 3 minutes. Clearing time refers to the amount of time it takes the fixer to clear the unexposed silver halide crystals off the film. If all the silver complexes have not been removed, the film will fog or even turn black when exposed to light. In general chemicals in hand-processing tanks should be changed every 4 to 6 weeks.

Biologic Growth

A common problem encountered in hand-processing tanks is the growth of bacteria and fungi, particularly during the warm seasons of the year. Bacterial and fungal growth can produce slime deposits that can build up in the tanks. The bacteria, fungi, and occasional algae originate from the air, personnel, or incoming water supply. If not controlled, they can cause corrosion of the metal surfaces, as well as artifacts on the films. The growth rate of the organisms is increased in stagnant water.

Biologic growth can be inhibited by good housekeeping. When the chemicals are changed and the processing tanks are drained, they should be scrubbed and quite possibly soaked with 1% chlorine bleach and water. The wash tank of automatic processors should be drained at the end of the day to reduce biologic growth. A simple filtering system can prevent organisms from entering through the water line.

Manual-Processing Procedure

The manual-processing procedure (by hand) should be standardized as much as possible. By establishing a routine and following it, mistakes made in the darkroom are less likely. Normally, the developing tanks are positioned so that the processing procedure starts at the left and ends at the right. In other words, the developing tank is on the left, the wash tank in the center, and the fix tank on the right (Fig. 7-6).

Figure 7-6 Bird’s-eye view of hand-processing tanks. The tank labeled A is the developer, B is the rinse and wash tank, and C is the fix tank.

Manual processing is not a difficult procedure, and the technique can be learned in a relatively short period.

Step 1—Preparation.

Before the film is processed manually, the chemicals should be at the proper temperature (normally 20° C [68° F]) and should be stirred. Because the chemicals are suspensions, they tend to settle to the bottom of the tanks (Fig. 7-7). The paddles used to stir should not be shared between tanks; the developer paddle should never go into the fix tank and vice versa. (Note: Even slight fixer contamination in the developer can render it useless.) At this point, the white lights should be turned off and the safelight turned on.

Figure 7-7 Stirring the chemicals before hand processing is important. The chemicals tend to settle to the bottom of the tanks. Processing without stirring the chemicals could result in unevenly developed film.

Step 2—Unloading the cassette.

Care should be taken when removing the film from the cassette. Fingernails should not be used as a tool to remove the film from the cassette corners. This technique can damage the sensitive intensifying screens. The proper method of removing the film is to open the backplate of the cassette and gently shake the top so that the film can be grasped by the corner between the thumb and the forefinger (Fig. 7-8). The x-ray film should be handled by the corners or edges only. The cassette should be closed while it is being labeled and loaded onto the film hanger. Film labeling is discussed at the end of this chapter.

Figure 7-8 Proper method of removing film from a cassette. The film should be “dumped” out of the cassette rather than “pried” out using fingers.

Step 3—Loading the film on a hanger.

A tension clip hanger is loaded by inserting the film into the bottom, stationary clips first, then rotating the hanger right side up and inserting the film into the movable spring clips (Fig. 7-9). The film should be stretched so that it is taut enough to “bounce a coin on it.” Taut mounting will prevent the film from touching adjacent films or walls in the processing tank.

Figure 7-9 Loading film on a clip film hanger.

If a channel hanger is used, it should be held in one hand while sliding the film into the channels with the other. All sides and corners of the film should be checked for correct placement in a channel. Once the film is in position, the top hinge can be closed.

Step 4—Developing the film.

The film is immersed in the developing tank, and the hanger is agitated two or three times to remove any air bubbles from the film surface (Fig. 7-10). The lid on the developer tank is replaced, and the timer is set for the appropriate development time. At this juncture, the hands should be dried and the cassette reloaded with film. Care should be taken in the reloading process. The replacement film should meet all four corners of the cassette before closing so that no portion of the film is compressed in the cassette seams.

Figure 7-10 Immerse the film into the developing tank, and agitate two or three times to remove any air bubbles that may be attached to the side of the film. The developer temperature should be 68°F.

Step 5—Rinsing the film.

When the timer sounds, the film should be removed from the developer rapidly to avoid excessive dripping back into the developer tank (Fig. 7-11). For fast drainage, the hanger should be tilted so that the chemical carryover (spent developer) goes into the rinse or stop bath. Preventing the used developer from adding volume to the developer tank assists accurate tank replenishment. The film is immersed in the rinse bath and agitated for 30 seconds.

Figure 7-11 Rinse the film for 30 seconds. Before moving the film to the fix tank, tilt the hanger to allow for faster water drainage.

Step 6—Fixing the film.

After the film has been in the rinse tank for 30 seconds, it should be drained of excess water and immersed in the fix tank (Fig. 7-12). The film is agitated two to three times to remove any air bubbles on the film surface, and the timer is set for the appropriate duration. The duration of the fixation process is usually twice the clearing time and until after the film has lost its “milky” appearance. The milky appearance refers to the unexposed silver halide crystals that remain on the film. Once the silver is removed, the image will appear clear or transparent. After the film has been in the fix for 1 minute, it may be viewed briefly to evaluate the quality of exposure and positioning. Putting the film back into the fix tank after evaluation for a total of at least 10 minutes is important to allow maximum hardening of the film surface.

Figure 7-12 Immerse the film into the fix tank, and agitate the film a few times to remove any air bubbles. The film should be fixed for twice the developing time or a minimum of 10 minutes.

Step 7—Washing the film.

The film is removed from the fix quickly so that chemical carryover (spent fixer) enters the wash tank. As with the developer, preventing carryover from entering the fix tank allows for accurate fix replenishment. The film should wash for 20 to 30 minutes (Fig. 7-13). The wash time depends on the water flow and exchange rate of the bath. The flow should have approximately eight complete changes per hour.

Figure 7-13 Wash the film for 20 to 30 minutes. If more than one film is being washed at once, provide enough space between each film to allow adequate washing.

Step 8—Optional final rinse.

If facilities permit, a wetting agent can speed the drying time and prevent water marks on the film surface. The film is briefly dipped in the wetting agent before drying (Fig. 7-14).

Figure 7-14 A wetting agent. An optional step in the hand-processing procedure is to immerse the film briefly into a final rinse known as a wetting agent. The wetting agent decreases drying time and the chance of streaking while the film is drying.

Step 9—Drying the film.

The film should be dried in a dust-free area to prevent artifacts from sticking to the wet film surface. If channel hangers are used, the films should be removed from the hangers and hung with clips on a tension wire (similar to a clothesline). Tension clip hangers can be hung on a drying rack (Fig. 7-15).

Figure 7-15 Films drying on a drying rack.

The films should be well separated and never allowed to touch each other while wet. When the films are dry, the sharp points on the corners of those processed with tension clip hangers must be trimmed before filing. Trimming the sharp points prevents scratching the emulsion of adjacent films. The films can now be inserted into the appropriately labeled envelope.

Automatic Processing

Automatic processing involves the same basic principles as manual processing: the film is developed, fixed, washed, and dried. However, automatic processing has two major advantages over the manual method: (1) It is a highly standardized procedure with consistent quality, and (2) it can produce a dry radiograph in a short time period. In a practice or clinic that has a high radiographic output, the amount of saved labor hours is remarkable. However, the cost of an automatic processor is a primary factor that precludes many veterinary practices from having this convenience. For a low-volume veterinary practice, the expense of an automatic processor may not be justified.

A darkroom is still necessary for automatic processing, except that a much smaller space is required. A counter is necessary on the dry side to unload and load the cassettes, but the wet side consists of the processor only. Because the processor has its own drying mechanism, which uses heated forced air or infrared methods, an exhaust system or extractor fan is necessary to prevent excessive heat and fume accumulation while in operation. Some automatic processors are designed to protrude through the darkroom wall so that a special exhaust system is unnecessary (Fig. 7-16). In this case the film is introduced into the processor in the darkroom, and a finished, dry film exits in the adjacent room.

Figure 7-16 Cross section of an automatic processor that is designed to protrude through the darkroom wall. The film is loaded into the processor in the darkroom, and the finished product is delivered to the adjacent room.

Metallic Replacement

The metallic replacement method of recovery removes the silver from the exhausted fix by replacing the silver in the solution with another metal. The metal is normally iron in the form of steel wool. The steel wool dissolves in the acid fix solution and physically replaces the suspended silver, thus allowing the silver metal to precipitate to the bottom of the recovery unit.

A metallic replacer unit usually consists of a cartridge loaded with steel wool. The fix is poured into a top receptacle and allowed to “trickle” through the steel wool (Fig. 7-19). The fix containing no silver can then be discarded (according to local pollution control ordinances). Beware, the acid from the fix solution can harm pipes if water flow is low.

Figure 7-19 A Vault Junior trickle silver recovery system. The exhausted fix is poured into empty tank A and “trickled” through tank B. The fix containing no silver can then be discarded.

Up to 99% of the available silver can be recovered with the metallic replacement method, but the purity of recovered silver is low. This method is relatively inexpensive and is recommended for low-volume hand-processing systems.

Electrolytic Recovery

Electrolytic recovery involves two electrodes (an anode and a cathode) placed either directly into the fix tank or into a separate holding container for the exhausted fix solution. As an electric current passes between the two electrodes, the silver is attracted to the cathode and the silver is plated (collected) on the cathode. The advantage of electrolytic recovery is that the fix solution may be reused; however, this requires much chemical analysis. This method of reclamation recovers high-purity silver but is generally less efficient than metallic replacement.

Chemical Precipitation

The chemical precipitation technique of silver recovery involves addition of more chemical compounds to precipitate the silver from the fix solution. As the chemicals are added to the fix, the silver floats to the bottom of the receptacle and forms a sludge. The sludge is then filtered, dried, and packed to be sold to a refiner.

Gold and silver refiners and dealers often purchase exhausted fix solution and old radiographs to reclaim the silver that they contain. Companies that purchase fix solutions and discarded radiographs usually are listed in the telephone directory’s Yellow Pages.

Before radiographs can be sold for reclamation, however, the veterinary practice is legally required to keep them for a specific length of time. The legal requirement for retaining radiographs is 7 years, but it is advisable to keep them until the patient is deceased.

Lead Markers

One of the simplest methods of film labeling is with lead letters and numbers that are placed directly on the cassette before exposure. The lead digits can be placed in a holder or taped directly to the cassette (Fig. 7-20). The lead absorbs the primary radiation from the x-ray beam so that the film directly under the lead is left unexposed and appears transparent. It is possible to purchase prepared holders that include the name and address of the clinic spelled out permanently in lead letters. With a permanent prepared holder, only the date and identification of the patient must be changed.

Figure 7-20 Lead letters placed in a holder designed to be placed on top of a cassette for film identification.

The disadvantages of this method of labeling are that it can be time consuming and that the small lead digits are easily lost. In addition, it limits tight collimation because the area outside the patient must be exposed to provide an image of the label.

Lead-Impregnated Tape

Another method of labeling a radiograph during exposure is with disposable lead-impregnated tape. Writing on the tape with a ballpoint pen or pencil displaces the soft lead, leaving indentations. These indentations create a difference in density that allows x-rays to penetrate to the film. The tape is then placed on a holder that has the name and address of the facility permanently attached (Fig. 7-21). The lead tape can be used to label left or right, time intervals for a series of radiographs, and markers to indicate the direction of the x-ray beam for oblique views.

Figure 7-21 Lead-impregnated tape. A, The lead is displaced by means of a pointed writing instrument, leaving indentations. B, After the information is written, the tape is adhered to the cassette and exposed during the radiographic procedure.

The lead-impregnated tape is available in 50- or 100-foot rolls or in precut 3-inch strips. The manufacturer of the tape will usually supply the lead-tape holder with the specified facility information.

Photoimprinting Label System

One label system uses a lead blocker placed on the outside of the cassette, an identification card, and a photimprinter. The lead blocker prevents exposure to a 3 × 7 cm area, to which identification can be exposed in the darkroom. Following the removal of the film in the darkroom, but before processing, a typed or written card is placed between the unexposed portion of the film and a light source (photoimprinter). The light is “flashed,” and the written information on the card is recorded on the previously unexposed area (Fig. 7-22).

Figure 7-22 Film identification method known as photoimprinting. A, The identification card is placed on the photoimprinter with the film placed over the card. The imprinter is then closed, and a light is “flashed” under the card, which exposes the information onto the film. B, After the film is processed, the identification can be clearly seen in the corner of the film.

New cassettes can be purchased with the lead blocker already installed, or 3 × 7 cm pieces of lead can be purchased and installed on the cassette face. It is important that the lead blockers be placed in the same corner of all the cassettes. This consistency will prevent “flashing” the wrong corner in the darkroom. Caution should also be taken to ensure that no anatomic area of interest is positioned in the blocker area, where no exposure will be made.

Miscellaneous Markers

Right (R) and left (L) markers are essential to identify a right or left limb or to identify the right or left side of the thorax or abdomen (Fig. 7-23). Markers may also be necessary at times to identify a unique view, orientation, or beam direction. Labeling the front and rear limbs may be necessary, particularly in equine radiography, because the anatomic structures of the distal limbs of the horse are virtually identical. Time-sequence labels are also available for special procedures to indicate time elapsed after the administration of radiopaque contrast media.

Figure 7-23 Left (L) and right (R) markers for anatomic orientation.

Other Identification Methods

If identification was forgotten before exposure, it is possible to write on the film before development with a pencil or other pointed device. The pressure from the pointed device will distort the film emulsion and record the information, which will remain on the film throughout the processing procedure. Scratching the information into the emulsion while it is still wet is also possible. Other methods include a permanent marker or a piece of adhesive tape on the dry film.

All of these identification techniques are considered temporary and are undesirable for routine film identification. They are not considered adequate markings should legal proceedings arise regarding the patient.