Film-Screen Image Processing & Analysis

• The text discusses film-screen image acquisition, processing, and image analysis.

Processing the Latent Image

• Latent Image: An invisible image induced in the silver halide crystal.
• Manifest Image: The image that appears on film after processing.
• The purpose of processing is to change silver halide crystals in the film emulsion into black metallic silver after X-ray or light photon exposure.
• Processing develops the latent image (invisible until processed) into a visible image.
• Improper or careless processing can negatively affect the quality.

Pako Company

• Pako Company was the first to introduce the automatic processor in 1942.
• The processor could handle 120 films per hour.
• The cycle time for processing one film was about 40 minutes.

Eastman Kodak Company

• In 1956, Eastman Kodak Company introduced the first roller transport system for processing medical radiographs with a 6-minute processing time.
• In 1965, Eastman Kodak introduced a 90-second rapid processing system.

Processing Steps

• Wetting: The first step involves swelling the emulsion to allow chemical baths to reach all parts of the emulsion uniformly.
• In automatic processing, the wetting step is omitted because the wetting agent is added during the second step, developing.
• Developing: Converts the latent image into a visible image.
• Rinsing: The film is rinsed in an acid solution after developing to halt the developing process and eliminate excess chemicals from the emulsion and is called the stop bath by photographers.
• Fixing: This clears unexposed silver halide crystals from the emulsion and hardens the emulsion to preserve the image, also increasing its structural soundness.
• Washing: This removes any leftover chemicals from the previous processing steps.
• Drying: This final step removes the water used to wash the film and make it suitable for handling and viewing.

Processing Chemistry

• The chemicals that are used to process films are designed to penetrate to the emulsion, which causes and effect.
• Radiographs must first be treated with a wetting agent to allow the chemicals to penetrate the emulsion.
• Water is used as the most common wetting agent which causes the gelatin to swell.
• In automatic processing, the wetting agent is in the developer.
• Development is a chemical process that amplifies the latent image which changes the silver ions of exposed crystals into metallic silver.
• The developer supplies electrons to change the silver ions to silver, which must be supplied to the silver ion for the ionic silver to change to metallic silver.

Chemicals

• Reducing agents: reduce exposed silver halide to black metallic silver.
• Hydroquinone: acts rather slowly but is responsible for the very blackest shades.
• Phenidone: acts rapidly and influences the lighter shades of gray.
• Synergism occurs when the action of two agents working together is greater than the sum of the action of each agent working independently, for example, Phenidone and Hydroquinone. Buffering Agents
• Enhance the action of the developing agent by controlling the concentration of hydrogen ions very corrosive and can cause a skin burn
• Sodium carbonate and Sodium hydroxide (Lye)

Restrainers

• restrict the action of the developing agent to only those silver halide crystals that have been irradiated will be reduced
• Potassium bromide and potassium iodide

Preservative

• control the oxidation of the developing agent by air (aerial oxidation )
• Sodium sulfite
• Hardener - controls swelling and softening of the emulsion.
• When films that drop from the processor are damp, the usual cause is depletion of the hardener.
• Lack of sufficient glutaraldehyde may be the biggest cause of problems with automatic processing.
• Glutaraldehyde

Sequestering agents

• Agents form stable complexes with these metallic ions and salts
• Chelates
• Film development is basically a chemical reaction
• Unexposed crystals are unaffected
• Film development is governed by three physical characteristics: time, temperature, and concentration of the developer.
• Long development time and high developer temperature increases reduction of the silver in each grain and promotes the development of the total number of grains.
• An increase in any of these factors beyond manufacturer recommendations will increase development fog.
• Fog also can be produced by chemical contamination of the developer (chemical fog), unintentional exposure to radiation (radiation fog), or improper storage at an elevated temperature and humidity.

FIXING

• Fixing gives archival quality.
• Archival quality refers to the permanence of the radiograph whereas the image does not deteriorate with age, remaining in its original state.
• When the film is removed from the developer, developer can be trapped in the emulsion and continue its reducing action
• If development is not stopped, development fog results. (Chemical Fog)

CHEMICALS

• Stop bath - to neutralize the residual developer in the emulsion and stop its action.
• In automatic processing, a stop bath is not used because the rollers of the transport system squeeze the film clean.
• Acetic acid Activator
• Activator neutralizes the pH of the emulsion and stops developer action.
• Acetic acid Fixing agent / Clearing agent
• Fixing agents remove unexposed and undeveloped silver halide crystals from the emulsion.
• Sodium thiosulfate is classically known as hypo, but ammonium thiosulfate is the fixing agent that often sees use in most fixer chemistries.
• Hypo retention is the term used to describe the undesirable retention of the fixer in the emulsion.
• Excess hypo slowly oxidizes and causes the image to discolor to brown over a long time.
• Fixing agents retained in the emulsion combine with silver to form silver sulfide, which appears yellow-brown.
• Silver sulfide stain is the most common cause of poor archival quality.
• Hardener - accelerates this shrinking process and causes the emulsion to become more rigid or hardened.
• Ensure that the film is transported properly through the wash-and-dry section and that rapid and complete drying occurs.
• Potassium alum, aluminum chloride, and chromium alum.
• Preservative has the same composition and serves the same purpose as the preservative in the developer.
• Preservative is needed to maintain the chemical balance because of the carryover of developer and fixer from one tank to another.
• Sodium sulfite

Buffer agents

• The buffer agents maintains the alkalinity and acidity—the pH—of the fixer
• Acetate components

WASHING

• Wash away any residual chemicals remaining in the emulsion, particularly hypo that clings to the surface of the film.
• Water is used as the wash agent.
• In automatic processing, the temperature of the wash water should be maintained at approximately 3°C (5°F) below the developer temperature.
• Inadequate washing leads to excessive hypo retention and the production of an image that will fade, turn brown with time, and leads to poor archival quality.

DRYING

• Dry is the Final step in processing
• Warm dry air is blown over both surfaces of the film as it is transported through the drying chamber.

AUTOMATIC PROCESSING

• The principal components of an automatic processor are the:
• Transport system
• Temperature control system
• Circulation system
• Replenishment system
• Dryer system

Transport System

• Transport begins at the feed tray
• Entrance rollers grip the film to begin its trip through the processor
• Microswitch is engaged to control the replenishment rate of the processing chemicals.
• Feed the film evenly using the side rails of the feed tray and alternate sides from film to film
• From the entrance rollers, the film is transported by rollers and racks through the wet chemistry tanks and the drying chamber and is finally deposited in the receiving bin.
• The shorter dimension of the film should always be against the side rail, so the proper replenishment rate is maintained.
• Controls processing by controlling the time the film is immersed in each wet chemical.
• The transport system consists of the three principal subsystems: rollers, transport racks, and a drive motor
• A master roller, with a diameter of 3 inches makes the film turn in the processor
• A number of planetary rollers and metal or plastic guide shoes are usually positioned around the master roller
• Guide shoes a curved metal lip with smooth grooves guides the film around the bend.
• Crossover rack - The crossover rack is a smaller rack assembly that is composed of rollers and guide shoes.

Temperature Control System

• The developer temperature is most critical, and it is usually maintained at 35°C (95°F).
• Wash water is maintained at 3°C (5°F) lower.

Filtration of Circulation System

• The Circulation System Continuously pumps the developer and the fixer, thus maintaining constant agitation.
• Agitation continually mix the processing chemicals, maintains a constant temperature throughout the processing tank, and aid exposure of the emulsion to the chemicals

Developer filtration

• Flecks of gelatin
• Cleaning the tanks and the transport system should be a part of the routine maintenance of any processor
• Fresh tap water is piped into the tank at the bottom and overflows out the top
• The minimum flow rate for the wash tank in most processors is 12 L/min (3 gal/min).

Replenishment System

• Meters the proper quantities of chemicals into each tank to maintain volume and chemical activity.
• Replenishment rates are approximately 60 to 70 mL of developer and 100 to 110 mL of fixer for every 35 cm (14 in) of film.

Types

• Volume replenishment
• Most common type
• Controlled by the size of the film (microswitch)
• Chemicals are replenished for each size of the film Flood replenishment (timed or standby replenishment)
• Processors that are not in constant use
• Controlled by a timer

Dryer System

• The Dryer System Consists of a Blower ventilation ducts, drying tubes, and exhaust system
• All extracts all residual moisture from the processed radiograph
• Heating Coils - 2500 W capacity
• Most processing faults leading to damp film are because of depletion of glutaraldehyde, the hardener in the developer.