Presentation on theme: "Chapter 20 Film Processing. The primary purpose of radiographic processing is to deposit enough black metallic silver at the latent image sites to permit."— Presentation transcript:
Chapter 20 Film Processing
The primary purpose of radiographic processing is to deposit enough black metallic silver at the latent image sites to permit a permanent visible image to form. Four steps: –Developing –Fixing –Washing –Drying
Developing Silver is deposited at the latent image sites and an image becomes visible. –The deposition of silver amplifies the density of the image. The action of the developer is controlled by the immersion time, solution temperature and chemical activity. Primary agents of the developer: –Reducing agents –Activator –Restrainer –Preservative –Hardener –Water- solvent
Developer: Reducing agents Phenidone (P developer): quickly reduces silver, enhancing fine detail and subtle shades of gray and works only in areas of light exposure. Hydroquinone: slowly reduces silver and produces heavy density. Superadditivity: when two agents are combined, ability greater than the sum of independent ability.
Developer: Reducing agents The process of reducing agents giving up electrons to neutralize the positive silver ions at the sensitivity speck to become black metallic silver. –Provide electrons to the silver ions attached to the sensitivity specks of the silver halide crystals (the latent image). Saturated speck opens gate to allow electrons in When silver obtains electron converted into black metallic silver Negative exterior of crystal prohibits the reducing agent from supplying electrons to the silver ions because the bromine and iodine repel electrons. More exposure more silver saturation larger gate faster reduction more black metallic silver. –Produces a film with varying degrees of blackness.
Developer: Reducing agents Chemical fog: the effect on the film when unexposed silver halides are reduced –Reducing agents permitted to work too long or –Too fast due to high temperature
Developer: Activator / Restrainer Activator –Sodium carbonate Maintains developer in alkaline solution. Assists the reducers in reaching the silver halides by causing the gelatin to swell and become more permeable. Restrainer –Potassium bromide Restrict the reducing agents action to those crystals with sensitivity speck gates. –Permits overactive reducers to attack it, instead of unexposed silver halides. Antifogging agent
Developer: Preservative / Hardener Preservative –Sodium sulfite Decrease oxidation of the reducing agents Hardener –Glutaraldehyde Controls the swelling of the gelatin to prevent scratches and abrasions to the emulsion during processing Maintains uniform film thickness to assist in transport though an automatic processor
Developer: Contamination As little as 0.1 percent fixer from adjacent tank into the developer will destroy the ability of the reducing agents. Films appear gray- extremely low contrast.
Fixing Removes undeveloped silver halides from the emulsion to permanently fix the image before exposure to light for viewing. Primary agent: clearing agent
Fixing: Clearing Agent Clearing agent: –Ammonium thiosulfate Bonds with the unexposed silver halides and removes them from the emulsion. Uses silver in the emulsion to form ammonium thiosilversulfate. When fixer is given insufficient time to remove unexposed silver halides- milky appearance appears Clearing time: twice the time necessary for the milky appearance present) to disappear.
Fixing: Activator / Preservative Activator –Acetic acid Provides acidic pH to enhance functioning of clearing agent Stops reduction (reducing agents function in alkaline solution) Preservative –Sodium sulfite- same as developer Dissolves silver from the ammonium thiosilversulfate. Maintains pH
Fixing: Hardener Hardener –Potassium alum Controls the swelling of the gelatin to prevent scratches and abrasions to the emulsion during processing. Maintains uniform film thickness to assist in transport though an automatic processor. Insufficient hardener will cause films to exit the processor with moist softened surfaces.
Archiving: Washing / Drying Archiving prepares the film for long term storage as a medical record by protecting it from deterioration by chemical, fading, and physical forces. Washing: uses water to remove developing and fixing chemicals. Drying: uses hot air to evaporate water and harden/seal emulsion.
Automatic Processing: Transport System Designed to move a film through the developer, fixer, wash and dryer sections of the processor. Controls the length of time the radiograph is immersed in each of the solutions and agitates the chemistry to ensure maximum reaction. Subsystems: –Transport racks –Crossover networks –Drive system
Automatic Processing: Transport System Transport racks: move the film down into and up out of solution tanks Crossover networks: turn the film down into the next tank. –Entrance rollers: designed to start the film traveling from the feed tray down into the developer section –Films should always be fed with the short axis along the feed tray guide. Drive system: turn rollers –Speed controls the time the film is immersed in each chemical.
Automatic Processing: Dryer System Begins with a series of squeegee crossover rollers removing excess wash water from the surface of the film Then driven between hot air tubes –Film emulsion shrinks and seals dry
Automatic Processing: Replenishment System Replaces chemicals that are depleted through the chemical reactions of processing, oxidation, and evaporation. Starter solution: acetic acid and potassium bromide added to the replenisher solution when starting a fresh tank of developer. –Potassium bromide needed to develop films properly Initial source is from emulsion on films. A newly mixed tank does not have, must be added.
Automatic Processing: Replenishment System Volume replenishment- high volume units –Activated when films enter the processor Flood replenishment- low volume units –Automatically floods the developer and fixer tanks with replenisher solutions at a regular interval regardless of the number of films processed
Automatic Processing: Circulation System Designed to stabilize temperatures, agitate solutions, mix the chemistry, and filter the solutions. Constant mild agitation is required so the chemicals will enter and exit the emulsion. Fresh chemicals added by replenishment systems need to be agitated to avoid overdevelopment, underdevelopment, and under fixation.
Automatic Processing: Temperature Control System Heat exchanger: uses a thermostat to heat the developer, developer then routed through circulation coil in the bottom of fixer and wash tanks. Developer temperature critical, change of 0.5* is noticable.
Darkroom Safelights: film is designed to be insensitive to orange-red light. Amount of this light controlled by type of filter, wattage of light source, distance from working surface Entrance: single door, double interlocking doors, revolving doors and light proof mazes. Pass box: light proof container set in darkroom wall for delivery of film cassettes. Ventilation: to vent hazardous fumes
Silver Recovery Systems Operate by providing electrons that can be used by the silver in the fixer solution to form black metallic silver. –Metallic Replacement: fixer acid breaks down the iron in a steel screen or in steel wool and displaces it with silver. Iron oxide gives up electrons for silver Low volume situations –Electrolytic: passes a current from a cathode to an anode through the fixer, the ionized silver is attracted to the negatively charged cathode. Moderate to high volumes –Chemical Precipitation: uses chemicals to break down in the fixer and release electrons, the silver is heavy and falls to the bottom of the tank. –Resin: uses acid to form resin ions, the silver is attracted to the resin, the resin is processed to remove the silver.