Chapter 20

Film Processing

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.