Q. 2. Describe the composition of developing and fixing solution and their functions.
Write in detail the composition and actions of developer and fixer used in dental radiography.
Ans. Developer and fixer are two special chemical solutions that are necessary for film processing.
The developer solution contains the following five basic chemicals and other ingredients as follows:
i. Developing agent
vi. Fungicide, buffers, and solvent.
• The developing agent is also known as the reducing agent, which contains the following chemicals:
a. Hydroquinone (para-dihydroxybenzene)
b. Elon or Metol (monomethyl-para-amino phenol sulphate)
c. Metol/phenindione (l-phenyl-3-pyrazolidone).
• The purpose of the developing agent is to reduce the exposed silver halide crystals chemically to black metallic silver.
• Hydroquinone is a benzene derivative and acts at slowrate but generates the black tones and the sharp contrast of the radiographic image. Hydroquinone is temperature sensitive; it is inactive below 60°F and very active above 80°F. Films are best developed at 70°F for 5 min.
• Elon is the product of aniline dyes and acts quickly to produce a visible radiographic image. It helps to develop shadow areas or shades of grey on the film and brings detail. It is less sensitive to temperature changes and generates grey tones in the image.
• Metal phenindione: It is a by-product of aniline dyes, works at faster rate but gives a low contrast. It is an efficient activator for hydroquinone at a very low concentration and works at lower alkalinity. It is more commonly used in automatic processor.
• The image produced will have shades of grey if only Elon is used; if only hydroquinone is used, the image will be black and white and if a combination is used, image will have all black, white, and grey shades.
• Sodium sulphite is used as preservative. As it has great affinity for oxygen, it prevents oxidation of developer solution and forms sulphonates, when combined with oxygen.
• Sodium carbonate is used as an activator. It provides alkaline medium usually above a pH of 11, which is required for hydroquinone to act, and it also softens the gelatin of the emulsion.
• It causes the emulsion to swell, makes the penetration of developing agent, and diffusion of the reaction product out, easier. This component of developer makes it soapy to touch.
• Other activators used are sodium hydroxide, sodium metaborate, and sodium tetraborate.
• Excessive alkalinity causes rapid reduction even of the unexposed silver bromide crystals and produces fog.
• Potassium bromide or benzothiazole is used as the restrainer. It prevents chemical fog, which is also called developmental fog.
• The added bromide serves to depress the reduction of the unexposed crystals and hence acts as an antifog agent and it restricts the action of the developing agent only to those silver halide crystals that are irradiated.
• Glutaraldehyde is added as a hardener, particularly in automatic processing to prevent emulsion from softening and sticking to the rollers.
• It is added to prevent bacterial growth.
• These are added to maintain the pH of developer.
• Distilled water is used as the solvent and as a medium in which the chemicals can react with the silver bromide of the emulsion.
The alkaline developer solution should be concentrated as recommended by the manufacturer’s instructions. Solution should not be used for more than 10–14 days irrespective of the number of films processed during that time.
Composition of developer in brief
i. Converts exposed silver halide crystals to black metallic silver. Slowly generates the black tones and contrast in the image
ii. Converts exposed silver halide crystals to black metallic silver. Quickly generates the grey tones in the image
||Prevents rapid oxidation of the developing agents
||Activates developer agents. Provides alkaline environment for developing agents. Softens gelatin of the film emulsion
||Prevents the developer from developing the unexposed silver halide crystals.
||Used in automatic processing, to prevent emulsion from softening and sticking to the rollers
||Prevents bacterial growth
||It dissolves chemicals
Composition of fixer
The function of fixer is to remove the undeveloped silver halide crystals from the emulsion and harden the emulsion.
The fixer solution contains four basic ingredients:
i. Fixing agent
iii. Hardening agent, and
• The fixing agent is also known as the clearing agent and is made up of sodium thiosulphate (hypo) or ammonium thiosulphate.
• Its purpose is to remove or clear all unexposed and undeveloped silver halide crystals from the film emulsion allowing light to pass through the film image and permitting viewing of the radiographic image on a view box.
• This chemical “clears” the film so that the black image produced by the developer becomes readily distinguished.
• Sodium sulphite, the same preservative as in the developer solution, is also used in the fixer solution.
• The purpose of the preservative is to prevent the chemical deterioration of the fixing agent.
• It also helps to clear the film by binding with any oxidized developer, which is carried to the fixing solution.
• Potassium alum, aluminium chloride etc. are used as the hardening agent in the fixer solution.
• It hardens and shrinks the gelatin in the film emulsion to prevent its oxidation and protects it against the scratches.
• It also shortens the drying time. It reduces the swelling of the emulsion during the final wash resulting in less mechanical damage to the emulsion, hence limiting water absorption.
• It also neutralizes any contaminating alkali from the developer.
• The acidifier used in the fixer solution is acetic acid or sulphuric acid. Its purpose is to neutralize the alkaline developer.
• It provides necessary acidic medium for diffusion of the thiosulphate into the emulsion.
• The acidifier also produces the necessary acidic environment required by the fixing agent.
||Sodium thiosulphate; ammonium thiosulphate
||Removes all unexposed undeveloped silver halide crystals from the emulsion
||Prevents deterioration of fixing agent
||Shrinks and hardens the gelatin in the emulsion
||Acetic acid; sulphuric acid
||Neutralizes the alkaline developer and stops further development
||It dissolves chemicals
Q. 3. Describe the processing of X-ray film.
Ans. Processing is the term used to describe the sequence of events required to convert the invisible latent image contained in the sensitized emulsion into the visible permanent radiographic image.
The purpose of processing is:
• Visible image formation and
• Preservation of image permanently for later correspondence.
Types of processing
There are basically two types of processing methods. They are:
i. Manual processing
a. Visual method
b. Time–temperature method
ii. Automatic processing.
i. Manual processing
a. Visual method:
• The visual method of manual processing is carried out in a darkroom with safelighting conditions.
• In this method, an exposed X-ray film is immersed in the developing solution and periodically viewed under the safelight for the emergence of a clear image.
• When the image appears, the film is washed and immersed in the fixing solution.
b. Time–temperature method:
• Time–temperature method is a type of manual processing method in which effective standardization may be achieved without any automatic aids.
• It is a simple technique of immersing the film in the developer kept at a constant temperature for a fixed duration of time.
The time–temperature chart is as follows:
• The advantage of manual processing is that the action of development is under the direct control of the operator.
• Disadvantages: Handling wet film, the requirement of a darkroom, and time consuming.
ii. Automatic processing
• In automatic processing machines, the exposed film is fed at one end and it passes successively through the developer, fixer, water, and drier.
• The roller system has a squeezing action; the developing solution absorbed by the gelatin of the emulsion will be less as it is transported from the developer to the fixer.
• The automatic processing machines make use of roller system for the transport of film. The film comes out through the other end of the processor, processed, dry, and ready for viewing.
The steps involved in manual processing of X-ray film consists of:
i. Developing the film
ii. Rinsing in water
iii. Fixing of film
iv. Washing film in running water
v. Drying and mounting of film.
i. Developing the film
• The exposed film is immersed in the developing solution until the image emerges.
• Depending on the exposure time of film and concentration of the developing solution, the time taken for development ranges from a few seconds to few minutes.
ii. Rinsing in water
• After developing the film, it is rinsed in water for 15 to 20 s before placing in the fixer. This slows down the development process and removes any alkali of the developing solution before placing in acidic fixer.
iii. Fixing of film
• Film is placed in the fixer for about 8–10 min.
• The action of the fixing solution is to remove the unexposed silver halide crystals and harden the emulsion.
• Too long fixing time can cause film fog and loss of proper contrast.
iv. Washing of film
• The film should be washed thoroughly for sufficient length of time in running water to remove residual fixing solution.
• If the silver compounds are not removed, there can be stains on the film. Discolouration of the image can also result due to the presence of thiosulphate and its products.
v. Drying and mounting of film
• The last step in the processing is drying the film and mounting for viewing.
• The film should be dried in a relatively dust-free environment.
• Commercially, driers are available for drying the film.
• Drying a film is very important as sometimes the water marks can result in artefacts. The processed films should be properly identified, mounted, and then viewed under transillumination.
Q. 4. Describe the darkroom chemistry.
Ans. The main function of a darkroom is to provide a completely darkened environment where X-ray film can be handled and processed to produce diagnostic radiographs in an efficient, precise, and standardized procedure.
• The darkroom should contain the following infrastructure:
ii. Visible light source (tube lights)
iii. Working area to load extraoral cassettes
iv. Processing tanks
v. Thermometer and stop clock
vii. Storage facility for unexposed films
viii. Exhaust and appropriate ventilation.
Requirements of a darkroom
The darkroom must be properly designed and well equipped. A well-planned darkroom makes processing easier. An ideal darkroom must have the following characteristics:
i. Convenient location
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