Radiographic film


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Radiographic film

  1. 1. History  By the time x-rays were discovered, photography was already an art.  Photographic film with a nitrocellulose base was already being marketed by George Eastman.
  2. 2.  The first x-rays were recorded on glass plates.  These were coated with emulsion on one side only.  The exposure dose was quite high.
  3. 3.  Glass plates were used until World war I.  During WWI, nitrocellulose based film was found to be a more feasible choice for recording x-rays.  This film was single-emulsion.
  4. 4.  It was later discovered that double-emulsion responded to x-rays faster.  The flaw with nitrocellulose based film was its easy flammability.  In 1924, cellulose acetate replaced the nitrocellulose based film.
  5. 5. The Radiographic Film  A Radiographic film is similar in construction and characteristics to a photographic film.  Its spectral response is different from photographic film but its mechanism of operation is the same.
  6. 6.  The film is sandwiched between the radiographic intensifying screens in a protective cassette.  The intensifying screens change the x-rays into visible light. The visible light exposes the radiographic film.
  7. 7. Film Structure Radiographic Film has two basic parts.  Base  Emulsion Most films have two layers of emulsion so these are referred as Double Emulsion Film.
  8. 8. An adhesive layer attaches the emulsion to the base. The emulsion is enclosed in a protective layer of gelatin called the overcoat.
  9. 9. Transparent Base  It is the foundation of the film. 150 to 300 µm thick.
  10. 10. Transparent Base  Provides a surface and support for the emulsion.  It must have strength, but it should be flexible.  Most film bases are composed of polyester.
  11. 11.  Polyester can withstand higher temperatures and is more fireproof.  The base is usually tinted blue to reduce light glare.  Amount of tint varies per specifications of different manufacturers.
  12. 12. Emulsion  The emulsion is the heart of the film.  The x-rays or light from the intensifying screens interact with the emulsion and transfer information to the film.  A homogeneous mixture of gelatin and silver halide crystals and is about 3 to 5 µm thick.
  13. 13. Gelatin  The gelatin is clear so it transmits the light to the silver halide crystals.  It is porous so the processing chemicals can easily penetrate to the silver halide crystals.  The primary function of the gelatin is to provide a support medium for the silver halide crystals by holding them in place.
  14. 14. Silver Halide Crystals  98% Silver Bromide  2% Silver Iodide  May be tabular,cubic,octahedr al,polygonal in shape.  Tabular shape used most commonly for general radiography.  About 0.1µm thick and 1µm in diameter.
  15. 15. Silver Halide Crystals  The differences in speed, contrast and resolution depends upon the process by which the silver halide crystals are manufactured.  From the time the emulsion ingredients are brought together until the film is packaged, the whole process occurs in complete darkness.
  16. 16. The Latent Image  Invisible image produced on the film after exposure prior to development.  The latent image is the invisible change in the silver halide crystals.  The interaction between the photons and the silver halide crystals produces the latent image.
  17. 17. The Latent Image Formation  This interaction is sometimes referred to as the photographic effect.  This process is not well understood and is still under research.  The Gurney-Mott theory is presently an acceptable explanation of the photographic effect.
  18. 18.  Ionic structure of silver halide crystal  Bromide and iodide ions are mostly concentrated on the surface of crystal giving it a negative charge.  The silver ions are inside known as interstitial silver ions,so inside is positively charged.
  19. 19. The Latent Image Formation A Radiation interaction releases electrons. B Electrons migrate to the sensitivity center(contaminant in the silver halide crystal,usually silver sulfide).
  20. 20. C At the sensitivity centre, atomic silver is formed by attracting an interstitial silver ion--- latent image centre.
  21. 21. D The process is repeated many times resulting in the build up of silver atoms. E The remaining silver halide is converted to silver during processing.
  22. 22. F The resulting silver grain is formed. Silver halide that is not irradiated remain inactive. The irradiated and non-irradiated silver halide produces the latent image.
  23. 23. Types of Films  Screen films most commonly used.  Screen film used with intensifying screens.  Single emulsion- emulsion on one side of base.  Double emulsion used with two screens.  Direct exposure film or non-screen film.  Special purpose films
  24. 24. Standard screen-film sizes English Units SI Units 7 x 7 in 18 x 18 cm 8 x 10 in 20 x 25 cm 10 x 12 in 24 x 30 cm 14 x 14 in 35 x 35 cm 14 x 17 in 35 x 43 cm
  25. 25. Screen Film Factors  Main factors to be considered when selecting film a) Contrast & Speed b) Crossover c) Spectral matching d) Reciprocity Law e) Safelights
  26. 26. Contrast  Contrast of a film depends on its latitude.  Latitude is the range of exposure techniques that produce an acceptable image.  Latitude is inversely proportional to contrast.
  27. 27.  High contrast film has low latitude  Medium contrast film has medium latitude  Low contrast film has high latitude  High contrast has small uniform grains  Low contrast has larger grains and wide range in size.
  28. 28. Speed  It is the sensitivity of film to x-rays and light.  The size and shape of the silver halide crystals are the main factors that determine speed.  Faster speed films are almost always double emulsion.  Light spectrum from screens must match to achieve optimum speed.
  29. 29. Crossover  Crossover is the exposure of an emulsion by light from the opposite radiographic intensifying screen.
  30. 30.  Reducing crossover by adding a dye to the base
  31. 31.  Crossover causes blurring of the image.  Can be reduced by  Tabular grains---flat, large surface area to vol ratio  Addition of a light absorbing dye in crossover control layer.
  32. 32. Spectral Matching  The most important consideration in selecting screen film is spectral absorption matching.  The material in the screen will determine the color of light emitted by the screen.  Special dyes in the film are used to match the screen to the film.
  33. 33. Spectral Matching  Calcium Tungstate screens emit blue and blue violet light.  Replaced by Rare earth screens.  Rare earth screens emit ultraviolet, blue, green and red light.
  34. 34. Spectral Matching  If the light spectrum does not match, there will be a significant loss of speed alongwith increased patient dose.
  35. 35. Reciprocity Law  In radiography, it is generally assumed that the total exposure of a film depends only on the total quantity of radiation (mAs) and not on the exposure time. This is known as the reciprocity law.  Reciprocity law Exposure=intensity x time =Constant Optical Density
  36. 36. Reciprocity Law  The reciprocity law is true for film exposed directly to x-rays.  It fails when film is exposed to light from radiographic intensifying screens.
  37. 37.  Reciprocity law failure is important when the exposure times are very long (as in mammography)or very short (angiography).  The result is a loss of speed. .
  38. 38. Safelights  Working with film in the darkroom requires special lighting to avoid exposure of the film.  Filters are used to avoid exposure of the film.
  39. 39. Safelights  An amber filter can be used for blue sensitive film only.  A red filter is used for blue-green sensitive film.  The color is not the only concern, the wattage of the bulb and distance between the lamp and work surface is also very important.
  40. 40. Special Film Types  Direct exposure film: used without intensifying screen.  were used for small body parts.  Requires 10 to 100 times more exposure. The emulsion is thicker than screen film.  Renders excellent detail. No longer used.
  41. 41.  Single emulsion film: once used for extremities but now most extremity cassettes are double screen type.  Again required more exposure.
  42. 42.  Mammography Film: Only single emulsion film currently used in modern radiography.  Laser Film: Used in with a laser printer for digital radiography, CT and MRI.
  43. 43.  Subtraction Film: used in angiography to do subtraction where the bone is removed for better visualization of the arteries.
  44. 44.  Spot film: Special roll film of 70 to 105 mm width used in fluoroscopy.  Can be processed in x-ray film processor.
  45. 45.  Cine film:  35 mm black & white film supplied in rolls of 100 and 500 ft  used in coronary angiography.  Requires motion picture film processor.
  46. 46. Handling and Storage of Radiographic Film  X-ray film is a sensitive radiation detector and it must be handled in an area free of radiation.  Film storage must be shielded.  The darkroom adjacent to the x-ray room must be shielded.
  47. 47.  Improper handling of the film will result in poor image quality due to artifacts.  Avoid bending, creasing or rough handling of the film.  Avoid sharp objects contacting the film.
  48. 48.  Hands must be clean and dry.  Avoid hand creams, lotions or water free hand cleaners.  Static electricity or a dirty processor can cause artifacts.
  49. 49.  Film is sensitive to heat and humidity.  Heat and humidity causes fog or a loss of contrast.  Film should be stored at less than 20º C (68ºF)  Humidity should be between 40% and 60%.
  50. 50.  Film must be handled and stored in the dark.  Low level diffuse light causes fog.  Bright light causes gross exposure.  Luminous watches, cell phone and darkroom light leaks should be avoided.
  51. 51.  Films should be used no longer than the stated Shelf life.  The oldest film in stock should always be used first.  Expired film results in loss of speed and contrast and an increase in fog.