Radiographic film


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Radiographic film

  1. 1. Radiographic Film <ul><li>Remnant Radiation: the x-rays that interact with the x-ray film. </li></ul><ul><li>Few of the original x-rays actually make the image. The remnant radiation is the image forming radiation that passes completely through the patient. </li></ul>
  2. 2. Types of X-ray <ul><li>A- X-rays scatter by Compton interactions </li></ul><ul><li>B- x-rays absorbed by photoelectric absorption </li></ul><ul><li>C- X-rays that exit the patient without interaction. </li></ul>
  3. 3. Remnant Radiation <ul><li>The beam started as a relatively uniform intensity as it exited the tube. </li></ul><ul><li>Upon striking the patient, the beam is attenuated by the patient; some were absorbed, others are scattered. </li></ul><ul><li>Those that actually hit the film are referred to as the useful or remnant radiation . </li></ul>
  4. 4. Remnant Radiation <ul><li>The remnant radiation consists of x-rays scattered away from the receptor and the useful beam. </li></ul><ul><li>The film is sandwiched between radiographic intensifying screens in a protective cassette. </li></ul>
  5. 5. Intensifying Screens & Film <ul><li>The intensifying screens change the x-rays into visible light. The visible light exposes the radiographic film. </li></ul><ul><li>Radiographic film is similar in construction and characteristics to photographic film. </li></ul><ul><li>Its spectral response is different from photographic film but is mechanism of operation is the same. </li></ul>
  6. 6. Film Construction <ul><li>Radiographic Film has two basic parts. </li></ul><ul><li>Base </li></ul><ul><li>Emulsion </li></ul><ul><li>Most film has two layers of emulsion so it is referred to as Double Emulsion Film </li></ul>
  7. 7. Film Construction <ul><li>An adhesive layer attaches the emulsion to the base. </li></ul><ul><li>The emulsion is enclosed in a protective layer or overcoat. </li></ul>
  8. 8. Radiographic Film Base <ul><li>Initially x-ray were taken on glass plates. </li></ul><ul><li>In 1918 cellulose nitrate bases film replaced glass due to WWI and a shortage of glass. Cellulose Nitrate was flammable so x-ray film was a fire hazard. Several severe hospital fire were caused by the x-ray film. </li></ul>
  9. 9. Radiographic Film Base <ul><li>1920 Cellulose triacetate or safety base was introduced. Not as flammable. </li></ul><ul><li>Polyester base replaced Cellulose Triacetate in the 1960’s. Still used today. It is semi-rigid and about 150 to 300 µm thick. </li></ul>
  10. 10. Emulsion <ul><li>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 </li></ul><ul><li>The emulsion consists or a very homogeneous mixture of gelatin and silver halide crystals about 3 to 5 µm thick. </li></ul>
  11. 11. Gelatin <ul><li>The gelatin is clear so it transmits the light to the silver halide crystals. </li></ul><ul><li>It is porous so the processing chemicals can penetrate to the silver halide crystals. </li></ul><ul><li>The primary function of the gelatin is to provide a support medium for the silver halide crystals by holding them in place. </li></ul>
  12. 12. Silver Halide Crystals <ul><li>98% Silver Bromide </li></ul><ul><li>2% Silver Iodide </li></ul><ul><li>Tabular shape used most commonly for general radiography. </li></ul><ul><li>About 1µm thick for screen film exposure. </li></ul>
  13. 13. Silver Halide Crystals <ul><li>The differences in speed, contrast and resolution depend upon the process by which the silver halide crystals are manufactured and by the mixture of these crystals into the gelatin. </li></ul><ul><li>Size and concentration of crystals have a primary influence on speed. </li></ul>
  14. 14. Manufacture <ul><li>The manufacturers closely guard the mixture they use to manufacture their film. </li></ul><ul><li>Manufacture is in total darkness with protection for radiation. From the time that the emulsion ingredients are brought together until the film is packaged, no light is present. </li></ul>
  15. 15. The Latent Image <ul><li>The latent image is the invisible change in the silver halide crystals. </li></ul><ul><li>The interaction between the photons and the silver halide crystals produces the latent image or manifest image. </li></ul><ul><li>This interaction is sometimes referred to as the photographic effect. </li></ul>
  16. 16. The Latent Image <ul><li>This process is not well understood and is the subject of considerable research. </li></ul><ul><li>The following is the Gurney-Mott theory. </li></ul>
  17. 17. Producing the Latent Image <ul><li>Radiation interaction releases electrons. </li></ul><ul><li>Electrons migrate to the sensitivity center. </li></ul><ul><li>At the sensitivity center, atomic silver is formed by attracting an interstitial silver ion. </li></ul>
  18. 18. Producing the Latent Image <ul><li>The process is repeated many times resulting in the build up of silver atoms. </li></ul><ul><li>The remaining silver halide is converted to silver during processing. </li></ul>
  19. 19. Producing the Latent Image <ul><li>The resulting silver grain is formed. </li></ul><ul><li>Silver halide that is not irradiated remain inactive. The irradiated and non-irradiated silver halide produces the latent image. </li></ul>
  20. 20. Types of X-ray Film <ul><li>Two main types: </li></ul><ul><li>Screen film used with intensifying screens. </li></ul><ul><ul><li>Single emulsion- emulsion on one side of base. </li></ul></ul><ul><ul><li>Double emulsion used with two screens. </li></ul></ul><ul><li>Direct exposure film or non-screen film. </li></ul><ul><li>Special purpose: Duplication, Cine, Dental </li></ul>
  21. 21. Standard Screen-Film Sizes <ul><li>English Units </li></ul><ul><li>8” x 10” </li></ul><ul><li>10” x 12” </li></ul><ul><li>11” x 14” </li></ul><ul><li>7” x 17” </li></ul><ul><li>14” x 17” </li></ul><ul><li>14” x 36” </li></ul><ul><li>SI Units </li></ul><ul><li>20 x 25 </li></ul><ul><li>24 x 30 </li></ul><ul><li>30 x 35 </li></ul><ul><li>18 x 43 </li></ul><ul><li>35 x 43 </li></ul>
  22. 22. Screen Film Factors <ul><li>Main factors to be considered when selecting film </li></ul><ul><li>Contrast & Speed </li></ul><ul><li>Crossover </li></ul><ul><li>Spectral matching </li></ul><ul><li>Reciprocity Law </li></ul><ul><li>Safelights </li></ul>
  23. 23. Contrast <ul><li>Most manufacturers offer multiple contrast levels in their film lines. </li></ul><ul><ul><li>High contrast film has low latitude </li></ul></ul><ul><ul><li>Medium contrast has medium latitude </li></ul></ul><ul><ul><li>Low contrast has high latitude </li></ul></ul><ul><li>High contrast has small uniform grains </li></ul><ul><li>Low contrast has larger grains and wide range in size. </li></ul>
  24. 24. Speed <ul><li>The size and shape of the silver halide crystals are the main factors that determine speed. </li></ul><ul><li>Faster speed films are almost always double emulsion. </li></ul><ul><li>Light spectrum from screens must match to achieve optimum speed. </li></ul>
  25. 25. Crossover <ul><li>Crossover is the exposure of an emulsion by light from the opposite-side radiographic intensifying screen. </li></ul><ul><li>Modern tabular grain film with a dye or crossover control layer has reduced crossover. </li></ul>
  26. 26. Spectral Matching <ul><li>The most important consideration in selecting screen film is spectral absorption matching. </li></ul><ul><li>The material in the screens will determine the color of light emitted by the screens. </li></ul><ul><li>Special dyes in the film are used to match the screens to film. </li></ul>
  27. 27. Spectral Matching <ul><li>Calcium Tungstate screen emit blue and blue violet light. All film will respond to blue and violet light. </li></ul><ul><li>Rare earth screens emit blue-green light. Green sensitive film must be used. It is referred to as orthochromatic film. It will respond to blue & green light spectrums. </li></ul>
  28. 28. Spectral Matching <ul><li>If the light spectrum does not match, there will be a significant loss of speed. </li></ul><ul><li>Kodak Lanex Regular Screens are rated at 400 speed with orthochromatic film and 200 speed with blue sensitive film. </li></ul>
  29. 29. Reciprocity Law <ul><li>Exposure = Intensity x time = Constant Optical Density </li></ul><ul><li>So mA x time (s) = mAs </li></ul><ul><li>As long as the product of mA and time are the same, the optical density should be the same. Right? </li></ul><ul><li>Wrong !!! </li></ul>
  30. 30. Reciprocity Law <ul><li>There are times when the reciprocity law does not work with screen film. </li></ul><ul><li>Very short exposure times (1 ms) and long exposure times (1 second or more). </li></ul><ul><li>The result is a loss of speed. </li></ul>
  31. 31. Approximate Reciprocity Law Failure <ul><li>Exposure Time </li></ul><ul><li>1 ms </li></ul><ul><li>10 ms </li></ul><ul><li>100 ms </li></ul><ul><li>1 s </li></ul><ul><li>10 s </li></ul><ul><li>Relative Speed (%) </li></ul><ul><li>95 </li></ul><ul><li>100 </li></ul><ul><li>100 </li></ul><ul><li>90 </li></ul><ul><li>60 </li></ul>
  32. 32. Safelight <ul><li>Working with film in the darkroom requires special lighting to avoid exposure of the film. </li></ul><ul><li>Filters are used to avoid exposure of the film. </li></ul>
  33. 33. Safelights <ul><li>A red filter is used for blue-green sensitive film. </li></ul><ul><li>The color is not the only concern, the wattage of the bulb and distance from the counter top is also very important. </li></ul><ul><li>Maximum wattage is 15w. </li></ul><ul><li>Distance 60 “ from counter top. </li></ul>
  34. 34. Special Film Types <ul><li>Direct exposure: once used for small body parts measuring less than 10 cm. Requires 10 to 100 time more exposure. No longer used. </li></ul><ul><li>Single emulsion film: once used for extremities but now most extremity cassettes are double screen type. Again required more exposure. </li></ul>
  35. 35. Special Film Types <ul><li>Mammography Film: Only single emulsion film currently used in modern radiography. </li></ul><ul><li>Laser Film: Used in medical radiography with a laser printer for digital radiography, CT and MRI. Modern units are dry chemical printers. Similar to laser printers except image is printed on film. </li></ul>
  36. 36. Special Film Types <ul><li>Duplication Film: special single emulsion film used to copy x-ray films. Sensitive to UV or blue light. Never used in cassettes. </li></ul><ul><li>Subtraction Film: used in angiography to do subtraction where the bone is removed for better visualize the arteries. Not used in chiropractic. </li></ul>
  37. 37. Special Film Types <ul><li>Spotfilm : Special roll film of 70 to 105 mm width used in fluoroscopy in medical radiography. Can be processed in x-ray film processor. </li></ul><ul><li>Cine film : 16 mm or 135 mm black & white film used in coronary angiography. Requires motion picture film processor. </li></ul>
  38. 38. Handling and Storage of Radiographic Film <ul><li>X-ray film is a sensitive radiation detector and it must be handled in an area free of radiation. </li></ul><ul><ul><li>Film storage must be shielded. </li></ul></ul><ul><ul><li>The darkroom adjacent to the x-ray room must be shielded. </li></ul></ul><ul><ul><li>If film use is low more shielding may be required. </li></ul></ul>
  39. 39. Handling and Storage of Radiographic Film <ul><li>Improper handling of the film will result in poor image quality due to artifacts. </li></ul><ul><ul><li>Avoid bending, creasing or otherwise rough handling the film. Avoid sharp objects contacting the film. </li></ul></ul><ul><ul><li>Hands must be clean and dry. </li></ul></ul><ul><ul><li>Avoid hand creams, lotions or water free hand cleaners. </li></ul></ul><ul><ul><li>Static electricity or a dirty processor can cause artifacts. </li></ul></ul><ul><li>Artifacts must be avoided. </li></ul>
  40. 40. Handling and Storage of Radiographic Film <ul><li>Heat and Humidity must be controlled. Film is sensitive to heat and humidity from the time it is manufactured until the time it is viewed. </li></ul><ul><ul><li>Heat and humidity causes fog or a loss of contrast. Film should be stored at 20 º C (68º F). </li></ul></ul><ul><ul><li>Humidity should be between 40% and 60%. </li></ul></ul>
  41. 41. Handling and Storage of Radiographic Film <ul><li>Light will expose the film. Film must be handled and stored in he dark. </li></ul><ul><ul><li>If low level diffuse light exposes the film, fog is increased. </li></ul></ul><ul><ul><ul><li>Luminous watches, cell phone and darkroom light leaks should be avoided. </li></ul></ul></ul><ul><ul><li>Bright light causes gross exposure. </li></ul></ul>
  42. 42. Handling and Storage of Radiographic Film <ul><li>Shelf life. All film is supplied in boxes with an expiration date. </li></ul><ul><ul><li>Most film is supplied in boxes of 100 sheets. </li></ul></ul><ul><ul><li>The 14” x 36” size is supplies in 25 sheet boxes with each sheet interleaved with paper. </li></ul></ul><ul><li>The oldest film in stock should always be used first. Rotation is important. </li></ul><ul><li>Expired will loose speed and contrast and have increased fog. </li></ul>
  43. 43. End of Lecture Return to Lecture Index Return to Physics Homepage