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Radiographic film
 

Radiographic film

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    Radiographic film Radiographic film Presentation Transcript

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