Presentation on theme: "Week 4 B Chapter 14 Radiographic Film Remnant Radiation: the x-rays that interact with the x-ray film. Few of the original x-rays actually make the image."— Presentation transcript:
Week 4 B Chapter 14 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 1960s. 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 ARadiation interaction releases electrons. BElectrons migrate to the sensitivity center. C At the sensitivity center, atomic silver is formed by attracting an interstitial silver ion.
Producing the Latent Image D The process is repeated many times resulting in the build up of silver atoms. EThe remaining silver halide is converted to silver during processing.
Producing the Latent Image FThe 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 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 20º 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.
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