Transcript Document
Image Formation Differential Absorption of X-rays by the Patient Absorption of non-attenuated x-rays by screen Emission of light Formation of Film Latent Image Centers Processing - Formation of Visible Image Cassettes Cassettes are rigid, light-tight devices that hold xray film in contact with the intensifying screen Cassettes Configuration varies Cassettes - Cross Section Cassettes An important function of the cassette is to hold the screen in contact the film in an air tight seal. Loss of contact between the screen and film will result in increased light spread away from the screen and thereby decrease detail. Poor Screen Contact Screen Phosphors Screen color differs based on the type of phosphor and dyes imbedded in screen. Emits blue, ultraviolet or green light that exposes the film. Decreases exposure by 100 fold compared to non-screen film technology. Screen - Cross Section Screens The x-ray photon will strike the phosphor crystal in the screen. The excited phosphor will emit a specific wavelength of light that exposed the film. The efficiency of the screen is based on: Thickness of phosphor layer The type of phosphor The size of the phosphor crystal Trade-off: increased efficiency, decreased spatial resolution (thicker layer, larger crystal size). Screens The thicker the phosphor layer the less detail of the image Note the spreading of the light in the thicker screen layers Screens - Resolution Resolution (detail) can be measured using a line resolution phantom. Resolution is defined and the smallest number of line pairs than can be seen High detail screen (50 speed) may resolve 16 LP/mm Rapid screen (400 speed) may resolve 5 LP/mm but require 1/8th the radiation Spectral Matching Intensifying screen phosphors contain different elements The color of light emitted will also differ Yttrium tantalate - Violet Barium lead sulfate – Blue Lanthanum oxybromide – Blue Gadolinium oxysulfide – Green Calcium Tungsten - Blue Activators Thulium activated - Blue Terbium activated - Green Spectral Matching It is important that the wavelength of light emitted by the screen matches the sensitivity of the film. Specific filters required for safety light for green versus blue sensitive film. Latent Image Formation - Film Base - plastic/Mylar base Emulsion - silver halide crystals lattice work suspended in a gelatin. 3 to 5 µm Single or double emulsion films Copy film - single emulsion and solarized A surface imperfection (AgSulfide) in each silver grain (contains large number of silver halide ion lattice work) called the sensitivity speck. Site of reduction of silver ions to metallic silver (black). Three to six reduced silver ions are required to render a silver grain developable. Film Processing Film processing is a series of chemical reactions that result in the formation of a manifest image The processing will convert the exposed silver crystal into neutral or metallic silver which is black Steps in Film Processing Step Purpose Manual (Time) Automatic (time) Wetting Swelling of the emulsion to permit subsequent chemical penetration 12 s _ Development Production of a manifest image from the latent image 5 min 22 s Stop Bath Termination of the development and removal of excess chemical from the emulsion 30 s _ Fixing Removal of remaining silver halide from the emulsion and hardening of the gelatin Removal of Excess Chemicals 15 min 22 s 20 min 20 s Removal of water and preparation of the radiograph for viewing 30 min 26 s Washing Drying Development The developer chemicals will change the exposed silver crystal into metallic silver. The developer is a reducing agent which supplies electrons to convert the reminder of the silver ions within a grain into metallic silver. Ag+ + e- Ago Developer Chemicals Phenidone Primary reducing agent Produces shades of gray rapidly Hydroquinone Secondary reducing agent Produces black tones slowly Potassium bromide Antifog agent Keeps unexposed crystals from being chemically attacked Glutaraldehyde (Automatic Processor) controls the swelling of the emulsion Proper Development All exposed crystals containing a latent image center are reduced to metallic silver All unexposed crystals remain unaffected Fixer Chemicals The main purpose of fixing is to clear the film of all unexposed, undeveloped silver crystals. Fixing an unexposed film will remove all the silver making it a clear or a blue tint depending on the color of the base material. Ammonium Thiosulfate Removes undeveloped silver bromide from the emulsion Acetic acid Neutralizes the developer and stops its action Potassium Alum Stiffens and shrinks the emulsion Washing Important to wash the film in order to remove residual chemicals remaining in the emulsion. Inadequate washing of the fixer will result in silver oxidation with the remaining hypo from the fixer. Ag0 + NaHYPO ==> AgSulfide (brown) Acid smell White particulate precipitate on film Washing - Inadequate Incomplete removal of the thiosulfate will cause discoloring of the film resulting in poor archival quality of the film - occurs over time. Steps in Film Processing Unexposed and undeveloped film Developer No exposed crystals so no conversion to metallic silver Fixer Clears unexposed Emulsion leaving clear base Steps in Film Processing Exposed, Undeveloped - Latent image Developer - Only exposed Fixer - Clears unexposed crystals converted to metallic silver emulsion leaving Manifest image Steps in Film Processing Completely Exposed & undeveloped film Latent image Not visible Developer Majority of crystals converted to metallic silver Fixer Clears unexposed Emulsion leaving Manifest image