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Image Formation
Differential Absorption of X-rays by the PatientAbsorption of non-attenuated x-rays by
screenEmission of lightFormation of Film Latent Image CentersProcessing - Formation of Visible Image
Cassettes
Cassettes are rigid, light-tight devices that hold x-ray 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 PhosphorsScreen 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 layerThe type of phosphorThe 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 MatchingIt 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 baseEmulsion - 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 ProcessingFilm 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
15 min 22 s
Washing Removal of Excess Chemicals 20 min 20 s
Drying Removal of water and preparation of the radiograph for viewing
30 min 26 s
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
WashingImportant 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 - InadequateIncomplete 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 Fixer
Clears unexposed Emulsion leaving clear base
No exposed crystalsso no conversionto metallic silver
Steps in Film Processing
Exposed,Undeveloped - Latent
image
Developer - Only exposed crystals converted to
metallic silver
Fixer - Clears unexposed emulsion leaving Manifest
image
Steps in Film Processing
Completely Exposed &undeveloped film
Developer Fixer
Clears unexposed Emulsion leaving Manifest image
Majority of crystalsconverted to metallic silver
Latent imageNot visible