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