Mohamed Ahmed Mohamed Ahmed Mostafa

Intensifying Screen

Introduction

•Early in the history of radiography, scientists discovered that various inor-ganic salts or phosphors fluoresce (emit visible light) when exposed to an x-ray beam.

•In1897, screens were invented by Tomas Edison used to increase the photo-graphic effect of roentgen radiation.

•The intensity of this fluorescence is proportional to x-ray energy absorbed.

Function of Intensifying Screen

1- X-ray absorptionThe first function performed by the intensifying screen is to absorb the x-ray beam (energy) emerging from the patient's body.

2-Light ProductionThe second function performed by the intensifying screen is to convert a portion of the absorbed x-ray energy into light. This is the fluores-cent process.

The ideal intensifying screen would absorb all x-ray energy that enters it; real in-tensifying screens are generally not thick enough to absorb all of the photons.

Exposure ReductionSince film is more sensitive to light than to x-ray exposure, film can be exposed with much less radiation if an intensifying screen is used.

Conventional x-ray film has an x-ray exposure sensitivity in the range of 50 mR to

150 mR if exposed directly by the x-radiation.. When the film is combined with

intensifying screens, the sensitivity ranges from approximately 0.1 mR to 10 mR,

depending on the type of screen and film used.

Conversion EfficiencyThe Efficiency with which the phosphors convert the X-rays into

Light.

Absorption EfficiencyThe ability of phosphor Materials to absorb X-rays.

Screen EfficiencyThe ability of the light emitted by the phosphor to escape from

the screen and expose the film.

Intensification FactorIt is the result of division of Exposure required when screens

are not used to the exposure required with screens

Screen Speed: the time taken for the screen to emit light following exposure to x-

rays. The faster the screen, the lower the radiation dose to the patient.

A Good Doctor is a good Observer

CompositionIn all dental applications, intensifying screens are used in pairs, one on each side of the film, and they are positioned inside a cassette to be hold in contact with the x-ray film to maximize the sharpness of the image.

1- The BaseThe Base material of most intensifying screens is some form of polyester plastic that is about 0.25 mm thick. The base provides mechanical support for the other layers.2- The Reflective LayerIt is a thin layer of white material (Titanium Dioxide or Magnesium Oxide) between the base and the phosphor layer. It reflects light emitted from the phosphor layer back toward the x-ray film.This reflective base increases the light emission of intensifying screens but also result in some image unsharpness because of the divergence of light rays reflected back to the film.

Composition3- The Phosphor Layer

It is composed of phosphorescent crystals suspended in a polymeric binder. When the crystals absorb X-ray photons, they fluoresce.

Three Main phosphor materials are used in intensifying screens:1-Rare Earth Phosphors

2-Yttrium (a non rare earth phosphor but having similar properties).3-Calcium Tungstate CaWO4.

Rare Earth screensModern screens employ these phosphors which produce very fast screen speeds, en-abling a substantial reduction in radiation dose to patients, without excessive loss of image detail. The Rare Earth group of elements includes :

Lanthanum (Z=57)Gadolinium (Z=64)Terbium (Z=65)Thulium (Z=69)

The Term Rare Earth is used because it is difficult and ex-pensive

to separate these elements from earth and from each other,

not because the elements are scarce.

Terbium-activated screens emit Green Light, While Thulium-activated screens emit Blue light.

Calcium Tungstate ScreenThis was the original material used but it is no longer recommended.

-The speed of these screens depends upon:1-The thickness of the phosphor layer.

2-The size of the phosphor crystals.3-The presence or absence of light-absorbing dyes within the screen.

4-The conversion efficiency of the crystals.

-All calcium tungstate screens emit Blue light and must be used with blue-light-sensetive monochromatic radiographic film.

-Slower than Rare Earth Screens.

Composition

4- The Protective CoatA protective polymer coat (<15 micrometer thick) is placed over the phosphor layer to protect the phosphor and to provide a surface that can be cleaned. As the intensifying screen should be cleaned as described to avoid debris which cause light spots on the resultant radiograph.

Effect of Intensifying screen on the final image

1- The use of intensifying screen in extra-oral radiography has an ad-verse effect on image sharpness because the diverging visible light emitted by the screen spread out beyond the point of origin exposing film area larger than phosphor crystals.

2-This diverging light causes Blurring of fine details on the radiographs.

This light diffusion can be minimized by ensuring intimate contact be-tween intensifying screen and the film.

3-Another Factor that causes image Blurring is the crossover of the light emitted from one screen over the film base and reach the emulsion on the opposite side.

This can be solved by incorporation of Absorbing dyes into the Base preventing light crossover.

Screen Maintenance

1- Regular cleaning with appropriate cleaning agent (Alco-hol).2- Careful handling to avoid scratching or damaging of the screen.3- Regular check up of cassettes for detection of any loose contact between Film and screen.

Thank YouMohamed Ahmed Mohamed Ahmed

Mostafa

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