INTENSIFYING SCREEN
1) Definition:
It is a device that transfers X-ray energy into visible light; the visible light, in turn exposes the screen film.
- These screens intensify the effect of X-rays on the film, thus less radiation required to expose a screen film, thus less radiation exposure to patient
- Action of radiation on photographic film is supplemented by means of special screens which intensify the action of X-rays
- An intensifying screen is a smooth plastic sheet coated with minute fluorescent crystals known as phosphors.
- When exposed to X-rays the phosphors fluoresce and emit visible light in the blue or green spectrum, the emitted light then exposes the film.
2) Composition:
- Base –
- Made of either a stiff sheet of cardboard or polyster
- Supporting component of screen
- Reflecting layer-
- Thin layer of white material (i.e. magnesium oxide or titanium oxide) between the base and the luminescent layer
- Serves to redirect a large fraction of emitted light to the film
- Increases sensitivity but some degree of unsharpness is created
- Phosphor layer-
- This layer consists of light sensitive phosphor crystals suspended in plastic material
- Types of phosphor used in dental screen:
- a) Crystalline calcium tungstate
- Fluoresces in the blue portion
- Conventional screens(Kodak X-Omatic Regular screens) are used with blue sensitive films
- b) Rare earth intensifying screens
- Fluoresce in the green portion of spectrum
- Designed for use with green sensitive films
- Four times more efficient than calcium screen
- Considered faster and less exposure is required
- Coat: Protect the phosphor layer from mechanical insult such as abrasion, scratching, etc.
3) Effects of screens:
1) Unsharpness
– Three types of screens are available:
a) High definition
b) Normal
c) High speed
– The range of unsharpness produced varies from 0.15mm for high definition to 0.45mm for high speed screen
2) Mottle:
– Consists of faint irregular pattern of density variations which are not present in X-ray beam
– Screen mottle: non uniformity in the fluorescent layer may show up on the radiograph since the intensification factor will vary over the surface of the screen.This effect is not as marked in the newer screens.
– Quantum mottle: there is variation in the X-ray intensity over surface of the screen and it becomes apparent on the film
4) Care and Use of screen:
- No gapping between the screen and the film to avoid excessive blurring of image
- Cassette after use may become buckled or bent, and then it is difficult even for felt lining in cassette to maintain close contact between the screen and the film
- Scratches, dust or grease should be prevented
Lead intensifying screens: these are used only with high kV, radiography, i.e. with 250kV X-rays or Cobalt 60 gamma rays