1. INTENSIFYING SCREENS

2. INTENSIFYING SCREEN
INTENSIFYING SCREEN is a device that converts X-rays to visible light. It converts a higher energy electromagnetic radiation to a lower energy electromagnetic radiation.
In general, films that are exposed using screens has an image that is produced 95% by light and 5% by X-rays.

3. INTENSIFYING SCREEN ADVANTAGES OF USING SCREENS
reduces the dose required for a particular examination.
short exposure time
less movement unsharpness
DISADVANTAGE OF USING SCREEN
introduces the screen unsharpness

4. INTENSIFYING SCREEN

5. CONSTRUCTION OF SCREENS

6. CONSTRUCTION OF SCREENS
BASE
This acts as a support for all other layers of intensifying screen.
Made of polyester.
250 microns () thick for cassettes and 175  for screens used for automatic film changers.

7. CONSTRUCTION OF SCREENS
REFLECTIVE OR ABSORPTIVE LAYER
modern technology has already incorporated the reflective or absorptive layer in the upper part of the base.
Reflective layer intercepts light going away from the film and redirects it towards the film.
Increases speed but increases also the amount of unsharpness.
Made from thin (30 ) coating of titanium dioxide (TiO2) or similar compound.
Absorptive layer absorbed the light travelling away from the film.
This layer is made of dye incorporated on the base material.
This slows down the speed of the system, but has the advantage of improving the sharpness of the image.

8. CONSTRUCTION OF SCREENS
SUBSTRATUM
Attach the phosphor layer to the base.
This should be as thin as possible but should provide adequate adhesion.
PHOSPHOR LAYER
This is a dispersion of the phosphor crystals within a suitable binder.
It is approximately 150 .

9. CONSTRUCTION OF SCREENS
Binder material that is commonly used by manufacturer is the acetate acrylate as this has all the necessary characteristic:
Flexible
Inert to phosphor crystal and the light they emit
Provide even, known dispersion of the phosphor in the binder
Allows the phosphor layer to be coated onto the base at the required thickness.
Phosphor crystal is a metallic crystalline solid, naturally occurring or artificially made, that exhibits the property of fluorescence when exposed to X-rays and can be manufactured in useful form to produce high image quality.
Calcium tungstate (CaWO4)
Rare earths

10. CONSTRUCTION OF SCREENS
SUPERCOAT
This is the top protective layer of the screen.
It is approximately 8 thick.
It is made of cellulose acetobiturate, or other polymer.
It serves three functions:
protects the delicate phosphor layer from mechanical damage,
provides a surface which can be cleaned without damaging the phosphor layer, and
provides a smooth evacuation of entrapped air resulting in a good film-screen contact.
It must be a poor conductor of static electricity.
The surface of protective coating can be made with varying “roughness”.

11. INTENSIFYING ACTIONS OF SCREENS
CONVERSION EFFICIENCY OF PHOSPHOR
The efficiency with which the phosphor converts X-rays to light.
Efficiency = absorption x conversion x emission

12. INTENSIFYING ACTIONS OF SCREENS
SCREEN EFFICIENCY
The ability of the intensifying screen to absorb X-rays, to converts it to light, and to allow the light to escape from the screen and expose the film.
Exposure required to produce ND 1.0: No screens
IF =
Exposure required to produce ND 1.0: With screens
INTENSIFICATION FACTOR
It is the ratio of the x-ray exposure needed to produce the same density on a film with and without the screen.

13. SCREEN SPEED AND DETAIL
The relationship between screen speed and detail is a reciprocal one: as the speed of the screen increases, the amount of detail decreases.
FACTORS AFFECTING SPEED
Phosphor type
Phosphor grain size
Thickness of phosphor layer
Coating weight
Presence of reflective/absorptive layer
Dye tint in binder
Exposure technique

14. SCREEN DETAIL SCREEN UNSHARPNESS CROSSOVER EFFECT
Due to divergent emission of light coming from intensifying screen.
CROSSOVER EFFECT
It is a result of the widening light beam emitted by the crystal as it passes from one emulsion to the other, causing a shadowy, less sharp image in the emulsion layer furthest from the intensifying screen.

15. SCREEN DETAIL STRUCTURE MOTTLE SCREEN-FILM CONTACT
It is caused by the fact that it is not possible to evenly dispersed the phosphor crystal throughout the binder medium.
SCREEN-FILM CONTACT
Poor film-screen contact , causes the light emitted by the intensifying screen to diffuse before it reaches the film, so that the image produce is unsharp.

16. TYPES OF PHOSPHORS CALCIUM TUNGSTATE (CaWO4)
It is a naturally occurring phosphor and produces a continuous spectrum principally in the blue part of visible spectrum, with a peak output at approximately 425 nm.
Discovered by Thomas Edison.

17. TYPES OF PHOSPHORS
Spectral sensitivity of blue sensitive film and spectral emission of CaWO4

18. TYPES OF PHOSPHORS RARE EARTHS
Are soft, malleable metals that can be made to emit light upon stimulation by X-rays.
First introduced in 1970’s by Wickersheim, Alves, and Buchanan.
RARE EARTH SCREENS: principal phosphors, symbols, emission

19. TYPES OF PHOSPHORS
Spectral sensitivity of green sensitive film and spectral emission of Gd2O2S.Tb

20. THANK YOU 