INTENSIFYING SCREENS.pptx

INTENSIFYING SCREENS
Mr. Sameer Ahmad Ganaie
ASSISTANT PROFESSOR COPMS Adesh University, Bathinda Punjab 151001
sameerganie2635@gmail.com 28-04-2023
1

CONTENTS
• Introduction
• Purpose
• Advantages and disadvantages
• Intensifying Screens
• Construction of screen
• Intensifying actions of screens
• Screen speed and detail
• Types of phosphors
2

INTRODUCTION
• 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

PURPOSE
• Intensifying screens are used in the x-ray cassette to intensify the effect of the x-ray
photon by producing a large number of light photons.
4

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
5

INTENSIFYING SCREEN
6

CONSTRUCTION OF SCREENS
7

• BASE
This acts as a support for all other layers of intensifying screen.
Made of polyester.
250 microns thick for cassettes and 175 microns for screens used for automatic film
changers.
8

• 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 microns ) 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.
9

• 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.
10

• 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
11

• SUPERCOAT
This is the top protective layer of the screen.
It is approximately 8 microns 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”.
12

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
13

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.
IF = Exposure required to produce ND 1.0: No screens
________________________________________
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.
14

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
15

SCREEN DETAIL
• SCREEN UNSHARPNESS
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.
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
• 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
18

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INTENSIFYING SCREENS.pptx

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