Intensifying screen avinesh shrestha

INTENSIFYING SCREEN

Presented By:Avinesh Shrestha

B.Sc.MIT(1st year) Roll No:90

IOM

18th June 2015

There are three key parts of the Image Receptor for Conventional Radiography:

Film to record the image Intensifying Screens to

expose the film Cassette to protect the

screens and film Most conventional radiographic cassettes have a pair of screens that sandwich the film. This design use double emulsion film.

Introduction:

Screens Film

A part of the cassette used in the conventional X-ray film radiography containing fluorescent phosphors as active material

Different types of I.S. emit different intensities & color of light when irradiated by x-ray.

Radiographic I.S resemble flexible sheets of plastic or cardboard

I.S. come in sizes that correspond to film sizes

What is an Intensifying Screen?

Michael PupinIn early February 1896,

Direct film radiography

Screen film radiographyEssentials of radiologic Science

Contains FOUR distinct layers: Protective coating Phosphor Substratum Base

Constituents of I.S.:

Essentials of radiologic Science

In 1921, the cleanable fluorescent intensifying screen was developed by applying a thin protective coating over the phosphor. This innovation extended screen life and decreased the high cost of screen replacement

Layer closest to the radiographic film

About 10 to 20 µm thick

Made up of acetate

Protective Coating:

Protection: It resists surface abrasion & damage caused by handling.

It also helps to eliminate the buildup of static electricity & provides a surface for routine cleaning without disturbing active phosphor layer.

It includes qualities like:-Thin coating to reduce distance between film

& phosphor & minimize unsharpness.Transparent to light so that light produced in

fluorescent layer will reach the film. Waterproof to protect sensitive phosphor

crystals.

Functions of Protective coating

This is a bounding layer between the base & phosphor layer.

It is 25 µm thick.It is made up of shiny substance & highly

reflective white pigment, magnesium oxide or titanium dioxide.

The function of reflective layer is:Maximize effect of screen by reflecting light

back towards film emulsion which would otherwise be lost through screen base.

Substratum Layer

Reflective layer

It is made by adding a special dye to it.

This dye prevents any light travelling backwards towards the screen base from being reflected by base & is instead absorbed.

It has less photographic unsharpness.

It reduces screen speed.

Absorptive Layer

It is the farthest layer from the film. It is made from paper, cardboard or more usually a clear plastic such as polyester.

It is about 1mm thick.The function of base is to provide a strong, smooth but flexible support for fluorescent layer.

The base, sometimes contains titanium dioxide (e.g. Kodak Lanex screens) has same function as the reflective layer.

Base

Favorable properties of Base Chemically inert, not prone to interact with the

phosphor layer.

moisture resistant.

Resistant to radiation damage & discoloration with age.

Strong and flexible.

Lacking impurities that would be imaged by x-rays.

Uniformly radioparent.

Active layer of the I.S.Layer from which the X-ray photons are converted into visible light photons

Fluorescent crystals present in this layer is responsible for the conversion

For each absorption, large number of visible light photons are emitted

Phosphor layer vary in thickness from 50-300 µm depending on the type of screen. Individual phosphor crystals are 5-15µm thick.

Phospher layer

It refers to the emission of light from a substance due to stimuli like electric current, stress on the crystals ,biochemical reactions, electromagnetic radiation

Divided into many types like Chemiluminescence, Electroluminescence, Mechanoluminescence, Photoluminescence, etc

Involves excitement of outer shell electrons to different energy levels emitting light of different wavelength by de-exciting(thus different colors)

Out of these, we will be discussing only about Photoluminescence

Luminescence

The amazing Northern Lights, officially known in the Northern hemisphere as Aurora Borelias, are natural phenomena that features amazing colored light displays

It is the process of luminescence due the absorption of photons i.e. electromagnetic radiation

In the case of I.S . the electromagnetic radiation are the X ray photon emitted from the x ray tube

Photoluminescence

Fluorescence

Phosphorescence

Photoluminescence

A form of luminescence when light is produced instantaneously, i.e. it lasts only as long as the radiation exposure is given

Flurosence

The light emission continues for some time even after the radiation exposure has stopped

This phenomena is also known as Lag or Afterglow.

An undesirable phenomena in X ray imaging because the delayed emission of light fogs the film in the cassette before the radiographer can get it to the processor.

Phosphoresence

Visible Light Photon striking on the x ray film

Vacancy is created

Electron at the excited state

Fig: An Atom of a fluorescent crystal (Bhor’s Model)

De-excitation of electron emitting visible light

Interaction of X-ray and the Electron

X ray Photon

Reflective layer

Interaction of X-ray with phosphor

W. C. Roentgen’s interest in studying the glow of various substances led to his discovery of X rays; in 1895,

He had darkened his laboratory andcompletely enclosed his Crookes tube with black photographic paper A plate coated with barium platinocyanide, happened to be glowingseveral meters from the Crookes tube Eventually, because of the luminescent

property he discovered X rays, naming the cause of this glow as X ray

Discovery of X-ray due to fluroscence

It is a substance that exhibit the phenomenon of luminescence(photoluminescence)

Phosphor are hygroscopic so, they are suspended in a transparent binder ,polyurethane

It prevents any moisture penetration to prevent reduced luminescence

Polyurethane binds or hold the crystals together.

The binder in high resolution screen may contain carbon or colored granules called ‘acutance dye’.

Laterally scattered light(irradiation)must be absorbed by the dye to prevent image unsharpness, but reduces speed of the screen.

Phosphor

At first in 1895, William Conrad Roentgen observed luminescence of barium platinocyanide & was applied to diagnostic radiography.

Edison and his associates tested approximately 8500 different materials, during this work, about 1800 substances were found to be fluorescent.

Out of these substance calcium tungstate was found to have fluorescence approximately 6 times more intense and was used in I.S

Later in 1948, I.S. were introduced that were composed of a barium lead sulfate phosphor; they were the first commercially successful radiographic screens not made of calcium tungstate and provided greater speed

Breif history of phosphor used in I.S

in the early 1970s the use of rare-earth screens for medical radiography was introduced, first brought to market in 1974

It significantly changed screen-film radiography Since then, rare earth series of

elements(gadolinium oxysulphide,lanthanum oxysulphide, lanthnaum oxybromide,yttrium oxysulphide, yttrium tantalate) are the phosphor material used

Thomas Edison

Contd:

High atomic number so x-ray absorption will be high. Detective Quantum Efficiency(DQE) Emit a large amount of light per x-ray

absorption. Conversion Efficiency(CE) Light must be of proper wavelength to match

the sensitivity of the film Spectral Matching Phosphor Afterglow should be minimal.

Phosphor should not be affected by heat, humidity or other environmental conditions.

Properties of phosphor used in I.S.

Detective Quantum efficiency(DQE): The ratio of x-ray absorbed to the amount of

incident x-ray photon When the x-ray energy equals the K-shell

electron binding energy, absorption is high. Rare earth screens exhibit better absorption

properties than Ca tungstate in between the respective K-shell absorption edges.

DQE mainly depends on the type of phosphor. Other factors are thickness of phosphor,

coating wt. & photon energy of incident beam

Contd:

:Radiologic Science for Technologist

Conversion Efficiency(CE):

It is the ratio of visible light energy emitted to the x-ray energy absorbed.

When x-ray interacts photoelectrically, its energy reappears as heat or light through rearrangement of electrons in crystal lattice of phosphor. If all energy reappears as heat, the phosphor would be worthless.

CE of Ca tungstate is 5% & that of rare earth phosphors is about 20%.

Contd:

Elements(transitional metal) with atomic number between57(lanthanum) and 71(lutetium)

Has two important physical attributes which gives them advantage over conventional phosphorsHigh DQEHigh CE

Those used in I.S. are principally Gadolinium, Lanthanum, & Yttrium.

requires low patient dose, low thermal stress on x-ray tube & less lead shielding for x-ray rooms.

Rare earth screens are faster as compared to Ca tungstate but no significant increase in noise.

Rare earth phosphors

Use of activators•phosphors are used invariably inconjunction with activatorswhich are small quantitiesof some foreign elementadded to the phosphorduring manufacture.•The phosphor-activator combination not only determine the intensity of luminescence obtainable from the screen but also the color of the light emitted.


Low exposure factors can be used as compared to non-screen film thusReduced patient dose, Fine focal spots can be usedSuccessful use of low output equipments i.e. no need of

powerful generators & high heat capacity tubes &Long x-ray tube lifeReduced costs

Low exposure time & reduce movement unsharpness Exposure to personnel from scattered x-rays is reduced

Lower spatial resolutionLess detail than direct exposure film

Disadvantage

Advantage of I.S

IF is defined as the ratio of exposure required to produce the same optical density without a screen, to exposure required with screen.

IF= Exposure required without screenExposure required with screen

For e.g.: if exposure required to give density of 2 on a film used without screen is 150 mAs & 10 mAs when used with screen, then

IF= 150 = 15 10

Intensifying Factor(IF)

Mainly three types of screen of different speedSCREEN SPEEDHigh Resolution SlowRegular or Standard

Medium

Fast Fast

Types of Screen

It is made by making substratum layer absorptive. It has slow speed. It requires greater exposure than regular screens. It can be used when radiation dose is less important & high tube loading is not necessary.Contraindicated when there is risk of patient movement. It can be used in extremity radiography.

High resolution

It has medium speed.

It provides adequate speed & sharpness.

It can be used for most general radiographic application.

It provides the base from which the speed of I.S. can be calculated.

Regular

It produce greater film blackening

Sharpness will be decreased

It can be used in high risk of image blur from movement. (e.g. paedriatric radiography)

It use rare earth phosphors.

Fast

In mammography film single screen cassette is used in conjunction with single coated emulsion film.

Reduces photographic unsharpness.

The screen is usually placed at back, rather than at the front.

This results in light being produced in those phosphor nearest to the film

Single screen radiography


Primary characteristics of radiographic I.S. are screen speed, image noise, spatial resolution & image unsharpness.

Screen Charecteristics

The capability of a screen to produce visible light is called screen speed

The properties which determine nature & quality of screen in terms of speed areType of phosphor usedThickness of layerDensity with which the crystals are packedCoating weight:- It is an expression of quantity

of phosphor grain incorporated with phosphor layer and therefore depends on grain size and coating thickness

Screen Speed

Presence of reflective/absorptive layer

Dye in the layer Crystal size

OTHER FACTORS AFFECTING SPEED

It occurs as speckled background.

It occurs mainly when fast screens & high KV techniques are used.

It increase with higher CE but not with higher DQE.

Principal component of image noise is quantum mottle.

Quantum mottle often is a direct result of use of very fast speed screen-film systems that require very small amounts of exposure & result in a grainy, mottled or splotchy image.

Image Noise

Spatial resolution refers to how small the object can be imaged whereas contrast resolution refers to the ability to image similar tissues.

IS lowers spatial resolution compared with direct exposure.

High speed screens have low spatial resolution & fine detail screens have high spatial resolution

Spatial Resolution


The use of I.S. introduces certain degree of unavoidable unsharpness into image in comparison to non-screen film material due to light divergence.

The unsharpness produced is considered as intrinsic unsharpness.

It is also caused when cassette becomes damaged & the close contact between film & screen is lost.

Image Unsharpness

Image blur due to poor screen film contact

Procedure:Place cassette on table.Place wire mesh tool on cassette.Set collimation to film size.Make exposure and process film.View the film in view boxAreas with loss of resolution indicates poor

contact or stained screens.

Wire mesh test for screen film contact


Unsharpness Real image

Effect of the thickness of phosphor layer in speed and unsharpness

Crossover effect

Anti-cross overCrossover can be

decreased by placing a light-absorbing layer between the film emulsion and film base, using a base material that selectively absorbs the light

wavelengths emitted by the intensifying screens, and designing the film emulsion to increase light absorption.

Changing the shape of the crystal improved light absorption and reduced crossover.

It is vitally important that films are matched to colour of I.S. emission to obtain maximum film blackening for the least radiographic exposure and optimum speed.

It refers to using blue sensitive film with blue light-emitting screens and likewise with other colours

Failure to match the screen and film results in inappropriate radiographic density.

Luminescence is concentrated in narrow bands of wavelength in rare earth phosphors unlike the calcium tungstate having continuous spectrum.

Thus calcium tungstate known as broad-band emitters, while rare earth phosphors are line emitters.

Spectrum Matching


Spectral sensitivity of blue sensitive film and spectral emission of CaWO4 (450nm to 495nm is for the blue color of light)

Screens should be handled only when they are being installed in cassettes & when they are being cleaned. There should be the record of type of I.S., time of install & cleaning

When the screens are mounted in cassette, manufacturer's instruction must be followed carefully.

Finger marks, stains, dust, or foreign fragments affect screen’s fluorescent emission.

Small scratch made by fingernail, edge of film, etc can cause artifacts in radiograph so special care should be taken.

Care of Screen

The only way screens become useless is through improper handling & maintenance.

X-ray interaction do not cause the I.S. to wear out i.e. no radiation fatigue.

While replacing I.S. the only remedy is to replace both screens as they are sold in pairs correctly matched.

Contd:

I.S. is provided with adhesive tape on the back of the surface for better positioning of screen.

The front screen marked by the manufacturer is mounted first.

The screen should be handled on the edges.

The prepared screen is dropped carefully into the well of the cassette. Active surface of front & back screen should face each other.

Mounting the screen

We should not leave the cassette open as screen can be damaged by any chemicals, liquids, dust or other particles that fall on it.

We should not store the cassettes near the source of heat.

Contd….

Essentials of radiologic Science

I.S. must be cleaned periodically on the basis of amount of use & level of dust in work environment.

Special screen cleaning materials containing antistatic compound or mild soap & water are used under the guidance of manufacturers.

CAUTION: NEVER USE ALCOHOL. Procedure:

Moisten some cotton wool with solution & gently wipe the surfaces.Water must not reach the back or the edge of the

screen.Wipe the screen with fresh cotton.Wipe till dry.

Cleaning of screens

Worn radiolucent foamLoose, bent, or broken hingesWarped screens caused by excessive moistureWarped cassette frontSprung or cracked cassette frameForeign matter under the screen

Most common cause of poor screen film contact

Features Screen Speed

Recorded Detail

Patient Dose

Thick phosphor layer ↑ ↓ ↓

Large phosphor crystal ↑ ↓ ↓

High density layer ↑ ↓ ↓

Higher DQE ↑ ↓ ↓

High CE ↑ ↓ ↓

Absorbing layer ↓ ↑ ↑

Dye in phosphor layer ↓ ↑ ↑

Today, the film-screen cassette is gradually being replaced by alternative X-ray detectors such as the photostimulable phosphor plate and selenium plate.

Although appearing quite similar to a regular intensifying screen, an imaging plate(IP) functions quite differently.

Intensifying screens use fluorescent material;

whereas, An imaging plate uses a phosphorescence

material. By using a europium activated Barium Fluoro halide Phosphor(BaFX:Eu2+,X=Cl,Br,I), to trap and hold electrons,

A latent image is formed directly on the imaging plate itself; therefore, the term filmless.

Photo-stimuable phosphor (Imaging Plate)

Cross section of CR plateLike intensifying screens, imaging plates are coated with several different layers to achieve its optimum luminescent quality.

Chesney's’ Radiographic Imaging, John Ball & Tony Price, 6th edition, Blackwell Publishing, Oxford, UK.

Radiologic Science for Technologists-Stewart Carlyle Bushong

Essentials of Radiologic Science-Robert Fosbinder, Denise Orth

Screen Film Processing Systems for Medical Radiography: A Historical Review-Arthur G. Haus and John E. Cullinan

www.e-radiography.net

References

http://www.e-radiography.net/

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Intensifying screen avinesh shrestha