Intensifying screen and cassette

Dr. Satendra RautFirst year residentDepartment of Radio diagnosis

Intensifying screen :Introduction• An intensifying screen works by

converting x-ray energy into light energy.

• X-ray photon-absorbed by phosphor-become excited and fluoresce –emits UV and /or visible light.

• A small no of x-ray photon produce a very large no of light.

• Film is more sensitive to light then x-radiation.

• Thus radiographic exposures can be considerably reduced over those required for non-screen technique.

luminescence

• The emission of light from a substance bombarded by radiation is termed luminescence.

• Fluorescence-light emission lasts only as long as the radiation.

• Phosphorescence-afterglow-light continues to be emitted for sometime even after radiation exposure has ended. Undesirable phenomenon

Intensifying screens and unsharpness

• The use of intensifying screen causes divergence of light thus unsharpness.

• The further away each source of fluorescence is from the film ,the greater the degree of light divergence and thus the greater the unsharpness.

• Another reason-non uniform screen film contact due to damage , misuse .

• Unsharpness which arises as a consequence of using film and screen material is referred to as intrinsic or photographic unsharpness.

Screen construction• Base

– High grade cardboard or polyester plastic– Provide a smooth ,strong and flexible

support– Chemically inert, uniformly radioparent,

moisture resistant, not discolor• Substratum layer

– Bonding layer between base and phosphor layer

– Reflective substratum layer• Titanium dioxide-maximize the effect of

screen by reflecting light • Increases the speed • Extends the real image boundary –increased

photographic unsharpness.

– Absorptive substratum layer• Any light passing is absorbed• Slower but has less photographic unsharpness• Made absorptive by adding a colored dye to it

• Base:10 mil• Reflecting layer:1 mil• Phosphor layer:4-6 mil• Protective layer:0.7-0.8mil• 1mil=0.001in=0.0254mm

Cont..• Phosphor layer(fluorescent layer)

– Active layer of screen– Suspended in a transparent binder such as polyurethane– Binder in a high-resolution screen may also contain carbon granules or a colored

pigment-acutance dye- absorb any laterally scattered light within the fluorescent layer

– Coating weight is an expression of the quantity of phosphor grains incorporated in a phosphor layer and depends on • Grain size• Coating thickness

• Supercoat– Has protective function and made from acetate– Resist surface abrasion , anti-static quality– Must be as thin as possible– Is extended around the edges and back of screen –minimize edge wear and

provide anti-curl backing

Types of phosphor

• Materials which convert invisible radiation into luminous radiation are known as phosphors.

• Two qualities– Very efficient at x-ray absorption– Fluoresce, with little afterglow

• Examples– Calciim tungstate,barium fluorochloride,barium lead sulfate,

barium stronium sulphate,– rare earth phosphors-gadolinium oxysulphide, lanthanum

oxysulphide, lanthaum oxybromide, yttrium oxysulphide, yttrium tantalate

Rare earth phosphors

• Elements with atomic number between 57(lanthanum) and 71(lutecium).

• Has two important physical attributes which gives them advantage over conventional phosphors– Are more efficient at absorbing X-ray photons

(Absorption efficiency or quantum detection efficiency)

– Are more efficient at converting X-ray photons to light (conversion efficiency)

• Quantum detection efficiency– Depends not only on type of

phosphor but on thickness and coating weight of that phosphors

– Dependent on the photon energy of the incident beam

– While absorption may decrease , conversion difference remain superior

– High QDE are faster then low QDE

• Conversion efficiency– More efficient at

converting x-ray interactions into light,

– e.g-15-20% light conversion efficiency compared to calcium tungstate3-5%

Use of activators• Rare earth phosphors are

used invariably in conjunction with activators which are small quantities of some foreign element added to the phosphor during manufacture.

• The phosphor-activator combination not only determine the intensity of luminescence obtainable from the screen but also the color of the light emitted.

Phosphor Activator Emission

Gadolinium oxysulphide

Terbium Green

Lanthanum oxysulphide

Terbium Green

Yttrium oxysulphide

Terbium Blue

Yttrium tantalate

Niobium Blue

Lanthaum oxybromide

Thulium Blue

Matching film to intensifying screen

• To obtain maximum film blackening for the least radiographic exposure, the film are matched to the color of intensifying screen

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

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

Types of screen and their application

• Variety of screen speed by the choice of • Phosphor and size of phosphor grain• The addition/exclusion of absorptive/reflective layers• By varying the amount of reflective/absorptive material

• High resolution (Detail)– Making the substratum layer absorptive– Slow speed , high tube loading

• Regular– Medium speed screen – Most general radiographic application

• Fast– Greater film blackening – Detail sharpness diminished due to reflective layer– Ideal to image dense body parts as well maintain less patient dose

Single screen radiography• Mammography• Screen is placed at the

back • This results in light being

produced in those phosphor nearest to the film –less light divergent-less irradiation ad a sharper image is produced.

Less common types of screen • Graduated

– Speed gradually decreases across the screen from edge to edge, indicated on the screen by + and – signs.

– Achieved by adding to the supercoat a pigment of gradually increasing density– Used in pelvimetry and orthodontics– Fast end being placed adjacent to the densest part of patient(vertebrae end in

pelvimetry)– This allows a large range of tissue density in one film

• Screens for multisection cassette– Radiation is progressively attenuated as it passes through several pairs of

screens in multisection cassette– To obtain similar film blackening the screen pairs are arranged in increasing

speed from top to bottom– Done either by adding varying amounts of pigment to the supercoat or by

gradually increasing the coating weight of each fluorescent layer

Intensification factor(IF)• Is the number by which one must

multiply an exposure(mAs) used with screens, in order to produce a film of similar density if exposing the same type of film without screen

• The intensification factor are seldom referred -– Only has meaning when assessing

one particular type of film to be used either with or without the benefit of screen

– Increase in kV results in increase in intensification factor

– Not constant for all densities

• Thus the intensification factor for a given screen is only meaningful when considering one particular density and one particular kV value

Quantum mottle

• The more photons per cm2 or higher the density of photons present in the transmitted beam ,the greater its information content.

• If the photons are spread too thinly ,the gaps between them may become significant, and lead to a grainy or mottled appearance

• Factors contributing to fall in x-ray absorption by screen and thus a mottled appearance are-– Increasing kV– Poor absorption efficiency– Combination of fast film-screen and low radiographic exposure

• Is a function of fast radiographic systems and very small radiation exposure.

Factors of screen construction affecting speed and sharpness

• Phosphor grain size– Large crystal size-increased light

emission-increased light divergence-loss of sharpness

• Thickness of intensifying screen– Greater the screen thickness –

greater the vertical distance-greater the lateral spread of light –loss of sharpness

• Nature of substratum layer• Presence or absence of carbon

granules or pigment dye in binder

Cont..• Crossover effect

– Crossover is image degradation caused by light produced in one intensifying screen passing through the film base and producing an image in the opposite emulsion layer.

– Produce increased image blurring

– Screens incorporating reflective layer aggravate the problem

– Absorbing layer into the film between the base and emulsion to prevent crossover

Features of screen construction which affect speed and sharpness

Feature Speed Sharpness

Crystal size Increases Decreases

Phosphor layer thickness Increases Decreases

Reflective layer Increases Decreases

Absorptive layer Decreases Increases

Carbon granules or binder pigments

Decreases Increases

Crossover Negligible effect Decreases

QDE Increases No effect

Conversion efficiency Increases No effect

Care and maintenance of intensifying screens

• Do not open cassettes in the vicinity of chemicals or other liquids

• Do not leave cassettes open on the bench, exposed to contamination

• Do not store cassettes near source of heat such as radiator

• Mounting intensifying screen• Cleaning screens• Checking a screen for artifact

Advantages of using intensifying screens

• Lower exposure factors may be used , thus reduced patient dose, more frequent use of fine focal spots, the successful use of low-output equipment and less wear and tear on the x-ray tube

• Permits the use of very short exposure times, useful where movement unsharpness is a problem

• Higher contrast image compared to non-screen imaging

Film Cassette: Introduction• A film cassette is a container for exposed or unexposed films.• Function

– Hold intensifying screen and damage protection– Exclude all light and fogging– Maintain close and uniform contact– Exclude dust and dirt

• Features of the ideal cassette– Strong and rigid– Lightweight– Easy to open and close– No sharp edges– Provide minimal beam attenuation– Have a sliding aperture– Adequate lead foil in rear surface to reduce back scatter– Foam sponges for good contact– Vinyl covering for warmth– Range of sizes

Construction

• Front and a back hinged at one long edge

• Back-thin sheet of lead foil , a plastic foam pressure pad and an intensifying screen

• Front-cassette-well-front intensifying screen and a short lead blocker

• Some have additional pressure pad in front

• Range of locking • Internal black coating to prevent

internal light reflection• Slightly curved cassette back –for

good screen-film contact

Materials in cassette construction• Cassette front

– Metals (Al), plastic laminate, carbon fiber– Uniform thickness and density– British standards(BS 4304/1968)-cassette front if metal ,should have an Al equivalent

of no more than 1.6mm when used at 60kVp, or if plastic no more than 0.2 mm Al equivalent

– Material advantage• Strength and stiffness• Light in weight• Low beam absorption(Esp. carbon fiber)

• Cassette back– Metal or plastic –lined with lead foil(prevent from back scatter)– BS-should have a lead equivalent of at least 0.12 mm when used with equipment

operating at 150 kV constant potential• Cassette fittings

– Clips or fasteners-usually stainless steel– Hinges-metal or plastic– Pressure pads-plastic foam sponge

Types of X-Ray cassettes available• Single screen cassette

– Single intensifying screen –mammography• Curved cassettes

– Two types• 1st type-necessary close cannot be achieved –intercondyler projection of knee in flexion• 2nd type-panaromic views of mandible and maxilla in orthodontic

radiograph(orthopantamography)

• Gridded cassette– Have a stationary secondary radiation grid in front of cassette– Used where conventional bucky system is unavailable(mobile works)

• Multisection cassette– Tomography

• Set of 3-7 film with intensifying screen with 5-10 mm spacer– Multisection radiography

• Loaded with a set of films of varying speed intensifying screen but no spacer- for desired range of image density(bone to soft tissue)

Care and maintenance

Care of x-ray cassette– Treat gently– Carry securely– Stored in vertical position– When placed directly under patient-use of cassette tunnel– Waterproof

Cassette maintenance– Record date of introduction– Identify each cassette with intensifying screen– Keep record of cassette maintenance – Inspect regularly for signs of damage

Loading and unloading a cassette

• Unloading– Under safelight– Cassette face down ward– Clip released– Turned over– Front of the cassette tipped so the film falls from cassette well– Film removed with free hand

• Loading– Under safe light– Cassette face down ward as before and opened from back– Unexposed film lowered gently into cassette well– Closed by bringing over the back and engaging the locking clip

Other types of cassette• Vacuum cassette

– Supplied with vacuum pump– Made from flexible vinyl material

with a valve attached at one edge– Inside is a removable plastic

folder with single intensifying screen

– Single side emulsion film inserted with emulsion surface facing the intensifying screen

– Sealed and a sliding clamp place– Air expelled with vacuum pump-

thus very close contact – Mammography , where flexibility

needed

• Formatter cassette– For imaging from CRT, TV monitors

in USG, nuclear medicine, CT, DSA, MRI.

– Used in conjunction with a formatter or video imager

– Consist a frame designed to hold two single sided emulsion films & two removable slides(to protect from light exposure)

– Do not contain intensifying screen– Emulsion side facing outward.

• Photofluorography cassette– Recording of images from the

output phosphor of an image intensifier tube , carried out on 70mm or 105 mm roll films or 100 mm sheet films

– Details later..

Angiography casstettes• Siemens Elema AOT

system– Film feed or supply

cassette hold up to 30 sheets of unexposed films 35 x 35 cm in sixe , each positioned between metal spacers

• Puck system– Films are rapidly conveyed

from supply to take-up cassette.

Thank you .