INTENSIFYING SCREEN

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INTENSIFYING SCREEN : 

-SHAILENDRA R. PANDEYA BMIT, INTERN HOSMAT College of radiography HEI, Bangalore INTENSIFYING SCREEN

Introduction : 

Introduction The fluorescing materials which are used to increase the photographic response of the silver halide emulsion The main function of the intensifying screen is to reinforce the action of x-rays by subjecting the sensitive emulsion to the effects of light When x-rays falls on certain substances light is emitted The emission of light from a substance bombarded by radiation is termed as luminescence

luminescence : 

luminescence The emission of light from the substances bombarded by radiation is called luminescence ( i.e. invisible spectrum is changed into visible spectrum) Luminescence has two effects:- -Luminescence played a part in discovery of the x-ray; -card coated with barium platino-cynide, key factor

Slide 4: 

Fluorescence:- luminescence is excited only during the period of irradiation & will die upon the termination of the x-ray exposure Phosphorescence:- It is afterglow; i.e. the irradiated materials continues to emit light for sometime after cessation of exposure (Afterglow in any appreciable degree is not acceptable in luminescent materials esp. in imaging)

Fluorescence: Comparison To Phosphorescence : 

Fluorescence: Comparison To Phosphorescence

Luminescence Effect In Radiology : 

Luminescence Effect In Radiology Used in two ways;- - To obtain an image on a fluorescent screen which can be observed at fluoroscopy or may be recorded by camera as in MMR To increase the photographic response of the silver halide emulsion

Color Of Luminescence : 

Color Of Luminescence All the phosphors do not emit the same color. While manufacturing the screens the spectral sensitivity of the film is always considered 25 100 50 75 4000 5000 6000 7000

Fluorescent Materials : 

Fluorescent Materials These are the materials which converts invisible radiation into luminous radiation Commonly named as phosphors Used in crystalline & very pure form Even small traces (1/1000) of impurities will diminish the efficiency of the phosphors Expensive The term fluorescence was coined in 1852, derived from the mineral fluorspar

X-ray Phosphors : 

X-ray Phosphors To achieve its purpose, these materials must posses two characteristics It must absorb x-ray energy (absorption coefficient) It must effectively converts the absorbed x-ray energy to light to which the photographic emulsion is sensitive (brightness gain) It must also be transparent to its own luminescence & must allow the luminescent radiation easily to leave the affected crystal

Phosphors Used For Intensifying Screen : 

Phosphors Used For Intensifying Screen Calcium tungstate Barium fluorochloride Gadolinium oxysulphide Lanthanum oxysulphide Lanthanum oxybromide Yattrium oxysulphide

Calcium Tungstate (CaWO4) : 

Calcium Tungstate (CaWO4) 1st substance used in the preparation of intensifying screen n still continues It luminance in the pure state & requires no activators It emits from ultra violet to yellow green band of spectrum (max. fluorescence is about 420 nm) (x-ray film is most sensitive in this region of the spectrum) Color of fluorescence is independent on kv of radiation but the intensity of light is dependent on the intensity efficiency;- 5%

Barium Fluorochloride (BaFCl) : 

Barium Fluorochloride (BaFCl) It absorbs much more x-rays, i.e. higher absorption coefficient) Has greater efficiency to convert x-rays to light (greater brightness gain) Europium activated BaFCl emits ultraviolet & blue light in a narrow band with max.380 nm To obtain same densities it requires half the exposure value than Calcium tungstate

The Rare Earths : 

The Rare Earths Rare !!! ??? They are difficult to isolate & costly to prepare Difficult to get in pure form

Slide 14: 

The screens to which is given the collective description of rare earth consists essentially of four phosphors Gadolinium oxysulphide, activated by terbium(Gd2O2S:Tb) Lanthanum oxysulphide, activated by terbium(LaO2S:Tb) Lanthanum oxybromide , activated by thulium & terbium(LaOBr:Tb) Yattrium oxysulphide , activated by terbium(YO2S:Tb) Rare earth materials based on lanthanum & gadolinium exhibits the desired efficiency in absorbing x-rays & usually employed in medical imaging The rare earths

Slide 15: 

The gadolinium oxysulphide with terbium is most useful combination & it emits blue emission as fast calcium tungstate Lanthanium oxysulphide with terbium emits green light Lanthanium oxybromide with thulium emits ultra violet & blue band & with terbium emits mainly in blue region The rare earths

Slide 16: 

Yttrium is much lighter than the others Its absorption of x-rays is lower & has no better performance than calcium tungsaste But its conversion power is much higher which makes screen as fast as other rare earth’s Yttrium oxysulphide with terbium emits white light Disadvantages: More restricted in choice of film Amber light cannot be used as darkroom safe light , green or red can be used The rare earths

Construction : 

Construction Intensifying screen consists of; Base White pigment layer Fluorescent layer Super coat Cross section of screen Cross section through cassette

Base : 

Base May be polyester of cellulose acetate Must be radioparent & chemically inert Tough & flexibility

White Pigment Layer : 

White Pigment Layer The base is coated with a smooth white pigment layer of magnesium oxide or titanium dioxide Its function is to direct the emitted light to the film

Fluorescent Layer : 

Fluorescent Layer Chief component of the screen Over the white pigment layer uniform homologous layer of the fluorescent material is coated

Super coat : 

Super coat Over the fluorescent layer a thin transparent tough waterproof layer is coated It protects the fluorescent layer from scratching or any kind of abrasion

Types : 

Types Standard Intensifying Screen: -described as standard or regular or par speed - calcium tungastate is used as florescent material -gives adecuate speed with satisfactory details -suitable for general radiography

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High Speed Intensifying Screen; -calcium tungstate screen which have speed as their special features -since it is fast there will be loss of sharpness & detail of the image - used where there is a risk of motion unsharpness Types

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High Definition Intensifying Screen; -deigned in such a way that it gives fine detail & sharpness -these are slower in speed -used for immobilized parts as it takes long exposure Types

Slide 25: 

Graduated Intensifying Screen;- -designed with progressively decreasing intensifying factor -graduated longitudinally -available in large sizes & are intended for long scale of radiation intensities Types

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Other Intensifying Screens; -these are rare earth screens -increase in speed while maintaining the details & sharpness -requires less exposure -are used where there is risk of radiation to the patient & in rapid film changer Types

Slide 27: 

High K V Screen; -also called as lead intensifying screen -barium lead sulphide is used as fluorescent material -never used at low kv range but are used with 250 KV x-rays or cobalt gamma ray Types

Intensifying Factor : 

Intensifying Factor Density D1 EB EA B A It is the ratio of the energy( mAs) necessary to produce certain contrast on a film exposed alone, relative to that required by the same film with screen exposure with out screen Exposure with screen IF= i.e. IF= EA EB

Example… : 

Example… If a screen has an IF 15.. & a film is exposed without screen with 60KV & 150 mAS. Without screen it requires…. 60KV & 50 mA for 3sec i.e. 50x3=150 With screen it requires…60KV & 50mA for 0.2sec i.e. 50x0.2x15=150

Factors Affecting IF : 

Factors Affecting IF The amount of x-ray absorbed by screens The efficiency of its conversion to visible light How much of this light reach the film & its affect on film The IF value alone is hardly helpful information but if same film is exposed with out screen or with screen, it definitely show difference

Speed Of Intensifying Screen : 

Speed Of Intensifying Screen Ability to produce light i.e. conversion of invisible rays to the visible spectrum We can increase the speed of screen by increasing.. -thickness of florescent layer - crystal size - types of fluorescent material used But increasing crystal size it lead to unsharpness of the image * it is overcome by manufacturer by; -use of crystalline powder - use of dye esp. yellow or red

Types Of Screen With Respect To Speed : 

Types Of Screen With Respect To Speed Ultra fast High speed Medium or par speed Detail or slow speed Ultra detail

Factors Affecting The Speed Of Screen : 

Factors Affecting The Speed Of Screen Divided into two; - Intrinsic factors - Extrinsic factor

Intrinsic Factors : 

Intrinsic Factors Phosphor Thickness of layer Size of the phosphor crystal Reflectance of backing

Extrinsic Factor : 

Extrinsic Factor Kvp Temperature -mAs value have to be increased by 40% at 50^C - have to be decrease by 25% at 10^C

Care Of Intensifying Screen : 

Care Of Intensifying Screen In dark room cassettes are opened, stored or loaded in the vicinity of chemicals Use of other equipments carefully near the cassettes Cassettes should not be placed near the hot pipes Finger them as low as possible Protect them from exposure to damaging agents such as chemicals or any sorts of physical assault of any kind Proper and regular cleaning of screens

Method Of Cleaning Screens : 

Method Of Cleaning Screens Undust with soft brush Wipe with cotton wool swab Scentless soap or detergent can be used but never use any kinds of organic agents Lukewarm water can be used Finally wipe with dry soft cloth

Test For Screen Contact : 

Test For Screen Contact The screen contact test should be done to see whether the film & screen are in good contact or not, if not it will cause blur of the image causing unsharpness Requirements;- -sheet of zinc or copper which is 0.91mm thickness & continuously perforated with holes of 2.4mm

Method : 

Method Take a sheet enough to cover the cassette Suspected cassette is loaded & placed on the x-ray table with zinc sheet over it in close contact Expose with FFD not less than 59” having centre of the cassette using 50KV & 5mAs Process the film & observe Results:- -Where there is lack of contact in the cassette between film & screen appears dark, the darkness will cover the holes of the sheet

References & Further Studies : 

Radiographic imaging - D N & M O Chesney Radiographic photography - Manipal press www.radiopolosis.com www.wikiradiography.com References & Further Studies

Slide 41: 

Presenter:, Shailendra Raj Pandeya

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