INVENTED BY DR. GUSTAV BUCKYDEVICES THAT REDUCE THE AMOUNT OF SCATTERED RADIATION REACHING THE IMAGE RECEPTORGRIDS DO NOT REDUCE THE AMOUNT OF SCATTER RADIATION REACHING THE IMAGING PERSONNELGRIDS DO INCREASE RADIATION EXPOSURE OF PATIENTSFACTORS AFFECTING SCATTER PRODUCTION WITHIN THE PATIENT KILOVOLTAGEBEAM SIZETHICKNESS OF IRRADIATED TISSUECOMPOSITION OF IRRADIATED TISSUEZ# OF IRRADIATED TISSUEGRIDS STRIPS ARE MADE OF LEAD (Pb) LEAD HAS Z#AND ABSORBS SCATTER RADIATION THROUGHphoto electric INTERACTION CONS HIGHER PATIENT DOSE

PROSHIGHER CONTRASTINCREASED VISIBILITY OF DETAIL (CONTRAST RESOLUTION)GRID RADIOGRAPHY IS RECOMMENDED FOR:ANATOMICAL PARTS > 10 cmWITH HIGH kVp ( NOT ALWAYS—MAMMO)SOFT TSSUE STRUCTURES TO INCREASE CONTRASTSTRUCTURES AFFECTED BY PATHOLOGICAL CONDITION THAT WOULD INCRESE SCATTER PRODUCTION SCATTER RADIATION-PRODUCED WITHIN THE PATIENT THROUGH compton INTERATION SCATTER

GRIDS

GRID CONSTRUCTION

GRID RATIO G.R.= H/DGRID SURFACE X-RAY ABSORPTION—TYPICAL GRID STRIPS-50 MICROMETERSINTERSPACE –350 MICROMETERSX-RAY ABSORPTION (%)= WIDTH OF STRIPS X 100 ___________________________________ WIDTH OF STRIPS + WIDTH OF INTERSPACEHIGH QUALITY GRIDS CAN ATTENUATE 80%-90% OF SCATTER RADIATIONGRID RATIO VS CLEANUP

BUCKY FACTOR

BF=I.R./T.R. GRID CONVERSIONSNO GRID5:18:112:116:11 X MAS , KVP X 12 X MAS , + 8-10 KVP4 X MAS , + 12-15 KVP5 X MAS , + 20-25 KVP6 X MAS , + 30-40 KVPNEW MAS= ORIGINAL MAS X NEW GRID FACTOR/OLD FACTORGRID FREQUENCY# OF LEAD STRIPS PER INCH OR CMG.F.= 10,000 MICROMETERS/CMT + D MICROMETER/LINE PAIRT-STRIP WIDTH D-INTERSPACE WIDTHGRID TYPESSTATIONARYMOVING-SINGLE STROKE & RECIPROCATINGLINEARCROSSHATCHFOCUSEDMOVING GRID DR. HOLLIS POTTER MODIFIED BUCKY DIAPHGRAM POTTER-BUCKY DIAPHGRAM

GRID RATIO VS CLEANUPRATIO SCATTER

CLEAN-UP

CANTING =TILTING OF THE LEAD STRIPS TO CREATE FOCUSED GRID

CROSSHATCH GRID

LINEAR GRID

BIGGEST DISADVANTAGE OF LINEAR & CROSS GRID= GRID CUT-OFF

GRID EFFICIENCY= CIF and GSCONTRAST IMPROVEMENT FACTOR CIF= CONTRAST WITH GRID/CONTRAST WITHOUT GRIDGRID SELECTIVITY GS=NONSCATTER TRANSMITTED/SCATTER TRANSMITTEDGRID ERRORSOFF LEVEL OFF CENTEROFF FOCUS

UPSIDE DOWNOFF FOCUS & OFF CENTERCUTOOF ACROSS ENTIRE IMAGE, LIGHT IMAGECUTOOF ACROSS ENTIRE IMAGE, LIGHT IMAGECUTOFF TOWARD THE EDGE OF THE IMAGESEVERE CUTOFF TOWARD THE EDGE OF THE IMAGEDARK ON ONE SIDE & LIGHT ON THE OTHER

GRID ARTIFACTSMOIRE EFFECT PLACING GRID IN BUCKY MECHANISMSTROBOSCOPIC EFFECTDAMAGED GRIDSTROBOSCOPIC EFFECT (MOVING GRIDS) -MOTION OF THE GRID IS FROZENWHEN USING SHORT EXPOSURE TIME ( SHORTER THAN MOVEMENT OF THE GRID) RECIPROCATING MECHANISM IS BROKENAIR GAP TECHNIQUE

AIR GAP TECHNIQU

E

Salford University, Imaging BSC1 N.J. OldnallIntensifying Screens.docNick Oldnall Page 1 24/01/991Intensifying ScreensIntroductionThe efficiency of X-Ray film to absorb x-ray photons is only ª 1%.Purpose PurposeTo amplify the film blackening effect on the film of an X-ray exposure by the conversion of X-ray photons to light photons to which the film emulsion is sensitive. This depends on theefficiency of the screen phosphor to absorb X-Ray photons and convert them to ultra violetand visible light to which the film emulsion is optimally sensitised.LuminescenceThe emission of light by a substance when excited by any form of energyWhen certain materials absorb various kinds of energy, some of the energy may be emittedas light. This process involves two steps: (1) the initial energy causes the electrons of theatoms of the absorbing material to become excited and jump from the inner orbits of theatoms to the outer orbits; (2) when the electrons fall back to their original state, a photonof light is emitted. The interval between the two steps may be short (less than 1/100,000 ofa sec) or long (many hours). If the interval is short, the process is called fluorescence; if theinterval is long, the process is called phosphorescence. In either case the light produced isalmost always of lesser energy, that is, of longer wavelength, than the exciting light.1Fluorescence FluorescenceThe emission of light which ceases within 10-8 seconds of the removal of the luminescentenergy source. I.e. X-Ray screens. (No afterglow)Phosphorescence PhosphorescenceThe emission of light which continues on or is delayed after the removal of the luminescentenergy source is removed, i.e. luminous watch dialsIntensifying Screen Structure 1"Luminescence," Microsoft® Encarta® 96 Encyclopedia. © 1993-1995Microsoft Corporation. All rights reserved. © Funk & WagnallsCorporation. All rights reserved.Salford University, Imaging BSC1 N.J. OldnallIntensifying Screens.docNick Oldnall Page 2 24/01/992Diagrammatic representation of Screen structure, not to scale. Diagrammatic representation of Screen structure, not to scale.1 1 Supercoat or the Protective layer 5-10 Supercoat or the Protective layer 5-10 m mm mA strong smooth protective layer of cellulose acetate encapsulating and sealing the wholescreen, resistant to abrasion, moisture protecting the fluorescent layer, minimal thicknessreduces image unsharpness.2 Fluorescent layer 100 -200 2 Fluorescent layer 100 -200 m mm mAn even coating of microscopic phosphor dispersed and suspended in a binding material e.g.Cellulose acetate, nylon or polyurethane. Some phosphor materials are hygroscopic so needto be sealed in, by coating all sides dimensional stability is maintained.Desirable features of phosphors Desirable features of phosphors· High X-Ray absorption efficiency

· High X-Ray to light efficiency· Emission spectre matched to film sensitivity· Fast light emission· Absence of afterglow· Uniform light output i.e. uniform dispersion in suspension media.Types of phosphor Types of phosphorEarly Phosphors mainly blue emitters around 380 - 420 nmCalcium Tungstate, Discovered in 1897, Silver Zinc Sulphide, Barium lead sulphate,now superseded in most cases by the “rare earth” phosphors.Lanthanide Series Lanthanide Series or Rare Earth Elements, Rare Earth Elements, series of chemical elements of the periodictable. The rare earth elements (or rare earth metals) include the elements with atomicnumbers 57 to 71, and, named in order, are lanthanum, cerium, praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium,thulium, ytterbium, and lutetium. Yttrium (atomic number 39) and scandium (atomicnumber 21) are sometimes included in the group of rare earth elements. The elementsSalford University, Imaging BSC1 N.J. OldnallIntensifying Screens.docNick Oldnall Page 3 24/01/993cerium (atomic number 58) to lutetium (atomic number 71) are commonly known as thelanthanide series.2Rare earth Phosphors Rare earth Phosphors3 principal elements have been explored.Gadolinium (Gd), Yttrium (Y) and Lanthanum (La) and the most used compounds are theiroxy-sulphides (O2S) with activators Terbium and Europium with Terbium the most common.The function of the activator is to enhance the light output both in speed and thewavelength of the light to match the application of the screens use.Chart of the principle Characteristics of the most common “Rare earth” based Chart of the principle Characteristics of the most common “Rare earth” basedscreen phosphors. screen phosphors.AtomicWeight./AtomicNo:PrincipleEmissioncolourRelativeSharpnessEmissionefficiencyX-rayAbsorptionefficiency %@ 60 Kv.X-rayAbsorption

efficiency %@80 Kv.Gadolinium 157/64 Blue@550nmExcellent 18% 51 28Lanthanum 138/57 Green@600nmModerate 13% 33 17Yttrium 88/39 Green@600nmPoor 24% 12 5CalciumTungstate(Tungsten)*183/74* Blue@550nmExcellent 4% 13 27*X-Ray absorption generally increases with atomic number of the principal element.The emission spectra are line spectra with a series of peaks.For example lanthanum oxysulphide with terbium activator produces the main peak at600nm with smaller peaks at 550 and 625 nm.The thickness of the phosphor layer influences speed and definition i.e. increasing thethickness up to a point increases speed but also increases halation hence unsharpness,pigmentation of the suspension medium and the substratum can help to reduce halation.Recent advances include double phosphor and multiple layers. 2"Lanthanide Series," Microsoft® Encarta® 96 Encyclopedia. © 1993-1995 Microsoft Corporation. All rights reserved. © Funk & WagnallsCorporation. All rights reserved.Salford University, Imaging BSC1 N.J. OldnallIntensifying Screens.docNick Oldnall Page 4 24/01/9943 Substratum Layer 10 - 20 3 Substratum Layer 10 - 20 m mm mThe are 2 types of substratum and in some case there is no substratum layer and thefunction is incorporated into the phosphor layer.Reflective layer Reflective layerIn order to maximise the amount of light emitted by the phosphor layer a bright whitereflective layer is placed between the base and the phosphor layer to reflect as much of thelight produced towards the film, (note halation)Absorptive layer Absorptive layerIn order to minimise the halation effect hence minimise unsharpness caused by halationthere is sometimes an inert light absorptive dye place between the base and the phosphorto minimise internal reflections within the screen.Absorptive layers which control the screens light output can be used to match screens intoa graduated series for specialist applications such as multi-section tomography.4 Base layer 200 - 400 4 Base layer 200 - 400 m mm mThe base layer acts as a support for the other layers and common materials include card,polyester and plastic. The substratum layer is sometimes incorporated into the base.

All layers have to be designed or treated to adhere strongly together.Salford University, Imaging BSC1 N.J. OldnallIntensifying Screens.docNick Oldnall Page 5 24/01/995General Characteristics of ScreensScreen effects on Image QualityScreen characteristics are governed by various factors, such as phosphor particle size,suspension medium, layer thickness, phosphor /suspension packing density and the presenceor absence of the reflective/absorptive substratum.Screen Speed Screen SpeedScreen speed is s separate measured quantity to film speed or film screen combinationspeed. Generally this is expressed in terms of the amount luminance for a given X-Rayexposure. There is increasing support for a relative speed indication system where areference screen is set at 100 and others have their speed indicted relative to this i.e. 200requiring half the X-Ray exposure to produce the same level of luminance.The five following methods increase screen speed but will in most cases cause a change inother factors such as image sharpness and contrast. It must be remembered that anyindication of comparative screen speed must be for identical exposure conditions especiallythe Kv and the voltage waveforms of the generator used in the tests.Screen speed may be increased by the following methods.1. Selection of an efficient phosphor / activator combination.2. Increasing phosphor particle size (up to point)3. Minimising the binder volume4. Increasing the phosphor layer thickness5. Employing a reflective layer.Image Sharpness Image SharpnessRadiographic unsharpness is determined by the three factors.Movement unsharpnessGeometric unsharpnessFilm screen combination unsharpness, including the related film screen contact element.The following five methods increase screen sharpness but must be considered with the otherinfluences.1. Decreasing phosphor layer thickness2. Decreasing phosphor particle size3. Maximising protective layer transparency4. Addition of absorptive dyes to the phosphor layer5. Omitting the reflective layer.Note the interactions between the previous two lists.Salford University, Imaging BSC1 N.J. OldnallIntensifying Screens.docNick Oldnall Page 6 24/01/996Image contrast Image contrastScreen influence on Image contrast is one of the principal influences on image contrast theothers include exposure Kv and generator waveform and film properties patient type.Screen contrast is affected by1. X-Ray photon Kv.2. Phosphor type3. Speed difference between front and back screen,(A=B=X)(A@B=X-)Quantum Mottle Quantum Mottle

A film screen combination exposed to a uniform field of exposure produces an image with amottled appearance which is not due to the grain structure of the film, mottle is muchcoarser than film grain and is due to non even light emission from the intensifying screen.The light from individual phosphor particles produces a spot of density on the film if the X-Ray exposure is small as when very fast screens are used then this irregular spread ofimage points presents as mottle.Mounting and Care of ScreensMounting MountingIntensifying screens are in general mounted in pairs into a cassette and normally there is nodistinction between front and back. Exceptions to this may include mammography or highdetails usage applications where there may only be a single screen and in specialistapplications such as the chest imaging where there may be pairs of screens with distinctfront and back screen usage.Screens may be mounted in new cassettes by the manufacturers but sometimes it may benecessary to mount your own. It may be you have two cassettes with a damaged screen ineach and you want to make a single good set from the damaged pair.Screens should be mounted carefully ensuring they are not going to foul the cassette, aremounted on good pressure pads, are mounted with radiolucent adhesive which emits nofumes which may cause film fogging, air is mot trapped in the mounting adhesive,care should be taken when mounting lead blockers not to damage the screen or compromisefilm screen contact, if the screen is cut the exposed edge should be sealed with a minuteamount of glue or varnish to prevent hygroscopic damage.After mounting the screen apply the required identifier and then perform the film screencontact test as per the departmental quality assurance programme.Salford University, Imaging BSC1 N.J. OldnallIntensifying Screens.docNick Oldnall Page 7 24/01/997Care CareScreens are expensive objects and may cost several hundred pounds for a pair to fit a35cm x 43cm cassette.Screens should be cleaned as per the manufactures recommendations with particularregard to the use of solvent cleaners which may damage the protective coating.In general any dust and particles should be blown out, the cassette should then be brushedout with a soft paintbrush then a lightly moistened cotton wool ball should be wiped gentlyacross the surface, then the same with a dry one. Finally the cassette should be left openon it’s edge to dry in a convenient place.Do not scratch at any surface muck.Do not make the screen wet.Do not use unproved solvents.Ensure the screen is fully dry before reloading the cassette.Record the date of cleaning.

Scatter Control&Grid Use

Denise Ogilvie October 2007Objectives Identify factors that affect the amount of scatter radiation produced Describe methods used to control the amount of scatter radiation Describe the effect of beam restriction on image quality and patient dose Compare advantages and disadvantages of different beam restricting devisesObjectives Describe the purpose of a grid Explain the construction of a grid, including materials used, grid ratio and grid frequency Differentiate between parallel and focused grids, stationary and moving grids Calculate changes in technical factors to compensate for changes in grid selection Be able to identify common errors made when using a grid on an image Know when to use a grid and when not to use.Scatter Radiation Scatter is radiation which is changed in direction as a result of interaction with some medium. Some of the photon’s energy is absorbed, leaving the resultant photon with a change in its direction and with less energy These scattered photons are detrimental to contrast of the image and also increase the patient doseScatter Radiation Other sources of scatter – materials beyond the image receptor – table top – may cause scatter to go back to the image. Two primary factors affecting the amount of scatter produced –kvp and the irradiated materialScatter Radiation Kvp Affects the penetrability of the beam. Higher kVp, more photons go through patient to the IR, less absorbed by patient, higher scatter and less contrast on image Lower the kVp, increase in dose absorbed by patient, less fog on film, more contrasty imageScatter Radiation Irradiated Material

Amount of scatter affected by volume and atomic number of irradiated material Volume is controlled by field size and patient thickness Increase in volume if field size increases and patient thickness increases. Scatter Radiation To reduce scatter – smallest field size, compression of body part The higher the atomic number of the material the greater the absorption of photons and the less scatter eg bone compared to soft tissueScatter RadiationScatter Radiation Beam Restriction Aperture diaphragms, cones/cylinders, collimators – 3 types of beam restricting devices to control scatter and reduce patient doseScatter Radiation Aperture Diaphragm Simplest, low cost Flat piece of lead with hole (of different sizes) Slides into slot at bottom of collimator Some resultant penumbraScatter Radiation Cones and Cylinders Similar to diaphragm with extension cone or cylinder Slides into slot bottom of collimator Reduces penumbraScatter RadiationScatter Radiation Collimater More complex, most commonly used form of beam restriction Set of adjustable lead shutters Light & mirror to show area of beam and collimationScatter Radiation The bevelled edges of lead diaphragm compared to vertical edge.Radiographic Grids A device to absorb scatter radiation before it strikes the IR Made of thin Pb strips interspaced with

radiolucent material – usually aluminium Frequency – number of lines per inch or cm eg 60 lines per inch Grids with higher frequency have thinner Pb strips – better for stationary grids so you don’t see the lines The more Pb the better the scatter reductionRadiographic Grids Types Parallel – Pb & interspace running parallel to one another Focused – central strips parallel, then become more angled as you move away from the centre – angle matching that of divergent rays – allows more transmitted photons to reach the IRRadiographic Grids Crossed grid – 2 parallel grids on top of each other. May be parallel or focusedRadiographic Grids Focal range –recommended SID for that particular grid. For parallel grid focal range is from certain SID to infinity –function better at longer SIDRadiographic Grids Grid Ratio Ratio of height of Pb lines to distance between them Grid ratio increases, contrast increasesRadiographic Grids The higher the grid ratio the more exposure is requiredRadiographic Grids Potter Bucky – moving grid for better scatter clean up and improved image quality Grid is moved during the exposure to blur out grid lines. Movement must commence before exposure can be madeRadiographic Grids Air gap technique

Between patient and film Eliminates need of grid Gap of at least 15cm –increase SID to reduce magnification The scatter from the body does not hit the IRRadiographic Grids Grid Errors Upside down grid –peripheral grid cut off with a focus grid Check front of grid –upper side has line down centre indicating direction of grid linesRadiographic Grids Off centre – tube not centred to middle of grid. Result in decrease in exposure across entire image and visible grid lines The greater the decentering the greater the grid cut offRadiographic Grids Off level grid – tube angled across long axis of Pb strips Show grid lines with decrease in exposure on imageReferences Burns, E, Radiographic imaging a guide for producing quality images, Saunders 1992 1stedn Carlton, R, Adler, A, Principles of radiographic imaging an art and a science, 4th edn Fauber, T, Radiographic imaging & exposure, 2000 Kodak, The fundementals of radiography,11th edn Stockley, S, A manual of radiographic equipment,1stedn, 1986

INTENSIFYING SCREENS, CASSETTES AND SCREEN FILMS N. Serman & S. Singer

X-rays were discovered by W.C. Roentgen because of their ability to cause fluorescence. X-ray photons cannot be seen. The image produced by X-rays may be captured on a film, may be viewed directly (fluoroscopy) or on a monitor with digital radiology. Roentgen initially used a sheet of platinocyanide to view the fluorescence produced by X-ray photons. It was shortly thereafter that photographic plates were adapted for radiographic purposes. The combination of screen films, intensifying screens and cassettes are used in making extraoral images. The main function of screens is to reduce radiation to the patient. Currently, there are two groups of X-ray films for dental purposes: 1. Non-screen - Those with emulsions more sensitive to direct exposure of X - rays. These are primarily used as intraoral films and provide excellent image quality. 2. Screen - Those with emulsions more sensitive to blue [standard] OR green [rare earth] light. Emitted when X-rays strike the intensifying screens. The X-ray photons are converted to visible light photons. Screen film is used for extraoral views, such as panoramic, cephalometric and TMJ imaging. It is manufactured with dyes in the emulsion that absorb specific wavelengths of visible light. Unfortunately, it does not produce the image detail of non-screen films. Screen films are always used IN COMBINATION with intensifying screens. With screen-film, it is mainly the light photons from the intensifying screens that produces the image on the film and not the photons. Intensifying screens permit a good radiograph to be produced with the patient receiving a much lower dose of radiation. This film is very sensitive to both X-ray photons and light photons but much more sensitive to light photons. INTENSIFYING SCREENS. An intensifying screen is a plastic sheet coated with fluorescent material called phosphors. Phosphors are materials which convert photon energy to light. LUMINESCENCE is the emission of light from a substance bombarded by radiation. There are two types; fluorescence and phosphorescence. Fluorescence means that luminescence is excited only during the period of irradiation and will terminate at completion of the X-ray exposure. The phosphors in intensifying screens produce fluorescence. Phosphorescence is afterglow. The irradiated material continues to emit light for a time after cessation of exposure to radiation and will continue to produce an image which you do not want. The luminescent effect is used radiographically in two ways: - 2 - 1. To obtain an image on a fluorescent screen as in fluoroscopy, and 2. To increase the photographic response of the silver halide emulsion. In this case the fluorescent material is placed in the emulsion layer on the intensifying screen, in

direct contact with the film during exposure. Since X-ray films are coated with emulsion on both sides, intensifying screens are employed in pairs. Each emulsion surface is placed in close contact with the effective surface of one intensifying screen, to avoid loss of image sharpness. Speeds of Intensifying Screens. 1. Fast screens - thick layer, and relatively large crystals used, maximum speed is attained but with some sacrifice in definition. 2. Slow screens or high definition screens - a thin layer and relatively small crystals are used; detail is the best, but speed is slow necessitating a higher dose of ionizing radiation. 3. Medium screens - medium thick layer of medium sized crystals in order to provide comprise between speed and definition. There are three types of intensifying screens: a) Standard - slow screens b) Rare earth - fast screens c) Combination Standard screens use calcium tungstate phosphors, while rare earth screens use gadolinium or lanthanum phosphors. The commercial name for rare earth screens is Lanex. Rare earth phosphors are more efficient at converting X-rays to visible light thus reducing the radiation further to the patient. The manufacturers name and the type of screen are printed on the one side of the screen and this information appears on the radiograph. The intensifying screen is placed in a cassette in close contact with a film. The visible light from its fluorescent image will add to the latent image on the film. Its function is to reinforce the action of X-rays by subjecting the emulsion to the effect of light as well as ionizing radiation. The benefit is the reduction in dose of ionizing radiation to the patient. Characteristics of Intensifying Screens 1) An intensifying screen consists of a base of polyester or cellulose triacetate similar to radiographic film 2) This base must be radioparent 3) and chemically inert. 4) It must combine characteristics of toughness and flexibility 5) should neither curl 6) not discolor with age. 7) The base is first coated with a reflective layer of titanium dioxide to bounce light back onto the film. Divergence of the light rays causes unsharpness of the image. 8) with a uniform homogeneous phosphor layer- standard or rare earth. 9) this is covered with a thin transparent supercoat consisting of gelatin. The

purpose of the latter is protective, and is very thin and care is always required in handling intensifying screens to avoid any kind of abrasion 10) The flexibility of the material is important to allow the screen to bend without cracking - an intensifying screen of this type is used in the panoramic cassette. Each crystal on the screen emits bluish light for regular screens (or green light for rare-earth-screens) Brightness is related directly to the intensity of the X-rays in that minute portion of the image. Thus, over the entire surface of the screen, differences in X-ray intensities are transformed into differences of bluish light (green light) brightness to which the film is highly sensitive. The entire image is thus intensified for recording by the film. The larger the crystals and the thicker the fluorescent layer on the screen, the more light is produced and the greater the intensification. However, the light spreads more widely and the sharpness of detail of the image is decreased accordingly. Manufactures have attempted to improve image quality without sacrifice to film speed by using phosphor crystals of different shapes. An example of this is the T-Mat film that we use for panoramic and extraoral radiographs. Increasing Film Speed. 1. Thicker phosphor layers. 2. Higher conversion efficiency. 3. Higher absorption phosphor. 4. Decreased resolution of image The film-screen combination must be matched so that the emission characteristics of the screen match the spectral sensitivity of the film. It is also important to note that when double-loading cassettes, one must use a faster film (e.g.: T-Mat H) or increase the kVp as only one side of each film will be in contact with the intensifying screens. Regular Inspection 1. Intensifying screens in a flat cassette may come loose and should be re-attached immediately. Loose screens are an invitation to error in the darkroom. It is easy, when loading a cassette, to slip the film on top of both screen if they are unattached. 2. The felt pad or foam rubber in the back of the cassette may have become insecure or worn. This can result in failure of the intensifying screens to maintain uniform contact with the film and this causes a localized area of unsharpness on the radiograph, due to the spread of fluorescent light between the screens and the emulsion. There is nothing that can be done for a cassette which is failing to maintain contact between the intensifying screens. 3. Screens which are old or cracked can be seen to have fairly mottled appearance and this will be reproduced on radiographs. When this is noticed it is time to discard the screens. As they are sold in pairs, there is little to be done except to replace both screens in the cassette.

(Like one glove on its own, it may subsequently never have a match.) Care of Intensifying Screens Screens are easily damaged. Their fluorescent emission will be affected if the active surface is soiled even slightly. Screens must thus be kept clean otherwise light photons will be prevented from reaching the screen and creating an image and the screen in that area will appear clear. Dirt will also create “high” spots which will create wear. Screens are best cleaned with antistatic solution. Use a damp cloth and rub gently. Ensure that the screen is dry before closing the cassette otherwise the gelatin on the surface of the screens will stick together. Never leave the cassette open as it will accumulate dirt and dust on the screen. CASSETTES Definition: A flat, light-tight container in which x-ray films are placed for exposure to ionizing radiation and usually backed by lead to eliminate the effects of back scatter radiation. Cassettes are used in association with intensifying screens and screen films. They have related functions: 1. to contain a film 2. to exclude light, 3. to maintain the film in close, uniform contact with both screens during the exposure 4. to protect the intensifying screens from physical damage. The structure of a standard cassettes suggests a book as it consists of two flat rectangular plates hinged along one edge. The front aspect of the cassette faces the x-ray tube and consists of a sturdy metal frame into which is fixed a sheet of either light metal such as aluminum, or plastic material; the critical point being that it must be transparent to x-rays. The frame constitutes a shallow container into which can be placed thin intensifying screens and a film. The back of the cassette is constructed from a strong metal. It is customary to spray the internal surface of the back of the cassette with lead paint, the purpose of which is to absorb secondary radiation, preventing it from being scattered back onto the film. The back of the cassette contains a felt pad. The intensifying screen at the back of the film lies on this felt and is usually glued to it. The function of the felt is to maintain this screen, the film and its fellow screen in uniform, firm contact. The front and back of the cassette are held tightly together, either by spring clips on the edge opposite to the hinge or by means of pivoted resilient metal bars on the back of the cassette. These fit into grooves in the frame. The cassette utilized with most panoramic radiographs are made from plastic that can bend. However, when they are placed on the drum of the panoramic machine they become rigid, and the functions as stated above apply.

Properties of a cassette: a) weight - It should be light for easy manipulation b) robust structure - cassettes in daily use are subject to considerable stress and wear. Screens may fail to maintain contact with the film or leakage of light at the edges can occur. Cassettes deserve and should have stringent care in handling. c) i. Non flexible - so as not to allow the film to bend. ii. Flexible cassettes - for panoramic machines. Flexible cassettes are necessary for the specialized equipment associated with panoramic radiography. They are mounted within a simple envelope of plastic material, folded at one end and fastened with press buttons or velcro of conventional design. The cassette is attached to a drum and is rigid for the duration of the exposure. d) Size - Slightly larger than the x-ray beam and area to be radiographed. e) Ease of operation. Care of cassettes: Treated with care x-ray cassettes [and intensifying screens] are good for years of hard work. Their general care is aimed at the avoidance of rough handling by all who use them. It is helpful to mark each cassette, with identifying numerals which are inconspicuous - it makes it easy to eliminate, if radiographic faults are observed, ascribable to damage of some kind. e.g. cracks in the intensifying screens or light leaks. Screens come with a sticker indicating the film speed and this sticker is placed on the outside of the cassette. Regular Inspection Cassettes should be inspected at regular intervals to maintain them in serviceable condition. Hinges and clips are subject to stress and their proper functioning should be checked frequently to assure that wear has not occurred. Intensifying screens may come adrift and should be re-fixed immediately. Loose screens are an invitation to error in the darkroom for even with the best of intentions it is easy, when loading a cassette, to slip the film on top of both screens if these are unattached. The felt pad in the back of the cassette may have become insecure or worn. This can result in failure of the intensifying screens to maintain uniform contact with the film and this in turn causes a localized area of unsharpness on the film due to the spread of fluorescent light between the screens and the emulsion. Flexible cassettes may tear at the edges allowing the entry of light, and this must be regularly checked. Test for light leaks

Following a number of fogged films, physical inspection of a cassette usually makes evident the admission of extraneous light (black areas) and the points at which it has entered. Broken clips or hinges, buckled corners or loose fronts are the likely causes. Tears often occur at seams and other stress points of flexible cassettes. Heat diminishes the efficiency of intensifying screens. Cassettes should not be left lying close to radiators or stored near hot pipes. It may be noted that film emulsions gain speed with increased temperatures, but screens lose speed. If the cassette has a plastic front, this may warp and spoil the contact with the front screen, thus reducing the detail of the image. In the darkroom cassettes should never be stored, opened or reloaded in the vicinity of chemicals. An open cassette lying on the bench almost certainly will be a victim to anything falling onto it (including dust). It is good practice always to leave cassettes closed, and placed on a shelf. Cassettes that are not loaded should not be left locked. Written and posted darkroom instructions should be posted near the darkroom and included in the office manual. Cassettes need to have the letters R or L (made of lead) placed on the exposure (front) side to indicate the right or left side of the patient. This is for both diagnostic and medico-legal purposes. In the panoramic machines the "L" and "R" are part of the head support of the machine. A grid is sometimes placed in a cassette to avoid scatter radiation from reaching the film and diminishing the detail of the image. The screen speed is always recorded on the outside of the cassette to ensure that the film and the screen speed correspond, otherwise the detail and density of the image will be affected.