RADG 316 VI: a PowerPoint presentation for Third Year Radiography Students

Radiographic Intensifying Screens

1. RADG 316 VI The Recording System: Intensifying Screens

2. Table Contents Iuminescence intensifying screens and unsharpness Screen construction Types of phosphor Matching film to intensifying screen Types of screen and their applications Variation in speed of front and bad screen Single-screen radiography Other less common types of screen Intensification factor (IF) Quantum mottle Factors of screen construction affecting speed and sharpness Assessing the relative speed of a new film-screen system Care and maintenance of intensifying screens Advantages of using intensifying screens

3. Intensifying Screen Because X-rays arc so penetrative, only about I% of them are absorbed by the emulsion layers. Thus, when X-rays alone are used to create radiographic images (non-screen radiography), most pass straight through the film without causing any film blackening. In order, therefore, to create adequate radiographic density, comparatively large exposures have to be made. An intensifying screen works by converting X-ray energy into light energy. As X-ray photons pass through the phosphor layer of the screen, they are absorbed by the phosphor crystals which then become excited and fluoresce, emitting ultraviolet and/or visible light

4. Why is it Called Intensifying Screen? For every X-ray photon absorbed, hundreds of light photons are emitted by the screen, so the screen can increase the effect of a beam of radiation emerging from a patient by creating a very large number of light photons for a relatively small number of X-ray photons incident upon the screen. This feature, in addition to the fact that film is more sensitive to light than X-radiation anyway, means that the screen is effectively intensifying the photographic effect of the beam. As a consequence, radiographic exposures can be reduced considerably over those required for non-screen imaging.

5. The interaction of an X-ray photon with a screen phosphor cr ys tal results in the emission of light in all directions. That light tra ve lling towards the film emulsion is absorbed h,· the film's si lve r halide crystals producing the latent image. An intensifying screen works by converting X-ray energy into light energy. As X-ray photons pass through the phosphor layer of the screen, they are absorbed by the phosphor crystals which then become excited and fluoresce, emitting ultraviolet and/or visible light

6. Luminescence The emission of light from a substance bombarded by radiation is termed luminescence and includes two effects: fluorescence and phosphorescence. Fluorescence, which is the light emission we described earlier, lasts only as long as the radiation exposure. Phosphorescence, on the other hand, is afterglow, i.e. light continues to be emitted for some time even after radiation exposure has ended. Phosphorescence is an undesirable phenomenon in X-ray imaging. The consequences of afterglow occurring in intensifying screens would make the use of a rapid film changer, for example, impossible.

7. Intensifying screens and unsharpness The use of intensifying screens inevitably means that a certain degree of unavoidable unsharpness will be introduced into the image in comparison to that obtained where non-screen film material is used. This unsharpness is due to light divergence. Light emitted from a phosphor crystal diverges on its way to the film. The further away each source of fluorescence is from the film, the greater the degree of light di v ergence and thus the greater the unsharpness .

8. Intensifying screens and unsharpness Another cause of unsharpness , but one which is avoidable, arises when a cassette becomes damaged and the close and uniform contact between the film and the screen is lost. When this happens the light arising in the _screen phosphor as a consequence of an X-ray photon interaction is allowed to spread to too great an extent and the image so produced is unsatisfactory. Radiographic unsharpness which arises as a consequence of using film and screen materials is referred to as intrinsic or photographic unsharpness , in contradistinction to geometric and movement unsharpness .

9. The effect of poor film/screen contact on image sharpness. The area of poor contact has resulted in the increased spread of light and the subsequent b lurring of image boundaries.

10. Screen construction A magnified cross-section through an intensifying screen.

11. Base Screen base is made from paper, cardboard, or more usually a clear plastic such as polyester, the function of which is to provide a strong, smooth, but flexible support for the fluorescent layer. The thickness of the base is about 0.18 mm. Additional characteristics required of screen base are that it should: • Be chemically inert; • Be uniformly radioparent; • Be moisture resistant; • Not discolour with age or on exposure to X-rays.

12. Substratum layer This is a bonding layer between the base and the phosphor layer. It may be reflective, absorptive or simply transparent in nature, depending on the manufacturers' intended characteristics for the screen. Use of a reflective substratum layer The function of a reflective layer is to maximize the effect of the screen by reflecting light which would otherwise be lost through the base, back towards the film emulsion. To give the screen this characteristic, a highly reflective white pigment, titanium dioxide, is added to the substratum layer.

13. Reflective substratum layer A reflective layer thus increases the speed of the screen since, for a given X-radiation input, the screen produces a proportionately greater blackening effect on a film than will a similar screen without a reflective layer. The reflected light, however, extends the real image boundary, so leading to an increase in photographic unsharpness . A magnified cross-section through tw o different intensifying screens. (a) No reflective layer so light is lost through base of screen. (b) The use of a reflective layer maximizes the in te nsifying effect of the screen by reflecting light back towards the film.

14. Use of an absorptive substratum layer An absorptive layer has a directly opposite effect to a reflective layer. Any light travelling backwards towards the screen base is prevented from being reflected by the base/phosphor interface and is instead absorbed. The use of an absorpti v e layer in an intensifying screen. Iight travelling backwards is effecti v ely absorbed. A screen with an absorptive v e layer is thus slower than a screen with a ref l ective layer, but has less photographic unsharpness . The substratum layer is made absorpti v e by adding a coloured dye to it.

15. Phosphor layer (fluorescent layer) This is the active layer of the screen. It consists of fluorescent crystals which emit light when struck by X-radiation. They are suspended in a transparent binder such as polyurethane. The binder material effectively holds the fluorescent crystals together and, because many phosphors are hygroscopic, polyurethane is an ideal choice because it prevents any moisture penetration which would otherwise cause reduced luminescence. In addition to the phosphor crystals, the binder in a high-resolution screen may also contain carbon granules or a coloured pigment (known as an 'acutance' dye). The function of both of these is to absorb any laterally scattered light within the fluorescent layer (irradiation) which, if it were to reach the film, would contribute to image unsharpness .

16. Phosphor layer (fluorescent layer) The use of carbon granules thus minimizes photographic unsharpness but reduces the speed of the screen. The type of phosphor used, the thickness of the layer and the density with which the crystals are packed, are all variable properties which will determine the nature and quality of a particular screen's performance in terms of speed. The term coating weight is often used in connection with screen phosphors. It is an expression of the quantity of phosphor grains incorporated in a phosphor layer, and will therefore depend on: (1) Grain size; (2) Coating thickness. In other words, the greater the number of grains, the greater is said to be the coating weight.

17. Supercoat This has a protective function and is made from acetate. It helps to resist surface abrasion and may include anti-static qualities, a valuable characteristic where the film is to be used with rapid sequence exposure equipment, for example. The coating must be thin ( - 8 µm) in order to reduce the distance between film and phosphor layer and thus minimize photographic unsharpness . It must be transparent, so that the light produced in the fluorescent layer may reach the film, and also waterproof in order to protect the sensitive phosphor crystals. The supercoat is extended around the edges and back of the screen, where it is effective both in minimizing edge wear and providing a non-curl backing.

18. Types of phosphor Materials which convert invisible radiation into luminous radiation are known as phosphors, and whilst a number of such substances exist, only a few of them have applications in radiography. Two qualities common to the selected phosphors are: (1) They are very efficient at X-ray absorption; (2) They fluoresce strongly, with little afterglow. Examples of some of the phosphors available are: calcium tungstate; barium fluorochloride ; barium lead sulphate; barium strontium sulphate; gadolinium oxysulphide; lanthanum oxysulphide; lanthanum oxybromide ; yttrium oxysulphide; yttrium tantalate. The last five named are commonly known as rare earth phosphors.

19. The rare earths In the late I 960s and early 1970s, new phosphors were developed from the rare earth series of elements, i.e. those elements with atomic numbers between 57 (lanthanum) and 71 (lutecium). These phosphors have two important physical attributes which give them an advantage over conventional phosphors such as calcium tungstate: ( 1) They are more efficient at absorbing X-ray photons (absorption efficiency or quantum detection efficiency); (2) They are more efficient at converting X-ray photons to light (conversion efficiency).

20. Quantum detection efficiency (QDE) Rare earth screens are generally more efficient at absorbing incident radiation than are conventional screens of calcium tungstate. This absorption property is known as the screen's QDE. The QDE depends not only on the type of phosphor but on the thickness and coating weight of that phosphor, an increase in either resulting in an increase in the QDE. In addition to these three factors, the QDE is also dependent upon the photon energy of the incident beam,

21. Comparison of the absorption characteristics of calcium tungstate and gadolinium o xy sulphid e phosphors at v arious photon energy le ve ls. However, whilst absorption differences may decrease, con v ersion differences, i.e. the ability of the rare earth screen to convert X-ray energy to light energy, remain far superior.

22. Conversion efficiency The rare earths are more efficient at converting X-ray photon interactions into light, e.g. 15-20%, the light conversion efficiency compared to calcium tungstate 3-5%. This means that for a given radiographic exposure much more light is produced with rare earth screens and so exposure factors can be reduced considerably when using them.

23. Use of activators Rare earth phosphors are invariably used in conjunction with activators which are small quantities of some foreign element added to the phosphor during manufacture. The choice of phosphor-activator combination not only determines the intensity of luminescence obtainable from the screen but also the colour of the light emitted. Common rare earth phosphors and activators.

24. Matching film to intensifying screen In order to obtain optimum speed from a film-screen system, i.e. to obtain maximum film blackening for the least radiographic exposure, it is vitally important that films are matched to the colour of intensifying screen emission. For instance, a screen phosphor emitting light towards the green end of the spectrum is best matched with an orthochromatic film. This will mean that the film's maximum sensitivity matches the screen's maximum light emission and consequently optimum performance, in terms of speed, will be obtained from that film-screen combination. If the same screen were inadvertently mismatched with a monochromatic film, then the light emission from the screen would have a much diminished effect on the film. In other words, for the same exposure, less film blackening occurs (i.e. system speed is decreased).

25. (a) Orthochromatic film correctly matched to green emitting screen phosphor. (b) mi smatching of film and screen. A monochromatic film emulsion has been used (maximum sensitivity around 420 nm) whilst the screen's greatest emission is in the 540 nm wa ve length range.

26. Types of screen and their applications All screen manufacturers produce at least three types of screen of differing speed. Screen manufacturers are able to produce a variety of screen speeds by the choice of phosphor and size of phosphor grain, the addition/ exclusion of absorptive/ reflective layers, and by varying the amount of reflective/absorptive material used in screen construction. Common screen speeds

27. High resolution ('Detail') High-resolution screens are manufactured by making the substratum layer absorptive. Slow-speed screens can he used when the fine detail is required, when radiation dose is less important and when the body part being radiographed does not necessitate a high tube loading (e.g. extremity radiography). These screens may require exposures two or three times greater than those used with 'regular' screens. Their use will therefore be contraindicated where there is a risk of patient movement, unless effective immobilization can be applied.

28. Regular & Fast Regular: This is a medium-speed screen which aims to give the radiographer the best of both worlds (adequate speed and sharpness). It is suitable for most general radiographic applications, and often provides the base from which, in practice, the speeds of other intensifying screens are calculated. Fast: These screens produce greater film blackening for a given radiographic exposure than do 'high-resolution' or 'regular' screens. However, detail sharpness will be diminished, due to the presence of the reflective layer and/ or the larger size phosphor grains used in their construction.

29. Application of Fast Screen These screens are chosen for radiographic examinations where the risk of unsharpness from movement is high, e.g. paediatric radiography. They are also ideal where there is the need to image dense body parts and yet maintain as low a patient dose as possible. The abdomen is a good example. Variation in speed of front and back screen

30. Single-screen radiography Although intensifying screens are nearly always used in pairs, one exception is in mammography where a single-screen cassette is used in conjunction with a single coated emulsion film. Eliminating the extra screen and emulsion reduces photographic unsharpness . If you examine one of these cassettes you will see that the screen is placed at the back, rather than at the front of the cassette as might at first be expected. The reason for the use of a back intensifying screen can be seen in Fig below.

31. The ad vantage of a back screen arrangement. Because the photon is absorbed ear ly in its tra v el through the fluoresc ent layer a front fitted screen affords more opportunity for the lateral spread of light and therefore increases image unsharpness . A hack screen, howe v er, means a reduction in light di v ergenc e and therefore a sharper image.

32. Other less common types of screen Graduated This is a screen where the speed gradually decreases across the screen from edge to edge, and is indicated on the screen by '+' and '-' signs. It is achieved by adding to the supercoat a pigment of gradually increasing density across the screen. Applications for this screen lie in pelvimetry and orthodontics. The fast 'end' of the screen being placed adjacent to the densest part of the patient (the vertebrae in the case of the pelvimetry), whilst the slower part of the screen is sited adjacent to the less-dense anterior abdomen.

33. Screens for multisection cassettes Because the radiation is progressively attenuated as it passes through the several pairs of screens of a multisection cassette, the bottom pair of screens will receive less radiation than the top pair, and the film thus exposed at this level will be underexposed in comparison to the uppermost films in the cassette. In order to obtain similar film blackening on all films in the cassette, the screen pairs are arranged in increasing speed from top to bottom, the film thus furthest away from the X-ray tube being exposed by the fastest pair of screens. The manufacturers produce these varying screen speeds by either adding varying amounts of pigment to the supercoat or by gradually increasing the coating weight of each fluorescent layer

34. Quantum mottle X-ray beam can be thought of as a stream of particles (quanta or photons) of radiation energy. The more photons per square centimetre or the higher the density of photons present in the transmitted beam, the greater will be 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.

35. Factors that Contributes to Quantum Mottle Any factor which contributes to a fall in X-ray absorption by the screen, e.g. increasing kV, poor absorption efficiency, or the combination of fast film-screen and low radiographic exposure, will lead to a fall in the number of X-ray quanta used to create the image, so the light photons produced by the screen will be less and the risk of producing a radiograph of non-uniform density with a mottled appearance will be increased.

36. Factors of screen construction affecting speed and sharpness Phosphor grain size Larger crystal sizes mean increased light emission (so an increase in speed) but increased light divergence, the latter factor contributing to loss of sharpness. The smaller the particle size, the greater the amount of light scattering within the phosphor layer leading to greater light absorption by this layer (Eastman Kodak, In other words, light emission is diminished and image sharpness improved.

37. Thickness of intensifying screen Effect of screen thickness on sharpness. The thicker the intensifying screen, the greater will be the lateral spread of light before it can reach the film and consequently the greater will be image unsharpness .

38. Other Factors Nature of substratum layer which can be reflective or absorptive layer can influence screen sharpness and speed earlier in this section. Presence /absence of carbon granules or pigment dye in binder

39. 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. Crossover thus produces increased blurring because of the longer path taken by the light in reaching the opposite emulsion and the consequent lateral spread of such light. Screens incorporating a reflective layer aggravate this problem. As we saw with film construction, some film manufacturers put an absorbing layer into their films between the base and the emulsion to prevent crossover

40. Cr o sso v er in an intensifying screen-film system. Light produced in one intensifying screen passes through film emulsion and base to produce a latent image in the opposite emulsion. Such extended light travel results in increased di v erg e nce and thereby increased image unsharpness . In (b) the addition of an absorbing layer in the film helps to pr ev ent crosso v er.

41. Crossover effect Film emulsions are very efficiency at absorbing UV light in particular, so screens of barium lead sulphate which emit strongly in the UV region of the spectrum are able to minimize crossover because most of the light emitted by the individual screens is absorbed in the adjacent emulsion. Features of screen construction which affect speed and sharpness.

42. Care and maintenance of intensifying screens An intensifying screen damaged in any way is irreparably damaged. Since screens are often sold in pairs correctly matched, the only remedy is to replace both. A very expensive exercise. Finger marks, stains, dust or other foreign fragments will all adversely affect the screens' fluorescent emission. The radiographic evidence will be of sites of reduced density, owing to the inhibition of normal fluorescence in the affected areas. The following rules will help to eliminate many of these hazards: • Do not open cassettes in the vicinity of chemicals or other liquids. • Do not leave cassettes open on the bench, exposed to contamination. Do no/ store cassettes near sources of heat such as radiators ( cassettes may warp and film/screen contact will be lost).

43. Cleaning screens Should screens require cleaning, the following procedure should be adopted using a mild liquid detergent or proprietary screen cleaner: (1) Moisten (not saturate) some cotton wool with the solution and gently wipe the screen surfaces. At no stage must the screen be allowed to become excessively wet, nor must water reach the back or edges of the screen. (2) Wipe the screen free of soap or cleaner with fresh cotton wool. (3) Wipe dry. ( 4) Stand cassette on edge, partly open, and allow to dry completely. (5) Record date of cleaning on back of cassette.

44. Advantages of using intensifying screens In comparison with the use of non-screen film materials, these advantages can be summarized as follows: (I) Lower exposure factors may be used, thus allowing for 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. (2) Permits the use of very short exposure times, which is useful where movement unsharpness may pose a problem. (3) Higher contrast image compared to non-screen imaging.