intensifying screen and its types by T.R.B.

Construction ,advancements, handling ,safe use, advantages and some demerits of intensifying screen are discussed.

1. Image intensifying screen Name: Thumba Raj Baruwal Roll no: 162 BSc MIT MMC ,IOM

2. Contents : Introduction History Construction of Radiographic intensifying screen Screen New phosphor technology Spectral matching of screen and film Care of I.S. screen Summery References .

3. Introduction 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.

4. Introduction A device that converts the energy of x-ray beam into visible light photons. This visible light then interacts with the radiographic film , forming the latent image The visible light is emitted from the phosphor of the screens, which is activated by the x-ray passing through the patient. 4

5. Introduction Less than 1% of the incident x-rays interact with the film to contribute to the latent image. The radiographic intensifying screen amplifies the effect of image-forming x-rays that reach the screen –film image receptor. Intensifying screen lowers the patient radiation dose considerably; but the image is slightly blurred. The radiographic film is sandwiched between two screens.

6. Orientation of intensifying screen and cassette The x-ray film used with intensifying screen has photosensitive emulsion on both side Film is sandwiched between two screens in a cassette so that emulsion on each side is exposed to the image forming light. In mammography, the film are single emulsion & are matched with single back screen. 6

7. Screen ,Film and cassette combination.

8. History Sir W.C. Roentgen discovered x-rays quit by accident. He observed the luminescence of barium platinocyanide , a phosphor that was never successfully applied to diagnostic radiology. Within 1 year of Roentgen’s discovery of x-rays , the American inventor sir Thomas Alva Edison developed calcium tungstate phosphor. Screen film combinations did not come into general use until about the time of World War I.

9. History With improved manufacturing techniques and quality control procedures , calcium tungstate proved superior for nearly all radiographic techniques and , until the 1970s, was used almost exclusively as the phosphor. Since then , rare earth screens have been used in diagnostic radiology.

10. Luminescence Any material that emits light in response to some outside stimulation is called a phosphor, and the emitted visible light is called luminescence . A number of stimuli, including electric current, biochemical reactions , visible light and x-rays cause luminescence in materials. Absorption of a single x-rays causes emission of thousands of light photons. Luminescent materials emit light of a characteristic color.

11. Luminescence Fluorescent The material which gives off light only during stimulus. Good for screen. It is the ability of the crystals of certain inorganic salts (Called phosphor) to emit light within 10 -8 second when excited by x-rays Phosphorescence : the material which continues to give off light after stimulus. Bad for screens called Lag or Afterglow. It is afterglow, i.e. light continues to be emitted for some time even after radiation exposure has ended and is an undesirable phenomenon in X-rays imaging system. 11

12. What actually happens?? The x-ray photon is absorbed by the target atom. The outer shell electron is raised to an excited state. It returns to a ground state with emission of a light photon. The light photon that is able to reach the film exposes the film and forms a latent image. 12

13. Screen Construction Most screens have four distinct layers; Protective coating Phosphor layer Substratum layer Base

14. Protective Coating(Super Coat) The layer of the radiographic intensifying screen closest to the radiographic film is the protective coating. It is made up of plastic ,largely composed of cellulose compound mixed with other polymers and it is transparent to light. It is 10 -20 micro-meter thick.

15. Functions Of Protective Coating 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. Thin coating to reduce distance between film & phosphor & minimize un-sharpness. Transparent to light so that light produced in fluorescent layer may reach the film. Waterproof to protect sensitive phosphor crystals. Extended around the edges & back of the screen to minimize edge wear & provide non-curl backing.

16. Phosphor Layer (Fluorescent Layer) The active layer of the radiographic intensifying screen that converts the x-rays beam into light. The phosphor layers vary in thickness from 50 to 300 micro meter , depending on the type of screen. The active substance of most phosphors before about 1980 were crystalline calcium tungstate embedded in a polymer matrix. The rare earth elements gadolinium, lanthanum, and yttrium are the phosphor material in newer ,faster screens.

17. Phosphor Layer Phosphor are hygroscopic so polyurethane prevents any moisture penetration to prevent reduced luminescence. Polyurethane binds or hold the crystals together. The binder in high resolution screen may contain carbon granules or colored pigment dye ‘acutance’. Laterally scattered light(irradiation)must be absorbed by the dye to prevent image Un-sharpness, but reduces speed of the screen. Phosphor layer vary in thickness from 50-300 micron depending on the type of screen. Individual phosphor crystals are 5-15 micron thick.

18. Phosphors Rare earth phosphors are widely used in conjunction with activators. Activators 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 color of the light emitted.

19. Properties of Phosphors used in Intensifying Screens Absorption(DQE): Ideal intensifying screen would absorb all x-rays energy that enters it ; real intensifying screens are generally not thick enough to absorb all of photons. Increasing thickness of an intensifying screen to increase its absorption capabilities degrades image quality . DQE= Absorption efficiency is determined by screen material ,screen thickness and photon energy spectrum.  

20. Properties of Phosphors used in Intensifying Screens Light production (CE) Intensifying screens convert a portion of absorbed x-rays energy into light by fluorescent process when exposed to high energy x-rays photons. It converts approximately 5% to 20% of absorbed x-ray energy into light. Conversion efficiency (CE)= Higher conversion efficiency results in increased noise.  

21. DQE and CE

22. Properties of Phosphors used in Intensifying Screens Exposure Reduction Film is more sensitive to light than to x-rays exposure , so film can be exposed with much less radiation if an intensifying screen is used. Conventional x-ray film has an x-ray exposure sensitivity in range of 50mR to 150mR if exposed directly by x-radiation. When film is combined with intensifying screens, sensitivity ranges from approximately 0.1mR to 10mR , depending on type of screen and film used. Intensification factor increases with exposure.

23. Intensification Factor(IF) The intensification factor is defined as the ratio of the exposure required to produce the same OD with as screen to the exposure required to produce an OD without screen. IF= The value of the IF can be used to determine the dose reduction accompanying the use of a screen .  

24. Principal factors that affect IF An increase in KV results in increase in IF. For greater density, exposure required to produce comparable densities on film with screen & without screen is greater , so IF will also be greater. IF should not be used to compare screen type film with non-screen type nor should it be used to contrast different types of screen-type film. It should be used only with one particular type of film used either with or without screens.

25. Intensification Factor

26. Properties of Phosphors used in Intensifying Screens Phosphor afterglow , the continuing emission of the light after exposure of the phosphor to x-rays , should be minimal. The phosphor should not be affected by heat, humidity, or other environmental conditions. The light emitted must be of proper wavelength (color) to match the sensitivity of the x-ray film. This is called spectral matching.

27. New Phosphor Technology This is the advancement in conventional phosphor to gain good speed and image quality. Which is concerned with : Thickness of phosphor layer. Conversion efficiency . Absorption efficiency.

28. Effect of Phosphor Thickness Medium speed calcium tungstate phosphor has 20% absorption efficiency and the fast screen calcium tungstate phosphor has 40% absorption efficiency. Practically it is found that, fast screen thickness =2.3 par speed thickness. Abrupt increase in the thickness is not the solution because it will increase the diffusion of light and hence the degrade image quality.  

29. Conversion Efficiency Conversion efficiency of the some common phosphor. Phosphor Conversion Efficiency Calcium tungstate 5% Lanthanum oxy bromide 18% Gadolinium oxysulfide 18% Yttrium oxysulfide 18%

30. Absorption Phosphors with higher absorption efficiency result in the good image quality. Calcium tungstate phosphor has the absorption efficiency 20% for the 150 micro meter thickness and on increasing the thickness by 2.3 times to the original ones the absorption efficiency will be 40%. Rare earth phosphor will have the 60% absorption efficiency for the 150 micro meter thickness. The improvement in the absorption efficiency is due to the binding energy of the electrons .

31. Absorption Efficiency Elements used in phosphor and their K-shell binding energies

32. Absorption Efficiency

33. Rare Earth Phosphor Newer phosphor materials have become the material of choice for most radiographic applications. The term rare earth describes those elements of group IIIA in the periodic table that have atomic numbers of 57 to 71 . These elements are transitional metal that are scarce in nature. Those used in rare earth screens are gadolinium , lanthanum and yttrium.

34. Phosphor Used in Intensifying Screen

35. Substratum Layer This is a bounding layer between the base & phosphor layer. It is 25 micron meter thick. It may be reflective, absorptive or simply transparent in nature, depending on the manufacturer’s policy.

36. Reflective Layer It is made of shiny substance & highly reflective white pigment, magnesium oxide or titanium dioxide. The functions of reflective layer are: Isotopic emission of light To maximize effect of screen by reflecting light which would otherwise be lost through screen base, back towards film emulsion. Increase the speed of screen. But the reflected light, however extends the real image boundary so leads to an increase in photographic Un-sharpness.

37. Reflective Layer

38. Absorptive Layer It is made by adding a special dye to it. An light travelling backwards towards the screen base is prevented from being reflected by base & is instead absorbed. It has less photographic Un-sharpness. It is mainly useful for absorbing light photons emitted at large angle to film. It reduces screen speed.

39. Base The base is the farthest layer from the radiographic film. The base is approximately 1mm thick . It is usually made of polyester. The base should be: Rugged and moister resistant. Can not be damaged by radiation or discoloration. Chemically inert, flexible and free of impurities.

40. Screen characteristics Radiologic technologist are concerned with three primary characteristics of radiographic intensifying screen; Screen speed Image noise Spatial resolution.

41. Screen speed Many types of radiographic intensifying screen are available , and each manufacturer uses different names to identify them . However screens usually are identified by their relative speed expressed numerically . Screen speeds range from 50 (slow , detail) to 1200 ( Very fast). The speed of a radiographic intensifying screen conveys no information regarding patient dose. This information is related by the intensification factor (IF).

42. Types of Screen Mainly three types of screen of different speed SCREEN SPEED High resolution or detail Slow Regular or standard Medium Fast Fast

43. High Resolution (Detail) It is made by making substratum layer absorptive. It has slow speed. It can be used for fine detail. It can be used when radiation dose is less important & high tube loading is not necessary. It requires greater exposure than regular screens. Due to exposure its use will be contraindicated when there is risk of patient movement. It can be used in extremity radiography.

44. Regular (Standard) 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

45. Fast It has fast speed. It produce greater film blackening for given exposure than by high resolution or regular screen. Detail sharpness will be reduced due to presence of reflective layer or larger size phosphor grain in screen construction. It can be used in high risk of un-sharpness from movement. (e.g. pediatric radiography) It can be used for imaging dense body parts maintaining low exposure. (e.g. abdomen radiography) It uses rare earth phosphor.

46. Variation In Speed Of Front & Back Screen I.S. used in pairs, with each emulsion surface being placed in closed contact with the effective surface of one I.S. Radiation which has transverse 1st screen, 1st emulsion, film base & 2nd emulsion have diminished intensity & will produce decreased fluorescence in 2nd screen, resulting in unequal OD of two emulsions. To obtain equal OD, Speed of back screen of cassette is increased by providing greater coating wt. for back screen Speed of front screen is reduced by adding pigment to supercoat of front screen.

47. Single Screen Radiography Although I.S. are usually used in pairs, one exception is in mammography film where single screen cassette is used in conjunction with single coated emulsion film. Elimination of extra emulsion & screen reduces photographic un-sharpness. The screen is usually placed at back, rather than at the front.

48. Asymmetric Screens Screens in the cassette can be of two types or speeds. Some people use two different speeds in cassette for full spine radiography. When types of screens are different, they are referred to as Asymmetric screens. One side may be high contrast and the other side with wide latitude. The combined image is superior.

49. Image Noise Image noise on a radiograph as a speckled background. Rare earths screens speed is higher because the percentage of the X-ray absorbed by the screen (QDE) is higher and also the amount of the light emitted for each X-ray absorbed (CE) is also higher. Higher CE results in increased noise, whereas increase in QDE doesn’t. Quantum mottle, a principal component of image noise is a direct result of use of very fast speed screen film systems that require very small amounts of exposure and results in a grainy, mottled, or splotchy images.

50. Spatial Resolution The image detail or the visibility of detail combines the quantitative measures of spatial resolution and contrast resolution. 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.

51. Spatial Resolution Generally those conditions that increase the IF reduce the spatial resolution. In mammography the screen is positioned in contact with the emulsion on the side of the film away from the X-ray source to reduce screen blur and improve the spatial resolution

52. Feature of screen ,variation in speed and sharpness FEATURE SPEED SHARPNESS CRYSTAL SIZE INCREASES DECREASES PHOSPHOR THICKNESS INCREASES DECREASES REFLECTIVE LAYER INCREASES DECREASES ABSORPTIVE LAYER DECREASES INCREASES CARBON GRANULES DECREASES INCREASES CROSSOVER NEGLIGIBLE EFFECT DECREASES QDE INCREASES NO EFFECT CONVERSION EFFICIENCY INCREASES NO EFFECT FEATURE SPEED SHARPNESS CRYSTAL SIZE INCREASES DECREASES PHOSPHOR THICKNESS INCREASES DECREASES REFLECTIVE LAYER INCREASES DECREASES ABSORPTIVE LAYER DECREASES INCREASES CARBON GRANULES DECREASES INCREASES CROSSOVER NEGLIGIBLE EFFECT DECREASES QDE CONVERSION EFFICIENCY INCREASES INCREASES NO EFFECT NO EFFECT

53. Crossover Effect If the film emulsion does not completely absorb the light from the intensifying screen, the unabsorbed light from one side can pass through the film base and expose the emulsion on the other side. This is commonly referred to as crossover. As the light passes through the film base, it can spread and introduce image blur.

54. Anti-crossover Crossover 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 increase light absorption. 

55. Crossover Reduction: Changing the shape of the crystal improves light absorption and reduced crossover.

56. Halation When light encounters a boundary between materials, reflection can occur at the boundary surface. Reflections at boundaries between film emulsion, film base, intensifying screens, and cassette surfaces are known as halation and contribute to image blur. Single-emulsion films generally have a light-absorbing layer coated on the other side of the base to prevent halation.

57. Screen film Combinations Screens in pairs and double emulsion film is the standard of the industry. Less than 1% of the image is produced by the x-ray photons. Each screen contributes relatively evenly in the production of the image.

58. Spectrum Matching For the optimum performance of the film screen combination that is speed of film, the color of light emitted by IS must be matched with sensitivity of film. The film, screens and safelight must match. Calcium Tungstate emits a broad blue spectrum.

59. Spectrum Matching

60. Spectrum Matching

61. Poor Screen Contact Poor screen contact will cause an area of the image to appear cloudy and blurry. Common reasons for poor contact include: Worn contact felt Loose, bent or broken hinges Loose bent or broken latches Warped screen Common reasons for poor contact include: Warped cassette front or frame. Sprung or cracked cassette frame. Foreign matter in the cassette.

62. Care of Intensifying Screen When loading cassettes, don’t slide the film in, a sharp corner or the edge can scratch the screen. Don’t dig the film out of the cassette with your finger nails. The screen must be periodically cleaned by antistatic compounds or mild soap and water. Maintaining the proper screen film contact, can be checked by radiographing a wire mesh.

63. Care of Intensifying Screen For high quality radiograph, proper care of radiographic I.S. is necessary. 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 on the back of cassette. 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.

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

65. Mounting the Screen 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. In order to use the cassette with actinic marker, a thin lead blocker is mounted in relation to front of the screen & corresponding in size & position to window of marker, usually lower left or upper right-hand corner.

66. Mounting the Screen A piece of new screen is carefully excised in order to accommodate the lead strip. While loading the film, we should not slide the film but just place the film inside the cassette. While removing film, we should rock the cassette on the hinged edge & it fall to our fingers. We should not dig the film with fingernails. 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 such as radiator.

67. Cleaning Of Screen 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. Never use the alcohol.

68. Cleaning of Screen 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 dry. Stand cassette on the edge, partly open & dry it thoroughly.

69. Advantages of intensifying screen INCREASED Adjustment of radiographic contrast Spatial resolution when small focal spots are used . Capacity for magnification radiography . Flexibility of kVp selection X-ray tube life DECREASED Patient dose Occupational exposure X-ray tube heat production X-rays exposure time X-ray tube mA Focal spot size

70. Disadvantages of Intensifying screen Lower spatial resolution and less detail than direct exposure film.

71. References Chesney's’ Radiographic Imaging , John Ball & Tony Price, 6 th edition, Blackwell Publishing, Oxford, UK. Christensen’s Physics of Diagnostic Radiology , Thomas S. Curry III, James E. Dowdey , Robert C. Murrey Jr, 4 th Edition, Lea & Fiber, Pennsylvania, USA. Radiologic science for technologists (Physics, biology & Protection),Stewart Carlyle Bushong , 9 th edition, MOSBY Elsevier, Canada. Radiographic imaging , DN & MO Chesney, 4 th edition, CBS Publishers & Distributors, India.