LUMINESCENT SCREENS............................

Intensifying screen

1. INTENSIFYING SCREEN

2. The x-ray photons making up the radiographic image cannot be seen by the human eye. This information is converted into visual image by one of the TWO methods Xray directly expose the photographic emulsion More Commonly , the energy of Xray is converted to Visible light which expose the xray film or Visible light is directly viewed( fluoroscopy) The sensitivity of film to direct x-ray exposure is low i.e < 1% interacts with film and contribute to Latent image formation. Intensifying screen is a device that convert the energy of the x-ray beam to visible light .

3. Cassette: Intensifying Screen are present inside a cassette. Cassettes  are rigid holders used in conventional and computed radiography (CR) for the screen film system and imaging plate respectively.  Fuctions : Prevent film from exposure to light Protect from bending and scratching Keeps the film in close contact with intensifying screen.

4. Features of Ideal Cassette It should be light weight It should not have sharp edges or corner ( to minimize injury) It should be easy to open and close in a dark environment It should maintain uniform and close contact between Intensifying screen and film Front part should allow low beam attenuation and be of uniform thickness with no irregularities Back surface should be made up of meterial with high atomic number (lead foil )to minimize back scatter It should be available in different film sizes

5. Construction FRONT PART: Made up of low atomic number material like aluminum, carbon fiber. Front Intensifying screen is present BACK SIDE : Lead foil to prevent backscatter which is further attached with plastic pressure pad and back I.S LOCKING SYSTEM: A rage of locking system is used from spring clip to sliding lock which serve to exclude light and helps to maintain good contact between film and I.S

6. Types of Cassette Single Screen Cassette - in mammography Double Screen Cassette - in general radiography Grided Cassette - in portable radiography Multisection Cassette - hold 3-7film with respective I.S and spacer . Used in tomography Vacuum Cassette – Primarily developed for mammography. Used in Joint Radiography Made up of flexible vinyl material Have a valve attached at one end Vacuum pump atteched to valve expell air that brings film in close contact with I.S

7. Type of Cassette Formatter Cassette- used in CT, MRI, DSA has frame to hold single emulsion film by their edge D o not have intensifying screen . Angiography cassette - can hold up to 30 films seperated by a spacer CR cassette – Photostimuable phosphor plate is used instead of film PSP plate is coated with europium activated barium fluoro halide

8. Intensifying screen: Developed first in1897 by Thomas Edison. It’s a part of cassette -converts x ray energy into visible light spectrum. ADVANTAGES : • reduces x ray dose to the patient short exposure times- reduced motion blur Higher contrast image compared to non-screen film

9. Two Types of LUMINISCENCE

10. Construction: An intensifying screen has four layers: 1. base or support layer 2. reflecting layer 3. phosphor layer 4. plastic protective coat The total thickness of a typical intensifying screen is about 15 or 16 mils (1 mil= 0.001in. = 0.0254 mm). i.e 0.381 to 0.4064 mm

11. 1.Base Layer: Made of high-grade cardboard or of a polyester plastic. The base is the thickest (1mm) and farthest from the film. The base should be: Rugged and moisture resistant Can not be damaged by radiation or discoloration Chemically inert, flexible and free of impurities

12. 2.Reflecting Coat. Made of a white substance, such as titanium dioxide (Ti02) The light produced by the interaction of x-ray photons and phosphor crystals is emitted in all directions. The reflecting layer acts to reflect light back toward the film. Thin layer, about 25 µ m thick .

13. 3.Phosphor Layer. Phosphor layers vary in thickness from 50 to 300 µ m Active layer of radiographic intensifying screen B efore 1980 crystalline calcium tungstate was used as phosphor layer . The rare earth elements gadolinium, lanthanum, and yttrium are the phosphor material in newer, faster screens .

14. 4.Protective Layer C losest to the radiographic film Largely composed of a cellulose compound that is mixed with other polymers. 10 to 20 µ m thick . This layer serves three functions: it gives physical protection to the delicate phosphor layer It helps to prevent static electricity it provides a surface that can be cleaned without damaging the phosphor layer.

15. Properties of Phosphor • The phosphor should have a high atomic number so that x-ray absorption is high. This is called detective quantum efficiency (DQE). • The phosphor should emit a large amount of light per x-ray absorption. This is called the x-ray conversion efficiency (CE). • The light emitted must be of proper wavelength (color) to match the sensitivity of the x-ray film. This is called spectral matching . • Phosphor afterglow, the continuing emission of 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.

16. Properties of phosphors: Detective Quantum efficiency(DQE): The ratio of x-ray absorbed to the amount of incident x-ray photon When the x-ray energy equals the K-shell electron binding energy, absorption is high. Rare earth screens exhibit better absorption properties than Ca tungstate in between the respective K-shell absorption edges. DQE mainly depends on the type of phosphor. Other factors are thickness of phosphor, coating wt. & photon energy of incident beam 

17. Properties of phosphors: Conversion Efficiency(CE): It is the ratio of visible light energy emitted to the x-ray energy absorbed. When x-ray interacts photoelectrically, its energy reappears as heat or light through rearrangement of electrons in crystal lattice of phosphor. If all energy reappears as heat, the phosphor would be worthless. CE of Ca tungstate is 5% & that of rare earth phosphors is about 20%.

18. Properties of phosphors: Screen efficiency: The ability of light emitted by phosphor to escape from the screen and expose the film is called screen efficacy. For typical screens about half the generated light reaches the film the rest is absorbed in the screen and is wasted.

19. Properties of phosphors: Spectral Matching For the screen to work at maximum efficiency, the light absorption characteristics of the film must be matched to the light emitted from the screens. This is called spectrum matching. Calcium Tungstate emits a blue spectrum. Rare earth emits a green spectrum. The film, screens and safelight must match.

20. Principle behind intensifying screens: Any material that gives of light in response to a stimulus is a luminescent material. The process of emission of by a luminescent material in response to X-Rays is luminescence.

21. Film-Screen Combination VS Film alone For constant kVp, Direct film exposure will require 34 times as many mAs as a film screen exposure

22. SCREEN CHARACTERISTICS Intensification factor (IF): Patient exposure is decreased greatly when I.S is used. A measure of this decrease in exposure is termed as IF The IF is defined as the ratio of the exposure required to produce the same OD with a screen to the exposure required to produce an OD without a screen The intensification factor of CaW04 screens will increase with kVp of the x-ray beam

23. SCREEN CHARACTERISTICS Screen Speed It is a relative number that describes how efficiently x-Ray are converted into Visible light. Usually ranges from 50 to 1200 Ca tungstate seldom are used anymore, they serve as the basis for comparison of all other screens Speed of Calcium Tungstate screen and its ability to record details are inversely related Screen Speed Details Low More Details Very Fast Less Details

24. Factor determining Screen Speed Thickness of phosphor layer Thicker Phosphor layer have Higher Speed Size of phosphor cryastal Increase in crystal size increases the Screen speed Presence or absence of light absorbing dye Light absorbing dye added to the phosphor layer decreases the Speed Phosphor Conversion Efficiency Increases the screen speed

25. Thick phosphor layer absorb more photon than thin layer So,thick screens will be faster but will cause a decrease in the clarity of the image recorded on the film. This decrease in image clarity is primarily caused by diffusion of light in the phosphor layer.

26. Screen-film contact: With good film-screen contact a dot of light produced in the screen will be recorded as a comparable dot on the film. If contact is poor, this dot of light will diffuse before it reaches the film, so that its radiographic image is unsharp.

27. Causes of poor screen film cotact : Worn radiolucent foam Loose, bent, or broken hinges Warped screens caused by excessive moisture Warped cassette front Sprung or cracked cassette frame Foreign matter under the screen

28. Screen Characteristics Spatial Resolution Image Details = Spatial Resolution + Contrast Resolution Spatial resolution refers to how small an object can be imaged Contrast resolution refers to the ability to image similar tissues, such as the liver and pancreas. Radiographic I.S have the disadvantage of lower spatial resolution compared with direct-exposure radiographs. A photograph in focus shows good spatial resolution. A line pair test can be done to test spatial resolution

29. Spatial resolution can be expressed by the number of lines pairs per mm (lp/mm) Higher these lines, the smaller the object can be imaged and the better the spatial resolution. Very Fast Screen can resolve 7 lp/mm & Slow Screen 15 lp/mm Factor improving spatial Resolution Smaller phosphor crystal Thin phosphor layer

30. Characteristics of typical intensifying Screen

31. Rare Earth Screen Newer phosphor materials have become the material of choice for most radiographic applications. These are elements of group IIIa in Periodic table and Z number 57 to 71 Those used in rare earth screen are principally gadolinium, lanthanum and yttrium. Compositions of 4 principal rare earth phosphor are Terbium activated gadolinium oxysulfide. Terbium activated lanthanum oxysulfide. T erbium activated yttrium oxysulfide L anthanum oxybromide

32. Absorption in Calcium Tungstate vs Rare Earth Phosphor Screen Rare Earth phosphor has better absorption between 35 to 70 keV and corresponds to most of the useful x-ray emitted during routine x-ray examination

33. Quantum mottle: commonly called "noise;’ results from statistical fluctuation in the number of x-ray photons absorbed by the intensifying screens to form the light image recorded on film. With the fast screens and films now available, it is possible to use a film-screen system so fast that noise makes the resulting image unsatisfactory.

34. Cleaning: Any foreign material on the screen ( such as blood) will block light photon and produce an area of underexposure on the film. I.S. must be cleaned periodically on the basis of amount of use & level of dust in work environment. Screen are best cleaned with a solution containing an antistatic compound and a detergent. Solution should be applied gently( never rub vigorously ) Cassette should be closed only if it is completely dry. CAUTION: NEVER USE ALCOHOL.

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