Intensifying Screen (x-ray)
Download as PPTX, PDF63 likes21,181 views
Cassettes contain intensifying screens and hold film during x-ray examinations. Intensifying screens convert a small percentage of x-ray photons that pass through the patient into visible light photons, which expose the film. This process increases the number of photons available to expose the film while significantly reducing the radiation dose to the patient compared to directly exposing the film. Modern rare earth screens are most efficient at this conversion but provide less image detail than earlier calcium tungstate screens or direct exposure of film.
Intensifying Screen (x-ray)
- 2. Cassettes serve 3 important functions: 1. Protect film from exposure to light 2. Protect film from bending and scratching during use. 3. Contain intensifying screens, keeps film in close contact to screen during exposure.
- 3. The CASSETTE is used to hold the film during examinations. It consist of front and back intensifying screens, and has a lead (Pb) backing. The cassette is light tight
- 4. Exposure side of cassette is the “front”. Made of radiolucent material – easily penetrated by x-rays, lightweight metal alloy or plastic material made of resin. Intensifying screen mounted to inside of front.
- 5. Back made of metal or plastic Inside back is a layer of lead foil – prevents backscatter that could fog the film Inside foil layer is a layer of padding – maintains good film/screen contact Back intensifying screen mounted on padding Has the ID blocker (patient identification)
- 6. 1% of xray photons that leave patient Interact with phosphors of intensifying screens 100’s of light photons created to make image on film Light photons expose silver halide crystals in the film emulsion Turn black metallic silver after procession
- 7. Flat surface coated with fluorescent crystals called phosphors that glow, giving off light when exposed to x-rays.
- 8. RARE EARTH – (emits green light) Developed in 1980’s Most efficient – most common in use today CALCIUM TUNGSTATE (blue light) Not as efficient
- 9. Gadolinium Lanthanum Yttrium Found in low abundance in nature
- 10. Direct x-ray exposure to film required 25 to 400 times more radiation to create an image on the film BETTER DETAIL THAN FILM SCREEN (NO BLURRING OF IMAGE FROM LIGHT) ALL EXPOSURE MADE FROM X-RAY PHOTONS BIG DOSE TO THE PATEINT
- 11. DISADVANTAGES: less detail than direct exposure (detail better with rare earth than calcuim tungstate screens) ADVANTAGES: 1. Reduce patient exposure 2. Increase x-ray tube life
- 12. Polyester plastic base – support layer Phosphor layer – active layer Reflective layer – increases screen efficiency by redirecting light headed in other directions Protective coating
- 14. Active layer – x-ray photons converted to light photons *Photoelectric Effect
- 15. A relative number that describes how efficiently x-rays are converted into usable light Ranges from 100 (slow) to 1200 (fast)
- 16. Greater efficiency = less exposure = faster -Standard screen speed class of 100 -200 screen speed is twice as fast Speeds for routine work: 200 – 800 Speeds for high detail: 50 – 100 Increasing speed also increases image noise
- 17. FASTER SPEED – REDUCES PATIENT EXPOSURE FASTER SPEED - REDUCES IMAGE DETAIL INCREASES NOISE (LIGHT BLURING AROUND IMAGE)
- 18. Relative Speed = Film speed & Screen speed mAs 1 = RS 2 mAs 2 RS 1 They are inversely related
- 19. Speckled background on the image Caused when fast screens and high kVp techniques are used. Noise reduces image contrast The percentage of x-rays absorbed by the screen is the detective quantum efficiency (DQE) The amount of light emitted for each x-ray absorbed is the conversion efficiency (CE)
- 20. Quantum Mottle causing a grainy, mottled or splotch image Often results of using very fast-speed screen-film systems
- 21. The light photons generated in the intensifying screen are emitted by phosphor crystals. These crystals are significantly larger than the silver halide crystals in the film use of a screen reduces image sharpness somewhat Some examinations requiring extremely fine detail use screens with small crystals.
- 29. Have higher DQE (detective quantum efficiency). Higher x-ray absorption abilities. Have higher CE (conversion efficiency). More light emitted per x-ray absorbed by the screen.
- 30. The use of intensifying screens lowers spatial resolution compared with direct-exposure radiographs. Spatial resolution expressed by the number of line pairs per millimeter (lp/mm)
- 31. Very fast screens = 7 lp/mm Fine-detail screens= 15 lp/mm Direct-exposure screens = 50 lp/mm
- 35. Warped cassette –poor contact
- 37. Image artifacts can appear if screens are modified Small scratches can leave artifacts Dirty screens can leave artifacts Screens should be cleaned once each month with manufacturer’s cleaner with antistatic compounds