Chapter 6 Image Receptors

1. Chapter 6Image Receptors Radiology

2. Introduction • To further understand radiography, one must know how a permanent record is produced using x-rays. • Radiograph is formed with light-sensitive film contained in a light-proof encasement. • Encasement in radiography is called a cassette.

3. The Cassette • Designed to hold a piece of x-ray film sandwiched between two fluorescent sheets of plastic called INTENSIFYING SCREENS. • Intensifying screens are responsible for converting the x-ray radiation into visible light, which creates a latent image on the x-ray film. • Film is then processed to convert latent image to visible image. • Only 5 % of exposure results from ionization of x-rays, rest is due to light emitted from intensifying screens.

4. The Cassette • Cassette- A rigid film holder designed to hold x-ray film and intensifying screens in close contact. • Constructed with light tight materials to prevent unwanted exposure, but yet still allow penetration of x-rays. • Originally cassettes were made with cardboard, but could not be reused. • Different forms today include aluminum, polycarbonate, magnesium, and carbon fiber.

5. The Cassette • Attached to the back with hinges and catches. • Forms light tight seal. • Back of cassette is constructed with heavier material than the front because is lined with material to absorb backscatter. • Sides are lined with felt or foam pressure pads that ensure close contact of the film to the intensifying screens.

6. The Cassette • Sizes vary. • Correspond to screen and film sizes. • Cost varies as well.

8. Cassette Care • Should always be handled with care. • Can be exposed to large amounts of physical abuse. • Common damage includes: • Dropping the cassette. • Leakage of fluid on the cassette. May want to place in plastic bag when a “messy” situation occurs.

9. Cassette Care continued • Should be cleaned on regular basis with mild soap and water. • Should clean exterior and interior of cassettes at least once monthly. • Cassettes should be numbered so that problems can be easily identified. • Intensifying screens are already numbered, but can add number onto intensifying screen.

10. The Cassette • Color coding is sometimes used to identify type of screen inside. • Front of cassette may be divided into four quadrants. • Shield other quadrants so that multiple views can be taken per film. • Lead blocker (approximately 3x7 cm) is found in corner of cassette.

11. Lead Blocker

12. Lead Blocker • Present to prevent irradiation of the part of the film necessary for identification. • Careful not to have image in area of lead blocker.

13. Intensifying Screens • Intensifying screens- sheets of luminescent phosphor crystals bound together and mounted on a cardboard or plastic base. • Two screens are normally in an x-ray cassette and sandwich the film. • Film has a coating of light sensitive emulsion on both sides (Double emulsion).

14. Intensifying Screens • When phosphor crystals in screen are hit by x-radiation, the crystals fluoresce, and x-rays are converted to visible light. • Visible light is what exposes the film. • Primary purpose of screen is to reduce the amount of radiation exposure required to produce a diagnostic radiograph.

15. Intensifying Screens • Reduces the amount of mAs needed, which in turn decreases the dose of radiation to the patient and the chance of motion on the radiograph. • Three properties determine efficiency of screen materials. • 1. They must have a high level of x-ray absorption. • 2. They must have high x-ray to light conversion with suitable energy and color. • 3. There must be little or no “afterglow” once radiation has ceased.

16. Screen Construction • Has four layers • A base or support • A reflective layer • A phosphor crystal layer • A protective coat

17. Base • Serves as flexible support to attach the phosphor layer to the cassette. • Base must have tough, moisture-resistant surface and not become brittle with extended use.

18. The Reflective Layer • Attached to the base. • Made of a white substance such as titanium dioxide. • Purpose is to reflect light emitted by phosphor layer back toward the x-ray film. • Increases the efficiency of the screen so that no light is lost through the base.

19. Phosphor Crystal Layer • Consists of uniformly distributed phosphor crystals held in place with binder material. • Layer can not change in thickness, crack or discolor with age as any variance will alter amount of light produced and alter exposure of film.

20. Phosphor types • Phosphor chosen must absorb x-rays efficiently and have minimum afterglow and emit sufficient light of the desired color. • Afterglow- tendency of phosphor to still give off light after x-radiation has stopped. • Absorption rate of phosphor refers to the extraction of x-ray photons from the beam. The more x-ray quanta absorbed, the greater the amount of light produced.

21. Protective Coat • Is a clear coating placed on the outer surface of the screen. • Provides protection to the phosphor layer. • Must be strong enough to resist marks and abrasions and must be easy to clean.

22. Screen Speed • General Rules: • Large crystal: Faster screens-less detail-high grain. • Small crystals: Slower screens-more detail- low grain.

23. Screen Speeds • Calcium tungstate- high x-ray absorption ability and physically strong but lacks in light conversion ability. • Rare-earth elements- higher x-ray to light conversion ability. • Other factors can affect screen speed aside from phosphor type. • Governed by crystal size, phosphor layer thickness, reflective layer efficiency, and dyes in the phosphor layer.

24. Crystal Size • Larger the crystal, greater the light emission. • X-ray strikes crystal, entire crystal starts to fluoresce. • Thus need less x-radiation. • However detail of image decreases as crystal size increases.

25. Phosphor Layer Thickness • When thickness is increased, x-ray absorption and light emission are increased. • Increase in thickness results in decrease of image detail.

26. Reflective Layer Efficiency • Is positioned between base and phosphor layer. • Purpose is to reflect all light emission from phosphor layer toward x-ray film. • Needs to be high in reflective ability in order to decrease x-radiation needed.

27. Dyes in the Phosphor Layer • Light absorbing dye may be incorporated into the binder material of the phosphor layer in some screens. • Dye is to decrease lateral spreading of the light emitted from the phosphor crystals. • May reduce light emitted and speed of screen.

28. Screen Speed Ratings • Many screens to choose from. • Screens divided into three categories: • Slow • Medium • Fast

29. Slow Screens • Also called high definition, ultra-detail, or fine grain. • Specifically designed from radiographic examinations that require optimal detail and in which exposure time is not critical.

30. Medium Screens • Also referred to as Regular, midspeed, normal or par speed. • Most common in private veterinary practice. • Provide good resolution with low exposures.

31. Fast Screens • Also referred to as High Speed. • Reduces exposure time to patient. • Able to penetrate thick tissue where more exposure is necessary. • Less detail.

32. Screen Speeds • May depend on manufacturer • Quantum Mottle- an artifact of faster screens that results in density variations due to random spatial distribution of the phosphor crystals within the screen. • Gives radiographs a spotted or mottled appearance.

33. Intensifying Screens Continued • Usually mounted in pairs in the cassette. • Labeled front and back and should be placed accordingly. • Should be firmly mounted in cassette.

34. Screen Care • Should be inspected and cleaned on regular basis. • Try to keep free from dust and animal hair. • After cleaning, should be left open to completely dry before reloading.

37. X-ray Film • Purpose is to provide a permanent record containing essential diagnostic information • Consists of polyester base coated on both sides with light-sensitive emulsion containing silver halide crystals. • When visible light or x-rays interact with silver halide crystals, latent image is formed. Through processing, this is converted to a visible image.

38. Film Composition • Layers include: • Base- flexible support with adhesive coating on each side. • Emulsion- next to base. Composed of gelatin containing silver halide crystals. • Supercoat- protective material to decrease the possibility of damage to emulsion.

39. Latent image • An invisible image on the x-ray film after it is exposed to ionizing radiation or visible light before processing. • After processing is converted to a visible image. • Silver hallide crystals converted to metallic silver that are exposed. • If not exposed, will be clear.

40. Film Types • Screen Film • Manufactured with silver crystals that are sensitive to fluorescent light emitted from intensifying screens and less sensitive to ionizing radiation. • Requires less exposure to x-rays to produce quality image.

41. Film Type Continued • Nonscreen Film • Exposed by the direct action of x-radiation. • More sensitive to ionizing radiation. • Requires greater exposure because there is no intensifying screens. • No real protective layer.

42. Film Speed • Speeds are determined from exposures required to produce an image with adequate density. • Film latitude- The exposure range of a film that will produce acceptable densities.

43. Film Speed Types • Fast Film (Ultraspeed) • Larger silver halide crystals • Requires less exposure to x-rays or fluorescent light from intensifying screens. • Produces grainier image that lacks definition. • Less latitude in exposure factors and processing.

44. Film Speeds • Medium Film (Standard or Pat speed) • Most widely used. • Compromises between fine grain and speed. • Has medium latitude. • Suitable for a wide range of examinations. • Standard by which manufacturers rate other films.

45. Film Speeds • Slow Film • Smaller halide crystals • Requires greater exposure by x-rays or fluorescent light from intensifying screens. • Less grainy and more defined image. • Greater latitude.

46. Film Care • Film boxes should be stored on end so film is vertical. • If stored horizontally too long, emulsions will blend together between the films. • Temp should be cool with low humidity. • Should not be stored close to ionizing radiation, or where vapors can reach it. • Should be checked regularly for expiration.