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Components of Radiographic Image Quality

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duy
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Components of Radiographic Image Quality. Radiologic Technology 244 created: Fall 2005 Rev 12-01-2009. Review handouts. Main Factors Affecting Recorded Detail. kVp & mAs Technique Selection (Time) Motion Object Unsharpness Focal Spot Size SID (Source to Image Distance)

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  1. Components of Radiographic ImageQuality Radiologic Technology 244 created: Fall 2005 Rev 12-01-2009

  2. Review handouts

  3. Main Factors Affecting Recorded Detail • kVp & mAs • Technique Selection (Time) • Motion • Object Unsharpness • Focal Spot Size • SID (Source to Image Distance) • OID (Object to Image Distance) • Material Unsharpness/ Film Screen Combo

  4. Factors that affectRecorded Detail • Geometric unsharpness OID SID SIZE SHAPE • Motion unsharpness (blurring) • Intensifying Screens • Film Speed / Composition • Film – Screen contact • Kvp & Mas (density / visibility)

  5. GEOMETRIC QUALITIES • DETAIL • DISTORTION • MAGNIFICATION

  6. DETAIL • The degree of sharpness in an object’s borders and structural details. • How “clear” the object looks on the radiograph

  7. Recorded Detail • Other names: -sharpness of detail -definition -resolution -degree of noise

  8. What are these What does they measure?

  9. Factors Affecting DENSITY • PATIENT THICKNESS,PATHOLOGY • MAS & KVP • SID

  10. POOR DETAIL GOOD DETAIL

  11. Motion • Can be voluntary or involuntary • Best controlled by short exposure times • Use of careful instructions to the pt. • Suspension of pt. respiration • Immobilization devices

  12. Decrease Motion Unsharpness • Instruct patient not to move or breath • Use Immobilization devices • Use Short exposure times • Lock equipment in place

  13. NAME 4 CAUSES

  14. Blurring of image due to patient movement during exposure.

  15. Focal Spot Size • Smaller x-ray beam width will produce a sharper image. • Fine detail = small focal spot (i.e. small bones) • General radiography uses large focal spot • Beam from penlight size flashlight vs. flood light beam

  16. FOCAL SPOT ANGLE

  17. Object Unsharpness • Main problem is trying to image a 3-D object on a 2-D film. • Human body is not straight edges and sharp angles. • We must compensate for object unsharpness with factors we can control: focal spot size, SID & OID

  18. SID Source to Image Distance • The greater the distance between the source of the x-ray (tube) and the image receptor (cassette), the greater the image sharpness. • Standard distance = 40 in. most exams • Exception = Chest radiography 72 in.

  19. SID • Shine a flashlight on a 3-D object, shadow borders will appear “fuzzy” On a radiograph it’s called ______________ • A true border – _____ • Farther the flashlight from object = sharper borders. Same with radiography.

  20. OIDObject to Image Distance • The closer the object to the film, the sharper the detail. • OID , penumbra , sharpness  • OID , penumbra , sharpness  • Structures located deep in the body, radiographer must know how to position to get the object closest to the film.

  21. Distortion • Misrepresentation of the true size or shape of an object MAGNIFICATION (size distortion) TRUE DISTORTION (shape distortion)

  22. Shape Distortion • Misrepresentation of the shape of an object • Controlled by alignment of the beam, part (object), & image receptor • Influences: Central ray angulation & body part rotation

  23. Elongation Foreshortened Normal

  24. Distortion (x-ray beam not centered over object & film) Distortion (object & film not parallel)

  25. Central Ray • Radiation beam diverges from the tube in a pyramid shape. • Photons in the center travel along a straight line – central ray • Photons along the beam’s periphery travel at an angle • When central ray in angled, image shape is distorted.

  26. Distortion of multiple objects in same image (right) due to x-ray beam not being centered over objects.

  27. Central Ray Angulation • Body parts are not always 90 degrees from one another • Central ray angulation is used to demonstrate certain details that can be hidden by superimposed body parts. • Body part rotation or obliquing the body can also help visualize superimposed anatomy. • NAME 3 EXAMPLES

  28. MAGNIFICATIONcaused by: • TUBE CLOSE TO THE PART (↓SID) • PART FAR FROM THE CASSETTE (↑ OID) Compensate for MAG : ↑ OID by ↑ SID = “increase SID 7” for every 1” OID”

  29. Size Distortion & SID • Major influences: SID & OID • As SID , magnification  • Standardized SID’s allow radiologist to assume certain amt. of magnification factors are present • Must note deviations from standard SID

  30. In terms of recorded detail and magnification, the best image is produced with a small OID and a large SID.

  31. What can be done to improve the detail with a large OID?

  32. Use a smaller FS

  33. 40” SID VS 72” SID

  34. Size Distortion & OID • If source is kept constant, OID will affect magnification • As OID , magnification  • The farther the object is from the film, the more magnification