Chapter 22 ScreenFilm Radiography: Magnification and Resolution - PowerPoint PPT Presentation

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Chapter 22 ScreenFilm Radiography: Magnification and Resolution

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Magnification occurs because of the divergence of the beam. We can increase the magnification (and the blur) of the image by moving an ... – PowerPoint PPT presentation

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Title: Chapter 22 ScreenFilm Radiography: Magnification and Resolution


1
Chapter 22Screen-Film RadiographyMagnification
and Resolution
2
1. Image is magnified because the beam is
divergent
  • Magnification occurs because of the divergence of
    the beam.
  • We can increase the magnification (and the blur)
    of the image by moving an object closer to the
    beam source and/or further from the receptor.
  • Unsharpness increases with the diameter of the
    beam source.
  • SOD ( source-to-object distance)
  • SID ( source-to-image receptor distance)
  • OB object size
  • IM image size
  • M magnification

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2. Distortion is caused by nonuniform
magnification
  • Identical circular coins in a plane (the object
    plane) produce, in a parallel image plane, images
    that are circular and all of the same size.
  • Because of distortion, balls and circular coins
    not lying parallel to the image plane will
    produce images that are neither circular nor of
    the same size.

5
3. Unsharpness arising from the shape of the
object being imaged
  • Four common sources of unsharpness of X-ray
    images
  • Unavoidable blur stemming from the geometric
    shapes of the structures being imaged
  • Relative motion of patient and the equipment
  • Penumbra arising from the finite size of the
    focal spot
  • Blur created by the image receptor itself, such
    as from the diffusion of light within the
    fluorescent screens of the film cassette.

6
  • Inherent unsharpness by the inherent 3D shape of
    the object being imaged
  • The sharpness of the edge of an image depends on
    the degree to which the corresponding edge of the
    object follows the lines of divergence of the
    X-ray beam.
  • Occasionally it is possible to reduce this effect
    somewhat through positioning of the patient.

7
4. Unsharpness resulting from motion of the
patient or of the imaging system
  • Unsharpness by motion of the patient or organ
    being imaged, or the imaging system.

Region of unsharpness in the image
8
  • Relative unsharpness degree to which the blur
    degrades its image quality
  • Ratio of the amount of the blur in the image to
    overall size of the image of the object.
  • Relative motion unshapness due to motion

9
  • Relative motion unsharness depends on the
    velocity of the motion and on the exposure
    duration.
  • It is independent of the magnification
  • (? Because the amount of blur increases with
    magnification, so also does the size of the
    objects image, and two effects cancel)
  • Relative motion unsharpness depends on the actual
    size of the object being imaged ? inconvenience.
  • Formal definition of the relative unsharpness for
    a 1 mm reference object in the body (OB 1 mm).
  • In most standard radiographic situation, motion
    is fortunately not a serious problem, because of
    the short exposure times involved.

10
For 10 mm/s motion and a 0.02 s exposure
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5. Geometric (penumbra) unsharpness caused by the
finite size of the focal spot
  • Geometric unsharpness
  • ???? ??? ???? ??? ??? ???? ??? ??? ???.
  • penumbra (??? ) ??? ??
  • It is the predominant source of unsharpness in
    normal radiography.

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  • The amount of geometric unsharpness thus
    increases with the size of the focal spot and
    with the amount of magnification.
  • When the object being imaged is against the image
    receptor (M1), there is no geometric unsharpness.

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  • Relative unsharpness caused by penumbra
  • Unlike Umot, Upen becomes larger with
    magnification.
  • If penumbra is the dominant source of
    unsharpness, then one produces the best pictures
    by using the least amount of magnification.

15
  • As the plane of the focal spot is not parallel to
    that of the film, penumbra unsharpness is
    somewhat less pronounced on the anode side of the
    image.
  • The magnitude of this effect depends on the anode
    angle.

16
6. Unsharpness produced by the image receptor
itself (screen blur)
  • Unsharpness by the image receptor is caused by
    the diffusion and scatter of light within the
    intensifying screens of the cassette.

17
7. Simple measurement of combined motion,
penumbra, and screen unsharpness
  • In the clinic, all three factors (motion,
    penumbra, and image receptor unsharpness) may
    combine to limit resolution.
  • Overall relative unsharpness
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