Chapter 22 ScreenFilm Radiography: Magnification and Resolution

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Chapter 22Screen-Film RadiographyMagnification
and Resolution
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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.

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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.

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• 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.

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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
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• 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

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• 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.

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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.

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• 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.

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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.

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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