Invisible X-ray image

1
Invisible X-ray image

• Formation
• Characteristics

2
X-ray tube
Plot of incident x-ray beam intensity
Object
Plot of transmitted x-ray beam intensity
Invisible x-ray image
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Invisible x-ray image
kV mA Sec FFD
E
Supporting tissue (m)
B
T3
B1
B2
T1
T2
Air
Invisible X-ray image consists of different x-ray
intensities
E B1
E B2
ET1
EM
EM
ET2
ET3
EA
4
Characteristics

• Subject contrast
• Sharpness
• Noise
• Resolution

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Subject contrast

• The difference in the x-ray intensities
  transmitted through the subject
• It is the shortened form of the radiation
  contrast of the subject
• Causes of subject contrast
• Differential attenuation
• Scattered radiation

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Differential attenuation

• Differential attenuation is the result of the
  attenuation caused by Photoelectric absorption
  and Compton scattering.
• Depends on
• Thickness of the anatomical structure
• Effective atomic number of the body tissues
• Physical density of the body tissues
• Presence of radiological contrast medium
• X-ray tube kilovoltage employed
• X-ray beam filtration

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Effective atomic number Subject contrast

• For a given Photon energy the photo electric
  absorption is higher when the atomic number is
  high ( bone absorbs more radiation than soft
  tissue)
• E.g. if the three tissues A,B,C have effective
  atomic numbers as Z1 gt Z2 gt Z3

Incident intensity
A Z1
B Z2
C Z3
Subject contrast A-B
Transmitted intensity
Subject contrast A-C
Subject contrast B-C
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X-ray tube kilovoltage subject contrast

• Photo electric absorption predominates at low
  kilovoltages, therefore at low kilovoltages the
  subject contrast is high, and when the
  kilovoltage is increased the subject contrast
  tend to be reduced.
• At high kilovoltages approaching 150kV the
  contrast is mainly caused by the compton effect
  which mainly depends on the density difference of
  the anatomical structures.

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kV subject contrast
Low kV
E
Supporting tissue (m)
B
T3
B1
B2
T1
T2
Air
E B1
E B2
Higher differences
ET1
EM
EM
ET2
ET3
EA
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kV subject contrast
High kV
E
Supporting tissue (m)
B2
B
T3
B1
T1
T2
Air
Lower differences
E B1
E B2
EM
EM
ET1
ET2
ET3
EA
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X-ray beam filtration Subject contrast

• Filtration reduces the low energy components of
  the x-ray beam. Hence increasing the filtration
  has the effect of increasing the effective photon
  energy of the beam. This influences the
  photoelectric absorption in a similar way as
  increasing the tube kilovoltage.
• Therefore increasing the filtration will decrease
  the subject contrast

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Scattered radiation subject contrast
Scattered radiation
Primary beam
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Scattered radiation subject contrast

• When the primary beam from x-ray tube interacts
  with matter scattered radiation is produced.
• Scattered radiation travels in different paths
  from the primary beam and will reduce the subject
  contrast of the invisible x-ray image.
• Not only the subject contrast but it will reduce
  the signal to noise ratio also.

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Scatter reduces the subject contrast
E
Supporting tissue (m)
B2
B
T3
B1
T1
T2
Air
Scatter Lowers the differences
E B1
E B2
EM
EM
ET1
ET2
ET3
EA
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How to minimize the effect of scatter on subject
contrast?

• Reduce the amount of scatter produced at the
  object (patient) by
• Collimating the primary beam
• Reducing the proportion of forward scatter using
  low kV
• Reducing the tissue thickness
• Avoiding other sources of scatter, such as bucky
  tray
• Protecting the image receptor by
• Use of secondary radiation grid
• Employing an air gap

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Use of grid
Lead strips
Image receptor
Radiolucent inter-space
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Employing Air gap
Image plane 2
Image plane 1
Object
Scatter
Air gap
Percentage of oblique ray reaching the image
receptor plane is reduced at image plane 2
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Sharpness of Invisible x-ray image

• The sharpness is determined first by the geometry
  of image formation
• The size of the source of radiation is of primary
  concerned
• Infinite size (Point source)
• Finite size ( larger than a point)
• When the size of the x-ray source (Focus) is
  large the sharpness of the image is less

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Image Geometry
Finite source
Point source
Image plane
Unsharpness (penumbra)
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Intensity distribution at previous situations
Intensity of x-rays at image plane
Intensity of x-rays at image plane
U
U
Distance across image plane
Distance across image plane
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Geometric unsharpness

• The formation of unsharpness due to a penumbra is
  a direct consequence of the finite size of the
  x-ray source.
• This form of unsharpness is known as Geometric
  unsharpness (UG)
• It can be shown that
• focal spot size x object-image
  distance
• Geometric -----------------------------------
  --------
• Unsharpness focus-object distance

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Evaluation of Geometric unsharpness
A
Source
B
Triangles OAB OCD are similar. AB/CD
OB/OC Re-arranging CD AB x OC/OB UG focal
size x OFD/FOB
O
Object
Image plane
C
D
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Factors governing geometric unsharpness

• Focal spot size
• Small focus gives minimum geometric unsharpness
• Object image (film) distance
• Shorter OFD gives less geometric unsharpness
• Focus to object ( Focal film) distance
• Longer the FFD lesser the geometric unsharpness
• Increase the FFD when OFD cannot be reduced, to
  minimize the geometric unsharpness
• Edge penetration

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Focal spot size Geometric unsharpness

• Unsharpness increases, when apparent focal area
  increases
• Apparent (effective) focal area Actual focal
  area x Sine of target angle
• Therefore Unsharpness increases when target angle
  increases for a given actual focal spot size
• Geometric Unsharpness can be reduced by using
  small focus but that reduces the maximum tube
  loading capacity

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Unsharpness due to Edge penetration

• This is due to the shape of the object
• The edges of the object absorb less amount of
  radiation and the absorption increases towards
  the centre
• This creates a intensity gradient producing
  inherent unsharpness

Intensity of x-rays at image plane
Distance across image plane
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Movement unsharpness

• Voluntary involuntary movement of the organs or
  body parts or the patient as a whole will cause
  changes in the pattern of x-ray intensities
  forming the invisible x-ray image
• This changes are referred to as movement
  unshrpness UM
• If they occur during image recording they will
  produce unsharpness in the final image

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Noise in the invisible x-ray image

• The kinds of noise present in the invisible x-ray
  image are
• Fog due to scatter radiation
• Quantum noise presence of less number of
  photons in the invisible x-ray image, making the
  identification of gaps between individual photons
  and finally making the recorded image looks
  grainy.
• Quantum noise can be avoided by using adequate
  exposure factors producing high enough x-ray
  intensity

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Resolution of invisible x-ray image

• The resolution depends on
• contrast,
• sharpness and
• noise.
• We must try to obtain maximum resolution at this
  stage because the resolution becomes less and
  less in the next stages of image production

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Conclusion

• It is important to know the details of production
  and characteristics of the invisible x-ray image
  because
• If the invisible x-ray image is of poor quality,
  it is extremely difficult to produce an adequate
  standard of final visible image.
• It is during the production of the invisible
  x-ray image that the radiographer has the
  greatest scope for control of image quality,
  particularly in conventional radiography.