Transcript Invisible X

Invisible X-ray image
1.
2.
Formation
Characteristics
X-ray tube
Plot of incident x-ray
beam intensity
Object
Invisible x-ray
image
Plot of transmitted x-ray
beam intensity
Invisible x-ray image
kV mA Sec FFD
E
B
B1
EM
E
B1
Supporting tissue (m)
B2
E
B2
T2
T1
ET1
EM
T3
Air
Invisible
X-ray
image
consists
of
different xray
intensities
ET2
ET3
EA
Characteristics
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Subject contrast
Sharpness
Noise
Resolution
Subject contrast
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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
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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
Effective atomic number & Subject
contrast
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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 > Z2 > Z3
Incident intensity
Subject
contrast A-B
A
Z1
B
Z2
C
Z3
Transmitted intensity
Subject
contrast A-C
Subject contrast B-C
X-ray tube kilovoltage & subject
contrast
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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.
kV & subject contrast
Low kV
E
B
B1
EM
E
B1
Supporting tissue (m)
B2
E
B2
T2
T1
ET1
EM
T3
Air
Higher
differen
ces
ET2
ET3
EA
kV & subject contrast
High kV
E
B
B1
EM
E
B1
Supporting tissue (m)
B2
E
B2
T1
ET1 EM
T2
T3
Air
Lower
differen
ces
ET2
ET3
EA
X-ray beam filtration & Subject
contrast
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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
Scattered radiation & subject
contrast
Scattered
radiation
Primary beam
Scattered radiation & subject
contrast
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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.
Scatter reduces the subject contrast
E
B
B1
EM
E
B1
Supporting tissue (m)
B2
E
B2
T1
ET1 EM
T2
ET2
T3
ET3
Air
Scatter
Lowers
the
differen
ces
EA
How to minimize the effect of
scatter on subject contrast?
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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
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Protecting the image receptor by
Use of secondary radiation grid
 Employing an air gap
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Use of grid
Lead strips
Radiolucent inter-space
Image receptor
Employing Air gap
Image plane 1
Image plane 2
Object
Air gap
Percentage of oblique ray reaching the image
receptor plane is reduced at image plane 2
Sharpness of Invisible x-ray image
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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)
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When the size of the x-ray source (Focus) is
large the sharpness of the image is less
Image Geometry
Point source
Image plane
Finite source
Unsharpness (penumbra)
Intensity of x-rays at image plane
Intensity of x-rays at image plane
Intensity distribution at previous
situations
Distance across image plane
U
U
Distance across image
plane
Geometric unsharpness
The formation of unsharpness due to a penumbra
is a direct consequence of the finite size of the xray 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
Source
A
B
Triangles OAB & OCD are similar.
AB/CD = OB/OC
Re-arranging
Object
CD = AB x OC/OB
O
UG = focal size x OFD/FOB
Image plane
C
D
Factors governing geometric
unsharpness
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Focal spot size
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Object image (film) distance
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Small focus gives minimum geometric unsharpness
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
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Edge penetration
Focal spot size & Geometric
unsharpness
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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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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
Unsharpness due to Edge
penetration
Distance across image plane
Movement unsharpness
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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
Noise in the invisible x-ray image
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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.
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Quantum noise can be avoided by using adequate
exposure factors producing high enough x-ray
intensity
Resolution of invisible x-ray image
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The resolution depends on
contrast,
 sharpness and
 noise.
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We must try to obtain maximum resolution at
this stage because the resolution becomes less
and less in the next stages of image production
Conclusion
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It is important to know the details of
production and characteristics of the invisible xray 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.