Invisible x ray-image

1. Radiographic Radiographic
PhotographyPhotography
Dr Hussein Ahmed HassanDr Hussein Ahmed Hassan

2. Invisible X-ray imageInvisible X-ray image
1.1.FormationFormation
2.2.CharacteristicsCharacteristics

3. X-ray tube
Object
Plot of incident x-ray
beam intensity
Plot of transmitted x-ray
beam intensity
Invisible x-ray
image

4. Invisible x-ray imageInvisible x-ray image
B2
T1 T2 T3
E
E
B1E
B2
E
T1
E
M
E
T2
E
T3
E
A
kV mA Sec FFD
BB1
Air
Supporting tissue (m)
E
M
Invisible
X-ray
image
consists
of
different x-
ray
intensities

5. CharacteristicsCharacteristics
Subject contrastSubject contrast
SharpnessSharpness
NoiseNoise
ResolutionResolution

6. Subject contrastSubject contrast
The difference in the x-ray intensities The difference in the x-ray intensities
transmitted through the subjecttransmitted through the subject
It is the shortened form of the It is the shortened form of the radiation radiation
contrast of the subjectcontrast of the subject
Causes of subject contrastCauses of subject contrast
Differential attenuationDifferential attenuation
Scattered radiationScattered radiation

7. Differential attenuationDifferential attenuation
Differential attenuation is the result of the Differential attenuation is the result of the
attenuation caused by attenuation caused by Photoelectric Photoelectric
absorptionabsorption and and Compton scattering.Compton scattering.
Depends onDepends on
Thickness of the anatomical structureThickness of the anatomical structure
Effective atomic number of the body tissuesEffective atomic number of the body tissues
Physical density of the body tissuesPhysical density of the body tissues
Presence of radiological contrast mediumPresence of radiological contrast medium
X-ray tube kilovoltage employedX-ray tube kilovoltage employed
X-ray beam filtrationX-ray beam filtration

8. Effective atomic number & Subject Effective atomic number & Subject
contrastcontrast
For a given Photon energy the photo electric For a given Photon energy the photo electric
absorption is higher when the atomic number is high ( absorption is higher when the atomic number is high (
bone absorbs more radiation than soft tissue)bone absorbs more radiation than soft tissue)
E.g. if the three tissues A,B,C have effective atomic E.g. if the three tissues A,B,C have effective atomic
numbers as Znumbers as Z
11 > Z > Z
2 2 > Z> Z
33
A
Z
1
B
Z
2
C
Z
3
Incident intensity
Transmitted intensity
Subject
contrast A-C
Subject contrast
A-B
Subject contrast B-C

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

10. kV & subject contrastkV & subject contrast
B2
T1 T2 T3
E
E
B1E
B2
E
T1
E
M
E
T2
E
T3
E
A
Low kV
BB1
Air
Supporting tissue (m)
E
M
Higher
differen
ces

11. kV & subject contrastkV & subject contrast
B2 T1 T2 T3
E
E
B1
E
B2
E
T1
E
M
E
T2
E
T3
E
A
High kV
BB1
Air
Supporting tissue (m)
E
M
Lower
differen
ces

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

13. Scattered radiation & subject Scattered radiation & subject
contrastcontrast
Scattered
radiation
Primary beam

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

15. Scatter reduces the subject contrastScatter reduces the subject contrast
B2 T1 T2 T3
E
E
B1
E
B2
E
T1
E
M
E
T2
E
T3
E
A
BB1
Air
Supporting tissue (m)
E
M
Scatter
Lowers
the
differen
ces

16. How to minimize the effect of scatter How to minimize the effect of scatter
on subject contrast?on subject contrast?
Reduce the amount of scatter produced at the object Reduce the amount of scatter produced at the object
(patient) by:(patient) by:
Collimating the primary beamCollimating the primary beam
Reducing the proportion of forward scatter using low kVReducing the proportion of forward scatter using low kV
Reducing the tissue thicknessReducing the tissue thickness
Avoiding other sources of scatter, such as bucky trayAvoiding other sources of scatter, such as bucky tray
Protecting the image receptor byProtecting the image receptor by
Use of secondary radiation gridUse of secondary radiation grid
Employing an air gapEmploying an air gap

17. Use of gridUse of grid
Lead strips
Radiolucent inter-space
Image receptor

18. Employing Air gapEmploying Air gap
Percentage of oblique ray reaching the image
receptor plane is reduced at image plane 2
Object
Image plane 2
Scatter
Image plane 1
Air gap

19. Sharpness of Invisible x-ray imageSharpness of Invisible x-ray image
The sharpness is determined first by the The sharpness is determined first by the
geometry of image formationgeometry of image formation
The size of the source of radiation is of primary The size of the source of radiation is of primary
concernedconcerned
Infinite size (Point source)Infinite size (Point source)
Finite size ( larger than a point)Finite size ( larger than a point)
When the size of the x-ray source (Focus) is When the size of the x-ray source (Focus) is
large the sharpness of the image is less large the sharpness of the image is less

20. Image GeometryImage Geometry
Point source Finite source
Unsharpness (penumbra)Image plane

21. Intensity distribution at previous Intensity distribution at previous
situationssituations
I
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Distance across image plane
Distance across image plane
U U

22. Geometric unsharpnessGeometric unsharpness
The formation of unsharpness due to a penumbra The formation of unsharpness due to a penumbra
is a direct consequence of the is a direct consequence of the finite sizefinite size of the x- of the x-
ray source.ray source.
This form of unsharpness is known as This form of unsharpness is known as Geometric Geometric
unsharpness (Uunsharpness (U
GG))
It can be shown thatIt can be shown that
focal spot size focal spot size xx object-image distance object-image distance
Geometric = -------------------------------------------Geometric = -------------------------------------------
Unsharpness focus-object distanceUnsharpness focus-object distance

23. Evaluation of Geometric Evaluation of Geometric
unsharpnessunsharpness
A
B
O
C D
Source
Image plane
Object
Triangles OAB & OCD are similar.
AB/CD = OB/OC
Re-arranging
CD = AB x OC/OB
U
G
= focal size x OFD/FOB

24. Factors governing geometric Factors governing geometric
unsharpnessunsharpness
Focal spot sizeFocal spot size
Small focus gives minimum geometric unsharpnessSmall focus gives minimum geometric unsharpness
Object image (film) distance Object image (film) distance
Shorter OFD gives less geometric unsharpnessShorter OFD gives less geometric unsharpness
Focus to object ( Focal film) distanceFocus to object ( Focal film) distance
Longer the FFD lesser the geometric unsharpnessLonger the FFD lesser the geometric unsharpness
Increase the FFD when OFD cannot be reduced, to Increase the FFD when OFD cannot be reduced, to
minimize the geometric unsharpnessminimize the geometric unsharpness
Edge penetrationEdge penetration

25. Focal spot size & Geometric Focal spot size & Geometric
unsharpnessunsharpness
Unsharpness increases, when apparent focal area Unsharpness increases, when apparent focal area
increases increases
Apparent (effective) focal area = Actual focal Apparent (effective) focal area = Actual focal
area x Sine of target anglearea x Sine of target angle
Therefore Unsharpness increases when target Therefore Unsharpness increases when target
angle increases for a given actual focal spot sizeangle increases for a given actual focal spot size
Geometric Unsharpness can be reduced by Geometric Unsharpness can be reduced by
using small focus but that reduces the maximum using small focus but that reduces the maximum
tube loading capacitytube loading capacity

26. Apparent (effective) focal area = Actual focal area x Apparent (effective) focal area = Actual focal area x
Sine of target angleSine of target angle

27. Unsharpness due to Edge Unsharpness due to Edge
penetrationpenetration
This is due to the shape This is due to the shape
of the objectof the object
The edges of the object The edges of the object
absorb less amount of absorb less amount of
radiation and the radiation and the
absorption increases absorption increases
towards the centretowards the centre
This creates a intensity This creates a intensity
gradient producing gradient producing
inherent unsharpnessinherent unsharpnessI
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Distance across image plane

28. Movement unsharpnessMovement unsharpness
Voluntary & involuntary movement of the Voluntary & involuntary movement of the
organs or body parts or the patient as a whole organs or body parts or the patient as a whole
will cause changes in the pattern of x-ray will cause changes in the pattern of x-ray
intensities forming the invisible x-ray imageintensities forming the invisible x-ray image
This changes are referred to as movement This changes are referred to as movement
unshrpness : Uunshrpness : U
MM
If they occur during image recording they will If they occur during image recording they will
produce unsharpness in the final imageproduce unsharpness in the final image

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

30. Resolution of invisible x-ray imageResolution of invisible x-ray image
The resolution depends on The resolution depends on
contrast, contrast,
sharpness and sharpness and
noise.noise.
We must try to obtain maximum resolution at We must try to obtain maximum resolution at
this stage because the resolution becomes less this stage because the resolution becomes less
and less in the next stages of image productionand less in the next stages of image production

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