1. Formation Characteristics
Invisible X-ray image
Formation
Characteristics

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

3. Invisible x-ray image kV mA Sec FFD E B B1 Air E B1 E B2 ET1 EM EM ET2
Supporting tissue (m) B B1 T3 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

5. Subject contrast Causes of 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

6. 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

7. 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 > Z2 > Z3
Incident intensity A Z1 B Z2 C Z3
Subject contrast A-B
Transmitted intensity
Subject contrast A-C
Subject contrast B-C

8. 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.

9. kV & subject contrast Low kV E B B1 Air E B1 E B2 Higher differences
Supporting tissue (m) B B1 T3 B2 T1 T2
Air
E B1
E B2
Higher differences
ET1 EM EM
ET2
ET3 EA

10. kV & subject contrast High kV E B B1 Air Lower differences E B1 E B2
Supporting tissue (m) B B2 B1 T3 T1 T2
Air
Lower differences
E B1
E B2 EM EM
ET1
ET2
ET3 EA

11. 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

12. Scattered radiation & subject contrast
Primary beam

13. 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.

14. Scatter reduces the subject contrast
Supporting tissue (m) B B2 B1 T3 T1 T2
Air
Scatter Lowers the differences
E B1
E B2 EM EM
ET1
ET2
ET3 EA

15. 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

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

17. Employing Air gap 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

18. 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

19. Image Geometry Finite source Point source Image plane
Unsharpness (penumbra)

20. 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

21. 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

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

23. 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

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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.