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Invisible X-ray image 1. Formation 2. Characteristics.

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Presentation on theme: "Invisible X-ray image 1. Formation 2. Characteristics."— Presentation transcript:

1 Invisible X-ray image 1. Formation 2. Characteristics

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

3 B2 T1 T2 T3 E E B1 E B2 E T1 EMEM E T2 E T3 EAEA kV mA Sec FFD B B1 Air Supporting tissue (m) EMEM Invisible X-ray image consists of different x- ray intensities

4 Characteristics Subject contrast Subject contrast Sharpness Sharpness Noise Noise Resolution Resolution

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

6 Differential attenuation Differential attenuation is the result of the attenuation caused by Photoelectric absorption and Compton scattering. Differential attenuation is the result of the attenuation caused by Photoelectric absorption and Compton scattering. Depends on Depends on Thickness of the anatomical structure Thickness of the anatomical structure Effective atomic number of the body tissues Effective atomic number of the body tissues Physical density of the body tissues Physical density of the body tissues Presence of radiological contrast medium Presence of radiological contrast medium X-ray tube kilovoltage employed X-ray tube kilovoltage employed X-ray beam filtration 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) 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 Z 1 > Z 2 > Z 3 E.g. if the three tissues A,B,C have effective atomic numbers as Z 1 > Z 2 > Z 3 AZ1AZ1 BZ2BZ2 CZ3CZ3 Incident intensity Transmitted intensity Subject contrast A-C Subject contrast A-B 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. 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. 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 B2 T1 T2 T3 E E B1 E B2 E T1 EMEM E T2 E T3 EAEA Low kV B B1 Air Supporting tissue (m) EMEM Higher differen ces

10 kV & subject contrast B2 T1 T2 T3 E E B1 E B2 E T1 EMEM E T2 E T3 EAEA High kV B B1 Air Supporting tissue (m) EMEM Lower differen ces

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. 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 Therefore increasing the filtration will decrease the subject contrast

12 Scattered radiation & subject contrast Scattered radiation Primary beam

13 Scattered radiation & subject contrast When the primary beam from x-ray tube interacts with matter scattered radiation is produced. 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. 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. Not only the subject contrast but it will reduce the signal to noise ratio also.

14 Scatter reduces the subject contrast B2 T1 T2 T3 E E B1 E B2 E T1 EMEM E T2 E T3 EAEA B B1 Air Supporting tissue (m) EMEM Scatter Lowers the differen ces

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

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

17 Employing 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

18 Sharpness of Invisible x-ray image The sharpness is determined first by the geometry of image formation The sharpness is determined first by the geometry of image formation The size of the source of radiation is of primary concerned The size of the source of radiation is of primary concerned 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 large the sharpness of the image is less When the size of the x-ray source (Focus) is large the sharpness of the image is less

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

20 Intensity distribution at previous situations Intensity of x-rays at image plane Distance across image plane UU

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

22 Evaluation of Geometric unsharpness A B O CD 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

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

24 Focal spot size & Geometric unsharpness Unsharpness increases, when apparent focal area increases Unsharpness increases, when apparent focal area increases Apparent (effective) focal area = Actual focal area x Sine of target angle 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 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 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 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 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 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 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 : U M This changes are referred to as movement unshrpness : U M If they occur during image recording they will produce unsharpness in the final image 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 The kinds of noise present in the invisible x-ray image are Fog due to scatter radiation 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 – 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 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 The resolution depends on contrast, contrast, sharpness and sharpness and noise. 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 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; 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. 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. 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.


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