invisible x ray image l.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Invisible X-ray image PowerPoint Presentation
Download Presentation
Invisible X-ray image

Loading in 2 Seconds...

play fullscreen
1 / 29

Invisible X-ray image - PowerPoint PPT Presentation


  • 178 Views
  • Uploaded on

Invisible X-ray image. Formation Characteristics. X-ray tube. Plot of incident x-ray beam intensity. Object. Plot of transmitted x-ray beam intensity. Invisible x-ray image. Invisible x-ray image. kV mA Sec FFD. E. Supporting tissue (m). B. B1. T3. B2. T1. T2. Air.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Invisible X-ray image' - dara


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
invisible x ray image

Invisible X-ray image

Formation

Characteristics

slide2

X-ray tube

Plot of incident x-ray beam intensity

Object

Plot of transmitted x-ray beam intensity

Invisible x-ray image

slide3

Invisible x-ray image

kV mA Sec FFD

E

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

characteristics
Characteristics
  • Subject contrast
  • Sharpness
  • Noise
  • Resolution
subject contrast
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
differential attenuation
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
effective atomic number subject contrast
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

x ray tube kilovoltage subject contrast
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.
slide9

kV & subject contrast

Low kV

E

Supporting tissue (m)

B

B1

T3

B2

T1

T2

Air

E B1

E B2

Higher differences

ET1

EM

EM

ET2

ET3

EA

slide10

kV & subject contrast

High kV

E

Supporting tissue (m)

B

B2

B1

T3

T1

T2

Air

Lower differences

E B1

E B2

EM

EM

ET1

ET2

ET3

EA

x ray beam filtration subject contrast
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
scattered radiation subject contrast
Scattered radiation & subject contrast

Scattered radiation

Primary beam

scattered radiation subject contrast13
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.
slide14

Scatter reduces the subject contrast

E

Supporting tissue (m)

B

B2

B1

T3

T1

T2

Air

Scatter Lowers the differences

E B1

E B2

EM

EM

ET1

ET2

ET3

EA

how to minimize the effect of scatter on subject contrast
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
use of grid
Use of grid

Lead strips

Image receptor

Radiolucent inter-space

employing air gap
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

sharpness of invisible x ray image
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
image geometry
Image Geometry

Finite source

Point source

Image plane

Unsharpness (penumbra)

intensity distribution at previous situations
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

geometric unsharpness
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

evaluation of geometric unsharpness
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

factors governing geometric unsharpness
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
focal spot size geometric unsharpness
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
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

Unsharpness due to Edge penetration

Intensity of x-rays at image plane

Distance across image plane

movement unsharpness
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
noise in the invisible x ray image
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
resolution of invisible x ray image
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
conclusion
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.