X-ray radiography Seminar

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X-ray radiographySeminar

• 22.10. 2008.

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Difficulties and problems of classical radiography

• The "geometrical" unsharpness of X-ray images is
  limited by finite dimension of the source, i.e.
  the window on X-ray tube. Therefore the optical
  shadow is always surrounded by half-shadow. By
  adjustment of distance between the tube, object
  and film we try to minimalize the half-shadow.
• Secondary photons, emitted by relaxation of atoms
  after
• Compton scattering, leave the body and
  uniformly increase the exposure of the film
  reducing the contrast.
• The quality of radiographs can be improved by the
  use of special filters, X-ray film coated with
  fluorescence layer and image intensifier.

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Contrast of the image

• The X-ray beams will be absorbed in different
  amount in particulate tissues. The transmitted
  beams will have different intensities - the basis
  for the contrast in the image.
• The contrast is satisfactory if the intensity
  difference is at least 10.
• Using the contrast agents we can improve the
  contrast.

I0
I1(bones)
I2 (soft tissues)
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• Soft tissues are absorbing X-radiation by the
  mechanism of Compton effect. The probability for
  absorption depends on electron density which is
  almost the same for the atoms of soft tissue and
  the contrast is rather low.
• It can be enhanced by introduction of substance
  with high density and high Z number (contrast
  agent) in organ under investigation. These agents
  are absorbing X-radiation by photoelectric
  effect. Using the lower voltage, only the soft
  tissues with contrast agent will have strong
  absorption.

intestines with solution of barium salt
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Influence of half-shadow on contrast

• half-shadow is the consequence of finite
  dimension of the source

half-shadow
x
L
shadow
D
film
window aperture
object
d
x D d/L
half-shadow is thinner if the distance from
object to film is small in comparison with
distance from object to source
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Use of intensifying screen on X-ray film

• Photographic emulsion is not enough sensitive for
  X-photons, only 1 of incident photons are
  absorbed in film - it would require the long
  irradiation of patient to get the good image
• Intensifying screen (phosphor in low Z matrix)
  enables the transformation of X-photons into
  light photons (1 X - 1000 VIS) which can be
  completely absorbed in film
• The intensity of image can be enhanced about 30
  times

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Image intensifier
- in fluorescent screen X-photons are
transformed in photons of visible light - these
photons eject electrons from photocathode - in
evacuated tube, electrons are accelerated toward
anode their motion is controlled by electron
optics - electrons hit the other fluorescent
screen and create the image of high intensity
which can be monitored in the dark
convenient for surgery halls, teaching, distance
learning
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Image intensifier in surgery
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Xerography
X-beam

• Xerography uses photoconductivity -
• photoelectric effect on a semiconductor
• the images have sharp boundary
• lines - the borders between different
• tissues
• The beam transmitted through the
• body contacts with positively charged
• selenium coated aluminum plate at the
• collision site, the photon energy causes
• selenium to become conductive and the
• charge is neutralized the distribution of
• positive charge density is inversely
• proportional to the number of incident
• photons (electrostatic image)
• by applying negatively charged dye
• powder we can get visible image

objects with different density
edges are sharp due to changes of electric field
as a step function