X - RAYS IN DIAGNOSTICS

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X - RAYS IN DIAGNOSTICS

• D. Krilov
• 22. 10. 2008.

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HISTORY

• W.C.Röntgen (1845-1923) 8.11. 1895. - discovered
  a new type of radiation in experiments on gas
  discharges he demonstrated that this radiation
• induces the ionization in the air
• penetrates through the matter
• does not deflect in electric and magnetic field
• foggs the photographic plate
• induces the burns on skin
• in January 1896. he produced the first anatomical
  X-ray picture of the hand
• Nature, Jan. 23 1896
• Science, Feb.14 1896

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The nature of X-rays

• X - rays are electromagnetic waves (1 pm 0,1
  nm)
• natural sources of X-rays do not exist the
  artificial source is X-ray tube
• X-rays are produced by two mechanisms
• 1. by rapid decceleration of fast electrons in
  electric fields generated by heavy nuclei
• 2. by relaxation of heavy atoms in tube target
  (anode)
• the medical application is based on specific
  interactions of incident X-ray photons with atoms
  in tissues the image is produced from the beams
  transmitted through the body

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X-ray tube
mv2/2 eUa
Ie 20-30 mA
cathode
anode
Ua 30-150 kV
Ig 3-5 A
tungsten disc
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Generation of X-rays

• Brehmsstrahlung (braking
• radiation)
• High speed electrons enter the crystal
• lattice of target atoms and are deccelerated
• in electric field of atomic nuclei.
• Energy of emitted photon depends on
• energy loss of photon.
• The photon with highest energy is
• generated when electron is stopped

h n
Ein
Eout
transmitted beam is composed of photons
with different energies - continuous spectrum
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• Collisions of incident electrons with electrons
  in target material
• Incident electron ejects one of electrons from
  inner shell of target atom.
• The empty state is filled by an electron from
  higher shell, the energy difference is emitted as
  X-photon
• Only the photons with energies equal to
  differences of particular atomic levels are
  emitted - line spectrum reflects the atomic
  structure of target
• The probability of such events is low - the
  intensity of line spectrum is low

Linear spectrum consists of a number of narrow
lines transitions into K shell Ka,
Kb transitions into L shell La, Lb
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X-ray spectrum

• The spectrum is the plot of spectral radiancy
  over wavelength
• It is composed of continuous and linear part
• continuous spectrum has well defined
  short-wavelength cutoff determined by anode
  voltage
• the highest radiancy is at wavelength
• linear spectrum is not important for medical
  diagnostics

Il(W/m3)
lmin
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Influence of tube voltage on X-ray spectrum
Beam power is determined by empiric relation Ie
is the current of electrons in tube which
depends on heating current of cathode Z is
atomic number of target atoms Intensity of beam
is the ratio of power and the
surface of the window on tube Increase of
voltage enhances the beam intensity. The spectrum
is shifted to shorter wavelengths the hardness
of the beam is higher
Il
spectral radiancy
l
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Influence of heating current and window filter
on X-ray spectrum
Il
Il
high heating current
without filter
with filter
low heating current
l
l
short wavelength cutoff is the same but
wavelength of highest radiancy is shifted to
shorter wavelengths. The intensity is lower but
the hardness is higher.
short wavelength cutoff and wavelength of highest
radiancy are not influenced by heating current
only the intensity of beam depends on the current
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Interaction of X-ray photons with atoms in tissue
the type of interaction depends on photon energy
and atomic composition of tissue
photoelectric effect predominates for the photons
with energies lower than 80 keV it is more
probable for heavy atoms in tissue which are
present in bones (Ca) Compton
scattering predominates for the photons with
higher energies it is more probable for lighter
atoms in soft tissue (O, N, C, H)
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Law of attenuation for X-rays

• Intensity decrease of monochromatic beam along
  its path through the tissue
• m(l) is linear absorption coefficient which
  depends on tissue and wavelength of radiation
• in medical diagnostics is commonly used mass
  absorption coefficient
• which depends on probabilities for photoelectric
  effect (t) i Compton
• scattering (s)

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Half value layer

• the thickness of absorber which attenuates half
  of the photons
• 
  parameter for determination of
• 
  hardness of polychromatic beam
• -
  higher x1/2 means harder beam

x
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Plot for attenuation of real polychromatic beam
rapid decay due to absorption of low energy
photons

• The analytical expression for the attenuation of
  polychromatic beam does not exist
• The average energy of polychromatic beam is
  chosen as the energy of corresponding
  monochromatic beam with equal half value layer.

I
x
along the path through tissue the beam becomes
harder due to predominate influence of higher
energy photons penetration power is increased
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X-ray diagnostics

• In classical radiography we get the image
  obtained from transmitted beams it displays the
  shadows of tissue structures - the image is 2D
  projection of 3D object therefore the shadows of
  bones overlay the shadows of soft tissue
• Intensity of transmitted beam depends on
  absorption coefficient
• The images of bones are obtained with high
  contrast if we apply lower tube voltage - low
  energy photons then, the absorption coefficient
  for photoelectric effect in bones is increased
  and the absorption in the soft tissue is very low
• the good images of soft tissue are obtained if we
  apply higher tube voltage - high energy photons
  in such conditions the absorption coefficient for
  Compton effect is increased we can see shadows
  of soft tissues but also the more pale shadows of
  the bones due to lower absorption of high energy
  photons however, the overall contrast is worse
  than for low voltage

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Computer Assisted Tomography (CT,CAT)

• Hounsfield and Cormack 1972.
• it is the combination of special way of
  recording, accumulation of data and mathematical
  processing for the image reconstruction
• The basic concept of the method
• The narrow beam propagates through the layer of
  the body, its thickness is determined by the beam
  width.
• At the other end is a detector which measures the
  intensity of transmitted beam.

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• The layer (perpendicular to the long axis of the
  body) is divided in volume elements - voxel (10
  mm3).The size of one voxel is determined by the
  cross section of the beam the voxel size
  determines the resolution of the image
• To each voxel is attributed its absorption
  cofficient. The beam propagates throuh the row of
  voxels and the intensity of transmitted beam is

pixel element of 2D image of the layer one
pixel contains the information from one voxel -
the number of pixels depends on the number of
voxels
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• By subsequent parallel translation of tube and
  detector across the layer, the whole layer is
  recorded. The process is repeated after each
  rotation of the pair tube-detector for a small
  angle. In that way we get enough data for the
  processing of the complex mathematical algorithm
  for the distribution of attenuation coefficients
  in the layer. The calculated data are transformed
  into pixels and displayed in grey scale.
• The number of counted photons determines the
  precision of absorption measurement along one row
  of voxels.
• The contrast depends on absorption differences in
  particluate tissues.
• The described procedure demanded time-consuming
  recording, so the technological improvement was
  based on building the devices for much faster
  recording which became possible after
  construction of light, small and cheap detectors.

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Novel equipments for CT

• They enable instantaneous recording for large
  number of directions in the layer simuntaniously.
  In that way the interval of patient irradiation
  is significantly shortened. Application of fan
  shaped beams and automatic rotation of the tube,
  makes the recording time even shorter.

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• CT equipment which is in use nowadays is
  constructed with immobile detectors arranged in a
  circle perpendicular to the long axis of the
  body, while the tube is rotating in that plane.
  By automatic shift of bed, the new layers are
  recorded.

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Spiral CT

• This novel method enables additional shortening
  of recording time, due to very fast computers.
  The data are taken and processed while the body
  is continuously shifted.
• The image of the heart can be obtained in 0.1 s.
• The computers enable reconstruction of 3D image