Tomography

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Tomography
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Limitation of Planar X-Rays
Angiography as one solution (Kandel et al.
Principles of Neural Science)
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Tomography

• If all we have is a single view (a) the
  information we have about 3-D structure can be
  represented by back-projecting those intensities
  along the direction of view
• If we add more directions, the quality of the
  image improves (g 160 views)
• (Russ, Image Processing Handbook.)

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Weighted back projection

• In the case of X-rays, there is a shadowing
  effect if all the X-rays are absorbed by a
  structure close to the source, there is no
  contrast for deeper-lying structures
• Solution distance-weighted back projection
  structures closer to the source are weighted more
  heavily in the reconstruction

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Computed axial tomography, or CT
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Brain CT
Wikipedia, Computed Axial Tomography
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Other tomographic techniques - MRI

• Nuclear dipole moment aligns parallel to
  applied magnetic field
• RF pulses applied perpendicular to magnetic
  field knock dipole movement off
• As nuclei process back to parallel, they emit
  radio waves
• T1 weighting time for nuclei to realign

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MR Scanner
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T1 weighting
Less dense tissue (CSF) relaxes fast
dark) Denser tissue (bone, myelin) relaxes more
slowly (white)
Wikipedia, Magnetic Resonance Imaging
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PET scanning

• Positron emission and positron annihilation
• Deoxyglucose as a metabolic marker
• 18F Deoxyglucose as a metabolic positron emitter

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PET scan of brain
Wikipedia, Positron Emission Tomography
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MicroCT

• Object is placed in a conical beam of X-rays
• The closer the object is to the X-ray source, the
  greater the magnification
• SkyScan 1072 5 micron resolution

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Fan beam reconstruction

• X-ray beams through specimen are not parallel
• Fan beam CT restricts X-rays to a fan
• Reconstruction back-projects along fan of beams
• Algorithm is fast, but not optimal

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Fan Beam reconstruction
Rosenfeld and Kak, Digital Picture Processing
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Feldkamp cone beam reconstruction

• X-ray beam is not constrained to a fan
• Reconstruction is done for a cone of projections
  through the specimen
• More accurate than fan-beam, but much slower
• Newer algorithms use wavelet or Fourier
  transforms, rather than weighted back-projection

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MicroCT of Rat Tibia
Skyscan Web Site (www.skyscan.be)