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Tomography

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Title: Tomography


1
Tomography
2
Limitation of Planar X-Rays
Angiography as one solution (Kandel et al.
Principles of Neural Science)
3
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.)

4
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

5
Computed axial tomography, or CT
6
Brain CT
Wikipedia, Computed Axial Tomography
7
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

8
MR Scanner
9
T1 weighting
Less dense tissue (CSF) relaxes fast
dark) Denser tissue (bone, myelin) relaxes more
slowly (white)
Wikipedia, Magnetic Resonance Imaging
10
PET scanning
  • Positron emission and positron annihilation
  • Deoxyglucose as a metabolic marker
  • 18F Deoxyglucose as a metabolic positron emitter

11
PET scan of brain
Wikipedia, Positron Emission Tomography
12
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

13
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

14
Fan Beam reconstruction
Rosenfeld and Kak, Digital Picture Processing
15
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

16
MicroCT of Rat Tibia
Skyscan Web Site (www.skyscan.be)
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