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

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limitation of planar x rays
Limitation of Planar X-Rays

Angiography as one solution

(Kandel et al. Principles of Neural Science)

tomography3
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.)

weighted back projection
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
brain ct
Brain CT

Wikipedia, Computed Axial Tomography

other tomographic techniques mri
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
t1 weighting
T1 weighting

Less dense tissue (CSF) relaxes fast = dark)

Denser tissue (bone, myelin) relaxes more slowly (white)

Wikipedia, Magnetic Resonance Imaging

pet scanning
PET scanning
  • Positron emission and positron annihilation
  • Deoxyglucose as a metabolic marker
  • 18F Deoxyglucose as a metabolic positron emitter
pet scan of brain
PET scan of brain

Wikipedia, Positron Emission Tomography

microct
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
fan beam reconstruction
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
fan beam reconstruction14
Fan Beam reconstruction

Rosenfeld and Kak, Digital Picture Processing

feldkamp cone beam reconstruction
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
microct of rat tibia
MicroCT of Rat Tibia

Skyscan Web Site

(www.skyscan.be)