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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 l.jpg
Limitation of Planar X-Rays

Angiography as one solution

(Kandel et al. Principles of Neural Science)


Tomography3 l.jpg
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 l.jpg
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 l.jpg
Brain CT

Wikipedia, Computed Axial Tomography


Other tomographic techniques mri l.jpg
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 l.jpg
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


Feldkamp cone beam reconstruction l.jpg
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 l.jpg
MicroCT of Rat Tibia

Skyscan Web Site

(www.skyscan.be)


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