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Tomography

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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Tomography

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  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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