CT

Card Set Information

Author:
stronginteraction
ID:
98789
Filename:
CT
Updated:
2011-08-28 22:24:38
Tags:
CT
Folders:

Description:
CT
Show Answers:

Home > Flashcards > Print Preview

The flashcards below were created by user stronginteraction on FreezingBlue Flashcards. What would you like to do?


  1. CT number and unit
    Example water, air, bone
    • CT# = (u-u_water)/u_water*1000
    • u_water = 0.1928 cm^-1
    • CT#(water) = 0
    • CT#(air) = -1000
    • CT#(bone) = +250...+1000
    • Hounsfield Units
  2. Attenuation
    Lambert-Beers Law
    • I = I_0 * exp[-(tau+sigma+sigma_gamma)]*L for monochromatic incident x-ray beam
    • L: thickness
    • Scattering attenuation coefficients:
    • tau: photoelectric
    • sigma: compton
    • sigma_gamma: coherent
    • Lambert-Beers Law
    • :I = I_0 * exp[-mu*L], mu = mu(E)
    • E: photon energy
  3. Sources of CT measurement error
    JH, ch.7
    • X-ray beam not monochromatic, mu=mu(E)
    • Scattered radiation, non-primary photons
    • Non-Linearity of detector/data acquisition system
    • Scanned object movement during scanning
    • Partial volume effect
    • Focal spot drift
    • Mechanical vibration
    • Off-focal radiation
    • CT gantry misalignment
    • Metal objects in scanning plane
    • X-ray tube arcing
    • X-ray photon starvation
    • Projection sampling deficiency
  4. Scattered Radiation
    Related Artifacts
    JH41
    • Not all detected photons are primary
    • Adds "low frequency bias" to true attenuation
    • This is non-linear when the logarithm is applied to the signal to obtain lin integrals
    • p = -ln(I/I_0) = Line integral [mu(x) dx]
    • Leads to shading and streaking artifacts
  5. Beam Hardening
    Reasons and Consequences
    • Back-projection assumes energy-independent attenuation coefficient mu or monoenergetic x-ray beam
    • Both conditions typically not fullfilled
    • Resulting artifacts in recontructed images:
    • Cupping, shading, streaking
  6. Total Intensity of Bremstrahlung Radiation
    Why x-ray pruduction with electrons?
    • X-ray intensity proportional to (Z2*z4*e6)/m2
    • Charged particle beam, mass m, charge z*e
    • Target charge Z*e
    • Electrons 3*106 times more efficient than protons
  7. Energy-wavelength conversion
    • E = h*nu = h * c / lambda
    • E = 1240 / lambda[nm] eV
    • lambda = 1240 / E_eV nm
    • 1 keV ~ 1.2 nm
  8. Low contrast differentiation of tissues
    Photoelectric effect
    HS32
    • Probability of photoelectric interaction, P
    • P ~ 1/E3
    • P ~ Z3
    • Need lower energy x-ray photons for differentiation
  9. X-ray energy transfer
    Compton vs Photoelectric
    HS34
    • Typically more energy is transferred in a photoelectric interaction than in a Compton interaction
    • Graph here
  10. CT Inventors
    Year and Nobel Prize
    • Allan M. Cormack, work 1955-1963, reconstruct attenuation coefficients, images disk w/ different materials, Tufts University
    • Godfrey N. Hounsfield, work 1967-1971, first clinically available CT scan, EMI Labs, England
    • Nobel prize 1979 (both), Physiology and Medicine
  11. 5 CT scanner generations
    • 1. Translate source and detector while scanning object at one given projection angle, rotate for new angle, repeat. One source. One detector cell.
    • 2. Translate less, rotate. Possible due to larger detector, several cells. Povides more coverage.
    • 3. Rotate both source and detector while scanning. Possible due to object fully covered in radiation "cone" and detector large enough to capture radiation.
    • 4. Stationary detector, rotate source
    • 5. Stationary detector, stationary source (Imatron)
  12. Mathematical formulation for reconstruction
    HS7
    • J. Radon, 1917, Austrian mathematician
    • "Object could be replicated from an infinite set of its projections"
    • Tomographic reconstruction problem = inverse Radon transform

What would you like to do?

Home > Flashcards > Print Preview