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BIOE 220/rad 220 Review session

BIOE 220/rad 220 Review session. January 30, 2011. Comments on HW 1. Problem 46-48 (the first set) were actually supposed to refer to a different picture, so don’t think that it’s a representative problem Can tell it’s T1 since CSF is dark

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BIOE 220/rad 220 Review session

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  1. BIOE 220/rad 220 Review session January 30, 2011

  2. Comments on HW 1 • Problem 46-48 (the first set) were actually supposed to refer to a different picture, so don’t think that it’s a representative problem • Can tell it’s T1 since CSF is dark • You can tell it’s spin echo since edges are well define and there aren’t susceptibility artifacts near the tissue-air boundaries • Remember to label left/right whenever it can be identified in the image • Other problems should have the correct solution written on your hw if you made a mistake. We can review any problems you’re interested in now • The homeworks and exams in this class will generally test whether you properly understand the concepts of the physics, rather than just solving equations

  3. How to Identify Modality - Review yes T2 Spin Echo Is CSF very bright? Are the bones bright? no MRI no yes Are CSF and Gray Matter the Same Brightness? yes Are we seeing a projection, or a slice? Proton Density Slice Projection CSF is dark Is gray matter brighter than white matter? CT Radiograph no T1 Weighting yes FLAIR (T2 contrast, dark CSF)

  4. X-Ray Overview: Radiography, CT • X-ray medical imaging measures x-rays that pass through the body to generate images • X-rays are electromagnetic energy in a band of energies much higher than visible light • X-rays that pass through the body can do one of several things • Pass through – These get detected properly • Get absorbed – These aren’t detected • Scattered – These can get detected improperly • Photoelectric effect – occurs more in high Z (atomic number) materials • Compton scattering – dominant effect in soft tissue

  5. Radiograph schematic

  6. Probability of interactions

  7. Linear attenuation coefficient

  8. Film convention

  9. X-ray source properties

  10. X-ray absorption

  11. Image properties • Contrast is determined by the energy of the photons (keV) and the tissue properties • SNR is determined by the number of photons that transmit (SNR = sqrt(N) ), which is a function of the transmission properties and the number of photons transmitted (mAs) • Scatter degrades SNR by adding detected events where there shouldn’t be any

  12. Magnification in radiography • Objects closer to source get magnified relative to farther objects

  13. CT takes a series of radiographs to reconstruct 3 dimensional object

  14. CT units are normalized to water attenuation (Hounsfield units) Contrast is the difference in Hounsfield units: C = A - B

  15. CT colormaps are windowed • The dynamic range in CT is too large to capture everything with a single colormap • The center (“level”) and extent (“window”) are varied to be able to see the type of tissue being imaged • Bone window has a high level and wide window • Soft tissue or brain window is close to 0 (water) and much smaller in extent

  16. White Matter Tracts • Crossing Structures • Corpus Callosum • Anterior commissure • Posterior commissure • Connections within brain • Optical radiation • Connections between brain and the rest of the body • Corticospinal tracts

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