MRI Image Formation Principles

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  1. What is the net magnetization vector (NMV)?
    Total number of excess protons aligned with the magnetic field
  2. What is the NMV proportional to?
    Magnetic field strength
  3. What is FID signal?
    the signal produced immediately following an RF pulse
  4. What is the process that digitizes the MR signals?
    Fourier transform
  5. 2D acquisitions usually require an _____
    interslice gap
  6. The overall SNR will _____ in 3d volume imaging when there is an increase in either ____ and/or and increase in the _____
    • increase
    • FOV
    • number of slices
  7. What is MPR?
    the ability to generate images in scan planes in addition to the acquisition plane
  8. The ETL is a scan time acceleration factor, but it also can lead to what?
    increased thermal induction in patients due to the increased RF power
  9. What is a gradient?
    a small change in the magnetic field along a particular axis
  10. A narrow receiver bandwidth increases ______
    susceptibility artifact
  11. Lower receiver bandwidth = _____SNR and _____chemical shift
    • high
    • more
  12. A wide receiver bandwidth generates _____SNR
  13. A wide receiver bandwidth should be applied in anatomical regions that contain
    high fat and water interfaces
  14. Decreasing receiver bandwidth has what effect on chemical shift?
    Increases chemical shift
  15. What is transmit bandwidth?
    The RF excitation pulse required for slice selection
  16. What is proportional to RF bandwidth?
    Slice thickness
  17. What happens to slice thickness when pulse bandwidth is lowered?
    Slice thickness is reduced
  18. What is receive bandwidth?
    the range of frequencies accepted by the receiver to sample the MR signal
  19. What is the relationship between receive bandwidth and SNR?
    • Direct relationship 
    • Decreasing bandwidth by factor of 2 increases SNR by square root of 2
  20. What are the advantages of increasing receiver bandwidth?
    • Shorter echo spacing (less blurring)
    • Reduced susceptibility artifacts
    • less chemical shift
    • fewer metal artifacts
    • increased available ETL
  21. What are the disadvantages of increasing receiver bandwidth?
    • Lower SNR
    • larger minimum required FOV
  22. What are advantages of decreasing receiver bandwidth?
    • Higher SNR
    • Smaller minimum required FOV
  23. What is needed in order to generate an MR image?
    the slice select gradient must be turned on during the RF energy application
  24. What does the slice selection gradient determine?
    • slice orientation
    • slice thickness
    • slice position
  25. If a steep slice gradient slope is used, what kind of slices are there?
    thin slices
  26. How are thin slices are achieved?
    • steep slice gradient
    • high amplitude 
    • narrow transmit bandwidth
  27. What is the frequency encoding (readout) gradient?
    The gradient that is on during the production of the echo
  28. When is the readout gradient usually turned on?
    During the sampling or readout peak echo and also during frequency encoding
  29. What is sampling time?
    the time to sample the entire k-space line
  30. What controls sample time?
    frequency/readout gradient
  31. Formula for sampling time
    Frequency matrix / Receive bandwidth
  32. What is sampling rate?
    Signal sampling frequency
  33. Nyquist theory
    sampling rate must be twice that of the sampling frequency in order to avoid aliasing in our image
  34. When does aliasing occur?
    In the phase direction
  35. What is gradient moment nulling?
    • Flow compensation
    • compensates for first order motion (protons moving with constant velocity) and slow flowing vessels
  36. What affects the flip angle?
    strength and duration of the RF field
  37. Increasing the FOV would _______spatial resolution because of _______
    • decrease
    • increase in pixel size
  38. What happens to SNR when the flip angle is increased?
    SNR increases to a point called the Ernst angle
  39. What is the Ernst angle?
    the Optimal flip angle that yields the maximum signal for a particular spin in the least amount of time
  40. What is Rectangular FOV?
    a technique reduces scan time by not filling as many lines of space in the phase direction while maintaining spatial resolution
  41. Why are flip angles used in gradient echo sequences?
    to control saturation effects

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MRI Image Formation Principles
2015-09-20 16:21:14
MRI image formation
Scan principles:Image formation
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