MRI Pulse sequences

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Author:
hrmckensey1015
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308489
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MRI Pulse sequences
Updated:
2015-09-26 09:21:49
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MRI pulse sequences
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mri
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MRI pulse sequences
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  1. A gradient echo sequence lacks a _______, making it ____________.
    • 180* refocusing RF pulse
    • more susceptible to magnetic field inhomogeneities
  2. Gradient echo sequences tend to be ______, due to the lack of 180* refocusing pulses, the TR______, and scan time______.
    • fast
    • shorter
    • shorter
  3. In a gradient echo sequence what is used to refocus the protons and create the echo?
    Gradient coils
  4. What kind of characteristics do gradient echo sequences yield?
    T1 or T2*
  5. When does the condition of steady state occur? Results in?
    • When the TR in a gradient echo is less than the T2
    • residual transverse magnetization at the time of the next excitation pulse
  6. What do T2* gradient echo require?
    high signal from fluid
  7. What do balanced steady state sequences result in?
    high SNR image with bright fluids
  8. What are balanced steady state sequences useful for evaluating?
    Cranial nerves
  9. What does a spoiled gradient echo sequence do?
    Removes any residual transverse magnetization prior to next excitation pulse
  10. Why is "spoiling" used?
    • to reduce stead state (T2*) effect
    • allows for fast scans with T1 contrast
  11. What is RF spoiling?
    When additional RF pulses spoil away the steady state effect (residual transverse magnetization) before the next excitation
  12. What is a spoiled gradient echo also called?
    Incoherent gradient echo imaging
  13. Conventional spin echo sequences begin with?
    90* RF excitation pulse
  14. What refocuses a decaying spin echo?
    180* RF pulse
  15. How many lines of Kspace are acquired during a conventional spin echo?
    one line
  16. How many lines of Kspace are filled per TR in fast spin echo sequences?
    multiple (2-50+) lines
  17. In a spin echo sequence, what is the time between the 90* RF pulse and the 180* RF pulse?
    1/2 TE time (Tau)
  18. How is an echo produced in a spin echo pulse sequence?
    from a combination of 2 or more RF pulses
  19. A dual echo spin echo sequence generates ______, both with _______, but the ______
    • two images for every slice
    • varying TE's
    • same TR
  20. In dual contrast spin echo sequences, the second echo image has ______, but ______
    than the first echo image
    • more T2 contrast
    • Lower SNR
  21. In Fast spin echo sequences if the ETL is increased, the scan time is _____, and SNR is _____
    • faster
    • decreased
  22. In a fast spin echo the effective TE are what?
    The echoes that are encoded with a low amplitude phase encoding gradient
  23. If ETL is increased what can occur?
    Increase in image blurring
  24. In FSE the number of shots is calculated by what formula?
    # phase encodings ÷ ETL
  25. What is an inversion recovery sequence?
    Sequence begins with 180* RF pulse followed by a 90* RF excitation pulse
  26. In and IR sequence what does the 180* RF pulse do?
    Nulls signal from fat or fluid
  27. What is the TI (inversion time)?
    The time interval between the 180* inverting RF pulse and the 90* RF excitation pulse
  28. If the desire is to null the signal fro a specific tissue using an inversion recovery, what should the inversion time be?
    69% of the T1 relaxation time of that tissue
  29. What are STIR sequences typically used for?
    Evaluation of compression fracture, lesions within retro-orbital fat, musculoskeletal contusions and fat suppression
  30. To null the signal from fluid in FLAIR sequence what TI should be used?
    ~2000ms
  31. What are T2 FLAIR sequences typically used to evaluate?
    Periventricular white matter
  32. What is the TR?
    time between two 90* RF excitation pulses
  33. What is the TE?
    The time between the 90* RF pulse and the peak of the signal in the receiver coil
  34. What will increase proton density weighting?
    Lengthening TR with a short TE in a spin echo sequence
  35. What affect does TE have on number of slices?
    TE increases = # of slices decreases
  36. What affect does TR have on # of slices?
    TR increases= # of slices increases
  37. Scan time Formula
    TR x Matrix x NEX ÷ ETL
  38. If ETL increases, scan time _____
    decreases
  39. What sequence is the most INSENSITIVE to inhomogeneity?
    Spin echo sequences
  40. What gradient is on during production of the echo?
    Frequency encoding gradient (readout)
  41. What is the fastest sequence commonly available?
    Echo planar imaging
  42. Spin echo
    90* RF followed by 180* RF
  43. Fast spin echo
    90*RF followed by train of 180* RF pulses
  44. Inversion recovery
    180* RF followed by 90* RF
  45. Gradient echo
    variable degree RF followed by gradient to produce echo
  46. EPI (echo planar imaging)
    a train of gradient echoes
  47. _____ the flip angle yields images with ____T1 information
    • Reducing
    • Less
  48. ______ the TE yields images with _____ T2 information
    • Reducing
    • Decreased
  49. ______the flip angle _____ SNR up to the _____
    • Increasing
    • Increases
    • Ernst angle
  50. What factors will lengthen scan time?
    • Increasing TR
    • Decreasing the # ETL
    • Increasing NEX
    • Decreasing parallel imaging factor
  51. What factors will shorten scan time?
    • Decreasing TR
    • Increasing the # ETL
    • Decreasing NEX
    • Enabling half scan 
    • Increasing parallel imaging factor
  52. What factors will improve SNR?
    • Increasing TR
    • Reducing phase matrix
    • Increasing FOV
    • Reducing (narrowing) receive bandwidth
    • Increasing NEX/NSA
    • Reducing # ETL
  53. What will decrease the SNR?
    • Decreasing TR
    • Reducing Pixel size
    • Decrease FOV
    • Increasing Receiver bandwidth
    • Decrease NEX
    • Increase # ETL

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