MRI Registry Review Mod 5 Imaging Parameters

  1. What are the four key influences on MRI sequence parameters selection?
    • SNR
    • image contrast
    • image resolution
    • scan time
  2. what is SNR?
    • signal to noise ratio
    • the amount of signal collected in the MR echoes compared to intrinsic electronic noise, from both the patient as well as system components
  3. what is image contrast?
    the ratio of the magnetization of one tissue to the magnetization of another tissue, at a given moment in time
  4. what is image resolution?
    a measure of the size of an object that can be visualized, or resolved, in an image
  5. what is scan time?
    best to keep scan time down to minimize motion artifacts
  6. how do you calculate scan time for a 2d study?
    (TR)x(#of signal averages)x(#lines in matrix)/# echoes contributing to single rawdata file per TR or echo train length
  7. what is TR?
    repetition time or is the time from the initial RF disturbance to the same RF disturbance during the next repetition
  8. what does TR control?
    the contrast between tissues in an image due to T1 relaxation
  9. what does short TR times give you in terms of high or low T1 contrast and high or low SNR?
    • high T1 contrast but reduced SNR
    • low SNR because the shorter TR gives a smaller longitudinal magnetization which gives a smaller transverse magnetization which gives a smaller MR signal
  10. what does long TR times give you in terms of high or low T1 contrast and high or low SNR?
    low T1 contrast but high SNR
  11. at low magnetic field strength, does the magnetization return to the longitudinal direction quicker or longer than a higher field strength?
    • quicker
    • T1 times are shorter
    • SNR are diminished in lower field magnets therefore a longer TR has to be used to maximize SNR
  12. is scan time directly or inversely proportional to TR?
    • directly proportional
    • longer TR equals longer scan time
  13. what happens when you increase TR?
    • increase longitudinal relaxation
    • decrease T1 contrast
    • increase SNR
    • increase maximum # slices allowable
    • increase scan time
  14. what is TE?
    • the time from the initial RF pulse to the center of the MR echo
    • also controls the contrast between tissues in an image due to T2 relaxation
  15. what does long TE times give you in terms of T2 contrast and SNR?
    • high T2 contrast
    • reduced SNR
  16. what happens when you increase TE?
    • increase transverse relaxation
    • increase t2 contrast
    • decrease SNR
    • decrease the maximum allowable # slices
    • increase gradient echo suceptibility
  17. what happens when you have a moderately short TI, a very short TI, and a long TI?
    • moderately short TI = increase T1 contrast
    • very short TI= null fat signal on T1 images (STIR)
    • long TI= decrease T1 contrast, null CSF signal on T2 images (FLAIR)
  18. what is the Ernst angle?
    • the flip angle which produces the best SNR for a given TR and tissue type
    • when the TR is decreased, as in gradient echo imaging, there is a specific flip angle which will generate the greatest SNR for a given tissue at a specific TR
  19. a larger flip angle, without exceeding 90 degrees, will have a higher or lower SNR, provided that the TR is relatively long?
    the larger the flip angle higher the SNR
  20. a thicker slice will yield a high or low SNR?
    • high SNR
    • thicker slice contains more protons therefore maintains a better SNR
  21. what happens to the resolution when increasing slice thickness?
    • goes down
    • produces an effect called partial volume averaging
  22. what is partial volume averaging?
    • results from the mathematical averaging of several millimeters of varying tissue contrast, which is displayed as a single shade of gray on the resulting image
    • within a specific location of a 10 mm T2 image, there may be short and long T2 tissues, as well as pathology but the fourier transform processes all of this information together and displays it as a single pixel in the T2 image.
    • this situation worsens when multiple tissues of different contrast are superimposed within the same slice
  23. what happens to anatomical coverage, SNR, partial volume artifacts and spatial resolution when you increase slice thickness?
    • increase anatomical coverage
    • increase SNR linearly
    • increase partial volume artifacts
    • decrease spatial resolution
  24. increasing the gap between slices will do what to crosstalk artifacts, anatomical coverage of the scan and important anatomy between slices?
    • decrease crosstalk artifacts
    • increase anatomical coverage of the scan
    • risk missing important anatomy between slices
  25. whats another method for reducing the effects of crosstalk artifacts?
    • to reorder the serial acquisition of the slices
    • rather than sampling the slices in their anatomical order. Instead of 1st slice, 3rd, 5th then 2nd, 4th, 6th
    • pg 32
  26. what changes when you change the frequency steps?
    • will affect the SNR and resolution
    • will not change the scan time
  27. what will be affected when you change the number of phase steps?
    • SNR
    • resolution
    • scan time
  28. how do you calculate resolution?
  29. what happens to SNR when matrix is increased and why?
    • SNR decreases
    • because only a finite amount of signal is collected, and by increasing the image matrix, we must distribute that signal across more pixels which decreases the overall signal per pixel
  30. for a given FOV: increasing the phase encoding steps will
    • decrease SNR
    • increase spatial resolution
    • increase scan time
  31. for a given FOV: increasing the frequency steps will:
    • decrease SNR
    • increase spatial resolution
    • not affect scan time
  32. increasing the FOV will:
    • increase the amount of anatomy displayed
    • increase SNR
    • increase truncation (ringing) artifacts
    • decrease in-plane resolution
  33. what is a rectangluar field of view (RFOV)?
    • is used to reduce the scan time by collecting fewer phase lines along the smaller dimension of the anatomy which in turn decreases scan time
    • for this to work, the phase encoding direction must be aligned with the smaller dimension of the anatomy. This technique reduces both the FOV along one direction in the image and the number of pixels along the phase direction of the image. Therefore the spatial resolution remains unaffected by selecting a RFOV and the pixels in the image remain square. However, the SNR decreases by a factor proportional to the square root of the fraction of sampled lines
  34. increasing the RFOV will
    • increase the amount of anatomy displayed
    • increase SNR
    • increase scan time
    • not affect spatial resolution
  35. what is parallel imaging?
    a scan time reduction technique that maintains the full square FOV
  36. what is one requirement for parallel imaging?
    a special type of RF coil must be used, called an array coil and it must have coil elements that are parallel to one another. THese parallel coil elements must be aligned relative to the pt's body along the direction that corresponds to the phase direction in the image
  37. which direction does phase have to go to be able to use parallel imaging?
  38. what happens during parallel imaging?
    • when parallel imaging is selected, a quick reference scan is completed before the desired scan. The reference scan stores the location of structures as sensitivities which correspond to the distance from each coil element. Next, when the actual scan is run using parallel imaging, the system applies a rectangular FOV that would ordinarily cause an artifact known as foldover or aliasing. By applying the RFOV (in the background) fewer phase lines are collected and the scan time is decreased. When the image is reconstructed, the known sensitivities acquired during the reference scan are used to "undo" the aliasing and reconstruct a normal, artifact free image that was acquired in a fraction of the time compared to if parallel imaging wasnt used.
    • So when parallel imaging is selected, the number of phase lines sampled is reduced. This reduces the scan time. But due to the orientation of the coils and through some complex calculations, the original, full number of phase lines are present in the image. Thus, parallel imaging allows for a shorter scan time, while the resolution and full field of view are maintained
  39. What is the parallel imaging "factor"?
    the scan time savings parameter that the technologist can select which is never more than the number of parallel coil elements located on the array coils
  40. increasing the parallel imaging factor will
    • decrease scan time
    • decrease SNR
    • maintain a full FOV
    • not affect spatial resolution
  41. what is an echo train length?
    • the number of echoes collected per slice, within a single TR perior
    • the longer the echo train, the faster the scan since fewer repetitions are required to completely fill the raw data
  42. what is the disadvantage to running a fast spin echo sequence with a long echo train
    • loss in signal in the later echoes which are formed at longer TEs
    • As each subsequent echo is formed at longer and longer echo times, the signal is reduced due to spin-spin interactions in the transverse plane
  43. increasing the echo train length will
    • decrease SNR
    • decrease scan time
  44. what is NSA?
    number of signal averages or number of excitations or the number of acquisitions
  45. sample question: in a 2D imaging sequence, what is the scan time of a spin echo sequence using a TR of 0.5 secs, 2 signal averages and a matrix of 256x256 matrix to image 12 slices?
    hint: the number of slices is there to fool you
    scan time = TR x signal avgs x matrix lines/ETL

    0.5 x 2 x 256 = 4.27 mins
  46. increasing the signal averages will
    • increase SNR
    • increase scan time
    • decrease motion artifacts
  47. what is the receiver bandwidth?
    • is the range of frequencies encoded in the MR echo
    • the range of precessional frequencies generated by the gradient that are collected in the echo while the readout gradient is applied is called the receiver bandwidth
  48. what happens when you lower the received bandwidth?
    • better SNR
    • but increased chemical shift
  49. increasing receiver bandwidth will
    • decrease SNR
    • decrease chemical shift artifacts
    • not affect scan time
  50. what is magnetization transfer contrast?
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MRI Registry Review Mod 5 Imaging Parameters
Imaging Parameters