Mark D. Herbst, M.D., Ph.D.

1
Chapter 5

• Mark D. Herbst, M.D., Ph.D.

2
The MR Imaging Process

• Two major functions
• Acquisition of RF signals
• Reconstruction of images

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k-space

• Where acquired signals are stored in the computer
• Filled one line at a time
• Must be filled completely before reconstruction
• The number of lines the resolution in the phase
  direction
• The number of dots per line the resolution in
  the frequency direction

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To get a 256 x 256 image

• Need 256 lines in k-space
• Each line must have 256 dots per line
• Each line takes time to get, and that time is TR
  in conventional single echo spin echo imaging, so
  this would take 256 TRs.

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To get a 192 x 256 image

• Need 192 lines in k-space
• Each line must have 256 dots per line
• Each line takes time to get, and that time is TR
  in conventional single echo spin echo imaging, so
  this would take 192 TRs.

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Acquisition

• Usually one line of k-space for each repetition
  of the imaging sequence each repetition time
  each TR
• Fast methods produce multiple lines for each TR

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Definition of TR

• Repetition time
• Time between 90 degree pulses in a spin echo
  technique

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Imaging Protocol

• Various methodsspin echo, gradient echo,
  inversion recovery
• Various image types-T1-weighted images (T1WI),
  T2WI, T2WI, STIR, FLAIR, MRA
• Spatial characteristicsslice thickness, number
  of slices, pixel size, voxel size
• Detail/noisematrix, pixel, and voxel size
• Selective signal suppression
• Artifact reduction techniques

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The Imaging Cycle

• Two distinct phases
• Longitudinal relaxation T1 weighting
• Transverse relaxation T2 weighting
• Amount of T1 or T2 weighting depends on TR and TE
• Echo event is produced by an RF pulse followed by
  either an RF pulse or a gradient pulse

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T1 contrast
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PD contrast
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T2 contrast
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T1, PD, and T2 contast all contribute to the
final brightness of the pixel on the MR image