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Chapter 5

Chapter 5. Mark D. Herbst, M.D., Ph.D. The MR Imaging Process. Two major functions Acquisition of RF signals Reconstruction of images. k-space. Where acquired signals are stored in the computer Filled one line at a time Must be filled completely before reconstruction

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Chapter 5

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  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

  3. 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

  4. 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.

  5. 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.

  6. 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

  7. Definition of TR • Repetition time • Time between 90 degree pulses in a spin echo technique

  8. Imaging Protocol • Various methods—spin echo, gradient echo, inversion recovery • Various image types-T1-weighted images (T1WI), T2WI, T2*WI, STIR, FLAIR, MRA • Spatial characteristics—slice thickness, number of slices, pixel size, voxel size • Detail/noise—matrix, pixel, and voxel size • Selective signal suppression • Artifact reduction techniques

  9. 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

  10. T1 contrast

  11. PD contrast

  12. T2 contrast

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

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