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EPI, Part 2: variants

EPI, Part 2: variants. Segmentation Partial Fourier Spin echo vs. gradient echo Inversion recovery Long vs. short TE. Last week. Chemical shift Subcutaneous fat resonates at a frequency 3.5 ppm different from water Shift in resonant frequency results in shift in location in image

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EPI, Part 2: variants

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  1. EPI, Part 2: variants • Segmentation • Partial Fourier • Spin echo vs. gradient echo • Inversion recovery • Long vs. short TE EPI, Part 2: variants

  2. Last week • Chemical shift • Subcutaneous fat resonates at a frequency 3.5 ppm different from water • Shift in resonant frequency results in shift in location in image • FLASH-like images: PE direction is impervious; errors show up in read-out direction • EPI-like images: PE direction is the bad guy • Idle curiosity: why does myelin not show up shifted? EPI, Part 2: variants

  3. This week • Review • Hardware • Pulse sequence diagrams • Read vs. phase encode • Pulse sequences • FLASH vs. EPI EPI, Part 2: variants

  4. Gradient coils (“gradients”) Superconducting coil (“magnet”) http://www.fmrib.ox.ac.uk/~peterj/lectures/hbm_1/sld010.htm RF coil (“body coil”) RF coil (“head coil”) (from http://www.novamedical.com/) Hardware: everything’s a coil Fancy fenders EPI, Part 2: variants

  5. Single-shot EPI pulse sequence TE RF GSS GPE GRO DAC EPI, Part 2: variants

  6. Spin Echo EPI pulse sequence TE/2 TE/2 RF GSS GPE GRO DAC EPI, Part 2: variants

  7. Inversion Recovery Spin Echo EPI TE/2 TE/2 RF GSS GPE GRO DAC EPI, Part 2: variants

  8. Inversion Recovery FLASH NPE RF GSS GPE GRO NRO DAC EPI, Part 2: variants

  9. Segmented EPI pulse sequence NPE/NSEG TE RF GSS GPE GRO DAC EPI, Part 2: variants

  10. Turbo Spin Echo (turbo factor = 4) TE? NPE/TF RF GSS GPE GRO DAC EPI, Part 2: variants

  11. Pulse sequence diagram: Nrep = 4 Flip angle = 40 degrees TR = 0.25 s RF Sequence type: segmented EPI Contrast: T1 and T2* Image resolution: 128 x 128 Segment acq. time: 32 x 0.75ms = 24 ms Image acq. time: 1 sec Sequence type: ___________ Contrast: ___________ Image resolution: ___________ Segment acq. time: Image acq. time: ___________ GSS … GPE TE = 20ms (16th line) … GRO (32 times) Echo spacing = 0.75ms 128 128 128 128 128 DAC EPI, Part 2: variants

  12. K-space trajectory and scaled T2* decay in gray matter: … M … … … 128 lines time (ms) Tacq,seg = 24ms TE = 20ms 128 (complex-valued) points EPI, Part 2: variants

  13. Image: EPI, Part 2: variants

  14. Pulse sequence diagram: 60 deg. TR = 1 s RF Sequence type: GE EPI (partial Fourier acquisition) Contrast: T2* Image resolution: 128 x 128 Image acq. time: 96 x 0.75ms = 72 ms (distortion!) Sequence type: ___________ Contrast: ___________ Image resolution: ___________ Image acq. time: ___________ GSS … GPE TE = 30ms (32nd line) … GRO (96 times) Echo spacing = 0.75ms 128 128 128 128 DAC EPI, Part 2: variants

  15. K-space trajectory and scaled T2 & T2* decay in gray matter: M T2 96 lines T2* … time (ms) Tacq = 72 ms TE = 30 ms 128 (complex-valued) points EPI, Part 2: variants

  16. Image: GE EPI, long TE and read-out GE EPI, shorter TE and read-out EPI, Part 2: variants

  17. Pulse sequence diagram: 180 deg. 90 deg. TR = 5 s RF Sequence type: ___________ Contrast: ___________ Image resolution: ___________ Image acq. time: ___________ Sequence type: SE EPI (partial Fourier acquisition) Contrast: T2 Image resolution: 128 x 128 Image acq. time: 96 x 0.75ms = 72 ms GSS … GPE TE = 60ms (32nd line) … GRO (96 times) Echo spacing = 0.75ms 128 128 128 128 DAC EPI, Part 2: variants

  18. K-space trajectory and scaled T2 & T2* decay in gray matter: M T2 96 lines T2* … time (ms) 90 180 Tacq = 72 ms TE = 60 ms 128 (complex-valued) points EPI, Part 2: variants

  19. Image: SE, more superior SE, more inferior GE EPI, Part 2: variants

  20. Pulse sequence diagram: Nrep = 4 90 deg. 180 180 TR = 2.5 s TI = 0.9 s RF Sequence type: IR SE EPI (4-shot) Contrast: T1 and (some) T2 Image resolution: 128 x 128 Image acq. time: 4 x 2.5s = 10 s Sequence type: ___________ Contrast: ______________________ Image resolution: ___________ Image acq. time: ___________ GSS … GPE TE = 25ms (16th line) … … GRO (32 times) Echo spacing = 0.75ms … DAC EPI, Part 2: variants

  21. K-space trajectory and scaled T2* decay in gray matter: … T2, white matter … … M … T2, gray matter 128 lines T2* time (ms) 90 180 Tacq,seg = 24ms TE = 40ms 128 (complex-valued) points EPI, Part 2: variants

  22. Image: EPI, Part 2: variants

  23. Pulse sequence diagram: Nrep = 176 x 224 TR = 10ms RF Sequence type: ___________ Contrast: ______________________ Image resolution: ___________ Image acq. time: ___________ Sequence type: 3D FLASH (slab-selective excitation) Contrast: T1 Image resolution: 256 x 224 x 176 Image acq. time: 176 x 224 x .01s = 6 min 34s GSS GPE GRO 256 points DAC EPI, Part 2: variants

  24. K-space trajectory and scaled T2* decay in gray matter: M 224 lines time (ms) Tacq = 2.5ms TE = 5ms 176 partitions 256 (complex-valued) points EPI, Part 2: variants

  25. Image: EPI, Part 2: variants

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