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EPI, Part 1

EPI, Part 1. Pulse sequences Nyquist ghost Chemical shift FLASH EPI. Overview: last week. Steady-state magnetization when TR is shorter than or comparable to T 1 , longitudinal magnetization cannot recover between repetitions Images have T1-weighting in addition to intended weighting

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EPI, Part 1

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  1. EPI, Part 1 • Pulse sequences • Nyquist ghost • Chemical shift • FLASH • EPI EPI, Part 1

  2. Overview: last week • Steady-state magnetization • when TR is shorter than or comparable to T1, longitudinal magnetization cannot recover between repetitions • Images have T1-weighting in addition to intended weighting • Ernst angle: angle that maximizes transverse magnetization at steady-state. M|| = M0 M|| = Mss EPI, Part 1

  3. This week • FLASH and EPI pulse sequences (review k-space trajectory) • Nyqiust ghost in EPI (and other issues …) • Relationship between total read-out time (TRO) and voxel displacement EPI, Part 1

  4. FLASH (TE ~ 5 ms) Flip angle ~ 7 deg. RF GSS excitation Read and phase pre-encode (refocusing) PE table increments each repetition GPE TE ~ 5ms Read-out GRO NRO DAC K-space trajectories: FLASH NPE EPI, Part 1

  5. EPI Nrep = 32 EPI (TE ~ 30 ms) Flip angle ~ 60 deg. excitation Read and phase pre-encode (refocusing) Phase blip Read-out TE ~ 30ms 64 pts 64 pts EPI, Part 1

  6. Nyquist (half FOV) ghost • Fourier relationships • Delta k determines FOV • Every-other-line modulation in k-space = ½ FOV modulation in image EPI, Part 1

  7. Chemical shift EPI, Part 1

  8. Chemical shift: fat water Subcutaneous fat 3.5 ppm = 3.5 x 128Hz @ 3T http://www.lipidlibrary.co.uk/nmr/1NMRsat/index.htm EPI, Part 1

  9. Chemical shift during gradient encoding Resonant frequency, no gradients Relative phase, kx = ky = 1.4 cm-1 Water, 127,680,000Hz Saturated fat, 127,680,447Hz EPI, Part 1

  10. Voxel displacement FLASH: BW = 1/(dwell time) = 1 / (10s) = 100,000 Hz Combination of gradient, sampling rate determine FOV, resolution 127.73MHz 127.68MHz 127.63MHz PE RO EPI, Part 1

  11. Voxel displacement FLASH: BW = 1/(dwell time) = 1 / (10s) = 100,000 Hz FLASH: frequency resolution = 100,000 Hz / 64 pixels = 1563 Hz/pixel identical calculation: TRO = 64*10s = 640s; Hz/pixel = 1/TRO Combination of gradient, sampling rate determine FOV, resolution 127.73MHz 127.68MHz 127.63MHz FLASH: frequency resolution in phase encode direction is effectively infinite since TPE = 0; PE RO EPI, Part 1

  12. FLASH & EPI: Tev Tev: time between measurement of first and last data point FLASH Tev,RO = NRO x dwellTime Tev,PE = 0 EPI Tev,RO = NRO x dwellTime Tev,PE = NPE x NRO x dwellTime excitation excitation Read and phase pre-encode (refocusing) Read and phase pre-encode (refocusing) Phase blip Read-out Read-out EPI, Part 1

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