Psy 8960, Fall ‘06 EPI, Part 11 Pulse sequences Nyquist ghost Chemical shift –FLASH –EPI

Psy 8960, Fall ‘06 EPI, Part 12 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 Ernst angle: angle that maximizes transverse magnetization at steady- state. M || = M 0 M || = M ss

Psy 8960, Fall ‘06 EPI, Part 13 This week FLASH and EPI pulse sequences (review k-space trajectory) Nyqiust ghost in EPI (and other issues …) Relationship between total read-out time (T RO ) and voxel displacement

Psy 8960, Fall ‘06 EPI, Part 14 K-space trajectories: FLASH FLASH (TE ~ 5 ms) excitation Read and phase pre-encode (refocusing) Read-out N RO RF G SS G PE G RO DAC PE table increments each repetition Flip angle ~ 7 deg. TE ~ 5ms N PE

Psy 8960, Fall ‘06 EPI, Part 15 EPI N rep = 32 Flip angle ~ 60 deg. TE ~ 30ms 64 pts EPI (TE ~ 30 ms) Read-out Read and phase pre-encode (refocusing) excitation Phase blip

Psy 8960, Fall ‘06 EPI, Part 16 Nyquist (half FOV) ghost Fourier relationships –Delta k determines FOV –Every-other-line modulation in k-space = ½ FOV modulation in image

Psy 8960, Fall ‘06 EPI, Part 17 Chemical shift

Psy 8960, Fall ‘06 EPI, Part 18 Chemical shift: fat waterSubcutaneous fat 3.5 ppm = 3.5 x 3T

Psy 8960, Fall ‘06 EPI, Part 19 Chemical shift during gradient encoding Saturated fat, 127,680,447Hz Water, 127,680,000Hz Relative phase, k x = k y = 1.4 cm -1 Resonant frequency, no gradients

Psy 8960, Fall ‘06 EPI, Part 110 Voxel displacement FLASH: BW = 1/(dwell time) = 1 / (10  s) = 100,000 Hz RO MHz MHz MHz Combination of gradient, sampling rate determine FOV, resolution PE

Psy 8960, Fall ‘06 EPI, Part 111 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: T RO = 64*10  s = 640  s; Hz/pixel = 1/T RO RO MHz MHz MHz Combination of gradient, sampling rate determine FOV, resolution FLASH: frequency resolution in phase encode direction is effectively infinite since T PE = 0; PE

Psy 8960, Fall ‘06 EPI, Part 112 FLASH & EPI: T ev FLASH T ev,RO = N RO x dwellTime T ev,PE = 0 excitation Read and phase pre-encode (refocusing) Read-out T ev : time between measurement of first and last data point Read-out Read and phase pre-encode (refocusing) excitation Phase blip EPI T ev,RO = N RO x dwellTime T ev,PE = N PE x N RO x dwellTime