Psy 8960, Fall ‘06 EPI, Part 11 Pulse sequences Nyquist ghost Chemical shift –FLASH –EPI.
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Transcript of Psy 8960, Fall ‘06 EPI, Part 11 Pulse sequences Nyquist ghost Chemical shift –FLASH –EPI.
EPI, Part 1 1Psy 8960, Fall ‘06
EPI, Part 1
• Pulse sequences• Nyquist ghost• Chemical shift
– FLASH– EPI
EPI, Part 1 2Psy 8960, Fall ‘06
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
1)cos( TTR
Ernst e
EPI, Part 1 3Psy 8960, Fall ‘06
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 4Psy 8960, Fall ‘06
K-space trajectories: FLASH
FLASH (TE ~ 5 ms)
excitationRead and phase pre-encode (refocusing)
Read-out
NRO
RF
GSS
GPE
GRO
DAC
PE table increments each repetition
Flip angle ~ 7 deg.
TE ~ 5ms
NPE
EPI, Part 1 5Psy 8960, Fall ‘06
EPINrep = 32
Flip angle ~ 60 deg.
TE ~ 30ms
64 pts 64 pts
EPI (TE ~ 30 ms)
Read-out
Read and phase pre-encode (refocusing)
excitation
Phase blip
EPI, Part 1 6Psy 8960, Fall ‘06
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 8Psy 8960, Fall ‘06
Chemical shift: fat
http://www.lipidlibrary.co.uk/nmr/1NMRsat/index.htm
water Subcutaneous fat
3.5 ppm = 3.5 x 128Hz @ 3T
EPI, Part 1 9Psy 8960, Fall ‘06
Chemical shift during gradient encoding
Saturated fat,127,680,447Hz
Water,127,680,000Hz
Relative phase, kx = ky = 1.4 cm-1Resonant frequency, no gradients
EPI, Part 1 10Psy 8960, Fall ‘06
Voxel displacementFLASH: BW = 1/(dwell time) = 1 / (10s) = 100,000 Hz
RO
127.73MHz
127.63MHz127.68MHz
Combination of gradient, sampling rate determine FOV, resolution
PE
EPI, Part 1 11Psy 8960, Fall ‘06
Voxel displacementFLASH: BW = 1/(dwell time) = 1 / (10s) = 100,000 Hz
FLASH: frequency resolution = 100,000 Hz / 64 pixels = 1563 Hz/pixel
identical calculation: TRO = 64*10s = 640s; Hz/pixel = 1/TRO
RO
127.73MHz
127.63MHz127.68MHz
Combination of gradient, sampling rate determine FOV, resolution
FLASH: frequency resolution in phase encode direction is effectively infinite since TPE = 0;
PE
EPI, Part 1 12Psy 8960, Fall ‘06
FLASH & EPI: Tev
FLASHTev,RO = NRO x dwellTime
Tev,PE = 0
excitationRead and phase pre-encode (refocusing)
Read-out
Tev: time between measurement of first and last data point
Read-out
Read and phase pre-encode (refocusing)
excitation
Phase blip
EPITev,RO = NRO x dwellTime
Tev,PE = NPE x NRO x dwellTime