Transcript Introduction to MRI: NMR - University of Minnesota

EPI, Part 1
• Pulse sequences
• Nyquist ghost
• Chemical shift
– FLASH
– EPI
Psy 8960, Fall ‘06
EPI, Part 1
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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
M|| = M0
M|| = Mss
• Ernst angle: angle that maximizes transverse magnetization at steadystate.
cos( Ernst )  e
Psy 8960, Fall ‘06
EPI, Part 1
TR
T1
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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
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K-space trajectories: FLASH
Flip angle ~ 7 deg.
NPE
FLASH (TE ~ 5 ms)
RF
GSS
GPE
PE table increments
each repetition
Read and phase
pre-encode
(refocusing)
excitation
TE ~ 5ms
Read-out
GRO
DAC
Psy 8960, Fall ‘06
NRO
EPI, Part 1
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EPI
Nrep = 32
Flip angle ~ 60 deg.
EPI (TE ~ 30 ms)
Read and phase
pre-encode
(refocusing)
Read-out
TE ~ 30ms
64 pts
Psy 8960, Fall ‘06
excitation
Phase
blip
64 pts
EPI, Part 1
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Nyquist (half FOV) ghost
• Fourier relationships
– Delta k determines FOV
– Every-other-line modulation in k-space = ½ FOV modulation in image
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Chemical shift
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Chemical shift: fat
water
Subcutaneous fat
3.5 ppm = 3.5 x 128Hz @ 3T
http://www.lipidlibrary.co.uk/nmr/1NMRsat/index.htm
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Chemical shift during gradient encoding
Relative phase, kx = ky = 1.4 cm-1
Resonant frequency, no gradients
Water,
127,680,000Hz
Saturated fat,
127,680,447Hz
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Voxel displacement
FLASH: BW = 1/(dwell time) = 1 / (10s) = 100,000 Hz
127.73MHz
127.68MHz
127.63MHz
Combination of
gradient, sampling
rate determine FOV,
resolution
PE
RO
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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
127.73MHz
127.68MHz
127.63MHz
FLASH:
frequency
resolution in
phase encode
direction is
effectively infinite
since TPE = 0;
Psy 8960, Fall ‘06
Combination of
gradient, sampling
rate determine FOV,
resolution
PE
RO
EPI, Part 1
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FLASH & EPI: Tev
Tev: time between measurement of first and last data point
FLASH
EPI
Tev,RO = NRO x dwellTime
Tev,PE = NPE x NRO x dwellTime
Tev,RO = NRO x dwellTime
Tev,PE = 0
Read and phase
pre-encode
(refocusing)
excitation
Read and phase
pre-encode
(refocusing)
Read-out
Read-out
Psy 8960, Fall ‘06
EPI, Part 1
excitation
Phase
blip
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