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EPI, Part 2: variants 1Psy 8960, Fall ‘06
EPI, Part 2: variants
• Segmentation• Partial Fourier• Spin echo vs.
gradient echo• Inversion recovery• Long vs. short TE
EPI, Part 2: variants 2Psy 8960, Fall ‘06
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 3Psy 8960, Fall ‘06
This week
• Review– Hardware– Pulse sequence diagrams
• Read vs. phase encode
– Pulse sequences• FLASH vs. EPI
EPI, Part 2: variants 4Psy 8960, Fall ‘06
Fancy fenders
Hardware: everything’s a coil
Superconducting coil (“magnet”)
http://www.fmrib.ox.ac.uk/~peterj/lectures/hbm_1/sld010.htm
Gradient coils (“gradients”)
RF coil (“body coil”)
RF coil (“head coil”)
(from http://www.novamedical.com/)
EPI, Part 2: variants 5Psy 8960, Fall ‘06
Single-shot EPI pulse sequence
RF
GSS
GPE
GRO
DAC
TE
EPI, Part 2: variants 6Psy 8960, Fall ‘06
Spin Echo EPI pulse sequence
RF
GSS
GPE
GRO
DAC
TE/2TE/2
EPI, Part 2: variants 7Psy 8960, Fall ‘06
Inversion Recovery Spin Echo EPI
RF
GSS
GPE
GRO
DAC
TE/2TE/2
EPI, Part 2: variants 8Psy 8960, Fall ‘06
Inversion Recovery FLASH
RF
GSS
GPE
GRO
DAC
NPE
NRO
EPI, Part 2: variants 9Psy 8960, Fall ‘06
Segmented EPI pulse sequence
RF
GSS
GPE
GRO
DAC
TENPE/NSEG
EPI, Part 2: variants 10Psy 8960, Fall ‘06
Turbo Spin Echo (turbo factor = 4)
RF
GSS
GPE
GRO
DAC
TE? NPE/TF
EPI, Part 2: variants 11Psy 8960, Fall ‘06
128
RF
GSS
GPE
GRO
DAC 128 128 128 128
…
… (32 times)
Echo spacing = 0.75ms
Pulse sequence diagram:
Flip angle = 40 degrees
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
TE = 20ms (16th line)
Nrep = 4
TR = 0.25 s
Sequence type: ___________
Contrast: ___________
Image resolution: ___________
Segment acq. time:
Image acq. time: ___________
EPI, Part 2: variants 12Psy 8960, Fall ‘06
K-space trajectory and scaled T2* decay in gray matter:
128 (complex-valued) points
Tacq,seg = 24msTE = 20ms
time (ms)
M
…………
128 lines
EPI, Part 2: variants 13Psy 8960, Fall ‘06
Image:
EPI, Part 2: variants 14Psy 8960, Fall ‘06
128
RF
GSS
GPE
GRO
DAC 128 128 128
…
… (96 times)
Echo spacing = 0.75ms
Pulse sequence diagram:
60 deg.
Sequence type: GE EPI(partial Fourier acquisition)
Contrast: T2*
Image resolution: 128 x 128
Image acq. time: 96 x 0.75ms = 72 ms (distortion!)
TR = 1 s
TE = 30ms (32nd line)
Sequence type: ___________
Contrast: ___________
Image resolution: ___________
Image acq. time: ___________
EPI, Part 2: variants 15Psy 8960, Fall ‘06
K-space trajectory and scaled T2 & T2* decay in gray matter:
128 (complex-valued) points
Tacq = 72 msTE = 30 ms
time (ms)
M
…
96 lines
T2*
T2
EPI, Part 2: variants 16Psy 8960, Fall ‘06
Image:
GE EPI, long TE and read-out GE EPI, shorter TE and read-out
EPI, Part 2: variants 17Psy 8960, Fall ‘06
128
RF
GSS
GPE
GRO
DAC 128 128 128
…
… (96 times)
Echo spacing = 0.75ms
Pulse sequence diagram:
90 deg.
Sequence type: SE EPI(partial Fourier acquisition)
Contrast: T2
Image resolution: 128 x 128
Image acq. time: 96 x 0.75ms = 72 ms
TR = 5 s
TE = 60ms (32nd line)
180 deg.
Sequence type: ___________
Contrast: ___________
Image resolution: ___________
Image acq. time: ___________
EPI, Part 2: variants 18Psy 8960, Fall ‘06
K-space trajectory and scaled T2 & T2* decay in gray matter:
128 (complex-valued) points
Tacq = 72 msTE = 60 ms
time (ms)
M
…
96 lines
18090
T2*
T2
EPI, Part 2: variants 19Psy 8960, Fall ‘06
Image:SE, more superior
GE
SE, more inferior
EPI, Part 2: variants 20Psy 8960, Fall ‘06
RF
GSS
GPE
GRO
DAC
…
Echo spacing = 0.75ms
Pulse sequence diagram:
90 deg.
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
TE = 25ms (16th line)
180 Nrep = 4
TR = 2.5 sTI = 0.9 s
180
…
…
…
Sequence type: ___________
Contrast: ______________________
Image resolution: ___________
Image acq. time: ___________
(32 times)
EPI, Part 2: variants 21Psy 8960, Fall ‘06
K-space trajectory and scaled T2* decay in gray matter:
128 (complex-valued) points
…………
128 lines
Tacq,seg = 24msTE = 40ms
time (ms)
M
18090
T2*
T2, white matter
T2, gray matter
EPI, Part 2: variants 22Psy 8960, Fall ‘06
Image:
EPI, Part 2: variants 23Psy 8960, Fall ‘06
Pulse sequence diagram:
Nrep = 176 x 224
256 points
RF
GSS
GPE
GRO
DAC
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
Sequence type: ___________
Contrast: ______________________
Image resolution: ___________
Image acq. time: ___________
TR = 10ms
EPI, Part 2: variants 24Psy 8960, Fall ‘06
K-space trajectory and scaled T2* decay in gray matter:
256 (complex-valued) points
Tacq = 2.5msTE = 5ms
time (ms)
M
224 lines
176 partitions
EPI, Part 2: variants 25Psy 8960, Fall ‘06
Image:
