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fMRI: Biological Basis and Experiment DesignLecture 9: Pulse sequences, Take 3
• Slice selection homework review
• Pulse sequence/k-space trajectory matching
• Spin echo
http://www-mrsrl.stanford.edu/~brian/mri-movies/
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Protons on water molcules: Ernst's "spies"
= B = B
= 267 x 106 (rad/s)/T = 42.6 MHz/T
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Slice selection - edges aren’t exactly hardf
x
Frequency profile of pulse = spatial profile of slice
BW thk = BW/GSS
G = 12 mT/m = 1.2 G/cm
= * x * G
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Pulse sequence diagram:
Nrep = 256
256 points
RF
GSS
GPE
GRO
DAC
Flip angle = 7 deg. TR = 10ms
TE ~ 5ms
Sequence type: FLASH
Contrast: T1
Image resolution: 256 x 256
Total acquisition time: 256 x 10ms = 2.5s
Sequence type: ___________
Contrast: ___________
Image resolution: ___________
Total acquisition time: ___________
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K-space trajectory and scaled T2* decay in gray matter:
256 (complex-valued) points
1234
256
…Tacq = 2.5msTE = 5ms
time (ms)
M
256 lines
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Image:
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64
RF
GSS
GPE
GRO
DAC 64 64 64 64 64
…
… (64 times)
Echo spacing = 0.57ms
Pulse sequence diagram:
Flip angle = 75 degrees
Sequence type: EPI
Contrast: T1 and T2*
Image resolution: 64 x 64
Image acq. time: 64 x 0.57ms = 36.48ms
TR = 1.5 s
TE = 30ms (32nd line)
Sequence type: ___________
Contrast: ___________
Image resolution: ___________
Image acq. time: ___________
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K-space trajectory and scaled T2* decay in gray matter:
64 (complex-valued) points
Tacq = 35msTE = 30ms
time (ms)
M
…
64 lines
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Image:
(resolution) (contrast)
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64
RF
GSS
GPE
GRO
DAC 64 64 64 64 64
…
… (64 times)
Echo spacing = 0.57ms
Pulse sequence diagram:
Flip angle = 75 degrees
Sequence type: GE EPI
Contrast: T2*
Image resolution: 64 x 64
Image acq. time: 64 x 0.57ms = 36.48ms
TR = 5 s
TE = 30ms (32nd line)
Sequence type: ___________
Contrast: ___________
Image resolution: ___________
Image acq. time: ___________
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K-space trajectory and scaled T2* decay in gray matter:
64 (complex-valued) points
Tacq = 35msTE = 30ms
time (ms)
M
…
64 lines
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Image:
(resolution) (contrast)
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128
RF
GSS
GPE
GRO
DAC 128 128 128 128
…
… (128 times)
Echo spacing = 0.75ms
Pulse sequence diagram:
Flip angle = 90 degrees
Sequence type: EPI
Contrast: T2*
Image resolution: 128 x 128
Image acq. time: 128 x 0.75ms = 96ms
TR = 5 s
TE = 55ms (65th line)
Sequence type: ___________
Contrast: ___________
Image resolution: ___________
Image acq. time: ___________
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K-space trajectory and scaled T2* decay in gray matter:
128 (complex-valued) points
Tacq = 96 msTE = 55 ms
time (ms)
M
…
128 lines
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Image:
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128
RF
GSS
GPE
GRO
DAC 128 128 128 128
…
… (96 times)
Echo spacing = 0.75ms
Pulse sequence diagram:
Flip angle = 90 degrees
Sequence type: EPI
Contrast: T2*
Image resolution: 128 x 128
Image acq. time: 96 x 0.75ms = 72 ms
TR = 5 s
TE = 30ms (32nd line)
Sequence type: ___________
Contrast: ___________
Image resolution: ___________
Image acq. time: ___________
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K-space trajectory and scaled T2* decay in gray matter:
128 (complex-valued) points
Tacq = 72 msTE = 30 ms
time (ms)
M
…
96 lines
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Image:
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Spin Echo
90 deg.180 deg.
time (ms)
M
T2*
T2
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128
RF
GSS
GPE
GRO
DAC 128 128 128
…
… (96 times)
Echo spacing = 0.75ms
Pulse sequence diagram:
90 deg.
Sequence type: SE EPI
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: ___________
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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
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Image:
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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: ___________
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K-space trajectory and scaled T2* decay in gray matter:
128 (complex-valued) points
Tacq,seg = 24msTE = 20ms
time (ms)
M
…………
128 lines
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Image:
But not really ... With the short read-out, the distortion would be much less noticeable. This acquisition had a short TE (using partial Fourier acquisition) but was not a segmented acquisition, so has more distortion than advertised.
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RF
GSS
GPE
GRO
DAC
…
Echo spacing = 0.75ms
Pulse sequence diagram:
90 deg.
Sequence type: IR SE EPI
Contrast: T1 and (small amount of) 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: ___________
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K-space trajectory and scaled T2* decay in gray matter:
128 (complex-valued) points
time (ms)
M
…………
128 lines
Tacq,seg = 24msTE = 40ms
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Image:
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Pulse sequence diagram:
Nrep = 176 x 224
256 points
RF
GSS
GPE
GRO
DAC
Sequence type: 3D FLASH
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
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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
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Image: