Transcript Introduction to MRI: NMR

EPI, Part 2: variants
• Segmentation
• Partial Fourier
• Spin echo vs.
gradient echo
• Inversion recovery
• Long vs. short TE
Psy 8960, Fall ‘06
EPI, Part 2: variants
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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 readout direction
– EPI-like images: PE direction is the
bad guy
– Idle curiosity: why does myelin not
show up shifted?
Psy 8960, Fall ‘06
EPI, Part 2: variants
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This week
• Review
– Hardware
– Pulse sequence diagrams
• Read vs. phase encode
– Pulse sequences
• FLASH vs. EPI
Psy 8960, Fall ‘06
EPI, Part 2: variants
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Hardware: everything’s a coil
Gradient coils (“gradients”)
Superconducting coil (“magnet”)
http://www.fmrib.ox.ac.uk/~peterj/lectures/hbm_1/sld010.htm
RF coil (“body coil”)
RF coil (“head coil”)
(from http://www.novamedical.com/)
Fancy fenders
Psy 8960, Fall ‘06
EPI, Part 2: variants
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Single-shot EPI pulse sequence
TE
RF
GSS
GPE
GRO
DAC
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EPI, Part 2: variants
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Spin Echo EPI pulse sequence
TE/2
TE/2
RF
GSS
GPE
GRO
DAC
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EPI, Part 2: variants
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Inversion Recovery Spin Echo EPI
TE/2
TE/2
RF
GSS
GPE
GRO
DAC
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Inversion Recovery FLASH
NPE
RF
GSS
GPE
GRO
NRO
DAC
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EPI, Part 2: variants
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Segmented EPI pulse sequence
TE
NPE/NSEG
RF
GSS
GPE
GRO
DAC
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EPI, Part 2: variants
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Turbo Spin Echo (turbo factor = 4)
TE?
NPE/TF
RF
GSS
GPE
GRO
DAC
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Pulse sequence diagram:
Nrep = 4
Flip angle = 40 degrees
TR = 0.25 s
RF
Sequence type: segmented
EPI
___________
Contrast: ___________
T1 and T2*
GSS
Image resolution: ___________
128 x 128
Segment acq. time: 32 x 0.75ms = 24 ms
…
GPE
Image acq. time: ___________
1 sec
TE = 20ms (16th line)
…
GRO
(32 times)
Echo spacing = 0.75ms
DAC
Psy 8960, Fall ‘06
128 128 128
128 128
EPI, Part 2: variants
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K-space trajectory and scaled T2* decay in gray
matter:
…
…
…
…
M
128
lines
time (ms)
Tacq,seg = 24ms
TE = 20ms
128 (complex-valued) points
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EPI, Part 2: variants
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Image:
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Pulse sequence diagram:
60 deg.
TR = 1 s
RF
GE EPI
Sequence type: ___________
(partial Fourier acquisition)
T2*
Contrast: ___________
GSS
128 x 128
Image resolution: ___________
96 x 0.75ms = 72 ms
Image acq. time: ___________
(distortion!)
…
GPE
TE = 30ms (32nd line)
…
GRO
(96 times)
Echo spacing = 0.75ms
DAC
Psy 8960, Fall ‘06
128 128
128 128
EPI, Part 2: variants
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K-space trajectory and scaled T2 & T2* decay
in gray matter:
M
T2
96
lines
T2*
…
time (ms)
Tacq = 72 ms
TE = 30 ms
128 (complex-valued) points
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Image:
GE EPI, long TE and read-out
Psy 8960, Fall ‘06
GE EPI, shorter TE and read-out
EPI, Part 2: variants
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Pulse sequence diagram:
90 deg.
180 deg.
TR = 5 s
RF
Sequencetype:
type: SE
___________
Sequence
EPI
(partial Fourier acquisition)
Contrast: T
___________
Contrast:
2
GSS
Imageresolution:
resolution: 128
___________
Image
x 128
Imageacq.
acq.time:
time: 96
___________
Image
x 0.75ms = 72 ms
…
GPE
TE = 60ms (32nd line)
…
GRO
(96 times)
Echo spacing = 0.75ms
DAC
Psy 8960, Fall ‘06
128 128
128 128
EPI, Part 2: variants
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K-space trajectory and scaled T2 & T2* decay
in gray matter:
M
T2
96
lines
T2*
…
90
time (ms)
180
Tacq = 72 ms
TE = 60 ms
128 (complex-valued) points
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Image:
SE, more superior
SE, more inferior
GE
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Pulse sequence diagram:
90 deg.
180
RF
Nrep = 4
180
TR = 2.5 s
TI = 0.9 s
Sequence type: IR
SE EPI
___________
(4-shot)
Contrast: ______________________
T1 and (some) T2
GSS
Image resolution: ___________
128 x 128
…
GPE
Image acq. time: ___________
4 x 2.5s = 10 s
TE = 25ms (16th line)
GRO
…
(32 times)
…
Echo spacing = 0.75ms
DAC
Psy 8960, Fall ‘06
…
EPI, Part 2: variants
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K-space trajectory and scaled T2* decay in gray
matter:
…
…
…
…
T2, white matter
M
T2, gray matter
128
lines
T2*
90
time (ms)
180
Tacq,seg = 24ms
TE = 40ms
128 (complex-valued) points
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EPI, Part 2: variants
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Image:
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Pulse sequence diagram:
Nrep = 176 x 224
TR = 10ms
RF
Sequence type: 3D
___________
FLASH
(slab-selective excitation)
Contrast: ______________________
T1
GSS
Image resolution: ___________
256 x 224 x 176
Image acq. time: ___________
176 x 224 x .01s =
6 min 34s
GPE
GRO
DAC
Psy 8960, Fall ‘06
256 points
EPI, Part 2: variants
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K-space trajectory and scaled T2* decay in gray
matter:
M
224
lines
time (ms)
Tacq = 2.5ms
TE = 5ms
176 partitions
256 (complex-valued) points
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EPI, Part 2: variants
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Image:
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