Psy 8960, Fall ‘06 EPI, Part 2: variants1 Segmentation Partial Fourier Spin echo vs. gradient echo Inversion recovery Long vs. short TE.

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: