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EPI, Part 2: variants. Segmentation Partial Fourier Spin echo vs. gradient echo Inversion recovery Long vs. short TE. 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

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Epi part 2 variants
EPI, Part 2: variants

  • Segmentation

  • Partial Fourier

  • Spin echo vs. gradient echo

  • Inversion recovery

  • Long vs. short TE

EPI, Part 2: variants


Last week
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


This week
This week

  • Review

    • Hardware

    • Pulse sequence diagrams

      • Read vs. phase encode

    • Pulse sequences

      • FLASH vs. EPI

EPI, Part 2: variants


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/)

Hardware: everything’s a coil

Fancy fenders

EPI, Part 2: variants


Single shot epi pulse sequence
Single-shot EPI pulse sequence

TE

RF

GSS

GPE

GRO

DAC

EPI, Part 2: variants


Spin echo epi pulse sequence
Spin Echo EPI pulse sequence

TE/2

TE/2

RF

GSS

GPE

GRO

DAC

EPI, Part 2: variants


Inversion recovery spin echo epi
Inversion Recovery Spin Echo EPI

TE/2

TE/2

RF

GSS

GPE

GRO

DAC

EPI, Part 2: variants


Inversion recovery flash
Inversion Recovery FLASH

NPE

RF

GSS

GPE

GRO

NRO

DAC

EPI, Part 2: variants


Segmented epi pulse sequence
Segmented EPI pulse sequence

NPE/NSEG

TE

RF

GSS

GPE

GRO

DAC

EPI, Part 2: variants


Turbo spin echo turbo factor 4
Turbo Spin Echo (turbo factor = 4)

TE?

NPE/TF

RF

GSS

GPE

GRO

DAC

EPI, Part 2: variants


Pulse sequence diagram
Pulse sequence diagram:

Nrep = 4

Flip angle = 40 degrees

TR = 0.25 s

RF

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

Sequence type: ___________

Contrast: ___________

Image resolution: ___________ Segment acq. time:

Image acq. time: ___________

GSS

GPE

TE = 20ms (16th line)

GRO

(32 times)

Echo spacing = 0.75ms

128

128

128

128

128

DAC

EPI, Part 2: variants


K space trajectory and scaled t 2 decay in gray matter
K-space trajectory and scaled T2* decay in gray matter:

M

128 lines

time (ms)

Tacq,seg = 24ms

TE = 20ms

128 (complex-valued) points

EPI, Part 2: variants


Image
Image:

EPI, Part 2: variants


Pulse sequence diagram1
Pulse sequence diagram:

60 deg.

TR = 1 s

RF

Sequence type: GE EPI (partial Fourier acquisition)

Contrast: T2*

Image resolution: 128 x 128

Image acq. time: 96 x 0.75ms = 72 ms

(distortion!)

Sequence type: ___________

Contrast: ___________

Image resolution: ___________

Image acq. time: ___________

GSS

GPE

TE = 30ms (32nd line)

GRO

(96 times)

Echo spacing = 0.75ms

128

128

128

128

DAC

EPI, Part 2: variants


K space trajectory and scaled t 2 t 2 decay in gray matter
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

EPI, Part 2: variants


Image1
Image:

GE EPI, long TE and read-out

GE EPI, shorter TE and read-out

EPI, Part 2: variants


Pulse sequence diagram2
Pulse sequence diagram:

180 deg.

90 deg.

TR = 5 s

RF

Sequence type: ___________

Contrast: ___________

Image resolution: ___________

Image acq. time: ___________

Sequence type: SE EPI (partial Fourier acquisition)

Contrast: T2

Image resolution: 128 x 128

Image acq. time: 96 x 0.75ms = 72 ms

GSS

GPE

TE = 60ms (32nd line)

GRO

(96 times)

Echo spacing = 0.75ms

128

128

128

128

DAC

EPI, Part 2: variants


K space trajectory and scaled t 2 t 2 decay in gray matter1
K-space trajectory and scaled T2 & T2* decay in gray matter:

M

T2

96 lines

T2*

time (ms)

90

180

Tacq = 72 ms

TE = 60 ms

128 (complex-valued) points

EPI, Part 2: variants


Image2
Image:

SE, more superior

SE, more inferior

GE

EPI, Part 2: variants


Pulse sequence diagram3
Pulse sequence diagram:

Nrep = 4

90 deg.

180

180

TR = 2.5 s

TI = 0.9 s

RF

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

Sequence type: ___________

Contrast: ______________________

Image resolution: ___________

Image acq. time: ___________

GSS

GPE

TE = 25ms (16th line)

GRO

(32 times)

Echo spacing = 0.75ms

DAC

EPI, Part 2: variants


K space trajectory and scaled t 2 decay in gray matter1
K-space trajectory and scaled T2* decay in gray matter:

T2, white matter

M

T2, gray matter

128 lines

T2*

time (ms)

90

180

Tacq,seg = 24ms

TE = 40ms

128 (complex-valued) points

EPI, Part 2: variants


Image3
Image:

EPI, Part 2: variants


Pulse sequence diagram4
Pulse sequence diagram:

Nrep = 176 x 224

TR = 10ms

RF

Sequence type: ___________

Contrast: ______________________

Image resolution: ___________

Image acq. time: ___________

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

GSS

GPE

GRO

256 points

DAC

EPI, Part 2: variants


K space trajectory and scaled t 2 decay in gray matter2
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

EPI, Part 2: variants


Image4
Image:

EPI, Part 2: variants


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