gradient spoiling - stanford university · 2014. 10. 14. · gradient-spoiler question • what if...

B.Hargreaves - RAD 229

Gradient Spoiling

• Average balanced SSFP magnetization• Reduce sensitivity to off-resonance

FFE, FISP, GRASS, GRE, FAST, Field Echo

219


Gradient-Spoiled Sequence (GRE, FFE, Fisp, GRASS)

RF

Gz

Gy

Gx

Signal

TR

220


Gradient Spoiling

RF

Gz

TR

Signal is NOT eliminated at the end of TR!221


Gradient Spoiling

Precession across a voxel dominated by spoiler:

• Each spin has a different precession

• Average of balanced SSFP

RF

Gz

Signal

TR

222


Mag

nitu

de

x

ySpin Distribution

Signal vs Balanced SSFP

Gradient Spoiled Signal

• Lower signal than balanced SSFP

• Flat signal vs. frequency profile

• No dark band artifacts!

Frequency

223


Gradient Spoiled vs Balanced SSFP

(Courtesy of Krishna Nayak, USC Electrical Engineering)

Gradient-Spoiled Balanced SSFP

224


Reversed Gradient Spoiling

RF

Gz

TR

Spoiler Gradient

Same as gradient-spoiling, but

• Precession before imaging

(SSFP Signal at t=TR)

• Some T2 contrast

Image Acquisition

Pre-RF225


Mag

nitu

deP

hase

-π

πx

ySpin Distribution Signal vs Balanced SSFP

Reversed Gradient Spoiled Signal

• Almost identical signal to gradient-spoiled imaging

• Flat signal vs. frequency profile

226


Double Echo Imaging: DESS/FADE

RF

Gz

Signal

TR

a b

c d

a b

c d

a b

c d

a b

c dEcho 1 Echo 2 Average Difference

227


Gradient-Spoiler Question

• What if we split the spoiler into two and sample mid-way between?

• No signal!

• Averaging the “TE=TR/2” bSSFP state -- sign flip with every band

• GRE and reversed GRE average bSSFP states just after and just before the RF pulses

228

RF

GzTR


Gradient-Spoiler Question (cont)

229

Pha

se

-π

π

Frequency

Pha

se

-π

π

Mag

nitu

deP

hase

-π

π

xy

x

y

x

y

Post-RF

Pre-RF

Center

Flipped!

Recall bSSFP Dynamics...Gradient-Spoiled

Distribution

Reverse Gradient-Spoiled Distribution

This Example


Summary: Gradient Spoiling

• Average balanced SSFP magnetization

• Reduce sensitivity to off-resonance

• Unbalanced gradients - more motion sensitivity

FFE, FISP, GRASS, GRE, FAST, Field Echo,T2-FFE, PSIF, CE-FAST

FADE, DESS230


RF Spoiled Imaging

• Goal: Pure T1 contrast with short TR

• Fast, 3D T1-weighted imaging

• Need to “zero” Mxy at end of TR

Frahm 1987, Zur 1991

SPGR, FLASH, T1-FFE, RF-spoiled FAST

231


RF-Spoiled Gradient Echo

RF

Gz

TR (Quadratic Phase Increment)S

igna

l

1

0

Mz

X XEliminate transverse magnetization: T1 contrast

232


Quadratic Phase RF

• RF axis of rotation, not flip angle!

• RF phase is “randomized”

Mx

My

Mx

My

Mz

233


RF Spoiling

• Goal: Eliminate transverse magnetization

• Quadratic phase increment with gradient spoiling:

φk = (0.5)117° k2

• Shifting, spoiled profile

• Transverse magnetization “cancels”

• T1 contrast

Frahm 1987, Zur 1991

234


0 50 100 150 2000

0.1

0.2

0.3

0.4

Excitation

Mag

nitu

de

0 50 100 150 200−1

0

1

2

3

Excitation

Phas

e (ra

d)

SPGR Signal Evolution (Bloch)

1 Pixel

Average(gradient-smoothed)

Excitation # Excitation #

Magnitude Phase

235


RF Spoiled Signal

Repetition Number

Repetition Number0 200

0 200

Mag

nitu

deP

hase

π

−π

Signal with residual transverse magnetization perfectly zero

236


RF-Spoiled Gradient Echo

237


RF-Spoiled Contrast-Enhanced MRPre-Contrast SPGR Post-Contrast SPGR

238

Courtesy Lewis Shin


RF Spoiling: Summary

• Gradient spoiling + Quadratic phase RF

• “Eliminates” transverse magnetization

• Lower signal than GRE or balanced SSFP

• Pure T1 contrast

239


Gradient Echo Sequence Comparison

Sequence

Spoiling

Transverse Magnetization

Contrast

SNR

RF-Spoiled

RF + Gradient

Cancelled

T1

Lower

Gradient Echo

Gradient

Averaged

T2/T1

Moderate

BalancedSSFP

None

Retained

T2/T1

High(but Banding)

240


Image Comparison

• Same TR, TE, flip angle

• Differences: Signal, Contrast, Dark-Bands

Balanced SSFP Gradient Spoiled RF Spoiled

241


Your Turn...Here are a balanced SSFP and RF-Spoiled post-contrast image. Which is the image on the left?

1) RF-Spoiled Post Gd

2) Balanced SSFP

242


Your Turn...An RF-Spoiled and balanced SSFP image are shown. Which is on the right?

1) RF-Spoiled

2) Balanced SSFP

243

• bSSFP: Bright fluid, Opposed-phase, dark bands


Flip Angle Selection

Ernst Angle Buxton 1990244


Flip Angle Selection?The best flip angle to use is found by:

1) Maximizing the image SNR

2) Maximizing contrast between certain tissues

3) Both 1 and 2

245


Flip Angle Examples

Balanced SSFP

Gradient Spoiled

RF-Spoiled

5∘ 10∘ 20∘ 30∘ 40∘ 50∘

Best?

Best?

246


Echo Time (TE) Considerations• Longer TE: T2* weighting (BOLD, Perfusion)

• BOLD Imaging for fMRI

• T2*-weighted perfusion

• Short TE

• Reduced flow/motion sensitivity

• Reduced T2* weighting

• In-phase and Out-of-phase TE

• Water/Fat cancellation, Dixon Imaging

247


Blood-Oxygen Level Dependent (BOLD) Imaging

Bandettini, et al. NMR Biomed (1994)248


tGx

t=TE

Full Echo

tGx

t=TE

Partial Echo

Clinical MRI, ed. Edelman, R et al., W.B. Saunders Co., 1996TE=9 TE=7

Echo Time Reduction

249


Short TE to Reduce Artifacts

• Fractional TE reduces motion and flow sensitivity

Fractional TEFull TEFreq

250


TE and balanced SSFP

• SSFP signal is refocused at TE = TR/2

• Fat / water are often opposed-phase

• SSFP is naturally flow-compensated

RF-SpoiledBalanced SSFP251


UTE

Ultrashort TE Imaging

Fast Spin Echo

(Courtesy of Jiang Du, Graeme Bydder)252


Preparation Options

• Fat Saturation

• Inversion - Recovery

• Myocardial Tagging

• T2-prep

• Magnetization Transfer

Mag Prep

Imaging Sequence

Mag Prep...

253


Cardiac: bSSFP and IR-RF-Spoiled

IR-Prep RF-SpoiledBalanced SSFP

254


Gradient Echo Summary

• Spoiling Variations

• Flip Angle Selection

• Echo Time (TE) Selection

• Magnetization Preparation

255

gradient spoiling - stanford university · 2014. 10. 14. · gradient-spoiler question • what if...

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