gradient spoiling - web.stanford.edu · section c2 b.hargreaves - rad 229 gradient spoiling...

B.Hargreaves - RAD 229Section C2

Gradient Spoiling

• Average balanced SSFP magnetization

• Reduce sensitivity to off-resonance

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

1


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

RF

Gz

Gy

Gx

Signal

TR

2


Gradient Spoiling

RF

Gz

TR

Signal is NOT eliminated at the end of TR!3


Gradient Spoiling

Precession across a voxel dominated by spoiler:

• Each spin has a different precession

• Average of balanced SSFP

RF

Gz

Signal

TR

4


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

5


Gradient Spoiled vs Balanced SSFP

(Courtesy of Suba Srinivasan, Stanford)

Gradient-SpoiledBalanced SSFP

6


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-RF7


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

8


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

9


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

10

RF

Gz

TR


Gradient-Spoiler Question (cont)

11

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, DESS 12


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

13


RF-Spoiled Gradient Echo

RF

Gz

TR (Quadratic Phase Increment)S

igna

l

1

0

Mz

X XEliminate transverse magnetization: T1 contrast

14


Quadratic Phase RF

• RF axis of rotation, not flip angle!

• RF phase is “randomized”

Mx

My

Mx

My

Mz

15


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

16


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

17


RF Spoiled Signal

Repetition Number

Repetition Number0 200

0 200

Mag

nitu

deP

hase

π

−π

Signal with residual transverse magnetization perfectly zero

18


RF-Spoiled Gradient Echo

19


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

20

Courtesy Lewis Shin


RF Spoiling: Summary

• Gradient spoiling + Quadratic phase RF

• “Eliminates” transverse magnetization

• Lower signal than GRE or balanced SSFP

• Pure T1 contrast

21


Gradient Echo Sequence Comparison

Sequence

Spoiling

Transverse Magnetization

Contrast

SNR

RF-Spoiled

RF + Gradient

Cancelled

T1

Lower

Gradient Spoiled

Gradient

Averaged

T2/T1

Moderate

Balanced SSFP

None

Retained

T2/T1

High (but Banding)

22


Just for Fun… Putting it Together!

23

Gradient- Spoiled

Constant RF Phase Increment

Quadratic Phase (Increasing Phase

Increment)

Average Signal Perfect Spoiling

117º

119º


Image Comparison

• Same TR, TE, flip angle

• Differences: Signal, Contrast, Dark-Bands

Balanced SSFP Gradient Spoiled RF Spoiled

24


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

25


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

1) RF-Spoiled

2) Balanced SSFP

26

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


Flip Angle Selection

Ernst Angle Buxton 1990

27


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

28


Flip Angle Examples

Balanced SSFP

Gradient Spoiled

RF-Spoiled

5∘ 10∘ 20∘ 30∘ 40∘ 50∘

Best?

Best?

29


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

30


Blood-Oxygen Level Dependent (BOLD) Imaging

Bandettini, et al. NMR Biomed (1994)31


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

32


Short TE to Reduce Artifacts• Fractional TE reduces motion and flow sensitivity

Fractional TEFull TE

Freq

33


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 SSFP34


Ultrashort TE Imaging

(Courtesy of Jiang Du, Graeme Bydder)UTEFast Spin Echo

35


Magnetization Preparation Options

• Fat Saturation

• Inversion - Recovery

• Myocardial Tagging

• T2-prep

• Magnetization Transfer

Mag Prep

Imaging Sequence

Mag Prep...

36


Cardiac: bSSFP and IR-RF-Spoiled

IR-Prep RF-SpoiledBalanced SSFP

37


Gradient Echo Summary

•Spoiling Variations •Flip Angle Selection •Echo Time (TE) Selection •Magnetization Preparation

38

gradient spoiling - web.stanford.edu · section c2 b.hargreaves - rad 229 gradient spoiling...

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