Scan Acceleration with Rapid Gradient-Echo · 1 of 214 Scan Acceleration with Rapid Gradient-Echo...

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Scan Acceleration with

Rapid Gradient-Echo

Hsiao-Wen Chung (鍾孝文), Ph.D., Professor

Dept. Electrical Engineering, National Taiwan Univ.

Dept. Radiology, Tri-Service General Hospital

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The Need for Faster Scan

• Patient comfort, motion artifacts,

efficiency, more information …

• EPI ? You know the difficulty now

• But there are a lot more ways to

accelerate the scanning

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Back to the Old Formula

• Scan time for single slice

= TR x (# phase encoding) x NEX

• Reduce phase encoding

– A little faster, trade in resolution

• Reduce NEX (1 or 0.5 at most)

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MRI Scan Time (1990 ?)

• Spin-echo : 256x256, 2 NEX

– PD, T2 : 16 min (TR 2000)

– T1 : 5 min (TR 600)

• Note : somewhat exaggerated

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Short Scan Time ?

• Reduce TR (2000 50 msec ?)

– 40 times faster ?

– 256x256, 1 NEX : 13 sec

• Sounds like an efficient way ?

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Short Scan Time ?

• Reduce TR

– Increased T1-weighting

– Reduced SNR

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Effects of Reduced TR on T1 Contrast

TR

Sig

nal In

ten

sit

y

Signal at this TR

Substantial reduction in TR leads to SNR loss

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How Low Is The SNR ?

• TR = T1 :

– ~ 63% of thermal equilibrium

• TR = 0.1 T1 :

– ~ 9.5% of thermal equilibrium

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Compensating SNR ?

• SNR loss due to slow T1 recovery

– CSF T1 = 0.7 ~ 4.0 sec

• Can magnetization recover from

nonzero (positive) values ?

• Can we retain Mz after RF pulsing ?

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Small Flip Angle RF Excitation

Mxy for data acquisition, some Mz for next excitation

z'

y'

x'

Bo Bo z'

y'

x'

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The FLASH Technique

• Reducing TR without sacrificing too

much SNR

• Achievable by lowering the flip angle

– via B1 amplitude adjustment

• Fast Low-Angle SHot (FLASH)

– Haase et al., 1985

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SNR Comparison

• TR = T1, a = 900 : ~ 63% of Mo

• TR = 0.1 T1 :

– a = 900 : ~ 9.5% of Mo

– a = 250 : ~ 22% of Mo

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Example

• CSF has T1 ~ 700 msec

– TR lowered to ~ 70 msec

– Slightly lowered quality

• Scan time ~ 18 sec allows

breath-hold exams

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Here Comes A Question

• Spin-echo no longer useable !

• Imaging has to be done with gradient-echo

– Bo inhomogeneity is going to affect

image quality

– But can also become another source of

diagnostic information

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Effects of 1800 Refocusing Pulse

Mz becomes negative (recovery takes even longer) !

z'

y'

x'

Bo Bo

z'

y'

x'

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Gradient-Echo Properties

• No refocusing function from 1800 pulse

• Image affected by Bo inhomogeneity

– T2* decaying (not T2)

– Instrumentation, air-tissue interface …

– Hemorrhage, hematoma, bone …

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Effects of Bo inhomogeneity

Non-uniform Bo in a voxel short T2*

Image voxel :

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Effects of TE

TE = 9 msec TE = 18 msec

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Gradient-echo Is Worse ?

• Image quality for gradient-echo is often

harder to control than for spin-echo

• But that does not mean “bad”

• Proper usage gives useful information that

cannot be provided by spin-echo

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Application Examples

• Hemorrhage (Iron in blood)

• Brain perfusion (Gd-based agent)

• Blood oxygenation (deoxy-Hb)

• Brain fMRI (BOLD contrast)

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T2* Signal Loss in Hemorrhage

T1 PD T2 GrE

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T2* Signal Loss & Blood Oxygenation

Normal air Pure oxygen

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Brain Oxygenation & Brain Function

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That Looks Good Then !

• Short TR faster scan

• Small flip angle not much SNR loss

• Gradient-echo more information

• Worth some more exploration !

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Effects of Flip Angle

• Small flip angle, partial flip

angle … (< 90)

• How small should it be ?

• 100 ? 300 ? 700 ? Arbitrary ?

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Flip Angle Contrast

• Short TR T1WI, long TR T2WI …

– Only true for 900 excitation

• TR already short in gradient-echo

• No longer use TR to alter T1 contrast

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How to Vary T1 Contrast?

• Magnetization vector goes into a steady

state after several RF pulsing

• Image intensity mainly determined by this

steady state behavior

• Steady state: T1 recovery for Mz in one TR

= Mz reduction due to RF pulsing

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Steady State with Many RF Pulses

Assuming no residual Mxy at the end of TR

Bo Bo

z'

y' x'

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The Formula Is Actually

• Signal proportional to

• a : flip angle

(1 - e -TR/T1) sin a

1 - e -TR/T1 cos a e -TE/T2*

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Simple Rule for PD or T1 Contrast

T1WI PDWI

z'

y'

x'

Bo Bo z'

y'

x'

recover from 0

little room for

recovery

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Control of T1 Contrast

• Large flip angle (~ 900)

– Similar to short-TR images (T1)

• Small flip angle (20 ~ 400)

– Reduced T1 weighting (PD)

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Flip Angle = 100

Proton-density-weighted image

z'

y'

x'

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Flip Angle = 200

z'

y'

x'

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Flip Angle = 300

z'

y'

x'

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Flip Angle = 400

z'

y'

x'

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Flip Angle = 500

T1-weighted image

z'

y'

x'

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Flip Angle = 600

z'

y'

x'

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Flip Angle = 700

Strong T1-weighted image

z'

y'

x'

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Comparison of PD & T1 Contrast

100 300 500

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Control of T2 Contrast

• Still use TE (< TR)

• Actually T2* in gradient-echo

– TE does not have to be too long

– T2* contrast very similar to T2WI

(other than Bo inhomogeneity)

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T2(*) Weighting

Decay of transverse magnetization

Bo z'

y'

x'

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Using TE to Control T2(*) Contrast

TE = 10 TE = 30 TE = 50

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Myth of Speeding Scan

• Is the examination time really

shortened ?

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Expansion of a Pulse Sequence …

TR >> TE : hardware mostly idle

Gp

B1 t

t

...

...

Gs t ...

Gr t ...

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Add Different Slices

Making best use of the idle time

Gp

B1 t

t

...

...

Gs t ...

Gr t ...

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Add Even More Slices

Multi-slice imaging (scan time not lengthened)

Gp

B1 t

t

...

...

Gs t ...

Gr t ...

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Myth of Speeding Scan

• As TR shortens, the number of slices

becomes smaller in a TR

• Multiple slice coverage repeat the

scan several times !

• Total exam time likely unchanged

totally useless ??

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Pros of Speeding Scan

• Faster single-slice scan

– Less motion influences

• 3D becomes possible !

• 2D examination time is not

necessarily shortened

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Scan Time Advantages

6.4 seconds 3.8 seconds

2.5 seconds 1.5 seconds

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Speeding Even Further ?

• TR shortened to ~10 msec :

– Flip angle reduced to ~100

– 2-sec scan time !!

– High-quality MR images for

uncooperative patients ?

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RF Pulsing with Very Small Flip Angle

Very small TR and flip angle : PDWI

z'

y'

x'

Bo Bo z'

y'

x'

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Clinical Restrictions

• TR ~ 10 msec :

– PDWI (seldom used clinically)

– TE < TR

– Then T2 contrast ... ?

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Gradient-echo with Very Short TR

TE < TR

Gp

B1 t

t

...

...

Gs t ...

Gr t ...

TR

TE

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If You Study Too Hard ...

• CE-FAST, PSIF, SSFP-echo ...

– TE can be larger than TR

• Complicated principles, questionable

applications out of scope !

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Clinical Restrictions

• TR ~ 10 msec :

– PDWI (seldom used clinically)

– TE < TR

– Then T2 contrast ... ?

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Back to Imaging Principle

• RF with very small flip angle

• T2 & T1 relaxation

• Next RF pulsing

• Repeat many times

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RF Pulsing with Very Small Flip Angle

Magnetization almost the same before/after RF

z'

y'

x'

Bo Bo z'

y'

x'

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When TR Is Very Short

• RF with very small flip angle

• T2 & T1 relaxation not obvious

• Next RF pulsing

• Repeat many times

• Contrast determined by initial Mo

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No Obvious Relaxation ?

• Can be useful !

• Use even smaller flip angle and TR

• Relaxation plays a minor role

• Manipulate the initial magnetization

vector to change contrast !

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How to Manipulate “M” ?

• RF pulsing, of course !

• 900 + 1800 + (-900) :

– T2-weighted magnetization !

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900-1800-(-900)

Spin echo principle

B1

900 1800

-900

z'

y' x'

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900-1800-(-900)

Mz becomes T2-dependent !

B1

900 1800

-900

z'

y'

x'

Short T2

Long T2

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Hints for the Processes

• Very small flip angle + very short TR

– T1/T2 do not affect signal intensity

– Contrast determined by initial Mz

• 900 + 1800 + (-900) :

– T2-weighted Mz !

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Combine These Two …

Preparation Fast acquisition

Gp

B1 t

t

...

...

Gs t ...

Gr t ...

TR 900 1800 -900

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Magnetization Preparation

Preparation Fast acquisition

B1 t ...

TR 900 1800 -900

TE ~ 200 msec TR x 256 ~ 2 sec

Total scan time ~ 2-3 sec

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Magnetization Preparation Images (T2)

PD (no prep) T2 (with prep)

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Names for The Technique

• Magnetization preparation

• Turbo-FLASH, MP-RAGE (Siemens)

• Driven-equilibrium fast SPGR (GE) ...

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Properties

• TR often very short (< 20 msec)

• Flip angle often very small (5~200)

• SNR often low (system-dependent)

• Contrast determined by the

“magnetization preparation” part

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The Reason for Low SNR

Not much Mxy available for sampling

z'

y'

x'

Bo Bo z'

y'

x'

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Expand the Applications !

• Different preparation modules

• Different acquisition modules

• To form many combinations

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Preparation Modules

• STIR/FLAIR (inversion recovery)

• Fat-Sat (off-reson pulse)

• Diffusion (RF + gradient)

• MTC (bipolar pulses) ...

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Inversion Recovery Preparation

T1-related preparation or suppression

B1

1800

z'

y' x'

TI

z'

y' x'

short T1

long T1

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STIR/FLAIR Turbo-FLASH

B1

1800 TI ~ 130 msec

B1

1800 TI ~ 2000 msec

fat relaxes to 0

CSF relaxes to 0

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Fat-Sat Preparation

B1

900 fat only

strong gradient for

spoiling

Gs

Gp

Gr

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Other Preparation Schemes

Some will be mentioned in the future

B1

900

Gs

1800

-900

B1

a0 -a0 a0 -a0 a0 -a0 a0 -a0

diffusion

magnetization

transfer

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Acquisition Modules

• FLASH, GRASS, SPGR, ...

• EPI (echo-planar imaging)

• FSE (TurboSE)

• Conventional spin-echo !

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FLASH (short TR)

Continual RF excitation

Gp

B1 t

t

...

...

Gs t ...

Gr t ...

TR

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Echo Planar Imaging

RF

Gs

Gp

Gr

t

t

t

t

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Fast Spin-Echo (Turbo Spin-Echo)

RF

Gs

Gp

Gr

t

t

t

t

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Preparation + Acquisition

FLAIR Fat-Sat EPI

Gp

B1

Gs

Gr

1800

TI ~ 2000

900

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FLAIR Fat-sat EPI

From Picker (Marconi Philips) brochure

Picker Vista

1800 + 2000 msec

TE = 120 msec

256x160

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Note (1)

• Short TR gradient-echo actually has

very complicated contrast behavior

• Greatly simplified in this course

• Complex parts saved for the future

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Note (2)

• TR > T2 ? TR < T2 ?

• Steady-state and non-steady-state

imaging families

– Approaching steady state

– Destroying steady state (spoiler)

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The Fast Spin-echo

Imaging Sequence




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Fast (Turbo) Spin-echo Sequence

Every echo forms one k-space line

RF

Gs

Gp

Gr

t

t

t

t

echo 1 echo 2 echo 3 ...

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Review: Accelerate Scan?

• Example : EPI

– Fill in the entire k-space after

one single RF excitation

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Echo Planar Imaging (EPI)

RF

Gz

Gy

Gx

t

t

t

t

kx

ky

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From EPI to FSE

• EPI : series of gradient echoes

– with proper encoding gradient

• FSE : series of spin echoes

– with proper encodign gradient

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Echo Planar Imaging (EPI)

RF

Gz

Gy

Gx

t

t

t

t

kx

ky

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Fast Spin-Echo (FSE)

RF

Gz

Gy

Gx

t

t

t

t

kx

ky

TR

TR

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Also Similar to Spin-Echo

• Spin-echo has the multi-echo option

– 900-1800-echo-1800-echo …

• Multi-echo : forms many images

• FSE : All echoes used in one image

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Multi-echo Sequence

Every echo belongs to a unique image

RF

Gs

Gp

Gr

t

t

t

t

image 1 image 2 image 3 ...

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Fast Spin-echo Sequence

All echoes belong to the same image

RF

Gs

Gp

Gr

t

t

t

t


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What You Can Infer

• Fast spin-echo sequence can be

easily modified from multi-echo

• FSE image behavior should be

similar to traditional spin-echo

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Comparison between FSE T2 & SE T2

SE (TE = 100) FSE (TE = 100)

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Also Similar to Spin-Echo

• Multiple k-space lines obtained with

every single RF excitation

– Just with several 1800 pulses

• Single-slice scan must be much

faster than spin-echo

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20-sec Scan for the Eye

No motion artifacts visible

GE 1.5 Tesla

Fast Spin-echo

ETL = 12

TR = 2000

Scan time = 20 sec

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Let’s Name It Then

• Turbo spin-echo (Siemens)

• Fast spin-echo (GE & others)

• RARE (Bruker)

– Rapid acquisition with relaxation

enhancement

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Why Is FSE Important ?

• Spin-echo : traditional MRI standard

• FSE similar to SE

• Much faster scan

– TR = 2000 : 7 min to 1 min

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FSE Similar to SE (256x128)

SE (6 min) FSE (48 sec)

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Acceleration Achieved

• 8 echoes (e.g.) with each 900

• 256x256 : 32xTR only

• 8 times faster than spin-echo

• Echo train length (ETL) = 8

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Multi-shot FSE

• Scan time = TR x (phase#) / ETL

• The larger ETL, the faster single-

slice scan

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How about Contrast ?

• Echoes have different TE !

• What determines T2 contrast ?

• Effective TE

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Multi-shot FSE Sequence

The k-space lines have different TE ??

RF

Gz

Gy

Gx

t

t

t

t

Signal t

kx

ky

TR

TR

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Don’t Forget about k-space …

Contrast mainly determined by central k-space

kx

ky

boundary

boundary

contrast

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A 256x256 Image Is Composed of …

Central k: contrast Outer k : boundary

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TE in FSE

• Central k-space determines the

image contrast

• Data passing central k-space

dominate the contrast

– despite of different TEs

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TE & Phase Encoding

• Data location in k-space

controlled by phase encoding

• Phase encoding order

determines TEeff

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k-space Filling Pattern in FSE

Early echo placed at central k-space: PDWI

RF

Gz

Gy

Gx

t

t

t

t

Signal t

kx

ky

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Late echo placed at central k-space: T2WI

RF

Gz

Gy

Gx

t

t

t

t

Signal t

kx

ky

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Expand: Dual-Contrast

• FSE is an expanded version of multi-

echo spin-echo …

• Dual echo naturally feasible in FSE

• T2 weighting also determined by

TEeff

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Dual-contrast FSE Sequence

RF

Gs

Gp

Gr

t

t

t

t


image 1 image 2

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Or Even ...

Data sharing

Contrast

Early echo : PD

Late echo : T2

kx

ky

boundary

(shared)

boundary

(shared)

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Data Sharing FSE Sequence

RF

Gs

Gp

Gr

t

t

t

t


image 1 image 2

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Data Sharing in Dual-Echo FSE

TEeff = 17 msec TEeff = 85 msec

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Data Sharing

• Only central k-space acquired

multiple times with different TE

– For different T2 weightings

• Outer k-space acquired only once

• Dual contrast with < 2 time penalty

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Move Further: Single-shot

• Entire acquisition + wasted time

< 1~2 T2 (100 ms to sec range)

• 256x256 : an echo every 4 msec

– Echo spacing (ESP)

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Multi-shot FSE Sequence

Scan time = TR x (phase #) / ETL

RF

Gz

Gy

Gx

t

t

t

t

TR

ESP

ETL = 3

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Single-shot FSE Sequence

No TR (or TR is infinite)

RF

Gz

Gy

Gx

t

t

t

t

ESP

ETL = # of phase encoding

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Certainly Possible But …

• ESP has ~4 ms lower limit

• ETL ~ 256 to yield 1-2 sec scan

• Most signals decay due to T2

relaxation

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Very late echoes show no signals at all

RF

Gz

Gy

Gx

t

t

t

t

Signal t

kx

ky

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ESP Can’t Be To Short !

• Specific Absorption Rate (SAR)

• RF power proportional to (flip angle)2

– 1800 power: 4x of 900, 36x of 300 !

• RF power deposition causes an

increase of local body temperature

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So many high-power RF pulses !

RF

Gz

Gy

Gx

t

t

t

t

ESP

ETL = # of phase encoding

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Single-shot FSE Usage

• You want only long T2 tissues

– Myelogram, MRCP

• Motion so severe that scan time

becomes the dominant factor

– Fetal imaging, GI imaging

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Only Long-T2 Tissues Have Signals

CSF in spinal cord : long T2 tissue

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Myelogram (Strongly T2-weighted FSE)

Original slices (heavy T2 images) MIP

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FSE MRCP (Same Principle)

Original slices MIP MRCP

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1-sec Fetal Scan

No artifacts from fetal motion

Siemens 1.5 Tesla

HASTE

ETL = 128

256x240

Scan time = 1 sec

22 weeks gestation

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… Including My Own Son

Courtesy Cheng-Yu Chen, M.D., Tri-Service General Hospital

5-month photo Future look?

28-week gestation 35-week gestation

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One Variation : HASTE

• Half-Fourier acquisition single-shot

TurboSE (Siemens)

• Single-shot fast spin-echo (GE)

• Half Fourier + TSE = ~1s scan

• Reduce 1800 to ~1300 for SAR

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Multi-shot FSE Usage

• Almost the new standard for T2

– Much faster than traditional SE

• HASTE best in GI

– Motion and susceptibility artifacts

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FSE Advantages

• FSE similar to traditional SE

– Spin-echo already a standard

– FSE widely accepted as well

– No gradient-echo artifacts

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Comparison between FSE T2 & SE T2

SE (TE = 100) TSE (TE = 100)

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Speed Advantages

• Overcome motion artifacts

• Multiple signal averages for SNR in

reasonable scan time

• Trade SNR for spatial resolution

• Long TR for proton density weighting

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Speed Advantages in terms of Motion

SE (ECG gating) FSE (no gating)

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Speed Advantage in GI Imaging

4:30 min scan, 512 matrix (readout)

R-L frequency encoding

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Resolution Advantage with SNR

High-resolution in reasonable scan time 256x256, 57 sec 512x512, 2:45 min

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Long TR Advantage in Nerve Roots

Strong CSF & high resolution for nerve roots

Siemens 1.5 Tesla

Turbo Spin-echo

512 matrix

3 mm slice

Scan time = 7 min

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FSE Properties

• Compared with SE at same TE

– Stronger magnetization transfer

contrast

– Weaker diffusion weighting

– Bright fat at long ETL

• No time to explain in this semester

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FSE Unique Artifacts

• Point-spread function blurring

– Will be briefly mentioned

• Pseudo edge enhancement

• Ghosts from data discontinuity

• No time to explain either

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Early echo placed at central k-space : blurring

RF

Gz

Gy

Gx

t

t

t

t

Signal t

kx

ky

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Blurring in FSE with Long ETL

HASTE (176x256) HASTE (128x256)

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ETL Comparison in Chest Imaging

ETL 15 (ECG, BH, 14 sec) ETL 85 (0.4 sec)

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Parallel MRI with

RF Phased Array Coils




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Review : Phased Array

• Surface coil: high SNR with limited

coverage

• Phased array: multi coils with geometric

arrangement to cancel mutual inductance

• Achieve high SNR and wide coverage

simultaneously

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Phased Array Coil

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Spine Phased Array

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Phased Array Image Formation

Signals received and processed separately

Receiver Receiver Receiver Receiver

Computer (reconstruction)

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Combine to Form Phased Array Image

Wide FOV for larger coverage

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Phased Array Imaging

• Coil elements receive signals

separately

• Send to individual receiver channel

• No other difference at all

– RF pulsing, phase encoding, etc.

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What Is Parallel Imaging ?

• Signals in different coils must be different

• If data in different coils show little

redundancy, can some steps be omitted?

• SMASH (1997)，SENSE (1999)

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Method 1

• Produce various spatial

frequency waveforms in k-space

using the coil profiles

• Multiple k-space lines in one

phase encoding

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Review : k-space & MRI

• Each point in the k-space coordinate

– (kx,ky) coordinate : specific waveform

– Signal intensity : relative weighting of

that waveform

• All MRIs are formed by these waveforms

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kx

ky

A k-space point represents a waveform

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Many waveforms summed to an MRI

Waveforms weighed by signal intensity

+

+

+

+ …

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Phased Array Helps in …

• Signals received at various locations

• Adjust weights of signals according

to the coil locations to “form”

different waveforms

– from one single acquisition

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Waveform Formation from Coil Profiles

8 elements arranged linearly

Coil arrangement

Equal weights

Form cosine

Form sine

High-freq cosine

High-freq sine

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Many Waveforms from One Acquisition

Many k-space lines from one phase encoding

kx

ky

freq encoding

phase encoding

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Parallel Imaging

• Multiple k-space lines with one phase

encoding due to separate signal

receiving with phased array coils

• N waveforms N times acceleration

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Let’s Name It

• Many “harmonics” formed at once

• SiMultaneous Acquisition of Spatial

Harmonics (SMASH)

– Sodickson 1997

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Acceleration Factor

• Theoretically, N coil elements

could form at most N harmonics

• Nothing is perfect in practice

acceleration factor < N

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SMASH Phantom Image (1997 MRM)

Usual scan (10 sec) 3 coils (5 sec)

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SMASH Body Image (1997 MRM)


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Philips ACS NT 1.5T

FLASH, 7.0/1.5/300

3D (128px256x20)

6 coils, R = 3

[Gd] = 0.13 mM/Kg

8 sec per 3D frame

SMASH CE-MRA (2000 Radiology)

Shorten breath-hold time or high temporal resolution

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SNR in SMASH

• Accelerate from reduced phase encoding

• SNR lowers according to the square root

relationship

• Half scan time SNR lowered to 70%

• Used when reducing motion effects

outweighs SNR loss

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SMASH Pitfalls

• Coil size, shape, arrangement

relatively restricted in order to form

perfect sinusoids

• Direction of multi coils often not

used for phase encoding

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Waveform Formation from Coil Profiles

8 elements arranged linearly

Coil arrangement

Equal weights

Form cosine

Form sine

High-freq cosine

High-freq sine

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Combine to Form Phased Array Image

Head-foot direction is often freq encoding

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Even Arrangement OK …

• Sinusoids formed by coil profiles

often non-perfect

• Imperfect reconstruction results

in residual aliasing

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Aliasing in SMASH with R = 2


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Aliasing in SMASH with R = 2


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SMASH Extensions

• Auto-SMASH

• VD Auto-SMASH

• GRAPPA (Siemens)

• Details omitted

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GRAPPA Lung Images

Usual scan 207 ms 150 ms (8-coil array)

HASTE 128x256 GRAPPA 256x256

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GRAPPA Liver Images

Usual scan 252 ms 252 ms (8-coil array)

HASTE 128x256 GRAPPA 256x256

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Method 2

• Coils have different sensitivity profiles, all

relatively small

• Reduce FOV for less phase encoding

– Accelerated, aliasing occurs

• Compute image according to the different

aliasing patterns

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3-coil Example

• Coil sensitivity profiles roughly

occupy 1/3 FOV

• Prescribe a small FOV (~1/3)

• Resolution unchanged

reduced matrix size

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Example Using 3 Coils

Phantom & coil locations Aliased images

1 2

3

1

2

3

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No Panic with Aliasing

• Aliased image =

signals within FOV + outside FOV

• Signals stronger within coil profile,

weaker outside weighted sum

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Aliased Image from Coil 1

Phantom & coil location

1

FOV

aliasing

aliasing

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Coil #1 local intensity

FOV

aliasing

(weaker)

aliasing

(even weaker)

1

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Phantom & coil image

aliasing

(weaker)

aliasing

(even weaker)

1

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Aliased Image = Weighted Sum

• An aliased images (D1) =

weighted sum of 3 sub-FOV images

• In the form of D1 = A1 x + B1 y + C1 z

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Coil #2 local intensity image

aliasing

(weaker)

aliasing

(weaker)

2

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Coil #3 local intensity image

aliasing

(weaker)

aliasing

(even weaker)

3

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3 Aliased Images from the 3 Coils

Phantom & coil location Aliased images

1 2

3

1

2

3

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3 Aliased Images (D)

• D1 = A1 x + B1 y + C1 z

• D2 = A2 x + B2 y + C2 z

• D3 = A3 x + B3 y + C3 z

• Solving the equations (matrix

inversion) gives (x, y, z)

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Algebraic Problem Now

• 3 aliased images (D)

– in “D = Ax + By + Cz” form

• A, B, C: known from coil profiles

• Matrix inversion to get (x, y, z)

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Don’t Forget …

• Each aliased pixel has one

unique set of D = Ax + By + Cz

equations

• Matrix inversion performed

256x256/3 times (for R = 3)

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Summary …

• 3 RF coils to receive signals

• 1/3 FOV prescribed with same resolution

• Scan accelerated 3 times from a reduction

in matrix size (phase encoding)

• Full FOV image can be computed

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3 Times Acceleration, Solve Equations

Full FOV image obtained from aliased images

1

2

3

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Let’s Name It

• It is about the usage of coil

sensitivity profiles …

• SENSitivity Encoding (SENSE)

– Pruessmann 1999

• Rumor has it that it’s Philips patent

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Acceleration Factor

• Theoretically, N coil elements

provide at most N aliased images

– Smallest prescribed FOV is FOV/N

• Like SMASH, < N in reality

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SENSE Brain Image (1999 MRM)


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SENSE Heart Image (Short Axis)

Usual scan (15 beats) 5 coils (5 beats)

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SNR in SENSE

• Acceleration thru reduced phase encoding

• SNR lowers according to square root

relationship

• Like SMASH, Used when reducing motion

effects outweighs SNR loss

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SENSE Heart Image (Axial)

Usual scan (128x128) 6 coils (R = 3)

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SENSE Coil Requirement

• D1 = A1 x + B1 y + C1 z

• D2 = A2 x + B2 y + C2 z

• D3 = A3 x + B3 y + C3 z

• Solvable as long as equations

are linearly independent

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SENSE RF Coil Arrangement

Phase direction can be either one

Phase

Phase

RF coil element

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SENSE Coil Requirement

• Linearly independent equations

• No need to form perfect sinusoid

• Easier than SMASH

• No many variations like GRAPPA

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SENSE Pitfalls

• Incompatible with restricted FOV

• Example : cardiac imaging

• Full FOV contains some aliasing

• Can’t distinguish after mixture

with 1/3 FOV aliasing

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Theoretical Comparison

• SMASH :

• Fast computation (Fourier transform)

• Artifact performance better than

SENSE at high acceleration factors

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Theoretical Comparison

• SENSE :

• Coil arrangement flexible

• Flexible slice orientation as in MRI

• General artifacts less than SMASH

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Practical Comparison

• SENSE & SMASH performance

highly depends on human

resources devoted to R&D

• No major difference when

commercialized

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Parallel Imaging Families

• Philips : SENSE

• Siemens : iPAT

– GRAPPA + mSENSE

• General Electric : ASSET

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Parallel MRI Advantages

• Phased array coils have long been

commercialized (’94)

• Matrix inversion software simple

• Acceleration is basically pulse-

sequence independent

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SENSE Coronary Angiogram

Usual scan (3.0 T) Similar quality at 3x

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Speed Advantage of SENSE

Abdominal CE-MRA 512x512 T2W FSE

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More Advantages

• Shorten EPI acquisition time

– Less EPI geometric distortion

• Reduce ETL in FSE

– Less blurring in HASTE

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SMASH EPI of Brain (Sagittal)

Usual scan Four coils

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SENSE DW-EPI of Brain (Axial)

8 coils (R = 4) to reduce distortions (3.0T)

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SMASH HASTE of Chest (192x256)

Usual scan Four coils

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Trade Scan Time for Resolution (R = 2)

Usual scan

192x256, 450 ms

4 coils

192x256, 225 ms

4 coils

384x256, 450 ms

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Parallel MRI Advantages

• Major medical centers will have it

soon after its first introduction

• Taiwan will have it after 3-5 years

at the latest time (a matter of $$)

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The Fast Imaging

Techniques




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