Post on 23-Mar-2020
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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
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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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
echo 1 echo 2 echo 3 ...
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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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k-space Filling Pattern in FSE
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
echo 1 echo 2 echo 3 ...
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
echo 1 echo 2 echo 3 ...
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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Single-shot FSE Sequence
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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Single-shot FSE Sequence
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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k-space Filling Pattern in FSE
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
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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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)
Usual scan (22 sec) 4 coils (11 sec)
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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
Usual scan (10 sec) 3 coils (5 sec)
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Aliasing in SMASH with R = 2
Usual scan (22 sec) 4 coils (11 sec)
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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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Aliased Image from Coil 1
Coil #1 local intensity
FOV
aliasing
(weaker)
aliasing
(even weaker)
1
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Aliased Image from Coil 1
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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Aliased Image from Coil 2
Coil #2 local intensity image
aliasing
(weaker)
aliasing
(weaker)
2
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Aliased Image from Coil 3
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)
Usual scan (170 sec) 2 coils (85 sec)
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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
Hsiao-Wen Chung (鍾孝文), Ph.D., Professor
Dept. Electrical Engineering, National Taiwan Univ.
Dept. Radiology, Tri-Service General Hospital