mri image artifacts and their remedies1 of 160 mri image artifacts and their remedies hsiao-wen...
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1 of 160
MRI Image Artifacts
and their Remedies
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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Example of MRI Artifacts
Expected image Motion ghosts
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MRI Artifacts
• Images find something that actually
does not exist in the patient
• Images do not find things that
actually exist in the patient
• MRI is know to contain quite a lot of
different artifacts
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Why Artifacts ?
• Retina receive photic stimulations
• Films receive photic exposure
• Anything that can get films exposed
is shown on the photograph and
seen by humans
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Why MRI Artifacts ?
• Complicated MRI image formation
• Factors unrelated to physiological
conditions but affecting image
formation can become visible on the
resulting MRI
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MRI Image Formation
• Magnetization
• RF excitation
• Spatial encoding repeat N times
• Signal receiving
• Image calculation
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MRI Image Formation
• Magnetization
• RF excitation
• Spatial encoding repeat N times
• Signal receiving
• Image calculation
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Artifacts from B0 or B1
• Nonuniform magnetization or RF
excitation = nonuniform signal
• Rarely seen in modern clinical MRI
– Surface coil receiving profile is
widely known to be nonuniform
and hence not treated as artifacts
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Comparison of Different RF Coils
Body coil Head coil 3-in surface
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One RF-related Artifact
• Flow void (in spin-echo)
• 900 excitation and 1800 refocusing
pulses are at different times
• Out-of-slice flow causes dislocation
and signal loss
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2D Spin Echo Sequence
Different timing for 900 and 1800 pulses
z gradient
RF (B1) t
t
y gradient t
x gradient t ...
900 1800
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Flow-Void in Spin-echo
• 900 – 1800 – signal receiving
• Timing difference
• Incomplete refocusing due to
flowing blood out of the slice
• Black-blood images (flow void)
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Bright-blood & Black-blood Images
Gradient-echo Spin-echo
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MRI Image Formation
• Magnetization
• RF excitation
• Spatial encoding repeat N times
• Signal receiving
• Image calculation
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Spatial Encoding Artifact
• Gradient for spatial encoding
• Local magnetic field strength
= frequency = location
• Frequency variations = location
misregistration
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Chemical-shift Artifacts
• Fat and water protons have
inherently different resonance
frequencies
• Location misregistration
• Will be addressed later today
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Chemical-shift Artifacts
R-L freq encoding A-P freq encoding
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RF Leakage
• EM wave interference from outside
– Larmor freq in FM band for radio
• Usually isolated by RF shielding
• Opened scan room door could result
in RF leakage
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RF Leakage
Zipper artifacts along phase direction
A-P : freq encoding
S-I : phase encoding
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Zipper Artifacts
• Zipper artifacts not necessarily
caused by RF leakage !
• Un-encoded stimulated echo ? It
gets complicated
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Blood Flow Artifacts
• Flow-related enhancement
– Detailed in a future lecture
• Flow void (spin-echo)
• Displacement artifacts
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Two Encoding Directions
• Phase and frequency encoding occur
at different times
• Blood flows to another location
before frequency encoding ?
• The vessels seem to be “displaced”
• Displacement artifacts
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2D Gradient Echo Sequence
Different timing for two spatial encodings
z gradient
RF (B1) t
t
y gradient t
x gradient t ...
< 900
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Displacement Artifacts
Displaced vessels especially for oblique ones
Note:
MRI displacement
artifacts occur for
in-plane flow
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Geometric Distortions
• Large range B0 inhomogeneity
• Resonance frequency changed
location mis-mapped
– Also sampling frequency related
– Particularly severe for EPI along
phase direction
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Large Range B0 Inhomogeneity
Shifting of resonance frequency locally
Image voxel :
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B0 Geometric Distortions
Arrow-head pattern along freq direction
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Distortions from Susceptibility
Distortion related to readout bandwidth
Round glass tubes
(diamagnetic)
CuSO4 solution
(paramagnetic)
1.5 Tesla
Gradient echo
Arrow-head pattern
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Severe Distortions from Iron Hair Pin
Spin echo Gradient echo
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MRI Image Formation
• Magnetization
• RF excitation
• Spatial encoding repeat N times
• Signal receiving
• Image calculation
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Signal Receiver Gain
• Receiver magnifies the weak
(~uV) MR signals
• Receiver gain often adjusted
automatically for clinical MRI
• Too high or too low artifacts
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Receiver Gain Artifacts
Desired normal image Receiver gain too high
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Aliasing Artifacts
• Sampling frequency too low
– High frequency mistaken as low
– “Front” mistaken as “back”
• Increasing sampling frequency
solves the problem
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Frequency Encoding
Freq = location; Amplitude = proton density
Bo
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The MRI Signal Received (Echo)
Signals = sum of all frequencies
+ + + + =
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The Sampling Process
One data point every fixed interval
. . .
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Sampling Too Slow : Freq Mistaken
+ + + +
=
. . .
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Mistaken of Location
Bo
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Aliasing Artifacts
FOV too small BW increased
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MRI Image Formation
• Magnetization
• RF excitation
• Spatial encoding repeat N times
• Signal receiving
• Image calculation
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Motion Ghosts
• Nonuniform signal intensity for each
repetition due to (periodic) motion
• k-space signal “modulated” by motion
• Modulation “frequency” reflected in image
“location”
• Multiple objects appear
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Pulse Sequence and k-space
RF
Gz
Gy
Gx
t
t
t
t
kx
ky
TR
TR
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Breathing Motion & Signal Modulation
Signal intensity changed by motion
Image slice location
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Signal intensity in k-space
kx
ky
Signal stronger ...
weaker ...
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Ghosts from Periodic Motion
Motion ghosts Respiratory gating
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The Modulation Pattern
• Periodic motion : clear ghosts
– Single frequencies in k-space
• Aperiodic motion : multiple
overlapped (blurred) ghosts
– Too many frequencies in k-space
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Ghosts from Respiratory Motion
Motion ghosts Respiratory gating
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Ghosts from Cardiac Motion
Motion ghosts ECG gating
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Artifacts from Aperiodic Motion
Desired image Motion ghosts
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Artifacts from Periodic Motion
Pulsation from abdominal aorta
Note :
1. Ghosts only for the
moving tissues
2. Always along phase
direction
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Motion Ghosts from CSF Flow
No Flow Comp Flow Comp
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Remedies
• Synchronized detection
– Respiratory or ECG gating
• Faster scan + breath-hold
• Ultrafast scanning
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Blood Flow Artifacts
• Flow-related enhancement
– Detailed in a future lecture
• Flow void (spin-echo)
• Displacement artifacts
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Flow-Related Enhancement
Neck Abdomen
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Cross Talk
• Frequently encountered in multi-angle
multi-slice imaging
• Some tissues excited multiple times within
one single TR
– TR effectively shortened low signal
• Also seen in contiguous-slice acquisition
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Single-slice Pulse Sequence Expanded
TR >> TE : scanner mostly idle
Gp
B1 t
t
...
...
Gs t ...
Gr t ...
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Adding Other Slices …
Making use of the idle time
Gp
B1 t
t
...
...
Gs t ...
Gr t ...
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… More Slices Added
Multi-slice imaging (scan time not lengthened)
Gp
B1 t
t
...
...
Gs t ...
Gr t ...
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Cross Talk for Multi-Angle Imaging
Excited by both slices
TR ~ halved
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Cross Talk
Lumbar image Sacrum image
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MRI Image Formation
• Magnetization
• RF excitation
• Spatial encoding repeat N times
• Signal receiving
• Image calculation
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k-space Discontinuity
• Image reconstruction = Fourier
transform = Combination using
sine & cosine waveforms
• Abrupt discontinuity = ringing
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Artifacts from Minor Data Error
k-space MR image
Value too large
(kx = - 29, ky = - 41)
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Remedies
• Minor error anyway
• Replaced using neighboring data
• Reconstructed images often very
satisfactory
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Another Type of k-space Discontinuity
Full k-space No data in outer k-space
discontinuity
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Gibb’s Ringing (Truncation Artifacts)
256x256 256x128
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Gibb’s Ringing
• Also called truncation artifacts
• k-space discontinuity due to data
omission
• Filling the k-space with more
data removes the artifacts
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Gibb’s Ringing (Truncation Artifacts)
256x128 256x192 256x256
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Syringomyelia (hole in spinal cord) ?
256x128 256x256
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MRI Image Formation
• Magnetization
• RF excitation
• Spatial encoding repeat N times
• Signal receiving
• Image calculation
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Not Only The Above !
• Image composed of “pixels”
• Huge number of protons in one pixel
• Signal = vector sum from all proton
magnetization
• What if inconsistent behavior ?
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Dephasing
• Protons = magnetization vectors
• Magnetization vectors oriented in all
different directions = 0 vector sum
– Reduced signals (dark)
• Intra-voxel phase dispersion
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Effects of TE on Dephasing
TE = 9 msec TE = 18 msec
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B0 Inhomogeneity
• Short T2* : Signal loss in gradient-echo
– Spin-echo unaffected due to 1800
refocusing pulse
– Signal loss more severe with long TE
– Also related to image resolution
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Air-Tissue Interface
Air distorts surrounding magnetic flux lines
Tissue :
diamagnetic
Air :
paramagnetic
Tissue :
diamagnetic
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Small Range Susceptibility Effect
B0 inhomogeneous in one voxel short T2*
Image voxel :
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Effects of Slice Thickness
B0 inhomogeneity is related to resolution
paramagnetic
Thin slice
Thick slice
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Effects of Slice Thickness
3 mm 5 mm 10 mm
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T2* Signal Loss in Hematoma
PDWI T2WI
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T2* Signal Loss in Hemorrhage
T1 PD T2 GrE
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Chemical-shift Dephasing
• Fat and water protons have inherent
different resonance frequencies
• 1.5 Tesla 220 Hz
– In-phase to out-phase every 2.27 msec
• Voxels containing both water fat have
variable signal intensities
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Chemical-shift Signal Loss
In-phase Out-phase
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Chemical-shift Artifacts
and Fat Suppression
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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Chemical Shift Effects
• Water and fat protons inherently
have slightly different resonance
frequencies
– Shielding effects from electron
cloud
– Difference in electron negativity
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Chemical Shift Phenomena
Shielding effects from electron cloud
O H C H
Oxygen nucleus attracts electron cloud,
reducing shielding effects on hydrogen nucleus
Carbon nucleus (methyl protons):
less electron negativity
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Proton MR Spectrum from Leg
Water & fat proton frequencies differ by 3.5 ppm
H2O
protons -CH2-
protons
from fat
ppm
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Chemical Shift Artifacts
• Spatial encoding in MRI :
– Field = frequency = location
• Water and fat protons have different
resonance frequencies
• Inherent location mis-registration
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Location = Magnetic Field = Frequency
Different frequencies imply different locations ??
water
fat
Bo
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Chemical Shift Artifacts
R-L freq encoding A-P freq encoding
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The Annoying Fat
• Inconsistent location with water tissues
• Inconsistent intensity in gradient-echo
• Fat is usually bright, obscuring lesion
– Short T1, moderate T2
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How Far Is It Displaced ?
• Qualitative description :
• Sampling needed before Fourier transform
• Sampling takes time
• Frequency difference more prominent as
time gets longer
• Displacement related to sampling speed
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How Far Is Artifact Displaced ?
Fast sampling,
frequency difference
less obvious
Slow sampling,
frequency difference
more obvious
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Quantitative Calculation
• Concept of “bandwidth per pixel” :
• Echo Fourier-transformed to form image
• Image = spectrum of echo
• A pixel occupies a spectral “interval”
– Bandwidth per pixel
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Image = Spectrum of Echo Signal
Every pixel occupies a “bandwidth”
Fourier
transform
time-varying
echo
pixel
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Chemical Shift Artifacts
• Displacement in number of pixels =
220 Hz / BW per pixel
– 3.5 ppm @ 1.5T = 220 Hz
• 32 KHz sampling freq ~ 125 Hz/pixel
• 220 / 125 ~ 2 pixels
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Fat Artifacts & Sampling Frequency
32 Hz/pixel 64Hz/pixel 128 Hz/pixel
Also note SNR difference !
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Fat Displacement & SNR
• High sampling freq low SNR
• Low sampling freq large
chemical shift displacement
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Freq & Phase Encoding
• No chemical shift displacement along the
phase encoding direction
• Identical time interval between RF
excitation to sampling
• Chemical shift artifacts occur only along
the freq encoding direction
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No Chemical Shift Artifact along Phase
t
t
t ...
RF
Gp
Gr
t
t ...
Gr
Gp
t RF
Identical time interval
between RF excitation
and readout
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EPI Is an Exception !
• EPI : continual sampling after one RF
• Long interval between phase encodings
• Severe chemical shift displacement along
phase direction for EPI
• Freq direction: less than one pixel
displacement due to very fast sampling
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EPI : Continual Sampling after One RF
RF
Gz
Gy
Gx
t
t
t
t
kx
ky
Large time interval between phase encodings
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EPI Fat Suppression
No Fat Sat With Fat Sat
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What Artifact Is This?
Soul out of skull ?
Answer :
Chemical shift
(fat) dislocation
artifact in EPI
Phase encoding
direction : A/P
Too severe that
EPI must use fat
suppression
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Likewise …
• Extremely severe fat artifact in EPI
• All other off-resonance artifacts get
“magnified” in EPI as well
– Susceptibility-induced geometric
distortion
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Distortions from Susceptibility
Distortion made visible using low bandwidth
Round glass tubes
(diamagnetic)
CuSO4 solution
(paramagnetic)
1.5 Tesla
Gradient echo
Arrow-head pattern
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Severe Distortions Using Iron Hair Pin
Spin echo Gradient echo
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But in EPI, no need for magnification
TSE T2WI EPI T2WI
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EPI Geometric Distortions
• Especially severe near skull base
– Ear, nose (air) …
• Stringent shimming requirement
• EPI less used outside brain
(although still some …)
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Not Only The Above !
• Image composed of “pixels”
• Huge number of protons in one pixel
• Signal = vector sum from all proton
magnetization
• What if inconsistent behavior ?
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Dephasing
• Protons = magnetization vectors
• Magnetization vectors oriented in all
different directions = 0 vector sum
– Reduced signals (dark)
• Intra-voxel phase dispersion
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Chemical Shift Dephasing
• Inherently different resonance freq
• 3.5 ppm 220 Hz at 1.5 Tesla
– In-phase changed to out-of-phase
every 2.27 msec
• Pixels having both fat and water show
varying signal intensity
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Chemical Shift Dephasing
In-phase to out-of-phase every 2.27 msec
z'
y' x'
z' z'
fat
water
RF t
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Chemical Shift Dephasing
In-phase Out-of-phase
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The Annoying Fat
• Inconsistent location with water tissues
• Inconsistent intensity in gradient-echo
• Fat is usually bright, obscuring lesion
– Short T1, moderate T2
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Signal Intensity vs. TE (Gradient Echo)
TE = 5 TE = 7 TE = 9 TE = 11
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Similar Behavior in Vertebral Bodies
Red marrow ~ water Yellow marrow ~ fat
TE = 9 msec TE = 10 msec TE = 11 msec TE = 12 msec TE = 13 msec
TE = 14 msec TE = 15 msec TE = 16 msec TE = 17 msec TE = 18 msec
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Note
• Chemical shift dephasing occurs in
gradient-echo images only
• Signal varies as a function of TE
• Spin-echo images unaffected due to
the refocusing 1800 pulse
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2D Gradient Echo Sequence
Chemical shift dephasing is present
z gradient
RF (B1) t
t
y gradient t
x gradient t ...
< 900
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2D Spin Echo Sequence
No chemical shift dephasing
z gradient
RF (B1) t
t
y gradient t
x gradient t ...
900 1800
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The Annoying Fat
• Inconsistent location with water tissues
• Inconsistent intensity in gradient-echo
• Fat is usually bright, obscuring lesion
– Short T1, moderate T2
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The Solution
• Why not just suppressing fat signal ?
– Fat suppression (Fat SAT)
• Comparison with before-suppression
provides diagnostic information
– Fatty or water-based mass?
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Comparison of Fat Suppression
Bone lesion clearly depicted after fat-SAT
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How to Suppress Fat ?
• Making use of the artifact origin
– Difference in frequencies
– Short T1 of fat
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CHESS Fat Suppression
• Chemical shift selective
• RF pulse to excite fat only without
touching water protons
• Use strong gradient to “spoil” the signal
• Perform imaging afterwards
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CHESS Principles
water
fat
900 fat only
RF
Gz
Gy
Gx
t
t
t
t
spin-echo fat-SAT
ppm
ppm
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Spin-Echo Fat Suppression
No fat sat With fat sat
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1-3-3-1 CHESS Pulse
• Simple means for frequency-
selective excitation
• Used in spatial modulation MRI
(SPAMM) for cardiac imaging
• You’ll see it in your homework set …
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Fat-SAT & Water-SAT
• Exactly the same principle
• Excite water without touching fat
• Strong gradient to “spoil” the excited
water signal
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Comparison of Fat-SAT & Water-SAT
Spin-echo Fat-SAT Water-SAT
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B0 & B1 Requirements
• Uniform B1 for complete saturation
– Good volume excitation RF coil
• Uniform B0 for consistent frequency
– Shimming before imaging
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Effects of RF Homogeneity
z'
y' x'
z'
y' x'
RF = 900
RF < 900
z'
z'
after spoiling
with strong gradient
residual
fat
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Spectra Before & After Shimming
water
H2O fat
-CH2-
ppm
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B0 Homogeneity Needed
• Fat-SAT according to frequency
– Frequency directly related to B0
• Air & tissue have different susceptibilities
– Incomplete fat-SAT very frequently
encountered near sinus
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Dixon (In- & Out-phase)
• Again using frequency difference
• Adjust TE to obtain in-phase (water+fat) &
out-phase (water-fat) gradient-echo
– Water image = in-phase + out-phase
– Fat image = in-phase – out-phase
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Dixon Method
z'
y' x'
z' z'
fat
water
RF t
Image #1 = water - fat
Image #2 = water + fat
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Remember Chemical Shift Dephasing ?
In-phase Out-of-phase
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The Dixon Method
• B0 homogeneity required as well
• Needs in- and out-phase images
twice scan time
– unless sampling within 2.27 msec
• Called in- (out-) phase clinically
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Water & Fat Images After Calculation
In-phase water image fat image
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Another Example in Abdomen
Out-phase water image fat image
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Multi-point Dixon Method
• If other factors interfere B0
– More variables come into play
• Multiple images at different TEs
– E.g. water + fat + B0 map
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How to Suppress Fat ?
• Making use of the artifact origin
– Difference in frequencies
– Short T1 of fat
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Short TI Inversion Recovery
• STIR imaging
• 1800 inversion (sounds familiar ?)
• Wait for T1 relaxation (TI : inversion time)
• Imaging started when fat passes null point
• TI ~ about 160 msec at 1.5T
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STIR Principles
TI
RF
Gz
Gy
Gx
t
t
t
t
spin-echo IR
fat
gray
matter
CSF
z'
y' x'
z'
y' x'
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STIR Fat-Sat Image (Lipoma)
T1-weighted image STIR image
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STIR & FLAIR Principles
Adjust TI to suppress different tissues
fat
gray matter
CSF
TI (STIR)
TI (FLAIR)
fluid attenuated inversion recovery
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FLAIR Depicting Lesion (Infarction)
T1WI T2WI FLAIR
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STIR Properties
• Homogeneous B0 not required
– T1 not a strong function of B0
• Image shows some T1 influence
• Inverse T1 weighting
– Opposite to common short-TR T1-
weighted images
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T1-weighting in STIR
• Long T1 tissues : bright
Short T1 tissues : dark
• CSF > gray matter > white matter
• Contrary to common spin-echo T1-
weighted images
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Why Inverse T1 weighting ?
CSF signal larger than gray matter at short TI
fat
gray matter
CSF
TI
Long T1 tissues not decayed much yet
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STIR Fat-sat Spin-Echo
Inversion recovery for fat-SAT
1.5 Tesla
IR Spin-echo
TI = 150 msec
TR = 2000
TE = 20
Periorbital fat is
suppressed.
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Fat-SAT Comparison
• CHESS : homogeneous B0 & B1
• Dixon : twice scan time (?)
• STIR : contrast altered
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Conventional Fat-SAT
• Suppression “preparation”
• Imaging follows suppression
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Special RF Pulses
• Shinnar-LaRoux RF pulse design
• k-space RF pulse design
• One RF to achieve > two purposes
• Example: simultaneous spatial & spectral
selection
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Spatial & Spectral Selection
Cost : 20 ~ 40 msec duration just for the RF
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MRI Image Formation
• Magnetization
• RF excitation
• Spatial encoding repeat N times
• Signal receiving
• Image calculation
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Other MRI Artifacts ?
• Come on ! Plenty of them !
• Many artifacts show no obvious origin or
remedies
– Just like human diseases
• What should I do if encountering strange
artifacts ?
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What Artifacts ?
Why do I get this ? Why do I get this ?
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What Artifacts ?
Thick water waves ? Thin water waves ?
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Artifacts Rule-of-Thumb
• Make sure reproducibility first
• Reasoning + adjusting parameters +
experiments, then back again
• Cost : Time + efforts
• Blame the manufacturer ?? Perhaps
never solved …
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MRI Image Artifacts
and their Remedies
Hsiao-Wen Chung (鍾孝文), Ph.D., Professor
Dept. Electrical Engineering, National Taiwan Univ.
Dept. Radiology, Tri-Service General Hospital