mri image artifacts and their remedies1 of 160 mri image artifacts and their remedies hsiao-wen...

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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

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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

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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

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

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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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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”

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2D Gradient Echo Sequence

Different timing for two spatial encodings

z gradient

RF (B1) t

y gradient t

x gradient t ...

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Displacement Artifacts

Displaced vessels especially for oblique ones

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

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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

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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

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Aliasing Artifacts

FOV too small BW increased

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MRI Image Formation

• Magnetization

• RF excitation

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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

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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

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)

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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

Gs t ...

Gr t ...

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Adding Other Slices …

Making use of the idle time

Gs t ...

Gr t ...

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… More Slices Added

Multi-slice imaging (scan time not lengthened)

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

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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

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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

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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and Fat Suppression

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Chemical Shift Effects

• Water and fat protons inherently

have slightly different resonance

frequencies

– Shielding effects from electron

– 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

protons -CH2-

protons

from fat

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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 ??

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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

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• 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

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

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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In-phase to out-of-phase every 2.27 msec

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In-phase Out-of-phase

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The Annoying Fat

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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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• 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

y gradient t

x gradient t ...

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2D Spin Echo Sequence

No chemical shift dephasing

z gradient

RF (B1) t

y gradient t

x gradient t ...

900 1800

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The Annoying Fat

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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

900 fat only

spin-echo fat-SAT

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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

RF = 900

RF < 900

after spoiling

with strong gradient

residual

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Spectra Before & After Shimming

H2O fat

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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

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

spin-echo IR

matter

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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

gray matter

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

gray matter

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

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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

mri image artifacts and their remedies1 of 160 mri image artifacts and their remedies hsiao-wen...

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