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MRI Artifacts and Solutions
Chen Lin PhD DABR
Indiana University School of Medicine & IU Health Partners
Declaration of Conflict of Interest or Relationship
Research support from Siemens Healthcare
Type of Artifacts
• K-space Error Artifacts
• Motion and Flow Artifacts
• bSSFP Artifacts
• EPI Artifacts
• …
Chen Lin 10/2011
K-SPACE ERROR ARTIFACTS
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Truncation of k-space Data
Ringing around high contrast objects
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K-space data Overflow
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K-space DC Offset
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K-space Spikes
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RF Interference
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Ga
llag
he
r e
t a
l A
JR
20
08
; 1
90
:13
96–
14
05
Nyquist (N/2) Ghosting
• Different phase shifts in odd and even frequency encoding lines
• Correct by: – Gradient calibration and/or eddy current compensation
– Reference scan (w/o phase encoding or shifting one phase step)
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ky
kx
FOV/2
EPI
T2 Blurring and Edge Ringing
ky
kx
ky Y
I I
Multi-shot FSE/TSE
I-space Point Spread Function
FT
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K-space Intensity Profile
Blurring
Ringing/Ghosting
FSE/TSE Artifact with respect to TE
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CSE TE=30ms TSE TE=30ms EL=30 EP=10ms TSE TE=130ms EL=30 EP=10ms
ky
I
ky
I
ky
I
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MOTION ARTIFACTS
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Motion Artifacts
How to deal with motion ?
1. Minimize the amount of motion:
– Explain the procedure to the patient ahead of time and give feedback during the exam.
– Ensure comfortable position (e.g with cushions)
– Use immobilization devices (e.g. straps, …, bit bar)
– Encourage patient cooperation (e.g. Breath-hold)
– Pharmacological intervention ( e.g. Sedation, GA, O2, …)
2. Suppress signal from moving tissue/organ:
– Spatial and/or chemical shift selective saturation
– Use/select appropriate coils
– Reduced FOV imaging
Chen Lin 10/2011
How to deal with motion ? (cont’d)
3. Change / hide of motion artifact
– Swap phase and frequency directions
– Use multiple averages
– Use pseudo random k-space sampling / random view order
4. Monitor and repeat inadequate scans (with adjusted protocols)
Chen Lin 10/2011
How to deal with motion ?
3. Apply motion compensation techniques
– Motion detection
– Correction schemes
Chen Lin 10/2011
Motion Compensation Techniques
1. Motion Detection:
– Physiological signal: ECG, Pulse, Respiratory Bellow
– Direct measurement: optical
– Navigator: 1D, 2D, 3D, Spherical, …
– Extract Motion info from acquired data.
– Comparison of overlapping k-space segments
– Image registration
– Combination of the above
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Motion Compensation Techniques
2. Correction scheme:
– Prospective triggering
– Retrospective gating
– Sorting data according to the phase of periodical motion (CINE)
– Reordering of phase encoding steps
– Update spatial encoding gradients
– Correct phase error due to motion
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Retro Gating with Arrhythmia Rejection
Min. RR Max. RR
Target RR
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Prospectively Triggered blurred by arrhythmias
Retrospectively Triggered
with arrhythmias rejection
Arrhythmia Rejection
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How to deal with motion ?
5. Use motion insensitive techniques:
– Use short essential protocols and scan critical series first.
– Protocol short scans and/or split a long scan (Use end-expiration for BH consistency)
– Use faster imaging techniques: • Acquire fewer k-space points per image (e.g. parallel imaging)
• Reduce TR (e.g. GRE, high performance gradient HW, higher BW)
• Acquire more k-space points per excitation (e.g. SS-EPI, SS-FSE (HASTE), etc )
– Motion insensitive k-space sampling (e.g. Radial sampling (PR), Spiral)
– Flow/motion compensating gradient waveform (e.g. GMN)
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Single Shot DB HASTE 1 heartbeat
Low spatial resolution Low temporal resolution Less sensitive to motion
Segmented DB TSE 8 heartbeats
High spatial resolution High temporal resolution
Sensitive to arrhythmia and breathing
Segmented versus Single Shot
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Breath Hold TSE Free Breathing HASTE
TSE with MBH versus HASTE
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Cartesian k-space
Radial k-space
Radial k-space Sampling
• No phase encoding and less sensitive to motion
• Higher spatial and/or temporal resolution
• Isotropic in-plane resolution
• No phase wrap at smaller FOV
• Radial streaks artifact, more pronounced near edge FOV
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Cartesian Real-Time Cine • Echo-sharing • 50 lines • 55 ms frame rate • 300mm x 300mm FOV • 2.3mm x 6.0mm res
Radial Real-Time Cine • Interleaved Echo-sharing • 50 lines • 55 ms frame rate • 300mm x 300mm FOV • 2.3mm x 2.3mm res
Higher Resolution with Radial
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A few lines
A few lines
A few lines
all k-space Lines
Single-shot
Segmented: Acquired data over several heart beat
Single-shot: Acquire all k-space data in ONE heart
beat i.e. segments = Yres Chen Lin 10/2011
Single-Shot Setup
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ECG
Trigger
R
slice # m
slice #1
slice #2
slice #3
#2 # n #1 #3 phase
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Acquired temp res about 150ms
Effective temp res about 75ms
With iPAT temp res about 50ms
Real-time Single-Shot
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Real-time Setup
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•
Real-time
Multiple slices
Free breathing
Lower resolution
Segmented View-Shared
Only 1 slice
8 sec breath-hold
Higher resolution
Real-time Single-shot
• Can be used non-triggered and non breath hold with multiple slices.
• Less temporal and spatial resolution than segmented view-shared. Chen Lin 10/2011
Real-time TrueFISP Example
• 7 short-axis cine slices covering from base to apex of heart, with matrix 70*128.
• After triggering, and all k-space views for each slice were acquired in rapid succession.
• All 7 slices in 14 heartbeats and single breath-hold.
In non-triggered mode, the number of phases define how long the scan will run.
1 2 3 4 5 6 7
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Retrospective Motion Correction
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Flow Artifact
Flow Artifact Correction
• Spatial pre-saturation pulses prior to entry of the vessel into the slices
• Motion Compensation Gradients
• Cardiac & respiratory gating
• Surface coil localization
SAT Bands
Imaging volume/slice
BSSFP OFF-RESONANCE ARTIFACTS
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Bright Blood Sequences
Gslice
Gphase
Gread
TR a a
TR -a a
Gslice
Gphase
Gread
SSFP
(FLASH)
Balanced SSFP
(TrueFISP) Chen Lin 10/2011
Un-spoiled GRE (SSFP)
SSFP-FID: SSFP-ECHO:
• Echo formed from un-spoiled Mxy of previous TR
• T2 weighted
Chen Lin, PhD 11/09
TR
a a
Gslice
TR
a a TR
Off-resonance Effect
To Reduce phase accumulation between excitation:
– Improve B0 homogeneity or shift center frequency
– Reduce TR Chen Lin 10/2011
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r J R
adio
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an
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bSSFP/TruFi Banding
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-50Hz
-100Hz
50Hz
100Hz
Local Shim
CISS – Constructive Interference Steady State
Chen Lin, PhD 10/2010
TR -a a
Gslice
Gphase
Gread
+a a MIP
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D co
chlea p
roto
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EPI ARTIFACTS
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Breast DWI
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N/2 Ghosting
Chemical shift
Distortion
Artifacts in EPI
More severe at higher field strength
Geometric Distortion in EPI
• Phase error accumulates in the echo train.
• Minimized with fewer echoes (ETL) and/or less echo spacing (ESP)
• Retrospective correction with measured B0 map Chen Lin 10/2011
Gphase
Gread
B0 inhomogeneity introduces local gradients
How to reduce Echo SPacing (ESP)
• High rBW (Faster sampling rate)
• Lower read resolution (Less frequency encoding points)
• Ramp Sampling
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How to reduce echo train length (ETL)
• Reduce phase resolution or phase FOV (Less phase encoding steps)
• Partial Phase Fourier
• Parallel Imaging
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ky
kx
Multi-shot EPI (MS-EPI)
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ky
kx
ky
kx
Single-shot EPI (SS-EPI) with low resolution in Y direction
Multi-shot EPI (MS-EPI) with higher resolution
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Examples with varying ESP & ETL
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rBW=752Hz/px p=None
rBW=1502Hz/px p=None
rBW=752Hz/px p=2
rBW=1502Hz/px p=2
Chemical Shift Artifact in EPI
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With Fat Suppression Without Fat Suppression
Other Artifact in EPI
• “Blurring”
– Due to T2* induced signal modulation in k-space.
– May not be obvious due to low spatial resolution and other artifacts.
• “Dark Spots”
– Intra-voxel de-phasing to due to off resonance.
– Reduce voxel size, i.e. slice thickness.
– SE-EPI.
Chen Lin 10/2011
What are the artifacts?
Chen Lin 10/2011
Thank You !
www.indiana.edu/~mri
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