11/7/2011

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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

Chen Lin 10/2011

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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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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FOV/2

EPI

T2 Blurring and Edge Ringing

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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

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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

Chen Lin 10/2011

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)

Chen Lin 10/2011

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

Chen Lin 10/2011

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

Chen Lin 10/2011

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

Chen Lin 10/2011

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

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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

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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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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

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Gphase

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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

Chen Lin 10/2011

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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Multi-shot EPI (MS-EPI)

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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

Chen Lin 10/2011

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