How to Learn MRI
An Illustrated Workbook
Exercise 6: Artifacts in MRI
Teaching Points:
Different types of artifacts and their
characteristics:
a.
b.
c.
d.
Metal
Radio Frequency Interference
Wrap around
Chemical shift
1
Artifacts are common in MR images. For this exercise
you will learn how different kinds of artifacts are
generated and how to avoid them in clinical scans.
Typical MR artifacts include metal artifact, Radio
Frequency (RF) interference artifact, wrap around
artifact, chemical shift artifact, Gibbs’ ringing artifact
and motion artifact. Each of them has distinct
characteristics which you will learn about in this
exercise.
A. Metal Artifact
Fig 6.1 Phantom model with metal
As you know MRI is heavily dependent on the
strength, direction and uniformity of the magnetic
field. Implants and other foreign objects in or near
the scanned area that have susceptibility different
from human tissue distort the magnetic field,
changing the frequency of precession of the protons.
Susceptibility is the ability of a material to alter
the magnetic field. Metals typically have positive
susceptibility, making the field stronger. Human
tissues have negative susceptibility which makes
the field weaker. Mixing metal together with human
tissues creates magnetic field distortions which
destroy the image. This is because the frequency
and phase encoding gradients that are used to
spatially encode the region of interest are no longer
accurately mapping the data. The good thing about
metal artifact is that they distort images predictably,
such that an inverting pulse, e.g. the 1800 spin echo,
will substantially reduce the distortion.
There are a lot of things that affect the quality of
images with susceptibility artifacts. Let’s examine
these factors in this part. Find a phantom and tape a
quarter coin on the surface (see Fig6.1).
2
Step 1: Run 3-Plane localizer
Refer to previous exercises on how to run the 3-Plane localizer. You can set it up yourself by
starting a new patient and filling up all the necessary information (Fig6.2). Be sure to select Body
Coil by GE and change the plane to 3 Plane.
Fig 6.2 3-Plane Localizer sequence parameter
Patient Information
Patient Protocols
Site
Accession 0000
Number
Patient
Position Supine
Patient
Entry Feet First
Head
Patient ID 0000
Neck/Cervical
Patient Name MR PLASTIC
Coil
Chest Thoracic
Upper Extremities
Auto Start
BODY
Series 3 PLANE LOC
Description
Abodomen/Lumbar
Plane 3-Plane
Mode 2D
Pelvis
LowerExtremities
Imaging
Options
Other
Protocol
# of TE(s)
per scan
TE
1
Max
1.0
2.0
1.5
1.5
TE2
0.0
0.0
TR
5.1
5.1
Inv. Time
0
0
TI2
0
0
30
30
Flip Angle
Echo Train
Length
Bandwidth
Bandwidth2
31.2
31.2
31.2
Aquisition Timing
Freq DIR Unswap
Freq 256
Graphic RX
OFF
Phase 128
Flow Comp
Direction
NEX 1.00
Shim
Image
Users CVs
Enhance
Screen
Auto
Phase Correct
Phase
FOV 1.00
Contrast
Acqs Before
Pause
ml
Amf
Agent
Scanning Range
FOV
31.2
HIS/RIS
Protocol
Additional Parameters
Min.
Seq, Fast
Psd Name
Full
Info
Scan Timing
Grad
Mode
Pulse Seq Gradient Echo
Slice
Thickness:
29.0
4.0
Min.
Max
5
48
S/I
Start
0.0
End 5.0
# of
Slices:
L/R Center
P/A Center
0.0
0.0
5
5
5
Table
Delta
0.00
3
Step 2: Change TE in gradient echo images
When metal is present, the local magnetic susceptibility will cause phase
dispersion which leads to image artifact. In a gradient echo sequence, the phase
dispersion is proportional to TE. So a longer TE results in larger more severe metal
artifact in a GRE sequence. Let’s experiment on this.
To create your own GRE series, click New Series in Rx Manager, select INRX,
and click View Edit. Change series description to Metal GRE TE 4. Go to Pulse
Sequence. On the popup window, select Gradient Echo for Pulse Sequence Family
and GRE for Pulse Sequence then click Accept (Fig6.3). Edit the parameters as
shown in Fig6.4. Don’t forget to set the TE to 4ms
Fig 6.3 Pulse Sequence popup window
Plane :
Axial
Imaging Mode :
Pulse Seq Family
2D
Pulse Sequence
None
No Phase Wrap
Echo Planar Imaging
Fast GRE
CCOMP
Respiratory Gating/Triggering
Fast Spin Echo
Fast GRE-ET
Cardiac Gating/Triggering
Sequential
Gradient Echo
Fast SPGR
Extended Dynamic Range
Square Pixel
MNS
Fiesta
Flow Compensation
ZIP512
Propeller
GRE
Magntization Transfer
Spin Echo
MERGE
Spiral
SPGR
Vascular
PSD Name:
BRAVO
SWIFT
BREASE
T2MAP
COSMIC
TRICKS
LAVA-XV
VIBRANT
MR-Echo
Accept
4
Fig 6.4 Metal GRE TE 4 sequence parameter
Patient Information
Patient Protocols
Site
Accession 0000
Number
Patient
Position
Patient
Entry
Head
Patient ID 0000
Supine
Feet First
Neck/Cervical
Patient Name MR PLASTIC
BODY
Coil
Chest Thoracic
Upper Extremities
Auto Start
Series
Description
METAL GRE TE 4
Abodomen/Lumbar
Plane
Axial
Mode 2D
Gradient Echo
Grad
Mode
Pelvis
LowerExtremities
Imaging
Options
Other
Protocol
TE
1
4.0
TE2
TR
500.0
Inv. Time
TI2
Flip Angle
30
Min.
Max
1.0
2.0
3.8
75.0
0.0
75.0
17.0
10733.3
0
0
50
4000
1
180
Echo Train
Length
Bandwidth
Bandwidth2
15.63
2.0
2.0
31.2
15.6
HIS/RIS
Protocol
Additional Parameters
Scan Timing
None
Psd Name
Full
Info
# of TE(s)
per scan
Pulse Seq
Aquisition Timing
Freq DIR R/L
Freq 256
Graphic RX
OFF
Phase 192
Flow Comp
Direction
NEX 1.00
Shim
Image
Users CVs
Enhance
Screen
Phase
FOV
Auto
Phase Correct
1.00
Contrast
Acqs Before
Pause
ml
Amf
Agent
Scanning Range
FOV
13.0
Slice
Thickness:
11.0
Spacing
1.5
Min.
Max
5
48
S/I
Start
I36.1
End
S26.4
# of
Slices:
L/R Center
R13.5
P/A Center
P45.o
!
6
Table
Delta
0.00
Click Save Series, Download, and Scan. While scanning, create Metal GRE TE 10 and Metal GRE
TE 34, by Copying and Pasting Metal GRE TE 4 twice and changing the TE to 10ms and 34ms
respectively.
5
TE = 10ms
TE = 4ms
TE = 34ms
Fig 6.5
Metal Artifact comparison showing
susceptibility artifact from a metal
quarter increase with increasing TE.
Note as TE gets longer, the artifact
gets bigger.
The TE is critical to the artifact in gradient echo sequence. As much as possible, we want to use short TE to
get rid of metal artifact. How to do that? Try changing the following parameters and see how they affect the
minimum TE.
a) Bandwidth
b) FOV
c) Frequency matrix
Figure 6.6 shows that increased FOV, narrowed bandwidth and small frequency encoding steps result in shorter
TE. Tune these parameters accordingly to lessen the metal artifact.
40
Fig 6.6
35
FOV = 8,
FOV = 8,
FOV = 8,
FOV = 16,
FOV = 16,
FOV = 16,
30
Min TE/ms
25
Freq = 192
Freq = 256
Freq = 512
Freq = 192
Freq = 256
Freq = 512
20
15
10
5
0
0
5
10
15
20
Bandwidth KHz
25
30
35
6
Step 3: Metal artifact in a spin echo image
A spin echo sequence has a 180˚ pulse which refocuses the transverse magnetization. This
reverses the dephasing caused by susceptibility. So spin echo images have less metal artifact
than gradient echo images.
Copy and paste Metal GRE TE 10. Change series description to Metal SE TE 10. Go to Pulse
Sequence. On the popup window, select Fast Spin Echo for Pulse Sequence Family and FSE-XL
for Pulse Sequence then click Accept (Fig6.7). Edit the parameters as shown on Fig6.8. Leave the
other parameters the same and run this series.
Fig 6.7 Pulse Sequence popup window
Plane :
Axial
Imaging Mode :
Pulse Seq Family
2D
Pulse Sequence
None
Multi-Phase
Echo Planar Imaging
FRFSE-XL
ASSET
No Phase Wrap
Fast Spin Echo
FSE-IR
Blood Suppression
Respiratory Gating/Triggering
Gradient Echo
FSE-XL
Cardiac Gating/Triggering
Sequential
MNS
SSFSE
Classic
Square Pixel
Propeller
SSFSE-IR
Extended Dynamic Range
Tailored RF
Spin Echo
T1Flair
Flow Compensation
ZIP1024
Spiral
T2Flair
Vascular
PSD Name:
BRAVO
SWIFT
BREASE
T2MAP
COSMIC
TRICKS
LAVA-XV
VIBRANT
MR-Echo
Accept
Full Echo Train
7
Fig 6.8 Metal SE TE 10 sequence parameter
Patient Information
Patient Protocols
Site
Accession 0000
Number
Patient
Position
Supine
Patient
Entry
Head
Patient ID 0000
Feet First
Neck/Cervical
Patient Name MR PLASTIC
BODY
Coil
Chest Thoracic
Upper Extremities
Auto Start
Series
Description
METAL SE TE 10
Abodomen/Lumbar
Plane
Axial
Mode 2D
FSE-XL
Grad
Mode
Pelvis
LowerExtremities
Imaging
Options
Other
Protocol
TE
Min.
Max
1
1.0
2.0
10.0
3.8
75.0
0.0
75.0
17.0
10733.3
0
0
50
4000
1
180
TE2
TR
500.0
Inv. Time
TI2
Flip Angle
Echo Train
Length
4
Bandwidth
15.63
Bandwidth2
2.0
2.0
31.2
15.6
HIS/RIS
Protocol
Additional Parameters
Scan Timing
Fast
Psd Name
Full
Info
# of TE(s)
per scan
Pulse Seq
Aquisition Timing
Freq DIR R/L
Freq 256
192
Flow Comp
Direction
NEX 1.00
Shim
Phase
Graphic RX
OFF
Image
Enhance
Users CVs
Screen
Phase
FOV
Auto
Phase Correct
1.00
Contrast
Acqs Before
Pause
ml
Amf
Agent
Scanning Range
FOV
13.0
Slice
Thickness:
11.0
Spacing
Min.
Max
5
48
1.5
S/I
L/R Center
P/A Center
Start
I34.7
I6.3
P42.2
End
S27.0
I6.3
P42.2
# of
Slices:
!
6
Table
Delta
0.00
To compare the images of GRE and SE both with TE=10ms, go to Display Desktop then click
Browser. Click Metal GRE TE 10 then Viewer. Click Compare. Click Metal SE TE 10 then Viewer.
Gradient Echo (TE = 10ms)
Spin Echo( TE = 10ms)
Fig6.9 Gradient Echo, Spin Echo comparison. Note higher SNR and less
artifact with spin echo.
8
B. RF interference artifact
Have you ever wondered why we keep asking you to close the door to the magnet room tightly before you start
scanning?
Closing the door completes what is known as a Faraday cage around the magnet. This cage prevents outside RF
radiation from disturbing the magnetic fields and electrical signal within the magnet. It helps shield the magnet room
from external RF sources such as power lines, radio waves and other sources of electromagnetic interference. Keeping
this interference out of the magnet room helps increase the consistency of fields applied intentionally in the magnet;
which allows accurate sampling of the desired MR signal.
So now you are probably wondering what would happen to your images if you left that door to the magnet room
open. Let’s now start scanning with the door open.
Copy and paste the Metal GRE TE 10 then View Edit. Change series description to RF interference. Make sure there are
adequate FOV, matrix size, and bandwidth to capture the images without wrapping, ringing or chemical shift artifacts.
Click Save Series, Download, and Scan.
Door Closed
Door Open = Big RF LEak
Door Closed
Fig 6.10 Artifact from RF leaking into
room via open scanner door
!
!
9
C. Wrap around artifact (Aliasing)
This kind of artifact occurs when the scanned object is larger than the FOV in the phase encoding
direction. As a result, the area beyond the FOV is under sampled erroneously superimposed over
the anatomy in the FOV on the final image; this is also known as aliasing. In order to sample a region
without aliasing the rate at which data is sampled must follow what is known as the Nyquist theorem;
this is so that sufficient frequency information is sampled to reconstruct the image accurately.
In order to eliminate aliasing artifact we have two choices, we can increase the FOV to include all of
the objects in the scanned area or we can remove the error causing component that we do not want
in the FOV from the scanned area, such as asking our volunteer to move his arms above his head or
using No Phase Wrap sleeves to attenuate the signal from your volunteer’s arms and lessen the artifact
(Refer to exercise 5).
From now on, remove the quarter from the phantom because we don’t want the metal artifact to
interfere with other artifacts. Landmark the phantom again then close the door tightly when you
leave the room. Copy and paste the 3 Plane loc on the Rx Manager done at the beginning of this
exercise then View Edit. Make sure that the FOV includes the axial surface of the phantom (See
Fig6.11a). Click Save Series, Download, and Scan.
Copy and paste Metal GRE TE 10 on the Rx Manager then View Edit. Change the series description to
Wrap Around then go to Graphic Rx and decrease the FOV (See Fig6.11b).
10
Fig 6.11 Adjusting the FOV
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Fig 6.12 Wrap around artifact
Phase
Encoding
Phase
Encoding
Note the aliasing
Frequency Encoding
Note the aliasing in this direction
Frequency Encoding
The No Phase Wrap sleeves allow patients to put their arms on the sides
especially during abdominal scan. It prevents wrap around artifact or
overlapping on the opposite side of the image of signals outside of the
FOV. Fig6.13a shows a coronal FIESTA image with small FOV. Note the
left arm appears as aliasing on the right side. Fig6.13b shows the same
scan with sleeves. No aliasing artifact appears.
Fig 6.13 Coronal Fiesta of the Abdomen
Without Sleeves
With Sleeves
!
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12
D. Chemical shift artifact
Protons may have a naturally lower procession frequency due to the electron shielding that occurs in different
molecular structures around the proton. This shielding prevents the proton from experiencing the same applied
external magnetic field strength exactly as other protons within different chemical structures. Since MRI imaging relies
on frequency and phase encoding to determine the proper spatial frequency of the protons this causes a shift in the
final image. This chemical shift artifact can appear in several different ways depending on the severity of the shift.
The artifact can appear as a ring of light and dark bands in the frequency encoding direction around the object or as a
ghost image of the object. Fat, for example, naturally has a lower frequency for this reason and an MRI image can have
a ghost image of the fat distribution within the image. The severity of the shift depends on the receive bandwidth.
Narrowing receiver bandwidth will increase the severity of the chemical shift artifact.
Step 1: Bandwidth VS. Chemical shift
Now try axial in phase gradient echo sequence as you learned in Exercise 5. Position the patient using the cardiac coil
and do the proper landmarking. Select AXL IN PHASE found in the MRCP Protocol of the Abdomen. Click View Edit,
change series description to Chem Shift BW 4 and set the bandwidth to 4 KHz. Adjust the localizer to 5 slices and the
Acqs before pause if the Rx Scan Time is not possible for a single breath hold. Click Save Series, Download, Prescan,
give instructions for breathholding, then Scan.
Repeat the same procedure to create Chem Shift BW 16 with Bandwidth equals 16 KHz and Chem Shift BW 64 with
Bandwidth equals 64 KHz. Compare the images.
Fig6.14 shows the effects of different bandwidths. A low bandwidth of 4 KHz results in severe chemical shift artifacts
while 64 KHz gives pretty good image.
Fig 6.14 Axial In phase with different bandwidths
BW = 4(KHz)
BW = 16(KHz)
BW = 64(KHz)
13
Step 2: India ink artifact on Gradient Echo
Try axial in phase and out of phase sequence together. Actually this is shown in exercise 5. Below is a comparison of
both sequences. You can see India ink artifact in out-of-phase image. It is different from chemical shift artifact because
the width of the black line at fat-water boundaries is always one voxel..
In-phase
Out-of-phase
Fig 6.15
E. Gibbs’ artifact
This artifact appears as a series of light and dark bands emanating away from high contrast edges. This is an artifact
caused by insufficient spatial frequency data; there is not enough high frequency data to accurately define the edge.
Since Fourier data is a particular set of sinusoids that describes the final image when too few sinusoids are used to
describe the final image the edges will tend to ring as sinusoids are oscillatory functions. This occurs when the matrix
size used to image the FOV is too small; too few steps are recorded. The remedy for this artifact is to increase the
size of the matrix. This is more often a problem in the phase encoding direction, since this dimension of the matrix is
usually kept smaller than the frequency encode direction to minimize scan time. A good rule of thumb is to keep the
phase encoding direction at least half the size of the frequency encoding direction.
You can see good Gibbs’ artifact from sagittal spinal scan because it contains many distinct bright and dark
boundaries. Figure 6.16 gives a comparison of different phase encoding steps from 512 to 64.
PE = 512
PE = 256
PE = 64
PE = 128
Figure 6.16 Gibbs’ Artifact
!
Note ringing with incorrect direction of spinal cord and CSF space when using only 128 phase encode step (PE=128)
compared to a more accurate depiction at PE = 512 or 256.
!