Fundamentals Behind the 10 Most Common Magnetic Resonance Imaging Artifacts with Correction ...

78 Pages·2011·6.55 MB·English

Fundamentals Behind the 10 Most Common Magnetic
Resonance Imaging Artifacts with Correction Strategies and
10 High-Yield Points
Poster No.: C-1248
Congress: ECR 2011
Type: Educational Exhibit
Authors: R. Javan, J. R. O'Rear, J. E. Machin; Memphis, TN/US
Keywords: Physics in radiology, MR
DOI: 10.1594/ecr2011/C-1248
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Learning objectives
The purpose of this exhibit is to describe the most common Magnetic Resonance Imaging
(MRI) artifacts under 10 categories, geared towards enhancing fundamental radiologic
physics knowledge, along with strategies for reducing these artifacts when possible. A set
of top 10 take-home high-yield points are provided in the end. Some images are obtained
from our institution's PACS, some from peer-reviewed journals with proper permission
obtained and some from educational web tutorials. The content is categorized in the
following fashion;
1. Magnetism (a. Magnetic Susceptibility, b. Main Magnetic Field
Inhomogeneity, c. Anatomic Distortion or Bending due to Gradient
Nonlinearity, d. Standing Wave)
2. Resonance Frequency (a. Chemical Shift, b."The 2nd Kind" or Black
Boundary)
3. Motion (a. Voluntary Patient Motion, b. Cardiac and Respiratory Cycle, c.
Blood and CSF Flow, d. Flow Enhancement or Entry Slice)
4. Voxel (a. Partial-Volume or Averaging, b. Cross-Talk or Cross-Excitation or
Slice-overlap)
5. K-Space (a. Spike or Herringbone, b. Zero Fill-In or at times called Zebra, c.
N/2 Ghost and Segmental K-Space)
6. Sampling (a. Wraparound or Aliasing, b. Truncation or Gibbs Ringing)
7. Coil (a. Inadequate Shimming, b. Improper Distance From the Skin,
c. Phased Array Coil Malfunction, d. Central Point or DC Offset and
Quadrature Ghost)
8. Radiofrequency (a. RF Noise or Zipper, b. RF Overflow)
9. Moire Fringes or Zebra
10. Magic Angle
Background
1. MAGNETISM
a. Magnetic Susceptibility: This results from differences in the field at interfaces
of different susceptibilities seen greatest surrounding ferromagnetic objects inside
diamagnetic materials; these gradients cause dephasing and frequency shifts of
surrounding tissues creating bright and dark areas (Fig. 1 on page 16) with spatial
distortion; greatest with long echo times and gradient echo sequences. It can be seen
due to dental fillings (Fig. 2 on page 16), deposited hemosiderin or even mascara on
eyelids (Fig. 3 on page 17). This effect can be minimized by a large receiver bandwidth
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and a decreased echo time. Fast spin-echo acquisitions with a high bandwidth typically
work well. The MARS technique is used in MSK radiology for periprosthetic evaluation
by RF increase, narrow section thickness, tilted view angle, and increased read gradient.
Fig.: Magnetic Susceptibility seen as high intensity areas with echo-planar imaging
technique in diffusion weighted images, due to abrupt change at interfaces of different
susceptibilities.
References: R. Javan; Radiology, Baptist Memorial Hospital, Memphis, UNITED
STATES OF AMERICA
b. Main Magnetic Field Inhomogeneity: There are inherent small main magnetic field
non-uniformities, which are essentially imperfections. Magnetic shimming is done to
reduce this effect. (See Coils)
c. Anatomic Distortion or Bending: This occurse due to gradient non-linearity
especially seen at the edges of the gradient (Fig. 4 on page 18, 5 on page 19).
These may be fixed by algorithms provided by the vendor.
d. Standing Wave: Occurrs with 3T magnets and is often incorrectly called a "dielectric
resonance" effect. Strong signal variations across an image can be seen, especially
brightening or dark "holes" in regions away from the receive coil. These are more
pronounced in obese patients with a distended abdomen, during pregnancy (Fig. 6
on page 20) or patients with ascites. Therefore, ultrahigh-field MR should no tbe
performed in patients with large ascites or during pregnancy.
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Fig.: Severe standing wave artifact in pregnant woman during fetal ultrahigh-field-
strength MRI at 3.0 T shows marked signal loss (long arrows). Short thin arrows mark
placenta; thick arrows mark fetal torso.
References: Merkle EM, Dale BM. "Abdominal MRI at 3.0 T: The Basics Revisited".
AJR 2006; 186:1524-1532
2. RESONANCE FREQUENCY
a. Chemical Shift: Fat containing structures are shifted in the frequency direction from
their true positions, due to differences in resonance frequencies. In the spine, this causes
one end plate to appear thicker than the opposite one; in the abdomen and orbits, this
causes a black border at one fat-water interface and a bright border at the opposite side
(Fig. 7 on page 21, 8 on page 22). It is greater at high main magnetic fields and low
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gradient strengths. Correction may be performed by using a fat suppression technique
or use a wide receiver bandwidth (e.g., +32 KHz)
b."The 2nd Kind" or Black Boundary: If a pixel is shared by tissues (e.g. fat and water)
with different resonant frequencies, the selection of certain TEs picks a point at which
they are out of phase and thus their spins are cancelled (Fig. 9 on page 23, 10 on
page 24). Therefore, avoid multiples of the TEs where they cancel each other. This
artifact at times is used for diagnostic purposes, such as in adrenal or liver imaging for
tissue characterization.
Fig.: Chemical Shift of "the 2nd kind" seen as black line along the interface of fat and
water.
References: R. Javan; Radiology, Baptist Memorial Hospital, Memphis, UNITED
STATES OF AMERICA
3. MOTION
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a. Voluntary Patient Motion: Ghost and blur appearance is seen (Fig. 11a on page
25). Coil may also move. It is very important to educate patient. Sedation may be
necessary, especially in children.
Fig.: Motion artifacts - on the left image, ghost lines seen anterior to the abdominal
wall due to breathing; on the right image, bright circles anterior to the aorta are seen
representing blood pulsation artifacts.
References: Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR
Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26:275-297.
b. Cardiac and Respiratory Cycle: These tend to be more prominent in the phase
direction; they can be reduced with presaturation pulses outside of FOV, swapping PEG
and FEG, using Propeller acquisition (Fig. 12 on page 26) (oversampling k-space
center), gradient moment nulling to rephase moving spins, and gated techniques (Fig.
13 on page 27).
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Fig.: Placement of the navigator section for respiratory motion compensation. Left
image with aqua overlay shows the navigator section from which the displacement
information is obtained to determine the diaphragmatic position. Graph shows
diagphragmatic movement, indicated by the white wave and green line. The yellow
boxes represent the best time to image ("window of opportunity").
References: Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR
Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26:275-297.
c. Blood and CSF Flow: Blood pulsation artifact may be seen in aorta (Fig. 11b on
page 25), internal carotid artery or any vascular structure with pulsatile flow. In CSF
this artifact can mimic intradural lesion and may be reduced by flow compensation and
presaturation.
Fig.: CSF flow artifact secondary to unsaturated protons moving randomly, seen as
dark signal areas, which may be mistaken as intradural lesions, such as AVMs.
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References: R. Javan; Radiology, Baptist Memorial Hospital, Memphis, UNITED
STATES OF AMERICA
d. Flow Enhancement or Entry Slice :Normally, flow voids (Fig. 14 on page 28) are
seen in vessels. But this artifact may mimic vessel occlusion and manifest as a bright
signal in the first slice that the vessel enters (usually seen on several slices and fades with
distance). It can be reduced with use of gradient echo flow technique and presaturation.
4. VOXEL
a. Partial-Volume or Averaging: Caused by voxel containing two different tissues and
therefore a signal average of both tissues (Fig. 15 on page 28). This can lead to
diagnostic misinterpretations. It can be corrected by obtaining thinner slices, which can
compromise SNR.
Fig.: Partial volume averaging artifact, left image obtained at 10mm thickness does not
show the VII and VIII cranial nerves while the left image does, obtained at 3 mm.
References: R. Javan; Radiology, Baptist Memorial Hospital, Memphis, UNITED
STATES OF AMERICA
b. Cross-Talk or Cross-Excitation or Slice-overlap: Occurs with multi-angle multi-slice
acquisition when there's overlap such that images include areas where the spins have
already been saturated resulting in a band of signal loss horizontally and usually greatest
posteriorly - as long as the saturated area is posterior to the spinal canal there is no harm.
This artifact looks like a big black stripe in the image (Fig. 16 on page 29). This is
corrected by continuous parallel imaging.
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Fig.: Cross-talk artifact seen in the lumbar spine due to overlapping intersection of
imaging slices, producing band-like areas of signal void posterior to the fifth lumbar
vertebral body. This artifact is eliminated by repeating sagittal and axial images using
parallel contiguous slices
References: Peh WCG, Chan JHM. "Artifacts in musculoskeletal magnetic resonance
imaging: identification and correction." Skeletal Radiology 2001; 30:179-191.
5. K-SPACE
a. Spike or Herringbone: Bad data points in k-space result in band artifacts (Fig. 17
on page 30). Spike noise usually occurs because of loose electrical connections that
produce arcs or because of the breakdown of interconnections in an RF coil, and it
is more evident with the use of high-duty-cycle sequences. This artifact can be used
advantagously for the purpose of tagging suring the cardiac cycle for assessing cardiac
motion.
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Fig.: Spike artifact. Bad data points in k-space (arrow on right) result in band artifacts
on the MR image on left.
References: Zhu J, Gullapalli RP. "AAPM/RSNA Physics Tutorial for Residents: MR
Artifacts, Safety, and Quality Control." RadioGraphics 2006; 26:275-297..
b. Zero Fill-In or at times called Zebra: This happens due to missing data or data that
is set to zero in k-space by the scanner. Abrupt change from signal to no signal can give
a zebra like artifact (Fig. 18 on page 30).
c. N/2 Ghost and Segmental K-Space: These are ghosts caused by phase errors as
discontinuities in k-space (Fig. 19 on page 30). Fast spin-echo sequences are also
susceptible to segmented k-space artifacts. Usually requires a call to service.
6. SAMPLING
a. Wraparound or Aliasing: Occurs when FOV is smaller than the body part imaged
(Fig. 20 on page 31, 21 on page 32, 25 on page 35). It can be corrected by
using a larger FOV and oversampling. In the phase (x) direction it can be corrected with
a higher number of phase encoding steps; in the frequency direction (y) can be fixed by
sampling the signal twice as fast. One can also apply spatial presaturation to undesired
tissue or swap frequency/phase-encoding directions.
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