MRI artifact


An MRI artifact is a visual artifact in magnetic resonance imaging. It is a feature appearing in an image that is not present in the original object. Many different artifacts can occur
during MRI, some affecting the diagnostic quality, while others may be confused with pathology. Artifacts can be classified as patient-related, signal processing-dependent and hardware -related.

Patient-related MR artifacts

Motion artifacts

A motion artifact is one of the most common
artifacts in MR imaging. Motion can cause
either ghost images or diffuse image
noise in the phase-encoding direction.
The reason for mainly affecting data
sampling in the phase-encoding
direction is the significant difference
in the time of acquisition in the frequency-
and phase-encoding directions.
Frequency-encoding sampling
in all the rows of the matrix takes place during a single
echo. Phase-encoded
sampling takes several seconds, or
even minutes, owing to the collection
of all the k-space lines to enable
Fourier analysis. Major physiological
movements are of millisecond to seconds
duration and thus too slow to
affect frequency-encoded sampling,
but they have a pronounced effect in
the phase-encoding direction.
Periodic movements such as cardiac
movement and blood vessel or
CSF pulsation cause ghost images,
while non-periodic movement causes
diffuse image noise. Ghost
image intensity increases with amplitude
of movement and the signal
intensity from the moving tissue.
Several methods can be used to
reduce motion artifacts, including
patient immobilisation, cardiac and
respiratory gating, signal suppression
of the tissue causing the artifact,
choosing the shorter dimension of the
matrix as the phase-encoding direction,
view-ordering or phase-reordering
methods and swapping phase and
frequency-encoding directions to
move the artifact out of the field of
interest.

Flow

Flow can manifest as either an altered
intravascular signal, or as
flow-related artifacts.
Flow enhancement, also known as
inflow effect, is caused by fully magnetised
protons entering the imaged slice
while the stationary protons have not
fully regained their magnetization.
The fully magnetized protons yield a
high signal in comparison with the
rest of the surroundings.
High velocity flow causes the protons
entering the image to be removed
from it by the time the 180-degree
pulse is administered. The effect is
that these protons do not contribute
to the echo and are registered as a signal
void or flow-related signal loss
.
Spatial misregistration manifests
as displacement of an intravascular
signal owing to position encoding of a
voxel in the phase direction preceding
frequency encoding by time TE/2.The
intensity of the artifact is dependent
on the signal intensity from the vessel,
and is less apparent with increased TE.

Metal artifacts

Metal artifacts occur at interfaces
of tissues with different magnetic susceptibilities,
which cause local magnetic
fields to distort the external
magnetic field. This distortion
changes the precession frequency in
the tissue leading to spatial mismapping
of information. The degree of
distortion depends on the type of
metal,
the type of interface,
pulse sequence and imaging parameters.
Metal artifacts are caused by external
ferromagnetics such as cobalt
containing make-up, internal ferromagnetics
such as surgical clips,
spinal hardware and other orthopaedic
devices, and in some cases, metallic objects swallowed by people with pica. Manifestation of these
artifacts is variable, including total
signal loss, peripheral high signal and
image distortion.
Reduction of these artifacts can be
attempted by orientating the long axis
of an implant or device parallel to the
long axis of the external magnetic
field, possible with mobile extremity
imaging and an open magnet.
Further methods used are choosing
the appropriate frequency encoding
direction, since metal artifacts are
most pronounced in this direction,
using smaller voxel sizes, fast imaging
sequences, increased readout bandwidth
and avoiding gradient-echo
imaging when metal is present. A
technique called MARS applies an
additional gradient, along the slice
select gradient at the time the frequency
encoding gradient is applied.

Signal processing dependent artifacts

The ways in which the data are
sampled, processed and mapped out
on the image matrix manifest these
artifacts.

Chemical shift artifact

Chemical shift artifact occurs at
the fat/water interface in the phase
encoding or section-select directions
. These artifacts arise due to
the difference in resonance of protons
as a result of their micromagnetic
environment. The protons of fat resonate
at a slightly lower frequency
than those of water. High field
strength magnets are particularly susceptible
to this artifact.
Determination of the artifact can
be made by swapping the phase- and
frequency-encoding gradients and
examining the resultant shift
of the tissues.

Partial volume

Partial volume artifacts arise from the size of the voxel over which the signal is averaged. Objects smaller than the voxel dimensions lose their identity, and loss of detail and spatial resolution occurs. Reduction of these artifacts is accomplished by using a smaller pixel size and/or a smaller slice
thickness.

Wrap-around

A wrap-around artifact also known as an aliasing artifact, is a result of mismapping
of anatomy that lies outside the
field of view but within the slice volume.
The selected field of view is
smaller than the size of the imaged
object. The anatomy is usually displaced
to the opposite side of the
image. It can be caused
by non-linear gradients or by undersampling
of the frequencies contained
within the return signal.
The sampling rate must be twice
the maximal frequency that occurs in
the object. If
not, the Fourier transform will assign
very low values to the frequency signals
greater than the Nyquist limit.
These frequencies will then ‘wrap
around’ to the opposite side of the
image, masquerading as low-frequency
signals. In the frequency encode
direction a filter can be applied to the
acquired signal to eliminate frequencies
greater than the Nyquist frequency.
In the phase encode direction, artifacts
can be reduced by an increasing
number of phase encode steps
. For correction,
a larger field of view may be chosen.

Gibbs artifacts

Gibbs artifacts or Gibbs ringing artifacts, also known as truncation artifacts are caused by the under-sampling
of high spatial frequencies at sharp boundaries in the image.
Lack of appropriate high-frequency components
leads to an oscillation at a
sharp transition known as a ringing
artifact. It appears as multiple, regularly spaced
parallel bands of alternating bright
and dark signal that slowly fade with
distance. Ringing artifacts are
more prominent in smaller digital
matrix sizes.
Methods employed to correct
Gibbs artifact include filtering the k-space
data prior to Fourier transform,
increasing the matrix size for a given
field of view, the Gegenbauer reconstruction
and Bayesian approach.

Machine/hardware-related artifacts

This is a wide and still expanding subject.
Only a few common artifacts are recognised.

Radiofrequency (RF) quadrature

RF detection circuit failure arises
from improper detector channel
operation. Fourier-transformed data
display a bright spot in the centre of
the image. If one channel of the detector
has a higher gain than the other it
will result in object ghosting in the
image. This is the result of a hardware
failure and must be addressed by a
service representative.

External magnetic field (B0) inhomogeneity

B0 inhomogeneity leads to mismapping
of tissues. Inhomogeneous
external magnetic field causes either
spatial, intensity, or both distortions.
Intensity distortion occurs when the
field in a location is greater or less than
in the rest of the imaged object
. Spatial distortion results
from long-range field gradients, which remain constant in the inhomogeneous
field.

Gradient field artifacts (B1 inhomogeneity)

Magnetic field gradients are used
to spatially encode the location of signals
from excited protons within the
volume being imaged. The slice select
gradient defines the volume.
Phase- and frequency-encoding gradients
provide the information in the
other two dimensions. Any deviation
in the gradient would be represented
as a distortion.
As the distance increases from the
centre of the applied gradient, loss of
field strength occurs at the periphery.
Anatomical compression occurs and
is especially pronounced on coronal
and sagittal imaging.
When the phase-encoding gradient
is different, the width or height of
the voxel is different, resulting in distortion.
Anatomical proportions are
compressed along one or the other
axis. Square pixels should
be obtained.
Ideally the phase gradient should
be assigned to the smaller dimension
of the object and the frequency gradient
to the larger dimension. In practice
this is not always possible because
of the necessity of displacing motion
artifacts.
This may be corrected by reducing
the field of view, by lowering the gradient
field strength or by decreasing
the frequency bandwidth of radio signal.
If correction is not achieved, the
cause might be either a damaged gradient
coil or an abnormal current
passing through the gradient coil.

RF inhomogeneity

Variation in intensity across the
image may be due to the failure of the
RF coil, non-uniform B1 field, non-uniform
sensitivity of the receive only
coil, or presence
of non-ferromagnetic material in
the imaged object.

Asymmetrical brightness

There is a uniform decrease in signal
intensity along the frequency encoding
axis. Signal drop-off is due
to filters that are too tight about the
signal band. Some of the signal generated
by the imaged section is, thereby,
inappropriately rejected. A similar
artifact may be caused by non-uniformity
in slice thickness.

RF noise

RF pulses and precessional frequencies
of MRI instruments occupy
the same frequency bandwidth
as common sources such as TV,
radio, fluorescent lights and computers.
Stray RF signals can cause various
artifacts. Narrow-band noise is
projected perpendicular to the frequency-
encoding direction. Broadband
noise disrupts the image over
a much larger area. Appropriate site
planning, proper installation and
RF shielding eliminate
stray RF interference.

Zero line and star artifacts

A bright linear signal in a dashed pattern
that decreases in intensity
across the screen and can occur as a
line or star pattern, depending on
the position of the patient in the
‘phase-frequency space’.
Zero line and star artifacts are
due to system noise or any cause of
RF pollution within the room
. If this pattern persists,
check for sources of system
noise such as bad electronics or
alternating current line noise, loose
connections to surface coils, or any
source of RF pollution. If a star pattern
is encountered, the manufacturer
needs to readjust the system
software so that the image is moved
off the zero point.

Zipper artifacts

Although less
common, zippers are bands
through the image centre due to an
imperfect Faraday cage, with RF
pollution in, but originating from
outside, the cage.
Residual free induction decay
stimulated echo also causes zippers.

RF tip angle inhomogeneity

These are patchy areas of
increased or decreased signal intensity.
This artifact is produced by
variations in RF energy required to
tip protons 90 or 180 degrees within
the selected slice volume.

Bounce point artifact

Absence of signal from tissues of
a particular T1 value is a consequence
of magnitude sensitive
reconstruction in inversion recovery
imaging. When the chosen T1
equals 69% of the T1 value of a
particular tissue, a bounce point
artifact occurs.
Use phase-sensitive reconstruction
inversion recovery techniques.

Surface coil artifacts

Close to the surface coil the signals
are very strong resulting in a
very intense image signal.
Further from the coil the signal
strength drops rapidly due to the
attenuation with a loss of image
brightness and significant shading
to the uniformity. Surface coil sensitivity
intensifies problems related
to RF attenuation and RF mismatching.

Slice-to-slice interference

Non-uniform RF energy received
by adjacent slices during a multi-slice
acquisition is due to cross-excitation
of adjacent slices with contrast loss in
reconstructed images.
To overcome these interference
artifacts, the acquisition of
two independent sets of gapped
multi-slice images need to be included, and subsequently
reordered during display of the full
image set.