Artifacts of mri

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Artifacts of mri Powered By Docstoc
					     By Samar Ali Shigairi MD
Sattam S. Lingawi, MD, FRCRP, ABR
  The chemical shift artifact is commonly noticed in
   the spine at the vertebral body end plates, in the
    abdomen, and in the orbits where fat and other
tissues form borders. In the frequency direction, the
    MRI scanner uses the frequency of the signal to
 indicate spatial position. Since water in organs and
  muscle resonate at a different frequency than fat,
 the MRI scanner mistakes the frequency difference
  as a spatial (positional) difference. As a result, fat
  containing structures are shifted in the frequency
          direction from their true positions.
    As a result, fat containing structures are
    shifted in the frequency direction from
    their true positions.
   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 border.
   This artifact is shown in
    the following axial
    image of a kidney where
    the bright border along
    the top of the kidney
    and the dark border
    along the bottom of the
    kidney represent the
    artifact. This artifact is
    greater at higher field
    strengths and lesser at
    higher gradient
    strengths. Practically
    about the only way to
    eliminate this artifact is
    to use a fat suppression
    technique.
       Aliasing or "Wrap-around

 Aliasing or wrap-around is a common artifact that
occurs when the field of view (FOV) is smaller than
  the body part being imaged. The part of the body
that lies beyond the edge of the FOV is projected on
      to the other side of the image. This can be
  corrected, if necessary, by oversampling the data.
 In the frequency direction, this is accomplished by
    sampling the signal twice as fast. In the phase
    direction, the number of phase-encoding steps
  must be increased with a longer study as a result.
 The first image
shows wrap-
around of the
back of the head
on to the front of
the head, where
the phase-
encoded
direction is
anterior-
posterior.The
The second image
has the phase and
frequency directions
reversed resulting in
absence of the
aliasing artifact.
Oversampling was
used in the
frequency direction
to eliminate the
aliasing.
Black Boundary Artifact
The Black Boundary Artifact is an artificially created black
line located at fat-water interfaces such as muscle-fat
interfaces. This results in a sharp delineation of the
muscle-fat boundary that is sometimes visually appealing
but not an anatomical structure. The following is a coronal
image through the upper body with an echo time of 7ms.
A black line is seen surrounding the muscles of the
shoulder girdle as well as around the liver.
This artifact can occur for a couple of reasons.
The most common reason is a result of selecting an
echo time (TE) in which the fat and water spins
(located in the same pixel at an interface) are out of
phase, cancelling each other. At 1.5 T, the 3.5 PPM
difference in frequency between water and
saturated fat results in cancellation of spins at 4.5
ms multiples, starting at about 2.3 ms; for example
at 6.8ms, 11.3ms, and 15.9 ms. To avoid this
artifact, TE's close to 4.5ms, 9ms, 13.6ms,... should
be chosen.
Gibbs or truncation artifacts are bright or dark lines that are
seen parallel and adjacent to borders of abrupt intensity
change, as when going from bright CSF to dark spinal cord
on a T2-weighted image. In the spinal cord, this artifact can
simulate a small syrinx to the unaware. It is also seen in
other locations as at the brain/calvarium interface. This
artifact is related to the finite number of encoding steps
used by the Fourier transform to reconstruct an image. The
more encoding steps, the less intense and narrower the
artifacts
The first axial image
is a phantom
containing water,
surrounded by air.
The image was
encoded 128 times
in the horizontal
direction and 256
times in the vertical
direction. Note the
prominent light and
dark line along the
sides that fade as
they approach the
top and bottom of
the phantom.
The second image
was encoded 256
times in both
directions. Minimal
artifact is seen
uniformly around
the periphery of the
phantom.
There are various causes for zipper artifacts in
images. Most of them are related to hardware or
software problems beyond the radiologist
immediate control. The zipper artifacts that can be
controlled easily are those due to RF entering the
scanning room when the door is open during
acquisition of images. RF from some radio
transmitters will cause zipper artifacts that are
oriented perpendicular to the frequency axis of
your image. Other equipment and software
problems can cause zippers in either axis.
An axial MRI of
the head in a
patient. The
scanner room
door was left
open during the
acquisition
causing the zipper
artifacts shown.
Phase-encoded motion artifacts appear as bright
noise or repeating densities oriented in the phase
direction, occurring as the results of motion during
acquisition of a sequence. These artifacts may be
seen from arterial pulsations, swallowing,
breathing, peristalsis, and physical movement of a
patient. They can be distinguished from Gibbs or
truncation artifacts because they extend across the
entire FOV, unlike truncation artifacts that
diminish quickly away from the boundary causing
them
 Phase-encoded artifacts can be reduced by
various techniques depending on their cause and
location. Arterial pulsation artifacts can be
reduced by spatial presaturation pulses prior to
entry of the vessel into the slices. Spatial
presaturation can also reduce some swallowing
and breathing artifacts. Surface coil localization
can reduce artifacts generated at a distance from
the area of interest. Pulse sequences can be
shortened, and respiratory and/or cardiac or
peripheral gaiting techniques may also help
The following axial
.

image of the head
shows a phase-
encoded motion
artifact running
transversely across
the back of the head
(posterior fossa) as a
result of venous
flow in the
transverse sinuses.
The slice-overlap artifact is a name I've given to
the loss of signal seen in an image from a multi-
angle, multi-slice acquisition, as is obtained
commonly in the lumbar spine. If the slices
obtained at different disk spaces are not parallel,
then the slices may overlap. If two levels are done
at the same time, e.g., L4-5 and L5-S1, then the
level acquired second will include spins that have
already been saturated. This causes a band of
signal loss crossing horizontally in your image,
usually worst posteriorly .
The dark horizontal
bands in the bottom
of the following
axial image through
the lumbar spine
demonstrates this
artifact.
As long as the
saturated area stays
posterior to the
spinal canal it causes
no harm.
Magic angle effects are seen most frequently in tendons and ligaments
that are oriented at about a 55 degree angle to the main magnetic field.
Signal from water molecules associates with the tendon collagen fibers
is not normally seen because of dipolar interactions that result in very
short T2 Times. At an angle of about 55 degrees to the main magnetic
field, the dipolar interactions become zero, resulting in an increase of
the T2 Times about 100 fold. This results in signal being visible in
tedons with ordinary pulse sequences
.A bright signal
from this artifact is
commonly seen in
the rotator cuff and
occasionally in the
patellar tendon and
elsewhere. The
following image
shows increase
signal in the distal
patellar tendon from
this magic angle
effect.
Moire fringes are an interference pattern most commonly seen when
doing gradient echo images with the body coil as shown in the figure.
Because of lack of perfect homogeneity of the main magnetic field from
one side of the body to the other, aliasing of one side of the body to the
other results in superimposition of signals of different phases that
alternatively add and cancel. This causes the banding appearance and
is similar to the effect of looking though two screen windows.
RF overflow artifacts cause a nonuniform, washed-out appearance to
an image as shown in the following axial image of a head. This artifact
occurs when the signal received by the scanner from the patient is too
intense to be accurately digitized by the analog-to-digital converter.
Autoprescanning usually adjusts the receiver gain to prevent this from
occurring but if the artifact still occurs, the receiver gain can be
decreased manually.
The central point artifact is a focal dot of increased signal in the center
of an image. It is caused by a constant offset of the DC voltage in the
receiver. After fourier transformation, this constant offset gives the
bright dot in the center of the image as shown in the diagram below.
The following axial MRI image of the head shows a central point artifact
projecting in the pons (bright dot in the middle of the image).
Susceptibility artifacts occur as the result of microscopic
gradients or variations in the magnetic field strength that
occurs near the interfaces of substance of different
magnetic susceptibility. Large susceptibility artifacts are
commonly seen surrounding ferromagnetic objects inside
of diamagnetic materials (such as the human body). These
gradients cause dephasing of spins and frequency shifts of
the surrounding tissues. The net result are bright and dark
areas with spatial distortion of surrounding anatomy.
These artifacts are worst with long echo times and with
gradient echo sequences.
* An axial MRI of the head
   in a patient with
   mascara on her eyelids.
   Susceptibility artifacts
   from the mascara
   obscure the front half of
   the globes.
Occasionally, data in the K-space array will be missing or will be set to
zero by the scanner as shown in the figure below. The abrupt change
from signal to no signal at all results in artifacts in the images such as
zebra stripes and other anomalies. The following coronal image of the
shoulder shows an example of a zero-fill artifact.

				
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posted:10/23/2012
language:English
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