2. Introduction
• An Artifact is something observed in a scientific
investigation that is not naturally present but occurs as
a result of the investigative procedure.
• It is a structure not normally present, but visible as a
result of malfunction in the hardware or software of the
device, or a consequence of environmental influences
8. • Cause: This type of artifact results from the
contamination of the MR scanner environment
(shielded MR suite) by spurious Radio Frequency
signals.
Zipper
artifact
9. A Faraday cage is a
conductive enclosure
used to shield a
space from
electromagnetic
interference.
In MR imaging, this
type of structure
provides
radiofrequency
shielding to the
scanning room to
minimize occurrence
of imaging artifacts.
10. • The contamination can result dueto
1)BreechintheRadiofrequencycagei.e. RF entering the shielded
room from outside
2) Illumination devices & other medical equipment (e.g pulse
oximeter )
11. Appearance- A dense broken line of alternatively dark
and bright pixels running across the image perpendicular
to the frequency encoding direction.
12.
13. Solution:
1) Scan door should be closed
2) Breech in RF cage should be located and repaired
3) Equipments in the scan room should be serviced
14. Herringbone
artifact
• Herringbone artifact (also known as ascrisscross
artifact or corduroyartifact)
• Due to interferences which occur while filing‐in of
the k‐space
• These interferences are usually RF spikes which
lead to points in k‐space with high intensity
• Causes ;
• Fluctuating power supply
• RF pulse discripencies
15. Appearance- It appears as obliquely oriented stripes seen throughout the image. Artifact is
scattered all over the image in a single slice or multiple slices .
16. Moire
fringes
• Moire fringes are an interference pattern most commonly seen when
doing gradient echo images with the body coil.
• 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 screenwindows.
18. Solutions:
1) Use of spin echo sequences
2) Use of anti Aliasing remedies
3) Plug on the receiver coil should be firmly put in the
socket
19. Slice-overlap
artifact
• Loss of signal in an image due to a multi‐slice, multi‐angle acquisition
or imperfect sliceprofile
• The mechanism behind this artifact is spinsaturation
20. Image depicts Two sets of spine images, at different angles, covering L4‐5 and L5‐S1 respectively. The
set acquired later(red) will have image spins that have already been excited and that are
saturated,leading to signal loss
If slices at different angles cross, then spins that have been previously
excited (saturated) can not be excited again, thus leading to a signal
drop in the crossing region
23. Cross excitation
artifact
• There is loss of signal within a slice due to pre-excitation from RF
pulse meant for an adjacent slice
• Appearance- ThisArtefact causes a reduction in SNR in adjacent
slices in a Slice stack.
.
24. Cause : Ideally the profile of a slice
should be square or rectangular
when viewed from the edge, but in
practice a radio frequency excitation
pulse is not able to achieve this.
The adjacent slices receive energy
from the RF excitation pulse of their
neighbors .
When the tissues in next slice receive
their own RF pulse they are unable to
produce a signal as they are already
saturated
25. Solution:-
• Cross excitation can be reduced by insuring that
there is at least at 30% gap between the slices
(Gap= 30% of the Slice thickness )
This reduces the likelihood of RF pulse exciting
adjacent slice.
• Interleaving between slices i.e.
FIrst acquiring the odd number slices(1,3,5,7..)
Then acquiring the even number slices( 2,4,6,8..)
26. Effect of cross-talk on image contrast. On left is
a SE image with 50% gap showing expected spin-
density contrast. On right the same sequence with 0%
gap demonstrating impairedcontrast.
27. On the coronal scout, the lumen of the gallbladder is sharply divided into low signal
medially and high signal laterally with a "level" appearance suggesting layering
sludge. However, the axial T2W shows no sludge in the gallbladder. This finding is
attributed to cross-excitation artifact.
28. Phase-encoded motion
artifact
• Phase-encoded motion artifact occurs as a result
of tissue/fluid moving during the scan. It manifests
as ghosting in the direction of phase-encoding.
• These artifacts may be seen from arterial pulsations,
swallowing, breathing, peristalsis, and physical
movement of a patient. When projected over
anatomy it can mimic pathology, and needs to be
recognized.
• Motion that is random such as the patient moving
produces a smear in the phase direction. Periodic
motion, such as respiratory or cardiac/vascular
pulsation, produces discrete, well-defined ghosts.
29. This is a typical example of
phase-encoded artifact secondary
to aortic pulsation. The ghost
images of the aorta are:
discrete, well defined, lining with
the aorta along the short axis of
the abdomen (AP axis) at regular
intervals and extending beyond
the field of view.
These features help recognize
them and not to confuse them
with any pathology.
30. Blood flow in the sagittal sinus and carotids causes Periodic ghosting which gives the
impression of an enhancing lesion in the occipital lobe as well as anterior to the right
temporal horn. Wide windowing demonstrates the ghosting extending beyond the confines
of the patient's anatomy.
Apparent enhancement in the
occipital lobe and right temporal
lobe in a patient with
malignancy.
windowed slice demonstrates
the phase mismapping artefact from
the superior sagittal sinus.
31. Swallowing artifact has
produced horizontal
band of noise along
phase-encoded
direction. Where it
passes over vertebral
body and spinal cord,
there are local intensity
and contrast changes,
rendering these portions
of scan suspect.
32. Solutions
cardiac/respiratory gating
spatial presaturation bands placed over moving tissues (e.g.
over the anterior neck in sagittal cervical spines)
spatial presaturation bands placed outside the FOV, especially
before the entry or after the exit slice for reducing ghosting
from vascular flow
scanning prone to reduce abdominal excursion
increasing the number of signal averages
shorten the scan time when motion is from patient movement
33. Cardiac Gating
This involves the acquistion of a series of MR images of a
single anatomic section during a specific point in the
cardiac cycle ,which is being monitored with ECG.
Data within the k-space is linked with specific time point
in cardiac cycle.
The acquired datasets are then sorted according to time
stamp to produce sequential images allowing a dynamic
evaluation of myocardial function.
34. Respiratory gating
respiration related motion is monitored, using bellows or
breathing belt, and image acquisition is timed to take
place at end expiration, when there is little or no motion.
however this is infrequently used in clinical imaging.
-Breath holding method most often used
-Sequences with short acquisition time may be used, or if
breath holding possible for limited time, exam may be
divided into several brief acquisitions.
35. Saturation pulses
Saturation pulses involve the application of RF energy to
suppress the MR signal from moving tissues outside the
imaged volume to reduce or eliminate motion artifacts.
37. -Most commonly used in abdominal imaging at the interface of lung and diaphragm
-Application of small, one dimensional spatial encoding gradient in a plane perpendicular to
diaphragm.
- Echo measured at this location allows correction of imaging dataset and ensures that only the
imaging data acquired when diaphragm is at its peak (end expiration), is used in image
reconstruction.
Navigator Echo
38. Entry slice
phenomenon
Entry slice phenomenon occurs when unsaturated spins in
blood first enter into a slice .
Unsaturated spins = protons without transverse magnetisation
When a large number of such protons encounter a RF pulse for
the first time(i.e. at the entry slice),they undergo transverse
magnetisation and emit a strong signal.
- It is characterized by the bright signal in a blood vessel
(artery or vein) at the first slice that the vessel enters.
-
39.
40.
41. This artefact can be confused with thrombosis
Solutions-
-The characteristic location and if necessary, the use of
gradient echo flow techniques can be used to
differentiate entry slice artefacts from occlusions.
-Spatial saturation bands place before the first slice and
after the last can also be used to eliminate this artefact.
42. Magic angle
artefact
• The magic angle effect is important in the clinical MR
imaging of certain tissues that are highly structured
and are oriented obliquely to the main magnetic field
(especially tendons, cartilage, and peripheral nerves).
• In tendons, the water molecules are aligned along the tendon sheath
and have strong dipolar interactions. Due to these dipolar
interactions, protons in the collagen rapidly dissipate their energy
after encountering a RF pulse i.e. tendons have a short intrinsic
T2 value (1-10 ms).
The signal of tendons is uniformly low on all conventional MR
sequences due to this rapid T2 decay.
43.
44.
45. however, when the tendons are aligned at an angle of 55degree to the
main magnetic field , these dipolar interactions go to zero, leading to
an increase in T2 relaxation time and consequent sudden increase in
signal.
46.
47.
48.
49.
50.
51. Common sites:
proximal part of the Posterior cruciate ligament
infrapatellar tendon at the tibial insertion
peroneal tendons as they hook around the lateral
malleolus
Supraspinatus tendon
52. High signal within a tendon can mimic tendinopathy so it
is important that radiologists are aware of the magic
angle phenomena.
Look for it in the outer portions of the rotator cuff,
supraspinatus tendon, the distil patellar tendon, tendons
of the ankle, extensor and flexor policis tendons of the
wrist or any part made of collagen that lies in a curve
53. Susceptibility
artifact
• Due to the presence of substances ( e.g.ferromagnetic materials) that
causes variation in the local magneticfield
• Results in increased magnetic field inhomogeneity ,leading to spin
dephasing and consequent signal drop
• Becomes worse for long echo times and GRE sequences( no RF 180
pulse)
• Less visible in fast spin echosequence
• Worse for higher fieldstrength
57. Remedies
• Use spin-echo sequences
A spin echo sequence uses a 180-degree RF
pulse .
The main purpose of this 180-degree pulse is
to correct local field inhomogeneities.
A gradient echo pulse sequence does not
utilize such a “refocusing” pulse.
As such, local field inhomogeneities are
much more readily apparent when gradient
echo sequences are utilized
58. Chemical Shift
-Chemical Shift is the difference between precessional
frequencies of water and fat.(3.5ppm)
-Artefact is the displacement of signal intensity due to this
difference
59. AT 1.5 T W>F by 220Hz
At 3T W>F by 440Hz
WATER FAT
Water molecules have low
electron shielding and their
nucleus is more susceptible to
external magnetic field.
As a result water molecules
precess at a higher frequency
at the same magnetic field.
Fat molecules have higher
electron shielding and their
nucleus is comparitively less
susceptible to external
magnetic field.
Fat molecules precess at a
lower frequency at the same
mag field
60. water and lipid
protons coexist at
the interface, the
signal emitted by
the lipid protons will
have a lower
frequency than that
of the water
protons,leading to
mismapping of fat.
61. CHEMICAL SHIFT TYPE 1: MISMAPPING ARTIFACT
-Most commonly seen around water containg structures
sorrounded bt Fat(Liver,Kidneys,Orbits)
-Artifact appears as a dark band on one side of the
water/fat interface and a bright band on the other side
62. a black band is seen on the left side of each kidney and a
white band on the right.
63. T2 weighted image demonstrates
chemical shift artefact around the
lipoma.
Note the signal loss anteriorly
(black arrows) and hyperintensity
posteriorly (white arrows).
This is the result of inaccurate
spatial encoding, resulting in the
lipoma being incorrectly placed
posterior to its actual location.
LIPOMA
64. axial T1 image also
demonstrates a chemical
shift artifact at the edges
between the mass and
the orbital fat.
ORBITAL HEMANGIOMA
65. Chemical Shift: 2nd Kind
The chemical shift artifact of the second kind is sometimes
known as the India Ink or black line artifact because it
looks like organs and muscle bundles have been outlined
with a black pen. The line does not correspond to a true
anatomic structure, but results from destructive
interference of signals at the boundary pixels that contain
both water and fat. For this reason it is sometimes often
called the phase cancellation artifact. Examples in the
spine, thigh, and abdomen.
67. Gibbs artifact /
truncationartifact
• It refers to a series of bright or dark lines, parallel & next to
borders of abrupt intensity change
Areas of abrupt intensity change such as
-the CSF-Spinal Cord Interface.
-the Skull-Brain Interface.
68. Bright and dark bands are
seen parallel and adjacent
to the outer convexity of
calvaria
70. . Figure illustrates a syrinx-like artifact
(red arrow) within the center of the cord, a
classic truncation artifact that appears as a
result of alternating dark and light signal
bands overlying the spinal cord.
This artifact can be easily mistaken for
hydromyelia (dilatation of the central canal,
more commonly referred to as a “syrinx”),
71. Truncation artifacts are reduced by
the use of a smaller field of view
(FOV) or a larger matrix size.
72. Aliasing / wrap
aroundartifact
• It 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 onto the
other side of the image
• Caused by under‐sampling in the phase encoding direction
73. -phase shifts of between 0° and 360°
encompass the field-of-view.
The subject's left flank in the drawing
extends outside the field-of-view and
encompasses phase shifts from 361°
to 450°.
Since all meaningful frequencies
have been defined over the range of
0° to 360°, a phase shift of 361° will
be assigned to the spatial position of
1°, and a shift of 450° will be
assigned to 450°-360° = 90°.
Hence ,the left side of the patient's
body will therefore be "wrapped
around" and spatially mismapped to
the opposite (right) side of the image.
74.
75. Solution
-Enlarging the field of view (FOV)
-Using pre-saturation bands on areas outside the FOV
-Anti-aliasing software
-Use a surface coil to reduce the signal outside of the
area of interest
76. PARTIAL VOLUME ARTIFACT
-Partial volume artefact occurs if slice thickness >
thickness of the tissue of interest.
-Occurs when multiple tissue types are encompassed
within a single voxel
-If a small structure is entirely contained within the slice
thickness along with other tissue of differing signal
intensities, the resulting signal displayed on the image is a
combination of these two intensities.
This reduces contrast of the small structure.
77. Slice thickness 10mm
Unable to depict finer
structures due to partial
volume artefact
Slice thickness 3mm
Depicts cranial nerves
