Magnetic resonance imaging (MRI) is a medical imaging technique that uses a magnetic field and computer-generated radio waves to create detailed images of the organs and tissues in your body. Most MRI machines are large, tube-shaped magnets. When you lie inside an MRI machine, the magnetic field temporarily realigns water molecules in your body. Radio waves cause these aligned atoms to produce faint signals, which are used to create cross-sectional MRI images — like slices in a loaf of bread. The MRI machine can also produce 3D images that can be viewed from different angles. Products & Services Sign up for Email: Get Your Free Resource – Coping with Cancer Why it's done MRI is a noninvasive way for your doctor to examine your organs, tissues and skeletal system. It produces high-resolution images of the inside of the body that help diagnose a variety of problems. MRI of the brain and spinal cord MRI is the most frequently used imaging test of the brain and spinal cord. It's often performed to help diagnose: Aneurysms of cerebral vessels Disorders of the eye and inner ear Multiple sclerosis Spinal cord disorders Stroke Tumors Brain injury from trauma A special type of MRI is the functional MRI of the brain (fMRI). It produces images of blood flow to certain areas of the brain. It can be used to examine the brain's anatomy and determine which parts of the brain are handling critical functions. This helps identify important language and movement control areas in the brains of people being considered for brain surgery. Functional MRI can also be used to assess damage from a head injury or from disorders such as Alzheimer's disease. MRI of the heart and blood vessels MRI that focuses on the heart or blood vessels can assess: Size and function of the heart's chambers Thickness and movement of the walls of the heart Extent of damage caused by heart attacks or heart disease Structural problems in the aorta, such as aneurysms or dissections Inflammation or blockages in the blood vessels

1. MRI ARTIFACTS

3. s

8. Gradient coils generate secondary
magnetic fields over the primary field.
The arrangement of these coils gives
mri the capability to image directionally
along the 3 axis.
Z gradient- long axis-axial
Y gradient – vertical axis –coronal
X gradient – horizontal axis - saggital

9. RF coils are
used for
transmitting
the RF pulse
and receiving
signals.

15. Saturated Spins

16. The Analog signal is converted to a
digital signal by the Analog to
digital converter(ADC)
The digital signal representing the
imaged body part is stored in the
temporary image space called K-
space.
The digital signal is then sent to an
image processor where a
mathematical formula called
fourier transformation is applied
and the image of the mri scan is
dispayed on a monitor

17. 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

18. Types of MRI image
artifacts
MR hardware
• Zipper artifact
• Herringbone artifact
• Zebrastripes
• Moire fringes
• Central Point artifact

19.  MR software
 Slice-overlap artifact (cross talkartifact)
 RF overflow artifact
 Cross excitation artifact

20.  Patient and physiologic motion
 -Phase-encoded motion artifact
 -Entry slice phenomenon

21.  Tissue heterogeneity and foreign bodies
 -Magic angleeffect
 -Magnetic susceptibility artifact
 -Chemical shift artifact

22. Fourier transform
-Gibbs artifact /truncation artifact
--Aliasing/wrap around artifact

23. • Cause: This type of artifact results from the
contamination of the MR scanner environment
(shielded MR suite) by spurious Radio Frequency
signals.
Zipper
artifact

24.  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.

25. • 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 )

26.  Appearance- A dense broken line of alternatively dark
and bright pixels running across the image perpendicular
to the frequency encoding direction.

28.  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

29. 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

30. 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 .

31. 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 occurs resultsing 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.

32. Wavy
appearance
at the edge
of the
images

33.  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

34. Central Point artifact
 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.

35.  T2 weighted gradient
echo sequence
showing a central
point artifact in the
pons.

36. Solution
 maintain a constant temperature in scanner
and equipment room for receiver amplifiers.
 software to estimate DC offset and adjust
the data accordingly in the k-space.

37. 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

38. 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

40. Solution
• Avoid using a steep change in angle between slices
• Increase Slice gap
• Interleaved acquisition
• Use separate acquistions

41. RF overflow artifact
RF overflow
artifact causes a
nonuniform, washed-out
appearance to an image.
 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.
The top and bottom of the echo
have been 'clipped off’

42.  If the peak signal does not
lie within the computational
range of the computer and
image reconstruction
software, severe arithmetic
errors occur, and incorrect
pixel values are assigned.
 Image has significantly low
spatial frequency distortion
giving it a ‘ghostlike’ unreal
quality with loss of contrast
and resolution.

43. Solution
 Autoprescanning usually adjusts the receiver gain to
prevent this from occuring or the receiver gain can be
reduced manually

44. 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.
.

45.  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

46. 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..)

47. 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
10% gap demonstrating impairedcontrast.

48. 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.

49.  In this case, the scouts were
acquired in the following
order: axial, sagittal and
finally coronal. The sagittal
FOV involved the medial
gallbladder.
 The protons in the medial half
of GB are already saturated.
 Therefore, the immediate
subsequent RF pulse of the
coronal acquisition will result
in a low signal in the medial
gallbladder and can mimic
sludge.

50. 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.

51. 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.

52. 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.

53.  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.

54. 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

55. 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.

56. 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.

57.  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.

58. Saturation pulse

59. -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

60. 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.
-

63.  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.

64. 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.

67.  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.

73. 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

74.  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

75. 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

76. Susceptibility artefact due
to hip prosthesis

77. Susceptibility artefact
due to dental fillings

78. Susceptibility artefact
due to dental fillings

79. 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

80. 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

81. 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

82.  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.

83. 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

84.  a black band is seen on the left side of each kidney and a
white band on the right.

85.  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

86.  axial T1 image also
demonstrates a chemical
shift artifact at the edges
between the mass and
the orbital fat.
ORBITAL HEMANGIOMA

87.  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.

88. spine
Abdomen
Thigh

89. Gibbs artifact /
truncationartifact
• It refers to a series of bright or dark lines, parallel & next to
borders of abrupt intensity change
This artefact occurs at areas of abrupt intensity change such as
-the CSF-Spinal Cord Interface.
-the Skull-Brain Interface.

90.  Bright and dark bands are
seen parallel and adjacent
to the outer convexity of
calvaria

91. Skull-Brain Interface

92. . 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”),

93. Truncation artifacts are reduced by
the use of a smaller field of view
(FOV) or a larger matrix size.

94. 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

95. -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.

97. 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

98.  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.

99. Slice thickness 10mm
Unable to depict finer
structures due to partial
volume artefact
Slice thickness 3mm
Depicts cranial nerves

100. THANKS