artifacts in MRI

1. MAGNETIC RESONANCE IMAGING
ARTIFACTS
MODERATOR-Dr. C.N PRADEEP KUMAR SIR
PRESENTER- DR. YASHWANTH NAIK M.B

3. K-SPACE

4. •256x256 matrix- 256 lines of Data
•Each line- 256 data points.
•The y-dimension -phase encoding
•x-dimension -frequency encoding
•The distance between neighboring
points in k-space determines the
field of view of the object imaged,
and
•The extent of k-space determines
the resolution of the image.

6. MRI Artifacts
 An artifact is a feature appearing in an image that is not
present in the original object.
 Suboptimal image quality may hinder accurate
diagnosis.
 Recognition of common MR imaging artifacts,
underlying physics and practical methods
for correcting artifacts.

7. SOURCES OF ARTIFACTS
 Depending on their origin, artifacts are typically
classified as
 Patient-related.
 Signal processing dependent.
 Hardware (machine)-related.

8.  Patient related artifacts :
- Motion artifacts.
- Flow artifacts.
- Metal artifacts.
-Entry slice phenomenon.
 Signal processing dependent artifacts:
- Chemical shift artifact
- Wrap around
- Gibbs phenomenon (ringing artifact)
- Cross excitation and cross talk artifacts

9.  Machine/hardware-related artifacts:
- Zipper artifacts
-Herringbone artifacts
-Moire Fringes
-Zebra artifacts
- RF overflow artifacts (clipping)
-Shading artifacts.
 Others :
--Tissue heterogeneity and foreign bodies
-Partial volume artifacts.
-Magic angle artifacts.
-Magnetic susceptibility artifacts.

10.  Artifacts Caused by Patient Motion
 Ghosting and smearing
- Ghosts are replica of something in the image.
-Common artifacts produced by voluntary or
involuntary motion of the patient.
- Random motion (patient motion) produces
smearing.
- Periodic motion (respiratory/cardiac/vascular)
produces discrete , well defined ghosts.
- Motion related artifacts are produced in phase
encoding direction.

11. -These motion-related artifacts may result from :
• Esophageal contraction and vascular pulsation
during head and neck imaging.
• Respiration and cardiac activity during thoracic and
abdominal imaging.
• Bowel peristalsis during abdominal and pelvic
imaging.

12. PATIENT MOTION

13. BREATH HOLD

14.  Corrective measures :
 Phase encoding axis swap i.e, changing the phase
encoding direction.
 Spatial presaturation bands placed over moving tissues.
-The area producing motion artifacts is saturated with RF
pulses before start of proper pulse sequence such that all
signals from this area are eliminated.

15. Spatial saturation band
D/A- increased specific
absorption rate (SAR) and
- reduction in the number of
slices that can be obtained per TR.
Eg : in sagittal image of the
cervical spine swallowing produces
ghosts along phase axis (anterior -
posterior) and obscures the spinal
cord.
Application of saturation band
anterior to the cervical spine over
esophagus eliminates this artifact.

16.  Spatial presaturation bands placed outside the FOV.
 Scanning prone to reduce abdominal excursion
 Cardiac/Respiratory gating
 Shorten the scan time when motion is from patient
moving.

17. AXIAL T2-WEIGHTED SPIN-ECHO FAST SPIN ECHO

18. Flow related artifacts
 Flow can manifest as altered
intravascular signal (flow
enhancement or flow-related
signal loss).
 Flow enhancement, also
known as inflow effect, is
caused by fully magnetised
protons(high signal).
 High velocity flow - protons
do not contribute to the echo
and are registered as a signal
void or flow-related signal
loss.
Flow physics

19. Altered T2W signal due to CSF flow
3D spoiled GRE sequence shows inflow
enhancement of arteries at the skull base

20. A common way to reduce the motion artifact caused by through-
plane flow is to apply a saturation band adjacent to the imaging
section

21.  Aliasing/wraparound :
- When the imaging field of view is smaller than the
anatomy being imaged, aliasing occurs.
- Anatomy that appears outside the FOV appears within
image and on opposite side.
- Aliasing can occur along any axis – frequency encoding
axis/ phase encoding axis.
- Basis of aliasing lies in analog to digital conversion.

23.  Aliasing in MRI can be compensated for by:
- Enlarging the field of view (FOV)
- Using pre-saturation bands on areas outside the
FOV
- Anti-aliasing software
- Switching the phase and frequency directions
- Use a surface coil to reduce the signal outside of
the area of interest

24.  Chemical shift artifacts
-Chemical shift is due to the
differences between resonance
frequencies of fat and water.
- -It occurs in the frequency
encode direction where a shift in
the detected anatomy occurs
because fat resonates at a slightly
lower frequency than water.
- At 1.5T chemical shift between
water and fat is 220 Hz.

25.  Chemical shift becomes source of artifacts.
 There are two types of chemical shift artifacts:
1. Chemical shift misregistration artifacts.
2. Interference from chemical shift (in-
phase/out- phase).

26. Chemical shift misregistration Interference from chemical shift
Along frequency encoding axis Along phase encoding axis
Dark edge at the interface between fat
and water
Dark edge around certain organs
Corrected by :
1.Fat supression
2.Increasing band width
1.Using spin echo sequences
2. Selecting TE that is multiple of
4.5 at 1.5T.
Good effects:
Forms the basis of MR spectroscopy In phase and out phase imaging is
used to detect fat in lesion.
Normal in-phase image with TE of 5 ms and out-of-phase image with
TE of 2.4 ms at 1.5T MRI scanner

28. CORRECTIVE MEASURES
 Chemical shift depends upon the bandwidth;
- Narrower the bandwidth higher is the chemical shift. -
Increasing the bandwidth will decrease the artifact.
 Fat suppressed imaging can be used to eliminate the
chemical shift misregistration and the black boundary
artifact.
 Use of a spin echo sequence instead of a gradient echo can
eliminate the black boundary artifact.

29. Gibb’s artifacts (Truncation, Edge and Ringing)
- Appear as multiple fine parallel lines immediately adjacent to
high-contrast interfaces such as CSF-spinal cord and the skull-
brain interface .
- These artifacts are problematic in long narrow structures such
as spinal imaging.

30. - Gibbs artifacts occur as a consequence of under
sampling of data for using Fourier transformation to
reconstruct.
-Insufficient collection of samples in either the phase
encoding direction or the readout direction leads to
Gibbs rings as a result of the Fourier transform

31. - These can appear
both phase-encode and
frequency-encode
directions.
- The more encoding
steps, the less intense
and narrower the
artifacts.
( eg :-256 x 256 matrix
instead of 256 x 128)

33.  Corrective measures :
- Increasing the matrix size (i.e. sampling frequency for
the frequency direction and number of phase encoding steps
for the phase direction).
- Use of smoothing filters
- If fat is one of the boundaries, use of fat suppression

34.  Magnetic susceptibility artifacts.
• Susceptibility (χ) is a measure of the extent a substance
becomes magnetized when placed in an external magnetic
field.
• This results from local magnetic field inhomogeneities
introduced by the metallic object into the otherwise
homogeneous external magnetic field B0.
• Materials that disperse the main field are
called diamagnetic.

35.  Materials that concentrate the
field are called
paramagnetic, superparamagn
etic, or ferromagnetic,
depending on the magnitude of
the effect.
 Since ferromagnetic materials
have the largest susceptibilities,
field distortions and MR
artifacts are usually prominent
around metal objects and
implants.
 Axis : Frequency and Phase
encoding

36.  MRI imaging characteristics of susceptibility
artifacts
1. Geometric distortion
2. Focal areas of signal void and regions of very
bright signal resulting from "piling up" signal
assigned to the wrong areas .

38.  Corrective measures :
- Use of spin echo sequence.
- Remove all metals.
- Increasing band width .
- Using thin slice and higher matrix.
- Use of parallel imaging and using radial k-space
sampling.

39.  In cases with metallic hardware, susceptibility artifacts
can be reduced to certain extent by
 Increasing bandwidth,
 Reducing TE,
 Using thin slices and
 Higher matrix.
 However, care should be taken to avoid the heating of
tissue that is adjacent to the metal

40.  Good effects :
 Used to diagnose hemorrhage , hemosiderin deposition
and calcification.
 Forms the basis of post contrast T2* weighted MR
perfusion studies.
 Used to quantify myocardial and liver iron overload.

41. ZIPPER ARTIFACT
 A common equipment related artifact caused by the
leakage of electromagnetic energy into the magnet
room.
 It appears as a region of increased noise with a width of
1 or 2 pixels that extends in the frequency encoding
direction, throughout the image series
 All magnet rooms are shielded to eliminate interference
from local RF broadcasting stations or from electronic
equipment that emits an electromagnetic signal that
could interfere with the MR signal.

42.  Leakage is usually caused by electronic equipment
brought into the MR imaging room and the frequency
generated by this equipment, which is picked up by the
receive chain of the imager subsystem.
 The persistence of the problem even after the removal or
electrical disconnection of all electronic equipment in the
imager room might indicate that the RF shield has been
compromised.

43. Zipper artifact (RF leakage artifact). Images show constant-frequency
artifacts produced by RF leakage from electronic components
brought into the magnet room.

44.  Corrective measures :
- Make sure the MR scanner room- door is shut
during imaging.
- Remove all electronic devices from the patient
prior to imaging.
- If RF shielding is compromised.
 this usually occurs at the contacts between the door and
the jam and may need to be cleaned or repaired.
 the penetration panel where the cables enter the room is
another site to be checked.

45.  K -space artifact/straight
line artifacts:
- Due to bad data point in k – space.
- Seen as regularly spaced straight
lines through MR image due to spike in
k-space.
- Due to loose electrical connections
and breakdown of interconnections in
RF coil.
- The location of spike and its
distance from centre of k space
determines the angulation b/w lines
and distance between them.

48.  Corrective measures :
- For RF interference , site of RF leak should be
located and corrected.
- Sources of RF inside the room should be removed.

49.  On the other hand, the pattern of stripes produced
by a spike in k-space can be used for tagging, an
important technique in cardiac imaging.
 Here, preparation pulses are applied prior to the
imaging sequence. The preparation pulse has an
excitation profile such that it forms echoes in
different parts of the k-space.
 Fourier transform of such images produces tags in
a gridlike pattern, with known spacing between the
stripes.

50.  In cardiac imaging, these tags are applied at the
start of each cardiac phase, and cardiac image
acquisition then is performed at multiple phases of
the cardiac cycle.
 Following the changes in tag position during the
cardiac cycle from the point of initial placement can
be useful for assessing cardiac motion

51.  Shading artifacts: ( dielectric artifact)
- The image will have uneven contrast with loss of
signal intensity in one part of the image.
- Frequency and phase encoding direction.
- Causes :
1. Uneven excitation of nuclei within patient .
2. Abnormal loading of coil/coupling of coil.
3. Inhomogeneity of magnetic field.
4. Over flow of analog to digital converter.

52. Improper loading (connection) of the coil

53. Corrective measures :
- Load the coil correctly.
- Shimming to reduce inhomogenecity of field
- Acquire the images with less amplification to avoid
ADC overflow

54. Cross excitation and cross talk artifacts :
 Loss of signal within a slice because of pre-excitation
from a RF pulse meant for the adjacent slice.
 The frequency profile of the RF pulse is imperfect.

56.  Cross talk artifacts :
- Same as cross excitation.
- Produced when RF pulse is switched off.
- Due to dessipation of energy to nuclei from
neighbouring nuclei.
- Seen in slice selection gradient.

57.  Corrective measures :
 Increase interslice gap.
 Scanning with interleaved slices with odd and even
numbered slice respectively.
 Optimized RF pulse that have a more rectangular
slice profile can be implemented.

58. Parallel imaging artifacts :
 Parallel imaging allows for faster image acquisition
and potentially improved image quality by
undersampling k-space.
 High acceleration factor and small FOV is used.
 Graininess in the image.
 Can be reduced by using lower acceleration factor
and increasing the FOV.

60.  Moire fringes :
- 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 through two screen windows.

61.  Corrective measures :
- Surface coil
- Shimming

62. Magic angle artifacts :
- An artifactual T2 bright signal seen in tendons and
ligaments that are oriented at about a 55 degree angle to
the main magnetic field.
- A bright signal from this artifact is commonly seen in the
rotator cuff and distal patellar tendon.
-Not to be mistaken for tear.
-Tends to occur only on short TE sequences (e.g. T1, GRE,
PD).
Remedy :
Sequences with a longer TE can be used to avoid this
artifact

64. To summarise:
- MR artifacts hinders the image quality.
- MR artifacts are confused with pathology.
- Identifying them and correcting them by radiologist.