Understanding Artefacts in MRI is very important for effective health service and outcome. dedicated for Radiography students

1. MRI ARTIFACTS
Anjan Dangal, B.Sc. MIT 3rd Year, NAMS,Bir Hospital,
Nepal

2.  Wrap Around
 Truncation
 Chemical Shift
 Susceptibility
 Metal
 Motion
 Flow
 Herring Bone

3. Artifacts
 Any feature that is not the true representation
of Anatomy.
Fourier
Transfor
m
Manifestation of artifacts can only be done by understanding
K Space.

4. Motion Artifact

5. Wrap Around ( Aliasing Artefacts )
 Occurs when the Field of
Fiew
Is smaller than the Physical
Extent of Anatomy.
Result: Anatomy wraps
around form one side to
another

6.  Wrap Around artefact only occurs in phase
encoding direction

7. Remedies
 Increase Field Of view
 Decrease Resolution
 Increase Scan time
 Swape Phase/ Frequency Encoding Direction
 Live it It !

8. Truncation Artifacts ( Gibbs
Ringing )

9.  Sharp Border ( From Dark to light ) / Edges
cannot be accurately represented by
prescribed resolution
 Ringing of the edge
Why is st called Truncation Artefact ?
When we acquire data , we fill them in K space

10. Worsens Spatial Resolution

11. Remedies for Truncation
 Reason : Insufficient Resolution
 Increase Resolution
 Increase Matrix size ( Scan Time )
 Decrease FOV ( Wrap Around Artefact )

12. Chemical Shift Artifact

13.  Chemical shift refers to small changes in
resonant frequency due to different molecular
environments of nuclei.
 alternating bands of white and dark bands as
indicated by the arrows on either side of the fat
water interface

14.  physical key fact of these artifacts is a shift of
the Larmor frequency of water and fat protons,
due to a little difference of the local magnetic
field which is caused by shielding electron
clouds (“chemical”).

15.  Reconstructed position of fat is shifted relative
to its true position.
water
fatwater
fat
True Position
Reconstructed
Position

16.  Amount of shift is related to difference in
larmor frequency of fat and water.
 Diffrence in frequency varies with magnetic
field strength

18. Remedies
 Fat saturation
 Increase Band Width ( decrease readout
direction )

20. but the minimized water fat shift produces
more noise and a less signal to noise ratio.

21.  The artifact is most commonly seen around
water-containing structures (liver, kidneys,
optic nerves, thecal sac, muscles, nerve
fascicles) surrounded by fat

22. Susceptibility Artifact
 Magnetic Susceptibility
 Proporties of substance that determines the magnetic field
it sees

23.  Tissues with different susceptibility can give
rise to susceptibility artifact
 As they would precess
 At different freq

24.  T2 Ã-weighted
magnetic resonance
imaging of the brain
demonstrates
susceptibility artifact
within the mass

25.  Dephase and signal
loss
Bo

27. Remedies
 Increase Bandwidth ( Decreases time of
Readout )
 Use Spin Echo Sequence ( Corrects for
Inhomogeneity )

28. Motion Artifact

29.  MR scans seconds to minutes
 Physiologic motion Seconds
 Data in convinent : one scan to next
 Leads to motion artifact

30. Nature
 Periodic motion: Blurring and Ghosting (
Repetition of Anatomy )
 Ghosting is dramatic and obscure anatomy of
interest
 Blurring is more subtle but degrade quality of
information

31. Remedies
 Depends on nature of motion
 Cardiac : Cardiac Gating
 Respiratory : Breath Holding, Motion
compensator ( Navigator Echoes)

32. Flow Artifact
 Periodic oscillation
by blood flow

33.  Signal void

35.  Spatial saturation

37. Zipper Artefact
 where one or more spurious bands of
electronic noise extend perpendicular to the
frequency encode direction and is present in
all images of a series.

39. Remedy
 make sure the MR scanner room-door is shut during imaging
 remove all electronic devices from the patient prior to imaging
 if the artifact persists despite all nearby electronic equipment
being turned off, it is possible that the 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

40. Herring Bone Artefact
 for some reason a particular
spatial frequency has a
larger than expected
contribution

42. Remedies
 IF Recurrent : Hard ware Problem
 Spurious Occurrence
 Re Scan