This document discusses various artifacts that can appear in MRI images and how to identify, explain, and address them. It covers common artifacts like motion artifacts from respiration or cardiac motion that can be solved with techniques like gating. Other artifacts discussed include chemical shift artifacts between fat and water addressed with fat suppression, truncation artifacts from under-sampling addressed by increasing matrix size, and magnetic susceptibility artifacts from implants addressed with choice of sequence. Identification of artifact location, cause, and solution approach is important for radiologists to properly interpret images.
4. introduction
• In routine MRI, the prescence of artifacts in images is
part of the practice
• If some are readily identified, others can be interpreted
as pathology
• There fore it is important to
identify where they appear in the image
know why they appear
know how they can be solved
where, why, how
6. How
• Some artifacts can be solved with the intervention of
the MR technician or MR physicist;
• But some of them have to be solved by an MR
engineer
• Therfore it is very important to learn where those
artifacts appear in the images in order to decide the
best cource of action
7. Motion artifact
• MRI is very sensitive to motion artifact since it takes time to
acquire the image using the conventional SE sequence.
• Also caused by – cardiac and respiratory movement,
peristalsis
- moving blood or csf.
8.
9. • Eliminated by using different techniques.
1. Respiratory compression
2. Respiratory gating
3. Cardiac gating
4. Phase & frequency direction swap
10. Phase Wrapping
• Caused
by phase encoding errors.
occur whenever the dimensions of an object
exceed the defined field of view
• Appears
Structures extending beyond the right
margin will be wrapped around to the left margin
of the image and vice versa.
11.
12. Chemical Shift
chemical shift effects can be used to
selectively suppress the signal from fat.
• cause
• occur on the basis of two mechanisms:
• 1, spatial mis-registration
• between fat and water (type 1) and
• 2, cancellation of the signal at the interface
between fat and
water (type 2)….on GE
13.
14. remedy
• Use SE sequence
• Use fat suppression techniques
• Use wider reciever bandwidth
Usage
tissue characterization
diagonsing fatty pathologies
15. Truncation Artifacts
• Its other names are Ringing, Gibbs Or Spectacle
Leakage Artifacts.
Cause
results from under - sampling of data (too few K space
lines filled) so that interfaces of high and low signal are
incorrectly represented on the image
Appear
as semicircular or straight bright or dark lines
immediately adjacent to high contrast interfaces..i.e CSF
and spinal cord
(it can mimic syrinx)
16.
17. Eliminated
Increasing matrix size along the phase-encoding
direction
Gradient reorientation will displace the artifacts to
another portion of the image.
18. Magnetic susceptibility
• Causes- foreign bodies/implants
-calsium hydroxyapatite
-gadolinium chelate accumulation
-iron oxide or hemosiderin.
• Appear - signal voids and/or image distortions
Remedy; SE & short TE
Usage
I. Detection of hematomas
II. Quantification of liver iron content
III. Contrast agents
•
19.
20.
21. Magic angle
Tendons and ligaments have a very short T2 relaxation time.
But, when they are oriented in 55o to the z axis their signal
may increase and mimic pathology.
Appear
as a bright signal
22.
23. EDDY ARTIFACT
• Caused
• By rapid on/off of the gradient coil produce a small
current.
• Appears
As a signal drop in the margin of the image.
• Remedy
Shield gradient coils
24.
25. Partial volume artifact
The signal intensity of different tissues and structures
that are located in the same voxel are averaged.
Remedy
use thinner slice thickness
26. Axial T1WI of the brain at exactly the same level. Second image shows 7th and 8th cranial nerves (arrow) but the first one merely depicts them.
The reason for this is the partial volume averaging. The first slice was taken at thickness of 10 mm while second slice was taken at 3 mm.
27. Zipper artifact
• Causes imperfectly closed room
• supportive equipments e.g pulsoximetry
• Appear : as a line of alternatively bright and dark pixels
running through the image in two ways,
• phase encoding direction
due to radio frequency noise and external effect
• frequency encoding direction
imperfect slice selection or inadequate radio frequency
pulse.
28.
29. Crisscross or Herringbone Artifacts
• caused
due to a data processing or
reconstruction error.
• Appears
It is characterized by an obliquely oriented
stripe that is seen
throughout the image.
• Eliminated
reconstructing the image again.
Truncation artifact. Sagittal fat-suppressed fast spin- echo T2-weighted image of
the cervical spine shows a band of increased signal intensity within the spinal cord. This
mimics a syrinx and is due to insufficient phase-encoding steps in the anterior-posterior
direction.
Fig. 15: This tiny piece of metal (square) visible on a shoulder x-ray caused the magnetic susceptibility artifact seen on the MR image (T2*). The loss of signal and
image distortion caused by that metal piece significantly impairs our interpretation of the images.
Axial T1WI of the brain at exactly the same level. Second image shows 7th and 8th cranial nerves (arrow) but the first one merely depicts them. The reason for this is the partial volume averaging. The first slice was taken at thickness of 10 mm while second slice was taken at 3 mm.