1. Distortion artifacts in MRI and
their correction
Vibha Chaswal, Ph.D.

2. MRI Gradient fields
• The gradient fields are superimposed over the static
magnetic field. For transverse images:
• Z gradient–selects the slice, and is applied whilst the 90° RF
pulse is on
• X gradient–creates a frequency change in emitted signal
producing lines (or columns) of data, and is applied when
the echo RF is being measured
• Y gradient–produces a change in the phase of rotation in
different voxels, and is applied between the 90° and 180°
RF pulses at a different strength during each cycle. The
number of phase steps (e.g. 256) affects scan time.
• “Frequency encoding” and “Phase encoding” directions (X
and Y) are interchangeable.

3. Gradient spatial encoding
Pulse sequencing diagram

4. MRI distortion Artifacts
A whole range……..
Equipment-related
• Non-uniformity of signal
• Wraparound (phase wrap,
aliasing)
• Peripheral signal artifacts
• Slice overlap (cross-excitation)
• Noise and RF interference
• Truncation (ringing, Gibbs
artifact)
• Phase smearing
Patient-related
• Signal void due to freezing
• Partial volume averaging
• Chemical shift
• Motion artifacts (blurring,
ghosting and pulsatility)
• Flow artifacts
• Susceptibility and image
distortion
• Magic angle

5. B0 inhomogeneity and susceptibility
• Extremely homogeneous static magnetic field is required
around isocenter of the magnet for MRI.
• B0 homogeneity of the empty magnet:
typical value of 1ppm (0.001%) in a 50 cm diameter
spherical volume.
• susceptibility = local changes in Bo due to a substance’s
own magnetic properties
• Artifacts occur at interfaces between substances of different
susceptibilities, e.g. air/bone/metal/ calcification/hemorrhage;
implants and previous surgery, tooth fillings, mascara, ear-rings,
hair-slides
– worse at higher field strengths

6. B0 inhomogeneity
• Decreased frequency-selective fat-saturation efficacy of pulses
• Effect more pronounced the further the image slice is positioned from
the isocenter of the magnet
(a) off-center acquisition with insufficient fat
suppression (arrows)
(b) Identical acquisition at isocenter
Reduction:
improve field shimming
Optimal positioning of the patient in z-direction
Dietrich et. al., Artifacts in 3-T MRI: Physical background and reduction strategies, EJR, 65, 2008

7. B0 inhomogeneity
•
Relatively small frequency shifts lead to substantial reduction of the
transversal steady-state magnetization and, thus, to band-shaped signal losses
in the image
Reduction:
Move the artifact out of the region of interest (apply frequency offset to nominal
resonant frequency)
Dietrich et. al., Artifacts in 3-T MRI: Physical background and reduction strategies, EJR, 65, 2008

8. Susceptibility: Metal artifacts
•
An important manifestation of a susceptibility-related artifact is the signal
loss in gradient-echo images around metallic implants or surgical clips
Right hip prosthesis is seen as region of signal drop
out (closed arrows)
Figure from: Mechlin et al, AJR 143, December 1984
STIR axial MRI image of a patient, arrows B point to
artifacts due to right hip prosthesis.
Figure from: Dr. Geller et al, Malignant Peripheral Nerve
Sheath Tumors (MPNST), ESUN, june 2006, 3(3)
http://sarcomahelp.org/learning_center/mpnst.html
Reduction:
Decrease Echo time, TE, of pulse sequence
Increase receiver band-width

9. Susceptibility: Geometric distortion
artifact
•
•
Usually in echo-planar imaging (EPI)
At interfaces between soft-tissue and bone/air e.g., in the
base of the skull or in the head-and-neck region
Reduction:
a) decrease the echo-spacing of the
read out train e.g., by increasing
the receiver bandwidth or by
parallel imaging techniques
b) Use fast-spin echo technique
(b) EPI acquisition without parallel imaging, severe distortion
artifacts are visible (arrows) and (c) EPI acquisition with
acceleration factor 2, distortion artifacts are still present but
considerably reduced (arrows).
Dietrich et. al., Artifacts in 3-T MRI: Physical background and reduction strategies, EJR, 65,
2008

10. B1 inhomogeneity
• Spatial inhomogeneity of the B1 field results in
flip-angle deviations depending on the spatial
position
• Reduced signal intensity in these areas or to
altered contrast particularly in FLASH sequences
whose T1-weighting depends on the flip angle

11. Dielectric resonance effects/ RF
interference/ standing wave effect
•
Constructive or destructive interferences of the transmitted RF field may be observed
resulting in either regional (e.g., central) brightening or regional signal loss,
respectively.
Signal loss due to wavelength effect
Reduction:
a) Effect can be mitigated by positioning a dielectric cushion close to the ROI, or
b) Manually modify the RF-transmitter amplitude in order to reduce B1-inhomogeneityinduced signal loss

12. Chemical Shift Artifact
• For MRI in vivo, two important groups of
molecules : protons in water and protons in
fat tissue.
• Larmor frequencies difference of these
protons ~ 3.5 ppm.
• chemical shift leads to a slight geometric shift
of the relative position of water and fat
protons in readout direction.

13. Chemical Shift Artifact Reduction
image-compromising superposition of
fat tissue onto other tissues
Reduction:
a)manually increase the RF-transmitter amplitude and apply image post-processing
filters to obtain more uniform image intensities.
b)Increase (~double) receiver bandwidth
c)Based on anatomy switch readout and phase encoding directions

14. Blood Flow related artifact
• Magnetohydrodynamic effect: The flow of blood ions
perpendicular to the strong static magnetic field, B0, gives rise to
induced voltages and currents;
“(a) Signal loss in the pulmonary vessels in non-gated 3-T single-shot fast-spin-echo
(FSE) acquisition possibly related to the magnetohydrodynamic effect. (b) Non-gated
1.5-T single-shot FSE acquisition demonstrating visual signal in pulmonary vessels. (c)
ECG-gated 3-T single-shot FSE acquisition of the same volunteer as in (a) with data
acquisition during diastole; the vessel signal is restored.” …….Deitrich et al, EJR
65(2008)

15. SNR related artifacts
• Artifacts to Noise ratio (ANR)
• Artifacts are masked by increased statistical
noise.
• Gibbs Ringing: caused by data clipping at the
edges of K-space (raw data readings are still over
noise level at the borders of acquired K-space)

16. Gibbs artifact
Reduction:
Increase spatial resolution or,
Apply reconstruction filters (Hanning filter)

17. Resources
Many more Artifacts but time restriction
⇒Resources for MR Artifacts ad their reduction
⇒MR-TIP: online database (open forum) from University
of Illinois, Excellent resource
⇒ Review article by Dietrich et.al,”Artifacts in 3-T MRI:
Physical Background and reduction strategies” EJR, Vol
65, 2008

18. Thank You