How does Phase mismapping (motion) appear?
Ghosting. Replicates anatomy along the image.
This always happens in the phase direction
What causes phase mismapping (motion)
Motion
What are some remedies for phase mismapping?
- Swap phase/frequency
- Use pre-sat pulses
- Respiratory compensation
- Cardiac gating
- Gradient motion re-phasing
- Immobilization
- Communication with patient
- Sedation
How does the aliasing (wrap-around) artifact appear on the image?
Anatomy outside of FOV wraps inside of FOV.
Appears on opposite side of image.
Can be in frequency or phase direction
What causes aliasing?
Anatomy outside of FOV produces signal. This signal is encoded and placed inside FOV
How can you remedy aliasing?
- Increase FOV.
- Use pre-saturation bands.
- Oversampling.
How does the truncation artefact appear on an image?
Banding at the interface of high and low signal. High and low intensity bands.
What causes the artefact truncation?
Under-sampling of data (too few lines of k-space)
How can you remedy truncation artefacts?
- Increase matrix size.
- Decrease pixel size
How does the RF artefact (zipper artefact)
RF lines will run parallel to the phase direction, perpendicular to the frequency direction.
Looks like a zipper. Alternating light and dark.
Happens in frequency direction
What causes RF (zipper) artefact?
RF interference entering room
How do you remedy RF artefact?
- Remove electronic objects
- Maintain proper humidity in room
- Call engineer
How does magnetic susceptibility artefact appear?
Distortion of image and voids of signal.
Frequency and phase direction.
What causes magnetic susceptibility artefact?
Ferromagnetic metal such as braces, screws, or pins.
Natural iron of a haemorrhage.
How can you remedy magnetic susceptibility artefact?
- Thorough screening
- Use SE or TSE (rather than GRE)
- increase bandwidth
What does the partial volume effect look like?
Inhomogenous signal. More pronounced in area of body where large difference in signals.
What causes partial volume effect?
Different anatomical structures with different signals in same pixel
How do you remedy partial volume effect?
Reduce pixel size
What artifact is seen in this image?
crosstalk
Dark bands of signal loss are seen crossing horizontally through the axial acquisition
What artifact is seen in this image?
magic angle artifact
What artifact is seen in this image?
aliasing
What artifact is seen in this image?
gibbs/ truncation
What artifact is seen in this image?
Zipper artifact
What artifact is seen in this image?
Phase mismapping (motion)
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MRI in Practice: Ch. 1 Basic Principle24
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MRI In Practice: Chapter 2 Image Weighting and Contrast26
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MRI in Practice: Ch 3 Encoding & Imaging Formation14
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MRI in Practice: Ch 4 Parameters and Trade-Offs20
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MRI in Practice: Ch 5 Pulse Sequences15
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Anatomy, Pathology and Vitals29
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MRI Safety17
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Scan Coverage6
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Handbook Of MRI Tecnique: Ch 25
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Sequence Parameters11
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Artefacts25
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Memorize These for CAMRT2
