SAQs - Chapter 7
1. What is magnetic susceptibility?
Magnetic susceptibility artefact is the ability of an object to become magnetized. Metal objects cause an artefact because they magnetize to a different degree than tissue.
2. How is magnetic susceptibility artefact reduced?
Using spin echo pulse sequences. Remove metal and reduce the TE.
3. What is the difference between chemical shift and out of phase artefact?
Chemical shift relates to the difference in precessional frequency between fat and water whereas out of phase artefact relates to the fact that as fat and water precess at different frequencies, they are only in phase with each other at certain periodicities.
4. What is the periodicity of fat and water at 1.5 T?
4.2 ms.
5. What is the relevance of this in parameter selection?
A TE that is a multiple of the periodicity must be selected to reduce out of phase artefact in gradient echo sequences although this artefact can be useful under certain circumstances.
6. Under what conditions would you get phase wrap or phase aliasing?
When anatomy that is producing signal occurs outside the FOV in the phase direction.
7. List the ways in which phase wrap or phase aliasing may be reduced.
• Use anti-aliasing software.
• Swapping phase and frequency may reduce this artefact; swapping encoding directions means that the shortest axis of the
anatomy is along the phase direction.
• Enlarge FOV in the phase direction.
8. Under what conditions would you get frequency wrap or frequency aliasing?
When frequencies are not sampled often enough to satisfy the Nyquist theorem.
9. How may frequency wrap or frequency aliasing be reduced?
Increase the digital sampling frequency or use frequency filters.
10. What is the difference between cross-talk and cross-excitation?
Cross-talk occurs when energy is given up to nuclei in adjacent slices during relaxation. Cross-excitation is caused by RF pulses exciting nuclei in adjacent slices.
11. Which can be reduced and how?
Only cross-excitation can be compensated for by having a gap between slices, squaring the RF pulses off so that they fit the slices or by interleaving the slice acquisition.
12. How is magic angle artefact compensated for?
Changing the TE or altering the angle of the anatomy relative to B0.