SAQs - Chapter 4
1. List the factors that affect SNR.
• Field strength
• Proton density
• Coil type
• TR
• TE
• Flip angle
• Slice thickness
• Matrix
• FOV
• Receive bandwidth
• NEX
2. Which could you change without affecting image contrast or scan time?
Slice thickness and FOV.
3. List three ways of improving the CNR between pathology and normal tissue.
MTC, T2 weighted images, contrast enhancement or chemical suppression.
4. How is K space filling altered in a 50% rectangular FOV?
The incremental step between each phase encode is doubled, which halves the FOV in the phase direction relative to frequency and halves the number of phase encodes performed. The phase resolution is however maintained because the most outer lines of K space are still filled but the scan time is halved because only the phase encoding steps are completed.
5. How would you achieve equal resolution in all reformatting planes in a volume acquisition?
Voxel must be isotropic, i.e. equal dimensions in all planes. Keep matrix square. Calculate pixel dimension by dividing FOV by number of pixels. Select slice thickness that equals this dimension.
6. List four consequences of decreasing the TR.
• Shorter scan time.
• Increased T1 weighting.
• Decreased SNR.
• Decreased slice number.
7. List four consequences of decreasing the phase matrix.
• Shorter scan time.
• Decreased spatial resolution.
• Increased SNR.
• Increased truncation artefact.