Respiration Artefact Reduction
ROPE
- Respiratory ordered phase encoding collects centre of K-space when respiratory motion is minimal.
- Moderate scan time penalty (25%) & not entirely effective.
- Can be useful
Respiratory Gating
Respiratory Triggering
- Data collection restricted to periods of minimal respiratory motion (excitation continues).
- Moderately effective.
- High scan time penalty (50%-100%) & not very effective.
- Increases SAR substantially.
- Essentially useless
Multiple Acquisitions
- Machine based triggering or user selected TR to match breathing period.
- Usable for long TR sequences only
- Quite effective but not robust
Breath Hold Sequences Average motion artefacts over multiple acquisitions Typically only works with NEX >4
[repeat phase step n times then increment, average over TR x NEX]Better with "SMART" averaging (Philips)
(collect all phase steps then repeat acquisition, average over TR x N.phase x NEX)Only moderately effective Increased scan time and opportunity for other patient motion
Ultrafast Acquisition
- Restrict sequence period to <25 seconds
- Extremely effective (no respiratory artefact or repeat the scan)
- Presents difficulties for sequence design and parameter choices.
- Practical application depends on scanner performance specifications.
- Gives sharper images by avoiding motion induced partial volume averaging effects
- Sequential acquisitions of single slice sequences running <1 second
- Obviates need for breath hold (better with it)
- Overcomes some limitations of breath hold sequences
- Practical with high performance equipment
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