Any artefact that can affect a 2D image can affect a 3D image in the same way, although they can appear differently due to the difference in spatial localisation method. The phase encoding for partitions is prone to the same motion ghosts as normal in-plane phase encoding, but they will extend in the slice select direction and be noted on adjacent images. These can be seen across the slice select direction so the ghosts of the pulsatile object may not appear in the same slice as the artefact, nor in every slice. In multi-slab 3DFT, ghosts in the slice select direction will be restricted to the slab containing the source.
Aliasing is common in the slice select direction whenever the object is larger than the slab width. The signals from outside one edge of the slab will wrap around to the slices at the opposite edge of the slab. This is commonly seen as extra ears on sagittal head slabs, or bilateral display of the fibulae in knee sequences. Restricted view coils can minimise this aliasing. Phase over-sampling in the slice select direction is more effective but costs time.
The profile of the slab is prone to the same distortions as 2DFT slice profiles, and the poor slice profile common to many short TR selective 3DFT sequences is frequently seen. Edge slices will appear with poor signal level and little contrast. The severity of the effect depends entirely on sequence design and needs to be assessed individually. Its appearance is often compounded by slice direction aliasing. It is overcome by extending the thickness of the slab by 10-30%, acquiring extra partitions and discarding the poor images.
The slice profile of a partition is rectangular, so there is no loss of contrast due to cross excitation (cross-talk) between partitions.
No comments:
Post a Comment