Artifacts either by distorting the
k-space trajectory (i.e. due to imperfect
shimming) or as a consequence of the reduced
bandwidth in the
phase encode direction, commonly with EPI
sequences.
While a standard
spin warp-based sequence has an infinitely large
bandwidth in the
phase encode direction (about 1 or 2 kH), the
bandwidth in EPI is related to the time between the
gradient echoes (about a millisecond).
Hence even small
frequency offsets can result in significant shifts of the signal in the
phase encoding direction.
Segmentation can introduce ghosting if there are significant difference in the
amplitude and
phase of the signal. This can be a particular problem when trying to acquire the segments in rapid succession.
Suitable choices of
excitation schemes and/or subsequent correction can help to reduce this
artifact.
The signal from fat can easily be offset by a large fraction of the
FOV, and must be suppressed. The effect of
frequency offsets can be reduced by collecting data with more than one
excitation, which effectively increases the
bandwidth in the
phase encoding direction.
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Case Study 1 :
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