When a multi
shot technique is applied, each
shot will have its own effect on the
prepulse, with a
scan time increase. Multiple shots allow a shorter IR delay but at the cost of increased
scan time.
In multi
shot technique (also called mosaic imaging), a group of samples, which are contiguous in k space are acquired in the same sequence repetition. The
phase encoding steps or profiles are split into 'shots' (sub-acquisitions). The
shot interval is the time between the shots. Usually kept as short as possible. Because the
acquisitions are divided into different shots, each
shot will have less T1 variation, thereby increasing T1
contrast. Two excitations, each requiring the data for one half of
k-space, are the simplest variation of multi
shot techniques (e.g. positive versus negative
phase encoding).
The alternative to this mosaic strategy for multi
shot EPI is interleaving. In interleaved
sequences, each repetition acquires every nth (n is the number of shots) line in
k-space and for the complete
raw data set the various repetition data are interlaced.
See also
Single Shot Technique.