Ideally, we would like to obtain our images from contiguous slices. In practice, there is always some excitation outside the desired slice boundaries (usually referred to as slice "cross-talk"). This means that when exciting a particular slice with an RF pulse, neighboring slices will also experience some partial excitation at the edges nearest to the excited slice. Each pulse sequence uses different RF excitation pulses, and the degree of out-of-slice excitation is therefore particular to the pulse sequence. In general, if slices are acquired using an interleaved ordering scheme*, it is possible to use zero gap without noticing any loss of signal. However, if a sequential acquisition is used, inserting small gaps between slices may be necessary. For most pulse sequences, we recommend using gaps of approximately 10% of the actual slice thickness. (See Figure 1.10)

Figure 1.10 Due to the finite length of RF pulses used in imaging pulse sequences, the slice profiles of the pulses are imperfect, resulting in some excitation of the neighbouring slices. An interleaved acquisition helps to reduce the amount of signal loss that occurs as a result of imperfect slice profiles.

In orthopedic imaging, a very long TR is often used in order to acquire the large number of thin slices that are required for the desired coverage. The default acquisition scheme for FSE is that the odd slices are acquired first, then the even slices within the same TR. If the TR is long (~4000ms), enough time will have passed between excitation of neighboring slices to allow significant recovery of the out-of-slice magnetization. Zero gap can usually be used in this case with very little noticeable signal loss from slice crosstalk.

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* An interleaved acquisition means that the odd slices are acquired in the first pass (i.e. Slices 1,3,5,7,...) and the even slices are acquired in the second pass (Slices 2,4,6,8,...)