TwinTree Insert

06-06 Multiple Slices

number of possible slice configurations in multiple slice imaging is depicted in Fi­g­­ure 06-18. Provided there is no overlap between the slices, then each slice should be totally in­­de­pen­dent of the other slices.

If there is an overlap, the RF pulses used for ex­ci­ta­tion of the different slices inter­fere with each other and interrupt, for in­stance, the T1 re­co­ve­ry in adjacent slices, leading to reduced signal-to-noise ratio and in­flu­enc­ing signal intensities.

Figure 06-18:
Multiple slices: (a) multiple slices with wide gaps; (b) multiple contiguous slices; and (c) multiple over­­lap­ping slices.

Commonly, a small gap is retained be­tween slices to avoid this kind of crosstalk, or the excitation pulses are distributed over two packages; the first one excites the odd, the se­cond one the even slices.

In many pulse sequences, there is a quite long TR between each excitation of a par­ti­cu­lar slice while the magnetization recovers. Because of the relatively long T1 re­la­xa­tion time of tis­sues, a delay of up to three seconds may be necessary before re­peat­ing the excitation. To make the most efficient use of this time, we can excite a number of parallel slices in each in­ter­val, which is achieved by changing the fre­quen­cy of the RF pulse. This procedure can be repeated to pro­duce a series of sli­ces (Figure 06-19).

Figure 06-19:
Top: Excitation of multiple slices within one TR cycle (SE pulse sequence: multi-slice single echo). In this case, five slices can be excited within one repetition cycle. The excitation frequency of the in­di­vi­dual pulses is slightly changed so that only selected nuclei, and thus slices, are excited (cf. Figure 06-17).
Bottom: Excitation of multiple slices within one TR cycle (SE pulse sequence). In each slice, one echo has been added to create a multi-slice double-echo sequence. The number of slices has been reduced to three because of the time restriction.

The number of slices obtainable can be calculated by dividing the repetition time TR by the time required for each slice. For example, if TR = 400 ms and TE = 50 ms, the theoretically possible number of slices is eight (in practice seven, since each slice requires slightly more than TE). If the repe­tition time is long enough, not only several slices but also se­ve­ral images with increas­ing echo times per slice can be created.

This is known as multi-slice multi-echo sequence.

Commonly in an in­ves­ti­ga­tion of the brain, 15 or 16 parallel slices in the trans­verse view with two echoes are ac­quired. The repetition time (TR) is between 2000 and 3000 ms, the times of the two echoes (TE) are 20 and 80 ms.

Multiple slice imaging is not restricted to spin-echo sequences, but can be per­­form­ed with practically all pulse sequences.

One can add inversion pulses to create in­version-recovery images with a relatively long inversion time (TI); during this time, additional slices can be inverted. How­ever, usually the number of slices is limited in IR sequences.