TwinTree Insert

10-03 Multiecho Sequences

pin-echo sequences are not limited to a single 180° pulse and echo. Their ad­van­t­a­ges lie in the possibility to form a multi­tude of echoes by trans­mit­ting a train of 180° pulses. Thus, we can create a number of images with in­creas­ing TE va­lues.

The best-known MSE sequence is the Carr-Purcell-Meiboom-Gill (CPMG) se­quence.

Single and multiple SE sequences can be ac­qui­red in single-slice, multiple-slice and 3D modes.

Multislice imaging is limited by TE and TR. Its contrast is also influenced by the gaps between the slices and flow in ves­sels. In 3D imaging, the entire sam­ple vo­l­u­me is excited si­mul­ta­ne­ous­ly and slices are obtained by the use of an additional phase-encoding gra­dient. However, 3D imaging with an SE sequence is time-con­­sum­ing and pro­­hi­­bi­­ti­­ve­­ly long in clinical settings.

The efficiency of a multiecho se­quence (MSE) is far higher than a single echo or an in­ver­sion-recovery sequence, both in terms of examination time and in the creation of con­trast. They allow the creation of T1- and T2-dependent (T1- and T2-weighted) ima­ge con­trast­  [⇒ Rinck 1983].

10-03-01 Rapid Spin Echo

Multiple spin-echo data acquisition can be accelerated by Fast or Rapid Spin Echo (RSE) sequences (e.g., RARE). A further addition to the RSE armory can use lon­ger echo trains (HASTE, GRASE, and similar sequences).

Rather than using the same amount of phase-encoding for each echo, and each echo as one line for an image associated with a particular TE, different amounts of phase-encoding can be applied. Thus, these echoes can be implemented as dif­­fe­­rent lines in k-space in a single image.

The numbers of echoes per ex­ci­ta­tion which are incorporated into one image de­ter­mine the time-saving factor of the sequence.

If one has a spin-echo sequence with a 256×256 image matrix and a da­ta ac­qui­si­tion time of 256 seconds, in an RSE se­quen­ce with 8 echoes data ac­qui­­si­tion will take 256/8 = 32 seconds; with 16 echoes, da­ta acquisition will only take 256/16 = 16 se­conds.

Multiple-slice se­quences will take longer, according to the number of slices.

RSE sequences add two more parame­ters to TR and TE: the number of echoes per ex­ci­ta­tion (also called echo train length, TSE factor, or turbo factor) and the echo spacing. Since the echo time is close to a time average in RSE sequences, TE is called effective TE (Figure 10-06 and Table 10-04).

Figure 10-06:
A rapid spin-echo sequence utilizes an initial 90° pulse followed by multiple 180° refocusing pul­ses, pro­duc­­ing an echo train. In our example, all echoes are used for one k-space. The echo with the 'ef­fec­tive TE' is as­signed to the center slab of k-space and determines overall image contrast; the data of other echoes are placed in the slabs further away.

Table 10-04:
The pulse-sequence parameters of an RSE sequence are different from those of a conventional SE se­quen­ce (with the exception of TR).

It is not as obvious as with conventional imaging techniques what sort of con­trast we can obtain in RSE sequences.

A straightforward signal-intensity cal­cu­la­tion, similar to those in conventional 'pure' pulse se­quences, is not possible.

Basically, the con­trast in Rapid Spin Echo sequences depends on the order in which we ap­ply the phase-encoding. Contrast manipulation is achieved by dif­fe­rent order­ing of the contributions in k-space; neither TR nor TE are changed, but dif­fe­rent echo­es are assigned to the reconstruction in k-space. Images can be proton den­­si­­ty- or T2-weighted.

There is less speed advantage in T1-weighted RSE compared to conventional SE ima­ges; T1-weighted contrast can only be obtained through inversion re­co­ve­ry or par­tial saturation with the application of shorter TR.

However, for a large number of clinical questions ρ-weighted images can sub­sti­tute T1-weighted ima­ges.

The trade-off of RSE is several, although subtle, dif­ferences in contrast, most im­por­tant­ly due to the different signal intensity of fatty tis­sues (Figure 10-07) [⇒ Hen­kel­man 1992].

Figure 10-07:
Comparison of rapid spin echo and con­ven­ti­o­nal spin echo. Left: RSE: ETL = 8; eff. TE = 64 ms; ES = 16 ms; TR = 3000 ms
Right: SE: TE = 64, TR = 3000 ms.
With the exception of the signal from sub­cu­ta­neous fat, contrast is very similar.

In RSE images, the lipid signal is usually higher than on si­mi­lar SE images. This is claimed to be caused by several factors, including spin coup­ling among glyceride pro­tons.

There are also magnetization-transfer phenomena that cause protein con­tai­ning tis­sues to appear darker than on similar SE images, whereas hemorrhage with he­mo­si­de­rin will appear less dark and CSF will appear relatively brighter, obliterating con­trast between the ventricles and, e.g., periventricular multiple sclerosis pla­ques.

One also might exchange image sharpness for either enhanced or blurred edges, de­­pend­ing on whether k-space is acquired first or last.