Fast Spin Echo

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The change in phase of spins during EPI data collection, which causes image warping, can be greatly reduced by applying 180º RF refocusing pulses (described above) multiple times during the middle of data acquisition. This strategy is less efficient than EPI because data cannot be collected during each RF pulse application. However, the improvement in the quality of a fast spin echo (FSE; Fig. 11B) is often worth the loss in efficiency.3 These RF pulses heat the patient like very inefficient bursts of microwave energy; thus, there is a limit to the number of refocusing pulses that can be applied in a given duration.

Figure 12. Illustration of the data collection coordinates in k-space for (A) conventional and (B) spiral techniques.  Position along each axis represents the number of phase cycles of the induced spatial wave in each respective direction.
Turbo Spin Echo Sequence with three echoes 

Applications

The FSE sequence is extremely versatile and is used everywhere. It is by far the most commonly used sequence in MRI today. The FSE can be tailored to T1, T2, or Proton Density Weighted imaging.    FSE can be pushed for high resolution imaging in extremities and neuro work and can result in very fast breath hold imaging in the abdomen.

Advantages

	Decreased scan times allowing for more signal averages and/or higher resolution
	Heavy T2 weighting
	Versatile - can be used for all weightings in all body regions/parts
	Long TR's (good for T2 weighted imaging)

Disadvantages

	Many more parameters to learn/understand
	Edge blurring (not as common with today's short IET's)
	Bright fat on T2 weighted imaging (due to lack of J-coupling)
	Long TR's (bad for T1 weighted imaging)