Fast spin echo

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Fast or turbo spin echo (FSE/TSE) is an adaptation of conventional spin-echo (SE) acquisition technique designed to reduce imaging time. It has largely supplanted the original spin-echo technique due to vastly improved imaging speed.

Basic spin echo sequence

In a basic SE sequence, a single echo is measured during each repetition time (TR). FSE is more efficient because multiple echoes are recorded after each 90-degree excitation pulse (multiple echoes per TR). This is achieved by transmitting a series of 180-degree inversion pulses at set intervals and measuring the corresponding echo according to a slightly different phase encoding gradient.  In this way, multiple lines of K space (corresponding to multiple phase-encoded steps) are encoded after a single 90-degree pulse.

Fast spin echo sequence

FSE results in reduced imaging times, with the extent of reduction dependent on the number of echoes produced in each cycle; this is also known as the echo train length. The improvement in imaging time is most powerful when used with a rectangular field of view (e.g. spinal imaging), and the phase encoding direction is chosen to correspond with the smallest matrix size dimension. This minimizes the number of excitation pulse repetitions required per image.

Because echoes are generated using 180-degree inversion pulses, FSE retains the benefit of correcting for external magnetic field inhomogeneity. However, the benefit of reduced imaging time comes at a few costs:

  • reduced signal to noise (SNR)
    • echo amplitude decreases as a function of time from excitation pulse. Thus, later echoes will be subject to lower SNR
  • non-specific T2 image weighting
    • time to echo (TE) affects the T2 weighting of an image, and is defined as the time interval between excitation pulse and peak echo
    • since multiple echoes are acquired per excitation pulse, FSE technique results in varying, progressively increased TE times during each TR acquisition cycle. This is more pronounced with longer echo train lengths
    • the apparent or "effective" TE of the image depends on how the FSE-generated echoes are used to fill K space. A common technique involves filling the center of K space, which contributes the most to image contrast, using the echoes that have desirable TE
  • TSE technique also reduces the number of interleaved slices that can be obtained