HyperechoesWhat are "hyper-echoes" and how do they differ from regular echoes?
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A hyperecho is a high amplitude stimulated echo that can be made to appear when RF-pulses are arranged in a particular symmetric order. The concept was developed by Juergen Hennig from the University Hospital of Freiburg and is offered as a Turbo SE option on some Siemens scanners.
The special arrangement of RF-pulses in a TSE sequence is shown in the diagram below. At the center of the pulse train is a 180º-pulse which is flanked on either side by a mirror images pulses with opposite polarities and phases. The inverse symmetry of the pulses after the 180º-pulse exactly undo the complex rotations produced by the pulses before the 180º-pulse, allowing the strong hyperecho to refocus at the expected echo time. The hyperecho has nearly same amplitude as a regular spin echo without all the intervening pulses.
Surprisingly, as long as these symmetry relations are maintained, hyperechoes will form regardless of the flip angles and phase shifts of the individual pulses. The hyperecho process may be repeated several times in a TSE echo train. Typically hyperechoes are generated to coincide with the lowest order phase-encoding steps to produce maximal signal-to-noise. Although currently primarily used in conjunction with TSE/FSE sequences, hyperechoes have potential application for diffusion-weighted imaging, GRE imaging, and MR spectroscopy.
References
Frank LR, Wong EC, Liu TT, Buxton RB. Increased diffusion sensitivity with hyperechos. Magn Reson Med 2003; 49:1098-1105.
Hennig J, Weigel M, Scheffler K. Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS). Magn Reson Med 2003;49:527–535.
Hennig J, Scheffler K. Hyperechoes. Magn Reson Med 2001;46:6–12.
Mugler JP III. Optimized three-dimensional fast spin echo MRI. J Magn Reson Imaging 2014;39:745-767. (Excellent up-to-date review).
Weigel M., Hennig J. Contrast behavior and relaxation effects of conventional and hyperecho-turbo spin echo sequences at 1.5 and 3T. Magn Reson Med 2006;55:826-35.
Frank LR, Wong EC, Liu TT, Buxton RB. Increased diffusion sensitivity with hyperechos. Magn Reson Med 2003; 49:1098-1105.
Hennig J, Weigel M, Scheffler K. Multiecho sequences with variable refocusing flip angles: optimization of signal behavior using smooth transitions between pseudo steady states (TRAPS). Magn Reson Med 2003;49:527–535.
Hennig J, Scheffler K. Hyperechoes. Magn Reson Med 2001;46:6–12.
Mugler JP III. Optimized three-dimensional fast spin echo MRI. J Magn Reson Imaging 2014;39:745-767. (Excellent up-to-date review).
Weigel M., Hennig J. Contrast behavior and relaxation effects of conventional and hyperecho-turbo spin echo sequences at 1.5 and 3T. Magn Reson Med 2006;55:826-35.
Related Questions
Why would you want to use variable flip angle FSE? Wouldn't smaller flip angles kill the MR signal?
Why would you want to use variable flip angle FSE? Wouldn't smaller flip angles kill the MR signal?