Improved signal-to-noise ratio in EPI sequences with highly asymmetric spin echo and highly asymmetric STEAM preparations (HASE-EPI and HASTEAM-EPI)

  • Manoj ShresthaEmail author
  • Ulrike Nöth
  • Ralf Deichmann
Research Article



Alternative versions of echo-planar imaging (EPI) sequences with standard spin echo (SE) or stimulated echo acquisition mode (STEAM) preparations are proposed, allowing for improved signal-to-noise ratio (SNR), especially in high-resolution imaging. The suitability of the proposed sequences, dubbed “highly asymmetric SE-EPI” (HASE-EPI) and “highly asymmetric STEAM-EPI” (HASTEAM-EPI), is tested in vivo, for anatomical imaging with \(T_{2}\) weighting and diffusion-weighted imaging (DWI).

Materials and methods

In HASE-EPI and HASTEAM-EPI, echo formation occurs prior to the EPI readout, rather than at k-space centre as in standard SE-EPI and STEAM-EPI. This allows for a constant preparation period, independent of the spatial resolution. The proposed sequences are compared to their standard counterparts, via simulations and experimentally via in vivo anatomical imaging and DWI.


HASE-EPI and HASTEAM-EPI yield SNR improvements for large matrix sizes and fully sampled EPI readout. Simulations and in vivo results show a signal gain in HASE-EPI versus SE-EPI or HASTEAM-EPI versus STEAM-EPI for white and gray matter, particularly for higher spatial resolution with full readout in anatomical imaging and DWI. However, simulations also show that in the case of very long EPI-readout trains, HASE-EPI and HASTEAM-EPI are more prone to pixel broadening due to relaxation effects.


In contrast to commonly used SE-EPI or STEAM-EPI, the proposed sequences facilitate the acquisition of anatomical imaging and DWI data with improved SNR, especially for full EPI readouts. However, the applicability of HASE-EPI and HASTEAM-EPI should be carefully assessed: while signal gains are less pronounced when using parallel imaging and/or reduced matrix sizes, there may be pixel broadening for very long EPI-readout trains.


Asymmetric echo Short echo time Signal-to-noise ratio Asymmetric imaging Diffusion-weighted imaging 


Authors’ contribution

Protocol/project development: MS, RD. Data collection and management: MS, UN. Data analysis: MS.

Compliance with ethical standards

Conflict of interest

The authors declare they have no conflict of interest.

Ethical approval

All procedures performed in the vivo study were approved by the local ethics committee of the University Hospital.

Informed consent

Written informed consent was obtained from all healthy participants before scanning.


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Copyright information

© European Society for Magnetic Resonance in Medicine and Biology (ESMRMB) 2019

Authors and Affiliations

  1. 1.Brain Imaging Center (BIC)Goethe University FrankfurtFrankfurt am MainGermany

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