Center for Magnetic Resonance Research, Department of Radiology
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In order to accelerate whole brain coverage for functional mapping in the human brain beyond what we achieved with the introduction of Multiband (MB) EPI method [1, 2], we combined the MB EPI approach with the simultaneous image refocused (SIR) scheme  which temporally interleaves signals from several slices within an EPI echo train. In this new approach, referred to as Multiplexed-EPI (M-EPI) , if multiband pulses with m bands are used for each of the s temporally interleaved RF pulses in the SIR technique (where m and s are positive non-zero integers), the result is simultaneous acquisition of m times s (i.e. m•s) slices in a single echo train.
FIGURE 1: Pulse Sequences for Multiband (MB) EPI and Multiplexed EPI
Figure 1 illustrates the MB EPI and Multiplexed-EPI pulse sequences. MB EPI looks just like single shot EPI except for the fact the excitation RF pulse is a multiband pulse that excites m slices simultaneously. Multiplexed-EPI sequence looks like the SIR sequence except that each of the s temporally interleaved pulses is now a multiband pulse with m bands. The net result is m•s fold speed-up.
Although we pursued the development of these slice accelerated sequences initially for high resolution imaging at 7 Tesla [1, 2], they have so far been employed mainly at 3T, driven by the needs of the Human Connectome Project that is carried out by the Washing University-University of Minnesota (WU-Minn) consortia (http://humanconnectome.org/). The Multiplexed-EPI sequence was used to demonstrate significantly improved resting state fMRI  for mapping functional connectivity in the human brain at 3T. However, these techniques clearly will enable high resolution whole brain imaging rapidly and will find numerous applications in high resolution fMRI and diffusion based tractography studies at ultrahigh fields.
FIGURE 2: 3T Multiplexed EPI images with different accelerations. PIPE designates, conventional acceleration along the phase encode direction
1. Moeller, S., E. Auerbach, P.-F. van de Moortele, G. Adriany, and K. Ugurbil: fMRI with 16 fold reduction using multibanded multislice sampling. Proc. Int. Soc. Magn. Reson. in Med. , 2008. 16: p. 2366.
2. Moeller, S., E. Yacoub, C.A. Olman, E. Auerbach, J. Strupp, N. Harel, and K. Ugurbil: Multiband multislice GE-EPI at 7 tesla, with 16-fold acceleration using partial parallel imaging with application to high spatial and temporal whole-brain fMRI. Magn Reson Med, 2010. 63(5): p. 1144-53.
3. Feinberg, D.A., T.G. Reese, and V.J. Wedeen: Simultaneous echo refocusing in EPI. Magn Reson Med, 2002. 48(1): p. 1-5.
4. Feinberg, D.A., S. Moeller, S.M. Smith, E. Auerbach, S. Ramanna, M.F. Glasser, K.L. Miller, K. Ugurbil, and E. Yacoub: Multiplexed echo planar imaging for sub-second whole brain FMRI and fast diffusion imaging. PLoS ONE, 2010. 5(12): p. e15710.