Multi-shot Echo-planar Imaging with Simultaneous MultiSlice Wave-Encoding
JaeJin Cho1, HyunWook Park1, Kawin Setsompop2,3, and Berkin Bilgic2,3

1Korea Advanced institute of Science and Technology, Daejeon, Korea, Republic of, 2Athinoula A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Boston, MA, United States, 3Harvard Medical School, Boston, MA, United States


We propose an imaging sequence for Simultaneous MultiSlice Multi-Shot EPI (SMS-MS EPI) with wave-CAIPI controlled aliasing, to significantly reduce the imaging time and geometric distortion in gradient echo imaging. We extend the MUSSELS low-rank constrained parallel imaging technique to SMS acceleration and exploit the similarities among the EPI shots for improved reconstruction. In simulations, we demonstrate the capability of our sequence to incorporate wave-CAIPI encoding, which allows higher acceleration rates by fully harnessing the three-dimensional encoding capability of multi-channel receive arrays. Using MUSSELS with wave-SMS, whole-brain T2*-weighted images at 1 mm isotropic resolution can be obtained at the total acceleration of Rtotal=24 (RinplanexRSMS=8x3), corresponding to an acquisition with high image quality and geometric fidelity.


Wave controlled aliasing in parallel imaging (wave-CAIPI) employs extra sinusoidal gradient modulations during the readout to effectively uses three-dimensional coil sensitivity to achieve higher accelerations through parallel imaging (1,2). Wave-CAIPI can be applied to echo planar imaging (EPI) to provide higher accelerations and reduce EPI-related distortion and blurring artifacts (3). Another approach to mitigate distortion and blurring in EPI is through multi-shot EPI (MS EPI) acquisition, where recent works in low-rank reconstruction with MUSSELS (4) has facilitated high-quality reconstruction that is robust to shot-to-shot phase variations. Recent simultaneous-multislice (SMS) imaging methods have also been widely used in EPI to reduce the imaging time (5,6) and have also been combined with MS EPI to speed up its acquisition (7). In this abstract, we propose SMS-MS EPI with wave-CAIPI for obtaining high-resolution images within a short imaging time. Simulations and a preliminary phantom acquisition were performed to investigate the performance of the proposed method.


We designed the imaging sequence for SMS-MS EPI with wave-CAIPI as shown in Figure 1. Multiband RF is used to collect the signals from multiple slices at the same time. The navigator signals are obtained during the first three readouts, and extra gradient modulations for blipped-CAIPI (6) and wave-CAIPI (1) encodings are incorporated into the EPI readout. With the sign of the readout gradient alternating from one EPI-line to the next, the wave-gradients were also made to alternate, with waveforms flipping in time and gradient-polarity directions from one readout to the next.

The forward model for SMS-MS EPI with wave-CAIPI is shown in Figure 2. The image data are modulated by the point-spread function (PSF) from Gy and Gz wave-gradients to generate the wave image, which is spatially shifted along y and slice-collapsed through blipped-CAIPI SMS encoding to obtain the SMS wave data. The SMS wave data is then sub-sampled in k-space through in-plane acceleration and multi-shot acquisition. The aliased SMS-MS-wave image can then be obtained by 2D Fourier transform of the k-space SMS-MS-wave data.

We simulated the k-space SMS-MS-wave data using the forward model in Figure 2. The high-resolution reference images used for the simulation were obtained from several single-band MS EPI acquisition which was previously reconstructed using the MUSSELS. The voxel size is 1mmⅹ1mmⅹ1mm, the FOV is 220mmⅹ208mmⅹ120mm, and the 32-channel head coil was used. Simulations were performed to create a 4-shot SMS-MS acquisition, where the acceleration in each EPI-shot is RinplaneⅹRSMS = 8ⅹ3, with shifted complementary ky sampling between shots and application of blipped-CAIPI to create N/3-FOV shift between slices before the sub-sampling. Data were simulated for acquisition with and without wave-CAIPI encoding and both data modulated by blipped-CAIPI. Wave-encoding with 1 sinusoidal cycle and gradient amplitude of 20 mT/m was used for both y and z directions.

Image reconstructions were performed using generalized parallel imaging reconstruction for the forward model described in Figure 2, with low-rank constraint added as per MUSSELS to mitigate reconstruction error due from shot-to-shot phase variations. Potential reduction in effective resolution of the final reconstructed image due to wave-encoding was also characterized using an intra-voxel phase simulation.

Results and Discussion

The reconstructed images from the simulated data are shown in Figure 3. Results from SMS-MS-EPI data without and with wave-CAIPI encoding are shown, with RMSE of 69 % and 14 %, respectively, compared to the reference images. As can be observed, SMS-MS-EPI without wave-encoding suffers from severe artifacts from the high accelerations, that wave-encoding is shown to nicely mitigate. The PSF estimation of the reconstructed images using the intra-voxel phase simulation is shown in Figure 4. The signals within a voxel experience small de-phasing due to the phase modulations from the wave-gradient as shown in Figure 4a and 4b. The amount of dephasing varies throughout the readout, which causes a small signal modulation along kx. It creates small side-lobes on the PSF of the final reconstructed image, as shown in Figure 4c. For the acquisition parameters used in the simulated reconstruction, the side-lobe amplitude in the PSF remains small at 1.6 %.


We proposed SMS-MS EPI with wave-CAIPI, which could help provide high accelerations to mitigate image distortion and blurring artifacts in EPI as well as increase acquisition speed. The reconstructions from the simulated data show high-quality results at high accelerations to demonstrate the potential benefits of the SMS-MS EPI with wave-CAIPI.


No acknowledgement found.


1. Bilgic B, Gagoski BA, Cauley SF, Fan AP, Polimeni JR, Grant PE, Wald LL, Setsompop K. Wave‐CAIPI for highly accelerated 3D imaging. Magnetic resonance in medicine 2015;73(6):2152-2162.

2. Gagoski BA, Bilgic B, Eichner C, Bhat H, Grant PE, Wald LL, Setsompop K. RARE/turbo spin echo imaging with simultaneous multislice Wave‐CAIPI. Magnetic resonance in medicine 2015;73(3):929-938.

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Figure 1. MS EPI with SMS wave-encoding. The sine and cosine waves are applied in the y and z directions, respectively. The first three DC readouts are used as the navigator for eliminating the N/2 ghost artifact. We assumed the three-band RF excitation and the slice shift by the blipped CAIPI.

Figure 2. The forward model for SMS-MS EPI with wave-CAIPI. PSFy,z and PSFblip indicate the convolutional operators using the PSF of wave-CAIPI and the blipped-CAIPI, respectively. ∑z and Sky indicate the summation along the z direction and the sub-sampling along the ky direction, respectively.

Figure 3. Reconstructed images from SMS-MS-EPI data without and with wave-CAIPI encoding

Figure 4. The intra-voxel phase simulation. a. The modulated phase within a voxel along the kx and the y or z directions. b. The phase modulation by wave gradient. c. PSF within a single voxel.

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)