Introduction

Spin-echo echo-planar imaging (EPI)¹ is the standard technique for diffusion-weighted acquisitions but suffers from geometric distortions in the presence of field inhomogeneities. The distortions increase with the echo train length and, thus, are more pronounced for larger fields-of-view (FOVs) and higher spatial resolutions which limits the applicability of EPI to small, inner structures like the optic nerve or the spinal cord. To shorten the echo train and reduced geometric distortions accordingly, zonal oblique multi-slice (ZOOM) EPI^2,3 has been proposed. It involves cross-sectional RF excitations to focus the refocussed magnetization to an inner volume which allows to reduce the FOV without aliasing in the phase-encoding direction. In this study, ZOOM-EPI is combined with simultaneous multi-slice (SMS)⁴, the boundary condition to avoid unwanted signal contributions is determined, and the feasibility to shorten acquisitions times is demonstrated for diffusion-tensor imaging (DTI) of the human spinal cord.

Methods

The pulse sequences and geometric setups used in the present study are sketched in Fig. 1 and 2. The plane of the initial RF excitation is tilted by an angle φ compared to the image plane defined by the refocussing RF pulse. On both sides of the desired inner FOV, transition zones are excited and refocused which requires phase-encoding oversampling to avoid aliasing. Furthermore, neighboured sections are partially covered and saturated by the tilted RF excitation. Thus, the angle φ must be chosen as a compromise between minimum saturation effects and small transition zones. To extend the conventional approach (Fig. 1a and 2a) to simultaneous multi-slice imaging, both RF pulses must cover multiple frequency bands (Fig. 1b and 2b). As an unwanted side effect, magnetization outside of the target region can be excited and refocused by the bands of different slices. However, they can be positioned outside of the object to avoid their unwanted signal contributions if the tilt angle φ obeys tan φ < g/(f+max(x1, x2)) (g: minimum gap between the simultaneously acquired slices, f: desired inner FOV; x_i: distances from inner FOV to object boundary on different sides in the different slices; cf. Fig. 2b). Furthermore, additional gradient pulses to use the blipped-CAIPI approach⁴ were applied in the slice direction. Experiments were performed on a 3T whole-body MR system (PrismaFit, Siemens Healthineers, Erlangen, Germany) using a 64-channel head-neck coil together with a 32-channel spine array coil with only those coil elements selected that provide significant signal contributions for the chosen measurement volume. A water phantom was used for test experiments, in vivo acquisitions (62 slices, no gap) were performed in healthy volunteers after their informed consent was obtained. Images were acquired with a voxel size of 1.0×1.0×4.0mm³, a tilt angle φ of 5°, an inner FOV of 40mm plus 20mm phase-encoding oversampling to account for the transition zones, and involved an interleaved excitation order due to saturation in directly neighboured sections. Diffusion-tensor imaging was performed with six different directions of the diffusion weighting, a b value of 625s mm^-2 yielding an echo time of 75ms, and 12 averages. SMS acquisitions were accelerated by a factor of 2 which allowed to shorten the repetition time from 7300ms to 3700ms and the total acquisition time from 10.5min to 5.7min, slightly less than a factor of 2 due to the extra reference acquisitions required for SMS imaging. Isotropic diffusion-weighted images and maps of the apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were obtained with the analysis framework provided by the manufacturer.

Conclusion

For an appropriately chosen tilt angle, ZOOM-EPI can be accelerated with SMS acquisitions without side effects. Thus, DTI acquisitions of small target regions like the optic nerve or spinal cord can be shortened which could improve the clinical applicability in patients.

Acknowledgements

This work was supported by a grant from Wings for Life.