INTRODUCTION

A novel multi-shot EPI technique, termed DIADEM (Distortion-free Imaging: A Double Encoding Method)^1,2 was proposed recently for distortion-free imaging. The DIADEM uses double encoding including the inherent EPI phase-encoding (EPI-PE) and additional spin-warp phase-encoding (SW-PE) to form a 3D data I(s,y,x). With this SW-PE, it can produce distortion-free and blurring-free T2-weighted and diffusion-weighted images. However, the extra loop of the SW-PE is time consuming. A common acceleration approach for DIADEM is the use of a reduced field of view (rFoV) in the SW-PE by skipping the step of SW-PE³. The unfolding process can be successful if the reduced FoV in the SW-PE direction still covers the entire range of geometric distortions. Otherwise, folding-over artifacts will appear in the calculated DIADEM image. A recent study⁴ demonstrated that further FoV reduction was achievable with view-angle-tilting (VAT) method by reducing the level of distortion in the EPI-PE direction.
In this study, we propose the VAT-DIADEM combined with the simultaneous multi slice (SMS)⁵ acquisition to achieve higher acceleration factor in the acquisition and less folding-over ghost in the SMS-DIADEM data.

METHODS

All the phantom scans were performed on a 7T scanner (Siemens Healthineers, Erlangen, Germany) using a 32-channel head coil (Nova Medical, Wilmington MA, USA). A SMS factor of 2 and no in-plane parallel imaging were applied to maximize the level of distortion and minimize the unfolding bias, which resulted in an echo-spacing of 1.0 ms. Other imaging parameters were: TR/TE = 1000.0/80.0 ms, 1.0×1.0×4.0 mm³ resolution with 300% distant factor, and the reduced resolution factor of 4 in EPI-PE. Figure 1 shows the sequence diagram with (a) SMS-DIADEM (b) SMS-VAT-DIADEM. The VAT gradient amplitude is set to the same as in the blipped-CAIPI gradient amplitude. As the SMS-DIADEM sequence is unfolded by single-band EPI reference, SMS-VAT-DIADEM is similarly unfolded by VAT-EPI.

RESULTS and DISCUSSION

Figure 2 shows the unfolded SMS-EPI and DIADEM images with blipped-CAIPI gradients and blipped VAT gradients for the same acceleration (i.e. rFoV) factor of 4. From Figure 2 (e,f) SMS-EPI and (g,h) SMS-VAT-EPI it can be seen that unfolded EPI images show less distortion, but increased image blurring for VAT gradient. From Figure 2 (a,b) SMS-DIADEM and (c,d) SMS-VAT-DIADEM show that fold-over artifacts only appears in the SMS-DIADEM for an acceleration factor of 4. The highest fold-over-free rFoV acceleration factor is 2 for SMS-DIADEM and 4 for SMS-VAT-DIADEM. The value is consistent with previous VAT studies^4,6. Although signal-to-noise ratio for SMS-VAT-DIADEM is around 40% of SMS-DIADEM, the differences could be minimized with a high VAT gradient amplitude and the use of a high in-plane parallel imaging factor⁴. Note that single-band images are in good consistence with multi-band images (data not shown).

CONCLUSION

In this work, SMS is additionally added into the VAT-DIADEM to further accelerate the DIADEM acquisition. Combining both the VAT and SMS gradients on the slice axis allows rapid DIADEM acquisition not only in the in-plane (SW-PE) but also in the though-plane direction.

Acknowledgements

This project is supported by DFG-grant SP 632/4-2.