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Distortion Correction of Multi-Shot Diffusion-Weighted Echo-Planar Imaging using Reversed Gradient Acquisition and Joint Reconstruction
Xiaoxi Liu1, Di Cui1, Edward S. Hui1,2, Queenie Chan3, and Hing-Chiu Chang1

1Department of Diagnostic Radiology, The University of Hong Kong, Hong Kong, China, 2The State Key Laboratory of Brain and Cognitive Sciences, The University of Hong Kong, Hong Kong, China, 3Philips Healthcare, Hong Kong, China

Synopsis

Multi-shot diffusion-weighted echo-planar imaging (DW-EPI) with multiplexed sensitivity encoding (MUSE) is a self-navigated technique that can achieve high resolution diffusion-tensor imaging (DTI) without the need of navigator echo. However, even with multi-shot acquisition, the effective echo spacing is still relatively long for acquisition of high resolution DTI, leading to significant geometric distortion. In this study, we aim to reduce the geometric distortion of multi-shot DW-EPI by 1) integrating the reversed gradient acquisition in multi-shot DW-EPI, and 2) developing a joint reconstruction method that can reconstruct non-uniform k-space data by taking the off-resonance effect into account.

Purpose

Multi-shot diffusion-weighted echo-planar imaging (DW-EPI) with multiplexed sensitivity encoding (MUSE) is a self-navigated technique that can achieve high resolution diffusion-tensor imaging (DTI) without the need of navigator echo [1]. However, even with multi-shot acquisition, the effective echo spacing is still relatively long for acquisition of high resolution DTI, leading to significant geometric distortion. Although increasing the number of shot can help to alleviate the geometric distortion in multi-shot DW-EPI, the inter-shot phase variations cannot be measured from each segment due to high acceleration factor (e.g., R=8 for 8-shot acquisition). To address this issue, a navigator echo is needed to measure the phase variations that can reduce the acquisition efficiency [2]. In this study, we aim to reduce the geometric distortion of multi-shot DW-EPI by 1) integrating the reversed gradient (RG) acquisition [3] in multi-shot DW-EPI, and 2) developing a joint reconstruction method that can reconstruct non-uniform k-space data by taking the off-resonance effect into account.

Methods

Pulse sequence design: The multi-shot DW-EPI sequence was modified to enable RG acquisition as shown in the Figure 1a. All odd segments were acquired with forward phase-encoding (PE) direction, and even segments with reversed PE direction. Figure 1b shows the design of non-uniform k-space sampling for 4-shot DW-EPI with RG acquisition for simulation study.

Data reconstruction: Figure 2 shows the flowchart of data reconstruction. First, the k-space data with reversed PE was realigned (Figure 1a). Second, all segment data were respectively reconstructed by using SENSE to estimate the inter-shot phase variations [4]. Third, odd and even segment data were respectively reconstruction with MUSE framework for producing two images with opposite PE direction. Fourth, the displacement map (DM) was derived from two images with opposite PE direction using RG method [6]. Fifth, the SPACE-RIP algorithm [7] was modified to jointly reconstruct all segment data by taking into account phase variations and off-resonance effect estimated from DM.

In-vivo experiments: Human brain data were acquired at 3.0T MRI scanner (Philips, Achieva) using a modified 4-shot DW-EPI sequence with RG acquisition (matrix size = 256x256 (TE/TR = 91/5000ms) and matrix size = 384x384 (TE/TR = 92/5000ms), partial Fourier factor = 60%, FOV = 240x240mm, b-value = 800/mm2, DTI directions = 6). A turbo-spin echo data set was acquired for reference (TE/TR = 80/3000ms matrix size = 512x512). For comparison, two conventional 4-shot DW-EPI data sets with opposite PE direction were acquired to produce images with three pipelines: 1) the data with either forward or reversed PE direction was respectively reconstructed using MUSE, 2) the RG method was applied to MUSE reconstructed images with opposite PE directions for distortion correction, and 3) the data with either forward or reversed PE direction was respectively reconstructed using MUSE with acceleration factor of 2 (i.e., two out of four segments), and then applied RG correction.

Simulation study: A 384x384 data with non-uniform k-space sampling was simulated following the design shown in figure 1b. The off-resonance effects associated with non-uniform k-space sampling was considered to produce the distorted images with either forward or reversed PE direction. Data reconstruction followed the same pipeline for in-vivo data, expect that only low frequency part were used for phase measurement and DM calculation.

Results

Figure 3 and Figure 4 show the T2W images, DW images, and corresponding cFA maps for different acquisition and reconstruction methods with resolution of 256x256 and 384x384. Figure 3(a) and figure 4(a) are the 4-shot DW-EPI data with forward PE direction reconstructed using MUSE. Figure 3(b) and figure 4(b) show two 4-shot DW-EPI data sets with opposite PE direction respectively reconstructed using MUSE and subsequent applied RG correction. Figure 3(c) and figure 4(c) show two 4-shot DW-EPI data sets with opposite PE direction respectively reconstructed using MUSE with acceleration factor of 2, and subsequent applied RG correction. Figure 3(d) and figure 4(d) show the 4-shot DW-EPI data with RG acquisition reconstructed by proposed method. Figure 3(e) and figure 4(e) are the gold-standard TSE-T2W images. Figure 5 shows the simulation result of non-uniform k-space sampling data reconstructed with proposed method.

Discussion

We have demonstrated that the proposed acquisition strategy and joint reconstruction method can effectively reduce the geometric distortion of multi-shot DW-EPI. The modified multi-shot DW-EPI sequence with RG acquisition can eliminate extra scan time for acquiring another data set with opposite PE direction (Figure 3a). Compared with conventional RG method (Figure 3c), our proposed method shows better SNR performance (Figure 3d). In the simulation study, we have demonstrated that the proposed method can successfully reconstruct the data with non-uniform k-space sampling, showing the advantage of our method. In conclusion, the proposed joint reconstruction algorithm is a robust reconstruction algorithm for multi-shot DW-EPI with distortion correction.

Acknowledgements

The work was in part supported by grants from Hong Kong Research Grant Council (GRF HKU17138616 and GRF HKU17121517).

References

[1] Chen, NK; Wyrwicz, AM. Optimized distortion correction technique for echo planar imaging. Magn Reson Med 2001; 45:525–528.

[2] Chang HC and Chen NK, "Joint correction of Nyquist artifact and minuscule motion-induced aliasing artifact in interleaved diffusion weighted EPI data using a composite two-dimensional phase correction procedure", Magnetic Resonance Imaging, 2016;34(7):974-9.

[3] Morgan, Paul S.; Bowtell, Richard W.; Mcintyre, Dominick J. O.; Worthington, Brian S. Correction of Spatial Distortion in EPI Due to Inhomogeneous Static Magnetic Fields Using the Reversed Gradient Method. Journal of Magnetic Resonance Imaging, April 2004, Vol.19(4), pp.499-507.

[4] Pruessmann, Klaas P.; Weiger, Markus; Scheidegger, Markus B.; Boesiger, Peter. SENSE: Sensitivity encoding for fast MRI. Magnetic Resonance in Medicine, November 1999, Vol.42(5), pp.952-962.

[5] Jenkinson, Mark; Beckmann, Christian F.; Behrens, Timothy E.J.; Woolrich, Mark W.; Smith, Stephen M. FSL. NeuroImage, 15 August 2012, Vol.62(2), pp.782-790.

[6] Chang, H.C.; Chuang, T.C.; Lin, Y.R.; Wang, F.N. et al. Correction of geometric distortion in Propeller echo planar imaging using a modified reversed gradient approach Quant Imag Med Surg, 3(2) (2013), p.73.

[7] Kyriakos, Walid E.; Panych, Lawrence P.; Kacher, Daniel F.; Westin, Carl‐Fredrick; Bao, Sumi M.; Mulkern, Robert V.; Jolesz, Ferenc A. Sensitivity profiles from an array of coils for encoding and reconstruction in parallel (SPACE RIP). Magnetic Resonance in Medicine, August 2000, Vol.44(2), pp.301-308.

[8] Chen, Nan-Kuei; Guidon, Arnaud; Chang, Hing-Chiu; Song, Allen W. A robust multi-shot scan strategy for high-resolution diffusion weighted MRI enabled by multiplexed sensitivity-encoding (MUSE). NeuroImage, 15 May 2013, Vol.72, pp.41-47.

Figures

Fig.1: a) The multi-shot DW-EPI sequence was modified to enable RG acquisition. All odd segments were acquired with forward phase-encoding (PE) direction, and even segments with reversed PE direction. b) shows the design of non-uniform k-space sampling for 4-shot DW-EPI with RG acquisition for simulation study. The oversampling is 64 lines and the ETL of high frequency region is 1.5 times higher than the k-space center region.

The reconstruction flowchart of data reconstruction process.

Fig.3: With the resolution of 256x256, T2W images, DW images, and corresponding cFA maps for different acquisition and reconstruction methods. (a) The 4-shot DW-EPI data with forward PE direction reconstructed using MUSE. (b) Two 4-shot DW-EPI data sets with opposite PE direction respectively reconstructed using MUSE and subsequent applied RG correction. (c) Two 4-shot DW-EPI data sets with opposite PE direction respectively reconstructed using MUSE with acceleration factor of 2, and subsequent applied RG correction. (d) The 4-shot DW-EPI data with RG acquisition reconstructed by proposed method. (e) Gold-standard TSE-T2W image.

Fig.4: With the resolution of 384x384, T2W images, DW images, and corresponding cFA maps for different acquisition and reconstruction methods. (a) The 4-shot DW-EPI data with forward PE direction reconstructed using MUSE. (b) Two 4-shot DW-EPI data sets with opposite PE direction respectively reconstructed using MUSE and subsequent applied RG correction. (c) Two 4-shot DW-EPI data sets with opposite PE direction respectively reconstructed using MUSE with acceleration factor of 2, and subsequent applied RG correction. (d) The 4-shot DW-EPI data with RG acquisition reconstructed by proposed method. (e) Gold-standard TSE-T2W image.

Fig.5: The simulated T2WI and DWI images of non-uniform k-space sampling data with (a) forward PE direction, and (b) reversed PE direction. The arrows show the artifacts associated with discontinuous off-resonance effects due to non-uniform k-space sampling. (c) The reconstruction result with proposed method. (d) The original data for simulation.

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