Introduction

Diffusion-weighted imaging (DWI) is an important tool for clinical diagnosis and neuroscience research.¹ The single-shot echo planar imaging (ssEPI) sequences have been widely used; however, they suffer from well-known motion-related artifacts as well as the subtle T2* blurring due to a significant T2* decay of the signal and phase shift accumulation. Therefore, these problems hinder them from achieving higher spatial resolution especially at high field strengths. A feasible approach to overcome such problems is to use diffusion-weighted, readout-segmented EPI (rsEPI) with 2-dimensional navigator correction,^2,3 which has lower susceptibility distortion, reduced T2* blurring and a robust correction for motion-induced phase artifact. In the current study, through evaluating image distortion, signal strength and contrast, we provide a DWI evaluation by employing the rsEPI for in vivo macaque brain imaging over a 7T human MR scanner.

Methods

All measurements were performed on a 7T research MR scanner (Siemens Healthcare, Erlangen, Germany) equipped with a 1Tx/28Rx QED knee coil (Mayfield Village, OH, USA). Images were acquired over a female macaque (2-year old, 3.5kg) that was maintained and anesthetized by 1.5~2% isoflurane – all procedures were in accordance with NIH standards and approved by our Institutional Animal Care Committee. The monkey was placed in the sphinx position with its head centered within the knee coil. T2w TSE sequence were obtained in the same slice position as a reference for distortion evaluation (TR/TE 9000/36 ms; FOV 128×128 mm²; matrix 128×128). Prototype sequence RESOLVE^4,5 was utilized for rsEPI, and DW images were acquired at 1×1×1 mm³ resolution with an acceleration rate of 2 and b-values of 0, 500 and 1000 s/mm². The following parameter were used for all rsEPI-DWIs: matrix 96×96, FOV 96 mm; TR 7300 ms, acceleration rate 2 along A-P phase encoding direction, 2 averages, bandwidth of 789 Hz/pixel. We evaluated DW image with a monopolar scheme but increased readout segmented numbers of 7, 13, and 19 with the echo spacing of 0.4 ms, 0.34 ms, and 0.32 ms, and the scan time of 13’44”, 25’04”, and 35’31”, respectively. We also acquired DW images with 7 shots but for contrast-comparison purpose by using the bipolar and monopolar schemes.

Results and Discussion

Figure 1 shows rsEPI images at b = 0 s/mm² acquired at 7T in 17'29" with a voxel size of 0.7×0.7×1 mm³. T2-weighted images by TSE were provided as a reference for distortion evaluation. The squares in Figure 1 were drawn on the same slice position to enclose the same region of interest for geometric distortion comparison. The rsEPI DWIs showed a certain level of image distortion in regions with air-tissue interfaces, such as located at the paranasal sinus, the nasal cavity and the ear cavity. Figure 2 shows a comparisons of image distortion with increased readout segmented number for rsEPI with a monopolar scheme and 1 mm isotropic resolution (25 slices). Increasing shots number with rsEPI leads to longer scan time but shorter readout echo spacing, which of the latter is supposed to alleviate image distortions due to the susceptibility effect^2,3. However, the DW images distortion observed in Figure 2 were not apparently reduced with increased shot number. Figure 3 shows a comparison of 1mm isotropic DWIs (54 slices) between a bipolar scheme (left) and a monopolar (right) with 7-shot rsEPI-DWI acquired at 7T. The monopolar scheme is more sensitive to eddy currents induced by the diffusion gradient. Therefore, the bipolar scheme is usually used to avoid eddy currents that can arise from switching the diffusion gradient rather than a monopolar^6,7; however, our results indicates that the monopolar scheme had slight distortion and higher signal intensity/contrast as compared to the bipolar scheme.

Conclusion

The results shown in this study demonstrate that, for in vivo macaque brain imaging at 7T, rsEPI makes it possible to perform DW imaging with multiple readout segmentation and 1mm isotropic resolution with restrained images distortion and satisfactory MR signal quality. However, distortion in rsEPI-DWI were not apparently reduced with increased shots number (7, 13, and 19). In addition, the DW images with a monopolar scheme outperforms the bipolar scheme in signal intensity and contrast. Overall, rsEPI-based diffusion imaging can play an important role in high-resolution macaque brain research at 7T.

Acknowledgements

We would like to thank Mr. Guohua Xu, Bin Xu, and Dengfeng Zhou for animal handling. This work was supported in part by National Natural Science Foundation of China (81701774, 61771423) and Fundamental Research Funds for the Central Universities (2016QN81018).

References

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