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An optimized slice-GRAPPA reconstruction method to reduce leakage artifacts in small-animal multiband imaging
Hiroshi Toyoda1, Sosuke Yoshinaga2, Naoya Yuzuriha2, Mitsuhiro Takeda2, and Hiroaki Terasawa2

1Center for Information and Neural Networks, National Institute of Information and Communications Technology, Suita, Japan, 2Department of Structural BioImaging, Kumamoto University, Kumamoto, Japan

Synopsis

Accurate slice separation for simultaneous multi-slice acquisition continues to be challenging, especially when animal scanners equipped with relatively few receiver coil elements are used. We propose an optimized slice-GRAPPA method to reconstruct the dual-band EPI of rat brains, in which the size-optimized kernel was iteratively estimated to reduce artifacts. The reconstructed images were evaluated in terms of inter-slice leakage, g-factor, and temporal variation across the repetitions. With the proposed method, inter-slice leakage artifacts and the g-factor were reduced, and the average signal-to-noise ratio was improved. Thus, the total reconstruction accuracy was improved in the multiband EPI in a small animal study.

Introduction

Simultaneous multi-slice (SMS) acquisition can be performed generally well on a recent high-specification MRI system for humans with redundant numbers of receiver coil elements. Accurate slice separation for SMS data thus continues to be challenging, especially when animal scanners equipped with relatively few receiver coil elements are used.In this study, we propose an optimized slice-GRAPPA (SG) reconstruction method for the dual-band EPI of in vivo normal rat brains scanned with an animal scanner having a four-element receiver array coil. This study aims to modify and optimize the SG technique1 and its variant2 for reducing artifacts, such as inter-slice signal leakage, and for maximizing and stabilizing the signal-to-noise ratio (SNR) in the reconstructed images. It also seeks to demonstrate the feasibility of the method through application to dual-band EPI measurements of in vivo rat brains.

Methods

Dual-band EPI measurements

In vivo male Wister rat brains (n=8) were scanned on a 7T animal scanner (Bruker, BioSpec70/20) equipped with a transceiver RF coil and a receiver array coil with four elements.A custom-built single-shot dual-band gradient echo 2D EPI sequence was used with blipped-controlled aliasing for the main scans, which were preceded by reference scans with a single-band 2D EPI sequence. Dual-band excitation pulses were designed on the basis of sync functions with frequency offsets, corresponding to a between-band spacing of 9.75 mm. A total of 78 slices (39 dual-band excitations) were imaged in a single repetition. The repetition time and echo time were 2,500 and 15.0 ms, respectively. The voxel size in the plane was 0.25 mm × 0.25 mm, and the slice thickness was 0.25 mm.The encoding and imaging matrix sizes in the read-out and phase-encoding directions were 96 and 56, respectively (rectangular FOV).Second-order shimming within the imaging region was performed prior to EPI acquisition.

EPI image reconstruction method

All the single-band and dual-band EPI raw data were processed with a custom-built reconstruction software program designed for the purpose of this study. The first step in EPI reconstruction was the phase correction between the odd and even lines in k-space. Then, the folded images were separated into each slice at the original location. In this study, a modified SG method was proposed on the basis of the leak-block technique2. We introduced an iterative calculation procedure to estimate and apply the kernels for slice separation. In the iterative SG method, the k-space data for each slice were estimated iteratively until convergence by applying the updated kernels. In the non-iterative SG method, the k-space data for each slice were estimated only once by applying the initial estimates of kernels. Then, in turn, the kernels were updated with the newly estimated k-space data to minimize the leakage artifacts. The estimated kernel size was optimized for better performance. The original SG method1 was also performed for comparison purposes.

Evaluation of the method

The performance of the reconstruction methods was evaluated in terms of inter-slice leakage and g-factor. Leakage maps3 and g-factor maps were generated for the evaluations. The value of the g-factor was calculated for each slice.

Results and Discussion

The proposed reconstruction method can provide accurate and stable results in reconstructed images for dual-band EPI scans obtained on a 7T animal scanner (equipped with a 4ch receiver coil). The magnitude images and g-factor maps of all the slices reconstructed with the use of the proposed SG method are shown in comparison with those of conventional SG methods (Fig. 1, 2, 3). The leakage maps of the proposed and the conventional methods are shown in Fig. 4. The proposed iterative kernel computation method for the slice separation was effective in reducing artifacts in the dual-band EPI in our animal scans.

Conclusion

An optimized reconstruction method was proposed to reduce leakage artifacts and improve SNR in the dual-band EPI of in vivo normal rat brains. The proposed method allows slice acceleration in animal imaging by improving reconstruction quality, which results in the faster acquisition of whole-brain coverage of small animal brains without sacrificing spatial resolution in the slice-encoding direction (isotropic voxels can be achieved). Furthermore, the proposed method improves reconstruction accuracy in multi-band imaging and thus widens the range of applications of multi-band imaging, such as functional and diffusion MRI, as well as arterial spin labelling. The method can be used not only for slice acceleration in animal imaging but also for a more efficient multi-band EPI in humans, which can be scanned even with a clinical scanner system equipped with relatively few receiver coil elements.

Acknowledgements

This study was supported by JSPS KAKENHI Grant Number 25351003.

References

1. Setsompop K, Gagoski BA, Polimeni JR, et al. Magn Reson Med (2012).

2. Cauley SF, Polimeni JR, Bhat H, et al. Magn Reson Med 72:93-102 (2014).

3. Moeller S, Xu J, Auerback EJ, et al., Annual meeting of ISMRM, p.519 (2012).

Figures

Fig. 1. Phase-corrected magnitude images for dual-band EPI with FOV/2 shift before (A) and after (B) the slice unfolding procedures of the proposed SG method. The images are shown as planar views in the coronal, sagittal, and transversal planes.

Fig. 2. Magnitude images of all the scan slices reconstructed with the proposed SG method (A) and the conventional SG methods (B) for comparison purposes.

Fig. 3. G-factor maps of all the scan slices reconstructed with the proposed SG method (A) and the conventional SG methods (B) for comparison purposes. The voxels with a higher g-factor are observed in the caudal parts in both slice groups, which are located in the areas around the boundary between the sensitivity profiles of the two adjacent elements of the receiver array coil.

Fig. 4. Leakage maps of the proposed SG method (A) and the conventional SG methods (B) for comparison purposes. The more significant inter-slice signal leakage errors can be observed in the calculated leakage maps for the conventional SG methods rather than the proposed SG method. The proposed SG method reduces the average inter-slice leakage errors from 5.2 to 4.6 compared with the conventional SG methods. The values of inter-slice leakage were averaged across all slices, all repetitions, and all subjects.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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