Accelerating radial MRI using GROG followed by ESPIRiT
Ibtisam Aslam1, Faisal Najeeb1, and Hammad Omer1

1Electrical Engineering, COMSATS Institute of Information Technology, Islamabad, Pakistan

Synopsis

Accelerated non-Cartesian parallel imaging plays a vital role to reduce data acquisition time in the MR imaging; however the resultant images may contain aliasing artifacts. In this work, the application of ESPIRiT with GROG is proposed to get good reconstruction results from highly under-sampled radial data. The proposed method is tested on 3T short–axis cardiac radial data at different acceleration factors (AF=4, 6 and 9) and compared with pseudo-Cartesian GRAPPA. The results show that the proposed method offers significant improved (e.g. 81% improvement in term of artifact power at AF=4) reconstruction results as compared to conventional pseudo-Cartesian GRAPPA.

Introduction

In parallel imaging, under-sampled non-Cartesian trajectories help to achieve faster scan times; however the resultant images may have aliasing artifacts. Pseudo-Cartesian GRAPPA 1 along with GRAPPA Operator Gridding (GROG) 2 has been used in the recent past to eradicate these artifacts. In this paper, the application of ESPIRiT 3 (an eigenvectors based reconstruction algorithm) in combination with GROG 2 is proposed to get good reconstruction results from highly under-sampled radial data.

Method

GROG 2 provides mapping of the acquired non-Cartesian data points in k-space to the nearest Cartesian locations. GROG exploits self-calibrated weight sets to grid the non-Cartesian data and leaves some empty spaces in the gridded k-space 2. Pseudo-Cartesian GRAPPA 1 employs several patterns to estimate the coil-by-coil weight sets from auto-calibration signal (ACS) lines to fill the empty points of the GROG 2 gridded k-space. These suitable pseudo-Cartesian GRAPPA weight estimating patterns are difficult to determine 1. ESPIRiT uses eigenvector based multiple sets of the sensitivity maps to get un-aliased image. In ESPIRiT, multiple sets of sensitivity maps are estimated from a small calibration area using eigenvalue decomposition approach 3. The eigenvectors corresponding to the most significant eigenvalues provide sensitivity maps. This paper proposes the application of ESPIRiT 3 with GROG 2 to recover MR images from under-sampled radial data. The first step in the proposed method is to apply GROG on the acquired accelerated radial data which provides an estimate of the Cartesian k-space. This is followed by the application of ESPIRiT on this GROG gridded data. Figure 1 provides block diagram of the proposed method. The performance of the proposed method and pseudo-Cartesian GRAPPA 1 is evaluated using Artifact Power (AP) 4 and Root Mean Square Error (RMSE).1

Results and Discussion

The proposed method is tested on the short-axis cardiac MRI radial data which has been acquired using 3T Skyra (Siemens, Case Western Reserve University, Cleveland) scanner at TR=2.94ms with real-time, free-breathing and no ECG gating. The radial data set contains 30 channel cardiac coils, 80 measurement frames, 256 readout points and 144 projections. This short-axis cardiac radial data is resized to using temporal averaging method on the measurement frames.5 Figure 2 shows the reconstruction results of the proposed method as well as pseudo-Cartesian GRAPPA 1 for different acceleration factors (AF= 4, 6 and 9). Figure 2A shows the reconstruction results of the proposed method and Figure 2B shows the results of pseudo-Cartesian GRAPPA.1 The results confirm that the proposed method provides significant improvement (e.g. 81% in term of AP, 57% in term of RMSE improvement at AF=4) in the reconstructed images as compared to pseudo-Cartesian GRAPPA.1

Conclusion

The application of GROG followed by ESPIRiT is proposed to reconstruct the MR images from under-sampled radial data. The results show that the proposed method provides significant improvement in reconstruction results as compared to conventional pseudo-Cartesian GRAPPA.

Acknowledgements

No acknowledgement found.

References

[1] N. Seiberlich, F. Breuer, R. Heidemann et al. Reconstruction of undersampled non-Cartesian data sets using pseudo-Cartesian GRAPPA in conjunction with GROG. Magn. Reson. Med. 2008;59(5):1127–1137.

[2] N. Seiberlich, F. Breuer, M. Blaimer et al. Self-calibrating GRAPPA operator gridding for radial and spiral trajectories. Magn. Reson. Med. 2008;59(4):930–935.

[3] M. Uecker, P. Lai, M. J. Murphy et al. ESPIRiT--an eigenvalue approach to autocalibrating parallel MRI: where SENSE meets GRAPPA. Magn. Reson. Med. 2014;71(3):990–1001.

[4] H. Omer and R. Dickinson A graphical generalized implementation of SENSE reconstruction using Matlab. Concepts Magn. Reson. Part A, 2010;36A(3):178–186.

[5] R. Otazo, E. Candès, and D. K. Sodickson Low-rank plus sparse matrix decomposition for accelerated dynamic MRI with separation of background and dynamic components. Magn. Reson. Med. 2015; 73(3): 1125–1136.

Figures

Figure 1: Block diagram of the Proposed Method

Figure 2: Reconstructions results of short-axis cardiac data using the proposed method (A) and pseudo-Cartesian GRAPPA (B)



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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