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).
1Results 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.
1Conclusion
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
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