We demonstrate 3D stack-of-spiral MRF acquisition with hybrid sliding-window and GRAPPA (SW+GRAPPA) reconstruction using a constant density spiral (CDS) rather than standard variable density spiral (VDS). The CDS was found to mitigates high-frequency artifacts after in-plane sliding-window (SW) combination and improve the subsequent GRAPPA and dictionary matching reconstruction. The proposed 3D constant density stack-of-spiral MRF allows whole-brain (240×240×192 mm3) parametric mapping at 1 mm isotropic resolution with high quality in 8 minutes.
Methods
Sequence: 3D FISP sequence8 was implemented for MRF. Figure 2(a) shows the diagram of this partition-by-partition sampled sequence that also incorporates a low-flip-angle training data acquisition into the wait period. The total acquisition time for each partition is 7 seconds for a 420 time-points (tps) acquisition. The TR and FA trains are shown in Fig.2(b&c). CDS k-space trajectory, which consisted of 30 interleaves with zero-moment nulling, was utilized for acquisition. Interleaves were rotated by 12° for each TR with 1400 points per interleaf (7ms readout). To compare the proposed CDS with VDS method, the zero-moment compensated VDS with the same 30 interleaves was used in this study.
SW+GRAPPA reconstruction: SW and NUFFT9 are used to remove in-plane aliasing and create a Cartesian dataset that is fully sampled in-plane. This then allows for a direct application of Cartesian GRAPPA reconstruction to overcome Rz acceleration10. More details can be found in [6].
Data acquisition: Two datasets were acquired on a healthy subject, both with VDS and CDS, using a Siemens Prisma 3T scanner with a 64-channel coil.
i) Accelerated 1 mm isotropic whole-brain data at Rz=3 and 420 time points per partition. Acquisition was performed sagittally, with FOV of 240×240×192 mm3 and Tacq = 8 minutes.
ii) Fully sampled 1×1×4 mm3 whole-brain data, acquired transversally with 48 fully sampled partitions and 1200 time points per partition. Tacq = 15.2 minutes. This data was used to characterize the performance of CDS and VDS at various accelerations. Reconstructions through both standard matching and SW+GRAPPA were performed and compare in each case.
Results
Fig.3a shows a comparison of fully sampled CDS and VDS k-space data at 1mm isotropic obtained via SW across 30 TRs. Significant portion of high k-space data is missing in VDS, which causes ringing/high-frequency artifacts, both in SW images and the resulting T1 and T2 maps as shown in Fig.3b(top-row). These artifacts are effectively removed through CDS as shown Fig.3b(bottom-row), which visibly improved the quality of our highly accelerated 8-minutes 1mm3 whole-brain MRF. Fig.4 shows comparison of results from VDS and CDS at different accelerations, with different reconstructions. For fully sampled data, standard dictionary matching achieves good results for VDS, but poor reconstruction for CDS (Fig.4a). With SW reconstruction, both VDS and CDS achieve high quality results (Fig.4b). Reconstructions at Rz=3 with 600 and 420 time-points are also shown in Fig.4c&d. Here, ringing artifacts, which increased at 420tps, are observed for VDS. On the other hand, CDS achieves good reconstruction even at 420tps. Fig.5 displays high-quality quantitative maps of four representative slices from CDS acquisition at Rz=3 and 420tps, which provide 1x1x4mm3 whole-brain mapping in 2.1 minutes.1. Ma D et al., Nature 2013; 495, 187-192.
2. Ma D et al., ISMRM 2016, p.3180.
3. Ma D et al., Magn. Reson. Med. 2016; 75, 2303-2314.
4. Buonincontri G et al., Magn. Reson. Med. 2016; 76, 1127-1135.
5. Ma D et al., Magn. Reson. Med. 2017; doi: 10.1002/mrm.26886.
6. Liao C et al., Neuroimage. 2017; 162, 13-22.
7. Cao X et al., Magn. Reson. Med. 2017; 78, 1579-1588.
8. Jiang Y et al., Magn. Reson. Med. 2015; 74(6), 1621-1631.
9. Fessler JA, J. Magn. Reson. 2007; 188, 191-195.
10. Griswold MA et al., Magn. Reson. Med. 2002; 47, 1202-1210.