Rapid 3D Parallel Imaging of Non-Cartesian Data

Introduction: The 3D Pretty Easy Parallel Imaging (PEPI) technique, previously presented as a 2D technique [1], is a coil sensitivity based method used for reconstructing arbitrary 3D k-space trajectories. PEPI is an iterative technique that enforces receive b1-field and sampled data consistency in each iteration (eg. [2-4]). The 3D trajectory used is Spiral Projection Imaging (SPI) [3]. Methods: SPI: The trajectory described in figure 1 consists of fully sampled spiral planes rotated about the kz axis. Undersampling in kx-ky is caused by omitting planes from the fully sampled sphere in figure 1(b,c). Scan Parameters are: FOV 24cm, 1mm Res., 384 planes (fully sampled), 75 spiral arms, TE/TR = 0.6/9.9ms, scan time 5min (fully sampled), GE 3T Signa Excite System, 8-channel head coil. Subsampling the full set by factors of 4, 8, and 12 required 96, 48, 32 planes respectively. PEPI: Figure 2 shows the iteration loop. Coil combination in image space pushes data out from the SPI trajectory into non-sampled k-space. The new data are kept by masking the skipped k-space locations and recombining with original data in the next iteration. This is performed without the need for degridding and gridding operations at each iteration, improving computation speed. Results: Figure 3 shows a comparison between fully sampled gridded k-space, undersampled gridded k-space, and PEPI reconstructed data. Total reconstruction time for undersampling by 4 is 56min. The number of PEPI iterations is 18 and takes 10min. Reconstruction was performed on an 8-CPU 3GHz Intel Xeon machine. Conclusion: PEPI improves image quality at each level of undersampling used with arbitrary 3D SPI trajectories. PEPI provides rapid reconstruction times with less than 20% of the total reconstruction taken by the iteration process. References: [1] Pipe, ISMRM 2009, abstract 2722; [2] Pruessmann et al., MRM, 46:638, 2001; [3] Lustig et al., ISMRM 2007, abstract 333; [4] Wajer et al., ISMRM 2001, abstract 767; [5] Irarrazabal, P., MRM, 33:656, 1995