A Spatiotemporal-Constrained Sorting Method for Motion-robust 4D-MRI: A Feasibility Study.

PURPOSE: To develop a spatiotemporal-constrained sorting technique for motion-robust 4D-MRI.

METHODS: This sorting method implemented two new approaches for 4D imaging: 1) an optimized sparse k-space acquisition trajectory with self-gating signal derivation, and 2) a retrospective k-space sorting for reconstruction using a novel spatiotemporal-constrained strategy to minimize breathing variation induced motion artifacts. Such sorting was regularized by a spatiotemporal index (STI). Volumetric reconstruction was implemented iteratively with a 2nd -order total generalized variation (TGV) penalty. The proposed method was evaluated and compared with the conventional phase-sorting and amplitude-sorting methods in two studies. In a computer simulation study, 6 abdominal motion scenarios, including 2 cosine and 4 patient breathing motion patterns, were studied. Reconstruction accuracy was evaluated quantitatively in reference to the ground-truth by average image relative error (IRE) in 10 phases and target Dice similarity coefficients (DSC) in end-of-exhalation/inhalation (EOE/EOI) phases. In addition, the proposed method was evaluated using a custom made motion phantom. Reconstruction accuracy was evaluated by motion range measurement and image quality comparison in both fast and slow breathing motions.

RESULTS: In the simulation study, stitching motion artifacts in restricted images were less using the proposed method than those using the conventional methods. The average IRE and target DSC (EOE/EOI) were 0.031 and 0.95/0.94, respectively, suggesting better motion reconstruction accuracy than the phase-sorted method (IRE=0.057, DSC=0.89/0.89) and the amplitude-sorted method (IRE=0.048, DSC=0.91/0.88). In the phantom study, the moving target reconstructed by the proposed method demonstrated better rendering with less edge blurring. With fast breathing motion, the range measured using the proposed method was more accurate than that of the phase-sorted method and was comparable to the result of amplitude-sorted method as well as ground truths.

CONCLUSION: Preliminary results suggested that the proposed sorting technique could reconstruct high quality images and accurate motion estimation with reduced artifacts in 4D-MRI.