Respiratory motion‐resolved, self‐gated 4D‐MRI using rotating cartesian k‐space (ROCK)

Purpose To propose and validate a respiratory motion resolved, self‐gated (SG) 4D‐MRI technique to assess patient‐specific breathing motion of abdominal organs for radiation treatment planning. Methods The proposed 4D‐MRI technique was based on the balanced steady‐state free‐precession (bSSFP) technique and 3D k‐space encoding. A novel rotating cartesian k‐space (ROCK) reordering method was designed which incorporates repeatedly sampled k‐space centerline as the SG motion surrogate and allows for retrospective k‐space data binning into different respiratory positions based on the amplitude of the surrogate. The multiple respiratory‐resolved 3D k‐space data were subsequently reconstructed using a joint parallel imaging and compressed sensing method with spatial and temporal regularization. The proposed 4D‐MRI technique was validated using a custom‐made dynamic motion phantom and was tested in six healthy volunteers, in whom quantitative diaphragm and kidney motion measurements based on 4D‐MRI images were compared with those based on 2D‐CINE images. Results The 5‐minute 4D‐MRI scan offers high‐quality volumetric images in 1.2 × 1.2 × 1.6 mm3 and eight respiratory positions, with good soft‐tissue contrast. In phantom experiments with triangular motion waveform, the motion amplitude measurements based on 4D‐MRI were 11.89% smaller than the ground truth, whereas a −12.5% difference was expected due to data binning effects. In healthy volunteers, the difference between the measurements based on 4D‐MRI and the ones based on 2D‐CINE were 6.2 ± 4.5% for the diaphragm, 8.2 ± 4.9% and 8.9 ± 5.1% for the right and left kidney. Conclusion The proposed 4D‐MRI technique could provide high‐resolution, high‐quality, respiratory motion‐resolved 4D images with good soft‐tissue contrast and are free of the “stitching” artifacts usually seen on 4D‐CT and 4D‐MRI based on resorting 2D‐CINE. It could be used to visualize and quantify abdominal organ motion for MRI‐based radiation treatment planning.