Evaluation of Liver Respiratory Biomechanics using 4D-MRI
<bold>Purpose:</bold> It is of clinical interest to study liver deformation during breathing by applying deformable image registration (DIR) on respiratory correlated four-dimensional (4D) images. This study aims to evaluate and compare the accuracy of DIR-derived liver deformation based on 4D computed tomography (CT) and 4D magnetic resonance imaging (MRI).
<bold>Methods:</bold> 4D CT, 4D MRI and cine magnetic resonance (MR) images of liver region were acquired from 5 patients with liver cancer under an IRB-approved protocol. ROIs containing tumors in each patient were tracked multiple times (3~5) in cine MR images. The trajectories from tracking, covering several breathing cycles, were converted to trajectories in one breathing cycle through phase sorting. The average phase sorted trajectories for each patient were used as reference trajectories after manual verification. Deformation vector fields (DVFs) from 4D CT and 4D MRI were generated via DIR implemented in Velocity AI. To enable comparison between DVFs and reference tumor trajectories, deformation vectors from each frame were linked together, forming DVF-based trajectories at each voxel. All DVF-based trajectories within each ROI were averaged to represent tumor motion. The single-phase difference, the trajectory difference and the correlation coefficient between each pair of DVF-based trajectory and reference trajectory were calculated. Wilcoxon signed-rank tests were conducted to determine whether there was significant difference between the single-phase differences, the trajectory differences and the correlation coefficients for 4D CT and 4D MRI.
<bold>Results:</bold> In the superior-inferior (SI) direction, 4D CT trajectories exhibit smaller trajectory differences (traj. diff.) in millimeters on average (traj. diff. (mm)= 2.09±0.75mm) but larger trajectory differences in number of voxels (traj. diff. (voxels)= 0.87±0.29) and smaller correlation coefficients (c. c. = 0.89±0.09) than 4D MRI trajectories (traj. diff. (mm)= 2.23±1.46mm, traj. diff. (voxels)= 0.45±0.29, c. c. = 0.93±0.06) whereas 4D MRI (traj. diff. = 1.09±1.23mm, traj. diff. (voxels)= 0.60±0.65, c. c. = 0.59±0.30) surpasses 4D CT (traj. diff. = 1.30±1.36mm, traj. diff. (voxels)= 1.02±1.07, c. c. = 0.15±0.64) in every metric in the right-left (RL) direction. In the anterior-posterior (AP) direction, 4D MRI trajectories have smaller trajectory differences in millimeters (traj. diff. (mm) = 1.11±0.70mm) and smaller trajectory differences in voxels (traj. diff. (voxels) = 0.61±0.36) but slightly smaller correlation coefficients (c. c. = 0.72±0.26) than 4D CT trajectories (traj. diff. (mm) = 1.47±0.63mm, traj. diff. (voxels) = 1.15±0.50, c. c. = 0.77±0.26). A trend that the trajectory differences in voxels for 4D MRI are smaller than those for 4D CT in every direction has been observed, though the results of Wilcoxon signed-rank tests do not support there is any significant difference between the accuracy of DVFs from 4D CT and 4D MRI.
<bold>Conclusion:</bold> We have implemented a novel approach for evaluating accuracy of DVFs based on 4D imaging for studying liver deformation. Current results indicate that the accuracy of DVFs from 4D CT and 4D MRI are comparable. Trends suggesting that the DVF from 4D MRI can be potentially more accurate than the DVF from 4D CT have been observed. Further study on more patients is warranted to determine whether there is significant difference between 4D CT and 4D MRI and to what degree the accuracy of the DVF from 4D MRI can be improved.
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