Motion estimation of the liver based on deformable image registration: a comparison between four-dimensional-computed tomography and four-dimensional-magnetic resonance imaging

Aim: The aim of this study was to evaluate deformable image registration (DIR)-based motion estimation of the liver for four-dimensional-computed tomography (4D-CT) and 4D-magnetic resonance imaging (MRI). Methods: Five liver cancer patients were included. Each patient was imaged with 4D-CT and 4D-MRI under an Institutional Review Board-approved protocol. Motion estimation of the liver was obtained by performing DIR on 4D-CT and 4D-MRI. A region of interest (ROI) encompassing the expert-determined gross tumor volume was used as surrogate to evaluate the accuracy of the motion estimation. ROI motion trajectories were estimated by averaging the displacement vector fields (DVFs) within the ROI during the breathing cycles for 4D-CT and 4D-MRI and were compared to those extracted from cine MR. Target registration error (TRE), correlation coefficient (CC) for phase agreement, difference in phase at maximum displacement (ΔPmax), and Dice's Similarity Coefficient (DSC) for overall motion agreement were determined. Results: As compared to 4D-CT, 4D-MRI resulted in smaller TRE in DVFs (anterior-posterior [AP]: 1.0 ± 0.4 mm vs. 1.5 ± 0.5 mm, superior-inferior [SI]: 1.9 ± 0.7 mm vs. 2.2 ± 0.8 mm), greater CC (AP: 0.67 ± 0.32 vs. 0.49 ± 0.26, SI: 0.84 ± 0.15 vs. 0.58 ± 0.28), smaller ΔPmax (AP: 1.4 ± 1.7 vs. 2.0 ± 1.0, SI: 0.4 ± 0.9 vs. 1.2 ± 0.8), and greater DSC (AP: 0.67 ± 0.08 vs. 0.61 ± 0.11, SI: 0.73 ± 0.12 vs. 0.67 ± 0.10). Conclusion: 4D-MRI can potentially provide more realistic respiratory DVFs of the liver than 4D-CT.