Deep learning‐based motion compensation for four‐dimensional cone‐beam computed tomography (4D‐CBCT) reconstruction

Background Motion‐compensated (MoCo) reconstruction shows great promise in improving four‐dimensional cone‐beam computed tomography (4D‐CBCT) image quality. MoCo reconstruction for a 4D‐CBCT could be more accurate using motion information at the CBCT imaging time than that obtained from previous 4D‐CT scans. However, such data‐driven approaches are hampered by the quality of initial 4D‐CBCT images used for motion modeling. Purpose This study aims to develop a deep‐learning method to generate high‐quality motion models for MoCo reconstruction to improve the quality of final 4D‐CBCT images. Methods A 3D artifact‐reduction convolutional neural network (CNN) was proposed to improve conventional phase‐correlated Feldkamp–Davis–Kress (PCF) reconstructions by reducing undersampling‐induced streaking artifacts while maintaining motion information. The CNN‐generated artifact‐mitigated 4D‐CBCT images (CNN enhanced) were then used to build a motion model which was used by MoCo reconstruction (CNN+MoCo). The proposed procedure was evaluated using in‐vivo patient datasets, an extended cardiac‐torso (XCAT) phantom, and the public SPARE challenge datasets. The quality of reconstructed images for XCAT phantom and SPARE datasets was quantitatively assessed using root‐mean‐square‐error (RMSE) and normalized cross‐correlation (NCC). Results The trained CNN effectively reduced the streaking artifacts of PCF CBCT images for all datasets. More detailed structures can be recovered using the proposed CNN+MoCo reconstruction procedure. XCAT phantom experiments showed that the accuracy of estimated motion model using CNN enhanced images was greatly improved over PCF. CNN+MoCo showed lower RMSE and higher NCC compared to PCF, CNN enhanced and conventional MoCo. For the SPARE datasets, the average (± standard deviation) RMSE in mm−1 for body region of PCF, CNN enhanced, conventional MoCo and CNN+MoCo were 0.0040 ± 0.0009, 0.0029 ± 0.0002, 0.0024 ± 0.0003 and 0.0021 ± 0.0003. Corresponding NCC were 0.84 ± 0.05, 0.91 ± 0.05, 0.91 ± 0.05 and 0.93 ± 0.04. Conclusions CNN‐based artifact reduction can substantially reduce the artifacts in the initial 4D‐CBCT images. The improved images could be used to enhance the motion modeling and ultimately improve the quality of the final 4D‐CBCT images reconstructed using MoCo.