Deformable image registration (DIR) of 4D-CT is very important in many radiotherapeutic applications including tumor target definition, image fusion, dose accumulation and response evaluation. It is a challenging task to performing accurate and fast DIR of lung 4D-CT images due to its large and complicated deformations. In this study, we propose an unsupervised multi-scale DIR framework with attention-based network (MANet). Three cascaded models used for aligning CT images in different resolution levels were involved and trained by minimizing the loss functions, which were defined as the combination of dissimilarity between the fixed image and the deformed image and DVF regularization term. In addition, attention gates were incorporated into the three models to distinguish the moving structures from non-moving or minimal-moving structures during registration. The three models were trained sequentially and separately to minimize the loss function in each scale to initialize the MANet, and then trained jointly to minimize the total loss function which incorporated an additional dissimilarity between fixed image and deformed image. Besides, an adversarial network was integrated into MANet to enforce the DVF regularization by penalizing the unrealistic deformed images. The proposed MANet was evaluated on the public dir-lab dataset, and the target registration errors (TREs) of the model were compared with convention iterative optimization-based methods and three recently published deep learning-based methods. The initial results showed that the MANet with an average of TRE of 1.53 ± 1.02 mm outperformed other registration methods, and its execution time took about 1 s for DVF estimation with no requirement of manual-tuning for parameters, which demonstrating that our proposed method had the ability of performing superior registration for 4D-CT.
Keywords: 4D-CT; convolutional neural network; deformable registration; multi-scale.
© 2021 Institute of Physics and Engineering in Medicine.