LungRegNet: An unsupervised deformable image registration method for 4D‐CT lung

PurposeTo develop an accurate and fast deformable image registration (DIR) method for four‐dimensional computed tomography (4D‐CT) lung images. Deep learning‐based methods have the potential to quickly predict the deformation vector field (DVF) in a few forward predictions. We have developed an unsupervised deep learning method for 4D‐CT lung DIR with excellent performances in terms of registration accuracies, robustness, and computational speed.MethodsA fast and accurate 4D‐CT lung DIR method, namely LungRegNet, was proposed using deep learning. LungRegNet consists of two subnetworks which are CoarseNet and FineNet. As the name suggests, CoarseNet predicts large lung motion on a coarse scale image while FineNet predicts local lung motion on a fine scale image. Both the CoarseNet and FineNet include a generator and a discriminator. The generator was trained to directly predict the DVF to deform the moving image. The discriminator was trained to distinguish the deformed images from the original images. CoarseNet was first trained to deform the moving images. The deformed images were then used by the FineNet for FineNet training. To increase the registration accuracy of the LungRegNet, we generated vessel‐enhanced images by generating pulmonary vasculature probability maps prior to the network prediction.ResultsWe performed fivefold cross validation on ten 4D‐CT datasets from our department. To compare with other methods, we also tested our method using separate 10 DIRLAB datasets that provide 300 manual landmark pairs per case for target registration error (TRE) calculation. Our results suggested that LungRegNet has achieved better registration accuracy in terms of TRE than other deep learning‐based methods available in the literature on DIRLAB datasets. Compared to conventional DIR methods, LungRegNet could generate comparable registration accuracy with TRE smaller than 2 mm. The integration of both the discriminator and pulmonary vessel enhancements into the network was crucial to obtain high registration accuracy for 4D‐CT lung DIR. The mean and standard deviation of TRE were 1.00 ± 0.53 mm and 1.59 ± 1.58 mm on our datasets and DIRLAB datasets respectively.ConclusionsAn unsupervised deep learning‐based method has been developed to rapidly and accurately register 4D‐CT lung images. LungRegNet has outperformed its deep‐learning‐based peers and achieved excellent registration accuracy in terms of TRE.