  Deformable image registration is a fundamental task in medical imaging. Due
to the large computational complexity of deformable registration of volumetric
images, conventional iterative methods usually face the tradeoff between the
registration accuracy and the computation time in practice. In order to boost
the registration performance in both accuracy and runtime, we propose a fast
convolutional neural network. Specially, to efficiently utilize the memory
resources and enlarge the model capacity, we adopt additive forwarding instead
of channel concatenation and deepen the network in each encoder and decoder
stage. To facilitate the learning efficiency, we leverage skip connection
within the encoder and decoder stages to enable residual learning and employ an
auxiliary loss at the bottom layer with lowest resolution to involve deep
supervision. Particularly, the low-resolution auxiliary loss is weighted by an
exponentially decayed parameter during the training phase. In conjunction with
the main loss in high-resolution grid, a coarse-to-fine learning strategy is
achieved. Last but not least, we introduce an auxiliary loss based on the
segmentation prior to improve the registration performance in Dice score.
Comparing to the auxiliary loss using average Dice score, the proposed
multi-label segmentation loss does not induce additional memory cost in the
training phase and can be employed on images with arbitrary amount of
categories. In the experiments, we show FDRN outperforms the existing
state-of-the-art registration methods for brain MR images by resorting to the
compact network structure and efficient learning. Besides, FDRN is a
generalized framework for image registration which is not confined to a
particular type of medical images or anatomy.
