An improved U-shaped network for brain tumor segmentation

Accurate segmentation of brain tumors from magnetic resonance images is the key to the clinical diagnosis and rational treatment of brain tumor diseases. Recently, convolutional neural networks have been widely used in biomedical image processing. 3D U-Net is sought after because of its excellent segmentation effect; however, the feature map supplemented by the skip connection is the output feature map after the encoder feature extraction, and the loss of original detail information in this process is ignored. In the 3D U-Net design, after each layer of convolution, regularization, and activation function processing, the detailed information contained in the feature map will deviate from the original detailed information. For skip connections, the essence of this design is to supplement the detailed information of the original features to the decoder; that is, in the decoder stage, the more original the skip connection-supplemented feature maps are, the more easily the decoder can achieve a better segmentation effect. To address this problem, this paper proposes the concept of a front-skip connection. That is, the starting point of the skip connection is adjusted to the front to improve the network performance. On the basis of this idea, we design a front-skip connection inverted residual U-shaped network (FS Inv-Res U-Net). First, the front-skip connections are applied to three typical networks, DMF Net, HDC Net, and 3D U-Net, to verify their effectiveness and generalization. Applying our proposed front-skip connection concept on these three networks improves the network performance, indicating that the idea of a front-skip connection is simple but powerful and has out-of-the-box characteristics. Second, 3D U-Net is enhanced using the front-skip connection concept and the inverted residual structure of MobileNet, and then FS Inv-Res U-Net is proposed based on these two ideas. Additionally, ablation experiments are conducted on FS Inv-Res U-Net. After adding the front-skip connection and the inverted residual module to the backbone network 3D U-Net, the segmentation performance of the proposed network is greatly improved, indicating that the front-skip connection and the inverted residual module help our brain tumor segmentation network. Finally, the proposed network is validated on the validation dataset of the public datasets BraTS 2018 and BraTS 2019. The Dice scores of the validation results on the enhanced tumor, whole tumor, and tumor core were 80.23%, 90.30%, and 85.45% and 78.38%, 89.78%, and 83.01%, respectively; the hausdorff95 distances were 2.35, 4.77, and 5.50 mm and 4, 5.57, and 6.37 mm, respectively. The above results show that the FS Inv-Res U-Net proposed in this paper achieves the same evaluation indicators as advanced networks and provides accurate brain tumor segmentations.