Review: RU-Net & R2U-Net — Recurrent Residual Convolutional Neural Network (Medical Image Segmentation)

Improve U-Net by Recurrent Convolutions and Residual Connections

Sik-Ho Tsang
Sep 21 · 4 min read

In this story, RU-Net & R2U-Net, by University of Dayton and Comcast Labs, is briefly reviewed.

  • RU-Net is Recurrent Convolutional Neural Network (RCNN) based on U-Net.
  • R2U-Net is Recurrent Residual Convolutional Neural Network (RRCNN) based on U-Net.

This is a 2018 arXiv tech report with more than 40 citations. (Sik-Ho Tsang @ Medium)


  1. RU-Net
  2. R2U-Net
  3. Experimental Results

1. RU-Net

  • As shown above, RU-Net is based on U-Net except that there are recurrent convolutions before downsampling, before upsampling and before outputting the segmentation map.
  • (If interested, please visit my review on U-Net.)
  • For the recurrent convolutional layer (RCL), the output O(t) is the output at time step t before ReLU. And it is equal to w * x(t) + w * x(t-1) + b.
  • F(x,w) is just O(t) after ReLU.

2. R2U-Net

2.1. R2U-Net

  • In R2U-Net, it is residual learning instead of the one in RU-Net.

2.2. Comparison of Different Kinds of U-Net

  • (a): Basic convolutional unit in U-Net.
  • (b): Convolutional unit in RU-Net.
  • (c): Convolutional unit in Residual U-Net.
  • (d): Convolutional unit in R2U-Net.
  • RU-Net and R2U-Net are having the same number of network parameters as U-Net but with better performance.

3. Experimental Results

3.1. Blood Vessel Segmentation

  • Three different popular datasets for retina blood vessel segmentation including DRIVE, STARE, and CHASH_DB1.
  • RU-Net and R2U-Net obtain the best performance.

3.2. Skin Cancer Segmentation

  • R2U-Net obtains the best segmentation performance.

3.3. Lung Segmentation

  • R2U-Net with t=3 obtains the best segmentation performance.

3.4. Computational Time

There is also Attention R2U-Net by combining Attention U-Net and R2U-Net, by LeeJunHyun.


[2018 arXiv] [RU-Net & R2U-Net]
Recurrent Residual Convolutional Neural Network based on U-Net (R2U-Net) for Medical Image Segmentation

My Previous Reviews

Image Classification [LeNet] [AlexNet] [Maxout] [NIN] [ZFNet] [VGGNet] [Highway] [SPPNet] [PReLU-Net] [STN] [DeepImage] [SqueezeNet] [GoogLeNet / Inception-v1] [BN-Inception / Inception-v2] [Inception-v3] [Inception-v4] [Xception] [MobileNetV1] [ResNet] [Pre-Activation ResNet] [RiR] [RoR] [Stochastic Depth] [WRN] [ResNet-38] [Shake-Shake] [FractalNet] [Trimps-Soushen] [PolyNet] [ResNeXt] [DenseNet] [PyramidNet] [DRN] [DPN] [Residual Attention Network] [DMRNet / DFN-MR] [IGCNet / IGCV1] [MSDNet] [ShuffleNet V1] [SENet] [NASNet] [MobileNetV2]

Object Detection [OverFeat] [R-CNN] [Fast R-CNN] [Faster R-CNN] [MR-CNN & S-CNN] [DeepID-Net] [CRAFT] [R-FCN] [ION] [MultiPathNet] [NoC] [Hikvision] [GBD-Net / GBD-v1 & GBD-v2] [G-RMI] [TDM] [SSD] [DSSD] [YOLOv1] [YOLOv2 / YOLO9000] [YOLOv3] [FPN] [RetinaNet] [DCN]

Semantic Segmentation [FCN] [DeconvNet] [DeepLabv1 & DeepLabv2] [CRF-RNN] [SegNet] [ParseNet] [DilatedNet] [DRN] [RefineNet] [GCN] [PSPNet] [DeepLabv3] [ResNet-38] [ResNet-DUC-HDC] [LC] [FC-DenseNet] [IDW-CNN] [DIS] [SDN]

Biomedical Image Segmentation [CUMedVision1] [CUMedVision2 / DCAN] [U-Net] [CFS-FCN] [U-Net+ResNet] [MultiChannel] [V-Net] [3D U-Net] [M²FCN] [SA] [QSA+QNT] [3D U-Net+ResNet] [Cascaded 3D U-Net] [Attention U-Net] [RU-Net & R2U-Net]

Instance Segmentation [SDS] [Hypercolumn] [DeepMask] [SharpMask] [MultiPathNet] [MNC] [InstanceFCN] [FCIS]

Super Resolution [SRCNN] [FSRCNN] [VDSR] [ESPCN] [RED-Net] [DRCN] [DRRN] [LapSRN & MS-LapSRN] [SRDenseNet]

Human Pose Estimation [DeepPose] [Tompson NIPS’14] [Tompson CVPR’15] [CPM]

Codec Post-Processing [ARCNN] [Lin DCC’16] [IFCNN] [Li ICME’17] [VRCNN] [DCAD] [DS-CNN]

Generative Adversarial Network [GAN]

Sik-Ho Tsang

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PhD, Researcher. I share what I've learnt and done. :) My LinkedIn:

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