Fig 2. Different variants of U-Net structure and backbone (convolutional and recurrent convolutional units).

(a) classical U-Net uses convolutional encoding and decoding unit, (b) RU-Net uses encoder-decoder structure with recurrent convolutional units, (c) R2UNet combines recurrent convolutional units and residual connections (yellow line) form residual RCNN network, and (d) proposed semi-Siamese U-Net which uses encoder-parallel decoders structure with convolutional units.