Fig. 2

Illustration of the network design of the U-Net segmentation network. The network accepts a greyscale frame (128 × 128 pixels) and produces segmentation maps of equal size for the heart chambers involved. The network consists of a contracting path with multiple 3 × 3 convolutions followed by ReLU (Rectified Linear Unit) activation and a max. Pooling operation (2 × 2). The number of channels is doubled at each step of the contraction path. In the expanding part, the feature maps are upscaled symmetrically, with 2 × 2 up-convolutions. In addition, channels of the expanding path are combined with the corresponding part of the contracting path through concatenation. The number on top corresponds to the number of channels, while the dimensions are given on the left of the respective boxes. For details see Ref. [13]