Rethinking U-net Skip Connections for Biomedical Image Segmentation
Frauke Wilm Jonas Ammeling Mathias Öttl Rutger H. J. Fick Marc Aubreville Katharina Breininger
The U-net architecture has significantly impacted deep learning-based
segmentation of medical images. Through the integration of long-range skip
connections, it facilitated the preservation of high-resolution features.
Out-of-distribution data can, however, substantially impede the performance of
neural networks. Previous works showed that the trained network layers differ
in their susceptibility to this domain shift, e.g., shallow layers are more
affected than deeper layers. In this work, we investigate the implications of
this observation of layer sensitivity to domain shifts of U-net-style
segmentation networks. By copying features of shallow layers to corresponding
decoder blocks, these bear the risk of re-introducing domain-specific
information. We used a synthetic dataset to model different levels of data
distribution shifts and evaluated the impact on downstream segmentation
performance. We quantified the inherent domain susceptibility of each network
layer, using the Hellinger distance. These experiments confirmed the higher
domain susceptibility of earlier network layers. When gradually removing skip
connections, a decrease in domain susceptibility of deeper layers could be
observed. For downstream segmentation performance, the original U-net
outperformed the variant without any skip connections. The best performance,
however, was achieved when removing the uppermost skip connection - not only in
the presence of domain shifts but also for in-domain test data. We validated
our results on three clinical datasets - two histopathology datasets and one
magnetic resonance dataset - with performance increases of up to 10% in-domain
and 13% cross-domain when removing the uppermost skip connection.