Medical image segmentation is a method for partitioning medical images into regions corresponding to anatomical structures or pathologies.Medical image segmentation automatically divides a scan into anatomical or pathological structures. Modern methods, especially deep learning architectures like U-Net, process images at multiple scales, capturing both fine details and overall context. The output is a probability map for each pixel—the chance of belonging to one class or another (tumor, liver). Training requires large annotated datasets, which are often augmented artificially (rotations, flips).
Threshold-based segmentation emerged in the 1970s. In 1988, Kass et al. proposed active contour models that optimize energy functions to find boundaries. With deep learning, the U-Net architecture (Ronneberger et al., 2015) revolutionized the field, enabling end-to-end training on limited data. Subsequent improvements include attention mechanisms and transformer-based models. Leading conferences: MICCAI, MIDL.
A CNN is trained on images with pixel-wise annotations. U-Net compresses the image into a bottleneck through convolutional and pooling layers, then expands it back with transposed convolutions. Skip connections concatenate high-resolution encoder features with the decoder, preserving fine details. The final layer uses softmax (for multiple classes) or sigmoid (for binary cases). Dice loss (1 - 2|X∩Y|/(|X|+|Y|)) handles class imbalance. Post-processing (connected components, CRF) refines boundaries.
Outlining tumor boundaries on MRI for radiation therapy planning or measuring organ volumes on CT.
The U-Net architecture, shaped like the letter 'U', won the ISBI cell tracking challenge in 2015 and now underlies most medical segmentation tools.
| ⟨r²⟩ | variable | Total mean square radius — mean square radius metre squared |
| w_i | variable | Weighting factor for component i — weighting factor |
| ⟨r_i²⟩ | variable | Mean square radius of component i — mean square radius component metre squared |
| ∑ | operator | Adds contributions from each component — summation |
















