CHAOS stands for Combined (CT-MR) Healthy Abdominal Organ Segmentation, which is a dataset and challenge of abdominal CT contrast images and abdominal MR contrast images.

The dataset consists of the following subsets:

• CT dataset: 20 cases in training set (with gold standard), 20 cases in test set (without gold standard), the task is abdominal single organ (liver) segmentation.

• MR dataset: 20 cases in the training set (with gold standard), 20 cases in the test set (without gold standard), the task is the segmentation of multiple abdominal organs (liver, left and right kidneys, spleen).