The identification of key features (e.g. organs and tumours) in medical scans (CT, MRI, etc.) is a vital first step in many other image analysis applications, but it is by no means easy to identify such features automatically. Using statistical properties of image regions alone, it is not always possible to distinguish between different features with overlapping greyscale distributions. To do so, it helps to make use of additional knowledge that may have been acquired (e.g. from a medic) abou...