Adaptable landmark localisation: Applying model transfer learning to a shape model matching system

We address the challenge of model transfer learning for a shape model matching (SMM) system. The goal is to adapt an existing SMM system to work effectively with new data without rebuilding the system from scratch. Recently, several SMM systems have been proposed that combine the outcome of a Random Forest (RF) regression step with shape constraints. These methods have been shown to lead to accurate and robust results when applied to the localisation of landmarks annotating skeletal structures in radiographs. However, as these methods contain a supervised learning component, their performance heavily depends on the data that was used to train the system, limiting their applicability to a new dataset with different properties. Here we show how to tune an existing SMM system by both updating the RFs with new samples and re-estimating the shape model. We demonstrate the effectiveness of tuning a cephalometric SMM system to replicate the annotation style of a new observer. Our results demonstrate that tuning an existing system leads to significant improvements in performance on new data, up to the extent of performing a well as a system that was fully rebuilt using samples from the new dataset. The proposed approach is fast and does not require access to the original training data.