The invention relates to a system (1) for assisting in assessing a state of a subject's lung. The system is adapted to process a dark field image, which comprises dark field values for different spatial positions and for different breathing state values, such that a differentiation image is generated by differentiating the provided dark field image with respect to the breathing state values. This differentiation can lead to a functional image which can be used for detecting changes of air-soft tissue interfaces, which might be caused by a lung disease, with high sensitivity. This allows for an improved assisting in assessing a state of a subject's lung.
