The invention provides a system (102) for accurately detecting a patient's (114) movement during imaging procedures. The system comprises a camera (126) for providing a stream of camera images of a part of a patient's exterior (206). The system (102) furthermore comprises a fiducial element (116), which fiducial element (116) is mountable on said part of the patient's exterior (206), and which fiducial element (116) is detectable in the stream of camera images, and an image processor (128) for detecting a displacement of the fiducial element based on consecutive camera images comprised in at least the stream of camera images and for generating an output signal (129) indicative for said displacement. Herein, the fiducial element (116) has an in-plane stiffness which is substantially larger than an in-plane stiffness of said part of the patient's exterior. In addition, the fiducial element (116) and said part of the patient's exterior are provided with substantially equal outer in-plane dimensions.