The invention relates to a monitoring device (105) for remotely monitoring a heartbeat of a subject, the monitoring device comprising a remote sensor (102) for receiving a monitored signal (103) indicative of a movement of the subject's thoracic wall (101) induced by the subject's heartbeat. In an embodiment of the invention, the monitoring device (105) further comprises a triggering device (401) arranged to access the monitored signal (403) to generate a triggering signal (402) representative of a phase of the heartbeat. The invention further relates to an imaging or spectroscopy system (901), for example a magnetic resonance or computed tomography system or a cardiac 3D X-ray angiography system, arranged to acquire data from a subject, the system comprising such a monitoring device (105), wherein the system is further arranged to utilize the triggering signal (402) to synchronize the acquisition of the data to the phase of the subject's heartbeat.