Abstract: Motion in Magnetic Resonance Imaging (MRI) scans causes image corruption and remains a barrier to clinical imaging. We propose Beat Pilot Tone (BPT), a simple serendipitous system exploitation that turns any MRI receiver chain into a radio frequency (RF) motion sensing system that can operate at arbitrary frequencies (up to several GHz). Our contact-free system can be implemented on any MRI scanner regardless of field strength. Through electromagnetic field simulations and experiments, we explain BPT’s novel mechanism: two or more transmitted RF tones form motion-modulated standing wave patterns that are sensed by the same receiver coil arrays used for MR imaging. These waves are incidentally mixed by intermodulation and digitized simultaneously to the MRI data. BPT achieves an order of magnitude greater sensitivity to motion than other methods in detecting and separating common motion types (respiratory, bulk, cardiac, and head motion) in volunteers. Moreover, BPT offers tunable sensitivity to motion based on the transmit frequencies; at microwave frequencies, BPT can detect millimeter-scale vibrations (ballistocardiograms). With multiple antennas and frequency-multiplexing, BPT can operate as a multiple-input multiple-output (MIMO) system. BPT significantly expands the capability of any MRI system, paving the way toward multi-modality, motion-robust, and simultaneous RF and MR imaging. Joint work with Suma Anan