  Purpose: To introduce a simple system exploitation with the potential to turn
MRI scanners into general-purpose RF motion monitoring systems.
  Methods: Inspired by Pilot Tone (PT), this work proposes Beat Pilot Tone
(BPT), in which two or more RF tones at arbitrary frequencies are transmitted
continuously during the scan. These tones create motion-modulated standing wave
patterns that are sensed by the receiver coil array, incidentally mixed by
intermodulation in the receiver chain, and digitized simultaneously with the
MRI data. BPT can operate at almost any frequency as long as the
intermodulation products lie within the bandwidth of the receivers. BPT's
mechanism is explained in electromagnetic simulations and validated
experimentally.
  Results: Phantom and volunteer experiments over a range of transmit
frequencies suggest that BPT may offer frequency-dependent sensitivity to
motion. Using a semi-flexible body receiver array, BPT appears to sense
cardiac-induced body vibrations at microwave frequencies (1.2 GHz and greater).
At lower frequencies, it exhibits a similar cardiac signal shape to PT, likely
due to blood volume changes.
  Other volunteer experiments with respiratory, bulk, and head motion show that
BPT can achieve greater sensitivity to motion than PT and greater separability
between motion types. Basic multiple-input multiple-output (4x22 MIMO)
operation with simultaneous PT and BPT in head motion is demonstrated using two
transmit antennas and a 22-channel head-neck coil.
  Conclusion: BPT may offer a rich source of motion information that is
frequency-dependent, simultaneous, and complementary to PT and the MRI exam.
