The inventors have developed a new flexible motion sensing method that exploits nonlinear intermodulation of MRI receiver coil preamplifiers to sense the motion of a subject in an MRI scanner without on-subject hardware.
The method transmits two tones at two different frequencies, f1 and f2, designed to be received at frequency f_BPT by the receiver via intermodulation, where f1 and f2 are much greater than the MRI center frequency. These signals are picked up by the receiver coils, mixed at the pre-amplification stage by intermodulation, then digitized by the receiver chain. The method is 20 times more sensitive to motion than the state-of-the-art Pilot Tone (PT) method of motion sensing.
The inventors have demonstrated the method with second order intermodulation.
Additionally, more transmitters can be used, each with a different set of frequencies. Higher frequency tones enable greater sensitivity to subject motion. This method enables the detection of motion at multiple temporal and spatial scales, for example, breathing and rigid motion of the head.
The method is used simultaneously with conventional MR imaging and does not adversely impact the signal-to-noise ratio (SNR) of the acquired MR image.
The method has been demonstrated using inexpensive consumer grade hardware for the 2.4GHz ISM band as a proof-of-concept. Since the MR signal is small (< -30dBm), little transmit power is necessary to induce an intermodulation signal similar in amplitude to the MR signal.
The advantages of this method include: