A wireless radio frequency triggered acquisition device (WRAD) for self-synchronised measurements of the rate of change of the MRI gradient vector field for motion tracking.

In this work we present a device that is capable of wireless synchronisation to the MRI pulse sequence time frame with sub-microsecond precision. This is achieved by detecting radio frequency pulses in the parent pulse sequence using a small resonant circuit. The device incorporates a 3-axis pickup coil, constructed using conventional printed circuit board (PCB) manufacturing techniques, to measure the rate of change of the gradient waveforms with respect to time. Using Maxwell-s equations, assuming negligible rates of change of curl and divergence, a model of the expected gradient derivative (slew) vector field is presented. A 3-axis Hall effect magnetometer allows for the measurement of the direction of the static magnetic field in the device co-ordinate frame. By combining the magnetometer measurement with the pickup coil voltages and slew vector field model, orientation and position can be determined to within a precision of 0.1 degrees and 0.1 mm, respectively, using a pulse series lasting 880 μs. The gradient pulses are designed to be sinusoidal enabling the detection of a phase shift between the time frame of the pickup coil digitisation circuit and the gradient amplifiers. Signal processing is performed by a low power microcontroller on the device and the results are transmitted out of the scanner bore using a low latency 2.4 GHz radio link. The device identified an unexpected 40 kHz oscillation relating to the pulse width modulation (PWM) frequency of the gradient amplifiers that is predominantly in the direction of the static magnetic field. The proposed Wireless Radio frequency triggered Acquisition Device (WRAD) enables users to probe the scanner gradient slew vector field with minimal hardware set-up and shows promise for future developments in prospective motion correction.