  Purpose: Real time monitoring of dynamic magnetic fields has recently become
a commercially available option for measuring MRI k-space trajectories and
magnetic fields induced by eddy currents in real time. However, for accurate
image reconstructions, sub-microsecond synchronization between the MRI and
trajectory data is required. In this work, we introduce a new model-based
algorithm to automatically perform this synchronization using only the MRI and
trajectory data. Methods: The algorithm works by enforcing consistency between
the MRI data, trajectory data, and receiver sensitivity profiles by iteratively
alternating between convex optimizations for (a) the image and (b) the
synchronization delay. A healthy human subject was scanned at 7T using a
transmit-receive coil with integrated field probes using both single shot
spiral and echo-planar imaging (EPI), and reconstructions with various
synchronization delays were compared to the result of the proposed algorithm.
The accuracy of the algorithm was also investigated using simulations, where
the acquisition delays for simulated acquisitions were determined using the
proposed algorithm and compared to the known ground truth. Results: In the in
vivo scans, the proposed algorithm minimized artefacts related to
synchronization delay for both spiral and EPI acquisitions, and the computation
time required was less than 30 seconds. The simulations demonstrated accuracy
to within tens of nanoseconds. Conclusion: The proposed algorithm can
automatically determine synchronization delays between MRI data and
trajectories measured using a field probe system.
