Iterative static field map estimation for off-resonance correction in non-Cartesian susceptibility weighted imaging.

Purpose: Patient-induced inhomogeneities in the magnetic field cause distortions and blurring during acquisitions with long readouts such as in susceptibility-weighted imaging (SWI). Most correction methods require collecting an additional field map to remove these artifacts.

Theory: The static field map can be approximated with an acceptable error directly from a single echo acquisition in SWI. The main component of the observed phase is linearly related to and the echo time (TE), and the relative impact of non- terms becomes insignificant with >20 ms at 3 T for a well-tuned system.

Methods: The main step is to combine and unfold the multi-channel phase maps wrapped many times, and several competing algorithms are compared for this purpose. Four in vivo brain data sets collected using the recently proposed 3D spreading projection algorithm for rapid k-space sampling (SPARKLING) readouts are used to assess the proposed method.

Results: The estimated 3D field maps generated with a 0.6 mm isotropic spatial resolution provide overall similar off-resonance corrections compared to reference corrections based on an external acquisitions, and even improved for 2 of 4 individuals. Although a small estimation error is expected, no aftermath was observed in the proposed corrections, whereas degradations were observed in the references.

Conclusion: A static field map estimation method was proposed to take advantage of acquisitions with long echo times, and outperformed the reference technique based on an external field map. The difference can be attributed to an inherent robustness to mismatches between volumes and external maps, and diverse other sources investigated.