B0 mapping estimates the B0 field from the expected field for every voxel. These B0 maps can be used to perform prospective B0 shimming to minimize B0 inhomogeneities Jezzard & Balaban, 1995, they can be used to retrospectively correct for geometric distortions (FSL FUGUE Jenkinson et al., 2012Smith et al., 2004) (e.g.: for EPI), or to perform retrospective correction for k-space readout trajectory (e.g.: for spiral readout). Moreover, they can be used for retrospective recovery of enhanced signal decay An & Lin, 2002Alonso-Ortiz et al., 2017, for T2* mapping and they are also vital to quantitative susceptibility mapping (QSM) where the goal is to map the susceptibility of the subject.
One of the most simple and widely adopted techniques used to perform B0 mapping is the 2-echo phase difference technique. This technique is faster and simpler than most other alternatives. Before we dive into the technique, let’s dip our toes in some theory.
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- Alonso-Ortiz, E., Levesque, I. R., Paquin, R., & Pike, G. B. (2017). Field inhomogeneity correction for gradient echo myelin water fraction imaging. Magn. Reson. Med., 78(1), 49–57.