A comparison of navigators, snap-shot field monitoring, and probe-based field model training for correcting B₀-induced artifacts in T2*-weighted images at 7T

Purpose: To compare methods for estimating B ₀ maps used in retrospective correction of high-resolution anatomical images at ultra-high field strength. The B ₀ maps were obtained using three methods: (1) 1D navigators and coil sensitivities, (2) field probe (FP) data and a low-order spherical harmonics model, and (3) FP data and a training-based model. Methods: Data from nine subjects were acquired while they performed activities inducing B ₀ field fluctuations. Estimated B ₀ fields were compared with reference data, and the reductions of artifacts were compared in corrected T ₂ ^* images. Results: Reduction of sum-of-squares difference relative to a reference image was evaluated, and Method 1 yielded the largest artifact reduction: 27 ± 15%, 20 ± 18% (mean ± 1 standard deviation) for deep breathing and combined deep breathing and hand motion activities. Method 3 performed almost as well (24 ± 18%, 15 ± 17%), provided that adequate training data were used, and Method 2 gave a similar result (21 ± 16%, 19 ± 17%). Conclusion: This study confirms that all of the investigated methods can be used in retrospective image correction. In terms of image quality, Method 1 had a small advantage, whereas the FP-based methods measured the B ₀ field slightly more accurately. The specific strengths and weaknesses of FPs and navigators should therefore be considered when determining which B ₀-estimation method to use. Magn Reson Med 78:1373–1382, 2017.