Hello Jennifer,
The field mapping toolbox requires complex data (real and imaginary or phase and magnitude) with different echo times as input. The B0 field inhomogeneity can be calculated from these. The blip up / blip down approach is different - it determines the distorting field from how it manifests in each of the images for which everything (e.g. echo time) is the same except the direction of traversal through k-space (defined by the blip polarity). Therefore, this type of data is not suitable input to the field mapping toolbox.
Hope that helps,
Martina
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