FASTMAP display two error messages that require user interaction. Both are related to the goodness of fit of the phase projections.

  1. Noise above 1.5 Hz in the <N> projection.
    This error message alerts you to relatively high errors (RMSSD > 1.5 Hz), which can be a sign of trouble, such as poor sensitivity. In most cases this error is prompted at the beginning of the shim process, where inhomogeneities are large. If you are sure everything is ok, you can enter y to indicate to FASTMAP that it is ok to proceed.

  2. Not sufficient signal in <N> projection.
    This error arises from a check FASTMAP does to determine that Signal-to-RMSnoise is above 10. This error is critical, as a result FASTMAP will set the RMSD to 1000 (to avoid the data point to be used for shimming) and set the shim change to 0. If you get this error be sure to check that you have a discernible echo, if not, check the RF amplifier status, that the position of the ROI (X0, Y0, Z0) is ok and that tau is small enough if there is a large inhomogeneity present. If you do see nice echoes and good projections, it is possible that lro is too small, since the SNR calculation assumes that the outermost 25% of the projection (the width of which is defined by lro) contain noise only.

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