Acquiring a high quality fieldmap of the mean magnetic field is crucial to obtaining a good measurement. Making sure you set the parameters so that this field map is obtained as part of your protocol is something we help you with. There are cases, including new sequences, where this can require some extra attention.
The problem: The CNI team recently helped diagnose a confusing fieldmap result. The user was acquiring a fieldmap using a spiral acquisition, but without first performing a high-order shim (HOS) acquisition or setting Autoshim (linear-order shim). They subsequently acquired EPI data with Autoshim. This resulted in acceptable imaging quality but also created some problems. The disadvantage was that high-order shim corrections were not used, and the spiral fieldmap data did not incorporate the linear-order shim corrections that were imposed by the Autoshim. As a result, the data quality were not optimized.
The solution: We recommend that a HOS be acquired as part of your protocol, especially when EPI sequences are used for subsequent data acquisition. We describe these procedures on our Wiki entries, as well as some troubleshooting steps in a blog entry:
If this overhead is too time-consuming, then it is important to ensure an Autoshim is acquired, which performs a linear-order shim correction. Please note: when Autoshim acquisition is set to “Auto”, and an HOS is acquired, the system should use the HOS acquisition rather than acquire a new linear correction.
Some details: A variation in the B0 main field from the desired B0 leads to an image shift in the phase encode direction of EPI acquired images. When the B0 field is spatially varying, this leads to varying image shifts which can result in signal pileup, voids, and image warping. There are two methods described in our Wiki for correcting these distortions. First, you can repeat your EPI data acquisition twice, with a reversal of the phase-encoding direction between data sets as we describe in a Wiki entry: CNI Data Processing
Secondly, you can acquire an explicit fieldmap using the spiral fieldmap sequence and then process the subsequently acquired EPI data as described in another Wiki entry: Fieldmaps
Please note that if the explicit fieldmap correction approach is adopted, it is important to ensure that the subsequent EPI acquisition does not modify the shim corrections. In this case setting Autoshim to “Off” in your protocol would be advisable.
Adam and Hua