Fig. 5.
Effect of varying the EPI parameters and the optimization constraints. The mean BS achievable in the ROI and the mean GPE observed in the WSA are reported as a function of the constraint on the field standard deviation. BS values are relative to BS0 = 100% in the absence of any field gradients. The standard EPI parameters were TC = 30 ms, Δt = 330 μs, FoV = 192 mm and Δz = 2 mm. Changing these parameters affected the result of the optimization, leading to relative change in the contributions of X, Y and Z shim components (Table 1) and in the maximally achievable BS. As an example, an increase of the FoV up to 256 mm (voxel size Δx = 4 mm) will affect αRO and αPE with opposite effect. Simulations showed that the decrease in αRO was almost exactly balanced by the increase in αPE. As a result, no BS improvement is observed compared to the standard EPI sequence.