On z-shimming: settings maximizing the signal amplitude may not be derivable from the linear field component and may depend on the echo time

Motivation: Z-shimming ameliorates through-slice dephasing in T2*-weighted functional neuroimaging but, surprisingly, a linear fit to the field in slice direction may not provide z-shim settings yielding the maximum signal. Goal(s): To demonstrate that optimum z-shim settings should be obtained from a reference acquisition stepping through a range of z-shims at the desired echo time. Approach: Numerical simulations of the MR signal were performed for different through-slice field distributions, echo times, and z-shims. Results: Z-shims derived from the linear fit minimize the dephasing but this is not equivalent to maximizing the signal. Furthermore, optimum z-shim settings may depend on the echo time. Impact: The results could help to optimize z-shim settings in T2*-weighted acquisitions used for BOLD-based functional neuroimaging by using a reference acquisition stepping through a range of z-shims at the desired echo time rather than a fit to the field distribution.