Figure 6.

B₀ shim simulations assessing the benefit of adding 6 shim-only four-turn coils over the face area. (A) Three array designs are compared to two cases: baseline static 2^nd-order shimming (2SH) and slice-optimized 0^th-to-1^st-order shimming applied on top of the 2SH shim. The three designs are: A 30ch shim array with all RF coils converted to DC shim coils; a hybrid coil with 25 AC/DC shim coils and 6 DC-only shim coils added over the face area; and a hybrid coil with all 30 AC/DC coils plus the 6 face coils. (B) ΔB₀ maps for global shimming of one representative subject are shown along with slice-wise (𝜎^SL) and whole-brain (𝜎^WB) st. dev. and residuals. The face loops improve the shim in anterior temporal cortex (“AT”) and prefrontal cortex (“PF”). The 30ch AC/DC coil generates unwanted field offsets in the most superior part of the head (“S”) when these coils attempt to shim the frontal area; the 31ch array largely mitigates this problem. Relative to 2SH global shimming, the 𝜎^WB is reduced by 9% and 21% without and with the face coils, respectively. (C) In slice-optimized shimming, the face coils provide a more modest benefit, achieving a 50% reduction in 𝜎_B0 versus 46% for the 30ch array. For both global and slice-opt. shimming, the addition of 5 more AC/DC coils on the inferior-posterior area in the 36ch array brings only modest gains compared to the 31ch array. For (B) and (C), metric are also reported for the whole set of 8 field maps used in the simulations.