Coil performance of a 32-element head array as a percentage of the UISNR for a central axial slice through a homogeneous spherical phantom (σ = 0.97 Ω−1·m−1, εr = 81.3) at 3 Tesla. (a) Coil performance map where each pixel represents the experimental SNR divided by its corresponding UISNR value, scaled to account for sequence and system related parameters (5). (b) Approximated performance map, where the experimental SNR at every pixel is divided by the UISNR value at the center, calculated using the exact formula and scaled appropriately. The scaling factor is discussed in Appendix C. The approximated map provides an accurate estimate of coil performance along an extended central region, with less than 5% error for pixels that are within ~14% of the sphere radius a (c,d).