Optimal echo times for multi‐gradient echo‐based B0 field‐mapping

B₀ field maps are used ubiquitously in neuroimaging, in disciplines ranging from magnetic resonance spectroscopy to temperature mapping and susceptibility‐weighted imaging. Most B₀ maps are acquired using standard gradient‐echo–based vendor‐provided sequences, often comprised of two echoes spaced a few milliseconds apart. Herein, we analyze the optimal spacing of echo times, defined as those maximizing precision—minimizing the standard deviation—for a fixed total acquisition time. Field estimation is carried out using a weighted least squares estimator. The standard deviation is shown to be approximately inversely proportional to the total acquisition time, suggesting a law of diminishing returns, whereby substantial gains are obtained up to a certain point, with little improvement beyond that point. Validations are provided in a phantom and a group of volunteers. Multi‐gradient echo sequences are readily available on all manufacturer platforms, making our recommendations straightforward to implement on any modern scanner.