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Purpose: Despite decades of collective experience, radiofrequency coiloptimization for MR has remained a largely empirical process, with clearinsight into what might constitute truly task-optimal, as opposed to merely'good,' coil performance being difficult to come by. Here, a new principle, theOptimality Principle, is introduced, which allows one to predict, rapidly andintuitively, the form of optimal current patterns on any surface surroundingany arbitrary body.  Theory: The Optimality Principle, in its simplest form, states that thesurface current pattern associated with optimal transmit field or receivesensitivity at a point of interest (per unit current integrated over thesurface) is a precise scaled replica of the tangential electric field patternthat would be generated on the surface by a precessing spin placed at thatpoint. A more general perturbative formulation enables efficient calculation ofthe pattern modifications required to optimize signal-to-noise ratio inbody-noise-dominated situations.  Methods and Results: The unperturbed principle is validated numerically, andconvergence of the perturbative formulation is explored in simple geometries.Current patterns and corresponding field patterns in a variety of concretecases are then used to separate signal and noise effects in coil optimization,to understand the emergence of electric dipoles as strong performers at highfrequency, and to highlight the importance of surface geometry in coil design.  Conclusion: Like the Principle of Reciprocity from which it is derived, theOptimality Principle offers both a conceptual and a computational shortcut. Inaddition to providing quantitative targets for coil design, the OptimalityPrinciple affords direct physical insight into the fundamental determinants ofcoil performance.

The Optimality Principle for MR signal excitation and reception: New physical insights into ideal radiofrequency coil design