Compact magnet design for magnetic resonance: A system-aware overview of optimization strategies, manufacturing technologies, and system-level trade-offs

Abstract

Compact permanent-magnet systems are key enablers of portable and application-specific magnetic resonance (MR), but their design is inherently multi-objective: field strength and homogeneity must be balanced against size, weight, sample accessibility, manufacturability, thermal stability, and the requirements of the surrounding MR instrumentation. This article provides a system-aware overview of compact magnet design, linking established principles of magnet topology, magnetic materials, and shimming with emerging manufacturing technologies and numerical optimization strategies.

Open unilateral and closed magnet configurations are compared in terms of their field characteristics, accessibility, and suitability for different measurement tasks. Particular attention is given to passive and active shimming, including classical pole-piece and discrete-element approaches as well as free-form and additively manufactured correction structures. We further discuss how advanced manufacturing can expand the realizable geometric design space and thereby motivate optimization methods ranging from mixed-variable and population-based algorithms to free-form and data-driven approaches. At the same time, we emphasize that numerical optimization and additive manufacturing are not universally superior to established analytical design and conventional manufacturing; their value depends on the structure of the design problem and the practical constraints of its realization.

The magnet is treated as the central component of a coupled MR system rather than as an isolated field source. Interfaces to the rf transducer, shimming architecture, and supporting electronics are therefore considered where their requirements feed back into magnet-design decisions, particularly in portable and battery-powered instruments. This perspective also highlights an important limit of nominal geometric optimization: once material variability, manufacturing tolerances, and assembly errors dominate the residual field error, further optimization of the ideal magnet can yield diminishing practical returns and the design focus should shift toward a sufficiently capable and robust shim system.

By combining established magnet-design practice with recent developments in optimization and manufacturing, this overview aims to provide practical guidance for selecting design strategies according to the requirements, constraints, and production context of the intended MR application.