From Repeatability to Reproducibility in Quantitative Low-Field MRI: A Metrology-Informed Framework for Phantom Evidence

Low-field magnetic resonance imaging includes portable and open architectures, but quantitative results remain sensitive to field strength, temperature, environment, system state, acquisition, reconstruction, and model fitting. This methodological review asks what evidence supports claims of repeatability, intermediate precision, reproducibility, qualified reference-linked bias, measurement uncertainty, and use relevance. We conducted a targeted synthesis of empirical low-field reports, quantitative-imaging methods, metrology guidance, and MRI standards using a documented search and explicit source-classification criteria. The resulting framework separates three independent axes: represented precision conditions, strength of reference linkage, and relevance to the intended technical or clinical use. Each claim is linked to an experimental design, statistical estimand, configuration record, uncertainty model, and decision rule. An explicitly synthetic low-field T1 example illustrates design identifiability, precision-based replication planning, mixed-effects analysis, an uncertainty budget, and a guard-banded technical decision; it is not experimental validation. A staged pathway allows laboratories to begin with provenance, constancy, and within-system repeatability before progressing to reference-linked or multicentre studies. Meeting a phantom threshold establishes only the specified technical property under the tested conditions; it does not establish diagnostic utility, clinical benefit, regulatory conformity, or CE marking. The framework is an operational synthesis, not a replacement standard or validated quality score.