Magnetization transfer explains most of the T 1 variability in the MRI literature

Abstract Purpose To identify the predominant source of theT 1$$ {T}_1 $$ variability described in the literature, which ranges from 0.6–1.1 s for brain white matter at 3 T. Methods 25T 1$$ {T}_1 $$ ‐mapping methods from the literature were simulated with a mono‐exponential and various magnetization‐transfer (MT) models, each followed by mono‐exponential fitting. A single set of model parameters was assumed for the simulation of all methods, and these parameters were estimated by fitting the simulation‐based to the corresponding literatureT 1$$ {T}_1 $$ values of white matter at 3 T. We acquired in vivo data with a quantitative magnetization transfer and threeT 1$$ {T}_1 $$ ‐mapping techniques. The former was used to synthesize MR images that correspond to the threeT 1$$ {T}_1 $$ ‐mapping methods. A mono‐exponential model was fitted to the experimental and corresponding synthesized MR images. Results Mono‐exponential simulations suggest good inter‐method reproducibility and fail to explain the highly variableT 1$$ {T}_1 $$ estimates in the literature. In contrast, MT simulations suggest that a mono‐exponential fit results in a variableT 1$$ {T}_1 $$ and explain up to 62% of the literature's variability. In our own in vivo experiments, MT explains 70% of the observed variability. Conclusion The results suggest that a mono‐exponential model does not adequately describe longitudinal relaxation in biological tissue. Therefore,T 1$$ {T}_1 $$ in biological tissue should be considered only a semi‐quantitative metric that is inherently contingent upon the imaging methodology, and comparisons between differentT 1$$ {T}_1 $$ ‐mapping methods and the use of simplistic spin systems—such as doped‐water phantoms—for validation should be viewed with caution.