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

Purpose: To identify the predominant source of the T ₁ variability described in the literature, which ranges from 0.6-1.1 s for brain white matter at 3 T.

Methods: 25 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 literature T ₁ values of white matter at 3 T. We acquired in vivo data with a quantitative magnetization transfer and three T ₁-mapping techniques. The former was used to synthesize MR images that correspond to the three 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 variable T ₁ estimates in the literature. In contrast, MT simulations suggest that a mono-exponential fit results in a variable 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 in biological tissue should be considered only a semi-quantitative metric that is inherently contingent upon the imaging methodology; and comparisons between different T ₁-mapping methods and the use of simplistic spin systems-such as doped-water phantoms-for validation should be viewed with caution.