 Pairs of R1 values for myocardial tissue and blood data were fit with a two-space water-exchange model of equilibrium transcytolemmal water exchange. The myocardial extracellular volume fraction ECV and the intracellular lifetime of water τic, a cell size-dependent parameter, were adjustable parameters of this model. The measured blood hematocrit was a fixed parameter of the model. All R1 measurements for each patient were used to fit the model to determine ECV and τic. 