Breakdown of the blood-brain barrier has been hypothesized as a key mechanism leading to neuronal dysfunction that underlies many neurological and psychiatric diseases. Compared with exogenous contrast agents, trans-capillary water permeability may provide a more direct and sensitive assessment of BBB integrity at disease onset and progression. While the current gold standard approach to measure water permeability is O15-H2O PET, its widespread use is limited by availability and cost. Here, we propose a novel MRI-based method, IDEALS, to non-invasively map BBB water permeability with high sensitivity and whole-brain coverage. This method was applied in healthy participants and brain tumor patients.