Motivation: Transfer rate of contrast agent and water plays complementary role in assessing the integrity of blood-brain barrier, however there is currently no means of evaluating them simultaneously in dementia patients. Goal(s): We aimed to develop a new pharmacokinetic model to comprehensively evaluate the blood-brain barrier damage in dementia patients. Approach: Transfer rate of contrast agent and water was calculated based on a new pharmacokinetic model by simultaneous fitting of the Bloch–McConnell equation and the Patlak model. Results: The proposed model was able to exhibit distinct patterns of blood-brain barrier damage in different types of dementia, which was significantly associated with cognitive impairment. Impact: The transfer rate of contrast agent and water based on water exchange DCE-MRI demonstrated distinct patterns of blood-brain barrier damage in patients with Alzheimer’s disease and dementia with Lewy bodies, providing promising new non-invasive imaging biomarkers for dementia.