Contrast-enhanced ASL (CE-ASL) has been proposed as a method for measuring blood-brain barrier (BBB) water exchange. Altering the T1 of blood using a gadolinium-based contrast agent should allow the label location to be identified as a function of post-labelling delay time; however, the shorter T1 of blood water leads to a trade-off between contrast dose and SNR. We show using simulations that there is an optimal T1 of blood post-contrast that balances this trade-off. Using this information we estimate the expected accuracy and precision of BBB water exchange measurements using CE-ASL. Finally, we demonstrate the method in vivo.