Blood–brain barrier water exchange measurements using contrast‐enhanced ASL.

A technique for quantifying regional blood–brain barrier (BBB) water exchange rates using contrast‐enhanced arterial spin labelling (CE‐ASL) is presented and evaluated in simulations and in vivo. The two‐compartment ASL model describes the water exchange rate from blood to tissue, kb, but to estimate kb in practice it is necessary to separate the intra‐ and extravascular signals. This is challenging in standard ASL data owing to the small difference in T1 values. Here, a gadolinium‐based contrast agent is used to increase this T1 difference and enable the signal components to be disentangled. The optimal post‐contrast blood T1 (T1,bpost) at 3 T was determined in a sensitivity analysis, and the accuracy and precision of the method quantified using Monte Carlo simulations. Proof‐of‐concept data were acquired in six healthy volunteers (five female, age range 24–46 years). The sensitivity analysis identified the optimal T1,bpost at 3 T as 0.8 s. Simulations showed that kb could be estimated in individual cortical regions with a relative error ϵ<1% and coefficient of variation CoV=30%; however, a high dependence on blood T1 was also observed. In volunteer data, mean parameter values in grey matter were: arterial transit time tA=1.15±0.49 s, cerebral blood flow f=58.0±14.3 mL blood/min/100 mL tissue and water exchange rate kb=2.32±2.49 s−1. CE‐ASL can provide regional BBB water exchange rate estimates; however, the clinical utility of the technique is dependent on the achievable accuracy of measured T1 values. [ABSTRACT FROM AUTHOR]