Simultaneous Measurements of Arterial Transit Times and Water Exchange Rates by Diffusion-Weighted ASL

Introduction Changes in the exchange rate of water across the blood-brain barrier (BBB), denoted kw, may indicate BBB dysfunction before the leakage of larger molecules, such as contrast agents, is observable. One approach for measuring kw is to combine diffusion-weighted (DW) imaging with arterial spin labelling (ASL) to measure the vascular and tissue fractions of labelled water (1,2). kw is determined from the vascular-to-tissue ratio by characterizing the exchange between the two pools (3). However, the accuracy of DW-ASL is affected by arterial blood contributions and the arterial transit time (τa) (2,4). To address these issues, we propose a two-stage method using combinations of flow-encoding gradients and post-labelling delays (τd) to measure τa and kw. First, τa is determined using bipolar gradients to suppress flow in the feeding arteries with a relatively short τd (5). Second, kw is determined using bipolar gradients strong enough to suppress all vascular signals and τd > τa. The aims of this study were to assess the feasibility of this two-stage method and to determine the reproducibility of the kw measurements.