Filter exchange imaging (FEXI) is a non-invasive method to measure water exchange among diffusion compartments by apparent exchange rate (AXR). However, in human white matter, it is still controversial whether the diffusion-encoding gradient direction will affect AXR measurement. In this study, we performed FEXI on human brain with 20 diffusion gradient directions to explore features of AXR in white matter. We found that AXR measured with diffusion direction perpendicular to fiber (~ 0.47 s-1) is significantly larger than that parallel to fiber (~ 0.15 s-1), suggesting FEXI at different directions might measure exchanges between different biological compartments in white matter.
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