3D MRI of whole-brain water permeability with intrinsic diffusivity encoding of arterial labeled spin (IDEALS)

Abstract

This work proposes a novel MRI method – Intrinsic Diffusivity Encoding of Arterial Labeled Spin (IDEALS) – for the whole-brain mapping of water permeability in the human brain without an exogenous contrast agent. Quantitative separation of the intravascular and extravascular labeled water MRI signal was achieved in arterial spin labeling experiments with segmented 3D-GRASE acquisition by modulating the relative sensitivity between relaxation, true diffusion, and pseudodiffusion. The intrinsic diffusivity encoding in k-space created different broadening of the image-domain point spread functions for intravascular and extravascular labeled spins, from which blood-brain barrier (BBB) water extraction fraction (Ew) and water permeability surface area product (PSw) were estimated. The feasibility and sensitivity of this method was evaluated in healthy subjects at baseline and after caffeine challenge. The estimated baseline Ew and PSw maps showed contrast among gray matter (GM) and white matter (WM). GM Ew was significantly lower than that of WM (78.8% ± 3.3% in GM vs. 83.9% ± 4.6% in WM; p < 0.05) and GM PSw was significantly higher than that of WM (131.7 ± 29.5 mL/100 g/min in GM vs. 76.2 ± 18.4 mL/100 g/min in WM; p < 0.05). BBB Ew was significantly lower for females than males (74.9% ± 3.7% for females vs. 81.3% ± 3.3% for males in GM; 80.5% ± 4.7% for females vs. 86.1 ± 3.0 for males in WM; p < 0.05 for both), while significant PSw differences were only observed in WM (143.8 ± 34.4 mL/100 g/min for females vs. 123.6 ± 24.4 mL/100 g/min for males in GM; 91.6 ± 15.0 mL/100 g/min for females vs. 65.9 ± 12.5 mL/100 g/min for males in WM; p = 0.20 and p < 0.05 for GM and WM respectively). Significant correlations between Ew and CBF (r = −0.32, p < 0.05) and between PSw and CBF (r = 0.89, p < 0.05) were observed, consistent with 15O-H2O PET findings. After caffeine challenge, reduced CBF, Ew and PSw were observed, demonstrating the sensitivity of IDEALS approach.