Enlarged perivascular spaces in the basal ganglia are associated with arteries not veins

Abstract

Enlarged perivascular spaces (EPVS) are common in cerebral small vessel disease (CSVD) and have been identified as a marker of dysfunctional brain clearance. However, it remains unknown if the enlargement occurs predominantly around arteries or veins. We combined in vivo ultra-high-resolution MRI and histopathology to investigate the spatial relationship of veins and arteries with EPVS within the basal ganglia (BG). Furthermore, we assessed the relationship between the EPVS and measures of blood-flow (blood-flow velocity, pulsatility index) in the small arteries of the BG. Twenty-four healthy controls, twelve non-CAA CSVD patients, and five probable CAA patients underwent a 3 tesla [T] and 7T MRI-scan, and EPVS, arteries, and veins within the BG were manually segmented. Furthermore, the scans were co-registered. Six autopsy-cases were also assessed. In the BG, EPVS were significantly closer to and overlapped more frequently with arteries than with veins. Histological analysis showed a higher proportion of BG EPVS surrounding arteries than veins. Finally, the pulsatility index of BG arteries correlated with EPVS volume. Our results are in line with previous works and establish a pathophysiological relationship between arteries and EPVS, contributing to elucidating perivascular clearance routes in the human brain.

Keywords: 7T MRI; cerebral small vessel disease; histopathology; hypertensive arteriopathy; perivascular clearance.

Figure 1.

MRI-processing pipeline and parameters: a) Pipeline summarizing the MRI-processing, consisting of i) the segmentation of arteries, veins, and enlarged perivascular spaces (EPVS) in native space, ii) registration of MRI scans to each other, iii) definition of the basal ganglia (BG) as region of interest (ROI), and iv), parameter extraction/analysis. Visual quality control was conducted at every step. b) Exemplary segmentation of arteries in Time-of-Flight angiography (ToF), veins in quantitative susceptibility mapping (QSM), and BG as well as EPVS in the T2-weighted image (T2-w). Segmentations are illustrated in the axial plane (B, first row) as well as three-dimensionally, observed from dorsal to ventral (B, second row). c) Combined visualization of the exemplary segmentations in b) on a T2-weighted image (T2-w). d) Simplified graphical visualization and formula of the Hausdorff distance quantifying the spatial distance of two segmentations. d1 represents the maximum distance from the boundary in a to the nearest boundary in b $:$ d1 = sup_a∈A inf_b∈B d(a,b). d2 represents the maximum distance from the boundary in b to the nearest boundary in a: d2 = sup_b∈B inf_a∈A d(b,a).

Figure 2.

Ex vivo MRI and histopathology analysis: (a) 3T T2-weighted ex vivo MRI was performed on the most intact hemisphere and presence of enlarged perivascular spaces (EPVS) was evaluated. The inlet displays EPVS, which appear as hyperintense stripes. (b) Formalin-fixed 1 cm thick coronal section of the same brain and level seen in a). The inlet displays the sample of the brain region which included the basal ganglia (BG). (c) Tissue stained for Luxol fast blue with Hematoxylin&Eosin (LHE) for the identification of vessels and diameter measurement of corresponding PVS. All vessels with a diameter >10 μm (bigger inset) were selected and the corresponding PVS diameter measured perpendicularly to the vessel and the space (example of a measurement in the small inset). If the PVS was wider than 30% of the vessel’s diameter it was considered enlarged and the vessel classified as related to an EPVS. and (d) smooth muscle actin (SMA) immunohistochemistry on adjacent sections allowed to differentiate between arteries and veins based on the presence of SMA-positive cells in the vessel’s wall.

Figure 3.

Proximity of arteries and veins to enlarged perivascular spaces (EPVS): (a) Significantly smaller Hausdorff distance between segmentations of arteries and EPVS in comparison to veins and EPVS revealing closer proximity of arteries and EPVS in comparison to veins and EPVS. (b) In histology, a significantly larger number of EPVS were associated with arteries, than with veins. (c) 3D rendering of segmented veins and arteries within the basal ganglia (BG). (d) 3D rendering of veins, arteries and EPVS within the BG illustrating closer proximity of EPVS to arteries than to veins. ****=p < 0.0001, datapoints indicate individual cases.

Figure 4.

Quantitative measures of enlarged perivascular spaces (EPVS): (a) A significantly larger number of EPVS was associated with arteries than with veins. (b) The total volume of artery-associated EPVS within the BG was significantly larger than for vein-associated EPVS. (c) The volume-fraction of BG occupied by artery-associated EPVS was significantly larger than that of vein-associated EPVS. (d) The mean volume of individual EPVS was not significantly different for EPVS associated with arteries or veins. (e) Significantly more EPVS were found in the BG in non-cerebral amyloid angiopathy cerebral small vessel disease (non-CAA CSVD) patients than in CAA patients or controls. (f) The total volume of EPVS was significantly larger in non-CAA CSVD patients in comparison to controls. (g) The volume of BG occupied by EPVS was significantly larger in non-CAA CSVD patients in comparison to controls. (h) The mean volume of EPVS in the BG non-CAA CSVD patients was significantly larger than in controls. (i) Exemplary 3D rendering displaying no EPVS associated with veins within the BG. (j) 3 D rendering of the same case displaying a large number of artery-associated EPVS (in red). ****=p < 0.0001. ***=p < 0.001, **=p < 0.01, datapoints indicate individual cases.