Overview of the Current Knowledge and Conventional MRI Characteristics of Peri- and Para-Vascular Spaces

Abstract

Brain spaces around (perivascular spaces) and alongside (paravascular or Virchow-Robin spaces) vessels have gained significant attention in recent years due to the advancements of in vivo imaging tools and to their crucial role in maintaining brain health, contributing to the anatomic foundation of the glymphatic system. In fact, it is widely accepted that peri- and para-vascular spaces function as waste clearance pathways for the brain for materials such as ß-amyloid by allowing exchange between cerebrospinal fluid and interstitial fluid. Visible brain spaces on magnetic resonance imaging are often a normal finding, but they have also been associated with a wide range of neurological and systemic conditions, suggesting their potential as early indicators of intracranial pressure and neurofluid imbalance. Nonetheless, several aspects of these spaces are still controversial. This article offers an overview of the current knowledge and magnetic resonance imaging characteristics of peri- and para-vascular spaces, which can help in daily clinical practice image description and interpretation. This paper is organized into different sections, including the microscopic anatomy of peri- and para-vascular spaces, their associations with pathological and physiological events, and their differential diagnosis.

Keywords: MRI; PVS; Virchow–Robin spaces; brain cysts; glymphatic system; magnetic resonance imaging; neurofluids; neuroradiology; paravascular spaces; perivascular spaces.

Figure 1

Schematic drawing of paravascular and perivascular spaces currently believed micro-anatomy along and around basal arterioles, cortical perforating arterioles, veins, and venules. SAS: subarachnoid space; SPS: subpial space; ○: paravascular space; ●: perivascular space; *: vessel lumen; dotted line: arterioles endothelium; dash-dot line: vein and venules endothelium; dashed line: meningeal membrane; black line: glia limitans; grey line: meningeal membrane plus glia limitans. (A) The brain’s basal arterioles originate from the subarachnoid space and are surrounded by two leptomeningeal membranes. The inner membrane is in close proximity to the arteriolar wall, while the outer membrane is connected to the pia mater, allowing the basal para-arteriolar spaces to have direct communication with the SAS. The perivascular space is another space within the arterial tunica media. (B) In cortical arterioles, where there is only one leptomeningeal membrane closely applied to the vessel wall and no outer layer, the para-arteriolar space is believed to communicate with the SPS rather than the SAS. The perivascular space is another space within the arterial tunica media. (C) Veins and venules have only one leptomeningeal membrane that is closely attached to the vessel wall without any outer layer, and thus it is believed that the paravenous space communicates with the SPS rather than the SAS. The venous vasculature does not contain perivascular space.

Figure 2

Typical appearance of symmetrically enlarged PVS at the level of the basal ganglia (type I) with densitometric and signal intensity characteristics typically comparable to those of cerebrospinal fluid (black arrows). (A) Unenhanced computed tomography shows hypodense PVS. (B) A 1.5 T magnetic resonance imaging (MRI) fluid attenuated inversion recovery image shows hypointense PVS. (C) A 1.5 T MRI apparent diffusion coefficient image shows no diffusion restriction. (D) A 1.5 T MRI T2-weighted turbo spin echo image shows hyperintense PVS with a microscopic central vessel inside, known as the “vessel sign”. (E) A 1.5 T MRI T1-weighted spin echo image shows hypointense PVS.

Figure 3

A 1.5 T magnetic resonance imaging appearance of giant PVS in T2 turbo spin echo images (A,C) and fluid attenuated inversion recovery (FLAIR) images (B,D). (A,B) Patient with giant PVS in the right mesencephalothalamic region (type III) that cause a mass effect on the aqueduct of Sylvius. (C,D) Patient with a right anterior temporal lobe enlarged perivascular space (type IV) characterized by an elevated perilesional FLAIR signal with no mass effect, which indicates gliosis.

Figure 4

The 1.5 T magnetic resonance imaging T2 turbo spin echo images in different patients show the Potter scoring system based on counting visible PVS. Each hemisphere is scored separately for PVS, and the higher score of the two is used, as indicated at the bottom. (A–D) Axial slices at centrum semiovale level (type II). (E–H) Axial slices at basal ganglia level (type I), where H corresponds to the “état criblé” or “status cribrosum” described by Durand-Fardel in the basal ganglia. Note also the presence of a lacune in (D) (black circle) and that PVS in the centrum semiovale can be observed as they approach the cortex; their dilation increases as they reach the inner edge of the cortex; however, they cannot be seen as they pass through the cortex.

Figure 5

The 1.5 T magnetic resonance images show the main differential diagnosis for enlarged PVS in the context of cerebral small vessel disease (A–F) and 2 examples of differential diagnosis for giant PVS (G–L) using T2 turbo spin echo images (A,D,G,J), FLAIR images (B,E,H,K) and apparent diffusion coefficient images (C,F,I,L). (A,B) Patient with a small area of altered signal in the left centrum semiovale (black circle) characterized by hyperintensity in T2 and FLAIR images and diffusion restriction, therefore compatible with a recent subcortical infarction. (D–F) Patient with a small area of altered signal in the right centrum semiovale (empty black arrow) characterized by hyperintensity in the T2 image, hyperintense rim in the FLAIR image, and no diffusion restriction, therefore compatible with a lacune. Note also the white matter hyperintensities in centrum semiovale bilaterally. (G–I) Patient with a dysembryoplastic neuroepithelial tumor in the right medial temporal lobe (thick black arrow), with a distinct “bubbly” cystic appearance associated with T2/FLAIR signal abnormality and no diffusion restriction. (J–L) Patient with a left parieto-occipital neurenteric cyst characterized by a slightly hyperintense content in FLAIR images and no diffusion restriction.