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1、imaging and clinical 8,VR spaces surround the walls of arteries, arterioles, veins, and venules as they course from the subarachnoid space through the brain parenchyma . Electron microscopy and tracer studies have given insight into the location of VR spaces and clarified that the subarachnoid space
2、 does not communicate directly with the VR spaces . Arteries in the cerebral cortex are coated by a layer of leptomeninges that is subtended from the pia mater; by this anatomic arrangement, the VR spaces of the intracortical arteries are in direct continuity with the VR spaces around arteries in th
3、e subarachnoid space . The lack of a similar coating of leptomeningeal cells around veins in the cerebral cortex suggests that VR spaces around veins are in continuity with the subpial space . In contrast to arteries in the cerebral cortex, arteries in the basal ganglia are surrounded by notone but
4、two distinct coats of leptomeninges, separated by a VR space that is continuous with the VR space around arteries in the subarachnoid space.,The inner layer of leptomeninges closely invests the adventitia of the vessel wall. The outer layer abuts on the glia limitans of the underlying brain and is c
5、ontinuous with the pia mater on the surface of the brain and the anterior perforated substance. Veins in the basal ganglia have no outer layer of leptomeninges (similar to cortical veins), which suggests that their VR spaces are continuous with the subpial space . Interstitial fluid within the brain
6、 parenchyma drains from the gray matter of the brain by diffusion through the extracellular spaces and by bulk flow along VR spaces. There is evidence from tracer studies and from pathologic analysis of the human brain that VR spaces carry solutes from the brain and are, in effect, the lymphatic dra
7、inage pathways of the brain .,Photomicrograph (original magnification, 20; hematoxylin-eosin stain) of a coronal section through the anterior perforated substance shows two arteries (straight arrows) with surrounding VR spaces (curved arrows).,Drawing shows a cortical artery with a surrounding VR sp
8、ace crossing from the subarachnoid and subpial spaces through the brain parenchyma. The magnified view on the right shows the anatomic relationship between the artery, VR space, subpial space, and brain parenchyma.,Dilatation of VR spaces was described by Durant-Fardel in 1843. These dilatations are
9、 regular cavities that always contain a patent artery. The mechanisms underlying expanding VR spaces are still unknown. Different theories have been postulated: segmental necrotizing angiitis of the arteries or another unknown condition causing permeability of the arterial wall , expanding VR spaces
10、 resulting from disturbance of the drainage route of interstitial fluid due to cerebrospinal fluid (CSF) circulation in the cistern , spiral elongation of blood vessels and brain atrophy resulting in an extensive network of tunnels filled with extracellular water , gradual leaking of the interstitia
11、l fluid from the intracellular compartment to the pial space around the metarteriole through the fenestrae in the brain parenchyma , and fibrosis and obstruction of VR spaces along the length of arteries and consequent impedance of fluid flow .,Small VR spaces (2 mm) . Some studies found a correlati
12、on between dilated VR spaces and neuropsychiatric disorders , recent-onset multiple sclerosis (MS) , mild traumatic brain injury , and diseases associated with microvascular abnormalities.The prevalence of VR spaces at MR imaging is also dependent on the applied technique. Heavier T2-weighted imagin
13、g results in better visualization of VR spaces . In addition, the use of thinner sections will demonstrate more VR spaces as well . Also, high-field-strength MR imaging is expected to have an increased clinical impact in the near future; the current magnetic field (1.5 T) is likely to be switched to
14、 3 or 4 T. The anticipated higher signal-to-noise ratio at higher magnetic field strengths may successfully improve spatial resolution and image contrast , leading to better visualization (and increased prevalence) of VR spaces on MR images.,Signal Intensity Characteristics Visually, the signal inte
15、nsities of the VR spaces are identical to those of CSF with all pulse sequences. However, when signal intensities are measured, the VR spaces prove to have significantly lower signal intensity than the CSF-containing structures within and around the brain , a finding consistent with the fact that th
16、e VR spaces represent entrapments of interstitial fluid. This difference in signal intensity can also be explained by partial volume effects, since a VR space with accompanying vessel is smaller than the contemporary volume of a voxel on MR images.VR spaces show no restricted diffusion on diffusion-
17、weighted images because they are communicating compartments. T1-weighted images with substantial flow sensitivity may show high signal intensity due to inflow effects, thereby helping confirm that one is indeed dealing with VR spaces . VR spaces do not enhance with contrast material. In patients with small to moderate dilatations of the VR spaces (25 mm), the surrounding brain parenchyma generally has normal signal intensity .,
