神经影像与临床

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 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 the 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 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 continuous 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 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 drainage 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 space 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 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 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 interstitial 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 correlation 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 imaging 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 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 CharacteristicsVisually, the signal intensities 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 the 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-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 .,Locations and Morphology Dilated VR spaces typically occur in three characteristic locations. The first type (type I) is frequently seen on MR images and appears along the lenticulostriate arteries entering the basal ganglia through the anterior perforated substance. Here, the tortuous lenticulostriate arteries change direction from a lateral to a dorsomedial path and are grouped closely together. A proximal VR space, containing several vessels, is the resulting physiologic finding . The second type (type II) can be found along the path of the perforating medullary arteries as they enter the cortical gray matter over the high convexities and extend into the white matter .The third type (type III) appears in the midbrain. In the lower midbrain, VR spaces at the pontomesencephalic junction surround the penetrating branches of the collicular and accessory collicular arteries . They are mainly located between the cerebral peduncles in the axial plane and correspond to the level of the tentorial margin as seen in coronal sections. In the upper midbrain, where the VR spaces are visible at the mesencephalodiencephalic junction, they appear along the posterior (interpeduncular) thalamoperforating artery or the paramedian mesencephalothalamic artery and short and long circumferential arteries originating from the upper basilar artery or proximal posterior cerebral artery VR spaces are mostly seen as well-defined oval, rounded, or tubular structures, depending on the plane in which they are intersected. They have smooth margins, commonly appear bilaterally, and usually measure 5 mm or less .,血管周围间隙 (perivascular space，PVS)，又称 V-R(Virchow-Robin)间隙最初由德国病理学家Rudolf Virchow (1821 1885年) 与法国解剖学家Charles Philippe Robin（18211885年）对其进行描述血管周围间隙包绕在经蛛网膜下腔进入脑实质的小血管壁周围血管周围间隙包绕在经蛛网膜下腔进入脑实质的小血管(动脉、小动脉、静脉、小静脉)壁周围，并不与蛛网膜下腔直接相通脑动脉皮质支外被以柔脑膜，它是由软脑膜延伸而来，由此可知皮质动脉的血管周围间隙是蛛网膜下腔内动脉的血管周围间隙的直接延续大脑皮质静脉周围缺乏柔脑膜，这表明静脉的血管周围间隙与软脑膜下腔直接相通基底节静脉周围没有柔脑膜，与皮质静脉一样，它的血管周围间隙与软脑膜下腔直接相通,基底节动脉被覆两层柔脑膜，两层柔脑膜之间为血管周围间隙内层的柔脑膜紧贴血管外膜，外层的柔脑膜毗连脑的神经胶质界膜，是脑和前穿支表面软脑膜的延续脑实质内的组织间液通过细胞外间隙弥散而从脑灰质中排出，之后沿着血管周围间隙流动动物示踪研究和人脑病理分析证实血管周围间隙从脑内运输溶质，事实上是脑的淋巴回流通道直径2mm)，可称为大血管周围间隙根据大小可将血管周围间隙分为3级，即：级：直径在2 mm以下；级：直径在23mm之间；级：直径超过3mm一些研究认为血管周围间隙的扩大可能与某些神经精神疾病、初发型多发性硬化、轻度外伤性脑损伤等存在相关性,节段性坏死性血管炎或其他原因引起动脉壁通透性增高；脑脊液回流受阻使组织间液排出障碍，从而导致血管周围间隙扩大；血管迂曲及脑萎缩；沿动脉长轴分布的血管周围间隙纤维化和闭塞阻碍了液体流动血管周围间隙的信号特征肉眼观察血管周围间隙在MRI的各种成像序列上与脑脊液信号一致在T2WI 测量这些信号值却比脑脊液信号低，这也说明血管周围间隙内包含的是组织间液DWI上弥散不受限增强后血管周围间隙无强化小至中等大小(直径 25mm)的血管周围间隙的邻近脑实质一般无信号异常,血管周围间隙的部位和形态扩大的血管周围间隙常分布于三个特征性的部位:型见于豆纹动脉经前穿支进入基底节处型分布于脑的穿髓动脉进入大脑凸面并延伸至皮质下白质处型见于脑干其他较少见部位：丘脑、小脑、岛叶、最外囊及海马根据扫描的层面不同血管周围间隙可呈椭圆形、圆形、线状或管状，通常双侧对称、边界清楚，直径在5mm以下,Bilateral type I VR spaces in a 6-year-old boy. (a) Axial proton-densityweighted image (repetition time msec/echo time msec 2375/100) shows hyperintense areas (arrows) in the anterior perforated substance on both sides. (b) Axial fluid-attenuated inversion-recovery (FLAIR) image (6606/100) obtained at the same level shows that these areas have CSF-like content (arrows). The signal intensity of the surrounding brain parenchyma is normal.,(c, d) Diffusion-weighted image (2574/81; b factor = 1000 sec/mm2) (c) and corresponding apparent diffusion coefficient map (d) show no restricted diffusion in these areas (arrows).,Bilateral type I VR spaces in a 53-year-old woman. Coronal T1-weighted image (500/30) shows symmetrical hypointense areas (arrows) in the anterior perforated substance,Type II VR spaces in a 73-year-old woman. (a) Axial proton-densityweighted image (2376/100) shows multiple hyperintense foci in the centrum semiovale in both hemispheres. (b) On an axial FLAIR image (6614/100) obtained at the same level, the VR spaces are seen as hypointense dots without any surrounding high signal intensity. Note the two small lesions with a hypointense center and a hyperintense rim (arrows) in the left hemisphere; these lesions are not VR spaces but old lacunar infarctions.,Type II dilated VR spaces in a 6-year-old boy. (a) Axial T2-weighted image (2620/100) shows linear to punctate hyperintense areas around the occipital horns, especially on the left side (arrow). (b) FLAIR image (7572/100) obtained at the same level shows no abnormal signal intensity (arrow), in accordance with the fact that these areas are true VR spaces.,Type III VR space in a 25-year-old man. (a) Axial proton-densityweighted image (2620/100) shows a hyperintense spot in the brainstem (arrow). (b) Axial FLAIR image (7292/120) obtained at the same level shows that the spot has CSF-like content without abnormal surrounding signal intensity (arrow). These findings confirm that the spot is a VR space.,Type III VR spaces in a 68-year-old man. (a) Axial proton-densityweighted image (2382/100) shows multiple punctate hyperintense areas in the brainstem (arrow). (b) Close-up T2-weighted image (4615/120) clearly shows the fine punctate pattern. (c) Axial FLAIR image (6609/100) shows the CSF-like content of the dots (arrow). No surrounding high signal intensity is seen. The typical configuration and the fact that no high signal intensity is seen on the FLAIR image confirm that the dots are VR spaces.,Giant VR spaces in the mesencephalothalamic region in a 19-year-old man. (a, b) Axial (a) and sagittal (b) T2-weighted images (5970/120) show a multicystic lesion in the mesencephalothalamic region. The lesion extends from the left cerebral peduncle to the left thalamus. The content of the cysts is CSF-like. The adjacent brain parenchyma has normal signal intensity. No solid components are identified. (c) Axial gadolinium-enhanced T1-weighted image (478/18) shows no enhancement. The process has caused obstruction of the sylvian aqueduct, resulting in hydrocephalus. The size of the lesion and the degree of hydrocephalus were unchanged compared with the appearance on MR images obtained 2 years earlier.,Chronic lacunar infarction of the pons in a 59-year-old man. (a) Axial protondensityweighted image (2200/100) shows a hyperintense lesion in the pons (arrow). (b) Axial FLAIR image (6614/100) shows that the lesion has a hypointense center with a hyperintense rim (arrow), an appearance that reflects gliosis.,Acute and chronic lacunar infarctions in a 66-year-old man. (a) Axial proton-densityweighted image (2385/100) shows multiple high-signal-intensity lesions bilaterally in the basal ganglia, internal capsule, and thalamus (arrows). The signal intensity of the periventricular white matter is abnormally increased. (b) Axial FLAIR image (6608/100) shows multiple small high-signal-intensity lesions and hypointense lesions surrounded by hyperintense rims in the same region (arrows). (c) Apparent diffusion coefficient map shows a recent infarction in the posterior limb of the right internal capsule (arrow).,Cystic Periventricular Leukomalacia(囊性脑室周围白质软化) Periventricular leukomalacia, usually seen in premature infants, is a leukoencephalopathy resulting from a pre- or perinatal hypoxic-ischemic event. In the acute stage, white matter undergoes vascular congestion and coagulative necrosis. Cavitation then occurs within necrotic regions. End-stage periventricular leukomalacia has a typical appearance at MR imaging : T2-weighted and FLAIR images show abnormally increased signal intensity in the periventricular white matter. There is marked loss of periventricular white matter, predominantly in the periatrial regions, and compensatory focal ventricular enlargement adjacent to regions of abnormal white matter signal intensity. The involvement tends to be symmetrical. Corpus callosal thinning can be seen as a secondary manifestation. There is relative sparing of the overlying cortical mantle. In more severe cases, cavitated infarcts have replaced the immediate periventricular white matter . These cystic components have surrounding gliosis, easily depicted on FLAIR images, which distinguishes them from enlarged VR spaces .,Cystic periventricular leukomalacia in a 3-year-old boy with a history of perinatal asphyxia who had delayed motor and mental development and epilepsy. (a) Axial proton-densityweighted image (2611/100) shows hyperintense lesions predominantly in the right peritrigonal area (straight arrow) but also in the left peritrigonal area (curved arrow). These lesions could be mistaken for type II VR spaces. (b) Coronal FLAIR image (11,000/140) shows gliosis around the cystic lesions (arrows), a characteristic finding in end-stage cystic periventricular leukomalacia.,Multiple Sclerosis MS lesions may be located anywhere in the central nervous system. Lesions in the periventricular and juxtacortical white matter correspond to the location of type II VR spaces. In addition, individual MS plaques often appear as ovoid lesions, mimicking the shape of dilated VR spaces . However, MS lesions are usually arranged like fingers pointing away from the walls of the lateral ventricles (Dawson fingers) and can easily be distinguished from enlarged VR spaces by signal intensity characteristics. In the acute stage, MS lesions are isointense or mildly hypointense to brain parenchyma on T1-weighted images. In the chronic phase, they have a hypointense center with a mildly hyperintense rim on T1-weighted images. T2-weighted and FLAIR images demonstrate hyperintense lesions. Both solid and ring enhancement may occur. Enhancement is dependent on the current degree of inflammation .,Ovoid MS lesion of the centrum semiovale in a 49-year-old man. Axial proton-densityweighted (2624/100) (a) and FLAIR (7291/120) (b) images show a hyperintense lesion (arrow) in the right centrum semiovale. Other MS lesions were located behind the left occipital horn and in the basal ganglia and brainstem,Cryptococcosis(隐球菌) Cryptococcosis is an opportunistic fungal infection caused by Cryptococcus neoformans, affecting the central nervous system in human immunodeficiency virusseropositive patients and in patients with other immunocompromised states. Central nervous system infection can be either meningeal or parenchymal. Infection usually starts as meningitis, most pronounced at the base of the brain . The infection often provokes little inflammatory reaction, owing to the hosts immunity and to the immunosuppressive effect of the organisms capsule . Infection of the meninges may spread to the adjacent brain through the subarachnoid space or along the ependymal surface. As the infection spreads along the VR spaces, they may become distended with mucoid, gelatinous material that originates from the organisms capsule .Therefore, cryptococcosis should be considered in the differential diagnosis in any immunocompromised patient with dilated VR spaces. Larger collections of organisms and gelatinous capsular（凝胶状的荚膜） material in the VR spaces have been termed gelatinous pseudocysts（凝胶状假囊轴） . Mass lesions representing cryptococcomas may occur, particularly in the deep gray matter,Imaging findings are primarily manifestations of meningitis. Hydrocephalus often develops as a result of the acute meningeal exudate and may also occur in the course of the infection because of meningeal adhesions. Punctate hyperintense areas representing dilated VR spaces or cryptococcomas are frequently seen in the basal ganglia, thalami, and midbrain on T2-weighted images . On FLAIR images they are also hyperintense, making it possible to differentiate them from normal VR spaces. Contrast enhancement is uncommon . On diffusion-weighted images, there may be restricted diffusion in some of the lesions due to the high viscosity of their contents.,Cryptococcosis in a 58-year-old woman with headaches and fever who was seropositive for human immunodeficiency virus. Parasagittal T2-weighted image (5963/120) shows multiple dilated VR spaces in the region of the basal ganglia (arrowheads). C neoformans was cultured from the CSF.,Mucopolysaccharidoses（黏多糖症） The mucopolysaccharidoses are inherited disorders of metabolism characterized by enzyme deficiency and inability to break down glycosaminoglycan (GAG), which results in the accumulation of toxic intracellular substrate. Clinical features are mental and motor retardation, macrocephaly, and musculoskeletal deformities. The urinary GAG level is elevated. Brain atrophy and abnormalities of the white matter may be present. Typically, the VR spaces are dilated by accumulated GAG, which results in a cribriform appearance of the white matter, corpus callosum, and basal ganglia on T1-weighted images. Occasionally, arachnoid cysts (due to meningeal GAG deposition) are seen. On T2-weighted and FLAIR images, the dilated VR spaces are isointense to CSF . However, the surrounding white matter may show increased signal intensity, representing gliosis, edema, or de- or dysmyelination . The latter helps in differentiating them from normal VR spaces. In addition, MR spectroscopy shows a broad peak around 3.7 ppm (higher than the chemical shift of myoinositol), considered to contain signals from