Pial Vasculature Research Articles

Pia-FLOW: Deciphering hemodynamic maps of the pial vascular connectome and its response to arterial occlusion

The pial vasculature is the sole source of blood supply to the neocortex. The brain is contained within the skull, a vascularized bone marrow with a unique anatomical connection to the brain meninges. Recent developments in tissue clearing have enabled detailed mapping of the entire pial and calvarial vasculature. However, what are the absolute flow rate values of those vascular networks? This information cannot accurately be retrieved with the commonly used bioimaging methods. Here, we introduce Pia-FLOW, a unique approach based on large-scale transcranial fluorescence localization microscopy, to attain hemodynamic imaging of the whole murine pial and calvarial vasculature at frame rates up to 1,000 Hz and spatial resolution reaching 5.4 µm. Using Pia-FLOW, we provide detailed maps of flow velocity, direction, and vascular diameters which can serve as ground-truth data for further studies, advancing our understanding of brain fluid dynamics. Furthermore, Pia-FLOW revealed that the pial vascular network functions as one unit for robust allocation of blood after stroke.

Imaging of the pial arterial vasculature of the human brain in vivo using high-resolution 7T time-of-flight angiography.

The pial arterial vasculature of the human brain is the only blood supply to the neocortex, but quantitative data on the morphology and topology of these mesoscopic arteries (diameter 50-300 µm) remains scarce. Because it is commonly assumed that blood flow velocities in these vessels are prohibitively slow, non-invasive time-of-flight magnetic resonance angiography (TOF-MRA)-which is well suited to high 3D imaging resolutions-has not been applied to imaging the pial arteries. Here, we provide a theoretical framework that outlines how TOF-MRA can visualize small pial arteries in vivo, by employing extremely small voxels at the size of individual vessels. We then provide evidence for this theory by imaging the pial arteries at 140 µm isotropic resolution using a 7 Tesla (T) magnetic resonance imaging (MRI) scanner and prospective motion correction, and show that pial arteries one voxel width in diameter can be detected. We conclude that imaging pial arteries is not limited by slow blood flow, but instead by achievable image resolution. This study represents the first targeted, comprehensive account of imaging pial arteries in vivo in the human brain. This ultra-high-resolution angiography will enable the characterization of pial vascular anatomy across the brain to investigate patterns of blood supply and relationships between vascular and functional architecture.

Pial Vessel-Associated Microglia/Macrophages Increase in Female Dahl-SS/Jr Rats Independent of Pregnancy History.

As the resident immune cells of the central nervous system, microglia have a wide range of functions such as surveillance, phagocytosis, and signaling through production of chemokines and cytokines. Recent studies have identified and characterized macrophages residing at the meninges, a series of layers surrounding the brain and spinal cord. While perivascular microglia within the brain parenchyma increase following chronic hypertension, there are no reports of changes at the meninges, and specifically, associated with the pial vasculature. Thus, we used female Sprague Dawley and Dahl salt-sensitive (SS/Jr) rat brains, stained for ionized calcium-binding adapter molecule (Iba1), and characterized microglia/macrophages associated with pial vessels in the posterior brain. Results indicate that Iba1+ pial vessel-associated microglia (PVAM) completely surrounded the vessels in brains from the Dahl-SS/Jr rats. PVAM density was significantly higher and distance between PVAMs lower in Dahl-SS/Jr compared to the Sprague Dawley rat brains. Pregnancy history did not affect these findings. While the functional role of these cells are not known, we contextualize our novel findings with that of other studies assessing or characterizing myeloid cells at the borders of the CNS (meninges and choroid plexus) and perivascular macrophages and propose their possible origin in the Dahl-SS/Jr model of chronic hypertension.

Abstract TP249: Early-lifechronic Intermittent Hypoxia Susceptibility For Anxiety-like Behaviors: Focus On Cerebrovascular Alterations

Premature birth is a major public health issue affecting 1 in 10 deliveries worldwide. One of the most common diagnoses in these infants is apnea of prematurity, with chronic intermittent hypoxia (CIH) as one hallmark, which includes interlacing hypoxia and normoxia episodes. The perinatal brain is vulnerable to hypoxia because it is a shifting landscape with marked growth and remodeling of the vasculature. Hypoxia provides a natural angiogenic stimulus during growth of cerebrovascular networks. However, it is unclear if exposure to CIH alters the structure and function of developing brain vascular networks. To further investigate this question, we used a neonatal mouse model that mimics the hypoxia/reoxygenation events occurring in CIH. From postnatal day P2 to P12, pups were exposed to 20.9% O 2 (normoxia) and 5.7% O 2 (hypoxia; 25-30 episodes of 5-7s per hour) for 8 hours a day during daylight hours. Our previous studies examining mouse pups (P8-P12) reared in normoxic conditions showed that ascending venules were a key locus for new vascular growth during brain capillary development. Concurrently, an initially amorphous plexus of pial venules on the cortical surface undergoes massive intussusceptive remodeling to create an optimized tree-like network for cortical blood drainage. During CIH, in vivo two-photon imaging revealed that angiogenic sprouts emerged not only from venules but also from cortical arterioles. CIH led to more active angiogenesis and increased brain capillary length and branching. In addition, arteriovenous shunts were present within the pial vasculature. Critically, we observed that mice exposure to CIH during the neonatal period presented anxiety and mood like-behaviors (interupted grooming and jump/scape behavior) at weaning age (P21). Curiously, mice exposed to CIH exhibited improved recognition memory (novel object recognition) at P30. These findings begin to clarify the vascular changes induced by CIH and their association with behavioral abnormalities after development with CIH.

Interplay between vascular hemodynamics and the glymphatic system in the pathogenesis of idiopathic normal pressure hydrocephalus, exploring novel neuroimaging diagnostics.

As the aging population continues to grow, so will the incidence of age-related conditions, including idiopathic normal pressure hydrocephalus (iNPH). The pathogenesis of iNPH remains elusive, and this is due in part to the poor characterization of cerebral spinal fluid (CSF) dynamics within the brain. Advancements in technology and imaging techniques have enabled new breakthroughs in understanding CSF physiology, and therefore iNPH pathogenesis. This includes understanding the hemodynamic and microvascular components involved in CSF influx and flow. Namely, the glymphatic system appears to be the great mediator, facilitating perivascular CSF flow via astrocytic aquaporin channels located along the endothelium of the pial vasculature. The interplay between glymphatics and both arterial pulsatilty and venous compliance has also been recently demonstrated. It appears then that CSF flow, and therefore glymphatic function, are highly dependent on cardiocirculatory and vascular factors. Impairment in any one component, whether it be related to arterial pulsatility, microvascular changes, reduced venous drainage, or astrogliosis, contributes greatly to iNPH, although it is likely a combination thereof. The strong interplay between vascular hemodynamics and CSF flow suggests perfusion imaging and cerebral blood flow quantification may be a useful diagnostic tool in characterizing iNPH. In addition, studies detecting glymphatic flow with magnetic resonance imaging have also emerged. These imaging tools may serve to both diagnose iNPH and help delineate it from other similarly presenting disease processes. With a better understanding of the vascular and glymphatic factors related to iNPH pathogenesis, physicians are better able to select the best candidates for treatment.

Automated Prediction of Ischemic Brain Tissue Fate from Multiphase Computed Tomographic Angiography in Patients with Acute Ischemic Stroke Using Machine Learning.

Background and Purpose Multiphase computed tomographic angiography (mCTA) provides time variant images of pial vasculature supplying brain in patients with acute ischemic stroke (AIS). To develop a machine learning (ML) technique to predict tissue perfusion and infarction from mCTA source images. Methods 284 patients with AIS were included from the Precise and Rapid assessment of collaterals using multi-phase CTA in the triage of patients with acute ischemic stroke for Intra-artery Therapy (Prove-IT) study. All patients had non-contrast computed tomography, mCTA, and computed tomographic perfusion (CTP) at baseline and follow-up magnetic resonance imaging/non-contrast-enhanced computed tomography. Of the 284 patient images, 140 patient images were randomly selected to train and validate three ML models to predict a pre-defined Tmax thresholded perfusion abnormality, core and penumbra on CTP. The remaining 144 patient images were used to test the ML models. The predicted perfusion, core and penumbra lesions from ML models were compared to CTP perfusion lesion and to follow-up infarct using Bland-Altman plots, concordance correlation coefficient (CCC), intra-class correlation coefficient (ICC), and Dice similarity coefficient. Results Mean difference between the mCTA predicted perfusion volume and CTP perfusion volume was 4.6 mL (limit of agreement [LoA], –53 to 62.1 mL; P=0.56; CCC 0.63 [95% confidence interval [CI], 0.53 to 0.71; P<0.01], ICC 0.68 [95% CI, 0.58 to 0.78; P<0.001]). Mean difference between the mCTA predicted infarct and follow-up infarct in the 100 patients with acute reperfusion (modified thrombolysis in cerebral infarction [mTICI] 2b/2c/3) was 21.7 mL, while it was 3.4 mL in the 44 patients not achieving reperfusion (mTICI 0/1). Amongst reperfused subjects, CCC was 0.4 (95% CI, 0.15 to 0.55; P<0.01) and ICC was 0.42 (95% CI, 0.18 to 0.50; P<0.01); in non-reperfused subjects CCC was 0.52 (95% CI, 0.20 to 0.60; P<0.001) and ICC was 0.60 (95% CI, 0.37 to 0.76; P<0.001). No difference was observed between the mCTA and CTP predicted infarct volume in the test cohort (P=0.67). Conclusions A ML based mCTA model is able to predict brain tissue perfusion abnormality and follow-up infarction, comparable to CTP.

Effects of transient ischemia‐reperfusion on in vivo mitochondrial fission and fusion in the murine cerebral vasculature

Mitochondria play important roles in the regulation of cerebral circulation. The effects of ischemia with and without reperfusion on mitochondrial morphology have been well documented in different cell types of the brain. However, the time course of these events, an important determinant of mitochondrial targeting, has been understudied due to technical limitations.The aim of this study was to determine how mitochondrial fission and fusion dynamics are affected by ischemia‐reperfusion, in endothelial cells that form the basis of the neurovascular unit.A thinned skull cranial window was implanted in young, male and female (3–6 month old; n = 5) offspring of B6;129S‐Gt(ROSA)26Sortm1(CAG‐COX8A/Dendra‐2)Dcc/J and B6.Cg‐Tg(Tek‐cre) 1Ywa/J mice expressing mitoDendra 2 fluorescent protein in endothelial mitochondria. Two‐photon microscopy was used to visualize the brain vasculature, via RhodamineB Dextran labelled blood plasma and mitoDendra2 simultaneously at 875 nm. After 2 imaging sessions taken 60 minutes apart, animals were allowed to recover. Then, mice were exposed to 30 minutes of ischemia via middle cerebral artery occlusion followed by 4 hours of reperfusion. Additional time‐lapse images were taken between the 2nd and 4th hour of the re‐perfusion period. Three stacks of 50 images, 1 μm apart, were taken of different regions of interests, containing arteries, veins, and capillaries using a 40X objective and a 1X digital zoom; whereas 3X digital zoom was used to take images for the analyses of mitochondrial fission and fusion events. Ischemia‐reperfusion resulted in a prominent vasoconstriction of the pial vasculature (15% decrease in vessel diameter; n = 66 diameter measurements) as well as the disintegration of green mitochondria signal, suggestive of fission. The in‐depth analysis of time‐lapse images (8–12 stacks of images per channel per imaging session per mouse using a 3X digital zoom) is currently ongoing.Overall, we expect to correlate in vivo mitochondrial changes derived by fission and fusion with vascular events in response to ischemia‐reperfusion in large, penetrating arteries and veins as well as in capillaries.Support or Funding InformationAHA 17SDG33410366; U54 GM104940; HL‐093554

The pial vasculature of the mouse develops according to a sensory-independent program

The cerebral vasculature is organized to supply the brain’s metabolic needs. Sensory deprivation during the early postnatal period causes altered neural activity and lower metabolic demand. Neural activity is instructional for some aspects of vascular development, and deprivation causes changes in capillary density in the deprived brain region. However, it is not known if the pial arteriole network, which contains many leptomeningeal anastomoses (LMAs) that endow the network with redundancy against occlusions, is also affected by sensory deprivation. We quantified the effects of early-life sensory deprivation via whisker plucking on the densities of LMAs and penetrating arterioles (PAs) in anatomically-identified primary sensory regions (vibrissae cortex, forelimb/hindlimb cortex, visual cortex and auditory cortex) in mice. We found that the densities of penetrating arterioles were the same across cortical regions, though the hindlimb representation had a higher density of LMAs than other sensory regions. We found that the densities of PAs and LMAs, as well as quantitative measures of network topology, were not affected by sensory deprivation. Our results show that the postnatal development of the pial arterial network is robust to sensory deprivation.

Improving Microcirculatory Reperfusion Reduces Parenchymal Oxygen Radical Formation and Provides Neuroprotection

Reperfusion is the most significant determinant of good outcome after ischemic stroke. However, complete reperfusion often cannot be achieved, despite satisfactory recanalization. We hypothesized that microvascular protection was essential for achieving effective reperfusion and, hence, neuroprotection. To test this hypothesis, we have developed an in vivo model to differentially monitor parenchymal and vascular reactive oxygen species (ROS) formation. By comparing the ROS-suppressing effect of N-tert-butyl-α-phenylnitrone (PBN) with its blood-brain barrier impermeable analog 2-sulfo-phenyl-N-tert-butylnitrone (S-PBN), we assessed the impact of vascular ROS suppression alone on reperfusion and stroke outcome after recanalization. The distal middle cerebral artery was occluded for 1 hour by compressing with a micropipette and then recanalized (n=60 Swiss mice). ROS formation was monitored for 1 hour after recanalization by intravital fluorescence microscopy in pial vasculature and cortical parenchyma with topically applied hydroethidine through a cranial window. PBN (100 mg/kg) or S-PBN (156 mg/kg) was administered shortly before recanalization, and suppression of the vascular and parenchymal hydroethidine fluorescence was examined (n=22). Microcirculatory patency, reperfusion, ischemic tissue size, and neurological outcome were also assessed in a separate group of mice 1 to 72 hours after recanalization (n=30). PBN and S-PBN completely suppressed the reperfusion-induced increase in ROS signal within vasculature. PBN readily suppressed ROS produced in parenchyma by 88%. S-PBN also suppressed the parenchymal ROS by 64% but starting 40 minutes later. Intriguingly, PBN and S-PBN comparably reduced the size of ischemic area by 65% and 48% (P>0.05), respectively. S-PBN restored the microvascular patency and perfusion after recanalization, suggesting that its delayed parenchymal antioxidant effect could be secondary to improved microcirculatory reperfusion. Promoting microvascular reperfusion by protecting vasculature can secondarily reduce parenchymal ROS formation and provide neuroprotection. The model presented can be used to directly assess pharmacological end points postulated in brain parenchyma and vasculature in vivo.

Cerebral Vasoactivity and Oxygenation with Oxygen Carrier M101 in Rats.

The severity of traumatic brain injury (TBI) may be reduced if oxygen can be rapidly provided to the injured brain. This study evaluated if the oxygen-carrier M101 causes vasoconstricton of pial vasculature in healthy rats (Experiment 1) and if M101 improves brain tissue oxygen (PbtO2) in rats with controlled cortical impact (CCI)-TBI (Experiment 2). M101 (12.5 mL/kg intravenous [IV] over 2 h) caused a mild (9 mm Hg) increase in the mean arterial blood pressure (MAP) of healthy rats without constriction of cerebral pial arterioles. M101 (12 mL/kg IV over 1 h) caused a modest (27 mm Hg) increase in MAP (peak, 123 ± 5 mm Hg [mean ± standard error of the mean]) of CCI-TBI rats and restored PbtO2 to near pre-injury levels. In both M101 and untreated control (NON) groups, PbtO2 was ∼30 ± 2 mm Hg pre-injury and decreased (p ≤ 0.05) to ∼16 ± 2 mm Hg 15 min after CCI. In NON, PbtO2 remained ∼50% of baseline but M101 administration resulted in a sustained increase in PbtO2 (peak, 25 ± 5 mm Hg), which was not significantly different from pre-injury until the end of the study, when it decreased again below pre-injury (but was still higher than NON). Histopathology showed no differences between groups. In conclusion, M101 increased systemic blood pressures without concurrent cerebral pial vasoconstriction (in healthy rats) and restored PbtO2 to 86% of pre-injury for at least 80 min when given soon after CCI-TBI. M101 should be evaluated in a clinically-relevant large animal model for pre-hospital treatment of TBI.

Abstract 3860: Magnetization transfer and T2-weighted MRI studies are useful for visualizing phenotypic presentations of orthotopic, patient-derived xenograft mouse models of glioblastoma

Abstract Introduction: Patient-derived xenograft (PDX) models for glioblastoma (GBM) from resected tumor tissues replicate several features of the original tumor. They are considered to be representative models to study tumor progression, and to test responses to putative therapies. Longitudinal noninvasive imaging can be useful in such investigations. To that end, we employed magnetic resonance imaging (MRI) to visualize and measure tumor burden in four different PDX models of GBM. Experimental procedures: Four orthotopic mouse PDX models, HF2587, HF2927, HF3077 and HF3253, developed from neurosphere cultures of four different human glioblastoma samples were used in the study. The neurosphere cells were implanted into the right striatum in immunocompromised nude mice (n=5-8 per model) and allowed to grow for 2-8 weeks, depending on their known growth rates from previous studies. They were imaged in a Varian 7T MRI system with the following weightings: T2, T1, magnetization transfer (MT), and contrast enhanced MRI (CE-MRI) with Magnevist as the contrast agent (CA). Following imaging, all the mice were sacrificed and their brains processed for hematoxylin and eosin (H&amp;E) histology and human major histocompatibility complex (MHC) immunohistochemistry. Results: Tumor masses were visible as hyperintense regions on MT and T2-weighted images. The extent of such masses matched the H&amp;E and MHC staining patterns. Ventricle enlargements seen on MRI in several mice were also confirmed by histology. Necrotic cores, when present, were observed on both imaging and on histopathology with good spatial correlations. Surprisingly, post-contrast T1 imaging did not enhance in the tumor mass or peritumorally, except in one mouse in which some intra-tumoral enhancement was observed. In all other instances from the four PDX models tested, enhancement was observed only when the tumor tissue or parts of it were contiguous with pial or dural vasculature. Conclusions: At 7 Tesla, MT-MRI and T2-weighted imaging, rather than CE-MRI, appear to be of better utility in visualizing these PDX models of GBM if MRI is chosen as the imaging modality. Since the parent tumors imaged at lower field strengths showed contrast enhancement, absence of a similar feature in these models needs additional studies to understand their vascular characteristics. Such properties may include low level of vascularization and/or relatively less leaky tumor vasculature. Another possible reason may be that the models tested represent the invasive features of GBM better than the vascular features, e.g. peritumoral ring enhancement, of larger clinical tumors with increased exposure to hypoxia. Citation Format: Tavarekere N. Nagaraja, Rasha Elmghirbi, Susan Irtenkauf, Laura Hasselbach, Baruch Tawil, Stephen L. Brown, Swayamprava Panda, Glauber Cabral, Tom Mikkelsen, James R. Ewing, Ana deCarvalho. Magnetization transfer and T2-weighted MRI studies are useful for visualizing phenotypic presentations of orthotopic, patient-derived xenograft mouse models of glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3860. doi:10.1158/1538-7445.AM2017-3860

Enhanced contractility of intraparenchymal arterioles after global cerebral ischaemia in rat - new insights into the development of delayed cerebral hypoperfusion.

Delayed cerebral hypoperfusion is a secondary complication found in the days after transient global cerebral ischaemia that worsens the ischaemic damage inflicted by the initial transient episode of global cerebral ischaemia. A recent study demonstrated increased cerebral vasoconstriction in the large arteries on the brain surface (pial arteries) after global cerebral ischaemia. However, smaller arterioles inside the brain (parenchymal arterioles) are equally important in the regulation of cerebral blood flow and yet their pathophysiology after global cerebral ischaemia is largely unknown. Therefore, we investigated whether increased contractility occurs in the intraparenchymal arterioles. Global cerebral ischaemia was induced in male Wistar rats by bilateral common carotid occlusion for 15min combined with hypovolaemia. Regional cerebral blood flow was determined by quantitative autoradiography. Intraparenchymal arterioles were isolated and pressurized, and concentration-response curves to endothelin-1 with and without the endothelin B receptor-selective antagonist BQ788 was generated. Endothelin B receptor expression was investigated by quantitative flow cytometry and immunohistochemistry. We observed increased endothelin-1-mediated contractility of parenchymal arterioles correlating with reduced cerebral blood flow of the cortex, hippocampus and caudate nucleus 48h after global cerebral ischaemia. The increased endothelin-1-mediated contractility was abolished by BQ788, and the vascular smooth muscle cell-specific expression of endothelin B receptors was significantly increased after global cerebral ischaemia. Increased endothelin-1-mediated contractility and expression of endothelin B receptors in the intraparenchymal vasculature contributes to the development of delayed cerebral hypoperfusion after global cerebral ischaemia in combination with vascular changes of the pial vasculature.

Tail artifact removal in OCT angiography images of rodent cortex.

Optical coherence tomography angiography (OCTA) is a surging non-invasive, label-free, in vivo volumetric imaging method, currently being translated to clinical ophthalmology and becoming popular in neuroscience. Despite its attractiveness, there is an inherent issue of using OCT angiograms for quantitative cerebrovascular studies: The dynamic scattering of moving erythrocytes within pial vasculature creates tail-like artifacts that shadow the capillary vessels in the deeper layers of cortex. This false flow effect is relatively benign for qualitative visualization purposes, but it might have a significant impact on quantitative interpretation of angiographic results. In this work, we propose a simple image processing method to remove these tail artifacts in depth-resolved OCTA images using an adaptive enface mask generated with OCT structural images. We demonstrate the effectiveness of our method by comparing vessel densities and vessel similarities of depth-resolved OCT angiograms in a stroke study in a rodent model, in vivo. Thanks to the ability of seeing through the tails of pial vessels, capillary vessels beneath these vessels could be recovered to some extend in the deeper layers of mouse cerebral cortex, leading to a more accurate quantification. Tail artifact removed enface OCT angiogram of deeper layer in vivo mouse cortex.

Abstract TP438: De Novo Ivy Sign Indicates Postoperative Hyperperfusion in Moyamoya Disease

Background and Purpose: The ivy sign on fluid-attenuated inversion recovery (FLAIR) magnetic resonance imaging (MRI) is a specific finding in moyamoya disease (MMD). This sign indicates decreased cerebral perfusion, dilated pial vasculature, and slow leptomeningeal collateral flow. This study aimed to clarify the characteristics of perioperative changes in the ivy sign in relation to cerebral hyperperfusion (HP), which frequently occurs in MMD of unknown etiology. Methods: This prospective study included patients with MMD who underwent superior temporal artery-middle cerebral artery single bypass. FLAIR MRI was performed to evaluate the appearance of the ivy sign in the ipsilateral hemisphere preoperatively and on postoperative days 2 and 30. The ivy sign was assessed in combination with perioperative symptoms and cerebral hemodynamics using single-photon emission computed tomography (SPECT). Results: Of 42 consecutive patients (55 sides) who underwent bypass surgery, 32 (58.2%) showed an increase in the ivy sign (de novo ivy sign) on postoperative day 2; this had disappeared by day 30. Interestingly, these 32 patients had a significantly higher incidence of HP on SPECT and HP syndrome, and there was no correlation between the de novo ivy sign and a preoperative ivy sign or the preoperative cerebral hemodynamics. In multivariate analysis, a de novo ivy sign was significantly correlated with postoperative HP. Conclusions: In MMD, a de novo ivy sign could indicate postoperative HP after bypass, which is not always correlated with preoperative hemodynamic impairment. Additional factors other than preoperative cerebral hemodynamics might be involved in postoperative HP in MMD.

Preoperative embolization of intracranial hemangiopericytomas: case series and introduction of the transtumoral embolization technique

Background and purposeHemangiopericytomas (HPCs) are rare dural-based neoplasms. Preoperative embolization of these notoriously hypervascular tumors can be challenging as they often receive their dominant blood supply from pial feeders arising...

A method for longitudinal, transcranial imaging of blood flow and remodeling of the cerebral vasculature in postnatal mice.

In the weeks following birth, both the brain and the vascular network that supplies it undergo dramatic alteration. While studies of the postnatal evolution of the pial vasculature and blood flow through its vessels have been previously done histologically or acutely, here we describe a neonatal reinforced thin‐skull preparation for longitudinally imaging the development of the pial vasculature in mice using two‐photon laser scanning microscopy. Starting with mice as young as postnatal day 2 (P2), we are able to chronically image cortical areas >1 mm2, repeatedly for several consecutive days, allowing us to observe the remodeling of the pial arterial and venous networks. We used this method to measure blood velocity in individual vessels over multiple days, and show that blood flow through individual pial venules was correlated with subsequent diameter changes. This preparation allows the longitudinal imaging of the developing mammalian cerebral vascular network and its physiology.

Investigation of the neurovascular coupling in positive and negative BOLD responses in human brain at 7 T

Decreases in stimulus-dependent blood oxygenation level dependent (BOLD) signal and their underlying neurovascular origins have recently gained considerable interest. In this study a multi-echo, BOLD-corrected vascular space occupancy (VASO) functional magnetic resonance imaging (fMRI) technique was used to investigate neurovascular responses during stimuli that elicit positive and negative BOLD responses in human brain at 7T. Stimulus-induced BOLD, cerebral blood volume (CBV), and cerebral blood flow (CBF) changes were measured and analyzed in ‘arterial’ and ‘venous’ blood compartments in macro- and microvasculature. We found that the overall interplay of mean CBV, CBF and BOLD responses is similar for tasks inducing positive and negative BOLD responses. Some aspects of the neurovascular coupling however, such as the temporal response, cortical depth dependence, and the weighting between ‘arterial’ and ‘venous’ contributions, are significantly different for the different task conditions. Namely, while for excitatory tasks the BOLD response peaks at the cortical surface, and the CBV change is similar in cortex and pial vasculature, inhibitory tasks are associated with a maximum negative BOLD response in deeper layers, with CBV showing strong constriction of surface arteries and a faster return to baseline. The different interplays of CBV, CBF and BOLD during excitatory and inhibitory responses suggests different underlying hemodynamic mechanisms.

De Novo Ivy Sign Indicates Postoperative Hyperperfusion in Moyamoya Disease

The ivy sign on fluid-attenuated inversion recovery MRI is a specific finding in moyamoya disease (MMD). This sign indicates decreased cerebral perfusion, dilated pial vasculature, and slow leptomeningeal collateral flow. This study aimed to clarify the characteristics of perioperative changes in the ivy sign in relation to cerebral hyperperfusion, which frequently occurs in MMD of unknown pathogenesis. This prospective study included patients with MMD who underwent superior temporal artery-middle cerebral artery single bypass. Fluid-attenuated inversion recovery MRI was performed to evaluate the appearance of the ivy sign in the ipsilateral hemisphere preoperatively and on postoperative days 2 and 30. The ivy sign was assessed in combination with perioperative symptoms and cerebral hemodynamics using single-photon emission computed tomography. Of 42 consecutive patients (55 sides) who underwent bypass surgery, 32 (58.2%) showed an increase in the ivy sign (de novo ivy sign) on postoperative day 2; this had disappeared by day 30. Interestingly, these 32 patients had a significantly higher incidence of hyperperfusion on single-photon emission computed tomography and hyperperfusion syndrome, and there was no correlation between the de novo ivy sign and a preoperative ivy sign or the preoperative cerebral hemodynamics. In multivariate analysis, a de novo ivy sign was significantly correlated with postoperative hyperperfusion. In MMD, a de novo ivy sign could indicate postoperative hyperperfusion after bypass, which is not always correlated with preoperative hemodynamic impairment. Additional factors other than preoperative cerebral hemodynamics might be involved in postoperative hyperperfusion in MMD.

The Leptomeningeal Ivy Sign on Fluid-Attenuated Inversion Recovery Images in Moyamoya Disease: Positron Emission Tomography Study

Background: The ivy sign is sometimes seen on fluid-attenuated inversion recovery (FLAIR) images in moyamoya disease (MMD). In recent studies using single-photon emission computed tomography, ivy sign proliferation correlated with decreases in cerebrovascular reserve. However, a decreased vascular reserve is not concrete. The purpose of this study was to evaluate the correlation between ivy sign proliferation and the findings of ¹⁵O gas positron emission tomography (PET). Methods: In 19 MMD patients (12 women, age 31-69 years) with ischemic symptoms, FLAIR magnetic resonance imaging and ¹⁵O gas PET were performed. We classified the middle cerebral artery (MCA) territory into 2 regions in each hemisphere, and the degree of the ivy sign (ivy sign score) in each region was classified into 3 grades (0-2), where grade 0 indicated an absence of the ivy sign, grade 1 indicated that the ivy sign was seen on less than half of the cortical surface in each region, and grade 2 indicated that the ivy sign was seen on more than half of the cortical surface. We examined the relationship among the ivy sign score, the severity of ischemic symptoms and PET parameters in 76 MCA regions of 19 patients. Results: Ivy sign scores of the regions were 0 (n = 19), 1 (n = 40), and 2 (n = 17). Total ivy sign score of a hemisphere increased as clinical symptoms became more severe. Cerebral blood flow (CBF) values were lower, cerebral blood volume (CBV) values were higher, and CBF/CBV values were lower than those of controls as symptoms became severe (p < 0.05). CBF and CBF/CBV values decreased and CBV values increased as the ivy sign score increased, and were significantly higher and lower, respectively, than control values (p < 0.05). No significant differences in cerebral metabolic rate of oxygen and oxygen extraction fraction were found between the 3 ivy sign scores. A positive correlation was found between ivy sign score and increases in CBV (p < 0.01), and a more obvious negative correlation was found between ivy sign score and decreases in CBF/CBV (p < 0.001). Conclusions: We evaluated the correlation between ivy sign proliferation and the findings of ¹⁵O gas PET. We suggested that ivy sign proliferation was associated with both dilated pial vasculature and the slow flow of developed leptomeningeal collaterals in patients with MMD.

Do Microemboli Reach the Brain Penetrating Arteries?

As they are "end arteries," microembolic obstruction of brain penetrating arteries would be expected to create ischemia. Yet the mammalian brain appears to have an impressive tolerance to experimental microembolization with ischemia occurring only after the injection of large numbers of particulates. Potential explanations could be that the majority of these particulates marginate along the pial vasculature or escape the cerebral circulation via arteriovenous (AV) fistulae. To test these theories, we first established the level of injury created by the injection of 20, 45, and 90 μm fluorescent microspheres in Sprague-Dawley rats. Brains were examined by immunohistochemistry for injury and for infarction. We then injected 1000 size 20 μm, 500 size 45 μm, and 150 size 90 μm and harvested the brains and lungs for assays of fluorescence. The location of microemboli within the brain was established by determining the percent of 20 and 45 μm fluorescent microspheres entering the superficial versus deeper layers of the brain. The location of larger microemboli was established by 2T-MRI after injection of 60-100 μm microthrombi labeled with supraparamagnetic iron oxide (SPIO) particles. With 20 μm microspheres there were no areas of injury or infarction after injection of 500 and rare areas of injury and no infarctions after injection of 1000 microspheres. With either 250 or 500 size 45 μm microspheres there were a few (≤ 6) small areas of injury per animal with ≤ 2 areas of infarction. After injection, 93%-96% of injected microspheres remained in the brain. Approximately 40% of either fluorescent or SPIO labeled microthrombi were found on the brain surface. As in humans, the rat brain has an impressive tolerance to microemboli, although this clearly varies with emboli size and number. Wash out of particulates through AV connections is not a major factor in brain tolerance in this model. Approximately 40% of microemboli remain in the larger pial vasculature where the more extensive collateralization may limit their effects on distal perfusion. However, the remaining 60% enter penetrating arteries but few create ischemia.