Vasodynamics of pial and penetrating arterioles in relation to arteriolo-arteriolar anastomosis after focal stroke

Dilated lenticulostriate artery on whole-brain vessel wall imaging differentiates pathogenesis and predicts clinical outcomes in single subcortical infarction

OBJECTIVES

This study aimed to investigate the dilation of lenticulostriate artery (LSA) identified by whole-brain vessel wall imaging (WB-VWI) in differentiating the etiologic subtypes of single subcortical infarction (SSI) and to determine whether the appearance of dilated LSA was associated with 90-day clinical outcomes in parental atherosclerotic disease (PAD)-related SSI.

METHODS

Patients with acute SSI were prospectively enrolled and categorized into PAD-related SSI and cerebral small-vessel disease (CSVD)-related SSI groups. The imaging features of LSA morphology (branches, length, dilation, and tortuosity), plaques (burden, remodeling index, enhancement degree, and hyperintense plaque), and CSVD (white matter hyperintensity, lacunes, cerebral microbleed, and enlarged perivascular space) were evaluated. The logistic regression was performed to determine the association of dilated LSA with PAD-related SSI and 90-day clinical outcomes.

RESULTS

In total, 131 patients (mean age, 52.2 ± 13.2 years; 99 men) were included. The multivariate logistic regression analysis revealed that the presence of dilated LSAs (odds ratio (OR), 7.40; 95% confidence interval (CI): 1.88-29.17; p = 0.004)) was significantly associated with PAD-related SSI. Moreover, after adjusting for confounding factors, the association of poor outcomes with the total length of LSAs (OR, 0.94; 95% CI: 0.90-0.99; p = 0.011), dilated LSAs (OR, 0.001; 95% CI: 0.0001-0.08; p = 0.002), and plaque burden (OR, 1.35; 95% CI: 1.11-1.63; p = 0.002) remained statistically significant.

CONCLUSION

The dilation of LSA visualized on WB-VWI could differentiate various subtypes of SSI within LSA territory and was a prognostic imaging marker for 90-day clinical outcomes for PAD-related SSI.

CLINICAL RELEVANCE STATEMENT

Evaluation of LSA morphology based on WB-VWI can differentiate the pathogenesis and predict clinical outcomes in SSI, providing crucial insights into the etiologic mechanisms, risk stratification, and tailored therapies for these patients.

KEY POINTS

The prognosis of SSIs within lenticulostriate territory depend on the etiology of the disease. LSA dilation on WB-VWI was associated with parental atherosclerosis and better 90-day outcomes. Accurately identifying the etiology of SSIs in lenticulostriate territory assists in treatment decision-making.

Chronic hypertension alters the relationship between collateral blood flow, cortical cerebral blood flow, and brain tissue oxygenation

This study measured the relationship between pial collateral (leptomeningeal anastomoses, LMA) flow, intraparenchymal cortical cerebral blood flow (cCBF) and brain tissue oxygenation (btO2) during acute ischemic stroke to investigate how pial flow translates to downstream cCBF and btO2 and examined how this relationship is altered in hypertension. Proximal transient middle cerebral artery occlusion (tMCAO) was performed in male Wistar (n = 8/group) and Spontaneously Hypertensive Rats (SHR, n = 8/group). A combination laser Doppler-oxygen probe was placed within the expected cortical peri-infarct in addition to a surface laser doppler probe which measured LMA flow. Phenylephrine (PE) was infused 30 minutes into tMCAO to increase blood pressure (BP) by 30% for 10 minutes and assessed CBF autoregulation. During the initial 30-minute period of tMCAO, btO2 and cCBF were lower in SHR compared to Wistar rats (btO2: 11.5 ± 10.5 vs 17.5 ± 10.8 mmHg and cCBF: -29.7 ± 23.3% vs -17.8 ± 41.9%); however, LMA flow was similar between groups. The relationship between LMA flow, cCBF and btO2 were interdependent in Wistar rats. However, this relationship was disrupted in SHR rats and partially restored by induced hypertension. This study provides evidence that cCBF and btO2 were diminished during tMCAO in chronic hypertension, and that induced hypertension was beneficial regardless of hypertensive status.

Association of Morphology of Lenticulostriate Arteries and Proximal Plaque Characteristics With Single Subcortical Infarction: A Whole-Brain High-Resolution Vessel Wall Imaging Study

BACKGROUND

We aimed to investigate the association of characteristics of lenticulostriate artery (LSA) morphology and parental atheromatous disease (PAD) with single subcortical infarction (SSI) and to explore whether the LSA morphology is correlated with proximal plaque features in asymptomatic PAD.

METHODS AND RESULTS

Patients with acute SSI were prospectively enrolled and classified as large- and small-SSI groups. The clinical data and imaging features of LSA morphology (branches, length, dilation, and tortuosity) and middle cerebral artery plaques (normalized wall index, remodeling index, enhancement degree, and hyperintense plaques) were evaluated. Logistic regression was performed to determine the association of large SSIs with morphologic features of LSAs and plaques. The Spearman correlation between the morphologic characteristics of LSAs and plaque features in asymptomatic PAD was analyzed. Of the 121 patients recruited with symptomatic PAD, 102 had coexisting asymptomatic contralateral PAD. The mean length of LSAs (odds ratio, 0.84 [95% CI, 0.73-0.95]; P=0.007), mean tortuosity of LSAs (odds ratio, 1.13 [95% CI, 1.05-1.22]; P=0.002), dilated LSAs (odds ratio, 22.59 [95% CI, 2.46-207.74]; P=0.006), and normalized wall index (odds ratio, 1.08 [95% CI, 1.01-1.15]; P=0.022) were significantly associated with large SSIs. Moreover, the normalized wall index was negatively correlated with the mean length of LSAs (r=-0.348, P<0.001), and the remodeling index was negatively correlated with the mean tortuosity of LSAs (r=-0.348, P<0.001) in asymptomatic PAD.

CONCLUSIONS

Our findings suggest that mean length of LSAs, mean tortuosity of LSAs, dilated LSAs, and normalized wall index are associated with large SSIs. Moreover, plaque features in asymptomatic PAD are correlated with morphologic features of LSAs.

The role of leptomeningeal collaterals in redistributing blood flow during stroke

Leptomeningeal collaterals (LMCs) connect the main cerebral arteries and provide alternative pathways for blood flow during ischaemic stroke. This is beneficial for reducing infarct size and reperfusion success after treatment. However, a better understanding of how LMCs affect blood flow distribution is indispensable to improve therapeutic strategies. Here, we present a novel in silico approach that incorporates case-specific in vivo data into a computational model to simulate blood flow in large semi-realistic microvascular networks from two different mouse strains, characterised by having many and almost no LMCs between middle and anterior cerebral artery (MCA, ACA) territories. This framework is unique because our simulations are directly aligned with in vivo data. Moreover, it allows us to analyse perfusion characteristics quantitatively across all vessel types and for networks with no, few and many LMCs. We show that the occlusion of the MCA directly caused a redistribution of blood that was characterised by increased flow in LMCs. Interestingly, the improved perfusion of MCA-sided microvessels after dilating LMCs came at the cost of a reduced blood supply in other brain areas. This effect was enhanced in regions close to the watershed line and when the number of LMCs was increased. Additional dilations of surface and penetrating arteries after stroke improved perfusion across the entire vasculature and partially recovered flow in the obstructed region, especially in networks with many LMCs, which further underlines the role of LMCs during stroke.

Global effects of aging on the hemodynamic response function in the human brain

In functional magnetic resonance imaging, the hemodynamic response function (HRF) is a transient, stereotypical response to local changes in cerebral hemodynamics and oxygen metabolism due to briefly (< 4 s) evoked neural activity. Accordingly, the HRF is often used as an impulse response with the assumption of linearity in data analysis. In cognitive aging studies, it has been very common to interpret differences in brain activation as age-related changes in neural activity. Contrary to this assumption, however, evidence has accrued that normal aging may also significantly affect the vasculature, thereby affecting cerebral hemodynamics and metabolism, confounding interpretation of fMRI aging studies. In this study, use was made of a multisensory stimulus to evoke the HRF in ~ 87% of cerebral cortex in cognitively intact adults with ages ranging from 22-75 years. The stimulus evokes both positive and negative HRFs, which were characterized using model-free parameters in native-space coordinates. Results showed significant age trends in HRF parameter distributions in terms of both amplitudes (e.g., peak amplitude and CNR) and temporal dynamics (e.g., full-width-at-half-maximum). This work sets the stage for using HRF methods as a biomarker for age-related pathology.

A systematic observation of vasodynamics from different segments along the cerebral vasculature in the penumbra zone of awake mice following cerebral ischemia and recanalization

Different segments of the cerebral vascular network may react distinctly to brain ischemia and recanalization. However, there are limited systematic observations of these vascular responses in mice under a physiological state following ischemic stroke. Herein, we aimed to investigate the vasodynamics among several segments along the cerebral vessels in awake mice following cerebral ischemia/recanalization via two-photon imaging. Plasma in the blood vessels were labelled with fluorescein isothiocyanate dextran. Smooth muscle cells and pericytes were labelled via a genetic mouse line (PDGFRβ-tdTomato). We observed a no-reflow phenomenon in downstream microcirculation, and the vasodynamics of different segments of larger cerebral vessels varied in the penumbra area following cerebral ischemia-reperfusion. Despite obtaining reperfusion from the middle cerebral artery, there were significant constrictions of the downstream blood vessels in the ischemic penumbra zone. Interestingly, we observed an extensive constriction of the capillaries 3 hours following recanalization, both at the site covered by pericyte soma and by the pericyte process alone. In addition, we did not observe a significant positive correlation between the changed capillary diameter and pericyte coverage along the capillary. Taken together, abnormal constrictions and vasodynamics of cerebral large and small vessels may directly contribute to microcirculation failure following recanalization in ischemic stroke.

Deep Imaging to Dissect Microvascular Contributions to White Matter Degeneration in Rodent Models of Dementia

The increasing socio-economic burden of Alzheimer disease (AD) and AD-related dementias has created a pressing need to define targets for therapeutic intervention. Deficits in cerebral blood flow and neurovascular function have emerged as early contributors to disease progression. However, the cause, progression, and consequence of small vessel disease in AD/AD-related dementias remains poorly understood, making therapeutic targets difficult to pinpoint. Animal models that recapitulate features of AD/AD-related dementias may provide mechanistic insight because microvascular pathology can be studied as it develops in vivo. Recent advances in in vivo optical and ultrasound-based imaging of the rodent brain facilitate this goal by providing access to deeper brain structures, including white matter and hippocampus, which are more vulnerable to injury during cerebrovascular disease. Here, we highlight these novel imaging approaches and discuss their potential for improving our understanding of vascular contributions to AD/AD-related dementias.

Sex Difference in Capillary Reperfusion After Transient Middle Cerebral Artery Occlusion in Diabetic Mice

BACKGROUND

Type 2 diabetes (DM2) exacerbates stroke injury, reduces efficacy of endovascular therapy, and worsens long-term functional outcome. Sex differences exist in stroke incidence, response to therapy, poststroke microvascular dysfunction, and functional recovery. In this study, we tested the hypotheses that poor outcome after stroke in the setting of DM2 is linked to impaired microvascular tissue reperfusion and that male and female DM2 mice exhibit different microvascular reperfusion response after transient middle cerebral artery occlusion (MCAO).

METHODS

Transient MCAO was induced for 60 minutes using an intraluminal filament in young adult DM2 and nondiabetic control male and female mice. Capillary flux in deep cortical layers was assessed using optical coherence tomography-based optical microangiography (OMAG), and associated regional brain infarct size was evaluated by hematoxylin and eosin staining.

RESULTS

Compared to baseline, MCAO reduced absolute capillary red blood cell flux by 84% at 24 hours post-MCAO in male DM2 (P<0.001) but not male control mice. When normalized to pre-MCAO baseline, red blood cell flux 24 hours after stroke was 64% lower in male DM2 mice than male nondiabetic controls (P<0.01). In females, MCAO decreased capillary flux by 48% at 24 hours post-MCAO compared with baseline in DM2 (P<0.05) but not in control mice. Red blood cell flux of female DM2 mice did not differ from that of nondiabetic controls either before or 24 hours after MCAO. Furthermore, normalized capillary flux 24 hours after MCAO failed to differ between female DM2 mice and nondiabetic controls. Concomitantly, male but not female DM2 mice experienced 25% larger infarct in caudate-putamen versus respective nondiabetic controls (P<0.05).

CONCLUSIONS

DM2 impairs capillary perfusion and exacerbates ischemic deep brain injury in male but not female young adult mice. Premenopausal females appear to be protected against DM2-related capillary dysfunction and brain injury.

Automated counting of cerebral penetrating vessels using optical coherence tomography images of a mouse brain in vivo

RATIONALE AND OBJECTIVES

Penetrating blood vessels emanating from cortical surface vasculature and lying deep in the cortex are essential vascular conduits for the shuttling of blood from superficial pial vessels to the capillary beds in parenchyma for the nourishment of neuronal brain tissues. Locating and counting the penetrating vessels is beneficial for the quantification of a course of ischemia in blood occlusive events such as stroke. This paper seeks to demonstrate and validate a method for automated penetrating vessel counting that uses optical coherence tomography (OCT).

MATERIALS AND METHODS

This paper proposes an OCT method that effectively identifies and grades the cortical penetrating vessels in perfusion. The key to the proposed method is the harnessing of vascular features found in the penetrating vessels, which are distinctive from those of other vessels. In particular, with an increase in the light attenuation and flow turbulence, the contrast in the mean projection of the OCT datacube decreases, whereas that in the maximum projection of the Doppler frequency variance datacube increases. By multiplying the inversion of the former with the latter, its binary thresholding is sufficient to highlight the penetrating vessels and allows for their counting over the projection image.

RESULTS

A computational method that leverages the decrease in mean OCT projection intensity and the increase in Doppler frequency variance at the penetrating vessel is developed. It successfully identifies and counts penetrating vessels with a high accuracy of over 87%. The penetrating vessel density is observed to be significantly reduced in the mouse model of focal ischemic stroke.

CONCLUSION

The OCT analysis is effective for counting penetrating blood vessels in mice brains and may be applied to the rapid diagnosis and treatment of stroke in stroke models of small animals. This article is protected by copyright. All rights reserved.

Quantitative Hemodynamic Measurements in Cortical Vessels Using Functional Ultrasound Imaging

Red blood cell velocity (RBCv), cerebral blood flow (CBF), and volume (CBV) are three key parameters when describing brain hemodynamics. Functional ultrasound imaging is a Doppler-based method allowing for real-time measurement of relative CBV at high spatiotemporal resolution (100 × 110 × 300 μm3, up to 10 Hz) and large scale. Nevertheless, the measure of RBCv and CBF in small cortical vessels with functional ultrasound imaging remains challenging because of their orientation and size, which impairs the ability to perform precise measurements. We designed a directional flow filter to overpass these limitations allowing us to measure RBCv in single vessels using a standard functional ultrasound imaging system without contrast agents (e.g., microbubbles). This method allows to quickly extract the number of vessels in the cortex that was estimated to be approximately 650/cm3 in adult rats, with a 55–45% ratio for penetrating arterioles versus ascending venules. Then, we analyzed the changes in RBCv in these vessels during forepaw stimulation. We observed that ∼40 vessels located in the primary somatosensory forelimb cortex display a significant increase of the RBCv (median ΔRBCv ∼15%, maximal ΔRBCv ∼60%). As expected, we show that RBCv was higher for penetrating arterioles located in the center than in the periphery of the activated area. The proposed approach extends the capabilities of functional ultrasound imaging, which may contribute to a better understanding of the neurovascular coupling at the brain-wide scale.

Laminar perfusion imaging with zoomed arterial spin labeling at 7 Tesla

Laminar fMRI based on BOLD and CBV contrast at ultrahigh magnetic fields has been applied for studying the dynamics of mesoscopic brain networks. However, the quantitative interpretations of BOLD/CBV fMRI results are confounded by different baseline physiology across cortical layers. Here we introduce a novel 3D zoomed pseudo-continuous arterial spin labeling (pCASL) technique at 7T that offers the capability for quantitative measurements of laminar cerebral blood flow (CBF) both at rest and during task activation with high spatial specificity and sensitivity. We found arterial transit time in superficial layers is ∼100 ms shorter than in middle/deep layers revealing the time course of labeled blood flowing from pial arteries to downstream microvasculature. Resting state CBF peaked in the middle layers which is highly consistent with microvascular density measured from human cortex specimens. Finger tapping induced a robust two-peak laminar profile of CBF increases in the superficial (somatosensory and premotor input) and deep (spinal output) layers of M1, while finger brushing task induced a weaker CBF increase in superficial layers (somatosensory input). This observation is highly consistent with reported laminar profiles of CBV activation on M1. We further demonstrated that visuospatial attention induced a predominant CBF increase in deep layers and a smaller CBF increase on top of the lower baseline CBF in superficial layers of V1 (feedback cortical input), while stimulus driven activity peaked in the middle layers (feedforward thalamic input). With the capability for quantitative CBF measurements both at baseline and during task activation, high-resolution ASL perfusion fMRI at 7T provides an important tool for in vivo assessment of neurovascular function and metabolic activities of neural circuits across cortical layers.

Perfusion Defects and Collateral Flow Patterns in Acute Small Subcortical Infarction: a 4D Dynamic MRI Study

The hemodynamic changes of acute small subcortical infarction (SSI) are not well understood. We evaluate the hemodynamic changes and collaterals in acute SSI using perfusion magnetic resonance imaging (MRI). A total of 103 patients with acute SSI in penetrating artery territories were recruited and underwent MRI within 24 h of stroke onset. Using 4D dynamic perfusion MRI, they were divided into three patterns: 25 (24%) with normal perfusion, 31 (30%) with compensated perfusion, and 47 (46%) with hypoperfusion. The development of anterograde or retrograde collaterals was also evaluated. Patients with hypoperfusion pattern had the highest rate of early neurological deterioration (32%, p = 0.007), the largest initial and final infarction volumes (p < 0.001 and p = 0.029), the lowest relative cerebral blood flow (0.63, p < 0.001), and the lowest rate of anterograde and retrograde collaterals (19%, p < 0.001; 66%, p = 0.002). The anterograde collaterals were associated with higher relative cerebral blood volume (0.91 vs. 0.77; p = 0.024) and a higher rate of deep cerebral microbleeds (48 vs. 21%; p = 0.028), whereas retrograde collaterals were associated with higher systolic and diastolic blood pressure (p = 0.031 and 0.020), smaller initial infarction volume (0.81 vs. 1.34 ml, p = 0.031), and a higher rate of lobar cerebral microbleeds (30 vs. 0%; p = 0.013). Both anterograde and retrograde collaterals may play a critical role in maintaining cerebral perfusion and can have an impact on patient clinical outcomes. Further studies are warranted to verify these findings and to investigate effective treatments.

High-sensitivity detection of optogenetically-induced neural activity with functional ultrasound imaging

Whole-brain imaging approaches and optogenetic manipulations are powerful tools to map brain-wide neural circuits in vivo. To date, functional magnetic resonance imaging (fMRI) provides the most comprehensive evaluation of such large-scale circuitry. However, functional ultrasound imaging (fUSI) has recently emerged as a complementary imaging modality that can extend such measurements towards the context of diverse behavioral states and tasks. Nevertheless, in order to properly interpret the fUSI signal during these complicated scenarios, it must first be carefully validated against well-established technologies, such as fMRI, in highly controlled experimental settings. Here, to address this need, we compared subsequent fMRI and fUSI recordings in response to direct neuronal activation via optogenetics in the same animals under an identical anesthetic protocol. Specifically, we applied various intensities of light stimulation to the primary motor cortex (M1) of mice and compared the spatiotemporal dynamics of the elicited fMRI and fUSI signals. Overall, our general linear model analysis (t-scores) and time series analysis (z-scores) revealed that fUSI was more sensitive than fMRI for detecting optogenetically-induced neuronal activation. Local field potential recordings in the bilateral M1 and striatum also better co-localized with fUSI activation patterns than those of fMRI. Finally, the fUSI response contained distinct arterial and venous components that provide vascular readouts of neuronal activity with vessel-type specificity.

Differences in cerebral blood vasculature and flow in awake and anesthetized mouse cortex revealed by quantitative optical coherence tomography angiography

BACKGROUND

Most of the in vivo neurovascular imaging studies are performed in anesthetized animals. However, anesthesia significantly affects cerebral hemodynamics.

NEW METHOD

We applied optical coherence tomography (OCT) methods such as optical microangiography (OMAG) and Doppler optical microangiography (DOMAG) to quantitatively evaluate the effect of anesthesia in cerebral vasculature and blood flow in mouse brain.

RESULTS

The OMAG results indicated the increase of large vessel diameter and capillary density induced by ketamine-xylazine and isoflurane, meaning that both anesthetics caused vasodilation. In addition, the preliminary results from DOMAG showed that isoflurane increased the baseline cerebral blood flow.

COMPARISON WITH EXISTING METHODS

In comparison with other in vivo imaging modalities, OCT can provide label-free assessment of cortical tissue including tissue morphology, cerebral blood vessel network and flow information down to capillary level, with a large field of view and high imaging speed.

CONCLUSIONS

OCT angiography methods demonstrated the ability to measure the differences in the baseline morphological and flow parameters of both large and capillary cerebrovascular networks between awake and anesthetized mice.

Optical microangiography reveals temporal and depth-resolved hemodynamic change in mouse barrel cortex during whisker stimulation

SIGNIFICANCE

Cerebral blood flow (CBF) regulation at neurovascular coupling (NVC) plays an important role in normal brain functioning to support oxygen delivery to activating neurons. Therefore, studying the mechanisms of CBF adjustment is crucial for the improved understanding of brain activity.

AIM

We investigated the temporal profile of hemodynamic signal change in mouse cortex caused by neural activation and its variation over cortical depth.

APPROACH

Following the cranial window surgery, intrinsic optical signal imaging (IOSI) was used to spatially locate the activated region in mouse cortex during whisker stimulation. Optical microangiography (OMAG), the functional extension of optical coherence tomography, was applied to image the activated and control regions identified by IOSI. Temporal profiles of hemodynamic response signals obtained by IOSI and OMAG were compared, and OMAG signal was analyzed over cortical layers.

RESULTS

Our results showed that the hemodynamic response to neural activity revealed by blood flow change signal signal through IOSI is slower than that observed by OMAG signal. OMAG also indicated the laminar variation of the response over cortical depth, showing the largest response in cortical layer IV.

CONCLUSIONS

Overall, we demonstrated the development and application of dual-modality imaging system composed of IOSI and OMAG, which may have potential to enable the future investigations of depth-resolved CBF and to provide the insights of hemodynamic events associated with the NVC.

Multiscale modeling of human cerebrovasculature: A hybrid approach using image-based geometry and a mathematical algorithm

The cerebral vasculature has a complex and hierarchical network, ranging from vessels of a few millimeters to superficial cortical vessels with diameters of a few hundred micrometers, and to the microvasculature (arteriole/venule) and capillary beds in the cortex. In standard imaging techniques, it is difficult to segment all vessels in the network, especially in the case of the human brain. This study proposes a hybrid modeling approach that determines these networks by explicitly segmenting the large vessels from medical images and employing a novel vascular generation algorithm. The framework enables vasculatures to be generated at coarse and fine scales for individual arteries and veins with vascular subregions, following the personalized anatomy of the brain and macroscale vasculatures. In this study, the vascular structures of superficial cortical (pial) vessels before they penetrate the cortex are modeled as a mesoscale vasculature. The validity of the present approach is demonstrated through comparisons with partially observed data from existing measurements of the vessel distributions on the brain surface, pathway fractal features, and vascular territories of the major cerebral arteries. Additionally, this validation provides some biological insights: (i) vascular pathways may form to ensure a reasonable supply of blood to the local surface area; (ii) fractal features of vascular pathways are not sensitive to overall and local brain geometries; and (iii) whole pathways connecting the upstream and downstream entire-scale cerebral circulation are highly dependent on the local curvature of the cerebral sulci.

Cerebral autoregulation in the complex vascular networks of the brain is an amazing achievement. We believe that numerical analysis of the cerebral blood circulation using an anatomically precise vascular model provides a powerful tool for evaluating the direct relationships between local- and global-scale blood flows. However, there is a lack of information about the overall vascular pathways in the human brain, preventing a monolithic model of the human cerebrovasculature from being established. This paper presents a multiscale model of human cerebrovasculature based on a hybrid approach that uses image-based geometries and a newly developed mathematical algorithm. One important argument of this paper is the validity of the cerebrovasculature represented in the model, which reflects anatomical features of major cerebral vasculatures and brain shape, and has strong similarities with available data for human superficial cortical vessels. Investigations of the reconstructed model allow us to derive some biological insights and associated hypotheses for the cerebral vasculature. The authors believe the present cerebrovascular model can be applied to numerical simulations of the entire-scale cerebral blood flow.

Optical coherence tomography angiography in preclinical neuroimaging

Preclinical neuroimaging allows for the assessment of brain anatomy, connectivity, and function in laboratory animals, such as mice and this imaging field has been a rapidly growing aimed at bridging the translation gap between animal and human research. The progress in the animal research could be accelerated by high-resolution in vivo optical imaging technologies. Optical coherence tomography-based angiography (OCTA) estimates the scattering from moving red blood cells, providing the visualization of functional micro-vessel networks within tissue beds in vivo without a need for exogenous contrast agents. Recent advancement of OCTA methods have expanded its application to neuroimaging of small animal models of brain disorders. In this paper, we overview the recent development of OCTA techniques for blood flow imaging and its preclinical applications in neuroimaging. In specific, a summary of preclinical OCTA studies for traumatic brain injury, cerebral stroke, and aging brain on mice is reviewed.

Monitoring Acute Stroke Progression: Multi-Parametric OCT Imaging of Cortical Perfusion, Flow, and Tissue Scattering in a Mouse Model of Permanent Focal Ischemia

Cerebral ischemic stroke causes injury to brain tissue characterized by a complex cascade of neuronal and vascular events. Imaging during the early stages of its development allows prediction of tissue infarction and penumbra so that optimal intervention can be determined in order to salvage brain function impairment. Therefore, there is a critical need for novel imaging techniques that can characterize brain injury in the earliest phases of the ischemic stroke. This paper examined optical coherence tomography (OCT) for imaging acute injury in experimental ischemic stroke in vivo. Based on endogenous optical scattering signals provided by OCT imaging, we have developed a single, integrated imaging platform enabling the measurement of changes in blood perfusion, blood flow, erythrocyte velocity, and light attenuation within a cortical tissue, during focal cerebral ischemia in a mouse model. During the acute phase (from 5 min to the first few hours following the blood occlusion), the multi-parametric OCT imaging revealed multiple hemodynamic and tissue scattering responses in vivo, including cerebral blood flow deficits, capillary non-perfusion, displacement of penetrating vessels, and increased light attenuation in the cortical tissue at risk that are spatially correlated with the infarct core, as determined by postmortem staining with triphenyltetrazolium chloride. The use of multi-parametric OCT imaging may aid in the comprehensive evaluation of ischemic lesions during the early stages of stroke, thereby providing essential knowledge for guiding treatment decisions.

Optimized depth-resolved estimation to measure optical attenuation coefficients from optical coherence tomography and its application in cerebral damage determination

The optical attenuation coefficient (OAC) reflects the optical properties of various tissues or tissues of the same type under different physiological conditions. Quantitative measurement of OAC from optical coherence tomography (OCT) signals can provide additional information and can increase the potential for OCT applications. We present an optimized depth-resolved estimation (ODRE) method that derives a precise mapping between the measured OCT signal and the OAC. In contrast to previous depth-resolved estimation (DRE) methods, the optimized method can estimate the OAC in any depth range and ignore whether the light is completely attenuated. Numerical simulations and phantom experiments are used to verify its validity, and this method is applied to detect cerebral damage. In combination with OCT angiography, real-time observation of the change of blood perfusion and the degree of cerebral damage in mice with focal cerebral ischemia provides important information to help us understand the temporal relationship between brain damage and ischemia.

Quantification of cerebral vascular perfusion density via optical coherence tomography based on locally adaptive regional growth

Optical coherence tomography (OCT) angiography is a noninvasive imaging modality that produces volumetric views of blood flow perfusion in vivo with resolution at capillary level, which has been widely adopted to monitor cerebral perfusion status after stroke in experimental settings. Accurate quantification of cerebral perfusion from OCT angiograms is important for understanding the cerebral vascular pathophysiology and assessing the treatment of ischemic stroke. Quantification of blood vessels from OCT angiography faces some problems; one is uneven backscatter (which causes some blood vessels to be very bright, some very dark), and the other is that the brightness in the same blood vessel also changes due to the difference in diameter or depth. In this paper, we proposed a locally adaptive region growing algorithm to solve this problem. The algorithm, which confines the region growing process to a local region, is used to segment blood vessels in different images to cope well with the intensity changes in blood vessels. During segmentation, the initial seed pixels were selected with the aid of the Otsu algorithm, the growth criterion considered both global and local information, and the thresholds were also adjusted adaptively as local regions varied. After these processes are completed, we can calculate the percentage of segmented blood vessels across field of view of the images, named cerebral vascular perfusion density, and use it as an indicator to evaluate the cerebral blood perfusion of middle cerebral artery occlusion in mice. This paper demonstrates that the algorithm can produce satisfactory vascular segmentation results, and CVPD can be used as an effective indicator for evaluating post-ischemic injury.

Implications for understanding ischemic stroke as a sexually dimorphic disease: the role of pial collateral circulations

We investigated structural and functional differences in primary and pial collateral circulations in adult normotensive male and female Wistar rats. Male ( n = 10) and female ( n = 7) rats were subjected to middle cerebral artery (MCA) occlusion and changes in relative cerebral blood flow in MCA and pial collateral territories were measured by multisite laser-Doppler flowmetry. Rats were then transcardially perfused with a mixture of carbon black and latex, perfusion fixed, and imaged to compare primary and pial collateral structure between male ( n = 4) and female ( n = 3) rats, including lumen diameters and number. To study pial collateral function, leptomeningeal anastomoses (LMAs) were isolated and pressurized from male ( n = 7) and female ( n = 6) rats. Myogenic tone and reactivity to pressure, vascular function to pharmacological activator, or inhibitor of ion channels was measured and compared. There was no difference between relative cerebral blood flow in both MCA and pial collateral territories during occlusion and reperfusion between groups. Compared with male LMAs, female LMAs had similar myogenic tone (24.0 ± 7.3% vs. 16.0 ± 3.7%, P > 0.05) and reactivity to increased pressure and similar vascular responses to vasoconstrictive and vasodilatory stimuli. Additionally, compared with female LMAs, male LMAs had similar numbers (21 ± 1 vs. 20 ± 2, P > 0.05) and diameters (30.5 ± 2.0 vs. 26.2 ± 0.6 μm, P > 0.05), and no sex difference was detected in the diameter of arterial segments of circle of Willis. Together, our data establish no sex difference of cerebral collateral structure or function, suggesting that the reduced severity of stroke outcome in female rats is not likely due to differences in the cerebral collateral circulation. NEW & NOTEWORTHY Our work compared the function of leptomeningeal anastomoses between male and female adult normotensive rats with no sex difference found. We also confirmed no sex difference in primary and pial collateral structure in Wistar rats. Our findings suggest that the reduced severity of stroke in premenopausal women and reproductively intact female rodents is not likely due to improved primary and pial collateral circulations.

Aging-associated changes in cerebral vasculature and blood flow as determined by quantitative optical coherence tomography angiography

Normal aging is associated with significant alterations in brain's vascular structure and function, which can lead to compromised cerebral circulation and increased risk of neurodegeneration. The in vivo examination of cerebral blood flow (CBF), including capillary beds, in aging brains with sufficient spatial detail remains challenging with current imaging modalities. In the present study, we use 3-dimensional (3-D) quantitative optical coherence tomography angiography (OCTA) to examine characteristic differences of the cerebral vasculatures and hemodynamics at the somatosensory cortex between old (16 months old) and young mice (2 months old) in vivo. The quantitative metrics include cortical vascular morphology, CBF, and capillary flow velocity. We show that compared with young mice, the pial arterial tortuosity increases by 14%, the capillary vessel density decreases by 15%, and the CBF reduces by 33% in the old mice. Most importantly, changes in capillary velocity and heterogeneity with aging are quantified for the first time with sufficiently high statistical power between young and old populations, with a 21% (p < 0.05) increase in capillary mean velocity and 19% (p ≤ 0.05) increase in velocity heterogeneity in the latter. Our findings through noninvasive imaging are in line with previous studies of vascular structure modification with aging, with additional quantitative assessment in capillary velocity enabled by advanced OCTA algorithms on a single imaging platform. The results offer OCTA as a promising neuroimaging tool to study vascular aging, which may shed new light on the investigations of vascular factors contributing to the pathophysiology of age-related neurodegenerative disorders.

Capillary flow homogenization during functional activation revealed by optical coherence tomography angiography based capillary velocimetry

Elaborate modeling study suggests an important role of capillary transit time heterogeneity (CTTH) reduction in brain oxygenation during functional hyperemia. Here, we use optical coherence tomography angiography (OCTA) capillary velocimetry to probe blood flow dynamics in cerebral capillary beds and validate the change in CTTH during functional activation in an in vivo rodent model. Through evaluating flow dynamics and consequent transit time parameters from thousands of capillary vessels within three-dimensional (3-D) tissue volume upon hindpaw electrical stimulation, we observe reductions in both capillary mean transit time (MTT) (9.8% ± 2.2) and CTTH (5.9% ± 1.4) in the hindlimb somatosensory cortex (HLS1). Additionally, capillary flow pattern modification is observed with a significant difference (p < 0.05) between the HLS1 and non-activated cortex regions. These quantitative findings reveal a localized microcirculatory adjustment during functional activation, consistent with previous studies, and support the critical contribution of capillary flow homogenization to brain oxygenation. The OCTA velocimetry is a useful tool to image microcirculatory dynamics in vivo using animal models, enabling a more comprehensive understanding as to hemodynamic-metabolic coupling.

In vivo high-resolution cortical imaging with extended-focus optical coherence microscopy in the visible-NIR wavelength range

Visible light optical coherence tomography has shown great interest in recent years for spectroscopic and high-resolution retinal and cerebral imaging. Here, we present an extended-focus optical coherence microscopy system operating from the visible to the near-infrared wavelength range for high axial and lateral resolution imaging of cortical structures in vivo. The system exploits an ultrabroad illumination spectrum centered in the visible wavelength range (λc  =  650  nm, Δλ  ∼  250  nm) offering a submicron axial resolution (∼0.85  μm in water) and an extended-focus configuration providing a high lateral resolution of ∼1.4  μm maintained over ∼150  μm in depth in water. The system's axial and lateral resolution are first characterized using phantoms, and its imaging performance is then demonstrated by imaging the vasculature, myelinated axons, and neuronal cells in the first layers of the somatosensory cortex of mice in vivo.

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.

Optical Coherence Tomography: Basic Concepts and Applications in Neuroscience Research

Optical coherence tomography is a micrometer-scale imaging modality that permits label-free, cross-sectional imaging of biological tissue microstructure using tissue backscattering properties. After its invention in the 1990s, OCT is now being widely used in several branches of neuroscience as well as other fields of biomedical science. This review study reports an overview of OCT's applications in several branches or subbranches of neuroscience such as neuroimaging, neurology, neurosurgery, neuropathology, and neuroembryology. This study has briefly summarized the recent applications of OCT in neuroscience research, including a comparison, and provides a discussion of the remaining challenges and opportunities in addition to future directions. The chief aim of the review study is to draw the attention of a broad neuroscience community in order to maximize the applications of OCT in other branches of neuroscience too, and the study may also serve as a benchmark for future OCT-based neuroscience research. Despite some limitations, OCT proves to be a useful imaging tool in both basic and clinical neuroscience research.

Testing the effects of sensory stimulation as a collateral-based therapeutic for ischemic stroke in C57BL/6J and CD1 mouse strains

Utilizing a rat model of ischemic stroke, we have previously shown that sensory stimulation can completely protect rats from impending ischemic damage of cortex if this treatment is delivered within the first two hours post-permanent middle cerebral artery occlusion (pMCAo). The current study sought to extend our findings in rats to mice, which would allow new avenues of research not available in rats. Thus, young adult C57BL/6J and CD1 mice were tested for protection from ischemic stroke with the same protective sensory stimulation-based treatment. Cortical activity and blood flow were assessed with intrinsic signal optical imaging (ISOI) and laser speckle imaging (LSI), respectively, and histological analysis (TTC) was performed at the completion of the experiments. Standing in stark contrast to the positive results observed in rats, in both strains we found that there were no differences between treated and untreated mice at 24 hours post-pMCAo in terms of infarct volume, negative functional imaging results, and major reduction in retrograde collateral blood flow as compared to pre-pMCAo baseline and surgical controls. Also, no differences were found between the strains in terms of theses variables. Potential reasons for the differences between rats and mice are discussed.

Optical coherence tomography angiography-based capillary velocimetry

Challenge persists in the field of optical coherence tomography (OCT) when it is required to quantify capillary blood flow within tissue beds in vivo. We propose a useful approach to statistically estimate the mean capillary flow velocity using a model-based statistical method of eigendecomposition (ED) analysis of the complex OCT signals obtained with the OCT angiography (OCTA) scanning protocol. ED-based analysis is achieved by the covariance matrix of the ensemble complex OCT signals, upon which the eigenvalues and eigenvectors that represent the subsets of the signal makeup are calculated. From this analysis, the signals due to moving particles can be isolated by employing an adaptive regression filter to remove the eigencomponents that represent static tissue signals. The mean frequency (MF) of moving particles can be estimated by the first lag-one autocorrelation of the corresponding eigenvectors. Three important parameters are introduced, including the blood flow signal power representing the presence of blood flow (i.e., OCTA signals), the MF indicating the mean velocity of blood flow, and the frequency bandwidth describing the temporal flow heterogeneity within a scanned tissue volume. The proposed approach is tested using scattering phantoms, in which microfluidic channels are used to simulate the functional capillary vessels that are perfused with the scattering intralipid solution. The results indicate a linear relationship between the MF and mean flow velocity. In vivo animal experiments are also conducted by imaging mouse brain with distal middle cerebral artery ligation to test the capability of the method to image the changes in capillary flows in response to an ischemic insult, demonstrating the practical usefulness of the proposed method for providing important quantifiable information about capillary tissue beds in the investigations of neurological conditions in vivo.

Dynamic laser speckle angiography achieved by eigen-decomposition filtering

A new approach is proposed for statistically analysis of laser speckle signals emerged from a living biological tissue based on eigen-decomposition to separate the dynamic speckle signals due to moving blood cells from the static speckle signals due to static tissue components, upon which to achieve angiography of the interrogated tissue in vivo. The proposed approach is tested by imaging mouse ear pinna in vivo, demonstrating its capability of providing detailed microvascular networks with high contrast, and high temporal and spatial resolutions. It is expected to provide further opportunities for laser speckle imaging in the biomedical and clinical applications where microvascular response to certain stimulus or tissue injury is of interest.

Impaired Collateral Flow Compensation During Chronic Cerebral Hypoperfusion in the Type 2 Diabetic Mice

BACKGROUND AND PURPOSE

The presence of collaterals is associated with a reduced risk of stroke and transient ischemic attack in patients with steno-occlusive carotid artery disease. Although metabolic syndrome negatively impacts collateral status, it is unclear whether and to what extent type 2 diabetes mellitus affects cerebral collateral flow regulation during hypoperfusion.

METHODS

We examined the spatial and temporal changes of the leptomeningeal collateral flow and the flow dynamics of the penetrating arterioles in the distal middle cerebral artery and anterior cerebral artery branches over 2 weeks after unilateral common carotid artery occlusion (CCAO) using optical coherent tomography in db/+ and db/db mice. We also assessed the temporal adaptation of the circle of Willis after CCAO by measuring circle of Willis vessel diameters.

RESULTS

After unilateral CCAO, db/db mice exhibited diminished leptomeningeal collateral flow compensation compared with db/+ mice, which coincided with a reduced dilation of distal anterior cerebral artery branches, leading to reduced flow not only in pial vessels but also in penetrating arterioles bordering the distal middle cerebral artery and anterior cerebral artery. However, no apparent cell death was detected in either strain of mice during the first week after CCAO. db/db mice also experienced a more severe early reduction in the vessel diameters of several ipsilateral main feeding arteries in the circle of Willis, in addition to a delayed post-CCAO adaptive response by 1 to 2 weeks, compared with db/+ mice.

CONCLUSIONS

Type 2 diabetes mellitus is an additional risk factor for hemodynamic compromise during cerebral hypoperfusion, which may increase the severity and the risk of stroke or transient ischemic attack.

Extended-focus optical coherence microscopy for high-resolution imaging of the murine brain

We propose a new method and optical instrumentation for mouse brain imaging based on extended-focus optical coherence microscopy. This in vivo imaging technique allows the evaluation of the cytoarchitecture at cellular level and the circulation system dynamics in three dimensions. This minimally invasive and non-contact approach is performed without the application of contrasting agents. The optical design achieved a resolution of 2.2 μm over a distance of 800 μm, which was sufficient to obtain a detailed three-dimensional image of a wild-type mouse's brain down to the layer III of the cortex. Intrinsically contrasted microvessels and structures similar to the bodies of neurons were distinguishable.

Optical coherence tomography based microangiography provides an ability to longitudinally image arteriogenesis in vivo

BACKGROUND

Arteriogenesis describes the active growth of the pre-existing collateral arterioles, which is a crucial tissue-saving process in occlusive vascular diseases.

NEW METHOD

We propose to use optical coherence tomography (OCT)-based microangiography (OMAG) to monitor arteriogenesis following artery transection in mouse ear and focal stroke in mouse brain.

RESULTS

Our longitudinal mouse ear study shows that the growth phase of arteriogenesis, indicated by changes in collateral vessel diameter and velocity, occurs between 12 and 24h after vessel transection. Additionally, the magnitude of local inflammation is consistent with the time course of arteriogenesis, judging by the tissue thickness measurement and lymphatic vessel signals in OCT. In the mouse brain study, collateral vessel morphology, blood flow velocity and directionality are identified, and an activation of the collateral flow at the arteriolo-arteriolar anastomoses (AAA) is observed during stroke.

COMPARISON WITH EXISTING METHODS

In comparison with histology and fluorescence imaging, OCT/OMAG is completely non-invasive and capable of producing consistent results of longitudinal changes in collateral vessel morphology and vasodynamics.

CONCLUSION

OCT/OMAG is a promising imaging tool for longitudinal study of collateral vessel remodeling in small animals. This technique can be applied in guiding the in vivo experiments of arteriogenesis stimulation to treat occlusive vascular diseases, including stroke.

Optical Coherence Tomography for Brain Imaging and Developmental Biology

Optical coherence tomography (OCT) is a promising research tool for brain imaging and developmental biology. Serving as a three-dimensional optical biopsy technique, OCT provides volumetric reconstruction of brain tissues and embryonic structures with micrometer resolution and video rate imaging speed. Functional OCT enables label-free monitoring of hemodynamic and metabolic changes in the brain in vitro and in vivo in animal models. Due to its non-invasiveness nature, OCT enables longitudinal imaging of developing specimens in vivo without potential damage from surgical operation, tissue fixation and processing, and staining with exogenous contrast agents. In this paper, various OCT applications in brain imaging and developmental biology are reviewed, with a particular focus on imaging heart development. In addition, we report findings on the effects of a circadian gene (Clock) and high-fat-diet on heart development in Drosophila melanogaster. These findings contribute to our understanding of the fundamental mechanisms connecting circadian genes and obesity to heart development and cardiac diseases.

Hemodynamic changes in a rat parietal cortex after endothelin-1-induced middle cerebral artery occlusion monitored by optical coherence tomography

A blockage of the middle cerebral artery (MCA) on the cortical branch will seriously affect the blood supply of the cerebral cortex. Real-time monitoring of MCA hemodynamic parameters is critical for therapy and rehabilitation. Optical coherence tomography (OCT) is a powerful imaging modality that can produce not only structural images but also functional information on the tissue. We use OCT to detect hemodynamic changes after MCA branch occlusion. We injected a selected dose of endothelin-1 (ET-1) at a depth of 1 mm near the MCA and let the blood vessels follow a process first of occlusion and then of slow reperfusion as realistically as possible to simulate local cerebral ischemia. During this period, we used optical microangiography and Doppler OCT to obtain multiple hemodynamic MCA parameters. The change trend of these parameters from before to after ET-1 injection clearly reflects the dynamic regularity of the MCA. These results show the mechanism of the cerebral ischemia-reperfusion process after a transient middle cerebral artery occlusion and confirm that OCT can be used to monitor hemodynamic parameters.

Automated segmentation and enhancement of optical coherence tomography-acquired images of rodent brain

BACKGROUND

Optical coherence tomography (OCT) is a non-invasive optical imaging method that has proven useful in various fields such as ophthalmology, dermatology and neuroscience. In ophthalmology, significant progress has been made in retinal layer segmentation and enhancement of OCT images. There are also segmentation algorithms to separate epidermal and dermal layers in OCT-acquired images of human skin.

NEW METHOD

We describe simple image processing methods that allow automatic segmentation and enhancement of OCT images of rodent brain.

RESULTS

We demonstrate the effectiveness of the proposed methods for OCT-based microangiography (OMAG) and tissue injury mapping (TIM) of mouse cerebral cortex. The results show significant improvement in image contrast, delineation of tissue injury, allowing visualization of different layers of capillary beds.

COMPARISON WITH EXISTING METHODS

Previously reported methods for other applications are yet to be used in neuroscience due to the complexity of tissue anatomy, unique physiology and technical challenges.

CONCLUSIONS

OCT is a promising tool that provides high resolution in vivo microvascular and structural images of rodent brain. By automatically segmenting and enhancing OCT images, structural and microvascular changes in mouse cerebral cortex after stroke can be monitored in vivo with high contrast.

Wide-field optical coherence tomography angiography enabled by two repeated measurements of B-scans

Optical coherence tomography angiography (OCTA) has increasingly become clinically important, particularly in ophthalmology. However, the field of view (FOV) for current OCTA imaging is severely limited due to A-scan rates that can be afforded by current clinical systems and, more importantly, the requirement of a repeated scanning protocol. This Letter evaluates the possibility of using only two repeated B-scans for OCTA for the purpose of an increased FOV. The effect of repeated numbers on the OCTA result is discussed through experiments on an animal model in vivo and evaluated using quantitative metrics for image quality. Demonstrated through in vivo imaging of a pathological human eye, we show that optical microangiography-based OCTA with two repeated B-scans can provide wide-field angiography up to 12×12  mm with clinically acceptable image quality.

Potential use of OCT-based microangiography in clinical dermatology

BACKGROUND

Optical coherence tomography (OCT) is a revolutionary imaging technique used commonly in ophthalmology, and on the way to become clinically viable alternative in dermatology due to its capability of acquiring histopathology level details of in vivo tissue, non-invasively. In this study, we demonstrate the capabilities of OCT-based microangiography in detecting high resolution, three-dimensional structural, and microvascular features of in vivo human skin with various conditions.

METHODS

A swept-source OCT system that operates on a central wavelength of 1310 nm with an A-line rate of 100 kHz is used in this study. We apply optical microangiography (OMAG) technique to visualize the structural and microvascular changes in tissue.

RESULTS

OMAG images provide detailed visualization of functional microvasculature of healthy human skin from cheek and forehead areas, abnormal skin conditions from face, chest and belly. Moreover, OMAG is capable of monitoring the progress of wound healing on human skin from arm, delivering unprecedented detail of microstructural and microvascular information during longitudinal wound healing process.

CONCLUSION

The presented results promise the clinical use of OCT angiography, aiming to treat prevalent cutaneous diseases, by detecting blood perfusion, and structural changes within human skin, in vivo.

Acute insertion effects of penetrating cortical microelectrodes imaged with quantitative optical coherence angiography

The vascular response during cortical microelectrode insertion was measured with amplitude decorrelation-based quantitative optical coherence angiography (OCA). Four different shank-style microelectrode configurations were inserted in murine motor cortex beneath a surgically implanted window in discrete steps while OCA images were collected and processed for angiography and flowmetry. Quantitative measurements included tissue displacement (measured by optical flow), perfused capillary density, and capillary flow velocity. The primary effect of insertion was mechanical perturbation, the effects of which included tissue displacement, arteriolar rupture, and compression of a branch of the anterior cerebral artery causing a global decrease in flow. Other effects observed included local flow drop-out in the region immediately surrounding the microelectrode. The mean basal capillary network velocity for all animals was 0.23 ( ± 0.05    SD ) and 0.18 ( ± 0.07    SD ) mm / s for capillaries from 100 to 300    μ m and 300 to 500    μ m , respectively. Upon insertion, the 2-shank electrode arrays caused a decrease in capillary flow density and velocity, while the results from other configurations were not different from controls. The proximity to large vessels appears to play a larger role than the array configuration. These results can guide neurosurgeons and electrode designers to minimize trauma and ischemia during microelectrode insertion.

OCT-based label-free in vivo lymphangiography within human skin and areola

Due to the limitations of current imaging techniques, visualization of lymphatic capillaries within tissue in vivo has been challenging. Here, we present a label-free high resolution optical coherence tomography (OCT) based lymphangiography (OLAG) within human skin in vivo. OLAG enables rapid (~seconds) mapping of lymphatic networks, along with blood vessel networks, over 8 mm x 8 mm of human skin and 5 mm x 5 mm of human areola. Moreover, lymphatic system’s response to inflammation within human skin is monitored throughout an acne lesion development over 7 days. The demonstrated results promise OLAG as a revolutionary tool in the clinical research and treatment of patients with pathologic conditions such as cancer, diabetes, and autoimmune diseases.

Review of optical coherence tomography based angiography in neuroscience

The brain is a complex ecosystem, consisting of multiple layers and tissue compartments. To facilitate the understanding of its function and its response to neurological insults, a fast in vivo imaging tool with a micron-level resolution, which can provide a field of view at a few millimeters, is desirable. Optical coherence tomography (OCT) is a noninvasive method for imaging three-dimensional biological tissues with high resolution ([Formula: see text]) and without a need for contrast agents. Recent development of OCT-based angiography has started to shed some new light on cerebral hemodynamics in neuroscience. We give an overview of the recent developments of OCT-based imaging techniques for neuroscience applications in rodents. We summarize today's technological alternatives for OCT-based angiography for neuroscience and provide a discussion of challenges and opportunities. Moreover, a summary of OCT angiography studies for stroke, traumatic brain injury, and subarachnoid hemorrhage cases on rodents is provided.

Minimizing projection artifacts for accurate presentation of choroidal neovascularization in OCT micro-angiography

Current optical coherence tomography (OCT) based micro-angiography is prone to a projection (or tailing) effect due to the high scattering property of blood within overlying patent vessels, creating artifacts that interfere with the interpretation of retinal angiographic results. In this work, the projection effect in OCT micro-angiography is examined and its causality is explained by strong light scattering and photon propagation within blood. A simple practical approach is then introduced to minimize these artifacts presented in the outer retinal avascular space, especially useful for examining clinical cases with choroidal neovascularization (CNV). Demonstrated through in-vivo human posterior eye imaging of healthy and CNV subjects, the proposed method is shown effective to eliminate the projection artifacts in outer retinal space of OCT micro-angiography, resulting in better visualization of the pathological neovascularization when compared with the current common approaches. In addition, it is also shown that the proposed method is applicable to minimize the projection artifacts appearing in deep retinal layers.