The concept of the penumbra: can it be translated to stroke management?

(±)-5-bromo-2-(5-fluoro-1-hydroxyamyl) Benzoate Protects Against Oxidative Stress Injury in PC12 Cells Exposed to H2O2 Through Activation of Nrf2 Pathway

Background: Oxidative stress is associated with the pathogenesis of ischemic stroke (±)-5-bromo-2-(5-fluoro-1-hydroxyamyl) benzoate (BFB) is a novel compound modified by dl-3-n-butylphthalide (NBP). Here, we hypothesized that BFB may protect the PC12 cells against H₂O₂-induced oxidative stress injury through activation of the Nrf2 pathway.

Methods: We measured the cell viability and levels of lactate dehydrogenase (LDH), malondialdehyde (MDA), glutathione (GSH), and reactive oxygen species (ROS) to determine the construction of the H₂O₂-induced models of oxidative stress in PC12 cells. Additionally, apoptotic cell death, mitochondrial membrane potential, and cellular morphology were examined to determine the effect of BFB on oxidative stress injury in H₂O₂-treated PC12 cells. The expression levels of Nrf2-related and autophagy-related genes and proteins were detected using real time quantative PCR (RT-qPCR), Western Blot, and immunofluorescence analyses.

Results: Our study showed that BFB treatment reduced the elevated levels of MDA, LDH, and ROS, and decreased cell viability and GSH in H₂O₂-treated PC12 cells. We also observed the elevated expression of Nrf2 pathway-related factors and intranuclear transitions and found that Nrf2 inhibitors (ML385) could block the protective effect of BFB. The inhibitory effect of BFB on oxidative stress may be partially regulated by Nrf2 activation, and the initiation and induction of autophagy.

Conclusion: BFB inhibited H₂O₂-induced oxidative stress injury in PC12 cells by activating the Nrf2 pathway, initiating and inducing autophagy, suggesting that BFB may be a promising therapeutic agent in treating neurological disorders like cerebral ischemia.

LncRNA CEBPA-AS1 knockdown prevents neuronal apoptosis against oxygen glucose deprivation/reoxygenation by regulating the miR-455/GPER1 axis

Ischemic stroke (IS) is a common nervous system disease, which is a major cause of disability and death in the world. In present study, we demonstrated a regulatory mechanism of CCAAT/enhancer binding protein-alpha antisense 1 (CEBPA-AS1) in oxygen glucose deprivation/reoxygenation (OGD/R)-induced SH-SY5Y cells, with a focus on neuronal apoptosis. CEBPA-AS1, miR-455, and GPER1 expressions were evaluated by using qRT-PCR and Western blotting. The binding relationship among CEBPA-AS1, miR-455, and GPER1 was determined by a dual luciferase reporter assay. Neuronal viability and apoptosis were examined using MTT and flow cytometry assays, followed by determination of apoptosis-related factors (caspase 3, caspase 8, caspase 9, Bax, and Bcl-2). CEBPA-AS1 and GPER1 levels were upregulated, and miR-455 level was downregulated in the cell model of OGD/R induced. CEBPA-AS1 knockdown increased SH-SY5Y viability and reduced OGD/R-induced apoptosis. CEBPA-AS1 could act as a sponge of miR-455, and CEBPA-AS1 knockdown was found to elevate miR-455 expression. miR-455 overexpression also promoted SH-SY5Y cell viability and rescued them from OGD/R-induced apoptosis by binding to GPER1. GPER1 overexpression or miR-455 inhibition reversed the anti-apoptotic effect of CEBPA-AS1 knockdown. These findings suggest a regulatory network of CEBPA-AS1/miR-455/GPER1 that mediates neuronal cell apoptosis in the OGD model, providing a better understanding of pathogenic mechanisms after IS.

Metabolic MRI with hyperpolarized [1-13C]pyruvate separates benign oligemia from infarcting penumbra in porcine stroke

Acute ischemic stroke patients benefit from reperfusion in a short time-window after debut. Later treatment may be indicated if viable brain tissue is demonstrated and this outweighs the inherent risks of late reperfusion. Magnetic resonance imaging (MRI) with hyperpolarized [1-13C]pyruvate is an emerging technology that directly images metabolism. Here, we investigated its potential to detect viable tissue in ischemic stroke. Stroke was induced in pigs by intracerebral injection of endothelin 1. During ischemia, the rate constant of pyruvate-to-lactate conversion, kPL, was 52% larger in penumbra and 85% larger in the infarct compared to the contralateral hemisphere (P = 0.0001). Within the penumbra, the kPL was 50% higher in the regions that later infarcted compared to non-progressing regions (P = 0.026). After reperfusion, measures of pyruvate-to-lactate conversion were slightly decreased in the infarct compared to contralateral. In addition to metabolic imaging, we used hyperpolarized pyruvate for perfusion-weighted imaging. This was consistent with conventional imaging for assessment of infarct size and blood flow. Lastly, we confirmed the translatability of simultaneous assessment of metabolism and perfusion with hyperpolarized MRI in healthy volunteers. In conclusion, hyperpolarized [1-13C]pyruvate may aid penumbral characterization and increase access to reperfusion therapy for late presenting patients.

Necroptotic-Apoptotic Regulation in an Endothelin-1 Model of Cerebral Ischemia

The primary forms of cell death seen in ischemic stroke are of two major types: a necrotic/necroptotic form, and an apoptotic form that is frequently seen in penumbral regions of injury. Typically apoptotic versus necroptotic programmed cell death is described as competitive in nature, where necroptosis is often described as playing a backup role to apoptosis. In the present study, we examined the relationship between these two forms of cell death in a murine endothelin-1 model of ischemia-reperfusion injury in wildtype and caspase-3 null mice with and without addition of the pharmacologic RIPK1 phosphorylation inhibitor necrostatin-1. Analyses of ischemic brain injury were performed via both cellular and volumetric assessments, electron microscopy, TUNEL staining, activated caspase-3 and caspase-7 staining, as well as CD11b and F4/80 staining. Inhibition of caspase-3 or RIPK1 phosphorylation demonstrates significant neural protective effects which are non-additive and exhibit significant overlap in protected regions. Interestingly, morphologic analysis of the cortex demonstrates reduced apoptosis following RIPK1 inhibition. Consistent with this, RIPK1 inhibition reduces the levels of both caspase-3 and caspase-7 activation. Additionally, this protection appears independent of secondary inflammatory mediators. Together, these observations demonstrate that the necroptotic protein RIPK1 modifies caspase-3/-7 activity, ultimately resulting in decreased neuronal apoptosis. These findings thus modify the traditional exclusionary view of apoptotic/necroptotic signaling, revealing a new form of interaction between these dominant forms of cell death.

Temporal clustering, tissue composition, and total variation for mapping oxygen extraction fraction using QSM and quantitative BOLD

PURPOSE

To improve the accuracy of quantitative susceptibility mapping plus quantitative blood oxygen level-dependent magnitude (QSM+qBOLD or QQ) based mapping of oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO₂ ) using temporal clustering, tissue composition, and total variation (CCTV).

METHODS

Three-dimensional multi-echo gradient echo and arterial spin labeling images were acquired from 11 healthy subjects and 33 ischemic stroke patients. Diffusion-weighted imaging (DWI) was also obtained from patients. The CCTV mapping was developed for incorporating tissue-type information into clustering of the previous cluster analysis of time evolution (CAT) and applying total variation (TV). The QQ-based OEF and CMRO₂ were reconstructed with CAT, CAT+TV (CATV), and the proposed CCTV, and results were compared using region-of-interest analysis, Kruskal-Wallis test, and post hoc Wilcoxson rank sum test.

RESULTS

In simulation, CCTV provided more accurate and precise OEF than CAT or CATV. In healthy subjects, QQ-based OEF was less noisy and more uniform with CCTV than CAT. In subacute stroke patients, OEF with CCTV had a greater contrast-to-noise ratio between DWI-defined lesions and the unaffected contralateral side than with CAT or CATV: 1.9 ± 1.3 versus 1.1 ± 0.7 (P = .01) versus 0.7 ± 0.5 (P < .001).

CONCLUSION

The CCTV mapping significantly improves the robustness of QQ-based OEF against noise.

Molecular mechanism of long-term neuroprotective effects of gradual flow restoration on cerebral ischemia reperfusion injury in MCAO rats

BACKGROUND AND PURPOSE

Ischemia-reperfusion injuries (IRIs) can aggravate the condition of some patients with acute occlusion of major intracranial artery (AOMIA) who received endovascular thrombectomy. Here, we provided data confirming the association of Repressor Element-1 Silencing Transcription factor (REST) with the long-term neuroprotective effect of the middle cerebral artery occlusion (MCAO) rats underwent Gradual Flow Restoration (GFR).

METHODS

Long term neuroprotective effects of GFR intervention were evaluated on MCAO rats model after 3d and 7d reperfusion. The neurological deficit score and TTC staining were performed to evaluate the degree of brain damage in GFR and other interventions at different time. Differentially expressed genes related to cerebral ischemia reperfusion injury (CIRI) were initially screened and identified using GSE32529 microarray analysis. REST protein expression in rat brain cortex infarction was detected by Western blot analysis.

RESULTS

MCAO rats intervened with GFR exhibited reduced neurological deficit (P < 0.05) and alleviated brain infarction volume (P < 0.01). The REST gene with up-regulated expression and its downstream genes with down-regulated expression were screened by Microarray analysis. The brain cortex infarction in MCAO rats produced high levels of REST expression. The GFR intervention inhibited REST expression, and alleviated brain injury on MCAO rats.

CONCLUSION

Our results demonstrated that GFR intervention plays a long-term neuroprotective role and reduces brain edema and damage at reperfusion, possibly by inhibiting REST expression.

AMPK activation protects astrocytes from hypoxia‑induced cell death

Adenosine monophosphate (AMP)-activated protein kinase (AMPK) is a major cellular energy sensor that is activated by an increase in the AMP/adenosine triphosphate (ATP) ratio. This causes the initiation of adaptive cellular programs, leading to the inhibition of anabolic pathways and increasing ATP synthesis. AMPK indirectly inhibits mammalian target of rapamycin (mTOR) complex 1 (mTORC1), a serine/threonine kinase and central regulator of cell growth and metabolism, which integrates various growth inhibitory signals, such as the depletion of glucose, amino acids, ATP and oxygen. While neuroprotective approaches by definition focus on neurons, that are more sensitive under cell stress conditions, astrocytes play an important role in the cerebral energy homeostasis during ischemia. Therefore, the protection of astrocytic cells or other glial cells may contribute to the preservation of neuronal integrity and function. In the present study, it was thus hypothesized that a preventive induction of energy deprivation-activated signaling pathways via AMPK may protect astrocytes from hypoxia and glucose deprivation. Hypoxia-induced cell death was measured in a paradigm of hypoxia and partial glucose deprivation in vitro in the immortalized human astrocytic cell line SVG. Both the glycolysis inhibitor 2-deoxy-d-glucose (2DG) and the AMPK activator A-769662 induced the phosphorylation of AMPK, resulting in mTORC1 inhibition, as evidenced by a decrease in the phosphorylation of the target ribosomal protein S6 (RPS6). Treatment with both 2DG and A-769662 also decreased glucose consumption and lactate production. Furthermore, A-769662, but not 2DG induced an increase in oxygen consumption, possibly indicating a more efficient glucose utilization through oxidative phosphorylation. Hypoxia-induced cell death was profoundly reduced by treatment with 2DG or A-769662. On the whole, the findings of the present study demonstrate, that AMPK activation via 2DG or A-769662 protects astrocytes under hypoxic and glucose-depleted conditions.

Brain ischemic insult induces cofilin rod formation leading to synaptic dysfunction in neurons

Ischemic stroke not only induces neuron death in the infarct area but also structural and functional damage of the surviving neurons in the surrounding peri-infarct area. In the present study, we first identified cofilin rod, a pathological rod-like aggregation, formed in neurons of in vivo ischemic stroke animal model and induced neuronal impairment. Cofilin rods formed only on the ipsilateral side of the middle cerebral artery occlusion and reperfusion (MCAO-R) rat brain and showed the highest density in peri-infarct area. Our real-time live cell imaging, immunostaining and patch clamp studies showed that cofilin rod formation in neurons led to dendritic mitochondrial transportation failure, as well as impairment of synaptic structure and functions. Overexpression of LIM kinase or activation of its upstream regulator Rho, suppressed ischemia-induced cofilin rod formation and showed protective effect on synaptic function and structure impairment in both cultured neurons and MCAO-R rat model. In summary, our results demonstrate a novel mechanism of ischemic stroke-induced neuron injury in peri-infarct area and provide a potential target for the protection of neuronal structure and function against brain ischemia insult.

Dl-3-N-butylphthalide attenuates ischemic reperfusion injury by improving the function of cerebral artery and circulation

Dl-3-N-butylphthalide (NBP) is approved in China for the treatment of ischemic stroke. Previous studies have shown that NBP promotes recovery after stroke via multiple mechanisms. However, the effect of NBP on vascular function and thrombosis remains unclear. Here, we aim to study the effect of NBP on vascular function using a rat model of transient middle cerebral artery occlusion (MCAO) and a state-of-the-art high-resolution synchrotron radiation angiography. Eighty SD rats underwent MCAO surgery. NBP (90 mg/kg) was administrated daily by gavage. Synchrotron radiation angiography was used to evaluate the cerebral vascular perfusion, vasoconstriction, and vasodilation in real-time. Neurological scores, brain infarction and atrophy were evaluated. Real-time PCR was used to assess the expression levels of thrombosis and vasoconstriction-related genes. Results revealed that NBP attenuated thrombosis after MCAO and reduced brain infarct and atrophy volume. NBP administrated at 1 and 4 h after MCAO prevented the vasoconstriction of the artery and maintained its diameter at normal level. Administrated at one week after surgery, NBP functioned as a vasodilator in rats after MCAO while displayed no vasodilating effect in sham group. Our results suggested that NBP attenuates brain injury via increasing the regional blood flow by reducing thrombosis and vasoconstriction.

Brief overview: Protective roles of astrocyte-derived pentraxin-3 in blood-brain barrier integrity

Stroke is one of the world's leading causes of mortality and morbidity. Greater understanding is required of the underlying relationships in ischemic brains in order to prevent stroke or to develop effective treatment. This review highlights new findings about the relationship of blood–brain barrier with astrocytes, pentraxin-3 (PTX3), and other factors expressed during or after ischemic stroke. These are discussed with respect to their ameliorative or deleterious effects. These effects are measured in vivo in animal models as well as in vitro in cell cultures. Evidence was found to suggest that astrocytes play a key role in stroke by expressing PTX3, which, in turn, enhances endothelial tightness, increases tight junction proteins, and inhibits vascular endothelial growth factor. The role of astrocytes and PTX3 is examined in relation to hypoxic stress and conditioning as well as mitochondrial transfer. Astrocytes and PTX3 are placed in the context of brain circulation and related areas.

Cross-Frequency Coupling Between Cerebral Blood Flow Velocity and EEG in Ischemic Stroke Patients With Large Vessel Occlusion

Background: Neurovascular coupling enables a rapid adaptation of cerebral blood flow (CBF) to support neuronal activities. Whether this mechanism is compromised during the acute phase after ischemic stroke remains unknown. In this study, we applied a phase-amplitude cross-frequency coupling (PAC) algorithm to investigate multimodal neuro signals including CBF velocity (CBFV), and electroencephalography (EEG). Methods: Acute ischemic stroke patients admitted to the Neurointensive Care Unit, Tiantan Hospital, Capital Medical University (Beijing, China) with continuous monitoring of 8-lead EEG (F3-C3, T3-P3, P3-O1, F4-C4, T4-P4, P4-O2), non-invasive arterial blood pressure (ABP), and bilateral CBFV of the middle cerebral arteries or posterior cerebral arteries were retrospectively analyzed. PAC was calculated between the phase of CBFV in frequency bands (0-0.05 and 0.05-0.15 Hz) and the EEG amplitude in five bands (δ, θ, α, β, γ). The global PAC was calculated as the sum of all PACs across the six EEG channels and five EEG bands for each patient. The hemispherical asymmetry of cross-frequency coupling (CFC) was calculated as the difference between left and right PAC. Results: Sixteen patients (3 males) met our inclusion criteria. Their age was 60.9 ± 7.9 years old. The mean ABP, mean left CBFV, and mean right CBFV were 90.2 ± 31.2 mmHg, 57.3 ± 20.6 cm/s, and 68.4 ± 20.9 cm/s, respectively. The PAC between CBFV and EEG was significantly higher in β and γ bands than in the other three bands. Occipital region (P3-O1 and P4-O2 channels) showed stronger PAC than the other regions. The deceased group tended to have smaller global PAC than the survival group (the area under the receiver operating characteristic curve [AUROC] was 0.81, p = 0.57). The unfavorable outcome group showed smaller global PAC than the favorable group (AUROC = 0.65, p = 0.23). The PAC asymmetry between the two brain hemispheres correlates with the degree of stenosis in stroke patients (p = 0.01). Conclusion: We showed that CBFV interacts with EEG in β and γ bands through a phase-amplitude CFC relationship, with the strongest PAC found in the occipital region and that the degree of hemispherical asymmetry of CFC correlates with the degree of stenosis.

Imaging the physiological evolution of the ischemic penumbra in acute ischemic stroke

We review the hemodynamic, metabolic and cellular parameters affected during early ischemia and their changes as a function of approximate cerebral blood flow ( CBF) thresholds. These parameters underlie the current practical definition of an ischemic penumbra, namely metabolically affected but still viable brain tissue. Such tissue is at risk of infarction under continuing conditions of reduced CBF, but can be rescued through timely intervention. This definition will be useful in clinical diagnosis only if imaging techniques exist that can rapidly, and with sufficient accuracy, visualize the existence of a mismatch between such a metabolically affected area and regions that have suffered cell depolarization. Unfortunately, clinical data show that defining the outer boundary of the penumbra based solely on perfusion-related thresholds may not be sufficiently accurate. Also, thresholds for CBF and cerebral blood volume ( CBV) differ for white and gray matter and evolve with time for both inner and outer penumbral boundaries. As such, practical penumbral imaging would involve parameters in which the physiology is immediately displayed in a manner independent of baseline CBF or CBF threshold, namely pH, oxygen extraction fraction ( OEF), diffusion constant and mean transit time ( MTT). Suitable imaging technologies will need to meet this requirement in a 10-20 min exam.

Neuroprotection via AT2 receptor agonists in ischemic stroke

Stroke is a devastating disease that afflicts millions of people each year worldwide. Ischemic stroke, which accounts for ~88% of cases, occurs when blood supply to the brain is decreased, often because of thromboembolism or atherosclerotic occlusion. This deprives the brain of oxygen and nutrients, causing immediate, irreversible necrosis within the core of the ischemic area, but more delayed and potentially reversible neuronal damage in the surrounding brain tissue, the penumbra. The only currently approved therapies for ischemic stroke, the thrombolytic agent recombinant tissue plasminogen activator (rtPA) and the endovascular clot retrieval/destruction processes, are aimed at restoring blood flow to the infarcted area, but are only available for a minority of patients and are not able in most cases to completely restore neurological deficits. Consequently, there remains a need for agents that will protect neurones against death following ischemic stroke. Here, we evaluate angiotensin II (Ang II) type 2 (AT2) receptor agonists as a possible therapeutic target for this disease. We first provide an overview of stroke epidemiology, pathophysiology, and currently approved therapies. We next review the large amount of preclinical evidence, accumulated over the past decade and a half, which indicates that AT2 receptor agonists exert significant neuroprotective effects in various animal models, and discuss the potential mechanisms involved. Finally, after discussing the challenges of delivering blood–brain barrier (BBB) impermeable AT2 receptor agonists to the infarcted areas of the brain, we summarize the evidence for and against the development of these agents as a promising therapeutic strategy for ischemic stroke.

Protective effects of the angiotensin II AT2 receptor agonist compound 21 in ischemic stroke: a nose-to-brain delivery approach

Significant neuroprotective effects of angiotensin II type 2 (AT₂) receptor (AT₂ receptor) agonists in ischemic stroke have been previously demonstrated in multiple studies. However, the routes of agonist application used in these pre-clinical studies, direct intracerebroventricular (ICV) and systemic administration, are unsuitable for translation into humans; in the latter case because AT₂ receptor agonists are blood-brain barrier (BBB) impermeable. To circumvent this problem, in the current study we utilized the nose-to-brain (N2B) route of administration to bypass the BBB and deliver the selective AT₂ receptor agonist Compound 21 (C21) to naïve rats or rats that had undergone endothelin 1 (ET-1)-induced ischemic stroke. The results obtained from the present study indicated that C21 applied N2B entered the cerebral cortex and striatum within 30 min in amounts that are therapeutically relevant (8.4-9 nM), regardless of whether BBB was intact or disintegrated. C21 was first applied N2B at 1.5 h after stroke indeed provided neuroprotection, as evidenced by a highly significant, 57% reduction in cerebral infarct size and significant improvements in Bederson and Garcia neurological scores. N2B-administered C21 did not affect blood pressure or heart rate. Thus, these data provide proof-of-principle for the idea that N2B application of an AT₂ receptor agonist can exert neuroprotective actions when administered following ischemic stroke. Since N2B delivery of other agents has been shown to be effective in certain human central nervous system diseases, the N2B application of AT₂ receptor agonists may become a viable mode of delivering these neuroprotective agents for human ischemic stroke patients.

Incorporation of relative cerebral blood flow into CT perfusion maps reduces false ’at risk' penumbra

Purpose

The region defined as ‘at risk’ penumbra by current CT perfusion (CTP) maps is largely overestimated. We aimed to quantitate the portion of true ‘at risk’ tissue within CTP penumbra and to determine the parameter and threshold that would optimally distinguish it from false ‘at risk’ tissue, that is, benign oligaemia.

Methods

Among acute stroke patients evaluated by multimodal CT (NCCT/CTA/CTP) we identified those that had not undergone endovascular/thrombolytic treatment and had follow-up NCCT. Maps of absolute and relative CBF, CBV, MTT, TTP and Tmax as well as summary maps depicting infarcted and penumbral regions were generated using the Intellispace Portal (Philips Healthcare, Best, Netherlands). Follow-up CT was automatically co-registered to the CTP scan and the final infarct region was manually outlined. Perfusion parameters were systematically analysed – the parameter that resulted in the highest true-negative-rate (ie, proportion of benign oligaemia correctly identified) at a fixed, clinically relevant false-negative-rate (ie, proportion of ‘missed’ infarct) of 15%, was chosen as optimal. It was then re-applied to the CTP data to produce corrected perfusion maps.

Results

Forty seven acute stroke patients met selection criteria. Average portion of infarcted tissue within CTP penumbra was 15%±2.2%. Relative CBF at a threshold of 0.65 yielded the highest average true-negative-rate (48%), enabling reduction of the false ‘at risk’ penumbral region by ~half.

Conclusions

Applying a relative CBF threshold on relative MTT-based CTP maps can significantly reduce false ‘at risk’ penumbra. This step may help to avoid unnecessary endovascular interventions.

An alternative surgical approach reduces variability following filament induction of experimental stroke in mice

Animal models are essential for understanding the pathology of stroke and investigating potential treatments. However, in vivo stroke models are associated, particularly in mice, with high variability in lesion volume. We investigated whether a surgical refinement where reperfusion is not reliant on the Circle of Willis reduced outcome variability. Mice underwent 60 min of transient middle cerebral artery occlusion avoiding ligation of the external carotid artery. During reperfusion, the common carotid artery was either ligated (standard approach), or it was repaired to allow re-establishment of blood flow through the common carotid artery. All mice underwent MRI scanning for assessment of infarct volume, apparent diffusion coefficient and fractional anisotropy, along with terminal assessment of infarct volume by 2,3,5-triphenyltetrazolium chloride (TTC) staining. Repairing the common carotid artery following middle cerebral artery occlusion enhanced reperfusion (P<0.01) and reduced the variability seen in both total (histological analysis, P=0.008; T2-weighted MRI, P=0.015) and core (diffusion tensor MRI, P=0.043) lesion volume. Avoiding external carotid artery ligation may improve animal wellbeing, through reduced weight loss, while using an alternative surgical approach that enabled reperfusion through the common carotid artery decreased the variability in lesion volume seen within groups.

Summary: An alternative surgical approach following middle cerebral artery occlusion, which allows reperfusion through the common carotid artery, decreases the variability in lesion volume seen within groups and reduces the number of animals required to detect a treatment effect.

Investigating potentially salvageable penumbra tissue in an in vivo model of transient ischemic stroke using sodium, diffusion, and perfusion magnetic resonance imaging

BACKGROUND

Diffusion magnetic resonance imaging (MRI) is the current-state-of-the-art technique to clinically investigate acute (0-24 h) ischemic stroke tissue. However, reduced apparent diffusion coefficient (ADC)-considered a marker of tissue damage-was observed to reverse spontaneously during the subacute stroke phase (24-72 h) which means that low ADC cannot be used to reflect the damaged tissue after 24 h in experimental and clinical studies. One reason for the change in ADC is that ADC values drop with cytotoxic edema (acute phase) and rise when vasogenic edema begins (subacute phase). Recently, combined ¹H- and ²³Na-MRI was proposed as a more accurate approach to improve delineation between reversible (penumbra) and irreversible ischemic injury (core). The aim of this study was to investigate the effects of reperfusion on the ADC and the sodium MRI signal after experimental ischemic stroke in rats in well-defined areas of different viability levels of the cerebral lesion, i.e. core and penumbra as defined via perfusion and histology. Transient middle cerebral artery occlusion was induced in male rats by using the intraluminal filament technique. MRI sodium, perfusion and diffusion measurement was recorded before reperfusion, shortly after reperfusion and 24 h after reperfusion. The animals were reperfused after 90 min of ischemia.

RESULTS

Sodium signal in core did not change before reperfusion, increased after reperfusion while sodium signal in penumbra was significantly reduced before reperfusion, but showed no changes after reperfusion compared to control. The ADC was significantly decreased in core tissue at all three time points compared to contralateral side. This decrease recovered above commonly applied viability thresholds in the core after 24 h.

CONCLUSIONS

Reduced sodium-MRI signal in conjunction with reduced ADC can serve as a viability marker for penumbra detection and complement hydrogen diffusion- and perfusion-MRI in order to facilitate time-independent assessment of tissue fate and cellular bioenergetics failure in stroke patients.

Transverse relaxation of selectively excited metabolites in stroke at 21.1 T

PURPOSE

This study seeks to evaluate in vivo T₂ relaxation times of selectively excited stroke-relevant metabolites via ¹ H relaxation-enhanced magnetic resonance spectroscopy (RE-MRS) at 21.1 T (900 MHz).

METHODS

A quadrature surface coil was designed and optimized for investigations of rodents at 21.1 T. With voxel localization, a RE-MRS pulse sequence incorporating the excitation of selected metabolites was modified to include a variable echo delay for T₂ measurements. A middle cerebral artery occlusion (MCAO) animal model for stroke was examined with spectra taken 24 h post occlusion. Fourteen echo times were acquired, with each measurement completed in less than 2 min.

RESULTS

The RE-MRS approach produced high-quality spectra of the selectively excited metabolites in the stroked and contralateral regions. T₂ measurements reveal differential results between these regions, with significance achieved for lactic acid.

CONCLUSION

Using the RE-MRS technique at ultra-high magnetic field and an optimized quadrature surface coil design, full metabolic T₂ quantifications in a localized voxel is now possible in less than 27 min. Magn Reson Med 77:520-528, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Performance and Predictive Value of a User-Independent Platform for CT Perfusion Analysis: Threshold-Derived Automated Systems Outperform Examiner-Driven Approaches in Outcome Prediction of Acute Ischemic Stroke

BACKGROUND AND PURPOSE

Treatment strategies in acute ischemic stroke aim to curtail ischemic progression. Emerging paradigms propose patient subselection using imaging biomarkers derived from CT, CTA, and CT perfusion. We evaluated the performance of a fully-automated computational tool, hypothesizing enhancements compared with qualitative approaches. The correlation between imaging variables and clinical outcomes in a cohort of patients with acute ischemic stroke is reported.

MATERIALS AND METHODS

Sixty-two patients with acute ischemic stroke and MCA or ICA occlusion undergoing multidetector CT, CTA, and CTP were retrospectively evaluated. CTP was processed on a fully operator-independent platform (RApid processing of PerfusIon and Diffusion [RAPID]) computing automated core estimates based on relative cerebral blood flow and relative cerebral blood volume and hypoperfused tissue volumes at varying thresholds of time-to-maximum. Qualitative analysis was assigned by 2 independent reviewers for each variable, including CT-ASPECTS, CBV-ASPECTS, CBF-ASPECTS, CTA collateral score, and CTA clot burden score. Performance as predictors of favorable clinical outcome and final infarct volume was established for each variable.

RESULTS

Both RAPID core estimates, CT-ASPECTS, CBV-ASPECTS, and clot burden score correlated with favorable clinical outcome (P < .05); CBF-ASPECTS and collateral score were not significantly associated with favorable outcome, while hypoperfusion estimates were variably associated, depending on the selected time-to-maximum thresholds. Receiver operating characteristic analysis demonstrated disparities among tested variables, with RAPID core and hypoperfusion estimates outperforming all qualitative approaches (area under the curve, relative CBV = 0.86, relative CBF = 0.81; P < .001).

CONCLUSIONS

Qualitative approaches to acute ischemic stroke imaging are subject to limitations due to their subjective nature and lack of physiologic information. These findings support the benefits of high-speed automated analysis, outperforming conventional methodologies while limiting delays in clinical management.

Injury and repair in perinatal brain injury: Insights from non-invasive MR perfusion imaging

Injury to the developing brain remains an important complication in critically ill newborns, placing them at risk for future neurodevelopment impairments. Abnormal brain perfusion is often a key mechanism underlying neonatal brain injury. A better understanding of how alternations in brain perfusion can affect normal brain development will permit the development of therapeutic strategies that prevent and/or minimize brain injury and improve the neurodevelopmental outcome of these high-risk newborns. Recently, non-invasive MR perfusion imaging of the brain has been successfully applied to the neonatal brain, which is known to be smaller and have lower brain perfusion compared to older children and adults. This article will present an overview of the potential role of non-invasive perfusion imaging by MRI to study maturation, injury, and repair in perinatal brain injury and demonstrate why this perfusion sequence is an important addition to current neonatal imaging protocols, which already include different sequences to assess the anatomy and metabolism of the neonatal brain.

Multimodal nanoprobes evaluating physiological pore size of brain vasculatures in ischemic stroke models

Ischemic stroke accounts for 80% strokes and originates from a reduction of cerebral blood flow (CBF) after vascular occlusion. For treatment, the first action is to restore CBF by thrombolytic agent recombinant tissue-type plasminogen activator (rt-PA). Although rt-PA benefits clinical outcome, its application is limited by short therapeutic time window and risk of brain hemorrhage. Different to thrombolytic agents, neuroprotectants reduce neurological injuries by blocking ischemic cascade events such as excitotoxicity and oxidative stress. Nano-neuroprotectants demonstrate higher therapeutic effect than small molecular analogues due to their prolonged circulation lifetime and disrupted blood-brain barrier (BBB) in ischemic region. Even enhanced BBB permeability in ischemic territories is verified, the pore size of ischemic vasculatures determining how large and how efficient the therapeutics can pass is barely studied. In this work, nanoprobes (NPs) with different diameters are developed. In vivo multimodal imaging indicates that NP uptakes in ischemic region depended on their diameters and the pore size upper limit of ischemic vasculatures is determined as 10-11 nm. Additionally, penumbra defined as salvageable ischemic tissues performed a higher BBB permeability than infarct core. This work provides a guideline for developing nano-neuroprotectants by taking advantage of the locally enhanced BBB permeability in ischemic brain tissues.

Drowning stars: reassessing the role of astrocytes in brain edema

Edema formation frequently complicates brain infarction, tumors, and trauma. Despite the significant mortality of this condition, current treatment options are often ineffective or incompletely understood. Recent studies have revealed the existence of a brain-wide paravascular pathway for cerebrospinal (CSF) and interstitial fluid (ISF) exchange. The current review critically examines the contribution of this 'glymphatic' system to the main types of brain edema. We propose that in cytotoxic edema, energy depletion enhances glymphatic CSF influx, whilst suppressing ISF efflux. We also argue that paravascular inflammation or 'paravasculitis' plays a critical role in vasogenic edema. Finally, recent advances in diagnostic imaging of glymphatic function may hold the key to defining the edema profile of individual patients, and thus enable more targeted therapy.

Tissue oxygen saturation mapping with magnetic resonance imaging

A quantitative estimate of cerebral blood oxygen saturation is of critical importance in the investigation of cerebrovascular disease. While positron emission tomography can map in vivo the oxygen level in blood, it has limited availability and requires ionizing radiation. Magnetic resonance imaging (MRI) offers an alternative through the blood oxygen level-dependent contrast. Here, we describe an in vivo and non-invasive approach to map brain tissue oxygen saturation (StO2) with high spatial resolution. StO2 obtained with MRI correlated well with results from blood gas analyses for various oxygen and hematocrit challenges. In a stroke model, the hypoxic areas delineated in vivo by MRI spatially matched those observed ex vivo by pimonidazole staining. In a model of diffuse traumatic brain injury, MRI was able to detect even a reduction in StO2 that was too small to be detected by histology. In a F98 glioma model, MRI was able to map oxygenation heterogeneity. Thus, the MRI technique may improve our understanding of the pathophysiology of several brain diseases involving impaired oxygenation.

Altered resting-state FMRI signals in acute stroke patients with ischemic penumbra

Background

Identifying the ischemic penumbra in acute stroke subjects is important for the clinical decision making process. The aim of this study was to use resting-state functional magnetic resonance singal (fMRI) to investigate the change in the amplitude of low-frequency fluctuations (ALFF) of these subjects in three different subsections of acute stroke regions: the infarct core tissue, the penumbra tissue, and the normal brain tissue. Another aim of this study was to test the feasilbility of consistently detecting the penumbra region of the brain through ALFF analysis.

Methods

Sixteen subjects with first-ever acute ischemic stroke were scanned within 27 hours of the onset of stroke using magnetic resonance imaging. The core of infarct regions and penumbra regions were determined by diffusion and perfusion-weighted imaging respectively. The ALFF were measured from resting-state blood oxygen level dependent (BOLD) fMRI scans. The averaged relative ALFF value of each regions were correlated with the time after the onset of stroke.

Results

Relative ALFF values were significantly different in the infarct core tissue, penumbra tissue and normal brain tissue. The locations of lesions in the ALFF maps did not match perfectly with diffusion and perfusion-weighted imagings; however, these maps provide a contrast that can be used to differentiate between penumbra brain tissue and normal brain tissue. Significant correlations between time after stroke onset and the relative ALFF values were present in the penumbra tissue but not in the infarct core and normal brain tissue.

Conclusion

Preliminary results from this study suggest that the ALFF reflects the underlying neurovascular activity and has a great potential to estimate the brain tissue viability after ischemia. Results also show that the ALFF may contribute to acute stroke imaging for thrombolytic or neuroprotective therapies.

Dynamic perfusion and diffusion MRI of cortical spreading depolarization in photothrombotic ischemia

Cortical spreading depolarization (CSD) is known to exacerbate ischemic damage, as the number of CSDs correlates with the final infarct volumes and suppressing CSDs improves functional outcomes. To investigate the role of CSD in ischemic damage, we developed a novel rat model of photothrombotic ischemia using a miniature implantable optic fiber that allows lesion induction inside the magnetic resonance imaging (MRI) scanner. We were able to precisely control the location and the size of the ischemic lesion, and continuously monitor dynamic perfusion and diffusion MRI signal changes at high temporal resolution before, during and after the onset of focal ischemia. Our model showed that apparent diffusion coefficient (ADC) and cerebral blood flow (CBF) in the ischemic core dropped immediately after lesion onset by 20±6 and 41±23%, respectively, and continually declined over the next 5h. Meanwhile, CSDs were observed in all animals (n=36) and displayed either a transient decrease of ADC by 17±3% or an increase of CBF by 104±15%. All CSDs were initiated from the rim of the ischemic core, propagated outward, and confined to the ipsilesional cortex. Additionally, we demonstrated that by controlling the size of perfusion-diffusion mismatch (which approximates the penumbra) in our model, the number of CSDs correlated with the mismatch area rather than the final infarct volume. This study introduces a novel platform to study CSDs in real-time with high reproducibility using MRI.

Positron emission tomography radiotracers for imaging hypoxia

Localized hypoxia, the physiological hallmark of many clinical pathologies, is the consequence of acute or chronic ischemia in the affected region or tissue. The versatility, sensitivity, quantitative nature, and increasing availability of positron emission tomography (PET) make it the preclinical and clinical method of choice for functional imaging of tissue hypoxia at the molecular level. The progress and current status of radiotracers for hypoxia-specific PET imaging are reviewed in this article including references mainly focused on radiochemistry and also relevant to molecular imaging of hypoxia in preclinical and clinical studies.

Defining an Acidosis-Based Ischemic Penumbra from pH-Weighted MRI

It has been proposed that the spatial mismatch between deficits on perfusion-weighted imaging (PWI) and diffusion-weighted imaging (DWI) in MRI can be used to decide regarding thrombolytic treatment in acute stroke. However, uncertainty remains about the meaning and reversibility of the perfusion deficit and even part of the diffusion deficit. Thus, there remains a need for continued development of imaging technology that can better define a potentially salvageable ischemic area at risk of infarction. Amide proton transfer (APT) imaging is a novel MRI method that can map tissue pH changes, thus providing the potential to separate the PWI/DWI mismatch into an acidosis-based penumbra and a zone of benign oligemia. In this totally noninvasive method, the pH dependence of the chemical exchange between amide protons in endogenous proteins and peptides and water protons is exploited. Early results in animal models of ischemia show promise to derive an acidosis penumbra. Possible translation to the clinic and hurdles standing in the way of achieving this are discussed.

Effect of norepinephrine on spinal cord blood flow and parenchymal hemorrhage size in acute-phase experimental spinal cord injury

PURPOSE

In the acute phase of spinal cord injury (SCI), ischemia and parenchymal hemorrhage are believed to worsen the primary lesions induced by mechanical trauma. To minimize ischemia, keeping the mean arterial blood pressure above 85 mmHg for at least 1 week is recommended, and norepinephrine is frequently administered to achieve this goal. However, no experimental study has assessed the effect of norepinephrine on spinal cord blood flow (SCBF) and parenchymal hemorrhage size. We have assessed the effect of norepinephrine on SCBF and parenchymal hemorrhage size within the first hour after experimental SCI.

METHODS

A total of 38 animals were included in four groups according to whether SCI was induced and norepinephrine injected. SCI was induced at level Th10 by dropping a 10-g weight from a height of 10 cm. Each experiment lasted 60 min. Norepinephrine was started 15 min after the trauma. SCBF was measured in the ischemic penumbra zone surrounding the trauma epicenter using contrast-enhanced ultrasonography. Hemorrhage size was measured repeatedly on parasagittal B-mode ultrasonography slices.

RESULTS

SCI was associated with significant decreases in SCBF (P = 0.0002). Norepinephrine infusion did not significantly modify SCBF. Parenchymal hemorrhage size was significantly greater in the animals given norepinephrine (P = 0.0002).

CONCLUSION

In the rat, after a severe SCI at the Th10 level, injection of norepinephrine 15 min after SCI does not modify SCBF and increases the size of the parenchymal hemorrhage.

Interleukin-10 facilitates the selection of patients for systemic thrombolysis

BACKGROUND

Clinical-Diffusion mismatch (CDM; NIHSS score ≥8 & DWI lesion volume ≤25 mL) and Perfusion-Diffusion mismatch (PDM; difference >20% between initial DWI and MTT lesion volumes) have been proposed as surrogates for ischemic brains that are at risk of infarction. However, their utility to improve the selection of patients for thrombolytic treatment remains controversial. Our aim was to identify molecular biomarkers that can be used with neuroimaging to facilitate the selection of ischemic stroke patients for systemic thrombolysis.

METHODS

We prospectively studied 595 patients with ischemic stroke within 12 h of the stroke onset. A total of 184 patients received thrombolytic treatment according to the SITS-MOST criteria. DWI and MTT volumes were measured at admission. The main outcome variable was good functional outcome at 3 months (modified Rankin scale <3). Serum levels of glutamate (Glu), IL-10, TNF-α, IL-6, NSE, and active MMP-9 also were determined at admission.

RESULTS

Patients treated with t-PA who presented with PDM had higher IL-10 levels at admission (p < 0.0001). In contrast, patients with CDM had higher levels of IL-10 (p < 0.0001) as well as Glu and TNF-α (all p < 0.05) and lower levels of NSE and active MMP-9 (all p < 0.0001). IL-10 ≥ 30 pg/mL predicts good functional outcome at 3 months with a specificity of 88% and a sensitibity of 86%. IL-10 levels ≥30 pg/mL independently in both patients with PDM (OR, 18.9) and CDM (OR, 7.5), after an adjustment for covariates.

CONCLUSIONS

Serum levels of IL-10 facilitate the selection of ischemic stroke patients with CDM and PDM for systemic thrombolysis.

Understanding the reasons behind the low utilisation of thrombolysis in stroke

BACKGROUND

Thrombolysis remains the only approved therapy for acute ischaemic stroke (AIS); however, its utilisation is reported to be low.

AIMS

This study aimed to determine the reasons for the low utilisation of thrombolysis in clinical practice.

METHOD

Five metropolitan hospitals comprising two tertiary referral centres and three district hospitals conducted a retrospective, cross-sectional study. Researchers identified patients discharged with a principal diagnosis of AIS over a 12-month time period (July 2009-July 2010), and reviewed the medical record of systematically chosen samples.

RESULTS

The research team reviewed a total of 521 records (48.8% females, mean age 74.4 ± 14 years, age range 5-102 years) from the 1261 AIS patients. Sixty-nine per cent of AIS patients failed to meet eligibility criteria to receive thrombolysis because individuals arrived at the hospital later than 4.5 hours after the onset of symptoms. The factors found to be positively associated with late arrival included confusion at onset, absence of a witness at onset and waiting for improvement of symptoms. However, factors negatively associated with late arrival encompassed facial droop, slurred speech and immediately calling an ambulance. Only 14.7% of the patients arriving within 4.5 hours received thrombolysis. The main reasons for exclusion included such factors as rapidly improving symptoms (28.2%), minor symptoms (17.2%), patient receiving therapeutic anticoagulation (6.7%) and severe stroke (5.5%).

CONCLUSION

A late patient presentation represents the most significant barrier to utilising thrombolysis in the acute stroke setting. Thrombolysis continues to be currently underutilised in potentially eligible patients, and additional research is needed to identify more precise criteria for selecting patients for thrombolysis.

Penumbra detection using PWI/DWI mismatch MRI in a rat stroke model with and without comorbidity: comparison of methods

Perfusion-diffusion (perfusion-weighted imaging (PWI)/diffusion-weighted imaging (DWI)) mismatch is used to identify penumbra in acute stroke. However, limitations in penumbra detection with mismatch are recognized, with a lack of consensus on thresholds, quantification and validation of mismatch. We determined perfusion and diffusion thresholds from final infarct in the clinically relevant spontaneously hypertensive stroke-prone (SHRSP) rat and its normotensive control strain, Wistar-Kyoto (WKY) and compared three methods for penumbra calculation. After permanent middle cerebral artery occlusion (MCAO) (WKY n=12, SHRSP n=15), diffusion-weighted (DWI) and perfusion-weighted (PWI) images were obtained for 4 hours post stroke and final infarct determined at 24 hours on T(2) scans. The PWI/DWI mismatch was calculated from volumetric assessment (perfusion deficit volume minus apparent diffusion coefficient (ADC)-defined lesion volume) or spatial assessment of mismatch area on each coronal slice. The ADC-derived lesion growth provided the third, retrospective measure of penumbra. At 1 hour after MCAO, volumetric mismatch detected smaller volumes of penumbra in both strains (SHRSP: 31 ± 50 mm(3), WKY: 22 ± 59 mm(3), mean ± s.d.) compared with spatial assessment (SHRSP: 36 ± 15 mm(3), WKY: 43 ± 43 mm(3)) and ADC lesion expansion (SHRSP: 41 ± 45 mm(3), WKY: 65 ± 41 mm(3)), although these differences were not statistically significant. Spatial assessment appears most informative, using both diffusion and perfusion data, eliminating the influence of negative mismatch and allowing the anatomical location of penumbra to be assessed at given time points after stroke.

Outcome After Mobilization Within 24 Hours of Acute Stroke

Background and Purpose—

Very early mobilization (VEM) is considered to contribute to the beneficial effects of stroke units, but there are uncertainties regarding the optimal time to start mobilization. We hypothesized that VEM within 24 hours after admittance to the hospital would reduce poor outcome 3 months poststroke compared with mobilization between 24 and 48 hours.

Methods—

We conducted a prospective, randomized, controlled trial with blinded assessment at follow-up. Patients admitted to the stroke unit within 24 hours after stroke were assigned to either VEM within 24 hours of admittance or mobilization between 24 and 48 hours (control group). Primary outcome was the proportion of poor outcome (modified Rankin scale score, 3–6), whereas secondary outcomes were death rate, change in neurological impairment (National Institutes of Health Stroke Scale score), and dependency (Barthel Index 0–17).

Results—

Fifty-six patients were included (mean age±SD, 76.9±9.4 years), 27 were in the VEM group and 29 were in the control group. VEM patients had nonsignificant higher odds (adjusted for age and National Institutes of Health Stroke Scale score on admission) of poor outcome (OR, 2.70; 95% CI, 0.78–9.34; P =0.12), death (OR, 5.26; 95% CI, 0.84–32.88; P =0.08), and dependency (OR, 1.25; 95% CI, 0.36–4.34; P =0.73). The control group, having milder strokes (National Institutes of Health Stroke Scale score±SD: control group, 7.5±4.2; VEM, 9.2±6.5; P =0.26), had better neurological improvement ( P =0.02).

Conclusions—

We identified a trend toward increased poor outcome, death rate, and dependency among patients mobilized within 24 hours after hospitalization, and an improvement in neurological functioning in favor of patients mobilized between 24 and 48 hours. Very early or delayed mobilization after acute stroke is still undergoing debate, and results from ongoing larger trials are required.

Magnetic resonance diffusion-perfusion mismatch in acute ischemic stroke: An update

The concept of magnetic resonance perfusion-diffusion mismatch (PDM) provides a practical and approximate measure of the tissue at risk and has been increasingly applied for the evaluation of hyperacute and acute stroke in animals and patients. Recent studies demonstrated that PDM does not optimally define the ischemic penumbra; because early abnormality on diffusion-weighted imaging overestimates the infarct core by including part of the penumbra, and the abnormality on perfusion weighted imaging overestimates the penumbra by including regions of benign oligemia. To overcome these limitations, many efforts have been made to optimize conventional PDM. Various alternatives beyond the PDM concept are under investigation in order to better define the penumbra. The PDM theory has been applied in ischemic stroke for at least three purposes: to be used as a practical selection tool for stroke treatment; to test the hypothesis that patients with PDM pattern will benefit from treatment, while those without mismatch pattern will not; to be a surrogate measure for stroke outcome. The main patterns of PDM and its relation with clinical outcomes were also briefly reviewed. The conclusion was that patients with PDM documented more reperfusion, reduced infarct growth and better clinical outcomes compared to patients without PDM, but it was not yet clear that thrombolytic therapy is beneficial when patients were selected on PDM. Studies based on a larger cohort are currently under investigation to further validate the PDM hypothesis.

Preclinical stroke research--advantages and disadvantages of the most common rodent models of focal ischaemia

This review describes the most commonly used rodent models and outcome measures in preclinical stroke research and discusses their strengths and limitations. Most models involve permanent or transient middle cerebral artery occlusion with therapeutic agents tested for their ability to reduce stroke-induced infarcts and improve neurological deficits. Many drugs have demonstrated preclinical efficacy but, other than thrombolytics, which restore blood flow, none have demonstrated efficacy in clinical trials. This failure to translate efficacy from bench to bedside is discussed alongside achievable steps to improve the ability of preclinical research to predict clinical efficacy: (i) Improvements in study quality and reporting. Study design must include randomization, blinding and predefined inclusion/exclusion criteria, and journal editors have the power to ensure statements on these and mortality data are included in preclinical publications. (ii) Negative and neutral studies must be published to enable preclinical meta-analyses and systematic reviews to more accurately predict drug efficacy in man. (iii) Preclinical groups should work within networks and agree on standardized procedures for assessing final infarct and functional outcome. This will improve research quality, timeliness and translational capacity. (iv) Greater uptake and improvements in non-invasive diagnostic imaging to detect and study potentially salvageable penumbral tissue, the target for acute neuroprotection. Drug effects on penumbra lifespan studied serially, followed by assessment of behavioural outcome and infarct within in the same animal group, will increase the power to detect drug efficacy preclinically. Similar progress in detecting drug efficacy clinically will follow from patient recruitment into acute stroke trials based on evidence of remaining penumbra.

Visualizing cell death in experimental focal cerebral ischemia: promises, problems, and perspectives

One of the hallmarks of stroke pathophysiology is the widespread death of many different types of brain cells. As our understanding of the complex disease that is stroke has grown, it is now generally accepted that various different mechanisms can result in cell damage and eventual death. A plethora of techniques is available to identify various pathological features of cell death in stroke; each has its own drawbacks and pitfalls, and most are unable to distinguish between different types of cell death, which partially explains the widespread misuse of many terms. The purpose of this review is to summarize the standard histopathological and immunohistochemical techniques used to identify various pathological features of stroke. We then discuss how these methods should be properly interpreted on the basis of what they are showing, as well as advantages and disadvantages that require consideration. As there is much interest in the visualization of stroke using noninvasive imaging strategies, we also specifically discuss how these techniques can be interpreted within the context of cell death.

Potential of Early [ 18 F]-2-Fluoro-2-Deoxy-D-Glucose Positron Emission Tomography for Identifying Hypoperfusion and Predicting Fate of Tissue in a Rat Embolic Stroke Model

Background and Purpose—

Experimental stroke models are essential to study in vivo pathophysiological processes of focal cerebral ischemia. In this study, an embolic stroke model in rats was applied (1) to characterize early development of regional cerebral blood flow and metabolism with positron emission tomography (PET) using [ 15 O]H 2 O and [ 18 F]-2-fluoro-2-deoxy-D-glucose (FDG); and (2) to identify potential parameters for predicting tissue fate.

Methods—

Remote occlusion of the middle cerebral artery was induced in 10 Wistar rats by injection of 4 TiO 2 macrospheres. Sequential [ 15 O]H 2 O-PET (baseline, 5, 30, 60 minutes after middle cerebral artery occlusion) and FDG-PET measurements (75 minutes after middle cerebral artery occlusion) were performed. [ 15 O]H 2 O-PET data and FDG kinetic parameters were compared with MRIs and histology at 24 hours.

Results—

Regional cerebral blood flow decreased substantially within 30 minutes after middle cerebral artery occlusion (41% to 58% of baseline regional cerebral blood flow; P <0.001) with no relevant changes between 30 and 60 minutes. At 60 minutes, regional cerebral blood flow correlated well with the unidirectional transport parameter K1 of FDG in all animals ( r =0.86±0.09; P <0.001). Tissue fate could be accurately predicted taking into account K1 and net influx rate constant Ki of FDG. The infarct volume predicted by FDG-PET (375.8±102.3 mm 3 ) correlated significantly with the infarct size determined by MRI after 24 hours (360.8±93.7 mm 3 ; r =0.85).

Conclusions—

Hypoperfused tissue can be identified by decreased K1 of FDG. Acute ischemic tissue can be well characterized using K1 and Ki allowing for discrimination between infarct core and early viable tissue. Because FDG-PET is widely spread, our findings can be easily translated into clinical application for early diagnoses of ischemia.

New insights in perinatal arterial ischemic stroke by assessing brain perfusion

Perinatal arterial ischemic stroke (AIS) is an important cause of long-term morbidity in children. Thus, there is an urgent need to better understand the mechanisms of stroke in newborns in order to develop effective treatment and prevention strategies. The purpose of this study was to assess brain perfusion within the first month of life in newborns with AIS. In this study, magnetic resonance imaging (MRI) and perfusion imaging by arterial spin labeling (ASL) were used to assess brain perfusion in four term newborns with AIS. One patient had a stroke within the territory of the right middle cerebral artery (MCA); the other three patients had a stroke within the territory of the left MCA. None of them displayed any hemorrhagic component. All four patients demonstrated abnormal brain perfusion in the stroke area. Cerebral blood flow (CBF) within the stroke area was increased in patient # 1. In all other three patients, CBF was decreased within the stroke center and increased in the periphery of the stroke area. These results show the feasibility of the ASL sequence in newborns with AIS and support its addition to the current MRI protocol used in these newborns as it provides useful information on brain hemodynamics. Its value for identifying salvageable tissue in newborns needs to be further assessed, as well as its potential role in stroke follow-up and for tissue-specific treatment screening.

Influence of 100% and 40% oxygen on penumbral blood flow, oxygen level, and T2*-weighted MRI in a rat stroke model

Accurate imaging of the ischemic penumbra is a prerequisite for acute clinical stroke research. T(2)(*) magnetic resonance imaging (MRI) combined with an oxygen challenge (OC) is being developed to detect penumbra based on changes in blood deoxyhemoglobin. However, inducing OC with 100% O(2) induces sinus artefacts on human scans and influences cerebral blood flow (CBF), which can affect T(2)(*) signal. Therefore, we investigated replacing 100% O(2) OC with 40% O(2) OC (5 minutes 40% O(2) versus 100% O(2)) and determined the effects on blood pressure (BP), CBF, tissue pO(2), and T(2)(*) signal change in presumed penumbra in a rat stroke model. Probes implanted into penumbra and contralateral cortex simultaneously recorded pO(2) and CBF during 40% O(2) (n=6) or 100% O(2) (n=8) OC. In a separate MRI study, T(2)(*) signal change to 40% O(2) (n=6) and 100% O(2) (n=5) OC was compared. Oxygen challenge (40% and 100% O(2)) increased BP by 8.2% and 18.1%, penumbra CBF by 5% and 15%, and penumbra pO(2) levels by 80% and 144%, respectively. T(2)(*) signal significantly increased by 4.56% ± 1.61% and 8.65% ± 3.66% in penumbra compared with 2.98% ± 1.56% and 2.79% ± 0.66% in contralateral cortex and 1.09% ± 0.82% and -0.32% ± 0.67% in ischemic core, respectively. For diagnostic imaging, 40% O(2) OC could provide sufficient T(2)(*) signal change to detect penumbra with limited influence in BP and CBF.

A fibrin-specific thrombolytic nanomedicine approach to acute ischemic stroke

AIM

To develop a fibrin-specific urokinase nanomedicine thrombolytic agent.

MATERIALS & METHODS

In vitro fibrin-clot dissolution studies were utilized to develop and characterize simultaneous coupling and loading of anti-fibrin monoclonal antibody and urokinase onto perfluorocarbon nanoparticle (NP) surface. In vivo pharmacokinetics and fibrin-specific targeting of the nanolytic agent was studied in dogs.

RESULTS

Simultaneous coupling of up to 40 anti-fibrin antibodies and 400 urokinase enzymes per perfluorocarbon NP produced an effective targeted nanolytic agent with no significant surface protein-protein interference. Fibrin clot dissolution was not improved by increasing homing capacity from 10 to 40 antibodies/NP, but increasing enzymatic payload from 100 to 400/NP resulted in maximized lytic effect. Fluorescent microscopy showed that rhodamine-labeled urokinase nanoparticles densely decorated the intraluminal thrombus in canine clots in vivo analogous to the fibrin pattern, while an irrelevant-targeted agent had negligible binding.

CONCLUSION

This agent offers a vascularly constrained, simple to administer, low-dose nanomedicine approach that may present an attractive alternative for treating acute stroke victims.