Imaging the penumbra - strategies to detect tissue at risk after ischemic stroke

Out of the core: the impact of focal ischemia in regions beyond the penumbra

The changes in the necrotic core and the penumbra following induction of focal ischemia have been the focus of attention for some time. However, evidence shows, that ischemic injury is not confined to the primarily affected structures and may influence the remote areas as well. Yet many studies fail to probe into the structures beyond the penumbra, and possibly do not even find any significant results due to their short-term design, as secondary damage occurs later. This slower reaction can be perceived as a therapeutic opportunity, in contrast to the ischemic core defined as irreversibly damaged tissue, where the window for salvation is comparatively short. The pathologies in remote structures occur relatively frequently and are clearly linked to the post-stroke neurological outcome. In order to develop efficient therapies, a deeper understanding of what exactly happens in the exo-focal regions is necessary. The mechanisms of glia contribution to the ischemic damage in core/penumbra are relatively well described and include impaired ion homeostasis, excessive cell swelling, glutamate excitotoxic mechanism, release of pro-inflammatory cytokines and phagocytosis or damage propagation via astrocytic syncytia. However, little is known about glia involvement in post-ischemic processes in remote areas. In this literature review, we discuss the definitions of the terms "ischemic core", "penumbra" and "remote areas." Furthermore, we present evidence showing the array of structural and functional changes in the more remote regions from the primary site of focal ischemia, with a special focus on glia and the extracellular matrix. The collected information is compared with the processes commonly occurring in the ischemic core or in the penumbra. Moreover, the possible causes of this phenomenon and the approaches for investigation are described, and finally, we evaluate the efficacy of therapies, which have been studied for their anti-ischemic effect in remote areas in recent years.

The ischemic penumbra: From concept to reality

The discovery that brain tissue could potentially be salvaged from ischaemia due to stroke, has led to major advances in the development of therapies for ischemic stroke. In this review, we detail the advances in the understanding of this area termed the ischaemic penumbra, from its discovery to the evolution of imaging techniques, and finally some of the treatments developed. Evolving from animal studies from the 70s and 80s and translated to clinical practice, the field of ischemic reperfusion therapy has largely been guided by an array of imaging techniques developed to positively identify the ischemic penumbra, including positron emission tomography, computed tomography and magnetic resonance imaging. More recently, numerous penumbral identification imaging studies have allowed for a better understanding of the progression of the ischaemic core at the expense of the penumbra, and identification of patients than can benefit from reperfusion therapies in the acute phase. Importantly, 40 years of critical imaging research on the ischaemic penumbra have allowed for considerable extension of the treatment time window and better patient selection for reperfusion therapy. The translation of the penumbra concept into routine clinical practice has shown that "tissue is at least as important as time."

Endovascular stroke treatment using balloon guide catheters may reduce penumbral tissue damage and improve long-term outcome

Objectives

During mechanical recanalization of large vessel occlusions (LVO), the use of proximal flow arrest with balloon guide catheters (BGC) was shown to be associated with better angiographic and even clinical outcome. The aim of the study was to analyze the impact of BGC use on microstructural alterations in the salvaged penumbra.

Methods

All patients who underwent mechanical recanalization of LVO of the anterior circulation were reviewed within a prospective stroke registry of a single comprehensive stroke center. Fifty-two patients received an admission CT perfusion together with post-interventional diffusion tensor imaging. Technical details such as BGC usage were correlated with microstructural integrity changes of the salvaged gray matter through the mean diffusivity (MD) index. Moderation analysis was performed to test the interaction of BGC on the correlation between angiographic and clinical outcomes.

Results

For all patients with complete reperfusion, microstructural integrity changes with lowered MD index were found within the salvaged penumbra for cases of non-BGC usage (mean − 0.02) compared to cases with BGC usage (0.01, p = 0.04). The importance of complete reperfusion for good clinical outcome is predominantly based on patients treated with BGC (effect 2.78, p = 0.01 vs. for non-BGC: 0.3, p = 0.71).

Conclusions

The lowered MD index early after mechanical recanalization without BGC usage can be interpreted as microstructural ischemic damage of the salvaged penumbra. It was shown that achieving complete reperfusion in a setting of BGC usage with proximal flow arrest minimizes penumbral damage and improves long-term outcomes.

Key Points

• Microstructural ischemic damage can be reduced by using proximal flow arrest during endovascular treatment with balloon guide catheter.

• Complete reperfusion in a setting of balloon guide catheter minimizes penumbral damage and improves long-term outcome.

Electronic supplementary material

The online version of this article (10.1007/s00330-020-07260-3) contains supplementary material, which is available to authorized users.

Stroke progression and clinical outcome in ischemic stroke patients with a history of migraine

Background

Patients with migraine might be more susceptible of spreading depolarizations, which are known to affect vascular and neuronal function and penumbra recovery after stroke. We investigated whether these patients have more severe stroke progression and less favorable outcomes after recanalization therapy.

Methods

We included patients from a prospective multicenter ischemic stroke cohort. Lifetime migraine history was based on the International Classification of Headache Disorders II criteria. Patients without confirmed migraine diagnosis were excluded. Patients underwent CT angiography and CT perfusion <9 h of onset and follow-up CT after three days. On admission, presence of a perfusion deficit, infarct core and penumbra volume, and blood brain barrier permeability (BBBP) were assessed. At follow-up we assessed malignant edema, hemorrhagic transformation, and final infarct volume. Outcome at three months was evaluated with the modified Rankin Scale (mRS). We calculated adjusted relative risks (aRR) or difference of means (aB) with regression analyses.

Results

We included 600 patients of whom 43 had migraine. There were no differences between patients with or without migraine in presence of a perfusion deficit on admission (aRR: 0.98, 95%CI: 0.77–1.25), infarct core volume (aB: -10.8, 95%CI: -27.04–5.51), penumbra volume (aB: -11.6, 95%CI: -26.52–3.38), mean blood brain barrier permeability (aB: 0.08, 95%CI: -3.11–2.96), malignant edema (0% vs. 5%), hemorrhagic transformation (aRR: 0.26, 95%CI: 0.04–1.73), final infarct volume (aB: -14.8, 95%CI: 29.9–0.2) or outcome after recanalization therapy (mRS > 2, aRR: 0.50, 95%CI: 0.21–1.22).

Conclusion

Elderly patients with a history of migraine do not seem to have more severe stroke progression and have similar treatment outcomes compared with patients without migraine.

Docosanoids Promote Neurogenesis and Angiogenesis, Blood-Brain Barrier Integrity, Penumbra Protection, and Neurobehavioral Recovery After Experimental Ischemic Stroke

Docosahexaenoic acid (DHA) and neuroprotectin D1 (NPD1) are neuroprotective after experimental ischemic stroke. To explore underlying mechanisms, SD rats underwent 2 h of middle cerebral artery occlusion (MCAo) and treated with DHA (5 mg/kg, IV) or NPD1 (5 μg/per rat, ICV) and vehicles 1 h after. Neuro-behavioral assessments was conducted on days 1, 2, and 3, and on week 1, 2, 3, or 4. BrdU was injected on days 4, 5, and 6, immunohistochemistry was performed on week 2 or 4, MRI on day 7, and lipidomic analysis at 4 and 5 h after onset of stroke. DHA improved short- and long-term behavioral functions and reduced cortical, subcortical, and total infarct volumes (by 42, 47, and 31%, respectively) after 2 weeks and reduced tissue loss by 50% after 4 weeks. DHA increased the number of BrdU⁺/Ki-67⁺, BrdU⁺/DCX⁺, and BrdU⁺/NeuN⁺ cells in the cortex, subventricular zone, and dentate gyrus and potentiated NPD1 synthesis in the penumbra at 5 h after MCAo. NPD1 improved behavior, reduced lesion volumes, protected ischemic penumbra, increased NeuN, GFAP, SMI-71-positive cells and vessels, axonal regeneration in the penumbra, and attenuated blood-brain barrier (BBB) after MCAo. We conclude that docosanoid administration increases neurogenesis and angiogenesis, activates NPD1 synthesis in the penumbra, and diminishes BBB permeability, which correlates to long-term neurobehavioral recovery after experimental ischemic stroke.

Recovery of Repressed Memories in Fibromyalgia Patients Treated With Hyperbaric Oxygen - Case Series Presentation and Suggested Bio-Psycho-Social Mechanism

Fibromyalgia Syndrome (FMS) is a condition considered to represent a prototype of central sensitization syndrome, characterized by chronic widespread pain and along with symptoms of fatigue, non-restorative sleep and cognitive difficulties. FMS can be induced by trauma, infection or emotional stress with cumulative evidence that dissociation is relatively frequent in FMS patients. Two randomized controlled trials have shown that hyperbaric oxygen therapy (HBOT) can induce neuroplasticity and be effective in patients suffering from FMS. In this paper we present, for the first time, case series of female fibromyalgia patients who, in the course of HBOT, suddenly recalled repressed traumatic memories of childhood sexual abuse (CSA). The surfacing of the repressed (dissociative) memories decades after the sexual abuse events was sudden and utterly surprising. No psychological intervention was involved. As the memories surfaced, the physical pain related to FMS subsided. In one patient who had brain single photon emission CT (SPECT) before and after HBOT, the prefrontal cortex appeared suppressed before and reactivated after. The 3 cases reported in this article are representative of a total of nine fibromyalgia patients who experienced a retrieval of repressed memory during HBOT. These cases provide insights on dissociative amnesia and suggested mechanism hypothesis that is further discussed in the article. Obviously, prospective studies cannot be planned since patients are not aware of their repressed memories. However, it is very important to keep in mind the possibility of surfacing memories when treating fibromyalgia patients with HBOT or other interventions capable of awakening dormant brain regions.

Ischemic stroke: experimental models and reality

The vast majority of cerebral stroke cases are caused by transient or permanent occlusion of a cerebral blood vessel (“ischemic stroke”) eventually leading to brain infarction. The final infarct size and the neurological outcome depend on a multitude of factors such as the duration and severity of ischemia, the existence of collateral systems and an adequate systemic blood pressure, etiology and localization of the infarct, but also on age, sex, comorbidities with the respective multimedication and genetic background. Thus, ischemic stroke is a highly complex and heterogeneous disorder. It is immediately obvious that experimental models of stroke can cover only individual specific aspects of this multifaceted disease. A basic understanding of the principal molecular pathways induced by ischemia-like conditions comes already from in vitro studies. One of the most frequently used in vivo models in stroke research is the endovascular suture or filament model in rodents with occlusion of the middle cerebral artery (MCA), which causes reproducible infarcts in the MCA territory. It does not require craniectomy and allows reperfusion by withdrawal of the occluding filament. Although promptly restored blood flow is far from the pathophysiology of spontaneous human stroke, it more closely mimics the therapeutic situation of mechanical thrombectomy which is expected to be increasingly applied to stroke patients. Direct transient or permanent occlusion of cerebral arteries represents an alternative approach but requires craniectomy. Application of endothelin-1, a potent vasoconstrictor, allows induction of transient focal ischemia in nearly any brain region and is frequently used to model lacunar stroke. Circumscribed and highly reproducible cortical lesions are characteristic of photothrombotic stroke where infarcts are induced by photoactivation of a systemically given dye through the intact skull. The major shortcoming of this model is near complete lack of a penumbra. The two models mimicking human stroke most closely are various embolic stroke models and spontaneous stroke models. Closeness to reality has its price and goes along with higher variability of infarct size and location as well as unpredictable stroke onset in spontaneous models versus unpredictable reperfusion in embolic clot models.

Intranasal Insulin and Insulin-Like Growth Factor 1 as Neuroprotectants in Acute Ischemic Stroke

Treatment options for stroke remain limited. Neuroprotective therapies, in particular, have invariably failed to yield the expected benefit in stroke patients, despite robust theoretical and mechanistic background and promising animal data. Insulin and insulin-like growth factor 1 (IGF-1) play a pivotal role in critical brain functions, such as energy homeostasis, neuronal growth, and differentiation. They may exhibit neuroprotective properties in acute ischemic stroke based upon their vasodilatory, anti-inflammatory and antithrombotic effects, as well as improvements of functional connectivity, neuronal metabolism, neurotransmitter regulation, and remyelination. Intranasally administered insulin has demonstrated a benefit for prevention of cognitive decline in older people, and IGF-1 has shown potential benefit to improve functional outcomes in animal models of acute ischemic stroke. The intranasal route presents a feasible, tolerable, safe, and particularly effective administration route, bypassing the blood-brain barrier and maximizing distribution to the central nervous system (CNS), without the disadvantages of systemic side effects and first-pass metabolism. This review summarizes the neuroprotective potential of intranasally administered insulin and IGF-1 in stroke patients. We present the theoretical background and pathophysiologic mechanisms, animal and human studies of intranasal insulin and IGF-1, and the safety and feasibility of intranasal route for medication administration to the CNS.

A fast multiparameter MRI approach for acute stroke assessment on a 3T clinical scanner: preliminary results in a non-human primate model with transient ischemic occlusion

Many MRI parameters have been explored and demonstrated the capability or potential to evaluate acute stroke injury, providing anatomical, microstructural, functional, or neurochemical information for diagnostic purposes and therapeutic development. However, the application of multiparameter MRI approach is hindered in clinic due to the very limited time window after stroke insult. Parallel imaging technique can accelerate MRI data acquisition dramatically and has been incorporated in modern clinical scanners and increasingly applied for various diagnostic purposes. In the present study, a fast multiparameter MRI approach including structural T1-weighted imaging (T1W), T2-weighted imaging (T2W), diffusion tensor imaging (DTI), T2-mapping, proton magnetic resonance spectroscopy, cerebral blood flow (CBF), and magnetization transfer (MT) imaging, was implemented and optimized for assessing acute stroke injury on a 3T clinical scanner. A macaque model of transient ischemic stroke induced by a minimal interventional approach was utilized for evaluating the multiparameter MRI approach. The preliminary results indicate the surgical procedure successfully induced ischemic occlusion in the cortex and/or subcortex in adult macaque monkeys (n=4). Application of parallel imaging technique substantially reduced the scanning duration of most MRI data acquisitions, allowing for fast and repeated evaluation of acute stroke injury. Hence, the use of the multiparameter MRI approach with up to five quantitative measures can provide significant advantages in preclinical or clinical studies of stroke disease.

Classification of lesion area in stroke patients during the subacute phase: a multiparametric MRI study

PURPOSE

Stroke imaging studies during the acute phase are likely to precede several vascular brain mechanisms, which have an important role in patient outcome. The aim of this study was to identify within the lesion area during the subacute phase (≥1 day) reactive tissue, which may have the potential for recovery.

METHODS

Twenty seven stroke patients from two cohorts were included. MRI performed during the subacute phase included conventional, perfusion and diffusion imaging. In cohort I, unsupervised multiparametric classification of the lesion area was performed. In cohort II threshold based classification was performed during the subacute phase, and radiological outcome was assessed at follow-up scan.

RESULTS

Three tissue classes were identified in cohort I, referred to as irreversibly damaged, intermediary, and reactive tissue. Based on threshold values defined in cohort I, the reactive tissue was identified in 11/13 patients in cohort II, and showed tissue preservation/partial recovery in 9/11 patients at follow-up scan. The irreversibly damaged tissue was identified in 7/13 patients in cohort II, and predicted tissue necrosis in all cases.

CONCLUSION

Identification of reactive tissue following stroke during the subacute phase can improve radiological assessment, contribute to the understanding of brain recovery processes and has implications for new therapeutic approaches.

Perfusion computed tomography relative threshold values in definition of acute stroke lesions

Background

Perfusion computed tomography (CT) is a relatively new technique that allows fast evaluation of cerebral hemodynamics by providing perfusion maps and gives confirmation of perfusion deficits in ischemic areas. Some controversies exist regarding accuracy of quantitative detection of tissue viability: penumbra (tissue at risk) or core (necrosis).

Purpose

To define brain tissue viability grade on the basis of the perfusion CT parameters in acute stroke patients.

Material and Methods

A multimodal CT imaging protocol; unenhanced CT of the brain, CT angiography of head and neck blood vessels, followed by brain perfusion CT and 24 h follow-up brain CT was performed. Perfusion deficits were detected first visually, with subsequent manual quantitative and relative measurements in affected and contra-lateral hemisphere in 87 acute stroke patients.

Results

Visual perfusion deficit on perfusion CT images was found in 78 cases (38 women, 40 men; mean age, 30–84 years). Penumbra lesions (n = 49) and core lesions (n = 42) were detected by increased mean transit time (MTT) on perfusion CT maps in comparison to contra-lateral hemispheres. Cerebral blood volume (CBV) mean values in the penumbra group were increased in the penumbra group and decreased in the core group. Cerebral blood flow (CBF) values were decreased in penumbra and markedly decreased in core lesion.

Conclusion

Perfusion CT measurements are reliable in estimation of penumbra and core lesions in acute stroke patients, if relative threshold values are used. The most accurate parameter of hypoperfusion is increased MTT above 190%. Relative threshold values for irreversible lesion are CBF <30–40% and CBV <40% in comparison to contra-lateral hemisphere. Penumbra lesion is characterized by MTT increase and CBF decrease, while CBV shows variable values.

The potential roles of 18F-FDG-PET in management of acute stroke patients

Extensive efforts have recently been devoted to developing noninvasive imaging tools capable of delineating brain tissue viability (penumbra) during acute ischemic stroke. These efforts could have profound clinical implications for identifying patients who may benefit from tPA beyond the currently approved therapeutic time window and/or patients undergoing neuroendovascular treatments. To date, the DWI/PWI MRI and perfusion CT have received the most attention for identifying ischemic penumbra. However, their routine use in clinical settings remains limited. Preclinical and clinical PET studies with [¹⁸F]-fluoro-2-deoxy-D-glucose (¹⁸F-FDG) have consistently revealed a decreased ¹⁸F-FDG uptake in regions of presumed ischemic core. More importantly, an elevated ¹⁸F-FDG uptake in the peri-ischemic regions has been reported, potentially reflecting viable tissues. To this end, this paper provides a comprehensive review of the literature on the utilization of ¹⁴C-2-DG and ¹⁸F-FDG-PET in experimental as well as human stroke studies. Possible cellular mechanisms and physiological underpinnings attributed to the reported temporal and spatial uptake patterns of ¹⁸F-FDG are addressed. Given the wide availability of ¹⁸F-FDG in routine clinical settings, ¹⁸F-FDG PET may serve as an alternative, non-invasive tool to MRI and CT for the management of acute stroke patients.

Novel aspirin-triggered neuroprotectin D1 attenuates cerebral ischemic injury after experimental stroke

Acute ischemic stroke triggers complex neurovascular, neuroinflammatory and synaptic alterations. Aspirin and docosahexaenoic acid (DHA), an omega-3 essential fatty acid family member, have beneficial effects on cerebrovascular diseases. DHA is the precursor of neuroprotectin D1 (NPD1), which downregulates apoptosis and, in turn, promotes cell survival. Here we have tested the effect of aspirin plus DHA administration and discovered the synthesis of aspirin-triggered NPD1 (AT-NPD1) in the brain. Then we performed the total chemical synthesis of this molecule and tested in the setting of 2h middle cerebral artery occlusion (MCAo) in Sprague-Dawley rats. Neurological status was evaluated at 24h, 48 h, 72 h, and 7 days. At 3h post-stroke onset, an intravenous administration of 333 μg/kg of AT-NPD1 sodium salt (AT-NPD1-SS) or methyl-ester (AT-NPD1-ME) or vehicle (saline) as treatment was given. On day 7, ex vivo magnetic resonance imaging (MRI) of the brains was conducted on 11.7 T MRI. T2WI, 3D volumes, and apparent diffusion coefficient (ADC) maps were generated. In addition, infarct volumes and number of GFAP (reactive astrocytes), ED-1 (activated microglia/macrophages) and SMI-71-positive vessels were counted in the cortex and striatum at the level of the central lesion. All animals showed similar values for rectal and cranial temperatures, arterial blood gases, and plasma glucose during and after MCAo. Treatment with both AT-NPD1-SS and AT-NPD1-ME significantly improved neurological scores compared to saline treatment at 24h, 48 h, 72 h and 7 days. Total lesion volumes computed from T2WI images were significantly reduced by both AT-NPD1-SS and AT-NPD1-ME treatment in the cortex (by 44% and 81%), striatum (by 61% and 77%) and total infarct (by 48% and 78%, respectively). Brain edema, computed from T2WI in the cortex (penumbra) and striatum (core), was elevated in the saline group. In contrast, both AT-NPD1 decreased water content in the striatum on day 7. 3D volumes, computed from T2WI, were dramatically reduced with both AT-NPD1 and the lesion was mostly localized in the subcortical areas. Treatment with both AT-NPD1-SS and AT-NPD1-ME significantly reduced cortical (by 76% and 96%), subcortical (by 61% and 70%) and total (69% and 84%, respectively) infarct volumes as defined by histopathology. In conclusion, a novel biosynthetic pathway that leads to the formation of AT-NPD1 mediator in the brain was discovered. In addition, administration of synthetic AT-NPD1, in either its sodium salt or as the methyl ester, was able to attenuate cerebral ischemic injury which leads to a novel approach for pharmaceutical intervention and clinical translation.

Thrombotic stroke in the anesthetized monkey (Macaca mulatta): characterization by MRI--a pilot study

BACKGROUND

The lack of a relevant stroke model in large nonhuman primates hinders the development of innovative diagnostic/therapeutic approaches concerned with this cerebrovascular disease. Our objective was to develop a novel and clinically relevant model of embolic stroke in the anesthetized monkey that incorporates readily available clinical imaging techniques and that would allow the possibility of drug delivery including strategies of reperfusion.

METHODS

Thrombin was injected into the lumen of the middle cerebral artery (MCA) in 12 anesthetized (sevoflurane) male rhesus macaques (Macaca mulatta). Sequential MRI studies (including angiography, FLAIR, PWI, DWI, and gadolinium-enhanced T1W imaging) were performed in a 3T clinical MRI. Physiological and biochemical parameters were monitored throughout the investigations.

RESULTS

Once standardized, the surgical procedure induced transient occlusion of the middle cerebral artery in all operated animals. All animals studied showed spontaneous reperfusion, which occurred some time between 2 h and 7 days post-ictus. Eighty percent of the studied animals showed diffusion/perfusion mismatch. The ischemic lesions at 24 h spared both superficial and profound territories of the MCA. Some animals presented hemorrhagic transformation at 7 days post-ictus.

CONCLUSION

In this study, we developed a pre-clinically relevant model of embolic stroke in the anesthetized nonhuman primate.

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.

Acute stroke thrombolysis: time to dispense with the clock and move to tissue-based decision making?

Currently, imaging is predominantly used to exclude patients for thrombolysis, rather than identify patients most likely to benefit. This means that patients are being selected for treatment without reference to tissue pathophysiology. Imaging of specific stroke pathophysiology may be the key to selecting patients most likely to benefit from thrombolysis, and could revolutionize acute stroke assessment and treatment. The technology is available to identify the acute infarct core and possibly the penumbra, via magnetic resonance diffusion-weighted imaging, and both magnetic resonance- and computed tomography-perfusion imaging techniques. However, these modalities require fine tuning before they can be reliably implemented in a routine clinical setting.

Estimation of the ischemic penumbra based on CT perfusion a pilot study

RATIONALE AND OBJECTIVES

Ischemic penumbra (IP), the target of thrombolytic therapies, could be estimated by the mismatch region between magnetic resonance imaging (MRI) diffusion- and perfusion-defined abnormalities; however, the accuracy of this method has been challenged recently. In this study, we try to establish a method for calculating IP size based on computed tomography perfusion (CTP) and to observe the early evolution of IP in detail.

MATERIALS AND METHODS

The middle cerebral artery occlusion (MCAO) model in monkey was used to compare the accuracy in estimating the IP between CTP and MRI methods. A receiver operating characteristic (ROC) curve was performed to calculate the IP threshold of the different CTP parameters, and then the best parameter was obtained. The dynamic evolutions of estimated size of IP by these two methods were compared.

RESULTS

Among the three CTP parameters, relative cerebral blood flow (rCBF) had the highest sensitivity (83.3%) and specificity (98.5%) in estimating the IP. The optimal cutoff threshold of rCBF was 0.203. During the first 15 hours of the MCAO model, the estimated size of IP by the rCBF was larger than that of the MRI method; however, this relationship was reversed 15 hours later.

CONCLUSION

This study suggests that the rCBF method is more accurate in estimating the IP since previous studies have reported that the MRI method underestimated the exact IP in the early stage of ischemia and overestimated the exact IP in the later stages. Further experimental and clinical studies are needed to validate the conclusion.

Docosahexaenoic Acid therapy of experimental ischemic stroke

We examined the neuroprotective efficacy of docosahexaenoic acid (DHA), an omega-3 essential fatty acid family member, in acute ischemic stroke; studied the therapeutic window; and investigated whether DHA administration after an ischemic stroke is able to salvage the penumbra. In each series described below, SD rats underwent 2 h of middle cerebral artery occlusion (MCAo). In series 1, DHA or saline was administered i.v. at 3, 4, 5, or 6 h after stroke. In series 2, MRI was conducted on days 1, 3 and 7. In series 3, DHA or saline was administered at 3 h, and lipidomic analysis was conducted on day 3. Treatment with DHA significantly improved behavior and reduced total infarct volume by a mean of 40% when administered at 3 h, by 66% at 4 h, and by 59% at 5 h. Total lesion volumes computed from T2-weighted images were reduced in the DHA group at all time points. Lipidomic analysis showed that DHA treatment potentiates neuroprotectin D1 (NPD1) synthesis in the penumbra 3 days after MCAo. DHA administration provides neurobehavioral recovery, reduces brain infarction and edema, and activates NPD1 synthesis in the penumbra when administered up to 5 h after focal cerebral ischemia in rats.

The science of stroke: mechanisms in search of treatments

This review focuses on mechanisms and emerging concepts that drive the science of stroke in a therapeutic direction. Once considered exclusively a disorder of blood vessels, growing evidence has led to the realization that the biological processes underlying stroke are driven by the interaction of neurons, glia, vascular cells, and matrix components, which actively participate in mechanisms of tissue injury and repair. As new targets are identified, new opportunities emerge that build on an appreciation of acute cellular events acting in a broader context of ongoing destructive, protective, and reparative processes. The burden of disease is great, and its magnitude widens as a role for blood vessels and stroke in vascular and nonvascular dementias becomes more clearly established. This review then poses a number of fundamental questions, the answers to which may generate new directions for research and possibly new treatments that could reduce the impact of this enormous economic and societal burden.

A simple positron emission tomography-based calibration for perfusion-weighted magnetic resonance maps to optimize penumbral flow detection in acute stroke

BACKGROUND AND PURPOSE

Perfusion-weighted (PW) MRI is increasingly used to identify the tissue at risk. The adequate PW-MRI map and threshold remain controversial due to a considerable individual variation of values. By comparative positron emission tomography, we evaluated a simple MR-based and positron emission tomography-validated calibration of PW maps.

METHODS

PW-MRI and quantitative positron emission tomography (15O-water) of patients with acute stroke were used to calculate averaged as well as individual thresholds of penumbral flow (positron emission tomography cerebral blood flow (<20 mL/100 g/min) for maps of time to peak, mean transit time, cerebral blood flow, and cerebral blood volume. A linear regression analysis studied the variability of the individual thresholds using 3 different PW reference regions (hemispheric, white matter, gray matter). The best model was used for volumetric analysis to compare averaged and scaled individual thresholds and to calculate look-up tables for PW maps.

RESULTS

In 26 patients, the averaged thresholds were (median/interquartile range): cerebral blood flow 21.7 mL/100 g/min (19.9 to 32); cerebral blood volume 1.5 mL/100 g (0.9 to 1.8); mean transit time seconds 5.2 (3.9 to 6.9); and relative time to peak 4.2 seconds (2.8 to 5.8). The large individual variability was best explained by the mean value of the hemispheric reference derived from a region of interest on a level with the basal ganglia of the unaffected hemisphere (R(2): cerebral blood flow 0.76, cerebral blood volume 0.55, mean transit time 0.83, time to peak 0.95). Hemispheric reference-corrected thresholds clearly improved the detection of penumbral flow. Look-up tables were calculated to identify the individual thresholds according to the hemispheric reference value.

CONCLUSIONS

The individual variation of PW values, even if calculated by deconvolution, remains a major obstacle in quantitative PW imaging and can be significantly improved by a simple MR-based calibration. Easily applicable look-up tables identify the individual best threshold for each PW map to optimize mismatch detection.

Occurrence and predictors of futile recanalization following endovascular treatment among patients with acute ischemic stroke: a multicenter study

BACKGROUND AND PURPOSE

Although recanalization is the goal of thrombolysis, it is well recognized that it fails to improve outcome of acute stroke in a subset of patients. Our aim was to assess the rate of and factors associated with "futile recanalization," defined by absence of clinical benefit from recanalization, following endovascular treatment of acute ischemic stroke.

MATERIALS AND METHODS

Data from 6 studies of acute ischemic stroke treated with mechanical and/or pharmacologic endovascular treatment were analyzed. "Futile recanalization" was defined by the occurrence of unfavorable outcome (mRS score of > or = 3 at 1-3 months) despite complete angiographic recanalization (Qureshi grade 0 or TIMI grade 3).

RESULTS

Complete recanalization was observed in 96 of 270 patients treated with IA thrombolysis. Futile recanalization was observed in 47 (49%). In univariate analysis, patients with futile recanalization were older (73 +/- 11 versus 58 +/- 15 years, P < .0001) and had higher median initial NIHSS scores (19 versus 14, P < .0001), more frequent BA occlusion (17% versus 4%, P = .049), less frequent MCA occlusion (53% versus 76%, P = .032), and a nonsignificantly higher rate of symptomatic hemorrhagic complications (2% versus 9%, P = .2). In logistic regression analysis, futile recanalization was positively associated with age > 70 years (OR, 4.4; 95% CI, 1.9-10.5; P = .0008) and initial NIHSS score 10-19 (OR, 3.8; 95% CI, 1.7-8.4; P = .001), and initial NIHSS score > or = 20 (OR, 64.4; 95% CI, 28.8-144; P < .0001).

CONCLUSIONS

Futile recanalization is a relatively common occurrence following endovascular treatment, particularly among elderly patients and those with severe neurologic deficits.

Targeting ischemic penumbra: part I - from pathophysiology to therapeutic strategy

Penumbra is the viable tissue around the irreversibly damaged ischemic core. The purpose of acute stroke treatment is to salvage penumbral tissue and to improve brain function. However, the majority of acute stroke patients who have treatable penumbra are left untreated. Therefore, developing an effective non-recanalizational therapeutics, such as neuroprotective agents, has significant clinical applications. Part I of this serial review on "targeting penumbra" puts special emphases on penumbral pathophysiology and the development of therapeutic strategies. Bioenergetic intervention by massive metabolic suppression and direct energy delivery would be a promising future direction. An effective drug delivery system for this purpose should be able to penetrate BBB and achieve high local tissue drug levels while non-ischemic region being largely unaffected. Selective drug delivery to ischemic stroke penumbra is feasible and deserves intensive research.

Penumbra, the basis of neuroimaging in acute stroke treatment: current evidence

In modern medicine brain imaging is an essential prerequisite not only to acute stroke triage but also to determining the specific therapy indicated. This article reviews the need for imaging the brain in acute stroke, penumbral pathophysiology, penumbral imaging techniques, as well as current status of various imaging modalities that are being employed to select patients for specific therapeutic approaches.

Long-term survival and serial assessment of stroke damage and recovery - practical and methodological considerations

Impairments caused by stroke remain the main cause for adult disability. Despite a vigorous research effort, only 1 thrombolytic treatment has been approved in acute stroke (<3h). The limitations of preclinical studies and how these can be overcome have been the subject of various guidelines. However, often these guidelines focus on the acute stroke setting and omit long-term outcome measures, such as behaviour and neuroimaging. The considerations and practicalities of including the serial assessment of these approaches and their significance to establish therapeutic efficacy are discussed here.