The time of maximum post-ischemic hyperperfusion indicates infarct growth following transient experimental ischemia

Acute Ischemic Stroke in the Clinic and the Laboratory: Targets for Translational Research

Ischemic stroke research has enabled significant advancements in diagnosis, treatment, and management of this debilitating disease, yet challenges remain standing in the way of better patient prognoses. In this narrative review, a fictional case illustrates challenges and uncertainties that medical professionals still face - penumbra identification, lack of neuroprotective agents, side-effects of tissue plasminogen activator, dearth of molecular biomarkers, incomplete microvascular reperfusion or no-reflow, post-recanalization hyperperfusion, blood pressure management and procedural anesthetic effects. The current state of the field is broadly reviewed per topic, with the aim to introduce a broad audience (scientist and clinician alike) to recent successes in translational stroke research and pending scientific queries that are tractable for preclinical assessment. Opportunities for co-operation between clinical and experimental stroke experts are highlighted to increase the size and frequency of strides the field makes to improve our understanding of this disease and ways of treating it.

Propofol anesthesia improves stroke outcomes over isoflurane anesthesia-a longitudinal multiparametric MRI study in a rodent model of transient middle cerebral artery occlusion

General anesthesia is routinely used in endovascular thrombectomy procedures, for which volatile gas and/or intravenous propofol are recommended. Emerging evidence suggests propofol may have superior effects on disability and/or mortality rates, but a mode-of-action underlying these class-specific effects remains unknown. Here, a moderate isoflurane or propofol dosage on experimental stroke outcomes was retrospectively compared using serial multiparametric MRI and behavioral testing. Adult male rats (N = 26) were subjected to 90-min filament-induced transient middle cerebral artery occlusion. Diffusion-, T2- and perfusion-weighted MRI was performed during occlusion, 0.5 h after recanalization, and four days into the subacute phase. Sequels of ischemic damage-blood-brain barrier integrity, cerebrovascular reactivity and sensorimotor functioning-were assessed after four days. While size and severity of ischemia was comparable between groups during occlusion, isoflurane anesthesia was associated with larger lesion sizes and worsened sensorimotor functioning at follow-up. MRI markers indicated that cytotoxic edema persisted locally in the isoflurane group early after recanalization, coinciding with burgeoning vasogenic edema. At follow-up, sequels of ischemia were further aggravated in the post-ischemic lesion, manifesting as increased blood-brain barrier leakage, cerebrovascular paralysis and cerebral hyperperfusion. These findings shed new light on how isoflurane, and possibly similar volatile agents, associate with persisting injurious processes after recanalization that contribute to suboptimal treatment outcome.

Hyperperfusion profiles after recanalization differentially associate with outcomes in a rat ischemic stroke model

Futile recanalization hampers prognoses of ischemic stroke after successful mechanical thrombectomy, hypothetically through post-recanalization perfusion deficits, onset-to-groin delays and sex effects. Clinically, acute multiparametric imaging studies remain challenging. We assessed possible relationships between these factors and disease outcome after experimental cerebral ischemia-reperfusion, using translational MRI, behavioral testing and multi-model inference analyses. Male and female rats (N = 60) were subjected to 45-/90-min filament-induced transient middle cerebral artery occlusion. Diffusion, T2- and perfusion-weighted MRI at occlusion, 0.5 h and four days after recanalization, enabled tracking of tissue fate, and relative regional cerebral blood flow (rrCBF) and -volume (rrCBV). Lesion areas were parcellated into core, salvageable tissue and delayed injury, verified by histology. Recanalization resulted in acute-to-subacute lesion volume reductions, most apparently in females (n = 19). Hyperacute normo-to-hyperperfusion in the post-ischemic lesion augmented towards day four, particularly in males (n = 23). Tissue suffering delayed injury contained higher ratios of hypoperfused voxels early after recanalization. Regressed against acute-to-subacute lesion volume change, increased rrCBF associated with lesion growth, but increased rrCBV with lesion reduction. Similar relationships were detected for behavioral outcome. Post-ischemic hyperperfusion may develop differentially in males and females, and can be beneficial or detrimental to disease outcome, depending on which perfusion parameter is used as explanatory variable.

Implications of Post-recanalization Perfusion Deficit After Acute Ischemic Stroke: a Scoping Review of Clinical and Preclinical Imaging Studies

The goal of reperfusion therapy for acute ischemic stroke (AIS) is to restore cerebral blood flow through recanalization of the occluded vessel. Unfortunately, successful recanalization does not always result in favorable clinical outcome. Post-recanalization perfusion deficits (PRPDs), constituted by cerebral hypo- or hyperperfusion, may contribute to lagging patient recovery rates, but its clinical significance remains unclear. This scoping review provides an overview of clinical and preclinical findings on post-ischemic reperfusion, aiming to elucidate the pattern and consequences of PRPD from a translational perspective. The MEDLINE database was searched for quantitative clinical and preclinical studies of AIS reporting PRPD based on cerebral circulation parameters acquired by translational tomographic imaging methods. PRPD and stroke outcome were mapped on a charting table, creating an overview of PRPD after AIS. Twenty-two clinical and twenty-two preclinical studies were included. Post-recanalization hypoperfusion is rarely reported in clinical studies (4/22) but unequivocally associated with detrimental outcome. Post-recanalization hyperperfusion is more commonly reported (18/22 clinical studies) and may be associated with positive or negative outcome. PRPD has been replicated in animal studies, offering mechanistic insights into causes and consequences of PRPD and allowing delineation of possible courses of PRPD. Complex relationships exist between PRPD and stroke outcome. Diversity in methods and lack of standardized definitions in reperfusion studies complicate the characterization of reperfusion patterns. Recommendations are made to advance the understanding of PRPD mechanisms and to further disentangle the relation between PRPD and disease outcome.

Ultrafast power Doppler imaging for ischemic encephalopathy: A case report

BACKGROUND

Severely elevated intracranial pressure due to various reasons, such as decreased cerebral perfusion, can lead to devastating neurological outcomes, such as brain herniation. Decompression craniectomy is a life-saving procedure that is commonly performed for such a critical situation, but the changes in cerebral microvessels after brain herniation and decompression are unclear. Ultrafast power Doppler imaging (uPDI) is a new microvascular imaging technology that utilizes high frame rate plane/diverging wave transmission and advanced clutter filters. uPDI significantly improves Doppler sensitivity and can detect microvessels, which are usually invisible using traditional ultrasound Doppler imaging.

CASE SUMMARY

In this report, uPDI was used for the first time to observe the brain blood flow of a hypoperfusion area in a 4-year-old girl who underwent decompression craniectomy due to refractory intracranial hypertension (ICP) after malignant brain tumor surgery. B-mode imaging was used to verify the increased densities of the cerebral cortex and basal ganglia that were observed by computed tomography.

CONCLUSION

uPDI showed the local blood supplies and anatomical structures of the patient after decompressive craniectomy. uPDI is potentially a more intuitive and noninvasive method for evaluating the effects of severe ICP on cerebral microvessels.

Persistent perfusion abnormalities at day 1 correspond to different clinical trajectories after stroke

BACKGROUND

Perfusion abnormalities after thrombolysis are frequent within and surrounding ischemic lesions, but their relative frequency is not well known.

OBJECTIVE

To describe the different patterns of perfusion abnormalities observed at 24 hours and compare the characteristics of the patients according to their perfusion pattern.

METHODS

From our thrombolysis registry, we included 226 consecutive patients with an available arterial spin labeling (ASL) perfusion sequence at day 1. We performed a blinded assessment of the perfusion status (hypoperfusion-h, hyperperfusion-H, or normal-N) in the ischemic lesion and in the surrounding tissue. We compared the time course of clinical recovery, the rate of arterial recanalization, and hemorrhagic transformations in the different perfusion profiles.

RESULTS

We identified seven different perfusion profiles at day 1. Four of these (h/h, h/H, H/H, and H/N) represented the majority of the population (84.1%). The H/H profile was the most frequent (34.5%) and associated with 3-month good outcome (modified Rankin Scale (mRS): 63.5%). Patients with persistent hypoperfusion within and outside the lesion (h/h, 12.4%) exhibited worse outcomes after treatment (mRS score 0-2: 23.8%) than other patients, were less frequently recanalized (40.7%), and had more parenchymal hematoma (17.8%). The h/H profile had an intermediate clinical trajectory between the h/h profile and the hyperperfused profiles.

CONCLUSION

ASL hypoperfusion within the infarct and the surrounding tissue was associated with poor outcome. A more comprehensive view of the mechanisms in the hypoperfused surrounding tissue could help to design new therapeutic approaches during and after reperfusion therapies.

Post-ischemic hyperemia following endovascular therapy for acute stroke is associated with lesion growth

A substantial proportion of acute stroke patients fail to recover following successful endovascular therapy (EVT) and injury to the brain and vasculature secondary to reperfusion may be a contributor. Acute stroke patients were included with: i) large vessel occlusion of the anterior circulation, ii) successful recanalization, and iii) evaluable MRI early after EVT. Presence of hyperemia on MRI perfusion was assessed by consensus using a modified ASPECTS. Three different approaches were used to quantify relative cerebral blood flow (rCBF). Sixty-seven patients with median age of 66 [59-76], 57% female, met inclusion criteria. Hyperemia was present in 35/67 (52%) patients early post-EVT, in 32/65 (49%) patients at 24 hours, and in 19/48 (40%) patients at 5 days. There were no differences in incomplete reperfusion, HT, PH-2, HARM, severe HARM or symptomatic ICH rates between those with and without early post-EVT hyperemia. A strong association (R2 = 0.81, p < 0.001) was found between early post-EVT hyperemia (p = 0.027) and DWI volume at 24 hours after adjusting for DWI volume at 2 hours (p < 0.001) and incomplete reperfusion at 24 hours (p = 0.001). Early hyperemia is a potential marker for cerebrovascular injury and may help select patients for adjunctive therapy to prevent edema, reperfusion injury, and lesion growth.

Plasma Homocysteine (Hcy) Concentration Functions as a Predictive Biomarker of SPECT-Evaluated Post-Ischemic Hyperperfusion in Acute Ischemic Stroke

Introduction

Homocysteine (Hcy) concentration has been reported to be associated with ischemic stroke. In this study, we aimed to investigate the potential of plasma Hcy in the prediction of post-ischemic hyperperfusion in AIS patients, which was diagnosed with the single-photon emission computed tomography (SPECT) method.

Methods

A total of 112 ischemic stroke patients were recruited in this study. According to whether the patients were subjected to post-ischemic hyperperfusion, all recruited subjects were divided into a post-ischemic hyperperfusion (+) group (N=48) and post-ischemic hyperperfusion (-) group (N=64). The basic demographical data, clinicopathological data and laboratory biochemical data were collected and compared. Level of homocysteine (Hcy) and cystatin-C (Cys-C) and their potential as predictive biomarker are also investigated.

Results

No significant differences were spotted between the post-ischemic hyperperfusion group (+) and post-ischemic hyperperfusion (-) group in respect to the basic demographical and clinicopathological data. And the serum Hcy levels were lower in the post-ischemic hyperperfusion (+) group. Moreover, ROC analysis indicated significant relationships between Hcy levels and the onset of post-ischemic hyperperfusion.

Conclusion

In conclusion, we validated that the plasma Hcy concentration can be used as a predictive biomarker of SPECT-evaluated post-ischemic hyperperfusion in patients suffering from acute ischemic stroke.

In vivo imaging of cerebral glucose metabolism informs on subacute to chronic post-stroke tissue status - A pilot study combining PET and deuterium metabolic imaging

Recanalization therapy after acute ischemic stroke enables restoration of cerebral perfusion. However, a significant subset of patients has poor outcome, which may be caused by disruption of cerebral energy metabolism. To assess changes in glucose metabolism subacutely and chronically after recanalization, we applied two complementary imaging techniques, fluorodeoxyglucose (FDG) positron emission tomography (PET) and deuterium (2H) metabolic imaging (DMI), after 60-minute transient middle cerebral artery occlusion (tMCAO) in C57BL/6 mice. Glucose uptake, measured with FDG PET, was reduced at 48 hours after tMCAO and returned to baseline value after 11 days. DMI revealed effective glucose supply as well as elevated lactate production and reduced glutamate/glutamine synthesis in the lesion area at 48 hours post-tMCAO, of which the extent was dependent on stroke severity. A further decrease in oxidative metabolism was evident after 11 days. Immunohistochemistry revealed significant glial activation in and around the lesion, which may play a role in the observed metabolic profiles. Our findings indicate that imaging (altered) active glucose metabolism in and around reperfused stroke lesions can provide substantial information on (secondary) pathophysiological changes in post-ischemic brain tissue.

Anaesthesia and haemodynamic management of acute ischaemic stroke patients before, during and after endovascular therapy

Endovascular therapy (EVT) is now standard of care for eligible patients with acute ischaemic stroke caused by large vessel occlusion in the anterior circulation. EVT can be performed with general anaesthesia (GA) or with monitored anaesthesia care, involving local anaesthesia with or without conscious sedation (LA/CS). Controversies remain regarding the optimal choice of anaesthetic strategy and observational studies suggested poorer functional outcome and higher mortality in patients treated under GA, essentially because of its haemodynamic consequences and the delay to put patients under GA. However, these studies are limited by selection bias, the most severe patients being more likely to receive GA and recent randomised trials and meta-analysis showed that protocol-based GA compared with LA/CS is significantly associated with less disability at 3 months. Unlike for intravenous thrombolysis, few data exist to guide management of blood pressure (BP) before and during EVT, but arterial hypotension should be avoided as long as the occlusion persists. BP targets following EVT should probably be adapted to the degree of recanalisation and the extent of ischaemia. Lower BP levels may be warranted to prevent reperfusion injuries even if prospective haemodynamic management evaluations after EVT are lacking.

Development of fast multi-slice apparent T1 mapping for improved arterial spin labeling MRI measurement of cerebral blood flow

PURPOSE

To develop fast multi-slice apparent T₁ (T_1app ) mapping for accurate cerebral blood flow (CBF) quantification with arterial spin labeling (ASL) MRI.

METHODS

Fast multi-slice T_1app was measured using a modified inversion recovery echo planar imaging (EPI) sequence with simultaneous application of ASL tagging radiofrequency (RF) and gradient pulses. The fast multi-slice T_1app measurement was compared with the single-slice T_1app imaging approach, repeated per slice. CBF was assessed in healthy adult Wistar rats (N = 5) and rats with acute stroke 24 hours after a transient middle cerebral artery occlusion (N = 5).

RESULTS

The fast multi-slice T_1app measurement was in good agreement with that of a single-slice T_1app imaging approach (Lin's concordance correlation coefficient = 0.92). CBF calculated using T_1app reasonably accounted for the finite labeling RF duration, whereas the routine T₁ -normalized ASL MRI underestimated the CBF, particularly at short labeling durations. In acute stroke rats, the labeling time and the CBF difference (ΔCBF) between the contralateral normal area and the ischemic lesion were significantly correlated when using T₁ -normalized perfusion calculation (R = 0.844, P = .035). In comparison, T_1app -normalized ΔCBF had little labeling time dependence based on the linear regression equation of ΔCBF = -0.0247*τ + 1.579 mL/g/min (R = -0.352, P = .494).

CONCLUSIONS

Our study found fast multi-slice T_1app imaging improves the accuracy and reproducibility of CBF measurement.

Transcranial contrast-enhanced ultrasound in the rat brain reveals substantial hyperperfusion acutely post-stroke

Direct and real-time assessment of cerebral hemodynamics is key to improving our understanding of cerebral blood flow regulation in health and disease states such as stroke. While a number of sophisticated imaging platforms enable assessment of cerebral perfusion, most are limited either spatially or temporally. Here, we applied transcranial contrast-enhanced ultrasound (CEU) to measure cerebral perfusion in real-time through the intact rat skull before, during and after ischemic stroke, induced by intraluminal filament middle cerebral artery occlusion (MCAO). We demonstrate expected decreases in cortical and striatal blood volume, flow velocity and perfusion during MCAO. After filament retraction, blood volume and perfusion increased two-fold above baseline, indicative of acute hyperperfusion. Adjacent brain regions to the ischemic area and the contralateral hemisphere had increased blood volume during MCAO. We assessed our data using wavelet analysis to demonstrate striking vasomotion changes in the ischemic and contralateral cortices during MCAO and reperfusion. In conclusion, we demonstrate the application of CEU for real-time assessment of cerebral hemodynamics and show that the ischemic regions exhibit striking hyperemia post-MCAO. Whether this post-stoke hyperperfusion is sustained long-term and contributes to stroke severity is not known.

Small Vessels Are a Big Problem in Neurodegeneration and Neuroprotection

The cerebral microcirculation holds a critical position to match the high metabolic demand by neuronal activity. Functionally, microcirculation is virtually inseparable from other nervous system cells under both physiological and pathological conditions. For successful bench-to-bedside translation of neuroprotection research, the role of microcirculation in acute and chronic neurodegenerative disorders appears to be under-recognized, which may have contributed to clinical trial failures with some neuroprotectants. Increasing data over the last decade suggest that microcirculatory impairments such as endothelial or pericyte dysfunction, morphological irregularities in capillaries or frequent dynamic stalls in blood cell flux resulting in excessive heterogeneity in capillary transit may significantly compromise tissue oxygen availability. We now know that ischemia-induced persistent abnormalities in capillary flow negatively impact restoration of reperfusion after recanalization of occluded cerebral arteries. Similarly, microcirculatory impairments can accompany or even precede neural loss in animal models of several neurodegenerative disorders including Alzheimer's disease. Macrovessels are relatively easy to evaluate with radiological or experimental imaging methods but they cannot faithfully reflect the downstream microcirculatory disturbances, which may be quite heterogeneous across the tissue at microscopic scale and/or happen fast and transiently. The complexity and size of the elements of microcirculation, therefore, require utilization of cutting-edge imaging techniques with high spatiotemporal resolution as well as multidisciplinary team effort to disclose microvascular-neurodegenerative connection and to test treatment approaches to advance the field. Developments in two photon microscopy, ultrafast ultrasound, and optical coherence tomography provide valuable experimental tools to reveal those microscopic events with high resolution. Here, we review the up-to-date advances in understanding of the primary microcirculatory abnormalities that can result in neurodegenerative processes and the combined neurovascular protection approaches that can prevent acute as well as chronic neurodegeneration.

Clinical outcome prediction after thrombectomy of proximal middle cerebral artery occlusions by the appearance of lenticulostriate arteries on magnetic resonance angiography: A retrospective analysis

Post-ischemic vasodynamic changes in infarcted brain parenchyma are common and range from hypo- to hyperperfusion. In the present study, appearance of the lenticulostriate arteries (LSAs) on postinterventional 3T time-of-flight (TOF)-MRA suggestive for altered post-stroke vasodynamics following thrombectomy was investigated. Patients who underwent thrombectomy for a proximal MCA occlusion and for whom postinterventional 3T TOF-MRA (median at day 3) was available, were included in this retrospective analysis (n=98). LSA appearance was categorized into presence (LSA-sign⁺) or absence (LSA-sign^-) of vasodilatation in the ischemic hemisphere. Functional outcome was determined using the modified Rankin scale (mRS). LSA-sign⁺ was observed in 64/98 patients. Hypertension (adjusted OR: 0.171, 95% CI: 0.046-0.645) and preinterventional IV rtPA (adjusted OR: 0.265, 95% CI: 0.088-0.798) were associated with absence of the LSA-sign⁺. In multivariate logistic regression, LSA-sign⁺ was associated with substantial neurologic improvement (adjusted OR: 10.18, 95% CI: 2.69-38.57) and good functional outcome (discharge-mRS ≤ 2, adjusted OR: 7.127, 95% CI: 1.913-26.551 and day 90 mRS ≤ 2, adjusted OR: 3.786, 95% CI: 1.026-13.973) after correcting for relevant confounders. For all clinical endpoints, model fit improved when including the LSA-sign term (p<0.05). Asymmetrical dilatation of LSAs following successful thrombectomy indicates favorable neurologic and mid-term functional outcomes. This may indicate preserved cerebral blood flow regulatory mechanisms.

Small cortical grey matter lesions show no persistent infarction in transient ischaemic attack? A prospective cohort study

OBJECTIVES

To find determining factors for persistent infarction signs in patients with transient ischaemic attack (TIA), herein initial diffusion lesion size, visibility on apparent diffusion coefficient (ADC) or fluid-attenuated inversion recovery (FLAIR) and location.

DESIGN

Prospective cohort study of patients with clinical TIA receiving 3T-MRI within 72 hours of symptom onset and at 8-week follow-up.

SETTING

Clinical workflow in a single tertiary stroke centre between February 2012 and June 2014.

PARTICIPANTS

199 candidate patients were recruited, 64 patients were excluded due to non-TIA discharge diagnosis or no 8-week MRI. 122 patients completed the study.

PRIMARY OUTCOME MEASURES

The primary outcome was visible persistent infarction defined as 8-week FLAIR hyperintensity or atrophy corresponding to the initial diffusion-weighted imaging (DWI) lesion.

RESULTS

50 patients showed 84 initial DWI lesions. 29 (35%) DWI lesions did not result in infarction signs on 8-week FLAIR. 26 (90%, P<0.0001) reversing lesions were located in the cortical grey matter (cGM). cGM location (vs any other location) strongly predicted no 8-week infarction sign development (OR 0.02, 95% CI 0.001 to 0.17) or partial lesion area decrease (>30% of initial DWI-area, OR 14.10, 95% CI 3.61 to 54.72), adjusted for FLAIR-visibility, DWI-area, ADC-confirmation and time to scan (TTS) from symptom onset to baseline MRI. Acute FLAIR-visibility was a strong associated factor for persistent infarction signs (OR 33.06, 95% CI 2.94 to 1432.34). For cGM lesions area size was sole associated factor for persistent infarction signs with a 0.31 cm² (area under the curve (AUC), 0.97) threshold. In eight (16%) DWI-positive patients, all lesions reversed fully.

CONCLUSIONS

16% of DWI-positive patients and one-third of acute DWI lesions caused no persistent infarction signs, especially small cGM lesions were not followed by development of persistent infarction signs. Late MRI after TIA is likely to be less useful in the clinical setting, and it is dubious if the absence of old vascular lesions can be taken as evidence of no prior ischaemic attacks.

TRIAL REGISTRATION NUMBER

NCT01531946; Results.

Mapping the dynamics of brain perfusion using functional ultrasound in a rat model of transient middle cerebral artery occlusion

Following middle cerebral artery occlusion, tissue outcome ranges from normal to infarcted depending on depth and duration of hypoperfusion as well as occurrence and efficiency of reperfusion. However, the precise time course of these changes in relation to tissue and behavioral outcome remains unsettled. To address these issues, a three-dimensional wide field-of-view and real-time quantitative functional imaging technique able to map perfusion in the rodent brain would be desirable. Here, we applied functional ultrasound imaging, a novel approach to map relative cerebral blood volume without contrast agent, in a rat model of brief proximal transient middle cerebral artery occlusion to assess perfusion in penetrating arterioles and venules acutely and over six days thanks to a thinned-skull preparation. Functional ultrasound imaging efficiently mapped the acute changes in relative cerebral blood volume during occlusion and following reperfusion with high spatial resolution (100 µm), notably documenting marked focal decreases during occlusion, and was able to chart the fine dynamics of tissue reperfusion (rate: one frame/5 s) in the individual rat. No behavioral and only mild post-mortem immunofluorescence changes were observed. Our study suggests functional ultrasound is a particularly well-adapted imaging technique to study cerebral perfusion in acute experimental stroke longitudinally from the hyper-acute up to the chronic stage in the same subject.

Recent advances in mesoscopic-scale imaging in animal models of ischemic stroke

PURPOSE OF REVIEW

This article provides an overview of the recent literature regarding the application of in-vivo brain imaging techniques to animal models of ischemic stroke.

RECENT FINDINGS

Major breakthroughs concerned the effects of sensory stimulation on neuronal function, local hemodynamics, and tissue outcome in the hyperacute phase of stroke; the novel application to stroke of hybrid scanners allowing simultaneous PET and magnetic resonance; the refinements of magnetic resonance-based oxygen imaging, allowing to map the ischemic penumbra in a completely noninvasive way; the implementation of new PET ligands to selectively map poststroke neuronal death and neuroinflammation; and the use of novel mesoscale imaging techniques to demonstrate the major role of interhemispheric connectivity in poststroke plasticity and functional recovery.

SUMMARY

The array of techniques to map in vivo the key pathophysiological brain processes involved in stroke is currently enlarging at an amazing pace. This is paralleled by ever-increasing sophistication in postprocessing tools. The combination of techniques allowing simultaneous access to several variables is particularly powerful as it affords unprecedented insights into the intimate processes underlying the tissue and neuronal changes that follow a stroke. These major leaps forward will hopefully lead to therapeutic breakthroughs aiming at improving functional outcome after stroke.

Magnetic Resonance Imaging of Cerebral Blood Flow in Animal Stroke Models

Perfusion could provide useful information on the metabolic status and functional status of tissues and organs. This review summarizes the most commonly used perfusion measurement methods: Dynamic susceptibility contrast (DSC) and arterial spin labeling (ASL) and their applications in experimental stroke. Some new developments of cerebral blood flow (CBF) techniques in animal models are also discussed.

How temporal evolution of intracranial collaterals in acute stroke affects clinical outcomes

OBJECTIVE

We compared intracranial collaterals on pretreatment and day 2 brain CT angiograms (CTA) to assess their evolution and relationship with functional outcomes in acute ischemic stroke (AIS) patients treated with IV tissue plasminogen activator (tPA).

METHODS

Consecutive AIS patients who underwent pretreatment and day 2 CTA and received IV tPA during 2010-2013 were included. Collaterals were evaluated by 2 independent neuroradiologists using 3 predefined criteria: the Miteff system, the Maas system, and 20-point collateral scale by the Alberta Stroke Program Early CT Score methodology. We stratified our cohort by baseline pre-tPA state of their collaterals and by recanalization status of the primary vessel for analysis. Good outcomes at 3 months were defined by a modified Rankin Scale score of 0-1.

RESULTS

This study included 209 patients. Delayed collateral recruitment by any grading system was not associated with good outcomes. All 3 scoring systems showed that collateral recruitment on the follow-up CTA from a baseline poor collateral state was significantly associated with poor outcome and increased bleeding risk. When the primary vessel remained persistently occluded, collateral recruitment was significantly associated with worse outcomes. Interestingly, collateral recruitment was significantly associated with increased mortality in 2 of the 3 grading systems.

CONCLUSIONS

Not all collateral recruitment is beneficial; delayed collateral recruitment may be different from early recruitment and can result in worse outcomes and higher mortality. Prethrombolysis collateral status and recanalization are determinants of how intracranial collateral evolution affects functional outcomes.

Temporal assessment of vascular reactivity and functionality using MRI during postischemic proangiogenenic vascular remodeling

Postischemic angiogenesis is an important recovery mechanism. Both arteries and veins are upregulated during angiogenesis, but eventually there are more angiogenic veins than arteries in terms of number and length. It is critical to understand how the veins are modulated after ischemia and then transitioned into angiogenic vessels during the proangiogenic stage to finally serve as a restorative strength to the injured area. Using a rat model of transient focal cerebral ischemia, the hypercapnic blood oxygen level-dependent (BOLD) response was used to evaluate vascular reactivity, while the hyperoxic BOLD and tissue oxygen level-dependent (TOLD) responses were used to evaluate the vascular functionality at 1, 3, and 7days after ischemia. Vessel-like venous signals appeared on R2* maps on days 3 and 7, but not on day 1. The large hypercapnic BOLD responses on days 3 and 7 indicated that these areas have high vascular reactivity. The temporal correlation between vascular reactivity and the immunoreactivity to desmin and VEGF further indicates that the integrity of vascular reactivity is associated with the pericyte coverage as regulated by the VEGF level. Vascular functionality remained low on days 1, 3, and 7, as reflected by the small hyperoxic BOLD and large hyperoxic TOLD responses, indicating the low oxygen consumption of the ischemic tissues. These functional changes in proangiogenic veins may be critical for angiogenesis.

3-Nitropropionic acid-induced ischemia tolerance in the rat brain is mediated by reduced metabolic activity and cerebral blood flow

Tissue tolerance to ischemia can be achieved by noxious stimuli that are below a threshold to cause irreversible damage ('preconditioning'). Understanding the mechanisms underlying preconditioning may lead to the identification of novel therapeutic targets for diseases such as stroke. We here used the oxidative chain inhibitor 3-nitropropionic acid (NPA) to induce ischemia tolerance in a rat middle cerebral artery occlusion (MCAO) stroke model. Cerebral blood flow (CBF) and structural integrity were characterized by longitudinal magnetic resonance imaging (MRI) in combination with behavioral, histologic, and biochemical assessment of NPA-preconditioned animals and controls. Using this approach we show that the ischemia-tolerant state is characterized by a lower energy charge potential and lower CBF, indicating a reduced baseline metabolic demand, and therefore a cellular mechanism of neural protection. Blood vessel density and structural integrity were not altered by NPA treatment. When subjected to MCAO, preconditioned animals had a characteristic MRI signature consisting of enhanced CBF maintenance within the ischemic territory and intraischemic reversal of the initial cytotoxic edema, resulting in reduced infarct volumes. Thus, our data show that tissue protection through preconditioning occurs early during ischemia and indicate that a reduced cellular metabolism is associated with tissue tolerance to ischemia.