Beyond mismatch: evolving paradigms in imaging the ischemic penumbra with multimodal magnetic resonance imaging

Modelling of the physiological response of the brain to ischaemic stroke

Identification of salvageable brain tissue is a major challenge when planning the treatment of ischaemic stroke. As the standard technique used in this context, the perfusion-diffusion mismatch, has not shown total accuracy, there is an ongoing search for new imaging protocols that could better identify the region of the brain at risk and for new physiological models that could, on the one hand, incorporate the imaged parameters and predict the evolution of the condition for the individual, and, on the other hand, identify future biomarkers and thus suggest new directions for the design of imaging protocols. Recently, models of cellular metabolism after stroke and blood-brain barrier transport at tissue level have been introduced. We now extend these results by developing a model of the propagation of key metabolites in the brain's extracellular space owing to stroke-related oedema and chemical concentration gradients between the ischaemic and normal brain. We also couple the resulting chemical changes in the extracellular space with cellular metabolism. Our work enables the first patient-specific simulations of stroke progression with finite volume models to be made.

Diagnostic imaging for acute ischemic stroke management

Although our clinical understanding remains our most important diagnostic tool, acute stroke therapy without neuroimaging is impossible. In most patients, only non-contrast computed tomography is used for diagnosis of acute stroke. However, findings based exclusively on clinical assessment and nonhemorrhagic computed tomography scans may no longer be appropriate for acute stroke treatment. From a pathophysiologic point of view, advanced computed tomography techniques and stroke magnetic resonance imaging provide much more information about the acute stroke patient as the basis of decision making in acute stroke treatment. Advanced computed tomography may provide information comparable with stroke magnetic resonance imaging, although a more detailed evaluation concerning these methods in clinical practice is required. This review gives the reader an integrated view on the current status of acute stroke imaging based on advanced computed tomography and multiparametric stroke magnetic resonance imaging protocols. These new imaging techniques allow for a far more individualized method of decision making according to the findings in each patient. This results in improved identification of patients with acute stroke syndromes, improved patient selection of those patients who are regarded suitable for thrombolysis, an extension of the rather strictly defined therapeutic time window for treatment, as well as a more sophisticated method of introduction of alternative therapies into clinical practice.

Effects of cerebral ischemic and reperfusion on T2*-weighted MRI responses to brief oxygen challenge

This study characterized the effects of cerebral ischemia and reperfusion on T2*-weighted magnetic resonance image (MRI) responses to brief oxygen challenge (OC) in transient (60 minutes) cerebral ischemia in rats. During occlusion, the ischemic core tissue showed no significant OC response, whereas the perfusion-diffusion mismatch tissue showed markedly higher percent changes relative to normal tissue. After reperfusion, much of the pixels with initial exaggerated OC responses showed normal OC responses, and the majority of these tissues were salvaged as defined by endpoint T2 MRI. The initial core pixels showed exaggerated OC responses after reperfusion, but the majority of the core pixels eventually became infarct, suggesting exaggerated OC responses do not necessarily reflect salvageable tissue. Twenty-four hours after stroke, basal T1 increased in the ischemic core. Oxygen challenge decreased T1 significantly in the core, indicative of the substantial increases in dissolved oxygen in the core as the result of hyperperfusion. We concluded that exaggerated T2*-weighted MRI responses to OC offer useful insight in ischemic tissue fates. However, exaggerated OC pixels are not all salvageable, and they exhibited complex dynamics depending on reperfusion status, hyperperfusion, and edema effects.

Central nervous system imaging in diabetic cerebrovascular diseases and white matter hyperintensities

Diabetes mellitus is an important vascular risk factor for cerebrovascular disease. This occurs through pathophysiologic changes to the microcirculation as arteriolosclerosis and to the macrocirculation as large artery atherosclerosis. Imaging techniques can provide detailed visualization of the cerebrovasculature using CT (computed tomography) angiography and MR (magnetic resonance) angiography. Newer techniques focused on advanced parenchymal imaging include CT perfusion, quantitative MRI, and diffusion tensor imaging; each identifies brain lesion burden due to diabetes mellitus. These imaging approaches have provided insights into the diabetes mellitus brain and cerebral circulation pathophysiology. Imaging has taught us that diabetics develop cerebral atrophy, silent infarcts, and white matter disease more rapidly than other patient populations. Longitudinal studies are needed to quantify the rate and extent of such structural brain and blood vessel changes and how they relate to cognitive decline. Diabetes prevention and treatment strategies will then be possible to slow the development of such changes.

Perfusion MRI: the five most frequently asked clinical questions

OBJECTIVE

This article addresses questions that radiologists frequently ask when planning, performing, processing, and interpreting MRI perfusion studies in CNS imaging.

CONCLUSION

Perfusion MRI is a promising tool in assessing stroke, brain tumors, and neurodegenerative diseases. Most of the impediments that have limited the use of per-fusion MRI can be overcome to allow integration of these methods into modern neuroimaging protocols.

Quantitative temporal profiles of penumbra and infarction during permanent middle cerebral artery occlusion in rats

The basic premise of neuroprotection in acute stroke is the presence of salvageable tissue, but the spatiotemporal volume profiles of the penumbra and infarction remain poorly defined in preclinical animal models of acute stroke used to evaluate therapies for clinical application. Our aim was to define these profiles using magnetic resonance imaging (MRI) quantitative cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) for dual-parameter voxel analysis in the rat suture permanent middle cerebral artery occlusion (pMCAO) model. Eleven male Sprague Dawley rats were subjected to pMCAO with MRI measurements of quantitative CBF and ADC at baseline, over the first 4 h (n=9) and at 7, 14, and 21 days (n=4). Voxel analysis of CBF and ADC was used to characterize brain tissue ischemic transitions. Penumbra, core, and hyperemic infarction volumes were significantly elevated (P<0.05) and unchanged over the first 4 h of pMCAO while the total lesion volume progressively rose. At 7, 14, and 21 days, tissue compartment transitions reflected infarction, tissue cavitation, and selective ischemic neuronal necrosis. Anatomical distribution of penumbra and core revealed marked heterogeneity with penumbra scattered within core and penumbra persisting even after 4 h of permanent MCAO.

Efficacy and safety of endovascular treatment versus intravenous thrombolysis for acute ischemic stroke: a meta-analysis of randomized controlled trials

Background and Purpose

Although endovascular therapy (ET) is increasingly used in patients with moderate to severe acute ischemic stroke, its efficacy and safety remains controversial. We performed a meta-analysis aiming to compare the benefits and safety of endovascular treatment and intravenous thrombolysis in the treatment of acute ischemic stroke.

Methods

We systematically searched PubMed, Embase, Science direct and Springer unitil July, 2013. The primary outcomes included good outcome (mRS ≤ 2) and excellent outcome (mRS ≤ 1) at 90 days or at trial end point. Secondary outcomes were occurrence of symptomatic hemorrhage and all-cause mortality.

Results

Using a prespecified search strategy, 5 RCTs with 1106 patients comparing ET and intravenous thrombolysis (IVT) were included in the meta-analysis. ET and IVT were associated with similar good (43.06% vs 41.78%; OR=1.14; 95% CI, 0.77 to 1.69; P=0.52;) and excellent (30.43% vs 30.42%; OR=1.05; 95% CI, 0.80 to 1.38; P=0.72;) outcome. For additional end points, ET was not associated with increased occurrence of symptomatic hemorrhage (6.25% vs. 6.22%; OR=1.03; 95% CI, 0.62 to 1.69; P=0.91;), or all-cause mortality (18.45% vs. 17.35%; OR=1.00; 95% CI, 0.73 to 1.39; P=0.99;).

Conclusions

Formal meta-analysis indicates that there are similar safety outcomes and functional independence with endovascular therapy and intravenous thrombolysis for acute ischemic stroke.

A review of current imaging methods used in stroke research

Stroke is a serious healthcare problem with high mortality and long-term disability. However, to date, our ability to prevent and cure stroke remains limited. One important goal in stroke research is to identify the extent and location of lesion for treatment. In addition, accurately differentiating salvageable tissue from infarct and evaluating therapeutic efficacies are indispensible. These objectives could potentially be met with the assistance of modern neuroimaging techniques. This paper reviews current imaging methods commonly used in ischemic stroke research. These methods include positron emission tomography, computed tomography, T1 MRI, T2 MRI, diffusion and perfusion MRI, diffusion tensor imaging, blood-brain barrier permeability MRI, pH-weighted MRI, and functional MRI.

Spatiotemporal dynamic simulation of acute perfusion/diffusion ischemic stroke lesions evolution: a pilot study derived from longitudinal MR patient data

The spatiotemporal evolution of stroke lesions, from acute injury to final tissue damage, is complex. Diffusion-weighted (DWI) and perfusion-weighted (PWI) imaging is commonly used to detect early ischemic changes and attempts to distinguish between permanently damaged and salvageable tissues. To date, 2D and 3D measures of diffusion/perfusion regions at individual timepoints have been widely used but may underestimate the true lesion spatio-temporal dynamics. Currently there is no spatio-temporal 4D dynamic model that simulates the continuous evolution of ischemic stroke from MR images. We determined whether a 4D current-based diffeomorphic model, developed in the field of statistical modeling for measuring the variability of anatomical surfaces, could estimate patient-specific spatio-temporal continuous evolution for MR PWI (measured as mean transit time, (MTT)) and DWI lesions. In our representative pilot sample, the model fitted the data well. Our dynamic analysis of lesion evolution showed different patterns; for example, some DWI/PWI dynamic changes corresponded with DWI lesion expansion into PWI lesions, but other patterns were much more complex and diverse. There was wide variation in the time when the final tissue damage was reached after stroke for DWI and MTT.

Early identification of potentially salvageable tissue with MRI-based predictive algorithms after experimental ischemic stroke

Individualized stroke treatment decisions can be improved by accurate identification of the extent of salvageable tissue. Magnetic resonance imaging (MRI)-based approaches, including measurement of a 'perfusion-diffusion mismatch' and calculation of infarction probability, allow assessment of tissue-at-risk; however, the ability to explicitly depict potentially salvageable tissue remains uncertain. In this study, five predictive algorithms (generalized linear model (GLM), generalized additive model, support vector machine, adaptive boosting, and random forest) were tested in their potency to depict acute cerebral ischemic tissue that can recover after reperfusion. Acute T2-, diffusion-, and perfusion-weighted MRI, and follow-up T2 maps were collected from rats subjected to right-sided middle cerebral artery occlusion without subsequent reperfusion, for training of algorithms (Group I), and with spontaneous (Group II) or thrombolysis-induced reperfusion (Group III), to determine infarction probability-based viability thresholds and prediction accuracies. The infarction probability difference between irreversible-i.e., infarcted after reperfusion-and salvageable tissue injury-i.e., noninfarcted after reperfusion-was largest for GLM (20±7%) with highest accuracy of risk-based identification of acutely ischemic tissue that could recover on subsequent reperfusion (Dice's similarity index=0.79±0.14). Our study shows that assessment of the heterogeneity of infarction probability with MRI-based algorithms enables estimation of the extent of potentially salvageable tissue after acute ischemic stroke.

Real-time ultrasound perfusion imaging in acute stroke: assessment of cerebral perfusion deficits related to arterial recanalization

We investigated whether real-time ultrasound perfusion imaging (rt-UPI) is able to detect perfusion changes related to arterial recanalization in the acute phase of middle cerebral artery (MCA) stroke. Twenty-four patients with acute territorial MCA stroke were examined with rt-UPI and transcranial color-coded duplex ultrasound (TCCD). Ultrasound studies were consecutively performed within 24 h and 72-96 h after stroke onset. Real-time UPI parameters of bolus kinetics (time to peak, rt-TTP) and of refill kinetics (plateau A and slope β of the exponential replenishment curve) were calculated from regions of interest of ischemic versus normal brain tissue; these parameters were compared between early and follow-up examinations in patients who recanalized. At the early examination, there was a delay of rt-TTP in patients with MCA occlusion (rt-TTP [s]: 13.09 ± 3.21 vs. 10.16 ± 2.6; p = 0.01) and a lower value of the refill parameter β (β [1/s]: 0.62 ± 0.34 vs. 1.09 ± 0.58; p = 0.01) in ischemic compared with normal brain tissue, whereas there were no differences of the parameters A and Axβ. At follow-up, the delay of rt-TTP was reversible once recanalization of an underlying MCA obstruction was demonstrated: rt-TTP [s], 13.09 ± 3.21 at 24 h versus 10.95 ± 1.5 at 72-96 h (p = 0.03). Correspondingly, β showed a higher slope than at the first examination: β [1/s]: 0.55 ± 0.29 at 24 h versus 0.71 ± 0.27 at 72-96 h (p = 0.04). We conclude that real-time UPI can detect hemodynamic impairment in acute MCA occlusion and subsequent improvement following arterial recanalization. This offers the chance for bedside monitoring of the hemodynamic compromise (e.g. during therapeutic interventions such as systemic thrombolysis).

Magnetic resonance imaging of perfusion-diffusion mismatch in rodent and non-human primate stroke models

Stroke is a leading cause of death and long-term disability. Non-invasive magnetic resonance imaging (MRI) has been widely used for the early detection of ischemic stroke and the longitudinal monitoring of novel treatment strategies. Recent advances in MRI techniques have enabled improved sensitivity and specificity to detecting ischemic brain injury and monitoring functional recovery. This review describes recent progresses in the development and application of multimodal MRI and image analysis techniques to study experimental stroke in rats and non-human primates.

Tissue at risk in acute stroke patients treated beyond 8 h after symptom onset

INTRODUCTION

The decision on thrombolytics administration is usually based on a generalized, rigid time-based rule rather than an individualized evaluation of the "tissue at risk of infarction" which is the target of the recanalization therapies. The goals of our article are to assess whether there is tissue at risk of infarction in a group of acute stroke patients treated beyond 8 h after symptom onset and to investigate the baseline imaging and clinical features that predict the fate of this tissue at risk.

METHODS

We retrospectively reviewed a series of patients with acute ischemic stroke treated with endovascular recanalization therapies beyond 8 h after symptom onset. The tissue at risk was calculated as the difference between the infarct volumes on baseline and follow-up imaging (infarct growth). We analyzed the epidemiological distribution of infarct growth, and we performed a multivariate regression analysis to identify the baseline variables that predict infarct growth.

RESULTS

Our study group included 75 patients (65 ± 13.8 years, baseline National Institutes of Health Stroke Scale 14 ± 4.9, time to treatment 15.2 ± 8.7 h). The mean infarct growth was 78.6 ± 95.0 cc (p < 0.001), and, overall, the infarct growth was greater when the baseline volume of infarct tissue was small (p < 0.001) and in the case of a unsuccessful arterial recanalization (p = 0.001).

CONCLUSIONS

There is potentially salvageable ischemic tissue at risk in acute stroke patients treated beyond 8 h after symptom onset.

Targeting gaseous molecules to protect against cerebral ischaemic injury: mechanisms and prospects

1. Ischaemic brain injury is a leading cause of death and disability in many countries. However, the pathological mechanisms underlying ischaemic brain injury, including oxidative stress, calcium overload, excitotoxicity and neuronal apoptosis, are perplexing and this makes it difficult to find effective novel drugs for the treatment of the condition. 2. Recently, gaseous molecules such as nitric oxide (NO), carbon monoxide (CO), hydrogen sulphide (H(2)S) and hydrogen (H(2)) have attracted considerable interest because of their physiological and pathophysiological roles in various body systems. Emerging evidence indicates that gaseous molecules are involved in the pathological processes of ischaemic brain damage. 3. In the present review, we summarize evidence regarding the involvement of gaseous molecules in ischaemic brain injury and discuss the therapeutic potential of targeting gaseous molecules. 4. Collectively, the available data suggest that the application of these biological gas molecules and their pharmacological regulators may be a potential therapeutic approach for the treatment of ischaemic brain injury.

Serial MR analysis of early permanent and transient ischemia in rats: diffusion tensor imaging and high b value diffusion weighted imaging

Objective

To evaluate the temporal evolution and diagnostic values of the diffusion tensor imaging (DTI) and the high b value diffusion weighted imaging (DWI) in the early permanent and transient cerebral ischemia.

Materials and Methods

For permanent or 30-minute transient-ischemia induced 30 rats, DTI and DWIs at both high b (b = 3000 s/mm²) and standard b value (b = 1000 s/mm²) were obtained at the following conditions: at 15, 30, 45, 60 minutes after the occlusion of what for hyperacute permanent ischemia; at 1, 3, 5, 7, 9 hours after the occlusion for acute permanent ischemia; and at 15 minutes before reperfusion, 0.5, 2.5, and 24 hours after reperfusion for transient ischemia. The diffusion parameters and their ratios were obtained and compared between different b values, and among different time points and groups, respectively.

Results

For both b values, the apparent diffusion coefficient (ADC) ratio decreased for first three hours, and then slightly increased until 9 hours after the occlusion during a gradual continuous increase of DWI signal intensity (SI) ratio, with excellent correlation between ADC ratios and DWI SI ratios. The DWI showed a higher contrast ratio, but the ADC map showed a lower contrast ratio for permanent ischemia at high b value than at standard b value. Fractional anisotropy (FA) increased for 1 hour, then gradually decreased until 9 hours after the occlusion in permanent ischemia and showed transient normalization and secondary decay along with change in ADC in transient ischemia.

Conclusion

This study presents characteristic initial elevation and secondary decay of FA, higher contrast ratio of DWI, and lower contrast ratio of ADC map at high b value, in addition to the time evolutions of diffusion parameters in early permanent and transient ischemia.

In vivo imaging of induction of heat-shock protein-70 gene expression with fluorescence reflectance imaging and intravital confocal microscopy following brain ischaemia in reporter mice

PURPOSE

Stroke induces strong expression of the 72-kDa heat-shock protein (HSP-70) in the ischaemic brain, and neuronal expression of HSP-70 is associated with the ischaemic penumbra. The aim of this study was to image induction of Hsp-70 gene expression in vivo after brain ischaemia using reporter mice.

METHODS

A genomic DNA sequence of the Hspa1b promoter was used to generate an Hsp70-mPlum far-red fluorescence reporter vector. The construct was tested in cellular systems (NIH3T3 mouse fibroblast cell line) by transient transfection and examining mPlum and Hsp-70 induction under a challenge. After construct validation, mPlum transgenic mice were generated. Focal brain ischaemia was induced by transient intraluminal occlusion of the middle cerebral artery and the mice were imaged in vivo with fluorescence reflectance imaging (FRI) with an intact skull, and with confocal microscopy after opening a cranial window.

RESULTS

Cells transfected with the Hsp70-mPlum construct showed mPlum fluorescence after stimulation. One day after induction of ischaemia, reporter mice showed a FRI signal located in the HSP-70-positive zone within the ipsilateral hemisphere, as validated by immunohistochemistry. Live confocal microscopy allowed brain tissue to be visualized at the cellular level. mPlum fluorescence was observed in vivo in the ipsilateral cortex 1 day after induction of ischaemia in neurons, where it is compatible with penumbra and neuronal viability, and in blood vessels in the core of the infarction.

CONCLUSION

This study showed in vivo induction of Hsp-70 gene expression in ischaemic brain using reporter mice. The fluorescence signal showed in vivo the induction of Hsp-70 in penumbra neurons and in the vasculature within the ischaemic core.

Simultaneous and noninvasive imaging of cerebral oxygen metabolic rate, blood flow and oxygen extraction fraction in stroke mice

Many brain diseases have been linked to abnormal oxygen metabolism and blood perfusion; nevertheless, there is still a lack of robust diagnostic tools for directly imaging cerebral metabolic rate of oxygen (CMRO(2)) and cerebral blood flow (CBF), as well as the oxygen extraction fraction (OEF) that reflects the balance between CMRO(2) and CBF. This study employed the recently developed in vivo (17)O MR spectroscopic imaging to simultaneously assess CMRO(2), CBF and OEF in the brain using a preclinical middle cerebral arterial occlusion mouse model with a brief inhalation of (17)O-labeled oxygen gas. The results demonstrated high sensitivity and reliability of the noninvasive (17)O-MR approach for rapidly imaging CMRO(2), CBF and OEF abnormalities in the ischemic cortex of the MCAO mouse brain. It was found that in the ischemic brain regions both CMRO(2) and CBF were substantially lower than that of intact brain regions, even for the mildly damaged brain regions that were unable to be clearly identified by the conventional MRI. In contrast, OEF was higher in the MCAO affected brain regions. This study demonstrates a promising (17)O MRI technique for imaging abnormal oxygen metabolism and perfusion in the diseased brain regions. This (17)O MRI technique is advantageous because of its robustness, simplicity, noninvasiveness and reliability: features that are essential to potentially translate it to human patients for early diagnosis and monitoring of treatment efficacy.

Thrombolytic therapy for acute ischaemic stroke: what can we do to improve outcomes?

Constant efforts are being made in the stroke community to aim for maximum benefit from thrombolytic therapy since the approval of intravenous recombinant tissue plasminogen activator (rt-PA; alteplase) for the management of acute ischaemic stroke. However, fear of symptomatic haemorrhage secondary to thrombolytic therapy has been a major concern for treating physicians. Certain imaging and clinical variables may help guide the clinician towards better treatment decision making. Aggressive management of some predictive variables that have been shown to be surrogate outcome measures has been related to better clinical outcomes. Achieving faster, safer and complete recanalization with evolving endovascular techniques is routinely practiced to achieve better clinical outcomes. Selection of an 'ideal candidate' for thrombolysis can maximize functional outcomes in these patients. Although speed and safety are the key factors in acute management of stroke patients, there must also be a systematic and organized pattern to assist the stroke physician in making decisions to select the 'ideal candidate' for treatment to maximize results.

Multiparametric MRI and CT models of infarct core and favorable penumbral imaging patterns in acute ischemic stroke

BACKGROUND AND PURPOSE

Objective imaging methods to identify optimal candidates for late recanalization therapies are needed. The study goals were (1) to develop magnetic resonance imaging (MRI) and computed tomography (CT) multiparametric, voxel-based predictive models of infarct core and penumbra in acute ischemic stroke patients, and (2) to develop patient-level imaging criteria for favorable penumbral pattern based on good clinical outcome in response to successful recanalization.

METHODS

An analysis of imaging and clinical data was performed on 2 cohorts of patients (one screened with CT, the other with MRI) who underwent successful treatment for large vessel, anterior circulation stroke. Subjects were divided 2:1 into derivation and validation cohorts. Pretreatment imaging parameters independently predicting final tissue infarct and final clinical outcome were identified.

RESULTS

The MRI and CT models were developed and validated from 34 and 32 patients, using 943 320 and 1 236 917 voxels, respectively. The derivation MRI and 2-branch CT models had an overall accuracy of 74% and 80%, respectively, and were independently validated with an accuracy of 71% and 79%, respectively. The imaging criteria of (1) predicted infarct core ≤90 mL and (2) ratio of predicted infarct tissue within the at-risk region ≤70% identified patients as having a favorable penumbral pattern with 78% to 100% accuracy.

CONCLUSIONS

Multiparametric voxel-based MRI and CT models were developed to predict the extent of infarct core and overall penumbral pattern status in patients with acute ischemic stroke who may be candidates for late recanalization therapies. These models provide an alternative approach to mismatch in predicting ultimate tissue fate.

Apparent diffusion coefficient thresholds and diffusion lesion volume in acute stroke

BACKGROUND

Apparent diffusion coefficient (ADC) thresholds are used to determine acute stroke lesion volume, but the reliability of this approach and comparability to the volume of the magnetic resonance diffusion-weighted imaging (MR-DWI) hyperintense lesion is unclear.

METHODS

We prospectively recruited and clinically assessed patients who had experienced acute ischemic stroke and performed DWI less than 24 hours and at 3 to 7 days after stroke. We compared the volume of the manually outlined DW hyperintense lesion (reference standard) with lesion volumes derived from 3 commonly used ADC thresholds: .55 × 10(-3)/mm(2)/second(-1), .65 × 10(-3)/mm(2)/second(-1), and .75 × 10(-3)/mm(2)/second(-1), with and without "editing" of erroneous tissue. We compared the volumes obtained by reference standard, "raw," and "edited" thresholds.

RESULTS

Among 33 representative patients, the acute DWI lesion volume was 15,284 mm(3); the median unedited/edited ADC volumes were 52,972/2786 mm(3), 92,707/6,987 mm(3), and 227,681/unmeasureable mm(3) (.55 × 10(-3)/mm(2)/second(-1), .65 × 10(-3)/mm(2)/second(-1), and .75 × 10(-3)/mm(2)/second(-1) thresholds, respectively). Subacute lesions gave similar differences. These differences between edited and unedited diffusion-weighted imaging and ADC volumes were statistically significant.

CONCLUSIONS

Threshold-derived ADC volumes require substantial manual editing to avoid over- or underestimating the visible DWI lesion and should be used with caution.

Clinical presentation and ischemic zone on MRI in cancer patients with acute ischemic stroke

AIMS

This study was conducted to evaluate the clinical and MRI profiles in acute cancer strokes, and to demonstrate our experience with thrombolytic therapy in cancer stroke patients.

METHODS

We prospectively studied active cancer patients with acute ischemic stroke who underwent MRI within 48 h of the onset of symptoms. Patients were grouped based on the presence of conventional stroke mechanisms (CSM). Clinical characteristics and MRI profiles were evaluated.

RESULTS

A total of 70 patients were finally included in this study. Patients without CSM were more frequently presented with encephalopathy than those with CSM (29.4 vs. 2.8%, p = 0.002). The diffusion-perfusion mismatch pattern was more prevalent in patients with CSM (21 patients, 58.3%) than in patients without CSM (8 patients, 23.5%). Patients who had a higher tertiles of D-dimer level were significantly less likely to have the diffusion-perfusion mismatch pattern (p = 0.015). Among patients who presented within 6 h of the onset of stroke, revascularization therapy was performed in 4 of 16 (25%) patients with CSM, but none of the patients without CSM.

CONCLUSION

Based on the stroke mechanisms, the optimal strategy of thrombolytic therapy should be considered differently in cancer patients with acute ischemic stroke.

Selection of possible responders to thrombolytic therapy in acute ischemic stroke

Ischemic stroke is one of the leading causes of death and morbidity worldwide, and systemic thrombolytic treatment is still the first-line therapy within 4.5 h from symptom onset. Selecting patients for treatment response is mandatory in any time window but challenging as well. The authors aim to support stroke physicians in their individual decision making. Besides evidence from clinical trials, some suggestions included here exclusively reflect the authors' opinions. This article presents clinical and imaging criteria of selecting patients reasonably, offering causal therapy to a growing number of patients.

Design and rationale of the Mechanical Retrieval and Recanalization of Stroke Clots Using Embolectomy (MR RESCUE) Trial

RATIONALE

Multimodal imaging has the potential to identify acute ischaemic stroke patients most likely to benefit from late recanalization therapies.

AIMS

The general aim of the Mechanical Retrieval and Recanalization of Stroke Clots Using Embolectomy Trial is to investigate whether multimodal imaging can identify patients who will benefit substantially from mechanical embolectomy for the treatment of acute ischaemic stroke up to eight-hours from symptom onset.

DESIGN

Mechanical Retrieval and Recanalization of Stroke Clots Using Embolectomy is a randomized, controlled, blinded-outcome clinical trial.

POPULATION STUDIED

Acute ischaemic stroke patients with large vessel intracranial internal carotid artery or middle cerebral artery M1 or M2 occlusion enrolled within eight-hours of symptom onset are eligible. The study sample size is 120 patients.

STUDY INTERVENTION

Patients are randomized to endovascular embolectomy employing the Merci Retriever (Concentric Medical, Mountain View, CA) or the Penumbra System (Penumbra, Alameda, CA) vs. standard medical care, with randomization stratified by penumbral pattern.

OUTCOMES

The primary aim of the trial is to test the hypothesis that the presence of substantial ischaemic penumbral tissue visualized on multimodal imaging (magnetic resonance imaging or computed tomography) predicts patients most likely to respond to mechanical embolectomy for treatment of acute ischaemic stroke due to a large vessel, intracranial occlusion up to eight-hours from symptom onset. This hypothesis will be tested by analysing whether pretreatment imaging pattern has a significant interaction with treatment as a determinant of functional outcome based on the distribution of scores on the modified Rankin Scale measure of global disability assessed 90 days post-stroke. Nested hypotheses test for (1) treatment efficacy in patients with a penumbral pattern pretreatment, and (2) absence of treatment benefit (equivalency) in patients without a penumbral pattern pretreatment. An additional aim will only be tested if the primary hypothesis of an interaction is negative: that patients treated with mechanical embolectomy have improved functional outcome vs. standard medical management.

Imaging brain oxygenation with MRI using blood oxygenation approaches: methods, validation, and clinical applications

SUMMARY

In many pathophysiologic situations, including brain neoplasms, neurodegenerative disease, and chronic and acute ischemia, an imbalance exists between oxygen tissue consumption and delivery. Furthermore, oxygenation changes following a stress challenge, such as with carbogen gas or acetazolamide, can yield information about cerebrovascular reactivity. The unique sensitivity of the BOLD effect to the presence of deoxyhemoglobin has led to its widespread use in the field of cognitive neurosciences. However, the high spatial and temporal resolution afforded by BOLD imaging does not need to be limited to the study of healthy brains. While the complex relationship between the MR imaging signal and tissue oxygenation hinders a direct approach, many different methods have been developed during the past decade to obtain specific oxygenation measurements. These include qBOLD, phase- and susceptibility-based imaging, and intravascular T2-based approaches. The aim of this review is to give an overview of the theoretic basis of these methods as well as their application to measure oxygenation in both healthy subjects and those with disease.

How to repair an ischemic brain injury? Value of experimental models in search of answers

The major aim of experimental models of cerebral ischemia is to study the cerebral ischemic damage under controlled and reproducible conditions. Experimental studies have been fundamental in the establishment of new concepts regarding the mechanisms underlying the ischemic brain injury, such as the ischemic penumbra, the reperfusion injury, the cell death or the importance of the damage induced on mitochondria, glial cells and white matter. Disagreement between experimental and clinical studies regarding the benefit of drugs to reduce or restore the cerebral ischemic damage has created a growing controversy about the clinical value of the experimental models of cerebral ischemia. One of the major explanations for the failure of the clinical trials is the reductionist approach of most therapies, which are focused on the known effect of a single molecule within a specific pathway of ischemic damage. This philosophy contrasts to the complex morphological design of the cerebral tissue and the complex cellular and molecular physiopathology underlying the ischemic brain injury. We believe that the main objective of studies carried out in experimental models of cerebral ischemic injury must be a better understanding of the fundamental mechanisms underlying progression of the ischemic injury. Clinical trials should not be considered if the benefit obtained in experimental studies is limited or weak.

Refining the mismatch concept in acute stroke: lessons learned from PET and MRI

In ischemic stroke, positron-emission tomography (PET) established the imaging-based concept of penumbra. It defines hypoperfused, but functionally impaired, tissue with preserved viability that can be rescued by timely reperfusion. Diffusion-weighted and perfusion-weighted (PW) magnetic resonance imaging (MRI) translated the concept of penumbra to the concept of mismatch. However, the use of mismatch-based patient stratification for reperfusion therapy remains a matter of debate. The equivalence of mismatch and penumbra, as well as the validity of the classical mismatch concept is questioned for several reasons. First, methodological differences between PET and MRI lead to different definitions of the tissue at risk. Second, the mismatch concept is still poorly standardized among imaging facilities causing relevant variability in stroke research. Third, relevant conceptual issues (e.g., the choice of the adequate perfusion measure, the best quantitative approach to perfusion maps, and the required size of the mismatch) need further refinement. Fourth, the use of single thresholds does not account for the physiological heterogeneity of the penumbra and probabilistic approaches may be more promising. The implementation of this current knowledge into an optimized state-of-the-art mismatch model and its validation in clinical stroke studies remains a major challenge for future stroke research.

The two pathophysiologies of focal brain ischemia: implications for translational stroke research

Brain injury after focal ischemia evolves along two basically different pathophysiologies, depending on the severity of the primary flow reduction and the dynamics of postischemic recirculation. In permanent and gradually reversed focal ischemia as after thromboembolic occlusion, primary core injury is irreversible but the expansion of the core into the penumbra can be alleviated by hemodynamic and molecular interventions. Such alleviation can only be achieved within 3 hours after the onset of ischemia because untreated core injury expands to near maximum size during this interval. In promptly reversed transient ischemia as after mechanical vascular occlusion, primary core injury may recover but a secondary delayed injury evolves after a free interval of as long as 6 to 12 hours. This injury can be alleviated throughout the free interval but the longer window is without clinical relevance because transient mechanical vascular occlusion is not a model of naturally occurring stroke. As this difference is widely ignored in stroke research, most clinical trials have been designed with a far too long therapeutic window, which explains their failure. Transient mechanical vascular occlusion models should, therefore, be eliminated from the repertoire of preclinical stroke research.

Current acute stroke trials and their potential impact on the therapeutic time window

Several trials in acute stroke are underway or have been completed recently. Among the latter, ECASS 3 was a milestone regarding the extension of the rigid 3-h time window out to 4.5 h for intravenous thrombolysis with recombinant tissue plasminogen activator. Several other approaches are being tested for thrombolytic therapy, among them modern imaging-based patient selection of patients and interventional approaches. Other pharmaceutical strategies include neuroprotection, and restoration, biophysical approaches, such as near infrared laser therapy, hemodynamic augmentation, and sphenopalatine ganglion stimulation. This perspective will cover the recently completed and currently recruiting acute stroke trials with respect to their potential role in expanding the therapeutic time window for acute ischemic stroke.

Incorporating ADC temporal profiles to predict ischemic tissue fate in acute stroke

Algorithms to predict ischemic tissue fate based on acute stroke MRI typically utilized data at a single time point. The goal of this study was to investigate the potential improvement in prediction accuracy when incorporating MRI diffusion data from multiple time points during acute phase to improve prediction accuracy. This study was carried out using MRI data from rats subjected to permanent, 60-min and 30-min of middle cerebral artery occlusion (MCAO). The sensitivity and specificity of prediction accuracy were calculated. In the permanent MCAO group, prediction with multiple time-point diffusion data improved sensitivity and specificity compared with prediction using a single time point. In the 60-min MCAO group, multiple time-point analysis improved specificity but decreased sensitivity compared to the single time-point analysis. In the 30-min MCAO group, multiple time-point analysis showed no statistically significant improvement in specificity and sensitivity compared with the single time point analysis. This is because reperfusion transiently or permanently reversed the decline in ADC values, resulting in increased uncertainty and thus decreased prediction accuracy. Incorporating this a priori information could further improve prediction accuracy in the reperfusion group. These findings suggest that incorporating MRI data from multiple time points could improve prediction accuracy under certain ischemic conditions.

CT Density Changes with Rapid Onset Acute, Severe, Focal Cerebral Ischemia in Monkeys

Computerized tomography (CT) is the most often used imaging modality in the evaluation of acute clinical stroke. However, the rapidity with which CT density changes occur after acute, severe, focal ischemia cannot be determined clinically. Even if the time of symptom onset is known, clinical stroke severity is highly variable. We studied the time course of CT density change after severe, rapid onset, acute, focal ischemia as documented by stable xenon CT cerebral blood flow (CBF) in monkeys. Eight monkeys (Macaca mulatta) were subjected to transorbital occlusion of the left posterior cerebral, anterior, middle, and internal carotid arteries to induce focal ischemia. CT density Hounsfield units (HU), CBF by stable xenon CT, arterial blood pressure, and blood gases were measured before occlusion, immediately after occlusion, at 30 min, and hourly for up to 6 h. Occlusion of the cerebral arteries decreased CBF to 8 ± 5 ml/100 g/ min within 15 min postocclusion. At 6 h, CBF was unchanged at 9 ± 4 ml/100 g/ min. CT density within the ischemic core fell from 42 to 38 HU immediately after occlusion (P < 0.05), rose transiently, then fell at 2 h (P < 0.01) and plateaued at 36 ± 5 HU for a total decrease of 4-5 HU between 4 and 6 h poststroke. Changes in CT density lag severe focal ischemia by 2 h. Thus, when CT hypodensity is seen in acute stroke, it is likely 2 h old. It also provides an explanation for the phenomenon of clinical CT mismatch with clinical deficits and normal CT.

Evolving role of biomarkers in acute cerebrovascular disease

The development of a clinically validated biomarker of acute cerebral ischemia would have the potential to facilitate the use of time-sensitive reperfusion strategies, allow for individualization of patient care by predicting relative risk of hemorrhage and volume of penumbral tissue, and add valuable prognostic information for patients presenting with acute stroke. Additionally, a stroke biomarker might benefit early stage clinical research by serving as a surrogate measure of ischemic injury. Although at present there are no clinically validated biomarkers of acute stroke, previous studies have focused on markers associated with different components of the ischemic cascade, including microglial activation, inflammation, oxidative stress, neuronal injury, hemostasis, and endothelial dysfunction. Evolving technologies have provided high throughput approaches to investigate potential gene and protein signatures, and methods to measure newly discovered markers of cell death and immune responses. Prior to defining the clinical utility of stroke biomarkers, it is critical to understand the inherent limitations of a biomarker-based approach and define its potential value for providing adjunctive diagnostic and prognostic information. The identification and validation of a clinically relevant biomarker, or panel of markers, of stroke will ultimately require incorporation of both stringent research design and assessment in the clinical context in which the marker will be used.

Arterial spin labeling for acute stroke: practical considerations

Arterial spin labeling (ASL) is a non-contrast method of measuring cerebral perfusion with MRI. It has several advantages over traditional contrast-based perfusion-weighted imaging, including non-invasiveness, more straightforward cerebral blood flow (CBF) quantification, and repeatability. However, because of its lower signal-to-noise ratio (SNR) per unit time and its high sensitivity to arterial transit delays, it has not been used frequently in acute stroke, where arterial delays and time-efficiency are of the essence. This is beginning to change, driven by higher SNR implementations of ASL and the increasing use of 3T scanners. Furthermore, velocity-selective ASL sequences that are largely insensitive to arrival times are beginning to be applied to patients with cerebrovascular disease and promise the ability to quantify CBF even in regions supplied by late-arriving collateral flow. Despite these advances, many practical issues must be addressed to optimize ASL for its use in acute stroke studies. These include optimizing the trade-off between time, SNR, imaging resolution, and sensitivity to slow flow. Rapid and robust post-processing of image data must be made routine, such that CBF maps are available in real time so that they can be considered when making treatment decisions. Lastly, automated software needs to be developed in order to delineate hypoperfused tissue volumes, which is challenging due to the inherent differences between gray and white matter CBF. Attention to these details is critical to translate this promising research tool into mainstream clinical trials and practice in acute stroke.

Elevated T2-values in MRI of stroke patients shortly after symptom onset do not predict irreversible tissue infarction

Distinct from signal alterations in diffusion-weighted images, T(2)-values are also dependent on tissue water content and known to increase with time from symptom onset in acute ischaemic stroke. The purpose of this study was to evaluate whether there is a detectable increase of T(2)-values in different regions in acute ischaemic stroke in the acute and subacute situation and to study the effect of recanalization on the evaluation of T(2)-values in the subacute phase. In addition, we sought to evaluate whether this increase in T(2)-values is reversible. For this purpose, 22 patients with acute ischaemic stroke in the territory of the middle cerebral artery underwent magnetic resonance imaging including diffusion-weighted imaging, perfusion-weighted imaging, fluid-attenuated inversion recovery to determine final infarct size, time-of-flight-angiography (acute and on day 1 or 2) and a triple echo-T(2)-sequence (calculation of T(2) maps) within 6 h after symptom onset. Images were co-registered and regions of diffusion restriction and prolonged time-to-peak as well as surviving tissue (surviving tissue = time-to-peak - final infarct size) and lesion growth (lesion growth = final infarct size-diffusion restriction) were defined and superimposed onto the quantitative T(2) map. In addition, patients were dichotomized according to recanalization information. Mean quantitative T(2)-values were derived for each patient within each region of interest. Mean T(2)-values for patients with recanalization (n = 15) in surviving tissue region of interest were 115.8 ± 7.2 ms (mean ± SD) and in the lesion growth region of interest 114.6 ± 7.0 ms. T(2)-values for patients without recanalization (n = 7) were 117.7 ± 11.4 ms in surviving tissue region of interest and 117.3 ± 12.1 ms in lesion growth region of interest. There was no significant difference between T(2)-values measured in lesion growth and surviving tissue region of interest for patients with or without recanalization. Even though it has been shown that T(2)-values increase with time from symptom onset within the infarct core, increased T(2)-values in areas of perfusion impairment do not identify irreversible damaged brain tissue and high T(2)-values are even found in tissue that is not part of the final infarct lesion and can therefore normalize. In conclusion, this study suggests that T(2)-values are not a valid imaging biomarker in acute stroke to predict tissue outcome.

Visual assessment of magnetic resonance imaging perfusion lesions in a large patient group

PURPOSE

Few magnetic resonance imaging (MRI) studies of stroke have evaluated the value of visual assessment of perfusion/diffusion mismatch, which is crucial for routine application. In this study an attempt was made to visually assess perfusion lesions resembling the acute clinical situation and identify parameters with the highest interobserver reliability when used to define a perfusion/diffusion mismatch and the highest accuracy for prediction of infarct growth.

METHODS

Magnetic resonance imaging was performed within 6 h of symptom onset and again 1-11 days thereafter in 86 consecutive stroke patients who received intravenous thrombolytic therapy. The MRI protocol included diffusion-weighted imaging apparent diffusion coefficient (DWI/ADC), fluid-attenuated inversion recovery (FLAIR) and perfusion imaging (PI). Maps for different perfusion parameters, e.g. cerebral blood volume (CBV), cerebral blood flow (CBF), mean transit time (MTT) and time to peak (TTP) were calculated. Areas of perfusion deficits of all perfusion parameters were visually compared to corresponding ADCs and final infarct size by two independent observers.

RESULTS

The final infarct size was overestimated by TTP (in 81/83 patients by raters 1 and 2, respectively), MTT (82/83) and CBF (65/74) lesions. The ADC lesions were rated smaller than the final infarct size in 43/38 cases by raters 1 and 2 and the CBV decrease was rated to underestimate final infarct size in 40/31 cases. The only significantly increased OR of 3.883 (95 % CI 1.466-10.819, p = 0.004, rater 1)/5.142 (95 % CI 1.828-15.142, p = 0.001, rater 2) for predicting infarct growth was observed for the presence of a CBV > ADC mismatch, which also showed the highest kappa value of 0.407.

CONCLUSIONS

All mismatch patterns were prone to high interrater variability when assessed under conditions resembling the clinical setting. Of all tested mismatch patterns the CBV > ADC mismatch was the strongest predictor of lesion growth while visual assessment of TTP and CBF generally resulted in an overestimation of infarct sizes and the presence of a TTP > ADC or CBF > ADC mismatch was not significantly predictive for lesion growth. Visual inspection of these most commonly used mismatch patterns has a low value for the prediction of infarct growth and thus the estimation of the penumbra in ischemic stroke patients.

Controversies in the neurosurgical management of cerebellar hemorrhage and infarction

Evidence-based guidelines for the management of hemorrhagic and ischemic cerebellar stroke are sparse, and most available data come from Class III studies. As a result, opinions and practices regarding the nature and role of neurosurgical intervention vary widely. A comprehensive literature review was conducted to adjudicate several contentious issues, such as the difference in the management of cerebellar hemorrhage versus infarction, criteria for imaging to exclude an underlying structural lesion, the value of MRI for patient selection, the role of external ventricular drainage, the indications for operative management, the timing of surgical intervention, and various options of surgical technique, among others. Treatment algorithms proposed in several different studies are compared and contrasted. This analysis is concluded by a summary of the recommendations from the American Stroke Association, which advises that patients with cerebellar hemorrhage who experience neurological deterioration or who have brainstem compression and/or hydrocephalus due to ventricular obstruction should undergo surgical evacuation of the hemorrhage as soon as possible, and that initial treatment of such patients with ventricular drainage alone rather than surgical removal of the hemorrhage is not recommended.

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.

Spatiotemporal dynamics of diffusional kurtosis, mean diffusivity and perfusion changes in experimental stroke

Diffusional kurtosis imaging (DKI), which measures the non-Gaussianity of water diffusion, has been demonstrated to be a sensitive biomarker in many neuropathologies. The goal of this study was to longitudinally examine the spatiotemporal dynamics of DKI in cerebral ischemia in an animal model of permanent and transient (45 min) middle cerebral artery occlusion (MCAO) during the hyperacute, acute and chronic phases. Diffusional kurtosis showed different spatiotemporal dynamics. In particular, mean kurtosis (MK) was sensitive to hyperacute and acute stroke changes, and exhibited different contrast than mean diffusivity (MD) and higher contrast than fractional anisotropy (FA) and T2. MK contrast persisted 1 to 7 days post-occlusion, whereas MD showed renormalization at day 1-2 and reversed contrast at day 7. The current study showed that DKI has the potential to complement existing stroke imaging techniques, particularly in the assessment of subacute to early chronic stroke evolution.

Brain and vascular imaging of acute stroke

Contemporary imaging technologies permit the rapid and accurate assessment of the acute stroke patient. These studies form the underpinning of all therapeutic approaches. Although unenhanced computed tomography remains the principal diagnostic examination to exclude hemorrhagic stroke, multimodal computed tomography and magnetic resonance imaging can be use to assess cerebral perfusion and may reveal the ischemic penumbra, thus leading to better patient selection for intravenous or intra-arterial reperfusion strategies.

Current diagnosis and management of symptomatic intracranial atherosclerotic disease

PURPOSE OF REVIEW

Intracranial atherosclerotic disease (IAD) is likely the most common cause of stroke world-wide and is associated with a very high risk of recurrence. It results in cerebral ischemia due to a variety of mechanisms, including artery-to-artery embolism, hemodynamic failure, and occlusion of penetrating arteries. New imaging modalities focused on physiological consequences of IAD have become available and recent treatment trials have been completed.

RECENT FINDINGS

We review the traditional imaging modalities, emphasizing the advantages and limitations of each method, and discuss the novel physiological approaches that interrogate physiological process to indicate specific mechanisms of ischemia. These allow deeper understanding of the pathophysiological processes that underlie IAD-related ischemia. The key findings of recent therapeutic trials are reviewed, including the landmark randomized studies showing advantage of antiplatelet agents and risk factor modification, and a significant risk of complications with endovascular approaches.

SUMMARY

Current evidence argues for aggressive medical management and suggests caution with interventional treatments. We propose that mechanistic information will further refine the risk assessment of patients with IAD to offer targeted therapy.