Pathophysiological topography of acute ischemia by combined diffusion-weighted and perfusion MRI

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.

Multimodal MRI for ischemic stroke: from acute therapy to preventive strategies

Background and Purpose

Conventional therapies for ischemic stroke include thrombolytic therapy, prevention of inappropriate coagulation and thrombosis, and surgery to repair vascular abnormalities. Over 10 years have passed since the US Food and Drug Administration approved intravenous tissue plasminogen activator for use in acute stroke patients, but most major clinical trials have failed during the last 2 decades, including large clinical trials for secondary prevention and neuroprotection. These results suggest the presence of heterogeneity among stroke patients. Neuroimaging techniques now allow changes to be observed in patients from the acute to the recovery phase. The role of MRI in stroke evaluation and treatment is discussed herein.

Main Contents

Three MRI strategies are discussed with relevant examples. First, the following MRI strategies for acute ischemic stroke are presented: diffusion-perfusion mismatch, deoxygenation (oxygen extraction and cerebral metabolic rate of oxygen), and blood-brain barrier permeability derangement in selected patients for recanalization therapy. Second, multimodal MRI for identifying stroke mechanisms and the specific causes of stroke (i.e., patent foramen ovale, infective endocarditis, and nonbacterial thrombotic endocarditis) are presented, followed by MRI strategies for prevention of recurrent stroke: plaque images and flow dynamics for carotid intervention.

Expectations

The studies reviewed herein suggest that using MRI to improve the understanding of individual pathophysiologies will further promote the development of rational stroke therapies tailored to the specifics of each case.

A practical assessment of magnetic resonance diffusion-perfusion mismatch in acute stroke: observer variation and outcome

MR diffusion/perfusion mismatch may help identify patients for acute stroke treatment, but mixed results from clinical trials suggest that further evaluation of the mismatch concept is required. To work effectively, mismatch should predict prognosis on arrival at hospital. We assessed mismatch duration and associations with functional outcome in acute stroke. We recruited consecutive patients with acute stroke, recorded baseline clinical variables, performed MR diffusion and perfusion imaging and assessed 3-month functional outcome. We assessed practicalities, agreement between mismatch on mean transit time (MTT) or cerebral blood flow (CBF) maps, visually and with lesion volume, and the relationship of each to functional outcome. Of 82 patients starting imaging, 14 (17%) failed perfusion imaging. Overall, 42% had mismatch (56% at <6 h; 41% at 12-24 h; 23% at 24-48 h). Agreement for mismatch by visual versus volume assessment was fair using MTT (kappa 0.59, 95% CI 0.34-0.84) but poor using CBF (kappa 0.24, 95% CI 0.01-0.48). Mismatch by either definition was not associated with functional outcome, even when the analysis was restricted to just those with mismatch. Visual estimation is a reasonable proxy for mismatch volume on MTT but not CBF. Perfusion is more difficult for acute stroke patients than diffusion imaging. Mismatch is present in many patients beyond 12 h after stroke. Mismatch alone does not distinguish patients with good and poor prognosis; both can do well or poorly. Other factors, e.g. reperfusion, may influence outcome more strongly, even in patients without mismatch.

Theoretic basis and technical implementations of CT perfusion in acute ischemic stroke, part 1: Theoretic basis

CT perfusion (CTP) is a functional imaging technique that provides important information about capillary-level hemodynamics of the brain parenchyma and is a natural complement to the strengths of unenhanced CT and CT angiography in the evaluation of acute stroke, vasospasm, and other neurovascular disorders. CTP is critical in determining the extent of irreversibly infarcted brain tissue (infarct "core") and the severely ischemic but potentially salvageable tissue ("penumbra"). This is achieved by generating parametric maps of cerebral blood flow, cerebral blood volume, and mean transit time.

Neuroimaging of ischemic stroke with CT and MRI: advancing towards physiology-based diagnosis and therapy

Acute ischemic stroke is the third leading cause of death and the major cause of significant disability in adults in the USA and Europe. The number of patients who are actually treated for acute ischemic stroke is disappointingly low, despite availability of effective treatments. A major obstacle is the short window of time following stroke in which therapies are effective. Modern imaging is able to identify the ischemic penumbra, a key concept in stroke physiology. Evidence is accumulating that identification of a penumbra enhances patient management, resulting in significantly improved outcomes. Moreover, unexpectedly large proportions of patients have a substantial ischemic penumbra beyond the traditional time window and are suitable for therapy. The widespread availability of modern MRI and computed tomography systems presents new opportunities to use physiology to guide ischemic stroke therapy in individual patients. This article suggests an evidence-based alternative to contemporary acute ischemic stroke therapy.

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

The aim of thrombolytic therapy after acute ischemic stroke is salvage of the ischemic penumbra. Several imaging techniques have been used to identify the penumbra in patients who may benefit from reperfusion beyond the currently narrow 3-hour time-window for thrombolysis. We discuss the advantages and disadvantages of positron emission tomography (PET), single photon emission computed tomography (SPECT), MRI and CT scans. We comment on concepts of clinical-imaging mismatch models and we explore the implications for clinical trials.

Perfusion CT: is it clinically useful?

Combining perfusion CT (CTP) with CT angiography (CTA) and noncontrast CT (NCCT) provides much more information about acute stroke pathophysiology than NCCT alone. This multimodal CT approach adds only a few minutes to the standard NCCT and is more accessible and rapidly available in most centres than MRI. CTP can distinguish between infarct core and penumbra, which is not possible with NCCT alone. A small infarct core and large penumbra, plus the presence of vessel occlusion on CTA may be an ideal imaging 'target' for thrombolysis. To date, multimodal CT has predominantly been assessed in hemispheric stroke due to its limited spatial coverage. This will become less of an issue as slice coverage continues to improve with new generation CT scanners. Apart from the concepts above, more specific CTP and CTA criteria that increase (or decrease) probability of response to thrombolytic treatment are yet to be determined. Nonetheless, CTP thus has the potential to improve patient selection for thrombolysis.

Selective neuronal loss in rescued penumbra relates to initial hypoperfusion

Selective neuronal loss (SNL) in the rescued penumbra could account for suboptimal clinical recovery despite effective early reperfusion. Previous studies of SNL used single-photon emission tomography (SPECT), did not account for potential volume loss secondary to collapse of the infarct cavity, and failed to show a relationship with initial hypoperfusion. Here, we obtained acute-stage computerized tomography (CT) perfusion and follow-up quantitative (11)C-flumazenil (FMZ)-PET to map SNL in the non-infarcted tissue and assess its relationship with acute-stage hypoperfusion. We prospectively recruited seven patients with evidence of (i) acute (<6 h) extensive middle cerebral artery territory ischaemia based on clinical deficit (National Institutes of Health stroke scale, NIHSS score range: 8-23) and CT Perfusion (CTp) findings and (ii) early recanalization (spontaneous or following thrombolysis) based on spectacular clinical recovery (DeltaNIHSS > or =6 at 24 h), good clinical outcome (NIHSS < or =5) and small final infarct (6/7 subcortical) on late-stage MRI. Ten age-matched controls were also studied. FMZ image analysis took into account potential post-stroke volume loss. Across patients, clusters of significantly reduced FMZ binding were more prevalent and extensive in the non-infarcted middle cerebral artery cortical areas than in the non-affected hemisphere (P = 0.028, Wilcoxon sign rank test). Voxel-based between-group comparisons revealed several large clusters of significantly reduced FMZ binding in the affected peri-insular, superior temporal and prefrontal cortices (FDR P < 0.05), as compared with no cluster on the unaffected side. Finally, comparing CTp and PET data revealed a significant negative correlation between FMZ binding and initial hypoperfusion. Applying correction for volume loss did not substantially alter the significance of these results. Although based on a small patient sample sometimes studied late after the index stroke, and as such preliminary, our results establish the presence and distribution of FMZ binding loss in ultimately non-infarcted brain areas after stroke. In addition, the data suggest that this binding loss is proportional to initial hypoperfusion, in keeping with the hypothesis that the rescued penumbra is affected by SNL. Although its clinical counterparts remain uncertain, it is tempting to speculate that peri-infarct SNL could represent a new therapeutic target.

Physiological noise in MR images: an indicator of the tissue response to ischemia?

PURPOSE

To determine whether measuring signal intensity (SI) fluctuations in MRI time series data from acute stroke patients would identify ischemic tissue.

MATERIALS AND METHODS

Prebolus perfusion-weighted MRI data from 32 acute ischemic stroke patients (N = 32) was analyzed as a time series. Ischemic and normal tissue regions were outlined and compared.

RESULTS

The magnitude of the measured SI fluctuations was significantly lower in ischemic regions relative to normal tissue. Spatial differences in these fluctuations occurred in a manner that was different than other perfusion-based metrics.

CONCLUSION

Prior studies have shown that SI fluctuations in MRI time series data correspond to the presence of physiological "noise," which includes vasomotion, an autoregulatory phenomenon that affects the tissue response to ischemia. In this study, SI fluctuations were found to decrease in ischemia, consistent with the notion that small vessels will remain open (fluctuations in vessel diameter will decrease) when there is a challenge to flow. Spatial variation in SI fluctuations appeared to be different from spatial variation seen on other perfusion-based metrics, suggesting that a separate contrast mechanism is responsible, one that might be of diagnostic and prognostic value in acute stroke in which the ability of tissue to withstand ischemia is currently not well visualized.

Optimal definition for PWI/DWI mismatch in acute ischemic stroke patients

Although the perfusion-weighted imaging/diffusion-weighted imaging (PWI/DWI) mismatch model has been proposed to identify acute stroke patients who benefit from reperfusion therapy, the optimal definition of a mismatch is uncertain. We evaluated the odds ratio for a favorable clinical response in mismatch patients with reperfusion compared with no reperfusion for various mismatch ratio thresholds in patients enrolled in the diffusion and perfusion imaging evaluation for understanding stroke evolution (DEFUSE) study. A mismatch ratio of 2.6 provided the highest sensitivity (90%) and specificity (83%) for identifying patients in whom reperfusion was associated with a favorable response. Defining mismatch with a larger PWI/DWI ratio may provide greater power for detecting beneficial effects of reperfusion.

Comparing two methods for assessment of perfusion-diffusion mismatch in a rodent model of ischaemic stroke: a pilot study

This stroke experiment was designed to define the mismatch between perfusion-weighted imaging (PWI) and diffusion-weighted imaging (DWI) in MRI by applying early or instantly acquired PWI. Eight rats were induced with stroke through photothrombotic occlusion of the middle cerebral artery and scanned serially between 1 h and day 3 after induction using DWI and PWI with a 1.5 T MR scanner. The relative lesion volumes (rLV) on MRI and triphenyl tetrazolium chloride-stained specimens were defined as the proportion of lesion volume over brain volume. Discrepancies in the rLV between PWI- and DWI-derived apparent diffusion coefficient (ADC) maps were expressed by subtraction of the ADC from PWI, resulting in three possible patterns: (i) (PWI-ADC > 10% of PWI) denoting a mismatch; (ii) (-(10% of PWI) <or= PWI-ADC <or= 10% of PWI) denoting a match; and (iii) (PWI-ADC < -(10% of PWI)) denoting a reverse mismatch. The differences were compared with the minuend being either early PWI (ePWI) or instant PWI (iPWI) and the subtrahend being instant ADC (iADC). The occurrence and evolution of PWI-ADC patterns were analysed. Over time, PWI-ADC discrepancies evolved from mismatch, through to match, to reversed mismatch. The PWI-ADC mismatch still existed 3 days after MCA occlusion in one to three of the eight cases. The rLVs and mismatch incidences between the ePWI-iADC and iPWI-iADC models were linear correlated. A higher mismatch rate occurred in iPWI-iADC within day 1 and in ePWI-iADC at day 3. Both ePWI and iPWI proved useful to define PWI-ADC patterns within day 1. At day 3, iPWI appeared more adequate.

Clinical-diffusion mismatch defined by NIHSS and ASPECTS in non-lacunar anterior circulation infarction

OBJECTIVES

Instead of the mismatch in MRI between the perfusion-weighted imaging (PWI) lesion and the smaller diffusion-weighted imaging (DWI) lesion (PWI-DWI mismatch), clinical-DWI mismatch (CDM) has been proposed as a new diagnostic marker of brain tissue at risk of infarction in acute ischemic stroke. The Alberta Stroke Program Early CT Score (ASPECTS) has recently been applied to detect early ischemic change of acute ischemic stroke. The present study applies the CDM concept to DWI data and investigated the utility of the CDM defined by the NIH Stroke Scale (NIHSS) and ASPECTS in patients with non-lacunar anterior circulation infarction.

METHODS

Eighty-seven patients with first ever ischemic stroke within 24 hours of onset with symptoms of non-lacunar anterior circulation infarction with the NIHSS score>or=8 were enrolled. Initial lesion extent was measured by the ASPECTS on DWI within 24 hours, and initial neurological score was measured by the NIHSS. As NIHSS>or=8 has been suggested as a clinical indicator of a large volume of ischemic brain tissue, and the majority of patients with non-lacunar anterior infarction with score of NIHSS<8 had lesions with ASPECTS>or=8 on DWI, so CDM was defined as NIHSS>or=8 and DWI-ASPECTS 8>or=. We divided patients into matched and mismatched patient groups, and compared them with respect to background characteristics, neurological findings, laboratory data, radiological findings and outcome.

RESULTS

There were 35 CDM-positive patients (P group, 40.2%) and 52 CDM-negative patients (N group , 59.8%). P group patients had a higher risk of early neurological deterioration (END) than N group patients (37.1% vs 13.5%, p<0.05), which were always accompanied by lesion growth defined by 2 or more points decrease on ASPECTS (36 to 72 hours after onset on CT). The NIHSS at entry were significantly lower in the P group, but there was no difference in the outcome at three months measured by the modified Rankin Scale. However, CDM was not an independent predictor of END by multiple logistic regression analysis.

CONCLUSIONS

Patients with CDM had high rate of early neurological deterioration and lesion growth. CDM defined as NIHSS>or=8 and DWI-ASPECTS>or=8 can be another marker for detecting patients with tissue at risk of infarction, but more work is needed to clarify whether this CDM method is useful in acute stroke management.

Endovascular thrombolysis and stenting of a middle cerebral artery occlusion beyond 6 hours post-attack: special reference to the usefulness of diffusion-perfusion MRI

Intra-arterial thrombolysis and percutaneous angioplasty is feasible in patients with acute middle cerebral artery (MCA) occlusion limited to 6 hours post-ictus, but there are some limitations such as reocclusion or hemorrhagic complications. In this report, we describe a stent placement in the treatment of a refractory artherothrombotic MCA occlusion beyond 6 hours of symptom onset. A 57-year-old man presented with a progressive left-sided weakness and verbal disturbance resulting from an acute thrombotic occlusion of the right MCA superimposed on severe proximal atheromatous stenosis. Diffusion-perfusion magnetic resonance imaging (MRI) demonstrated the significant diffusion-perfusion mismatch. After chemical and mechanical thrombolysis of the clot, balloon angioplasty of the underlying MCA stenosis was performed 2 days post-attack, without significant angiographic improvement. Percutaneous endovascular deployment of a stent (Driver 2.5 x 12 mm, MTI, Irvine, CA) was subsequently performed, with excellent angiographic results. Follow-up diffusion-perfusion MRI showed improved perfusion in the hypoperfused area. The patient's National Institutes of Health Stroke Scale (NIHSS) score was increased from 12 to 3. Clot thrombolysis and subsequent stenting in patients with refractory proximal MCA occlusion is feasible and allows for a significant reduction in the amount of thrombolytic drug required. In selective patients with acute MCA occlusion, the therapeutic window for recanalization procedures can be safely and effectively extended beyond the 'traditional 6 hours'. Diffusion-perfusion MRI in acute MCA occlusion is important for indication of therapy.

Delayed perfusion phenomenon in a rat stroke model at 1.5T MR: An imaging sign parallel to spontaneous reperfusion and ischemic penumbra?

INTRODUCTION

Delayed perfusion (DP) sign at MR imaging was reported in stroke patients. We sought to experimentally elucidate its relation to spontaneous reperfusion and ischemic penumbra.

METHODS

Stroke was induced by photothrombotic occlusion of middle cerebral artery in eight rats and studied up to 72 h using a 1.5 T MR scanner with T2 weighted imaging (T2WI), diffusion weighted imaging (DWI), and dynamic susceptibility contrast-enhanced perfusion weighted imaging (DSC-PWI). Relative signal intensity (rSI), relative lesion volume (rLV), relative cerebral blood flow (rCBF), PWI(rLV)-DWI(rLV) mismatch (penumbra) and DP(rLV) were quantified and correlated with neurological deficit score (NDS), triphenyl tetrazolium chloride (TTC) staining, microangiography (MA) and histopathology.

RESULTS

The rSI and rLV characterized this stroke model on different MRI sequences and time points. DSC-PWI reproduced cortical DP in all rats, where rCBF evolved from 88.9% at 1 h through 64.9% at 6 h to 136.3% at 72 h. The PWI(rLV)-DWI(rLV) mismatch reached 10+/-5.4% at 1 h, remained positive through 12 h and decreased to -3.3+/-4.5% at 72 h. The incidence and rLV of the DP were well correlated with those of the penumbra (p<0.01, r(2)=0.85 and p<0.0001, r(2)=0.96, respectively). Shorter DP durations and more collateral arterioles occurred in rats without (n=4) than with (n=4) cortex involvement (p<0.05). Rats without cortex involvement tended to earlier reperfusion and a lower NDS. Microscopy confirmed MRI, MA and TTC findings.

CONCLUSIONS

In this rat stroke model, we reproduced clinically observed DP on DSC-PWI, confirmed spontaneous reperfusion, and identified the penumbra extending to 12h post-ischemia, which appeared interrelated.

Induced hypertension for the treatment of acute MCA occlusion beyond the thrombolysis window: case report

BACKGROUND

A minority of stroke patients is eligible for thrombolytic therapy. Small pilot case series have hinted that elevation of incident arterial blood pressure might be associated with a favorable prognosis either in acute or subacute stroke. However, these patients were not considered for thrombolytic therapy and were not followed - up systematically. We used pharmacologically induced hypertension in a stroke patient with middle cerebral artery (MCA) occlusion ineligible for thrombolysis that was followed-up by radiological, clinical and functional outcome assessment.

CASE PRESENTATION

A patient with acute embolic MCA occlusion producing a large, ischemic penumbra confirmed by perfusion CT was treated by induced hypertension with phenylephrine started within 4 h of admission. Increase in the mean arterial pressure by 20% led to a reduction of neurological deficit by 3 points on the National Institute of Stroke Scale. MRI and CT scans performed during phenylephrine infusion showed the presence of limited subcortical and cortical infarct changes that were clearly less extensive than the perfusion deficit in the brain perfusion CT at baseline, found in the absence of MCA patency. No complications due to induced hypertension therapy occurred. Moderate functional improvement up to modified Rankin scale 2 at follow up took place.

CONCLUSION

Induced hypertension in acute ischemic stroke seems clinically feasible and may be beneficial in selected normo- or hypotensive stroke patients not eligible for thrombolytic recanalization therapy.

Evolving therapeutic approaches to treating acute ischemic stroke

Stroke contributes significantly to death, disability, and healthcare costs; however, recombinant tissue plasminogen activator (rt-PA) is the only approved thrombolytic therapy for acute ischemic stroke. One area of development for new ischemic stroke treatment options is focused on neuroprotection of viable tissue in the ischemic vascular bed. The ischemic penumbra is recognizable on MRI by decreased perfusion, in contrast to the core area of ischemia, which includes diffusion and perfusion abnormalities. Understanding the mechanisms of neuronal death, including the role of excitotoxic neurotransmitters, free radical production, and apoptotic pathways, is important in developing new therapies for stroke. This article reviews these causes and results of stroke, as well as current and future neuroprotective treatment options. Several compounds have shown neuroprotective effects in animal studies, but have failed to be effective in human clinical trials. Several promising therapeutic areas include targeting of free radicals, modulation of glutamatergic transmission, and membrane stabilization via ion channels.

MRI versus CT-based thrombolysis treatment within and beyond the 3 h time window after stroke onset: a cohort study

BACKGROUND

Thrombolytic treatment with recombinant tissue plasminogen activator (rtPA) is approved for use within 3 h after stroke onset. Thus only a small percentage of patients can benefit. Meta-analyses and more recent studies suggest a benefit for a subset of patients beyond 3 h. We assessed the safety and efficacy of an MRI-based selection protocol for stroke treatment within and beyond 3 h compared with standard CT-based treatment.

METHODS

We assessed clinical outcome and incidence of symptomatic intracerebral haemorrhage (ICH) in 400 consecutive patients treated with intravenous rtPA. Patients eligible for thrombolysis within 3 h were selected by CT or MRI and beyond 3 h only by MRI. 18 patients were excluded from analysis because of violation of that algorithm. The remaining 382 patients were divided into three groups: CT-based treatment within 3 h (n=209); MRI-based treatment within 3 h (n=103); and MRI-based treatment beyond 3 h (n=70).

FINDINGS

Patients in group 3 (MRI > 3 h) had a similar 90 day outcome to those in the other two groups (48% were independent in the CT < or = 3 h group, 51% in the MRI < or = 3 h group, and 56% in group 3), but without an increased risk for symptomatic ICH (9%, 1%, 6%) or mortality (21%, 13%, 11%). MRI-selected patients overall had a significantly lower risk than CT-selected patients for symptomatic ICH (3% vs 9%; p=0.013) and mortality (12% vs 21%; p=0.021). Time to treatment did not affect outcomes in univariate and multivariate analyses.

INTERPRETATION

Our data suggest that beyond 3 h and maybe even within 3 h, patient selection is more important than time to treatment for a good outcome. Furthermore, MRI-based thrombolysis, irrespective of the time window, shows an improved safety profile while being at least as effective as standard CT-based treatment within 3 h.

Technical aspects of perfusion-weighted imaging

There is increasing interest in using diffusion-weighted (DWI) MR imaging and perfusion-weighted MR imaging (PWI) to assist clinical decision-making in the management of acute stroke patients. Larger PWI than DWI lesions have been speculated to represent potentially salvageable tissue that is at risk of infarction unless nutritive flow is restored and presence of these mismatches have been proposed as inclusion criteria for identifying patients most likely to benefit from therapeutic intervention. Understanding the technical aspects of PWI may improve comprehension of the capabilities and limitations of this technique.

Pathophysiology of Stroke Rehabilitation: Temporal Aspects of Neurofunctional Recovery

Stroke almost always causes an impairment of motor activity and function. Clinical recovery, though usually incomplete, is often highly dynamic and reflects the ability of the neuronal network to adapt. Mechanisms that underlie neuro-functional plasticity are now beginning to be understood. Albeit the enormous efforts undertaken to support the natural course of re-convalescence through rehabilitation, little has been done to relate possible effects of these therapeutic approaches to mechanisms of adaptive pathophysiology. The review presented here focuses on these mechanisms during the course of recovery post stroke. Next to an unmasking of latent network representations, other adaptive processes, such as excitatory metabolic stress, an imbalance in activating and inhibiting transmission, leading to salient hyperexcitability or mechanisms that consolidate novel connections prime the system's plastic capabilities. These pathophysiological processes potentially interact with rehabilitative interventions. They therefore form the foundation of positive, but possibly also negative recuperation under therapy.

Initial Ischemic Event: Perfusion-weighted MR Imaging and Apparent Diffusion Coefficient for Stroke Evolution

PURPOSE

To prospectively determine if the degree of acute perfusion or diffusion abnormalities measured prior to treatment onset help predict the evolution of brain infarction on magnetic resonance (MR) images.

MATERIALS AND METHODS

Local ethics committee approval and informed consent were obtained. On parametric maps obtained in 64 patients (mean age, 64 years +/- 13 [standard deviation]; 37 men and 27 women) with acute middle cerebral artery infarction, lesion volumetry was performed to determine time to peak, mean transit time, cerebral blood volume, and apparent diffusion coefficient obtained within 3 hours of symptom onset. The infarct lesions were assessed on T2-weighted MR images obtained at follow-up on day 8. Cerebrovascular changes were determined on MR angiograms. Inferential and correlation statistics were used.

RESULTS

A perfusion delay of more than 6 seconds relative to the nonaffected hemisphere on time-to-peak maps helped to predict the lesion volume on T2-weighted images (r = 0.686, P < .001). In contrast, neither the volume nor the degree of the diffusion abnormality helped to predict the infarct volume (r < 0.46). This was because in one subgroup of patients there was an increase and in one subgroup there was a decrease in infarct volume on the T2-weighted images (P < .001). There was a greater prevalence (P < .02) of cerebral artery abnormalities in the patients with larger infarcts. Clinically, the neurologic impairment was more severe (P < .01) and the mean arterial pressure higher (P < .04) in these patients.

CONCLUSION

The results suggest that in acute stroke the severity of the initial ischemic event as determined on time-to-peak maps indicates hemodynamic compromise in addition to internal carotid artery or middle cerebral artery occlusion, because of abnormalities in other cerebral arteries.

Selection of thrombolytic therapy beyond 3 h using magnetic resonance imaging

PURPOSE OF REVIEW

Use of intravenous thrombolytic therapy in ischaemic stroke is restricted to a 3-h time window because of the proof of this time window in pivotal clinical trials. Thrombolysis is aimed at recanalization of occluded arteries and reperfusion of the ischaemic penumbra, a region of critically hypoperfused, functionally impaired, but potentially viable brain. There are a number of current prospective trials that are testing the hypothesis that the presence of the penumbra will predict thrombolytic responders beyond 3 h.

RECENT FINDINGS

Using magnetic resonance imaging, a mismatch between a larger perfusion-weighted imaging lesion and smaller diffusion-weighted imaging lesion is considered to represent the ischaemic penumbra. Perfusion-weighted imaging provides semiquantitative cerebral blood flow imaging and diffusion-weighted imaging is an index of the largely irreversible ischaemic core. This definition has been modified with the recognition that the perfusion-weighted imaging lesion includes benign oligaemia and that a portion of the diffusion-weighted imaging core is potentially salvageable with rapid reperfusion. Most acute stroke patients have a magnetic resonance imaging-penumbral signature within 6 h of stroke onset. The penumbra is commonly, but not invariably, associated with proximal arterial occlusion and is time-dependent. Preliminary studies have shown benefit from thrombolytic therapy beyond the established 3-h window.

SUMMARY

Penumbral imaging using magnetic resonance imaging with perfusion over diffusion weighted imaging mismatch can provide a physiological 'tissue clock' in individual patients. Based on this hypothesis, a number of prospective trials are being performed. These include EPITHET, DEFUSE, DIAS, MR RESCUE and ROSIE.

Stroke Research in the Modern Era: Images versus Dogmas

Recovery of function following ischaemic stroke is a fascinating clinical observation. It comprises several modes, e.g. spectacular recovery in a matter of hours or days and gradual recovery over months or even years. That a non-functioning neural system can regain its function, even partially so, is challenging because of the obvious therapeutic implications. Until the mid-70s, however, dogmas largely prevailed which underpinned the then nihilistic approach to stroke patients. Proving these dogmas wrong has been a major achievement of modern stroke research. Thanks particularly to physiological imaging, key observations from the basic neurosciences have translated into the clinical realm in ways immediately understandable to the clinician, allowing the emergence of pathophysiology-based management.

Foundations of advanced magnetic resonance imaging

During the past decade, major breakthroughs in magnetic resonance imaging (MRI) quality were made by means of quantum leaps in scanner hardware and pulse sequences. Some advanced MRI techniques have truly revolutionized the detection of disease states and MRI can now-within a few minutes-acquire important quantitative information noninvasively from an individual in any plane or volume at comparatively high resolution. This article provides an overview of the most common advanced MRI methods including diffusion MRI, perfusion MRI, functional MRI, and the strengths and weaknesses of MRI at high magnetic field strengths.

The Evolving Role of Acute Stroke Imaging in Intravenous Thrombolytic Therapy: Patient Selection and Outcomes Assessment

In early trials of thrombolysis, unenhanced CT was used to exclude patients with brain hemorrhage or large infarctions but was insensitive to stroke pathophysiology or early signs of cerebral ischemia or infarction. Currently, CT angiography, CT perfusion, and MR imaging can provide information about stroke mechanisms and prognosis, quantify penumbral tissue, and support risk stratification and patient selection. This article reviews the role of neuroimaging in the original intravenous thrombolytic trials, current application of these technologies, and the potential future role of imaging to extend the time window for thrombolysis and to augment therapeutic success.

Partial rescue of the perfusion deficit area by thrombolysis

PURPOSE

To investigate the evolution of the perfusion deficit area following systemic thrombolysis with recombinant tissue plasminogen activator (rtPA) in a clinical study on acute cerebral ischemia.

MATERIALS AND METHODS

We performed volumetric measurements of the acute ischemic lesions in MR images of perfusion (TTP, MTT, and rCBV) and in diffusion-weighted (DW) images, as well as the manifest stroke lesions in T2-weighted MR images on day 8. We compared the data of 29 patients who were subjected to systemic thrombolysis with those of 18 patients who were not eligible for thrombolysis.

RESULTS

In the treated patients there were prominent MTT/DWI and TTP/DWI mismatches (P < 0.0006). The acute TTP volumes were smaller than the acute MTT volumes, but as large as the T2 lesions on day 8. The MTT/T2 lesion volume reduction was significant (P < 0.03) in patients who received the GPIIb/IIIa receptor antagonist tirofiban (N = 13) in addition to the low-dose rtPA. This corresponded to a greater neurological improvement compared to patients who received rtPA alone (P < 0.05). In contrast, in the nontreated patients the initial MTT and TTP lesion volumes were of similar magnitude and predicted the T2 lesions on day 8. In the treated and nontreated patients the TTP lesion signified the viability threshold of acute ischemia, which corresponded to a rCBF of 25 +/- 11 mL/100 g/min.

CONCLUSION

The perfusion deficit area comprises the ischemic core that is destined to undergo necrosis, and an ischemic rim that is salvageable by systemic thrombolysis.

MRI measurements of water diffusion: impact of region of interest selection on ischemic quantification

OBJECTIVE

To investigate the effect of ADC heterogeneity on region of interest (ROI) measurement of isotropic and anisotropic water diffusion in acute (< 12 h) cerebral infarctions.

METHODS AND MATERIALS

Full diffusion tensor images were retrospectively analyzed in 32 patients with acute cerebral infarction. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values were measured in ischemic lesions and in the corresponding contralateral, normal appearing brain by using four ROIs for each patient. The 2 x 2 pixel square ROIs were placed in the center, the lateral rim and the medial rim of the infarction. In addition, the whole volume of the infarction was measured using a free hand method. Each ROI value obtained from the ischemic lesion was normalized using contralateral normal ROI values.

RESULTS

The localization of the ROIs in relation to the ischemic lesion significantly affected ADC measurement (P < 0.01, using Friedman test), but not FA measurement (P = 0.25). Significant differences were found between ADC values of the center of the infarction versus whole volume (P < 0.01), and medial rim versus whole volume of infarction (P < 0.001) with variation of relative ADC values up to 11%. The differences of absolute ADC for these groups were 22 and 23%, respectively. The lowest ADC was found in the center, followed by medial rim, lateral rim and whole volume of infarction.

CONCLUSION

ADC quantification may provide variable results depending on ROI method. The ADC and FA values, obtained from the center of infarction tend to be lower compared to the periphery. The researchers who try to compare studies or work on ischemic quantification should be aware of these differences and effects.

Penumbral topography in human stroke: methodology and validation of the 'Penumbragram'

The location as well as the volume of the ischemic penumbra in human stroke is likely to influence the outcome of therapeutic intervention but its spatial extent is poorly characterized. Based on the observation that infarct expansion progresses from the center to the periphery of the penumbra in animal stroke models, we describe a method of mapping the three-dimensional spatial extent of the penumbra relative to the infarct in a 'Penumbragram'. Central, peripheral and external zones of the final infarct were defined according to median voxel distance from the infarct center (IC) and were further subdivided by coronal, sagittal and axial planes through the IC. In 10 patients with hypoxic, viable (penumbral) tissue identified by (18)F-Fluoromisonidazole positron emission tomography within 48 h of stroke onset, 'Penumbragrams' displaying the percentage of penumbra in each region were generated using anatomically co-registered data sets. The correlation between penumbral percentage and time from stroke onset was negative in the central (P < 0.05) and peripheral (P > 0.05) zones of the infarct and positive in external zones (P < 0.05). The validity of infarct segmentation was assessed by factor analysis with no a priori grouping of regions. Negative and positive correlations of penumbra volume and time from stroke onset were observed in seven (five corresponding to central zone of the infarct) and four (all in external zone) infarct regions and were measured reliably (Cronbach's alpha 0.84 and 0.9, respectively). The 'Penumbragram' is a valid method for objectively mapping the spatial extent of the penumbra, which is applicable to other imaging modalities.

Imaging the ischaemic penumbra

PURPOSE OF REVIEW

Imaging the penumbra is essential, not only to identify patients who might benefit from thrombolysis, but also to further understanding of the ischaemic process, thereby potentially revealing new opportunities for therapeutic intervention. Here we review recent imaging studies of the acute stroke process.

RECENT FINDINGS

Perfusion-computed tomography and computed tomography angiography enable assessment of the haemodynamic status and site of occlusion, leading to their promising use in guiding thrombolysis. The magnetic resonance concept of the diffusion-perfusion 'mismatch' being representative of penumbra appears to be an oversimplification. The mapping of simple variables such as time-to-peak might not directly reveal true penumbral perfusion levels. Also, lesions seen with diffusion-weighted imaging may be reversible as a result of early reperfusion. This reversal with subsequent normalization may represent selective neuronal damage. Late secondary injury, as indicated by the reappearance of the diffusion-weighted imaging lesion, has recently been documented; the mechanisms are unknown but form potential targets for future therapies. Despite these caveats, diffusion-weighted imaging-perfusion-weighted imaging remains the most useful approach to map the pathophysiology of stroke in the clinical setting. Acute/subacute flumazenil positron emission tomography studies are being used as markers of neuronal integrity to help shed further light on infarction thresholds, and potentially document selective neuronal loss. F-labelled fluoromisonidazole positron emission tomography imaging of brain hypoxia documents the temporal and spatial progression of the penumbra.

SUMMARY

The goal of understanding the complex process that is acute ischaemia in stroke, and subsequently the development of therapeutic strategies, continues to be advanced by imaging the penumbra in novel ways.

Intravenous Thrombolysis for Acute Ischemic Stroke

Intravenous recombinant tissue-type plasminogen activator (rtPA, alteplase) is the only drug approved for the treatment of acute ischemic stroke. It should be administered within 3 hours of stroke. There is additional evidence, however, that administration at later times, by means of other methods, is effective. Herein, is a broad review of the knowledge gained and insights created from studies in which thrombolytic treatment was used in patients with stroke.

Cerebral Blood Flow Index

OBJECTIVES

To evaluate the feasibility of utilizing cerebral blood flow (CBF) index images, calculated automatically and quickly from dynamic perfusion imaging (DPI), to identify acute cerebral ischemia. We attempted to investigate (1) whether the CBF index has a threshold for assessing tissue outcome, (2) whether CBF index images can predict the resulting infracted area, and if so, (3) whether the predictive capacity of the CBF index image is comparable to the regional CBF (rCBF) image delivered from singular value decomposition (SVD) deconvolution methods, which are regarded as most accurate in predicting the final infarct area.

METHODS

Diffusion-weighted images (DWI) and DPI were obtained in 17 patients within 12 hours of stroke onset and follow-up magnetic resonance imaging (MRI). On 3 DPI-delivered images, namely relative regional cerebral blood volume (rrCBV), uncorrected mean transit time (MTTu) and CBF index images, univariate discriminant analysis was done to estimate cut-off values to discriminate between infarcted and noninfarcted areas. Subsequently, correlations between the initial lesion volume of 3 images together with rCBF images delivered with SVD methods and the final infarct volume on follow-up T2-weighted MRI taken at the 8th to 20th day were determined.

RESULTS

Among the 3 images, only the CBF index image was able reveal the threshold of the ischemic region. Lesion volume of CBF index images against follow-up infarct volume had the highest correlation (r = 0.995) to a linear fit and the slope was closest to 1.0 (0.91) among the 3 and had identical accuracy to the regression coefficient of rCBF images.

CONCLUSIONS

CBF index images can predict final infarct volume. Evaluating CBF index images together with DWI can guide the initial assessment in the acute stage of cerebral ischemia.

Tacrolimus, a potential neuroprotective agent, ameliorates ischemic brain damage and neurologic deficits after focal cerebral ischemia in nonhuman primates

Tacrolimus (FK506), an immunosuppressive drug, is known to have potent neuroprotective activity and attenuate cerebral infarction in experimental models of stroke. Here we assess the neuroprotective efficacy of tacrolimus in a nonhuman primate model of stroke, photochemically induced thrombotic occlusion of the middle cerebral artery (MCA) in cynomolgus monkeys. In the first experiment, tacrolimus (0.01, 0.032, or 0.1 mg/kg) was intravenously administered immediately after MCA occlusion, and neurologic deficits and cerebral infarction volumes were assessed 24 hours after the ischemic insult. Tacrolimus dose-dependently reduced neurologic deficits and infarction volume in the cerebral cortex, with statistically significant amelioration of neurologic deficits at 0.032 and 0.1 mg/kg and significant reduction of infarction at 0.1 mg/kg. In the second experiment, the long-term efficacy of tacrolimus on neurologic deficits and cerebral infarction was assessed. Vehicle-treated monkeys exhibited persistent and severe deficits in motor and sensory function for up to 28 days. A single intravenous bolus injection of tacrolimus (0.1 or 0.2 mg/kg) produced long-lasting amelioration of neurologic deficits and significant reduction of infarction volume. In conclusion, we have provided compelling evidence that a single dose of tacrolimus not only reduces brain infarction but also ameliorates long-term neurologic deficits in a nonhuman primate model of stroke, strengthening the view that tacrolimus might be beneficial in treating stroke patients.