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

Perfusion MRI Abnormalities in Speech or Motor Transient Ischemic Attack Patients

BACKGROUND AND PURPOSE

Transient ischemic attack (TIA) patients may deteriorate rapidly. MRI is being increasingly used to assess such patients. One possible mechanism of neurological worsening is the presence of perfusion abnormalities. We sought to identify what proportion of TIA patients had evidence of perfusion abnormalities on MRI.

METHODS

TIA patients were prospectively enrolled and had a MRI completed as soon as possible. The images were assessed for the presence of perfusion abnormalities.

RESULTS

Sixty-nine TIA patients were enrolled, and 62 had perfusion imaging. In 56 patients (81%), the symptoms had resolved before imaging. In 21 patients (33.9%), there was evidence of a perfusion abnormality defined by relative mean transit time delay. In 12 patients (19.4%), the perfusion abnormality was present despite having complete resolution of neurological symptoms. We found no relationship between the presence of a perfusion abnormality and the clinical outcome.

CONCLUSIONS

A proportion of TIA patients have perfusion abnormalities evident on MRI.

MR perfusion and diffusion in acute ischemic stroke: human gray and white matter have different thresholds for infarction

It is thought that gray and white matter (GM and WM) have different perfusion and diffusion thresholds for cerebral infarction in humans. We sought to determine these thresholds with voxel-by-voxel, tissue-specific analysis of co-registered acute and follow-up magnetic resonance (MR) perfusion- and diffusion-weighted imaging. Quantitative cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), and apparent diffusion coefficient (ADC) maps were analyzed from nine acute stroke patients (imaging acquired within 6 h of onset). The average values of each measure were calculated for GM and WM in normally perfused tissue, the region of recovered tissue and in the final infarct. Perfusion and diffusion thresholds for infarction were determined on a patient-by-patient basis in GM and WM separately by selecting thresholds with equal sensitivities and specificities. Gray matter has higher thresholds for infarction than WM (P<0.009) for CBF (20.0 mL/100 g min in GM and 12.3 mL/100 g min in WM), CBV (2.4 mL/100 g in GM and 1.7 mL/100 g in WM), and ADC (786 x 10(-6) mm(2)/s in GM and 708 x 10(-6) mm(2)/s in WM). The MTT threshold for infarction in GM is lower (P=0.014) than for WM (6.8 secs in GM and 7.1 secs in WM). A single common threshold applied to both tissues overestimates tissue at risk in WM and underestimates tissue at risk in GM. This study suggests that tissue-specific analysis of perfusion and diffusion imaging is required to accurately predict tissue at risk of infarction in acute ischemic stroke.

Extending reperfusion therapy for acute ischemic stroke: emerging pharmacological, mechanical, and imaging strategies

BACKGROUND AND PURPOSE

Reperfusion is the most beneficial of all therapeutic strategies for acute ischemic stroke. However, the standard cerebral reperfusion treatment of the first decade of the reperfusion era, noncontrast computed tomography (CT)-guided, < or =3 hours, intravenous tissue plasminogen activator, has many limitations. This review surveys emerging strategies that have the potential to extend cerebral reperfusion therapy to larger numbers of patients.

SUMMARY OF REVIEW

Innovative intravenous pharmacological reperfusion strategies include novel fibrinolytic agents (tenecteplase, reteplase, desmetolplase, plasmin, and microplasmin), glycoprotein (GP) IIb/IIIa antagonists with platelet disaggregating effects (abciximab and tirofiban), combination therapies to improve efficacy of clot lysis (fibrinolytics and GP IIb/IIIa agents, and fibrinolytics and direct thrombin inhibitors), increase the time window for clot lysis (fibrinolytics and neuroprotectants), and reduce the frequency of hemorrhagic complications (fibrinolytics and vasoprotectants), and externally applied ultrasound to enhance enzymatic fibrinolysis. Promising intra-arterial pharmacological reperfusion approaches include novel fibrinolytic agents, combined intravenous and intra-arterial fibrinolysis, and combined fibrinolytics and GP IIb/IIIa agents. Emerging endovascular mechanical reperfusion strategies include intra-arterial thrombectomy (clot retrieval devices and suction thrombectomy devices), mechanical disruption (micro-guidewire passage, laser photoacoustic emulsification, and primary intracranial angioplasty), and augmented fibrinolysis by endovascular ultrasound. Multimodal imaging, with magnetic resonance (MR) or CT, can rapidly assess infarct core, penumbra, site of vessel occlusion, and tissue hemorrhagic propensity, enabling improved selection of patients for reperfusion therapy beyond any arbitrary fixed time window.

CONCLUSIONS

Therapeutic reperfusion is emerging as a treatment strategy of remarkable power and scope for rescuing patients experiencing acute brain ischemia, applicable within and beyond the 3-hour time window.

[Diffusion and perfusion MR imaging in stroke]

Magnetic resonance imaging (MRI) in stroke makes it possible to visualize the initial infarct in cases of acute cerebral ischemia. Perfusion MRI serves to determine which tissues are additionally at risk of infarction due to persistent hypoperfusion. MRI also allows those examiners with limited experience to reliably confirm an infarct. The most important differential diagnosis of cerebral ischemia, intracerebral hemorrhage, can likewise be recognized with certainty using MRI. Although diffusion and perfusion MRI only demonstrate the pathophysiology of cerebral ischemia approximately, the method is suited for identifying those patients who would profit from reperfusion therapy. Whether MRI is also appropriate as an aid to reaching a prognosis on the risk of secondary hemorrhage has not yet been resolved.

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.

A novel method to derive separate gray and white matter cerebral blood flow measures from MR imaging of acute ischemic stroke patients

Perfusion-weighted imaging (PWI) measures can predict tissue outcome in acute ischemic stroke. Accuracy might be improved if differential tissue susceptibility to ischemia is considered. We present a novel voxel-by-voxel analysis to characterize cerebral blood flow (CBF) separately in gray (GM) and white matter (WM). Ten patients were scanned with inversion-recovery spin-echo EPI (IRSEPI), diffusion-weighted imaging (DWI), PWI<6 h from onset and fluid attenuated inversion-recovery (FLAIR) at 30 days. Image processing included coregistration to PWI, automatic segmentation of IRSEPI into GM, WM and CSF and semiautomatic segmentation of DWI/FLAIR to derive the acute and 30-day lesions. Five tissue compartments were defined: (1) 'Core' (abnormal acutely and at 30 days), (2) 'Growth' (or 'infarcted penumbra', abnormal only at 30 days), (3) 'Reversed' (abnormal acutely but normal at 30 days), (4) 'MTT-Delayed ' (tissue with delayed mean transit time but not part of the acute or 30-day lesion), and (5) 'Normal' brain. Cerebral blood flow in GM and WM of each compartment was obtained from quantitative maps. Gray matter and WM mean CBF in the growth region differed by 5.5 mL/100 g min (P=0.015). Mean CBF also differed significantly within normal and MTT-Delayed compartments. The difference in the reversed region approached statistical significance. In core, GM and WM CBF did not differ. The results suggest separate ischemic thresholds for GM and WM in stroke penumbra.

“Clinical-CT Mismatch” and the Response to Systemic Thrombolytic Therapy in Acute Ischemic Stroke

Background and Purpose— Mismatch between clinical deficits and imaging lesions in acute stroke has been proposed as a method of identifying patients who have hypoperfused but still have viable brain, and may be especially apt to respond to reperfusion therapy. We explored this hypothesis using a combined database including 4 major clinical trials of intravenous (IV) thrombolytic therapy.

Methods— To determine what the radiological correlates of a “matched” functional deficit are, we calculated the relationship between the ASPECT score of the 24-hour (follow-up) CT scan and the 24-hour National Institutes of Health Stroke Scale (NIHSS) score on the subsample with ASPECT scores performed at this time (n=820). Based on this empirical relationship, we computed the absolute difference between the observed baseline ASPECT score and the “expected” score (ie, matched) based on baseline NIHSS for all patients (n=2131). We tested whether patients with better than expected baseline ASPECTS were more likely to benefit from IV recombinant tissue plasminogen activation (rtPA).

Results— At 24 hours, there was a strong, linear, negative correlation between NIHSS and ASPECTS (r 2 =0.33, P <0.0001); on average, an increase of 10 points on NIHSS corresponded to a decrease of ≈3 points on ASPECTS. At baseline, the average degree of mismatch between the observed and “expected” ASPECTS was 2.1 points (interquartile range, 1.0 to 3.4). However, multiple analyses failed to reveal a consistent relationship between the degree of clinical-CT mismatch at baseline and a patient’s likelihood of benefiting from IV rtPA.

Conclusion— Clinical-CT mismatch using ASPECT scoring does not reliably identify patients more or less likely to benefit from IV rtPA.

Use of multimodal MRI and novel endovascular therapies in a patient ineligible for intravenous tissue plasminogen activator

BACKGROUND AND PURPOSE

Options are limited for individuals who present to the ED within 3 hours of ischemic stroke onset, but who are deemed ineligible for intravenous thrombolysis. Multimodal MRI has been shown to be of great help in identifying stroke patients with large areas of at risk "penumbral tissue", who may gain from the use of novel endovascular therapies. We report a patient who twice benefited from this management approach, in the setting of hemorrhagic risk following successive ischemic strokes.

CASE DESCRIPTION

The patient is a 78 year old male who experienced acute ischemic stroke on 2 separate occasions 5 months apart, and for whom perceived contraindications to appropriate thrombolytic therapy administration led to the successful use of different endovascular therapies at each encounter. Furthermore, following mechanical clot retrieval during the second encounter, the high intensity signal area noted on the baseline diffusion weighted imaging (DWI) in the posterior circulation territory, was almost completely resolved on the day 7 post-procedure MRI.

CONCLUSIONS

To our knowledge, this is the first reported case of reversal of a DWI abnormality in the posterior circulation territory.

Perfusion and diffusion imaging in acute focal cerebral ischemia: temporal vs. spatial resolution

High-resolution diffusion- (DWI) and perfusion-weighted (PWI) imaging may provide substantial benefits in accurate delineation of normal, ischemic, and at-risk tissue. We compared the capability of low (400 x 400 microm(2)) and high (200 x 200 microm(2)) spatial resolution imaging in characterizing the spatiotemporal evolution of the ischemic lesion in a permanent middle artery occlusion (MCAO) model in rats. Serial measurements of cerebral blood flow (CBF) and the apparent diffusion coefficient (ADC) were performed. Lesion volumes were calculated by using viability thresholds or by visual inspection, and correlated with infarct volume defined by TTC staining at 24 h after MCAO. At the very early phase of ischemia, high-resolution resulted in a significantly larger ADC-derived lesion volume and a smaller PWI/DWI mismatch. At 3 h after MCAO, ADC and CBF lesions showed similar robust correlations with TTC-defined infarct volumes for both groups using previously established thresholds. When lesions were determined visually, low-resolution resulted in a substantial overestimation of TTC-defined infarct volume and a lower inter-observer reliability (r = 0.75), whereas high-resolution produced an excellent correlation with TTC-defined infarct volume and inter-observer reliability (r = 0.96). In conclusion, high-resolution MRI resulted in substantial temporal averaging of the ischemic lesion during the early phase, but was clearly superior in visual determination of final infarct size. Low-resolution reasonably evaluated the temporal and spatial evolution of ischemia when thresholds were used.

Prehospital Care of the Acute Stroke Patient

Emergency medical services (EMS) is the first medical contact for most acute stroke patients, thereby playing a pivotal role in the identification and treatment of acute cerebrovascular brain injury. The benefit of thrombolysis and interventional therapies for acute ischemic stroke is highly time dependent, making rapid and effective EMS response of critical importance. In addition, the general public has suboptimal knowledge about stroke warning signs and the importance of activating the EMS system. In the past, the ability of EMS dispatchers to recognize stroke calls has been documented to be poor. Reliable stroke identification in the field enables appropriate treatment to be initiated in the field and potentially inappropriate treatment avoided; the receiving hospital to be prenotified of a stroke patient's imminent arrival, rapid transport to be initiated; and stroke patients to be diverted to stroke-capable receiving hospitals. In this article we discuss research studies and educational programs aimed at improving stroke recognition by EMS dispatchers, prehospital personnel, and emergency department (ED) physicians and how this has impacted stroke treatment. In addition public educational programs and importance of community awareness of stroke symptoms will be discussed. For example, general public's utilization of 911 system for stroke victims has been limited in the past. However, it has been repeatedly shown that utilization of the 911 system is associated with accelerated arrival times to the ED, crucial to timely treatment of stroke patients. Finally, improved stroke recognition in the field has led investigators to study in the field treatment of stroke patients with neuroprotective agents. The potential impact of this on future of stroke treatment will be discussed.

The Evolving Role of Advanced MR Imaging as a Management Tool for Adult Ischemic Stroke: A Western-European Perspective

New and more advanced diagnostic imaging techniques for acute stroke triage have the potential to not only improve the quality of care but also reduce health care costs. Although sufficiently large and methodologically sound studies with regard to cost effectiveness of MR imaging are lacking, the overall impression is that MR imaging has revolutionized not only the diagnosis but also the open and investigational management of neurologically ill patients.

Blood pressure in the artery distal to an intraarterial embolus during thrombolytic therapy for occlusion of a major artery: a predictor of cerebral infarction following good recanalization

Object. The aim in this study was the investigation of back pressure in arteries distal to the occlusion site during intraarterial thrombolysis as well as the usefulness of back pressure measurement in combination with diffusion-weighted (DW) magnetic resonance (MR) imaging to predict the occurrence of ischemic lesions following good recanalization.

Methods. Twenty-five consecutive patients with severe hemiparesis caused by embolism of the internal carotid artery (10 patients) and the proximal middle cerebral artery (15 patients) were treated using intraarterial thrombolysis. Systolic back pressure, measured through a microcatheter in the artery just distal to the emboli, ranged from 22 to 78 mm Hg. According to an angiographic inclusion criterion for good recanalization—that is, recanalization of the M2 or more distal arteries at the end of thrombolysis—21 of 25 patients underwent evaluation in this study. In 14 patients volumes of low-density areas on computerized tomography (CT) scans obtained 2 months postthrombolysis were smaller in comparison with volumes of hyperintense areas on DW MR images acquired before treatment, whereas these low-density areas were larger in seven patients. Compared with those on initial DW MR images, the volume of abnormalities on CT scans obtained 2 months posttreatment were significantly reduced in patients with a systolic back pressure greater than 30 mm Hg (16 patients) than in those with a back pressure of 30 mm Hg or less (five patients) (p < 0.05). Systolic back pressures greater than 30 mm Hg were associated with significantly better modified Rankin Scale scores than those 30 mm Hg or less (p < 0.05).

Conclusions. Back pressure measurement in combination with DW MR imaging can be used to predict the occurrence of infarction as demonstrated on CT scans following thrombolysis.

Advanced MR Imaging of Acute Stroke: The University of California at Los Angeles Endovascular Therapy Experience

Rapid implementation of multimodal MR imaging, including diffusion-perfusion imaging before and following endovascular therapies for treatment of acute ischemic stroke, has yielded novel observations and expanded contemporary knowledge of acute stroke pathophysiology. For more than a decade at the University of California at Los Angeles, an intensive imaging strategy has been used to establish stroke diagnosis, delineate early patterns of cerebral ischemia, and monitor response to various reperfusion techniques. Angiographic findings before thrombolysis have substantiated stroke subtype, and documentation of concomitant recanalization has provided considerable insight regarding the vascular and imaging correlates of intra-arterial therapies. MR imaging studies have progressively characterized the complex pathophysiology of acute ischemic stroke, providing imaging strategies and models that may be used to optimize acute stroke care. The ensuing review emphasizes salient aspects of these neuroimaging studies, addressing numerous facets of acute stroke pathophysiology.

Does the mismatch match the penumbra? Magnetic resonance imaging and positron emission tomography in early ischemic stroke

BACKGROUND AND PURPOSE

In ischemic stroke, diffusion-weighted (DW) and perfusion-weighted (PW) magnet resonance imaging (MRI) is used to define the mismatch as the therapeutic target. With positron emission tomography (PET), we characterized the metabolic patterns of tissue compartments identified by MRI and compared the volumes of mismatch to those of PET-defined penumbra.

METHODS

In 6 acute (median, 5.2 hours) and 7 chronic (median, 10 days) stroke patients in whom a mismatch was defined by PW/DW MRI, PET was performed (median, 120-minute delay). Cerebral blood flow (CBF), oxygen metabolism (CMRO2), and oxygen extraction fraction (OEF) was determined in the areas of DWI lesion, mismatch, and oligemia. Then, the mismatch volume was compared with the volume of penumbra.

RESULTS

DWI lesions showed impaired tissue integrity (low CMRO2 and low OEF). Mismatch areas were viable (normal CMRO2) but showed largely varying OEF. Oligemic areas had metabolic patterns comparable to normal tissue. A mismatch volume was found in all 13 patients. However, only 8 of 13 had a corresponding penumbra volume that covered only a part of the mismatch.

CONCLUSIONS

Our comparative PET/MRI study confirmed the current pathophysiological hypothesis for the DWI lesion and for the oligemic areas. However, the mismatch area did not reliably detect elevated OEF and overestimated the penumbra defined by PET.

Partial-volume effect on ischemic tissue-fate delineation using quantitative perfusion and diffusion imaging on a rat stroke model

Partial-volume effects (PVE) in stroke imaging could hinder proper delineation of normal, ischemic, and at-risk tissues. Cerebral-blood-flow (CBF) and apparent diffusion coefficient (ADC) were measured at high and low resolution (HR = 128 x 128, LR = 64 x 64) in focal ischemia in rats during the acute phase. The data were evaluated for PVE on ischemic tissue classification on a pixel-by-pixel basis and the misclassified pixels were quantified as ischemia progressed. The main drawbacks of high-resolution imaging are reduced temporal resolution and/or signal-to-noise ratio. The high- versus low-resolution scatterplots and histograms of pixels along the normal-abnormal boundaries in the ADC and CBF maps showed marked ischemia-related PVE. By comparison with the homologous regions in the contralateral normal hemisphere, the effect of increased noise and intrinsic tissue heterogeneity due to high resolution could be distinguished from ischemia-related PVE. Degrading the high-resolution (128 x 128) data to a 64 x 64 or 32 x 32 matrix increased the severity of PVE. Zero-filling of low-resolution (64 x 64) data to 128 x 128 also increased PVE. It was concluded that PVE: (1) misclassified substantial pixels along the normal-abnormal boundaries, (2) overestimated abnormal volumes at the expense of mostly "at-risk" and some "normal" tissues, (3) were more severe at the early time points postischemia, and (4) confounded the interpretation of the operationally defined ischemic penumbra.

Acute studies of a new primate model of reversible middle cerebral artery occlusion

The recent failure of many clinical trials of neuroprotective compounds may be due in part to poor animal models of human stroke. We have developed an endovascular stroke model in nonhuman primates that is compatible with serial magnetic resonance imaging (MRI) monitoring. Using cynomologous macaques (n = 4), a microcatheter was navigated transarterially (under fluoroscopic guidance) from the femoral artery to the middle cerebral artery (MCA). The microcatheter was wedged in a branch of the MCA for 3 hours to cause focal cerebral ischemia, as verified angiographically. During occlusion and/or reperfusion, animals were scanned with MRI, and imaging findings were compared with the stained brain sections. All animals demonstrated small stroke lesions in the expected vascular territory, as seen on diffusion-weighted MRI and confirmed by postmortem examination. Reperfusion after 3 hours was confirmed angiographically (n = 2) and also by MRI (n = 4). The mean initial lesion volume, measured on the postreperfusion MRI scans, was 2.3 +/- 1.3 mL (n = 4). There was good agreement between anatomic location of the lesion on MRI and postmortem histological staining (n = 3). A "minimally invasive" primate model of focal cerebral ischemia was developed that is ideally suited to MRI studies of both acute and chronic stroke. By using serial MRI scans to measure changes in lesion size over time, we will be able to control for variability in lesion size/location. This model should prove useful as a test bed for new stroke therapies, in which noninvasive imaging findings are readily comparable to human stroke.

The diffusion-weighted lesion in acute stroke: heterogeneous patterns of flow/metabolism uncoupling as assessed by quantitative positron emission tomography

BACKGROUND

To investigate what the hyperintense lesion in diffusion-weighted imaging (DWI) of acute ischaemic stroke represents metabolically, we prospectively imaged acute carotid-territory stroke patients with DWI along with fully quantitative positron emission tomography (PET), which gives physiological maps of cerebral blood flow (CBF), the cerebral metabolic rate of oxygen (CMRO2) and the oxygen extraction fraction (OEF).

METHOD

Of 10 patients who consented, 5 (3 males, 2 females, 53-84 years, NIHSS 6-16) completed the imaging protocol of back-to-back DWI and PET within 21 (mean 15.7, range 7-21) h of stroke onset. All images were co-registered with the DWI lesion forming a region of interest (ROI) that was transferred to the PET parametric maps (OEF, CBF, CMRO2). Patterns of blood flow and metabolism were assessed within the DWI ROI.

RESULTS

Within the DWI lesions, the following patterns were observed: very low CBF and CMRO2/variable OEF; low CBF/high OEF, and high CBF/low OEF. There was a heterogeneity of patterns between and within DWI lesions. In addition, areas of hyperperfusion (with low OEF) and areas of hypoperfusion (with high OEF) were seen outside the DWI lesions.

CONCLUSION

The DWI lesion does not have a single flow/metabolism counterpart, suggesting that it reflects various stages of the ischaemic process.

Feasibility of Stent Placement in Carotid Arteries with Real-time MR Imaging Guidance in Pigs

All examinations were performed with approval from the institutional animal care and use committee of Columbia University. To assess the feasibility of real-time magnetic resonance (MR) imaging-guided neurovascular intervention in a swine model, the authors placed stents in the carotid arteries of five domestic pigs. Seven-French vascular sheaths were placed in the target carotid arteries via femoral access by using active MR tracking. Ten nitinol stents (8-10 x 20-40 mm) were successfully deployed in the target segments of carotid arteries bilaterally. MR imaging and necropsy findings confirmed stent position. Necropsy revealed no gross vascular injury. Study results demonstrated the feasibility of performing real-time MR imaging-guided neurovascular intervention by using an active-tracking technique in an animal model.

Hyperacute imaging of ischemic stroke: Role in therapeutic management

Ischemic stroke remains a significant cause of morbidity and mortality. Current therapeutic options for acute ischemic stroke include intravenous thrombolysis and endovascular approaches for recanalization of proximal arterial occlusion. The rapid identification of underlying stroke etiology or mechanism may facilitate selection criteria for emergent therapy. Hyperacute imaging plays an integral role in the delineation of stroke pathophysiology and the formulation of rational stroke therapy. Hyperacute imaging of ischemic stroke may demonstrate proximal vascular occlusion, compensatory collateral circulation, residual or collateral tissue perfusion, and the differentiation of ischemic core from penumbral regions. Characterization of the ischemic field, including core and penumbra, with various mismatch models on multimodal computed tomography or MRI may refine current therapeutic strategies for cerebral ischemia. The diagnostic and therapeutic role of hyperacute imaging has emerged as a pivotal component in the evaluation and management of ischemic stroke.

Magnetic resonance imaging criteria for thrombolysis in acute cerebral infarct

BACKGROUND AND PURPOSE

Magnetic resonance imaging (MRI) selection of stroke patients eligible for thrombolytic therapy is an emerging application. Although the efficacy of therapy within 3 hours after onset of symptoms with intravenous (IV) tissue plasminogen activator (tPA) has been proven for patients selected with computed tomography (CT), no randomized, double-blinded MRI trial has been published yet.

SUMMARY OF REVIEW

MRI screening of acute stroke patients before thrombolytic therapy is performed in some cerebrovascular centers. In contrast to the CT trials, MRI pilot studies demonstrate benefit of therapy up to 6 hours after onset of symptoms. This article reviews the literature that has lead to current controlled MRI-based thrombolysis trials. We examined the MRI criteria applied in 5 stroke centers. Along with the personal views of clinicians at these centers, the survey reveals a variety of clinical and MRI technical aspects that must be further investigated: the therapeutic consequence of microbleeds, the use of magnetic resonance angiography, dynamic time windows, and others.

CONCLUSION

MRI is an established application in acute evaluation of stroke patients and may suit as a brain clock, replacing the currently used epidemiological time clock when deciding whether to initiate thrombolytic therapy. MRI criteria for thrombolytic therapy are applied in some cerebrovascular centers, but the results of ongoing clinical trials must be awaited before it is possible to reach consensus.

Applications of diffusion/perfusion magnetic resonance imaging in experimental and clinical aspects of stroke

The acute evaluation of stroke patients has undergone dramatic advances in the recent past. The increasing availability of novel magnetic resonance imaging (MRI) techniques, such as diffusion and perfusion MRI, provides a plethora of information to clinicians evaluating patients suspected of having an acute stroke. This review focuses on recent advances with experimental and clinical applications of perfusion and diffusion imaging and their utility in identifying potentially salvageable ischemic tissue in rat stroke model and stroke patients.

Improved Prediction of Final Infarct Volume Using Bolus Delay–Corrected Perfusion-Weighted MRI

Background and Purpose— Magnetic resonance imaging (MRI)-based perfusion measures using dynamic susceptibility contrast are extremely useful for identification of ischemic penumbral tissue in acute stroke. However, errors in the measurement of cerebral blood flow (CBF) and mean transit time (MTT) can occur. The aim of this study was to investigate whether bolus delay–corrected (BDC) perfusion measures enable better delineation of the ischemic penumbra.

Methods— Diffusion-weighted MRI (DWI) and perfusion-weighted MRI data were acquired from 19 acute stroke patients. Perfusion abnormalities were manually defined on BDC perfusion maps (corrected MTT [cMTT] and corrected CBF [cCBF]), and on maps derived from an arterial input function placed within the contralateral (CBF, MTT) and ipsilateral (ipsilateral CBF [iCBF] and ipsilateral MTT [iMTT]) middle cerebral artery. Perfusion lesion volumes were correlated with 30-day T2-weighted MRI lesion volumes and with clinical outcome using the National Institutes of Health Stroke Scale (NIHSS).

Results— Spearman correlation coefficients for comparing lesion volumes delineated on DWI, CBF, iCBF, cCBF, MTT, iMTT, and cMTT maps with 30-day T2-weighted lesion volumes were 0.72, 0.87, 0.88, 0.90, 0.84, 0.92, and 0.96, respectively (all P <0.001). The analogous correlation coefficients for comparing 30-day National Institutes of Health Stroke Scale (NIHSS) scores were 0.39 (NS), 0.69 (NS), 0.75 ( P <0.001), 0.62 (NS), 0.72 ( P <0.001), 0.78 ( P <0.001), and 0.83 ( P <0.001), respectively.

Conclusions— Uncorrected perfusion lesion volumes overestimated the extent of ischemic injury. BDC perfusion measures (cMTT) correlated more accurately with final lesion volume and clinical outcome. Such measures offer an improved estimation of the final infarct size in acute stroke.

Impact of Admission Hyperglycemia on Stroke Outcome After Thrombolysis

BACKGROUND AND PURPOSE

We evaluated the impact of admission hyperglycemia (HG) on stroke outcome in relation to the timing of reperfusion in patients treated with tissue plasminogen activator (tPA).

METHODS

We studied 138 consecutive stroke patients with a documented middle cerebral artery (MCA) occlusion treated with intravenous tPA <3 hours of stroke onset. Serum glucose was determined at baseline before tPA administration. HG was defined as a glucose level >140 mg/dL. National Institutes of Health Stroke Scale (NIHSS) scores were obtained at baseline and 24 hour. Transcranial Doppler monitoring of recanalization was conducted during the first 12 hour of stroke onset. mRS was used to assess outcome at 3 months.

RESULTS

Median baseline NIHSS score was 17 points. At baseline, 42 (37.3%) patients were hyperglycemic and 96 (62.7%) normoglycemic. Reperfusion was achieved <3 hours of stroke onset in 32 (23%) patients, between 3 to 6 hours in 49 (36%), 6 to 12 hours in 15 (12%), and in 32 (23%) the MCA remained occluded at 12 hours. A logistic regression model revealed that baseline NIHSS score >16 points (odds ratio [OR], 3.32; 95% CI, 2.18 to 24.7; P=0.032) and admission glucose level >140 mg/dL (OR, 5.65; 95% CI, 1.97 to 16.18; P=0.002) independently predicted poor outcome (modified Rankin scale, 3 to 6) at 3 months. After adjusting by age, stroke severity, site of MCA occlusion, and degree of recanalization, the contribution of HG for poor outcome was higher as shorter the time to reperfusion. The highest odds for poor outcome related to HG corresponded to patients who recanalized <3 hour (OR, 3.1; 95% CI, 1.8 to 14.3; P=0.002), as compared with those who recanalized between 3 and 6 hours (OR, 2.1; 95% CI, 1.1 to 16; P=0.034) and between 6 to 12 hours (OR, 1.1; 95% CI, 0.7 to 21; P=0.43). Moreover, baseline glucose level was negatively correlated (r=-0.45; P=0.001) with the degree of improvement in the NIHSS score at 24 hours after early (<3 hours) but not after delayed (>3 hours) or no recanalization.

CONCLUSIONS

The impact of admission HG on stroke outcome varies depending on the time to tPA-induced reperfusion. The detrimental effect of acute HG is higher after early than after delayed or no reperfusion. Ultra-early glycemic control before reperfusion may improve the efficacy of thrombolytic therapy.

Does the acute diffusion-weighted imaging lesion represent penumbra as well as core? A combined quantitative PET/MRI voxel-based study

In acute ischemic stroke, the diffusion-weighted imaging (DWI) lesion is widely held to represent the core of irreversible damage and is therefore crucial in selecting patients for thrombolysis. However, recent research suggests it may also represent penumbra. An illustrative patient was imaged 7 hours after stroke onset with back-to-back 3T diffusion tensor imaging and quantitative positron emission tomography, which showed a DWI lesion and misery perfusion, respectively. Using previously validated voxel-based probabilistic CBF, CMRO2, and Oxygen Extraction Fraction (OEF) thresholds, the authors show that the DWI lesion contained not only core but also substantial proportions of penumbra. Also, severe apparent diffusion coefficient reductions were present within the potentially salvageable penumbra as well as in the core. These findings have potential implications regarding treatment decisions.

Which time-to-peak threshold best identifies penumbral flow? A comparison of perfusion-weighted magnetic resonance imaging and positron emission tomography in acute ischemic stroke

BACKGROUND AND PURPOSE

In acute ischemic stroke, the hypoperfused but viable tissue is the main therapeutic target. In clinical routine, time-to-peak (TTP) maps are frequently used to estimate the hemodynamic compromise and to calculate the mismatch volume. We evaluated the accuracy of TTP maps to identify penumbral flow by comparison with positron emission tomography (PET).

METHODS

Magnetic resonance imaging (MRI) and PET were performed in 11 patients with acute ischemic stroke (median 8 hours after stroke onset, 60 minutes between MRI and PET imaging). The volumes defined by increasing TTP thresholds (relative TTP delay of >2, >4, >6, >8, and >10 seconds) were compared with the volume of hypoperfusion (<20 mL/100 g per min) assessed by 15O-water PET. In a volumetric analysis, each threshold's sensitivity, specificity, and predictive values were calculated.

RESULTS

The median hypoperfusion volume was 34.5 cm3. Low TTP thresholds included large parts of the hypoperfused but also large parts of normoperfused tissue (median sensitivity/specificity: 93%/60% for TTP >2) and vice versa (50%/91% for TTP >10). TTP >4 seconds best identifies hypoperfusion (84%/77%). The positive predictive values increased with the size of hypoperfusion.

CONCLUSIONS

This first comparison of quantitative PET-CBF with TTP maps in acute ischemic human stroke indicates that the TTP threshold is crucial to reliably identify the tissue at risk; TTP >4 seconds best identifies penumbral flow; and TTP maps overestimate the extent of true hemodynamic compromise depending on the size of ischemia. Only if methodological restrictions are kept in mind, relative TTP maps are suitable to estimate the mismatch volume.

Therapeutic time window of thrombolytic therapy following stroke

Stroke is the third leading cause of death after myocardial infarction and cancer and the leading cause of permanent disability and of disability-adjusted loss of independent life-years in Western countries. Thrombolysis is the treatment of choice for acute stroke within 3 hours after symptom onset. Treatment beyond the 3-hour time window has not been shown to be effective in any single trial; however, meta-analyses suggest a somewhat less but still significant effect within 3 to 6 hours after stroke. It seems reasonable to apply improved selection criteria that would allow one to differentiate patients with a relevant indication for thrombolytic therapy from those who do not have one. We present an overview of a diagnostic approach to acute stroke management that allows the clinician to individualize patient management based on pathophysiologic reasoning and not rigid time windows established by randomized controlled trials. Therefore, this review concentrates on giving the reader an integrated knowledge of the current status of thrombolytic therapy in stroke and then develops a treatment algorithm based on pathophysiologic information rendered by a multiparametric stroke magnetic resonance imaging protocol.

Assessing acute middle cerebral artery ischemic stroke by quantitative electric tomography

This paper focuses on the application of quantitative electric tomography (qEEGT) to map changes in EEG generators for detection of early signs of ischemia in patients with acute middle cerebral artery stroke. Thirty-two patients were studied with the diagnosis of acute ischemic stroke of the left middle cerebral artery territory, within the first 24 hours of their clinical evolution. Variable Resolution Electrical Tomography was used for estimating EEG source generators. High resolution source Z-spectra and 3- dimensional images of Z values for all the sources at each frequency were obtained for all cases. To estimate statistically significant increments and decrements of brain electric activity within the frequency spectra, the t-Student vs. Zero test was performed. A significant increment of delta activity was observed on the affected vascular territory, and a more extensive increment of theta activity was detected. A significant alpha decrement was found in the parieto-occipital region of the affected cerebral hemisphere (left), and in the medial and posterior region of the right hemisphere. These findings suggest that qEEGT Z delta images are probably related to the main ischemic core within the affected arterial territory; penumbra, diaschisis, edema, might explain those observed theta and alpha abnormalities. It was concluded that qEEGT is useful for the detection of early signs of ischemia in acute ischemic stroke.

Outcome of Acute Stroke Patients Without Visible Occlusion on Early Arteriography

BACKGROUND

The aim of this study was to determine the clinical and radiological outcome of acute stroke patients who had no vessel occlusion on arteriography and to define predictors of clinical outcome.

METHODS

We analyzed clinical and radiological data of stroke patients whose arteriography performed within 6 hours of symptom onset did not visualize any vessel occlusion.

RESULTS

Twenty-eight of 283 consecutive patients (10%) who underwent arteriography with the intention to perform intraarterial thrombolysis did not show any arterial occlusion. Their median baseline National Institutes of Health Stroke Scale (NIHSS) score was 7. Time from symptom onset to arteriography ranged from 115 to 315 minutes; on average, it was 226 minutes. Presumed stroke cause was cardiac embolism in 11 patients (39%), small artery disease in 6 (21%), coronary angiography in 1 (4%), and undetermined in 10 patients (36%). After 3 months, modified Rankin Scale score (mRS) was < or =2 in 21 patients (75%), indicating a favorable outcome. Six patients (21%) had a poor outcome (mRS 3 or 4) and 1 patient (4%) had a myocardial infarction and died. Twenty-seven patients had follow-up brain imaging. It was normal in 5, showed a lacunar lesion in 8, a striatocapsular infarct in 2, a small or medium-sized anterior circulation infarct in 6, multiple small anterior circulation infarcts in 2, and multiple posterior circulation infarcts in 4. No predictors of clinical outcome were identified.

CONCLUSIONS

Most acute stroke patients with normal early arteriography show infarcts on brain imaging; however, clinical outcome is usually favorable.

Improving the prediction of final infarct size in acute stroke with bolus delay-corrected perfusion MRI measures

PURPOSE

To investigate whether bolus delay-corrected dynamic susceptibility contrast (DSC) perfusion MRI measures allowed a more accurate estimation of eventual infarct volume in 14 acute stroke patients using a predictive tissue classifier algorithm.

MATERIALS AND METHODS

Tissue classification was performed using a expectation maximization and k-means clustering algorithm utilizing diffusion and T2 measures (diffusion-weighted imaging [DWI], apparent diffusion coefficient [ADC], and T2) combined with uncorrected perfusion measures cerebral blood flow ((CBF) and mean transit time [MTT]), bolus delay-corrected perfusion measures (cCBF and cMTT), and bolus delay-corrected perfusion indices (cCBF and cMTT with bolus delay).

RESULTS

The mean similarity index (SI), a kappa-based correlation statistic reflecting the pixel-by-pixel classification agreement between predicted and 30-day T2 lesion volumes, were 0.55 +/- 0.19, 0.61 +/- 0.15 (P < 0.02) and 0.60 +/- 0.17 (P <0.03), respectively. Spearman's correlation coefficients, comparing predicted and final lesion volumes were 0.56 (P < 0.05), 0.70 (P < 0.01), and 0.84 (P < 0.001), respectively. We found a more significant correlation between predicted infarct volumes derived from bolus delay-corrected perfusion measures than from conventional perfusion measures when combined with diffusion measures and compared with final lesion volumes measured on 30-day T2 MRI scans.

CONCLUSION

Bolus delay-corrected perfusion measures enable an improved prediction of infarct evolution and evaluation of the hemodynamic status of neuronal tissue in acute stroke.