Apparent Diffusion Coefficient Thresholds Do Not Predict the Response to Acute Stroke Thrombolysis

Refined Ischemic Penumbra Imaging with Tissue pH and Diffusion Kurtosis Magnetic Resonance Imaging

Imaging has played a vital role in our mechanistic understanding of acute ischemia and the management of acute stroke patients. The most recent DAWN and DEFUSE-3 trials showed that endovascular therapy could be extended to a selected group of late-presenting stroke patients with the aid of imaging. Although perfusion and diffusion MRI have been commonly used in stroke imaging, the approximation of their mismatch as the penumbra is oversimplified, particularly in the era of endovascular therapy. Briefly, the hypoperfusion lesion includes the benign oligemia that does not proceed to infarction. Also, with prompt and effective reperfusion therapy, a portion of the diffusion lesion is potentially reversible. Therefore, advanced imaging that provides improved ischemic tissue characterization may enable new experimental stroke therapeutics and eventually further individualize stroke treatment upon translation to the clinical setting. Specifically, pH imaging captures tissue of altered metabolic state that demarcates the hypoperfused lesion into ischemic penumbra and benign oligemia, which remains promising to define the ischemic penumbra's outer boundary. On the other hand, diffusion kurtosis imaging (DKI) differentiates the most severely damaged and irreversibly injured diffusion lesion from the portion of diffusion lesion that is potentially reversible, refining the inner boundary of the penumbra. Altogether, the development of advanced imaging has the potential to not only transform the experimental stroke research but also aid clinical translation and patient management.

Cerebral Hyperperfusion and Concomitant Reversible Lesion at the Splenium after Direct Revascularization Surgery for Adult Moyamoya Disease: Possible Involvement of MERS and Watershed Shift Phenomenon

Superficial temporal artery (STA)–middle cerebral artery (MCA) bypass is the standard surgical treatment for moyamoya disease (MMD). Local cerebral hyperperfusion (CHP) is one of the potential complications, which could enhance intrinsic inflammation and oxidative stress in MMD patients and accompany concomitant watershed shift (WS) phenomenon, defined as the paradoxical decrease in the cerebral blood flow (CBF) near the site of CHP. However, CHP and simultaneous remote reversible lesion at the splenium have never been reported. A 22-year-old man with ischemic-onset MMD underwent left STA–MCA bypass. Although asymptomatic, local CHP and a paradoxical CBF decrease at the splenium were evident on N-isopropyl-p-[¹²³I] iodoamphetamine single-photon emission computed tomography 1 day after surgery. The patient was maintained under strict blood pressure control, but he subsequently developed transient delirium 4 days after surgery. MRI revealed a high-signal-intensity lesion with a low apparent diffusion coefficient at the splenium. After continued intensive management, the splenial lesion disappeared 14 days after surgery. The patient was discharged without neurological deficits. Catheter angiography 2 months later confirmed marked regression of posterior-to-anterior collaterals via the posterior pericallosal artery, suggesting dynamic watershed shift between blood flow supplies from the posterior and anterior circulation. Mild encephalitis/encephalopathy with a reversible splenial lesion could explain the pathophysiology of the postoperative splenial lesion in this case, which is associated with generation of oxidative stress, enhanced inflammation, and metabolic abnormalities. Rapid postoperative hemodynamic changes, including local CHP and concomitant WS phenomenon, might participate in the formation of the splenial lesion.

The ischemic penumbra: From concept to reality

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

Reliability of Smartphone for Diffusion-Weighted Imaging-Alberta Stroke Program Early Computed Tomography Scores in Acute Ischemic Stroke Patients: Diagnostic Test Accuracy Study

Background

High-quality neuroimages can be viewed using a medical app installed on a smartphone. Although interdevice agreement between smartphone and desktop PC monitor was found to be favorable for evaluating computed tomography images, there are no interdevice agreement data for diffusion-weighted imaging (DWI).

Objective

The aim of our study was to compare DWI interpretation using the Join smartphone app with that using a desktop PC monitor, in terms of interdevice and interrater agreement and elapsed interpretation time.

Methods

The ischemic change in the DWI of consecutive patients with acute stroke in the middle cerebral artery territory was graded by 2 vascular neurologists using the Join smartphone app and a desktop PC monitor. The vascular neurologists were blinded to all patient information. Each image was categorized as either Diffusion-Weighted Imaging–Alberta Stroke Program Early Computed Tomography Scores (DWI-ASPECTS) ≥7 or DWI-ASPECTS <7 according to the Japanese Society for Neuroendovascular Therapy. We analyzed interdevice agreement and interrater agreement with respect to DWI-ASPECTS. Elapsed interpretation time was compared between DWI-ASPECTS evaluated by the Join smartphone app and a desktop PC monitor.

Results

We analyzed the images of 111 patients (66% male; median age=69 years; median National Institutes of Health Stroke Scale score on admission=4). Interdevice agreement regarding DWI-ASPECTS between the smartphone and the desktop PC monitor was favorable (vascular neurologist 1: κ=0.777, P<.001, vascular neurologist 2: κ=0.787, P<.001). Interrater agreement was also satisfactory for the smartphone (κ=0.710, P<.001) and the desktop PC monitor (κ=0.663, P<.001). Median elapsed interpretation time was similar between the smartphone and the desktop PC monitor (vascular neurologist 1: 1.7 min vs 1.6 min; P=.64); vascular neurologist 2: 2.4 min vs 2.0 min; P=.14).

Conclusions

The use of a smartphone app enables vascular neurologists to estimate DWI-ASPECTS accurately and rapidly. The Join medical smartphone app shows great promise in the management of acute stroke.

Reversible diffusion-weighted imaging lesions in acute ischemic stroke: A systematic review

OBJECTIVES

To systematically review the literature for reversible diffusion-weighted imaging (DWIR) lesions and to describe its prevalence, predictors, and clinical significance.

METHODS

Studies were included if the first DWI MRI was performed within 24 hours of stroke onset and follow-up DWI or fluid-attenuated inversion recovery (FLAIR)/T2 was performed within 7 or 90 days, respectively, to measure DWIR. We abstracted clinical, imaging, and outcomes data.

RESULTS

Twenty-three studies met the study criteria. The prevalence of DWIR was 26.5% in DWI-based studies and 6% in FLAIR/T2-based studies. DWIR was associated with recanalization or reperfusion of the ischemic tissue with or without the use of tissue plasminogen activator (t-PA) or endovascular therapy, earlier treatment with t-PA, shorter time to endovascular therapy after MRI, and absent or less severe perfusion deficit within the DWI lesion. DWIR was associated with early neurologic improvement in 5 of 6 studies (defined as improvement in the NIH Stroke Scale (NIHSS) score by 4 or 8 points from baseline or NIHSS score 0 to 2 at 24 hours after treatment or at discharge or median NIHSS score at 7 days) and long-term outcome in 6 of 7 studies (defined as NIHSS score ≤1, improvement in the NIHSS score ≥8 points, or modified Rankin Scale score up to ≤2 at 30 or 90 days) likely due to reperfusion.

CONCLUSIONS

DWIR is seen in up to a quarter of patients with acute ischemic stroke, and it is associated with good clinical outcome following reperfusion. Our findings highlight the pitfalls of DWI to define ischemic core in the early hours of stroke.

Utility of Minimum Apparent Diffusion Coefficient Ratios in Alberta Stroke Program Early CT Score Regions for Deciding on Stroke Therapy

BACKGROUND AND PURPOSE

Therapeutic indications for recombinant tissue plasminogen activator therapy and endovascular therapy need to be assessed for patients with hyperacute ischemic stroke. We investigated the relationship between the minimum apparent diffusion coefficient ratios in each Alberta Stroke Program Early CT Score region and reversible lesion in patients with hyperacute ischemic stroke receiving recombinant tissue plasminogen activator therapy and/or treated with endovascular therapy.

MATERIALS AND METHODS

We retrospectively evaluated 29 patients with first ischemic stroke due to stenosis/occlusion of the internal carotid artery or horizontal portion of the middle cerebral artery that was successfully recanalized by recombinant tissue plasminogen activator therapy and/or treated with endovascular therapy. We measured the minimum apparent diffusion coefficient value in each Alberta Stroke Program Early CT Score region (11 regions) and calculated the ratio.

RESULTS

There was a significant difference in minimum apparent diffusion coefficient ratios between regions that included and did not include infarction (P < .0001), which were distinguishable with a cutoff value of .808 (area under the curve = .80, P < .001). A statistical difference in the proportion of infarction with the cutoff value was observed between patients treated with endovascular therapy and receiving recombinant tissue plasminogen activator therapy alone (9.9% versus 24.6%, P = .0041) and between patients with affected middle cerebral and internal carotid arteries (7.0% versus 24.2%, P = .0002). The lowest apparent diffusion coefficient ratio was associated with the time to recombinant tissue plasminogen activator injection.

CONCLUSIONS

Minimum apparent diffusion coefficient ratios in Alberta Stroke Program Early CT Score regions are useful in predicting therapeutic effect.

Imaging the physiological evolution of the ischemic penumbra in acute ischemic stroke

We review the hemodynamic, metabolic and cellular parameters affected during early ischemia and their changes as a function of approximate cerebral blood flow ( CBF) thresholds. These parameters underlie the current practical definition of an ischemic penumbra, namely metabolically affected but still viable brain tissue. Such tissue is at risk of infarction under continuing conditions of reduced CBF, but can be rescued through timely intervention. This definition will be useful in clinical diagnosis only if imaging techniques exist that can rapidly, and with sufficient accuracy, visualize the existence of a mismatch between such a metabolically affected area and regions that have suffered cell depolarization. Unfortunately, clinical data show that defining the outer boundary of the penumbra based solely on perfusion-related thresholds may not be sufficiently accurate. Also, thresholds for CBF and cerebral blood volume ( CBV) differ for white and gray matter and evolve with time for both inner and outer penumbral boundaries. As such, practical penumbral imaging would involve parameters in which the physiology is immediately displayed in a manner independent of baseline CBF or CBF threshold, namely pH, oxygen extraction fraction ( OEF), diffusion constant and mean transit time ( MTT). Suitable imaging technologies will need to meet this requirement in a 10-20 min exam.

JOURNAL CLUB: Evaluation of Diffusion Kurtosis Imaging of Stroke Lesion With Hemodynamic and Metabolic MRI in a Rodent Model of Acute Stroke

OBJECTIVE

Diffusion kurtosis imaging (DKI) has emerged as a new acute stroke imaging approach, augmenting routine DWI. Although it has been shown that a diffusion lesion without kurtosis abnormality is more likely to recover after reperfusion, whereas a kurtosis lesion shows poor response, little is known about the underlying pathophysiologic profile of the kurtosis lesion versus the kurtosis lesion-diffusion lesion mismatch.

MATERIALS AND METHODS

We performed multiparametric MRI, including arterial spin labeling, pH-sensitive amide proton transfer, and DKI, in a rodent model of acute stroke caused by embolic middle cerebral artery occlusion. Diffusion and kurtosis lesions were semiautomatically segmented, and multiparametric MRI indexes were compared among the kurtosis lesion, diffusion lesion, kurtosis lesion-diffusion lesion mismatch, and the contralateral normal tissue area.

RESULTS

We confirmed a significant difference between diffusion lesion and kurtosis lesion volumes (mean [± SD] volume, 151 ± 65 vs 125 ± 47 mm³; p < 0.05). Although ischemic lesions have significantly reduced cerebral blood flow compared with contralateral normal tissue, we did not find a significant difference in cerebral blood flow between the kurtosis lesion and the kurtosis lesion-diffusion lesion mismatch (mean cerebral blood flow, 0.53 ± 0.10 vs 0.47 ± 0.14 mL/g of tissue per minute; p > 0.05). Of importance, the pH of the kurtosis lesion was significantly lower than that of the lesion mismatch (mean pH, 6.81 ± 0.08 vs 6.89 ± 0.09; p < 0.01).

CONCLUSION

The present study confirms that DKI provides an expedient approach for refining the heterogeneous DWI lesion that is associated with graded metabolic derangement, which is promising for improving the infarction core definition and ultimately helping to guide stroke treatment.

The diagnostic value of susceptibility-weighted imaging for ischemic penumbra in patients with acute ischemic stroke

BACKGROUND

The ischemia penumbra area is not easy to be detected accurately using noninvasive imaging methods.

OBJECTIVE

We aim to assess the diagnostic value of susceptibility-weighted imaging (SWI) for ischemic penumbra in patients with acute ischemic stroke.

METHODS

A retrospective analysis was carried out in 47 patients with ischemic stroke involving the middle cerebral artery. Mean transit time (MTT), time to peak, relative cerebral blood flow, and relative cerebral blood volume maps were created after image processing.

RESULTS

No significant difference was found in the mismatch between the SWI and diffusion-weighted imaging (SWI-DWI) or in the MTT-DWI mismatch scores (P= 0.056, Kappa = 0.864).

CONCLUSIONS

SWI provides information comparable to PWI and, thus, could serve as a reliable magnetic resonance technique for assessing ischemic penumbrae.

Early Change in Stroke Size Performs Best in Predicting Response to Therapy

BACKGROUND

Reliable imaging biomarkers of response to therapy in acute stroke are needed. The final infarct volume and percent of early reperfusion have been used for this purpose. Early fluctuation in stroke size is a recognized phenomenon, but its utility as a biomarker for response to therapy has not been established. This study examined the clinical relevance of early change in stroke volume and compared it with the final infarct volume and percent of early reperfusion in identifying early neurologic improvement (ENI).

METHODS

Acute stroke patients, enrolled between 2013 and 2014 with serial magnetic resonance imaging (MRI) scans (pretreatment baseline, 2 h post, and 24 h post), who received thrombolysis were included in the analysis. Early change in stroke volume, infarct volume at 24 h on diffusion, and percent of early reperfusion were calculated from the baseline and 2 h MRI scans were compared. ENI was defined as ≥4 point decrease in National Institutes of Health Stroke Scales within 24 h. Logistic regression models and receiver operator characteristics analysis were used to compare the efficacy of 3 imaging biomarkers.

RESULTS

Serial MRIs of 58 acute stroke patients were analyzed. Early change in stroke volume was significantly associated with ENI by logistic regression analysis (OR 0.93, p = 0.048) and remained significant after controlling for stroke size and severity (OR 0.90, p = 0.032). Thus, for every 1 mL increase in stroke volume, there was a 10% decrease in the odds of ENI, while for every 1 mL decrease in stroke volume, there was a 10% increase in the odds of ENI. Neither infarct volume at 24 h nor percent of early reperfusion were significantly associated with ENI by logistic regression. Receiver-operator characteristic analysis identified early change in stroke volume as the only biomarker of the 3 that performed significantly different than chance (p = 0.03).

CONCLUSIONS

Early fluctuations in stroke size may represent a more reliable biomarker for response to therapy than the more traditional measures of final infarct volume and percent of early reperfusion.

How Sustained Is 24-Hour Diffusion-Weighted Imaging Lesion Reversal?

Background and Purpose—

Here, we assessed how sustained is reversal of the acute diffusion lesion (RAD) observed 24 hours after intravenous thrombolysis, and the relationships between RAD fate and early neurological improvement.

Methods—

We analyzed 155 consecutive patients thrombolyzed intravenously 152 minutes (median) after stroke onset and who underwent 3 MR sessions: 1 before and 2 after treatment (median times from onset, 25.6 and 54.3 hours, respectively). Using voxel-based analysis of diffusion-weighted imaging (DWI) 1 , DWI 2 , and DWI 3 lesions on coregistered image data sets, we assessed the outcome of RAD voxels (hyperintense on DWI 1 but not on DWI 2 ) as transient or sustained on DWI 3 , and their relationships with early neurological improvement, defined as ΔNational Institutes of Health Stroke Scale ≥8 or National Institutes of Health Stroke Scale ≤1 at 24 hours. T max and apparent diffusion coefficient values were compared between sustained and transient RAD voxels.

Results—

The median (interquartile range) baseline National Institutes of Health Stroke Scale and DWI 1 lesion volume were 11 (7–18) mL and 15.6 (6.0–50.9) mL, respectively. The median (interquartile range) RAD volume on DWI 2 was 2.8 (1.1–6.6) mL, of which 70% was sustained on DWI 3 . Sixteen (10.3%) patients had sustained RAD ≥10 mL. As compared with transient RAD voxels, sustained RAD voxels had nonsignificantly higher baseline apparent diffusion coefficient values (median [interquartile range], 793 [717–887] versus 777 [705–869]×10 −6 mm 2 ·s −1 , respectively; P =0.08) and significantly better perfusion ( T max , mean±SD, 6.3±3.2 versus 7.8±4.0 s; P <0.001). At variance with transient RAD, the volume of sustained RAD was associated with early neurological improvement in multivariate analysis (odds ratio, 1.08; 95% confidence interval, [1.01–1.17], per 1-mL increase; P =0.03).

Conclusions—

After thrombolysis, over two-thirds of the DWI lesion reversal captured on 24-hour follow-up MR is sustained. Sustained DWI lesion reversal volume is a strong imaging correlate of early neurological improvement.

N-acetylaspartate decrease in acute stage of ischemic stroke: a perspective from experimental and clinical studies

N-acetylaspartate (NAA) appears in a prominent peak in proton magnetic resonance spectroscopy ((1)H-MRS) of the brain. Exhibition by NAA of time-dependent attenuation that reflects energy metabolism during the acute stage of cerebral ischemia makes this metabolite a unique biomarker for assessing ischemic stroke. Although magnetic resonance (MR) imaging is a powerful technique for inspecting the pathological changes that occur during ischemic stroke, biomarkers that directly reflect the drastic metabolic changes associated with acute-stage ischemia are strongly warranted for appropriate therapeutic decision-making in daily clinical settings. In this review, we provide a brief overview of NAA metabolism and focus on the use of attenuation in NAA as a means for assessing the pathophysiological changes that occur during the acute stage of ischemic stroke.

Acute Stroke Imaging Part I: Fundamentals

Neuroimaging is essential to stroke diagnosis and management. To date, the non-contrast CT has served as our main diagnostic tool. Although brain parenchymal changes visible on CT do provide valuable prognostic information, they provide limited insight into the potential for tissue salvage in response to reperfusion therapy, such as thrombolysis. Newer advanced CT and MRI based imaging techniques have increased the detection sensitivity for hyperacute and chronic parenchymal changes, including ischemia and hemorrhage, permit visualization of blood vessels and cerebral blood flow. This review outlines the basic principles underlying acquisition and interpretation of these newer imaging modalities in the setting of acute stroke. The utility of advanced brain parenchymal and blood flow imaging in the context of acute stroke patient management is also discussed. Part II in this series is a discussion of how these techniques can be used to rationally select appropriate patients for thrombolysis based on pathophysiological data.

Apparent diffusion coefficient threshold for delineation of ischemic core

BACKGROUND

MRI-based selection of patients for acute stroke interventions requires rapid accurate estimation of the infarct core on diffusion-weighted MRI. Typically used manual methods to delineate restricted diffusion lesions are subjective and time consuming. These limitations would be overcome by a fully automated method that can rapidly and objectively delineate the ischemic core. An automated method would require predefined criteria to identify the ischemic core.

AIM

The aim of this study is to determine apparent diffusion coefficient-based criteria that can be implemented in a fully automated software solution for identification of the ischemic core.

METHODS

Imaging data from patients enrolled in the Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution (DEFUSE) study who had early revascularization following intravenous thrombolysis were included. The patients' baseline restricted diffusion and 30-day T2 -weighted fluid-attenuated inversion recovery lesions were manually delineated after coregistration. Parts of the restricted diffusion lesion that corresponded with 30-day infarct were considered ischemic core, whereas parts that corresponded with normal brain parenchyma at 30 days were considered noncore. The optimal apparent diffusion coefficient threshold to discriminate core from noncore voxels was determined by voxel-based receiver operating characteristics analysis using the Youden index.

RESULTS

51,045 diffusion positive voxels from 14 patients who met eligibility criteria were analyzed. The mean DWI lesion volume was 24 (± 23) ml. Of this, 18 (± 22) ml was ischemic core and 3 (± 5) ml was noncore. The remainder corresponded to preexisting gliosis, cerebrospinal fluid, or was lost to postinfarct atrophy. The apparent diffusion coefficient of core was lower than that of noncore voxels (P < 0.0001). The optimal threshold for identification of ischemic core was an apparent diffusion coefficient ≤ 620 × 10(-6) mm(2) /s (sensitivity 69% and specificity 78%).

CONCLUSIONS

Our data suggest that the ischemic core can be identified with an absolute apparent diffusion coefficient threshold. This threshold can be implemented in image analysis software for fully automated segmentation of the ischemic core.

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.

Visualizing cell death in experimental focal cerebral ischemia: promises, problems, and perspectives

One of the hallmarks of stroke pathophysiology is the widespread death of many different types of brain cells. As our understanding of the complex disease that is stroke has grown, it is now generally accepted that various different mechanisms can result in cell damage and eventual death. A plethora of techniques is available to identify various pathological features of cell death in stroke; each has its own drawbacks and pitfalls, and most are unable to distinguish between different types of cell death, which partially explains the widespread misuse of many terms. The purpose of this review is to summarize the standard histopathological and immunohistochemical techniques used to identify various pathological features of stroke. We then discuss how these methods should be properly interpreted on the basis of what they are showing, as well as advantages and disadvantages that require consideration. As there is much interest in the visualization of stroke using noninvasive imaging strategies, we also specifically discuss how these techniques can be interpreted within the context of cell death.

Non-invasive optical imaging of stroke

The acute onset of a neurological deficit is the key clinical feature of stroke. In most cases, however, pathophysiological changes in the cerebral vasculature precede the event, often by many years. Persisting neurological deficits may also require long-term rehabilitation. Hence, stroke may be considered a chronic disease, and diagnostic and therapeutic efforts must include identification of specific risk factors, and the monitoring of and interventions in the acute and subacute stages, and should aim at a pathophysiologically based approach to optimize the rehabilitative effort. Non-invasive optical techniques have been experimentally used in all three stages of the disease and may complement the established diagnostic and monitoring tools. Here, we provide an overview of studies using the methodology in the context of stroke, and we sketch perspectives of how they may be integrated into the assessment of the highly dynamic pathophysiological processes during the acute and subacute stages of the disease and also during rehabilitation and (secondary) prevention of stroke.

Imaging stroke patients with unclear onset times

Stroke is a leading cause of death and adult morbidity worldwide. By defining stroke symptom onset by the time the patient was last known to be well, many patients whose onsets are unwitnessed are automatically ineligible for thrombolytic therapy. Advanced brain imaging may serve as a substitute witness to estimate stroke onset and duration in those patients who do not have a human witness. This article reviews and compares some of these imaging-based approaches to thrombolysis eligibility, which can potentially expand the use of thrombolytic therapy to a broader population of acute stroke patients.

Correlation between CT and diffusion-weighted imaging of acute cerebral ischemia in a rat model

BACKGROUND AND PURPOSE

The quantitative temporal relationship between changes in CT attenuation, ADC value, and DWI signal intensity of acute ischemic tissue has not yet been determined in an animal model. This study was performed to determine the temporal relationship between CT attenuation, ADC value, and DWI signal intensity in acute cerebral ischemia.

MATERIALS AND METHODS

CT and DWI were performed at 1, 3, 5, 7, and 9 hours after left MCA occlusion in 11 rats. Mean values for CT attenuation, ADC, and DWI signal intensity were determined for the ischemic hemisphere and contralateral normal hemisphere. Temporal changes in each mean value and the relationship between CT attenuation and ADC value and DWI signal intensity were evaluated.

RESULTS

The decrease of CT attenuation and the increase of DWI signal intensity occurred gradually after MCA occlusion, while ADC value decreased rapidly at 1 hour. Although correlation was significant between time and rCT or rDWI (P<.01, respectively), no correlation between time and rADC was found (P=.33). There was a significant linear correlation between rCT and rDWI (r=0.497, P<.01), but no significant correlation between rCT and rADC (P=.509) was found.

CONCLUSIONS

The temporal change in CT attenuation was different from that in ADC value with no significant linear correlation between CT attenuation and ADC value for acute cerebral ischemia. However, rCT and rDWI showed a modest correlation.

DWI lesion volume reduction following acute stroke treatment with transient partial aortic obstruction

BACKGROUND AND PURPOSE

Diffusion-weighted imaging (DWI) identifies acute cerebral ischemia and DWI lesions are thought to indicate irreversibly damaged areas. However, new evidence suggests that DWI lesions may be reversible, especially with reperfusion. We present a patient who showed substantial reversal of her acute DWI lesion following partial aortic occlusion with Neuroflo™, a novel dual balloon catheter (Neuroflo™, CoAxia, MN).

METHODS

Case report/literature review.

RESULTS

A 48-year-old woman presented with left-sided weakness and demonstrated an acute DWI lesion in the right middle cerebral artery territory, with diffusion-perfusion mismatch. She was enrolled into an experimental study in which a dual balloon catheter was inflated in the lower aorta. The patient improved and her postprocedure magnetic resonance image showed a significant reduction in lesion volume on diffusion and perfusion-weighted imaging. At 1 month, a repeat computed tomography scan showed a small infarction in the right insula, lentiform nucleus, and frontal cortex. The patient had recovered with no significant disability at her 3-month follow-up.

CONCLUSION

Reperfusion can improve DWI lesions. Partial aortic obstruction with a novel dual balloon catheter may be useful to promote reperfusion.

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

RATIONALE AND OBJECTIVES

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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

Low Cerebral Blood Volume Is Predictive of Diffusion Restriction Only in Hyperacute Stroke

Background and Purpose— Diffusion-weighted MRI (DWI) demonstrates ischemic tissue with high sensitivity. Although low cerebral blood volume (CBV) is also used as a marker for infarction, the quantitative relationship between diffusion abnormalities and CBV is unknown. We tested the hypothesis that CBV would decrease proportionally to the apparent diffusion coefficient in patients with acute stroke and thus could be used as a surrogate parameter for diffusion restriction.

Methods— Perfusion-weighted imaging and DWI was performed in 54 patients within 28 hours of symptom onset. Mean apparent diffusion coefficient, cerebral blood flow, and CBV were measured within DWI lesions and contralateral regions.

Results— Within DWI lesions, CBV (3.3±1.9 mL/100 g) was significantly decreased relative to contralateral regions (4.1±2.1 mL/100 g, P <0.001). Relative CBV was not decreased in patients with evidence of early reperfusion (1.2±0.5) or mild stroke (National Institutes of Health Stroke Scale <4, 1.1±0.6). Linear regression indicated that relative CBV was predictive of relative apparent diffusion coefficient only in patients imaged within 9 hours of symptom onset ( R =0.50, P =0.02). Ischemic tissue volumes generated using a CBV threshold of the 50th percentile of normal tissue were correlated with DWI lesion volumes ( R =0.73, P <0.001). The mean difference between the CBV threshold of the 50th percentile of normal tissue and DWI lesion volumes was 6.3 mL (95% limits of agreement, 0.1 to 12.6 mL).

Conclusions— Decreases in relative CBV are predictive of diffusion abnormalities in ischemic stroke. The pattern of CBV changes varies with clinical severity and symptom duration. Ischemic tissue volumes comparable to DWI lesions can be generated using CBV thresholds, but the use of this method is limited in patients with minor stroke.

Neuroimaging in acute ischaemic stroke: insights into unanswered questions of pathophysiology

The treatment of acute ischaemic stroke is based on the principle that there is ischaemic but still potentially salvageable tissue that could be rescued if blood flow could be restored quickly. It is assumed that salvage might only be possible in the first few hours, and that infarct expansion is a direct result of failed recanalization of the main artery. This concept arose from experimental work in the 1970s, supported more recently by studies using imaging to identify penumbral tissue. However, although magnetic resonance diffusion and perfusion imaging is a way of imaging penumbral tissue and has been around for over a decade, it is not an easy technique to apply in practice and its use has produced conflicting results. Computed tomography perfusion, and any other tissue perfusion imaging technique, is likely to encounter the same difficulties. Indeed many factors, other than the presence of a diffusion-perfusion mismatch acutely, may determine or influence ultimate tissue fate even days after the stroke, and in turn, clinical outcome. Many of these potential influences are beginning to emerge from studies using different forms of imaging at later times after stroke. This review will explore the information now emerging from imaging studies in large artery ischaemic stroke to summarize knowledge to date and indicate unresolved issues for the future.

Influence of fluid-attenuated inversion-recovery on stroke apparent diffusion coefficient measurements and its clinical application

BACKGROUND AND PURPOSE

The application of a fluid-attenuated inversion-recovery pulse with a conventional diffusion-weighted MRI sequence (FLAIR DWI) decreases the partial volume effects from cerebrospinal fluid on apparent diffusion coefficient (ADC) measurements. For this reason, FLAIR DWI may be more useful in the evaluation of ischemic stroke, but few studies have looked at the effect of FLAIR on ADC measurements in this setting. This study quantitatively compares FLAIR DWI and conventional DWI in ischemic stroke of varying ages to assess the potential advantages of this technique.

METHODS

We respectively analyzed 139 DWI studies in patients with ischemic stroke with and without FLAIR at varying time points ranging from hyperacute to chronic. ADC values were measured in each lesion, as well as in the contralateral normal side. Comparisons were made between the ADC values obtained from the DWI sequences with and without FLAIR for both the lesion and the normal contralateral side.

RESULTS

The ADC measurements within the ischemic lesion were very similar on FLAIR DWI and conventional DWI for lesions less than 14 days old (p>0.05), but were significantly decreased on FLAIR DWI for lesions between 15 and 30 days old and in lesions >31 days old (chronic stage) (p<0.01). The contralateral ADC values were all significantly decreased on the FLAIR DWI sequence compared with conventional DWI (p<0.01).

CONCLUSIONS

The application of an inversion pulse does not significantly affect the ADC values for early stage ischemic stroke (less than 14 days from symptom onset), but results in a more accurate relative ADC measurement by reducing the cerebrospinal fluid partial volume effects of the normal contralateral side. In addition, combined with the conventional DWI, FLAIR DWI may be helpful in determining the age of ischemic lesions.

Does diffusion-weighted imaging represent the ischemic core? An evidence-based systematic review

BACKGROUND AND PURPOSE

Diffusion-weighted(DWI) hyperintensity is hypothesized to represent irreversibly infarcted tissue (ischemic core) in the setting of acute stroke [corrected]. Measurement of the ischemic core has implications for both prognosis and therapy. We wished to assess the level of evidence in the literature supporting this hypothesis.

MATERIALS AND METHODS

We performed a systematic review of the literature relating to tissue outcomes of DWI hyperintense stroke lesions in humans. The methodologic rigor of studies was evaluated by using criteria set out by the Oxford Centre for Evidence-Based Medicine. Data from individual studies were also analyzed to determine the prevalence of patients demonstrating lesion progression, no change, or lesion regression compared with follow-up imaging.

RESULTS

Limited numbers of highly methodologically rigorous studies (Oxford levels 1 and 2) were available. There was great variability in observed rates of DWI lesion reversal (0%-83%), with a surprisingly high mean rate of DWI lesion reversal (24% of pooled patients). Many studies did not include sufficient data to determine the precise prevalence of DWI lesion growth or reversal.

CONCLUSIONS

The available tissue-outcome evidence supporting the hypothesis that DWI is a surrogate marker for ischemic core in humans is troublingly inconsistent and merits an overall grade D based on the criteria set out by the Oxford Centre for Evidence-Based Medicine.

Diffusion-weighted imaging in ischemic stroke: effect of display method on observers' diagnostic performance

RATIONALE AND OBJECTIVES

When evaluating ischemic stroke on diffusion-weighted magnetic resonance imaging (DWI), the display method has not been investigated. The purpose of this study was to determine whether standardization of the display method for DWI affects observers' diagnostic performance in detecting ischemic stroke on DWI.

MATERIALS AND METHODS

Twenty-six observers evaluated 40 DWI studies in 20 patients with acute (< 6 hours) middle cerebral arterial strokes and 20 controls for the presence of hyperintense lesions in 10 areas using the Alberta Stroke Programme Early CT Score (ASPECTS) system and one area in the corona radiata using a modified version of the ASPECTS system (ASPECTS-DWI). The images were reviewed using a standardized display method (SDM) and a conventional display method (CDM). The reading time was recorded for each session. The observers' performance was evaluated with receiver-operating characteristic analysis.

RESULTS

In all observers with ASPECTS-DWI scores of < or = 8 points, the value of the mean average area under the receiver-operating characteristic curve was slightly higher for the SDM than the CDM, but the difference was not statistically significant. In the insular ribbon, diagnostic accuracy was significantly higher with the SDM than the CDM (P = .036). In the other locations, there were no significant differences. With the SDM, the mean reading time was reduced by 7.5 seconds (P = .024).

CONCLUSION

The SDM improved diagnostic accuracy for the insular ribbon and shortened the reading time, although it did not improve observers' performance with the ASPECTS-DWI system.

Variability in absolute apparent diffusion coefficient values across different platforms may be substantial: a multivendor, multi-institutional comparison study

PURPOSE

To determine whether and to what degree absolute apparent diffusion coefficient (ADC) values vary between different imagers, vendors, field strengths, and intraimager conditions.

MATERIALS AND METHODS

Informed consent and institutional review board approval were obtained. Diffusion-weighted (DW) images with nearly identical parameters were obtained at 1.5 and 3.0 T from 12 healthy volunteers at seven institutions by using 10 magnetic resonance (MR) imagers provided by four different vendors. ADC maps were generated from isotropic DW maps, and images with a b value of 0 sec/mm(2) were generated by using in-house software. The mean pixel values for the brain tissues were calculated for evaluating the differences among coil systems, imagers, vendors, and magnetic field strengths.

RESULTS

The absolute ADC values of gray and white matter from the same vendor varied substantially: 4%-9% at 1.5 and 3.0 T. With the exception of one vendor, the intervendor variability at 1.5 T was as high as 7%. Moreover, there was substantial intraimager variability, up to 8%, depending on the coil systems in certain imagers.

CONCLUSION

There is significant variability in ADC values depending on the coil systems, imagers, vendors, and field strengths used for MR imaging. The relative ADC values may be more suitable than absolute ADC values for evaluating diffusion abnormalities in patients enrolled in multicenter acute ischemic stroke trials.

Acute Ischemic Stroke Lesion Measurement on Diffusion-weighted Imaging–Important Considerations in Designing Acute Stroke Trials With Magnetic Resonance Imaging

BACKGROUND

In acute ischemic stroke, magnetic resonance diffusion-weighted imaging (DWI) is increasingly used to select patients for inclusion or as a surrogate outcome marker in clinical trials, or in routine practice. Little is known of what factors might affect DWI lesion size measurement. We examined morphologic factors that might affect DWI lesion measurement.

METHODS

On DWI obtained less than 24 hours after stroke, we categorized lesions according to DWI appearance (solitary or multifocal; well-defined or ill-defined edges), lesion size (</>5 cm(3)), and time to imaging (<6, 6-12, and 12-24 hours). Two observers (senior neuroradiologist; less-experienced imaging neuroscientist) measured all lesions. In 4 representative cases we assessed DWI lesion volume using two apparent diffusion coefficient thresholds (0.55 and 0.65 x 10(-3) mm(2)/s).

RESULTS

Among 63 patients (33% imaged < 6 hours after stroke), the neuroradiologist measured larger lesion volumes than the imaging neuroscientist (median 4.29 v 3.50 cm(3), respectively, P < .01). Differences between observers were greatest in patients scanned within 6 hours of stroke, in multifocal ill-defined or large lesions (all P < .01). Both apparent diffusion coefficient thresholds underestimated lesion extent and included remote normal tissue, particularly in multifocal ill-defined large lesions.

CONCLUSION

DWI lesion characteristics influence lesion volume measurement. Large, multifocal, ill-defined DWI lesions obtained in less than 6 hours have the greatest variability. Trials using DWI should account for this in their study design.

Measurement of brain temperature with magnetic resonance spectroscopy in acute ischemic stroke

OBJECTIVE

Pyrexia is associated with poor outcome after stroke, but the temperature changes in the brain after stroke are poorly understood. We used magnetic resonance spectroscopic imaging (water-to-N-acetylaspartate frequency shift) to measure cerebral temperature noninvasively in stroke patients.

METHODS

We performed magnetic resonance diffusion, perfusion (diffusion- and perfusion-weighted imaging), and magnetic resonance spectroscopic imaging, compared temperatures in tissues as defined by the diffusion-weighted imaging appearance (definitely abnormal, possibly abnormal and immediately adjacent normal-appearing brain, and normal brain), and tested associations with lesion and patient characteristics.

RESULTS

Among 40 patients, temperature was higher in possibly abnormal (37.63 degrees C) than in definitely abnormal tissue (37.30 degrees C; p < 0.001) or in normal-appearing brain (ipsilateral, 37.16 degrees C; contralateral, 37.22 degrees C; both p < 0.001). Ischemic lesion temperature increased before normal brain temperature. Higher temperatures occurred in lesions that were large, had diffusion/perfusion-weighted imaging mismatch, had reduced cerebral blood flow, and in clinically severe strokes. Only 1 of 25 patients with ischemic lesion temperature greater than 37.5 degrees C was pyrexial.

INTERPRETATION

Temperature is elevated in acutely ischemic brain. More work is required to determine whether raised temperature results from ischemic metabolic reactions, impaired heat exchange from reduced cerebral blood flow, or early inflammatory cell activity (or a combination of these), but magnetic resonance spectroscopic imaging could be used in studies of temperature after brain injury and to monitor interventions.

Imaging the penumbra in acute stroke

Imaging continues to have a huge impact on the understanding of the ischemic penumbra and the management of acute stroke. Determinants of penumbral tissue fate, such as age, hyperglycemia, hematocrit, and oxygen concentration, are increasingly being recognized using neuroimaging. The significance of the penumbra in the white matter and in posterior circulation stroke is also becoming clearer. Neuroimaging is also making invaluable contributions to clinical decision making in acute stroke, especially in relation to reperfusion therapies in the 3- to 6-hour time window. Despite ongoing questions over the choice of parameters to identify the penumbra and their respective clinical usefulness, imaging is gaining widespread use in acute stroke management. However, definitive evidence of its benefit is still lacking. This review explores the recent progress and controversies relating to imaging of the penumbra.

Local Relationships Between Restricted Water Diffusion and Oxygen Consumption in the Ischemic Human Brain

BACKGROUND AND PURPOSE

MR is widely used to depict still ischemic but viable tissue in acute stroke. However, the relationship between the apparent diffusion coefficient (ADC) and energy failure from reduced oxygen supply are unknown in man.

METHODS

Acute carotid-territory stroke patients were studied prospectively with both diffusion tensor-imaging and back-to-back steady-state 15O-PET. Substantial numbers of voxels with oxygen extraction fraction >0.70 (ie, significant ongoing hypoxia) were identified in 3 patients (imaged at 7, 16 and 21 hours after stroke onset). In this voxel population, the quantitative relationships between the ADC and cerebral metabolic rate of oxygen (CMRO2), and ADC and cerebral blood flow (CBF), were assessed.

RESULTS

The ADC remained essentially unchanged until CBF reached values approximately 20 mls/100g per min, beyond which it declined linearly. In contrast, except when severely reduced, the ADC was a poorer predictor of CMRO2. For both CBF and CMRO2, however, the relationship with ADC became steeper with longer times since onset, ie, the same ADC reflected lower perfusion and CMRO2 with elapsed time.

CONCLUSIONS

Despite the small sample and late times from stroke onset, the findings indicate that the degree of restricted water diffusion reliably reflects the severity of oxygen deprivation below the penumbral threshold but is less strongly related to metabolic disruption, which may explain why the ADC does not reliably predict tissue outcome. However, the same degree of diffusion restriction may correspond to greater severity of tissue disruption with elapsing time, which has relevance for stroke therapy. Time elapsed since stroke onset should be taken into account when interpreting ADC declines and in voxel-based infarct prediction models.