Magnetic resonance imaging of acute stroke

The future of magnetic resonance-based techniques in neurology

Magnetic resonance techniques have become increasingly important in neurology for defining: 1. brain, spinal cord and peripheral nerve or muscle structure; 2. pathological changes in tissue structures and properties; and 3. dynamic patterns of functional activation of the brain. New applications have been driven in part by advances in hardware, particularly improvements in magnet and gradient coil design. New imaging strategies allow novel approaches to contrast with, for example, diffusion imaging, magnetization transfer imaging, perfusion imaging and functional magnetic resonance imaging. In parallel with developments in hardware and image acquisition have been new approaches to image analysis. These have allowed quantitative descriptions of the image changes to be used for a precise, non-invasive definition of pathology. With the increasing capabilities and specificity of magnetic resonance techniques it is becoming more important that the neurologist is intimately involved in both the selection of magnetic resonance studies for patients and their interpretation. There is a need for considerably improved access to magnetic resonance technology, particularly in the acute or intensive care ward and in the neurosurgical theatre. This report illustrates several key developments. The task force concludes that magnetic resonance imaging is a major clinical tool of growing significance and offers recommendations for maximizing the potential future for magnetic resonance techniques in neurology.

Relationships between choline magnetic resonance spectroscopy, apparent diffusion coefficient and quantitative histopathology in human glioma

This study sought to correlate quantitative presurgical proton magnetic resonance spectroscopic imaging (1H-MRSI) and diffusion imaging (DI) results with quantitative histopathological features of resected glioma tissue. The primary hypotheses were (1) glioma choline signal correlates with cell density, (2) glioma apparent diffusion coefficient (ADC) correlates inversely with cell density, (3) glioma choline signal correlates with cell proliferative index. Eighteen adult glioma patients were preoperatively imaged with 1H-MRSI and DI as part of clinically-indicated MRI evaluations. Cell density and proliferative index readings were made on surgical specimens obtained at surgery performed within 12 days of the radiologic scans. The resected tissue location was identified by comparing preoperative and postoperative MRI. The tumor to contralateral normalized choline signal ratio (nCho) and the ADC from resected tumor regions were measured from the preoperative imaging data. Counts of nuclei per high power field in 5-10 fields provided a quantitative measure of cell density. MIB-1 immunohistochemistry provided an index of the proportion of proliferating cells. There was a statistically significant inverse linear correlation between glioma ADC and cell density. There was also a statistically significant linear correlation between the glioma nCho and the cell density. The nCho measure did not significantly correlate with proliferative index. The results indicate that both ADC and spectroscopic choline measures are related to glioma cell density. Therefore they may prove useful for differentiating dense cellular neoplastic lesions from those that contain large proportions of acellular necrotic space.

Interrelations of T(1) and diffusion of water in acute cerebral ischemia of the rat

Interrelation of T(1) and diffusion of water was studied in rat models of acute global and focal cerebral ischemia. Cortical T(1), as quantified with an inversion recovery method, increased by 4-7% within a few minutes of global ischemia at 4.7 and 9.4 T, but a significantly smaller change was detected at 1.5 T. The initial T(1) change occurred within seconds of cardiac arrest, much earlier than the extensive diffusion drop after 1-2 min. Thus, the initial increase in T(1) upon acute cerebral ischemia is directly caused by cessation of blood flow. In transient middle cerebral artery occlusion (MCAO), prolonged T(1) relaxation was detected within 10 min, with a subsequent increase during the course of ischemia. Spin density did not change during the first hour, showing that T(1) increase was not caused by net accumulation of water. Interestingly, partial recovery of T(1) upon release of MCAO, occurring independent of long-term tissue outcome, was observed only in concert with diffusion recovery.

Dynamic contrast-enhanced brain perfusion imaging: technique and clinical applications

Magnetic resonance (MR) and computed tomographic (CT) perfusion imaging are evolving noninvasive imaging techniques that, unlike conventional MR and CT angiographic methods, can be used to evaluate capillary level tissue perfusion. These techniques can provide early, highly accurate delineation of ischemic tissue, allowing the underlying hemodynamic disturbances of disorders such as stroke and vasospasm to be further analyzed, as well as defining abnormal regions of blood pool in brain tumors. Because MR perfusion (MRP) and CT perfusion (CTP) imaging can assess physiologic parameters such as cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT), they offer additional data that can be useful in the detection and characterization of entities such as tumor, infection, inflammation, and infarction, which all can have similar appearances on both contrast and noncontrast enhanced conventional CT and MR images. They can also facilitate the further evaluation of processes such as early dementia, psychiatric illnesses, and migraine headaches, which may appear normal on routine CT and MR imaging. MRP and CTP might also be of value in distinguishing residual or recurrent tumor from treatment effects such as radiation-induced necrosis. This article reviews the background principles, scanning techniques, and clinical applications of noninvasive cerebral perfusion imaging.

Is early ischemic lesion volume on diffusion-weighted imaging an independent predictor of stroke outcome? A multivariable analysis

BACKGROUND AND PURPOSE

The heterogeneity of stroke makes outcome prediction difficult. Neuroimaging parameters may improve the predictive value of clinical measures such as the National Institutes of Health Stroke Scale (NIHSS). We investigated whether the volume of early ischemic brain lesions assessed with diffusion-weighted imaging (DWI) was an independent predictor of functional outcome.

METHODS

We retrospectively selected patients with nonlacunar ischemic stroke in the anterior circulation from 4 prospective Stanford Stroke Center studies evaluating early MRI. The baseline NIHSS score and ischemic stroke risk factors were assessed. A DWI MRI was performed within 48 hours of symptom onset. Clinical characteristics and early lesion volume on DWI were compared between patients with an independent outcome (Barthel Index score >/=85) and a dependent outcome (Barthel Index score <85) at 1 month. A logistic regression model was performed with factors that were significantly different between the 2 groups in univariate analysis.

RESULTS

Sixty-three patients fulfilled the entry criteria. One month after symptom onset, 24 patients had a Barthel Index score <85 and 39 had a Barthel Index score >/=85. In univariate analysis, patients with independent outcome were younger, had lower baseline NIHSS scores, and had smaller lesion volumes on DWI. In a logistic regression model, DWI volume was an independent predictor of outcome, together with age and NIHSS score, after correction for imbalances in the delay between symptom onset and MRI.

CONCLUSIONS

DWI lesion volume measured within 48 hours of symptom onset is an independent risk factor for functional independence. This finding could have implications for the design of acute stroke trials.

Clinical correlations of diffusion and perfusion lesion volumes in acute ischemic stroke

The aim of this study was to describe the clinico-radiological correlations of magnetic resonance (MR) perfusion and diffusion-weighted imaging (DWI) abnormalities in ischemic stroke. Eighteen patients had undergone MR imaging and clinical evaluation within 24 h of symptom onset and at or after 7 days. During the first 24 h the volume of perfusion abnormality (measured on the relative mean transit time map) was larger than the DWI lesion in 12/18 patients. In 6/18 patients the DWI lesion volume was larger. Acutely (<24 h) all lesion volumes showed a significant correlation with acute clinical severity measured by the National Institutes of Health Stroke Scale score. The correlations of the hypoperfusion volume (rho = 0.86, p = 0.0001) and the volume 'tissue at risk' (larger than the DWI and perfusion lesion volumes, rho = 0.86, p = 0. 0001) with acute clinical severity were slightly higher than for the DWI lesion volume (rho = 0.76, p = 0.0001). The difference between the volume of tissue at risk (acutely) and the infarct on follow-up T(2)-weighted imaging correlated significantly with change in clinical severity from acute to chronic time points (rho = 0.72, p = 0.001). Such clinico-radiological relationships may support the use of DWI and perfusion MR in decisions concerning the administration and evaluation of stroke therapies.

Changes in the apparent diffusion coefficient of water and T2 relaxation time in gerbil hippocampus after mild ischemia

Selective neuronal death of the hippocampal CA1 area after mild ischemia is known as delayed neuronal death (DND). Progression of DND was evaluated using 7T MRI in gerbils. In the CA1, the T2 relaxation time started to increase on day 3 and was significantly higher on day 4 than that of the control gerbils. However, the apparent diffusion coefficient of water (ADC) was significantly lower on day 1 than in the control and continued to decline up to day 4. Changes in T2 coincided with loss of MAP2 immunoreactivity, while the ADC decrease preceded these changes. After 1 month, the ADC had returned to within the normal range, while T2 had decreased to below the control level. These results suggest that MRI is useful for monitoring DND.

Relationship between apparent diffusion coefficient and subsequent hemorrhagic transformation following acute ischemic stroke

BACKGROUND AND PURPOSE

A method for identifying patients at increased risk for developing secondary hemorrhagic transformation (HT) after acute ischemic stroke could be of significant value, particularly in patients being considered for thrombolytic therapy. We hypothesized that diffusion-weighted MRI might aid in the identification of such patients.

METHODS

We retrospectively analyzed 17 patients with ischemic stroke who received diffusion-weighted MRI within 8 hours of symptom onset and who also received follow-up neuroimaging within 1 week of initial scan. The apparent diffusion coefficient (ADC) for each pixel in the whole ischemic area was calculated, generating a histogram of values. Areas subsequently experiencing HT were then compared with areas not experiencing HT to determine the relationship between ADC and subsequent HT.

RESULTS

A significantly greater percentage of pixels possessed lower ADCs (</=550x10(-)(6) mm(2)/s) in HT lesions compared with non-HT lesions (47% versus 19%; P:<0.001). Moreover, >40% of the pixels possessed values </=550x10(-)(6) mm(2)/s in all lesions experiencing secondary HT, compared with <31% of the pixels in the non-HT-destined lesions.

CONCLUSIONS

HT-destined stroke regions possess a significantly great percentage of low ADC values than non-HT-destined regions. Early measurement of ADC values may be a useful tool for assessing secondary HT risk.

Early detection of irreversible cerebral ischemia in the rat using dispersion of the magnetic resonance imaging relaxation time, T1rho

The impact of brain imaging on the assessment of tissue status is likely to increase with the advent of treatment methods for acute cerebral ischemia. Multimodal magnetic resonance imaging (MRI) demonstrates potential for selecting stroke therapy patients by identifying the presence of acute ischemia, delineating the perfusion defect, and excluding hemorrhage. Yet, the identification of tissue subject to reversible or irreversible ischemia has proven to be difficult. Here, the authors show that T1 relaxation time in the rotating frame, so-called T1rho, serves as a sensitive MRI indicator of cerebral ischemia in the rat. The T1rho prolongs within minutes after a drop in the CBF of less than 22 mL 100 g(-1) min(-1). Dependence of T1rho on spin-lock amplitude, termed as T1rho dispersion, increases by approximately 20% on middle cerebral artery (MCA) occlusion, comparable with the magnitude of diffusion reduction. The T1rho dispersion change dynamically increases to be 38% +/- 10% by the first 60 minutes of ischemia in the brain region destined to develop infarction. Following reperfusion after 45 minutes of MCA occlusion, the tissue with elevated T1rho dispersion (yet normal diffusion) develops severe histologically verified neuronal damage; thus, the former parameter unveils an irreversible condition earlier than currently available MRI methods. The T1rho dispersion as a novel MRI index of cerebral ischemia may be useful in determination of the therapeutic window for acute ischemic stroke.

Relationship between residual cerebral blood flow and oxygen metabolism as predictive of ischemic tissue viability: sequential multitracer positron emission tomography scanning of middle cerebral artery occlusion during the critical first 6 hours after stroke in pigs

OBJECT

The authors tested the hypothesis that oxygen metabolism is the key factor linking the long-term viability of ischemic brain tissue to the magnitude of residual blood flow during the first 6 hours following a stroke.

METHODS

Eleven anesthetized pigs underwent a series of positron emission tomography studies to measure cerebral blood flow (CBF) and metabolism before and for 7 hours after the animals were subjected to permanent middle cerebral artery (MCA) occlusion. The extent of collateral blood supply was assessed using angiography. Abnormal metabolism of the ischemic tissue progressed as a function of time in inverse proportion to the magnitude of residual CBF, and the volume of the infarct grew in inverse proportion to the residual blood supply. Ten hours after occlusion of the MCA, the infarct topographically matched the tissue with a cerebral metabolic rate of oxygen consumption below 50% of values measured on the contralateral side. This was also the threshold for the decline of the oxygen extraction fraction below normal, which was critical for the prediction of nonviable ischemic tissue. Mildly ischemic tissue (CBF > 30 ml/100 g/min) did not reach the cerebral metabolic rate of oxygen threshold of viability during the first 6 hours after MCA occlusion; moderately ischemic tissue (CBF 12-30 m1/100 g/ min) reached the threshold of viability in 3 hours; and severely ischemic tissue (CBF < 12 ml/100 g/min) remained viable for less than 1 hour.

CONCLUSIONS

The relationship between the residual CBF and both oxygen metabolism and extraction is critical to the evolution of metabolic deficiency and lesion size after stroke.

Postoperative neurologic assessment and management of the cardiac surgical patient

The neurologic evaluation of patients in the immediate postoperative period and postanesthetic state is unique and challenging. Neurologic assessment is complicated by the lingering residual effects of anesthetics as well as by the effects of narcotic analgesics, anxiolytics, and muscle relaxants, especially in ventilated patients. In this review we examine the suspected causes, clinical manifestations, diagnostic options, and intervention schemes for the common neurologic syndromes seen after cardiac operations.

Broad-spectrum cation channel inhibition by LOE 908 MS reduces infarct volume in vivo and postmortem in focal cerebral ischemia in the rat

Cation channels conduct calcium, sodium and potassium, cations that are likely deleterious in the evolution of focal ischemic injury. We studied the effects of a novel, broad-spectrum inhibitor of several cation channels, LOE 908 MS, on acute ischemic lesion development with diffusion-weighted magnetic resonance imaging (DWI) and on cerebral perfusion with perfusion imaging (PI) in vivo and on cerebral infarct size using 2,3,5-triphenyltetrazolium chloride (TTC) staining postmortem. A total of 18 male Sprague-Dawley rats underwent 90 min of middle cerebral artery occlusion (MCAO) and were randomly and blindly assigned to either LOE 908 MS or vehicle starting 30 min after inducing focal ischemia and continuing for 4 h. Whole-brain DWI and multislice PI were done before initiation of treatment and repeated frequently for the next 3.5 h. DWI-derived lesion volume at 4 h showed a significant difference in favor of the drug treated group (P=0.03), whereas PI-derived perfusion deficit volumes did not significantly differ between the groups. The postmortem infarct volume at 24 h was significantly attenuated in the treated group in comparison to controls (P=0.0001) and neurological score was significantly better in the treated group (P<0.02). Blocking several distinct cation channels with LOE 908 MS significantly reduced infarct size and improved neurological outcome without observable adverse effects in this focal ischemia model.

Apparent diffusion coefficient and MR relaxation during osmotic manipulation in isolated turtle cerebellum

The apparent diffusion coefficient (ADC) and relaxation times of water were measured by magnetic resonance imaging (MRI) in the isolated turtle cerebellum during osmotic cell volume manipulation. The aim was to study effects of cell volume changes, a factor in ischemia and spreading depression, in isolation from considerations of blood flow and metabolism. Cerebella were superfused at 12-14 degrees C with solutions ranging from 50-200% normal osmolarity. Hypotonic solutions, which are known to cause cell swelling, led to reductions of ADC and increases of T(2), while hypertonic solutions had the opposite effect. This supports the concept that ADC varies with the extracellular space fraction and, combined with published data on extracellular ion diffusion, is consistent with fast or slow exchange models with effective diffusion coefficients that are approximately 1.7 times lower in intracellular than in extracellular space. Spin-spin relaxation can be affected by osmotic disturbance, though such changes are not seen in all pathologies that cause cell swelling.

Ischemic penumbra: evidence from functional imaging in man

The ischemic penumbra is defined as tissue with flow within the thresholds for maintenance of function and of morphologic integrity. Penumbra tissue has the potential for recovery and therefore is the target for interventional therapy in acute ischemic stroke. The identification of the penumbra necessitates measuring flow reduced less than the functional threshold and differentiating between morphologic integrity and damage. This can be achieved by multitracer positron emission tomography (PET) and perfusion-weighted (PW) and diffusion-weighted magnetic resonance imaging (DW-MRI) in experimental models, in which the recovery of critically perfused tissue or its conversion to infarction was documented in repeat studies. Neuroimaging modalities applied in patients with acute ischemic stroke--multitracer PET, PW- and DW-MRI, single photon emission computed tomography (SPECT), perfusion, and Xe-enhanced computed tomography (CT)-- often cannot reliably identify penumbra tissue: multitracer studies for the assessment of flow and irreversible metabolic damage usually cannot be performed in the clinical setting; CT and MRI do not reliably detect irreversible damage in the first hours after stroke, and even DW-MRI may be misleading in some cases: determinations of perfusion alone yield a poor estimate of the state of the tissue as long as the time course of changes is not known in individual cases. Therefore, the range of flow values in ischemic tissue found later, either within or outside the infarct, was rather broad. New tracers--for example, receptor ligands or hypoxia markers--might improve the identification of penumbra tissue in the future. Despite these methodologic limitations, the validity of the concept of the penumbra was proven in several therapeutic studies in which thrombolytic treatment reversed critical ischemia and decreased the volume of final infarcts. Such neuroimaging findings might serve as surrogate targets in the selection of other therapeutic strategies for large clinical trials.

The measurement of diffusion and perfusion in biological systems using magnetic resonance imaging

The aim of this review is to describe two recent developments in the use of magnetic resonance imaging (MRI) in the study of biological systems: diffusion and perfusion MRI. Diffusion MRI measures the molecular mobility of water in tissue, while perfusion MRI measures the rate at which blood is delivered to tissue. Therefore, both these techniques measure quantities which have direct physiological relevance. It is shown that diffusion in biological systems is a complex phenomenon, influenced directly by tissue microstructure, and that its measurement can provide a large amount of information about the organization of this structure in normal and diseased tissue. Perfusion reflects the delivery of essential nutrients to tissue, and so is directly related to its status. The concepts behind the techniques are explained, and the theoretical models that are used to convert MRI data to quantitative physical parameters are outlined. Examples of current applications of diffusion and perfusion MRI are given. In particular, the use of the techniques to study the pathophysiology of cerebral ischaemia/stroke is described. It is hoped that the biophysical insights provided by this approach will help to define the mechanisms of cell damage and allow evaluation of therapies aimed at reducing this damage.

Sampling and reconstruction effects due to motion in diffusion-weighted interleaved echo planar imaging

Subject motion during diffusion-weighted interleaved echo-planar imaging causes k-space offsets which lead to irregular sampling in the phase-encode direction. For each image, the k-space shifts are monitored using 2D navigator echoes, and are shown to lead to a frequent violation of the Nyquist condition when an ungated sequence is used on seven subjects. Combining data from four repeat acquisitions allows the Nyquist condition to be satisfied in all but 1% of images. Reconstruction of the irregularly-sampled data can be performed using a matrix inversion technique. The repeated acquisitions make the inversion more stable and additionally improve the signal-to-noise ratio. The resultant isotropic diffusion-weighted images and average apparent diffusion coefficient (ADC) maps show high resolution and enable clear localization of a stroke lesion. Residual ADC artifacts with a slow spatial variation are observed and assumed to originate from non-rigid pulsatile brain motion. Magn Reson Med 44:101-109, 2000.

Cerebral hemodynamics in human acute ischemic stroke: a study with diffusion- and perfusion-weighted magnetic resonance imaging and SPECT

Nineteen patients with acute ischemic stroke (<24 hours) underwent diffusion-weighted and perfusion-weighted (PWI) magnetic resonance imaging at the acute stage and 1 week later. Eleven patients also underwent technetium-99m ethyl cysteinate dimer single-photon emission computed tomography (SPECT) at the acute stage. Relative (ischemic vs. contralateral control) cerebral blood flow (relCBF), relative cerebral blood volume, and relative mean transit time were measured in the ischemic core, in the area of infarct growth, and in the eventually viable ischemic tissue on PWI maps. The relCBF was also measured from SPECT. There was a curvilinear relationship between the relCBF measured from PWI and SPECT (r = 0.854; P < 0.001). The tissue proceeding to infarction during the follow-up had significantly lower initial CBF and cerebral blood volume values on PWI maps (P < 0.001) than the eventually viable ischemic tissue had. The best value for discriminating the area of infarct growth from the eventually viable ischemic tissue was 48% for PWI relCBF and 87% for PWI relative cerebral blood volume. Combined diffusion and perfusion-weighted imaging enables one to detect hemodynamically different subregions inside the initial perfusion abnormality. Tissue survival may be different in these subregions and may be predicted.

Diffusion- and perfusion-weighted MRI: influence of severe carotid artery stenosis on the DWI/PWI mismatch in acute stroke

BACKGROUND AND PURPOSE

Diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) have been used increasingly in recent years to evaluate acute stroke in the emergency setting. In the present study, we compared DWI and PWI findings in acute stroke patients with and without severe extracranial internal carotid artery (ICA) disease.

METHODS

Twenty-seven patients with nonlacunar ischemic stroke were selected for this analysis. DWI, PWI, and conventional MRI were performed in all patients within 24 hours of symptom onset and after 1 week. To exclude patients with partial or complete reperfusion, we included only patients with a PWI deficit larger than the DWI lesion. Severe ICA disease (>70% stenosis) was present unilaterally in 9 and bilaterally in 2 patients. Acute DWI lesion volume, the size of the acute PWI/DWI mismatch, and final infarct size (on T2-weighted images) were determined.

RESULTS

The PWI/DWI mismatch was significantly larger in patients with severe ICA disease than in patients without extracranial carotid stenosis, both when time-to-peak and mean transit time maps (P<0.01) were used to calculate the mismatch. Quantitative analysis of the time-to-peak delay in the mismatch indicated that a relatively smaller fraction of the total mismatch was critically ischemic in patients with carotid stenosis than in those without. Average lesion volume increased less in the stenosis group (P=0.14), despite the larger PWI/DWI mismatch, and final infarct size was smaller in the stenosis group (P<0.05). In the 2 patients with bilateral ICA disease, variable hemodynamic involvement of the contralateral hemisphere was found in addition to the ipsilateral PWI deficit.

CONCLUSIONS

In most acute stroke patients with severe ICA stenosis, a considerably smaller fraction of the total PWI/DWI mismatch is at risk than in patients without carotid disease.

Validity and reliability of a quantitative computed tomography score in predicting outcome of hyperacute stroke before thrombolytic therapy. ASPECTS Study Group. Alberta Stroke Programme Early CT Score

BACKGROUND

Computed tomography (CT) must be done before thrombolytic treatment of hyperacute ischaemic stroke, but the significance of early ischaemic change on CT is unclear. We tested a quantitative CT score, the Alberta Stroke Programme Early CT Score (ASPECTS).

METHODS

203 consecutive patients with ischaemic stroke were treated with intravenous alteplase within 3 h of symptom onset in two North American teaching hospitals. All pretreatment CT scans were prospectively scored. The score divides the middle-cerebral-artery territory into ten regions of interest. Primary outcomes were symptomatic intracerebral haemorrhage and 3-month functional outcome. The sensitivity and specificity of ASPECTS for the primary outcomes were calculated. Logistic regression was used to test the association between the score on ASPECTS and the primary outcomes.

FINDINGS

Ischaemic changes on the baseline CT were seen in 117 (75%) of 156 treated patients with anterior-circulation ischaemia included in the analysis (23 had ischaemia in the posterior circulation and 24 were treated outside the protocol). Baseline ASPECTS value correlated inversely with the severity of stroke on the National Institutes of Health Stroke Scale (r=-0.56, p<0.001). Baseline ASPECTS value predicted functional outcome and symptomatic intracerebral haemorrhage (p<0.001, p=0.012, respectively). The sensitivity of ASPECTS for functional outcome was 0.78 and specificity 0.96; the values for symptomatic intracerebral haemorrhage were 0.90 and 0.62. Agreement between observers for ASPECTS, with knowledge of the affected hemisphere, was good (kappa statistic 0.71-0.89).

INTERPRETATION

This CT score is simple and reliable and identifies stroke patients unlikely to make an independent recovery despite thrombolytic treatment.

Why do neuroprotective drugs work in animals but not humans?

Many neuroprotective agents that seemed promising in animal studies of ischemic brain injury prove to have no effect when tested in clinical trials, suggesting that fundamental elements of translational research require better definition. A number of modifications have led to improvements in preclinical and human studies since the earliest controlled trials failed to confirm hypotheses suggested by animal data. Continued re-evaluation and sharing of information derived from the laboratory bench or the patient's bedside should eventually lead to effective neuroprotection in acute stroke. Experimental data should be carefully studied to improve the quality of agents coming to clinical trials and to design trial phasing that effectively determines drug safety and efficacy. This article will examine preclinical modeling and its translation to prospective studies of acute stroke therapy and will focus on some potential solutions directed at clinical trial design.

Diffusion and perfusion magnetic resonance imaging in childhood stroke

Two magnetic resonance imaging techniques, diffusion and perfusion imaging, are being used increasingly for evaluation of pathophysiology of stroke. This article introduces these techniques and reports some initial studies using these approaches, together with conventional T2-weighted magnetic resonance imaging, for investigation of childhood stroke. It is shown that the combination of T2-weighted and diffusion images can provide information about the timing of stroke events in childhood, and perfusion imaging can detect abnormalities not visible by other magnetic resonance imaging techniques. These magnetic resonance methods therefore should play an important role in investigation of children with stroke and could be of particular value in studies of at-risk populations of children such as those with sickle cell disease.

Temporal changes of the apparent diffusion coefficients of water and metabolites in rats with hemispheric infarction: experimental study of transhemispheric diaschisis in the contralateral hemisphere at 7 tesla

The purpose of the present study was to clarify the temporal changes of the apparent diffusion coefficients (ADCs) of cerebral metabolites during early focal ischemia using stimulated echo acquisition mode with short echo time at a 7 T magnet to assess the pathophysiology of the reduction in diffusion properties observed both in the ischemic cerebral hemisphere and in the contralateral hemisphere. The ADCs of metabolites in the infarcted hemisphere 1 hour and 3 hours after the onset of ischemia decreased with 25% and 29% for choline containing compounds (Cho), 16% and 26% for creatine and phosphocreatine (Cre), and 19% and 19% for N-acetylaspartate (NAA), respectively, compared with the ADC values 2 hours later in the contralateral hemisphere. There were decreases in the ADC of Cho, Cre, and NAA with 21%, 7%, and 18% 8 hours later, respectively, in the noninfarcted hemisphere, which suggested transhemispheric diaschisis in rats with focal cerebral ischemia. The present study proposed that the diffusion characteristics of the brain metabolites might offer new insights into circulatory and metabolic alteration in the cerebral intracellular circumstance.

Transient and permanent resolution of ischemic lesions on diffusion-weighted imaging after brief periods of focal ischemia in rats : correlation with histopathology

BACKGROUND AND PURPOSE

The early ischemic lesions demonstrated by diffusion-weighted imaging (DWI) are potentially reversible. The purposes of this study were to determine whether resolution of initial DWI lesions is transient or permanent after different brief periods of focal brain ischemia and to evaluate histological outcomes.

METHODS

Sixteen rats were subjected to 10 minutes (n=7) or 30 minutes (n=7) of temporary middle cerebral artery occlusion or sham operation (n=2). DWI, perfusion-weighted imaging (PWI), and T(2)-weighted imaging (T(2)WI) were performed during occlusion; immediately after reperfusion; and at 0.5, 1.0, 1.5, 12, 24, 48, and 72 hours after reperfusion. After the last MRI study, the brains were fixed, sectioned, stained with hematoxylin and eosin, and evaluated for neuronal necrosis.

RESULTS

No MRI or histological abnormalities were observed in the sham-operated rats. In both the 10-minute and 30-minute groups, the perfusion deficits and DWI hyperintensities that occurred during occlusion disappeared shortly after reperfusion. The DWI, PWI, and T(2)WI results remained normal thereafter in the 10-minute group, whereas secondary DWI hyperintensity and T(2)WI abnormalities developed at the 12-hour observation point in the 30-minute group. Histological examinations demonstrated neuronal necrosis in both groups, but the number of necrotic neurons was significantly higher in the 30-minute group (95+/-4%) than in the 10-minute group (17+/-10%, P<0.0001).

CONCLUSIONS

Transient or permanent resolution of initial DWI lesions depends on the duration of ischemia. Transient resolution of DWI lesions is associated with widespread neuronal necrosis; moreover, permanent resolution of DWI lesions does not necessarily indicate complete salvage of brain tissue from ischemic injury.

Diffusion-weighted MRI and proton MR spectroscopic imaging in the study of secondary neuronal injury after intracerebral hemorrhage

BACKGROUND AND PURPOSE

Cerebral ischemia has been proposed as contributing mechanism to secondary neuronal injury after intracerebral hemorrhage (ICH). Possible tools for investigating this hypothesis are diffusion-weighted (DWI) and proton magnetic resonance spectroscopic imaging ((1)H-MRSI). However, magnetic field inhomogeneity induced by paramagnetic blood products may prohibit the application of such techniques on perihematoma tissue. We report on the feasibility of DWI and (1)H-MRSI in the study of human ICH and present preliminary data on their contribution to understanding perihematoma tissue functional and metabolic profiles.

METHODS

Patients with acute supratentorial ICH were prospectively evaluated using DWI and (1)H-MRSI. Obscuration of perihematoma tissue with both sequences was assessed. Obtainable apparent diffusion coefficient (Dav) and lactate spectra in perihematoma brain tissue were recorded and analyzed.

RESULTS

Nine patients with mean age of 63.4 (36 to 87) years were enrolled. Mean time from symptom onset to initial MRI was 3.4 (1 to 9) days; mean hematoma volume was 35.4 (5 to 80) cm(3). Perihematoma diffusion values were attainable in 9 of 9 patients, and (1)H-MRSI measures were obtainable in 5 of 9 cases. Dav in perihematoma regions was 172.5 (120.0 to 302.5)x10(-5) mm(2)/s and 87.6 (76.5 to 102.1)x10(-5) mm(2)/s in contralateral corresponding regions of interest (P=0.002). One patient showed an additional area of reduced Dav with normal T(2) intensity, which suggests ischemia. (1)H-MRSI revealed lactate surrounding the hematoma in 2 patients.

CONCLUSIONS

DWI and (1)H-MRSI can be used in the study of ICH patients. Our preliminary data are inconsistent with ischemia as the primary mechanism for perihematoma tissue injury. Further investigation with advanced MRI techniques will give a clearer understanding of the role that ischemia plays in tissue injury after ICH.

Advances in neuroimaging of acute stroke

As therapeutic options for treating acute stroke evolve, neuroimaging strategies are assuming an increasingly important role in the initial evaluation and management of patients. There is a recognized need for objective neuroimaging methods to identify the best candidates for early intervention. Both acute and long-term treatment decisions for stroke patients should optimally incorporate information provided by neuroimaging studies regarding tissue viability (eg, size, location, vascular distribution, degree of reversibility of ischemic injury, presence of hemorrhage), vessel status (site and severity of stenoses and occlusions), and cerebral perfusion (size, location, and severity of hypoperfusion). The ability to acutely identify the ischemic penumbra and to use this information to make treatment decisions may be within reach, particularly with the multimodal data provided by magnetic resonance techniques. This article will review recent developments in the field of neuroimaging of acute stroke and discuss the clinical applications of specific techniques of magnetic resonance imaging, computed tomography, positron emission tomography, single photon emission tomography, catheter angiography, and ultrasound imaging.

Magnetic resonance perfusion imaging in acute ischemic stroke using continuous arterial spin labeling

BACKGROUND AND PURPOSE

Continuous arterial spin-labeled perfusion MRI (CASL-PI) uses electromagnetically labeled arterial blood water as a diffusible tracer to noninvasively measure cerebral blood flow (CBF). We hypothesized that CASL-PI could detect perfusion deficits and perfusion/diffusion mismatches and predict outcome in acute ischemic stroke.

METHODS

We studied 15 patients with acute ischemic stroke within 24 hours of symptom onset. With the use of a 6-minute imaging protocol, CASL-PI was measured at 1.5 T in 8-mm contiguous supratentorial slices with a 3.75-mm in-plane resolution. Diffusion-weighted images were also obtained. Visual inspection for perfusion deficits, perfusion/diffusion mismatches, and effects of delayed arterial transit was performed. CBF in predetermined vascular territories was quantified by transformation into Talairach space. Regional CBF values were correlated with National Institutes of Health Stroke Scale (NIHSS) score on admission and Rankin Scale (RS) score at 30 days.

RESULTS

Interpretable CASL-PI images were obtained in all patients. Perfusion deficits were consistent with symptoms and/or diffusion-weighted imaging abnormalities. Eleven patients had hypoperfusion, 3 had normal perfusion, and 1 had relative hyperperfusion. Perfusion/diffusion mismatches were present in 8 patients. Delayed arterial transit effect was present in 7 patients; serial imaging in 2 of them showed that the delayed arterial transit area did not succumb to infarction. CBF in the affected hemisphere correlated with NIHSS and RS scores (P=0.037 and P=0.003, Spearman rank correlation). The interhemispheric percent difference in middle cerebral artery CBF correlated with NIHSS and RS scores (P=0.007 and P=0.0002, respectively).

CONCLUSIONS

CASL-PI provides rapid noninvasive multislice imaging in acute ischemic stroke. It depicts perfusion deficits and perfusion/diffusion mismatches and quantifies regional CBF. CASL-PI CBF asymmetries correlate with severity and outcome. Delayed arterial transit effects may indicate collateral flow.

Reduction in water and metabolite apparent diffusion coefficients during energy failure involves cation-dependent mechanisms. A proton nuclear magnetic resonance study of rat cortical brain slices

Proton (1H) nuclear magnetic resonance (NMR) diffusion spectroscopy was used to assess apparent diffusion coefficients (ADCs) in rat brain slices. Aglycemic hypoxia caused reductions in the ADC of N-acetylaspartate (NAA) (0.15 to 0.09 x 10(-3) mm2/s) and "slow" diffusion coefficient (D2) of tissue water (0.51 to 0.37 x 10(-3) mm2/s), together with a 32+/-11% increase in tissue water volume, attributable to tissue swelling. The ADC and D2 reductions were diminished, however, by removing external Ca2+, and under 10 mmol/L Mg2+, normoxic diffusion coefficients persisted until 40 minutes of hypoxia. The data suggest that the shift of water into the intracellular space alone cannot satisfactorily explain the reduced cerebral diffusion upon energy failure and that external Mg2+ and Ca2+ play crucial modulatory roles.

Early [(11)C]Flumazenil/H(2)O positron emission tomography predicts irreversible ischemic cortical damage in stroke patients receiving acute thrombolytic therapy

BACKGROUND AND PURPOSE

Central benzodiazepine receptor ligands, such as [(11)C]flumazenil (FMZ), are markers of neuronal integrity and therefore might be useful in the differentiation of functionally and morphologically damaged tissue early in ischemic stroke. We sought to assess the value of a benzodiazepine receptor ligand for the early identification of irreversible ischemic damage to cortical areas that cannot benefit from reperfusion.

METHODS

Eleven patients (7 male, 4 female, aged 52 to 75 years) with acute, hemispheric ischemic stroke were treated with alteplase (recombinant tissue plasminogen activator; 0.9 mg/kg according to National Institute of Neurological Disorders and Stroke protocol) within 3 hours of onset of symptoms. At the beginning of thrombolysis, cortical cerebral blood flow ([(15)O]H(2)O) and FMZ binding were assessed by positron emission tomography (PET). Those early PET findings were related to the change in neurological deficit (National Institutes of Health Stroke Scale) and to the extent of cortical damage on MRI or CT 3 weeks after the stroke.

RESULTS

Hypoperfusion was observed in all cases, and in 8 patients the values were below critical thresholds estimated at 12 mL/100 g per minute, comprising 1 to 174 cm(3) of cortical tissue. Substantial reperfusion was seen in most of these regions 24 hours after thrombolysis. In 4 cases, distinct areas of decreased FMZ binding were detected. Those patients suffered permanent lesions in cortical areas corresponding to their FMZ defects (112 versus 146, 3 versus 3, 2 versus 1, and 128 versus 136 cm(3)). In the other patients no morphological defects were detected on MRI or CT, although blood flow was critically decreased in areas ranging in size up to 78 cm(3) before thrombolysis.

CONCLUSIONS

These findings suggest that imaging of benzodiazepine receptors by FMZ PET distinguishes between irreversibly damaged and viable penumbra tissue early after acute stroke.

Graded reduction of cerebral blood flow in rat as detected by the nuclear magnetic resonance relaxation time T2: a theoretical and experimental approach

The ability of transverse nuclear magnetic resonance relaxation time, T2, to reveal acutely reduced CBF was assessed using magnetic resonance imaging (MRI). Graded reduction of CBF was produced in rats using a modification of Pulsinelli's four-vessel occlusion model. The CBF in cerebral cortex was quantified using the hydrogen clearance method, and both T2 and the trace of the diffusion tensor (Dav = 1/3TraceD) in the adjacent cortical tissue were determined as a function of reduced CBF at 4.7 T. A previously published theory, interrelating cerebral hemodynamic parameters, hemoglobin, and oxygen metabolism with T2, was used to estimate the effects of reduced CBF on cerebral T2. The MRI data show that T2 reduces in a U-shape manner as a function of CBF, reaching a level that is 2.5 to 2.8 milliseconds (5% to 6%) below the control value at CBF, between 15% and 60% of normal. This reduction could be estimated by the theory using the literature values of cerebral blood volume, oxygen extraction ratio, and precapillary oxygen extraction during compromised CBF. Dav dropped with two apparent flow thresholds, so that a small 11% to 17% reduction occurred between CBF values of 16% to 45% of normal, followed by a precipitous collapse by more than 20% at CBF below 15% of normal. The current data show that T2 can be used as an indicator of acute hypoperfusion because of its ability to indicate blood oxygenation level-dependent phenomena on reduced CBF.

Diffusion and perfusion magnetic resonance imaging for the evaluation of acute stroke: potential use in guiding thrombolytic therapy

Recent data indicate that diffusion/perfusion weighted imaging could eventually play a significant role in acute stroke management, particularly in determining the suitability of acute stroke patients for thrombolytic therapy. The evidence supporting these uses is reviewed, and the future role of diffusion and perfusion weighted imaging in acute stroke management is discussed.

Diffusion-weighted magnetic resonance imaging in brain death

BACKGROUND; Traditionally the diagnosis of brain death is established on the basis of a combination of clinical signs and paraclinical methods. Diffusion-weighted MRI is a new method sensitive to cerebral ischemia. Its value in brain death has not been demonstrated until now.

CASE DESCRIPTION

A patient was referred to MRI with suspicion of a brain stem stroke. Echo-planar whole-brain, multislice, diffusion-weighted MRI was performed in addition to conventional sequences and MR angiography sequences. In addition to the extensive bilateral hyperintensities observed on T2-weighted images, diffusion-weighted MRI showed diffuse hyperintensities involving both hemispheres as well as a severe drop in the apparent diffusion coefficient in both affected hemispheres. There was also transtentorial herniation with compression of the brain stem as well as absence of flow voids on the T2-weighted images and absence of intracranial vessels on MR angiography. On the basis of the clinical and imaging findings, it was concluded that the patient was in a state of brain death. The patient died the same day.

CONCLUSIONS

With the use of new fast techniques such as diffusion-weighted imaging, now MRI can not only display anatomic changes associated with severe brain suffering but can also demonstrate ultrastructural changes secondary to brain death and differentiate them from edematous changes seen on T2-weighted images.

Volumetric measurements of right cerebral hemisphere infarction: use of a semiautomatic MRI segmentation technique

The applications of a new segmentation software, Anatomatic, in the evaluation of volumetric measurements of brain infarctions and the new Medimag 3D software in the evaluation of 3D image representation of infarctions are described. These programs are applied to magnetic resonance imaging. The aim of this study is to evaluate the use of these software packages in making accurate volumetric measurements in 40 patients with right cerebral infarctions, in determining the correlations between the quantitated lesions and neurological/neuropsychological dysfunctions and in creating realistic 3D views of the infarctions. Using Anatomatic, reproducible infarction volumes were achieved with ease and within a reasonably fast time. Medimag helped achieve realistic 3D representations of the infarctions. When compared, the semiautomatic segmentation proved to be much faster and yielded higher infarction volumes than the manual segmentation technique. Significantly positive correlations between the infarction volumes and neurological dysfunctions and neuropsychological deficit (neglect) helped to explain the effect of volumes on the clinical status of the patients.

The relationship between the apparent diffusion coefficient measured by magnetic resonance imaging, anoxic depolarization, and glutamate efflux during experimental cerebral ischemia

A reduction in the apparent diffusion coefficient (ADC) of water measured by magnetic resonance imaging (MRI) has been shown to occur early after cerebrovascular occlusion. This change may be a useful indicator of brain tissue adversely affected by inadequate blood supply. The objective of this study was to test the hypothesis that loss of membrane ion homeostasis and depolarization can occur simultaneously with the drop in ADC. Also investigated was whether elevation of extracellular glutamate ([GLU]e) would occur before ADC changes. High-speed MRI of the trace of the diffusion tensor (15-second time resolution) was combined with simultaneous recording of the extracellular direct current (DC) potential and on-line [GLU]e from the striatum of the anesthetized rat. After a control period, data were acquired during remote middle cerebral artery occlusion for 60 minutes, followed by 30 minutes of reperfusion, and cardiac arrest-induced global ischemia. After either focal or global ischemia, the ADC was reduced by 10 to 25% before anoxic depolarization occurred. After either insult, the time for half the maximum change in ADC was significantly shorter than the corresponding DC potential parameter (P < 0.05). The [GLU]e remained at low levels during the entire period of varying ADC and DC potential and did not peak until much later after either ischemic insult. This study demonstrates that ADC changes can occur before membrane depolarization and that high [GLU]e has no involvement in the early rapid ADC decrease.

Echoplanar magnetic resonance imaging in acute stroke

Echoplanar magnetic resonance imaging (EPI) enables rapid, non-invasive imaging and analysis of cerebral pathophysiology in acute stroke. It represents an important clinical advance over computed tomography (CT) and conventional magnetic resonance (MR) scanning. It can rapidly delineate infarcted cerebral tissue and distinguish acute from chronic stroke. In addition, EPI has the potential to quickly determine the presence and degree of potentially viable brain tissue in the ischaemic penumbra. Thrombolysis is thought to reperfuse the penumbra and hence reduce infarct size. The thrombolytic agent tissue plasminogen activator (t-PA) improves outcome in ischaemic stroke when administered within the first 3 hours of onset. However, there is a significant risk of haemorrhage, and the time window for benefit may well exceed 3 hours in some patients. Hence, by facilitating diagnosis of 'at-risk' tissue in the ischaemic penumbra, a major clinical role of EPI may well become the rational selection of patients for acute interventional stroke therapy.

Voxel-based mapping of irreversible ischaemic damage with PET in acute stroke

Objective mapping of irreversible tissue damage in the acute stage of ischaemic stroke would be useful for prognosis and in assessing the efficacy of therapeutic manoeuvres in impeding extension of infarction. From our database of 30 patients studied with 15O-PET within 5-18 h after onset of first-ever middle cerebral artery territory stroke, we extracted a subgroup of 19 survivors (age 74.6 +/- 8.5 years) in whom late CT coregistered with PET was available to determine final infarct topography. By means of a voxel-based analysis of the PET data, we determined putative thresholds for irreversible tissue damage as the lower limit of the 95% confidence interval calculated from all voxels within the ultimately non-infarcted brain parenchyma ipsilateral to the insult. The following values were found: 8.43 ml/100 ml/min, 0.87 ml/100 ml/min, 1.64 ml/100 ml, 0.27 and 2.21/min, for cerebral blood flow (CBF), oxygen consumption (CMRO2), blood volume (CBV), oxygen extraction fraction and the ratio CBF : CBV, respectively. Voxels below these thresholds occurred significantly more frequently in the final infarct region than in the non-infarcted parenchyma for CBF and CMRO2 (P = 0.016 and P = 0.0045, respectively, Wilcoxon test), but not for the other PET variables. Furthermore, with both CBF and CMRO2, the percentage of irreversible tissue damage voxels in the affected hemisphere relative to the opposite hemisphere was significantly positively correlated to both the volume of final infarct and the neurological outcome at 2 months (all P < 0.005, Spearman ranked test). These findings validate our voxel-based CBF and CMRO2 thresholds for probabilistic mapping of irreversible tissue damage within the 5-18 h interval after stroke onset; however, whether they would be applicable to earlier intervals remains to be determined. Transfer of our procedure for determination of irreversible tissue damage thresholds to other imaging modalities such as single proton emission computed tomography and diffusion-weighted MRI should be straightforward.

MRI features of intracerebral hemorrhage within 2 hours from symptom onset

BACKGROUND AND PURPOSE

MRI has been increasingly used in the evaluation of acute stroke patients. However, MRI must be able to detect early hemorrhage to be the only imaging screen used before treatment such as thrombolysis. Susceptibility-weighted imaging, an echo-planar T2* sequence, can show intracerebral hemorrhage (ICH) in patients imaged between 2.5 and 5 hours from symptom onset. It is unknown whether MRI can detect ICH earlier than 2.5 hours. We describe 5 patients with ICH who had MRI between 23 and 120 minutes from symptom onset and propose diagnostic patterns of evolution of hyperacute ICH on MRI.

METHODS

As part of our acute imaging protocol, all patients with acute stroke within 24 hours from symptom onset were imaged with a set of sequences that included susceptibility-weighted imaging, diffusion- and perfusion-weighted imaging, T1- and T2-weighted imaging, fluid-attenuated inversion recovery (FLAIR), and MR angiography using echo-planar techniques. Five patients with ICH had MRI between 23 and 120 minutes from the onset of symptoms.

RESULTS

ICH was identified in all patients. Distinctive patterns of hyperacute ICH and absence of signs of ischemic stroke were the hallmark features of this diagnosis. The hyperacute hematoma appears to be composed of 3 distinct areas: (1) center: isointense to hyperintense heterogeneous signal on susceptibility-weighted and T2-weighted imaging; (2) periphery: hypointense (susceptibility effect) on susceptibility-weighted and T2-weighted imaging; and (3) rim: hypointense on T1-weighted imaging and hyperintense on T2-weighted imaging, representing vasogenic edema encasing the hematoma.

CONCLUSIONS

MRI is able to detect hyperacute ICH and show a pattern of evolution of the hematoma within 2 hours from the onset of symptoms.

Effects of blood sugar level on rat transient focal brain ischemia consecutively observed by diffusion-weighted EPI and (1)H echo planar spectroscopic imaging

The effects of blood sugar level on transient focal brain ischemia were examined by consecutive diffusion-weighted EPI and (1)H echo planar spectroscopic imaging. A remote-controlled rat intraluminal suture middle cerebral artery occlusion (MCAO) model was prepared. Animals were divided into three experimental groups: control, 1 g/kg, and 2 g/kg glucose groups (n = 6 for each). Saline or glucose was infused intraperitoneally 30 min prior to MCAO. The glucose-loaded groups showed increased lactate accumulation and marked decreases in average diffusion coefficient in the ischemic region during 40-min MCAO. These changes were correlated with blood sugar levels at the onset of MCAO. After reperfusion, all rats in the control and 1 g/kg groups recovered from the ischemic changes, but three rats with marked hyperglycemia in the 2 g/kg group showed irreversible changes. The adverse effects of hyperglycemia on transient focal brain ischemia were clearly demonstrated by sequential 2D images. Magn Reson Med 42:895-902, 1999.

Limitations of clinical diagnosis in acute stroke

Trials in acute stroke have recruited on the basis of clinical diagnosis. Using MRI we have shown that clinical diagnosis is more limited than previously appreciated, thus trials may have been underpowered or confounded.

CT assessment of cerebral perfusion: experimental validation and initial clinical experience

PURPOSE

To validate a dynamic single-section computed tomographic (CT) method to measure cerebral blood volume (CBV) and cerebral blood flow (CBF) by using a noncarotid artery as the input and to demonstrate the feasibility of this method in a pilot series of patients.

MATERIALS AND METHODS

Twelve dynamic contrast material-enhanced CT studies were performed in beagles. CBV, CBF, and mean transit time (MTT) values were calculated by using an internal carotid artery (ICA) and a noncarotid artery as the input artery to the brain. Patient studies with use of the radial artery as the input were performed (a) repetitively in two patients after subarachnoid hemorrhage, (b) in a patient with a symptomatic ICA occlusion before and after the intravenous injection of 1 g of acetazolamide, and (c) in a patient with a malignant brain tumor.

RESULTS

Linear regression analyses revealed highly significant correlations (P < .001) between CBV (r, 0.98; slope, 0.96), CBF (r, 0.89; slope, 0.87), and MTT (r, 0.80; slope, 0.76) values calculated with the ICA and the noncarotid inputs. The CT-derived patient data correlated well with ancillary clinical and neuroradiologic findings.

CONCLUSION

Dynamic single-section CT scanning to measure CBV and CBF on the basis of a noncarotid input is a highly accessible and cost-effective blood flow measurement technique.

Longitudinal magnetic resonance imaging study of perfusion and diffusion in stroke: evolution of lesion volume and correlation with clinical outcome

A prospective longitudinal diffusion-weighted and perfusion-weighted magnetic resonance imaging (DWI/PWI) study of stroke patients (n = 21) at five distinct time points was performed to evaluate lesion evolution and to assess whether DWI and PWI can accurately and objectively demonstrate the degree of ischemia-induced deficits within hours after stroke onset. Patients were scanned first within 7 hours of symptom onset and then subsequently at 3 to 6 hours, 24 to 36 hours, 5 to 7 days, and 30 days after the initial scan. Lesion evolution was dynamic during the first month after stroke. Most patients (18 of 19, 95%) showed increased lesion volume over the first week and then decreased at 1 month relative to 1 week (12 of 14, 86%). Overall, lesion growth appeared to depend on the degree of mismatch between diffusion and perfusion at the initial scan. Abnormal volumes on the acute DWI and PWI (<7 hours) correlated well with initial National Institutes of Health (NIH) stroke scale scores, outcome NIH stroke scale scores, and final lesion volume. DWI and PWI can provide an early measure of metabolic and hemodynamic insufficiency, and thus can improve our understanding of the evolution and outcome after acute ischemic stroke.

Clinical application of perfusion and diffusion MR imaging in acute ischemic stroke

With the advances in new neuroimaging modalities, the role of imaging of acute ischemic stroke has broadened and progressed from making diagnoses to providing valuable information for patient management. The goal of thrombolytic therapy for acute ischemic stroke should be to salvage the ischemic tissue reversibility that can respond to recanalization and avoid reperfusion of the dead (nonviable) tissue. It is essential to have rapid diagnostic modalities that can distinguish viable ischemic tissue from irreversibly damaged tissue, because there is a risk of reperfusion injury such as hemorrhagic complications with early intervention. Although diffusion magnetic resonance (MR) imaging has been reported to have a high sensitivity and specificity for acute ischemia in acute stroke patients without early reperfusion therapy, the capability to differentiate reversible from irreversible ischemia by diffusion MR imaging has not been established. Perfusion MR imaging techniques provide direct information on parenchymal perfusion status (adequacy of the collateral circulation) and may have the potential for providing important information about tissue viability and/or reversibility for selecting appropriate patients for thrombolytic therapy.

Neuroprotective effects of a novel broad-spectrum cation channel blocker, LOE 908 MS, on experimental focal ischemia: a multispectral study

Thirty-four rats undergoing 90 minutes of temporary middle cerebral artery occlusion were randomly and blindly assigned to vehicle or (RS)-(3,4-dihydro-6, 7-dimethoxyisoquinoline-1-gamma1)-2-phenyl-N,N-di-2-(2, 3, 4-trimethoxyphenyl)ethyl acetamide (LOE 908 MS; 0.5 mg/kg) i.v. bolus at 30 minutes after arterial occlusion followed by a 5 mg/kg/hr i.v. infusion for 3.8 hours (n =17/group). Perfusion-, diffusion- and T(2)-weighted magnetic resonance imaging was performed before treatment and repeatedly after treatment. Multispectral analysis was used to define ischemic abnormalities. The size of the ischemic abnormalities, including the ischemic core and penumbra, was not different between the two groups before treatment. However, a significant difference in ischemic lesion size was detected beginning 1.5 hours after treatment. The size of the ischemic core was significantly smaller in the treatment group, while the size of the ischemic penumbra was similar in the two groups at 85 minutes after arterial occlusion. Postmortem infarct size at 24 hours was significantly smaller in the drug-treated group than in the placebo group. These results demonstrate that LOE 908 MS can reduce ischemic lesion size, which is probably attributable to inhibition of expansion of the ischemic core. J. Magn. Reson. Imaging 1999;10:138-145.

Combined diffusion and perfusion MRI with correlation to single-photon emission CT in acute ischemic stroke. Ischemic penumbra predicts infarct growth

BACKGROUND AND PURPOSE

More effective imaging methods are needed to overcome the limitations of CT in the investigation of treatments for acute ischemic stroke. Diffusion-weighted MRI (DWI) is sensitive in detecting infarcted brain tissue, whereas perfusion-weighted MRI (PWI) can detect brain perfusion in the same imaging session. Combining these methods may help in identifying the ischemic penumbra, which is an important concept in the hemodynamics of acute stroke. The purpose of this study was to determine whether combined DWI and PWI in acute (<24 hours) ischemic stroke can predict infarct growth and final size.

METHODS

Forty-six patients with acute ischemic stroke underwent DWI and PWI on days 1, 2, and 8. No patient received thrombolysis. Twenty-three patients underwent single-photon emission CT in the acute phase. Lesion volumes were measured from DWI, SPECT, and maps of relative cerebral blood flow calculated from PWI.

RESULTS

The mean volume of infarcted tissue detected by DWI increased from 46.1 to 75.6 cm(3) between days 1 and 2 (P<0.001; n=46) and to 78.5 cm(3) after 1 week (P<0.001; n=42). The perfusion-diffusion mismatch correlated with infarct growth (r=0. 699, P<0.001). The volume of hypoperfusion on the initial PWI correlated with final infarct size (r=0.827, P<0.001). The hypoperfusion volumes detected by PWI and SPECT correlated significantly (r=0.824, P<0.001).

CONCLUSIONS

Combined DWI and PWI can predict infarct enlargement in acute stroke. PWI can detect hypoperfused brain tissue in good agreement with SPECT in acute stroke.

Diffusion- and perfusion-weighted MRI. The DWI/PWI mismatch region in acute stroke

BACKGROUND AND PURPOSE

Diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) are relatively new MR techniques increasingly used in acute stroke. During the first hours of stroke evolution, the regions with abnormal perfusion are typically larger than the DWI lesions, and this mismatch region has been suggested to be "tissue at risk." The aim of this study was to evaluate the PWI/DWI mismatch region in acute stroke patients and find parameters indicative of both infarct progression and functional impairment.

METHODS

Twenty patients with nonlacunar ischemic stroke were imaged with DWI, PWI, and conventional MRI within 24 hours of symptom onset and after 1 week; in addition, the European Stroke Scale (ESS) score was recorded. With PWI, the volumes of regions with "time-to-peak" (TTP) delays of >/=2, 4, 6, 8, and 10 seconds were measured; these volumes were compared with the acute DWI lesion volumes, final infarct size, and ESS score.

RESULTS

In 80% of patients the acute DWI lesion was surrounded by regions with abnormal TTP delays (PWI>DWI lesion). A TTP delay of >/=6 s in the mismatch region was found to be associated with lesion enlargement between the initial and follow-up MRI scans. Lesions increased in 9 of 12 patients (75%) in whom the area with TTP delay >/=6 s was larger than the DWI lesion, but they increased in only 1 of 8 (12.5%) of the remaining patients, in whom the area with a TTP delay >/=6 s was smaller than the DWI lesion. The volume of the regions with TTP delays of >/=4 s correlated better with ESS (r=-0.88, P<0.001) than other PWI (or DWI) volumes, which indicated that a TTP delay of approximately 4 s might be the threshold for functional impairment of brain tissue.

CONCLUSIONS

Only patients with severe perfusion deficits in the PWI/DWI mismatch (TTP delays of >/=6 s) are at high risk of lesion enlargement. Functionally, more moderate perfusion deficits (TTP delays >/=4 and <6 s) appear to also contribute to the acute clinical deficit.