Magnetic resonance imaging of acute stroke

Advantages of frequency-domain modeling in dynamic-susceptibility contrast magnetic resonance cerebral blood flow quantification

In dynamic-susceptibility contrast magnetic resonance perfusion imaging, the cerebral blood flow (CBF) is estimated from the tissue residue function obtained through deconvolution of the contrast concentration functions. However, the reliability of CBF estimates obtained by deconvolution is sensitive to various distortions including high-frequency noise amplification. The frequency-domain Fourier transform-based and the time-domain singular-value decomposition-based (SVD) algorithms both have biases introduced into their CBF estimates when noise stability criteria are applied or when contrast recirculation is present. The recovery of the desired signal components from amid these distortions by modeling the residue function in the frequency domain is demonstrated. The basic advantages and applicability of the frequency-domain modeling concept are explored through a simple frequency-domain Lorentzian model (FDLM); with results compared to standard SVD-based approaches. The performance of the FDLM method is model dependent, well representing residue functions in the exponential family while less accurately representing other functions.

A comparison study of multishot vs. single-shot DWI-EPI in the neonatal brain: reduced effects of ghosting compared to adults

In the neonatal brain, it is important to use a fast imaging technique to acquire all diffusion weighted images (DWI) for apparent diffusion coefficient (ADC) calculation. Taking into account the occurrence of typical echo planar imaging (EPI) artifacts, we have investigated whether single-shot (SSh) or multishot (MSh) DWI-EPI should be preferred. In 14 neonates, 17 adult patients and 5 adult volunteers, DWIs are obtained both with SSh and MSh EPI. The occurrence of artifacts and their influence on the ADC are explored and further quantified using simulations and phantom studies. Two radiologists scored overall image quality and diagnosability of all images. Single-shot and MSh DWI-EPI scored equally well in neonates with respect to overall image quality and diagnosability. In newborns, more motion artifacts in MSh can be noticed while N/2-ghost artifacts in SSh occur less frequently than in adults. Both N/2-ghost and motion artifacts result in significant ADC abnormalities. There is a serious risk that these artifacts will be mistaken for genuine diffusion abnormalities. N/2-ghost artifacts are hardly noticed in the neonatal brain, which might be due to smaller cerebrospinal fluid (CSF) velocity than in adults. Apparent diffusion coefficient values in MSh are unreliable if motion occurs. We conclude that for ADC calculations in neonates SSh DWI-EPI is more reliable than MSh.

Intra-arterial thrombus visualized on T2* gradient echo imaging in acute ischemic stroke

BACKGROUND

MR signal loss related to arterial thrombosis leading to vascular susceptibility artifacts (VSA) has recently been reported on gradient echo images. The time course and sensitivity of VSA in acute stroke patients has been scarcely investigated. The aim of this study was to assess the frequency and course of VSA in acute stroke patients, to compare its sensitivity to distinct features of arterial occlusion as detected on FLAIR images or on CT scan.

METHODS

Twenty-nine patients were scanned from 45 min to 6 h after stroke onset using identical MR parameters. All had an acute ischemic lesion identified on diffusion-weighted images, 25 had an occlusion of MCA or PCA confirmed by magnetic resonance angiography.

RESULTS

VSA was detected in 22/25 patients having an occluded artery at the time of MRI examination. Flair disclosed a hyperintense vessel in all of these 25 cases, but CT scan revealed a hyperdense artery in only 15 cases. Follow-up studies showed that VSA can vanish or disappear after partial recanalization. When the artery remains occluded, VSA can decrease, disappear or increase in the next hours, possibly related to structural modifications of the thrombus with time. Most occlusions were due to cardiac and arterial emboli or to intracranial extension of carotid occlusion.

CONCLUSIONS

VSA are frequent in the first hours of MCA or PCA occlusion in acute stroke patients. The sensitivity of VSA appears lower than the arterial hyperintensity on FLAIR images but higher than the hyperdense artery sign on CT scan. The extent and intensity of VSA can change with recanalization or structural modifications of the thrombus.

Ischemia-induced ADC changes are larger than osmotically-induced ADC changes in a neonatal rat hippocampus model

Diffusion-weighted imaging (DWI) is frequently used to diagnose stroke. However, the origin of the observed reduction in the apparent diffusion coefficient (ADC) in the acute phase following ischemia is not well understood. Although cell swelling is considered to play an important role, it is unclear whether this can completely explain the large ADC decrease. We developed a method to induce in neonatal rat hippocampal slices both osmotic perturbations, which lead to cell swelling, and oxygen/glucose deprivation (OGD), which simulates ischemia. A perfusion system was used to provide the hippocampal slices with nutrients and oxygen to maintain slice viability, which was verified with the use of fluorescent dyes (live/dead staining). Upon induction of OGD, the ADC decreased to approximately 57% of the initial value within 2 hr. The ADC reduction cannot fully be explained by changes due to cell swelling, since these led only to a maximum decrease of approximately 83%. Therefore, in addition to cell swelling, other changes must contribute significantly to the ADC reduction.

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

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

IRM de diffusion du système nerveux central : applications cliniques

Diffusion-weighted MR imaging is a technique in which image contrast is determined by the motion of water molecules within tissues. This motion is characterized by the apparent diffusion coefficient (ADC). This technique is particularly useful for the early detection of cerebral infarction but many other diseases of the central nervous system are associated with a change in water diffusion and may be assessed by diffusion-weighted MR imaging. This is an easy and fast pulse sequence providing useful data for early diagnosis and prognosis as well as information about underlying pathophysiology. After an overview of the basic concepts of diffusion imaging and the knowledge required for image interpretation, we will assess the potential value of this technique for the diagnosis of the main diseases of the central nervous system.

Relation quantitative des modifications de la diffusion et de la perfusion au sein du parenchyme cérébral au cours de l’accident ischémique aigu

MR-based diffusion- and perfusion-weighted imaging (DWI/PWI) has become the standard imaging technique to assess the individual brain pathophysiological status in acute ischemic stroke. The finding of a "mismatch" with larger PWI than DWI abnormality is thought to reflect the presence of tissue at-risk of infarction, i.e., penumbra. However, there has been no detailed study of the quantitative relationships between perfusion and diffusion changes in stroke patients. According to the experimental concept of penumbra, the ADC would be expected to remain unchanged despite decreasing perfusion until a critical threshold is reached. We have tested this hypothesis directly in man.

METHODS

DWI/PWI was performed in 7 patients with MCA territory stroke within 4-10 hrs from onset. Mismatch was defined on diffusion and rMTT maps, and circular ROIs were positioned within the ADC lesion (D), the mismatch area (M), and the normal appearing cortex (N); mirror ROIs were also obtained, and affected/unaffected ratios for ADC and rCBF were computed for each ROI.

RESULTS

The mean (+/-1 SD) ADC ratios were 0.60 +/- 0.09, 0.95 +/- 0.10 and 1.02 +/- 0.04 in L, M and N, respectively; the corresponding rCBF ratios were 0.32 +/- 0.12, 0.75 +/- 0.14 and 0.97 +/- 0.09, respectively. The relationship was non-linear, with the rCBF but not the ADC ratio for M being significantly lower (p < 0.01) than that for N. A threshold for decline in ADC was apparent around 0.50 rCBF ratio.

COMMENT

These results directly document in man that the ADC declines only after hypoperfusion has reached a certain degree (about 50%), consistent with the concept of the ischaemic penumbra.

Repetitive involuntary leg movements in patients with brainstem lesions involving the pontine tegmentum: evidence for a pontine inhibitory region in humans

Repetitive involuntary limbs movements have been mostly reported in patients with extensive brainstem pathologies, but the region responsible is unknown. We describe two patients with progressive basilar artery infarcts who showed automatic stepping and one patient with an osmotic demyelination disorder who showed periodic involuntary leg movements. By subtracting diffusion-weighted images before and after the development of repetitive involuntary leg movements, the brainstem lesion responsible for the involuntary movements was distinctively located in the vicinity of the pontine tegmentum, which is known as the pontine inhibitory region in animal studies.

Tracer delay correction of cerebral blood flow with dynamic susceptibility contrast-enhanced MRI

Cerebral blood flow (CBF) and vascular mean transit time (MTT) can be determined by dynamic susceptibility contrast-enhanced magnetic resonance imaging and deconvolution with an arterial input function. However, deconvolution by a singular value decomposition (SVD) method is sensitive to the tracer delay that often occurs in patients with cerebrovascular disease. We investigated the effect of tracer delay on CBF determined by SVD deconvolution. Simulation study showed that underestimation of CBF due to tracer delay was larger for shorter MTTs. We developed a delay correction method that determines tracer delay by means of least-squares fitting pixel-by-pixel. The corrected CBF was determined by SVD deconvolution after time-shifting of the measured concentration curve. The simulations showed that the corrected CBF was insensitive to tracer delay irrespective of the vascular model, although CBF fluctuation increased slightly. We applied the delay correction to the CBF and MTT images acquired for nine patients with hyperacute stroke and unilateral occlusion of the middle cerebral artery. We found in some patients that the delay correction modulated the contrast of CBF and MTT images. For hyperacute stroke patients, tracer delay correction is essential to obtain reliable perfusion image when SVD deconvolution is used.

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

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

Corpus callosal microstructural integrity influences interhemispheric processing: a diffusion tensor imaging study

Normal aging and chronic alcoholism result in disruption of brain white matter microstructure that does not typically cause complete lesions but may underlie degradation of functions requiring interhemispheric information transfer. We examined whether the microstructural integrity of the corpus callosum assessed with diffusion tensor imaging (DTI) would relate to interhemispheric processing speed. DTI yields estimates of fractional anisotropy (FA), a measure of orientation and intravoxel coherence of water diffusion usually in white matter fibers, and diffusivity (<D>), a measure of the amount of intracellular and extracellular fluid diffusion. We tested the hypothesis that FA and <D> would be correlated with (i) the crossed-uncrossed difference (CUD), testing visuomotor interhemispheric transfer; and (ii) the redundant targets effect (RTE), testing parallel processing of visual information presented to each cerebral hemisphere. FA was lower and <D> higher in alcoholics than in controls. In controls but not alcoholics, large CUDs correlated with low FA and high <D> in total corpus callosum and regionally in the genu and splenium. In alcoholics but not controls, small RTEs, elicited with equiluminant stimuli, correlated with low FA in genu and splenium and high <D> in the callosal body. The results provide in vivo evidence for disruption of corpus callosum microstructure in normal aging and alcoholism that has functional ramifications for efficiency in interhemispheric processing.

Functional activation within the PI–DWI mismatch region in recovery from ischemic stroke: preliminary observations

In this study, we sought to investigate if brain tissue affected by ischemia can accommodate areas of activation related to restoration of brain function following ischemic stroke. In two patients perfusion imaging (PI) and diffusion weighted imaging (DWI) obtained in the acute phase after stroke was coregistered with BOLD imaging of brain functions acquired when profound recovery had occurred. Both patients suffered from thrombembolic brain infarction due to dissection of the internal carotid artery (ICA) characterized by a severe PI-DWI mismatch in the acute stage of stroke. Following ICA recanalization and clinical recovery BOLD imaging showed task-specific activation adjacent to the infarct lesion within the former PI-DWI mismatch area. The data in these two stroke patients provide evidence that brain tissue at risk of infarction as shown by the PI-DWI mismatch can survive and, thereby, constitute the major site underlying post-ischemic recovery.

Magnetic resonance imaging criteria for thrombolysis in acute cerebral infarct

BACKGROUND AND PURPOSE

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

SUMMARY OF REVIEW

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

CONCLUSION

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

Motor response to amphetamine treatment, task-specific training, and limited motor experience in a postacute animal stroke model

Despite advances in acute treatment of ischemic cerebrovascular events, the most common clinical outcome is disabling neurological impairment. Despite experimental evidence that psychostimulant treatment can positively affect recovery rate after focal brain lesions, beyond rehabilitation therapies there are no currently accepted medical treatments indicated for diminishing neurological impairment after clinically established stroke. To test the effect of amphetamine, task-specific training, limiting motor experience, and their interaction on motor recovery in a postacute animal model of stroke, animals were nonaversively trained in beam walking before a unilateral photochemical sensorimotor cortex lesion and tested for 10 days after lesion. Animals were randomized to groups receiving: a single session of motor training 24 h after lesion; a single injection of amphetamine 2 mg/kg 24 h after lesion; beam-walking experience limited to testing on days 1 and 10 after lesion; and groups that received amphetamine treatment combined with training or combined with limited experience. Motor recovery was maximally enhanced by training, delayed by amphetamine treatment, and most negatively affected by limiting beam-walking experience during the recovery period. These findings support physical training after stroke, indicating that limiting physical activity negatively affects motor recovery and raises questions about the role of stimulant treatment to enhance motor recovery in the postacute phase after stroke.

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

OBJECTIVE

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

METHODS AND MATERIALS

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

RESULTS

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

CONCLUSION

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

Development of brain infarct volume as assessed by magnetic resonance imaging (MRI): Follow-up of diffusion-weighted MRI lesions

PURPOSE

To investigate the development of ischemic brain lesions, as present in the acute stroke phase, by diffusion-weighted magnetic resonance imaging (DWI), and in the subacute and chronic phases until up to four months after stroke, in fluid-attenuated inversion recovery (FLAIR)- and T2-weighted (T2W) magnetic resonance (MR) images.

MATERIALS AND METHODS

Twelve consecutive patients with their first middle cerebral artery (MCA) infarction were included. Lesion volumes were assessed on T2W images recorded with a turbo spin echo (TSE) and on images recorded with the FLAIR sequence on average on day 8 and after about four months. They were compared with acute lesion volumes in perfusion and DWI images taken within 24 hours of stroke onset.

RESULTS

On day 8, lesion volumes in images obtained with FLAIR exceeded the acute infarct volumes in DWI. The chronic lesion volumes were almost identical in T2W and FLAIR images but significantly reduced compared with the acute DWI lesions. The lesion volumes assessed on DWI images correlated highly with the lesions in the images obtained with TSE or FLAIR, as did the lesions in the images obtained with FLAIR and TSE. The secondary lesion shrinkage was accompanied by ventricular enlargement and perilesional sulcal widening, as most clearly visible in the images obtained with FLAIR.

CONCLUSION

Our results show that the acute DWI lesions are highly predictive for the infarct lesion in the chronic stage after stroke despite a dynamic lesion evolution most evident in MR images obtained with FLAIR.

Perihematomal edema in primary intracerebral hemorrhage is plasma derived

BACKGROUND AND PURPOSE

The mechanisms of perihematomal injury in primary intracerebral hemorrhage (ICH) are incompletely understood. An MRI study was designed to elucidate the nature of edema and blood flow changes after ICH.

METHODS

Perihematomal blood flow and edema were studied prospectively with perfusion-weighted MRI (PWI) and diffusion-weighted MRI in 21 ICH patients. MRI and computed tomography (CT) images were coregistered to ensure perfusion and diffusion changes were outside of the hematoma. Edema volumes were measured on T2-weighted images. Apparent diffusion coefficient (ADC) values of the edematous regions were calculated.

RESULTS

Mean patient age was 64.2 years (45 to 89), and median National Institutes of Health stroke scale score was 12 (3 to 24). Median time to MRI was 21 hours (4.5 to 110). Average hematoma volume on CT was 26.1 (4 to 84) mL. PWI demonstrated perihematomal relative mean transit time (rMTT) was significantly correlated with hematoma volume (r=0.60; P=0.004) but not edema volume. Perihematomal oligemia (rMTT >2 s) was present in patients with hematoma volumes of >15 mL (average rMTT 4.6+/-2.0 s). Perihematomal edema was present in all patients. ADC values within this region (1178+/-213x10(-6) mm2/s) were increased 29% relative to contralateral homologous regions. Increases in perihematomal ADC predicted edema volume (r=0.54; P=0.012) and this was confirmed with multivariate analysis.

CONCLUSIONS

Acute perihematomal oligemia occurs in acute ICH but is not associated with MRI markers of ischemia and is unrelated to edema formation. Increased rates of water diffusion in the perihematomal region independently predict edema volume, suggesting the latter is plasma derived.

Assessment of cerebral hemodynamics and oxygen extraction using dynamic susceptibility contrast and spin echo blood oxygenation level-dependent magnetic resonance imaging: applications to carotid stenosis patients

Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) has been recently used to quantify cerebral blood volume (CBV) and oxygen extraction ratio (OER). In the present study, we have exploited the intravascular BOLD model to assess gray matter (GM) OER at hemispheric level using parenchymal T(2) and CBV data at 1.5 T, obtained by single spin echo and dynamic susceptibility contrast (DSC) perfusion MRI, respectively. An OER of 0.40 +/- 0.07 was determined in gray matter for control subjects. A group of carotid stenosis (CS) patients (n = 22) was examined by multiparametric MRI. The degree of CS was determined by contrast agent-enhanced magnetic resonance angiography. Within the group, eight cases with <70% narrowing of a carotid lumen, nine cases with 70-99%, and five cases with complete occlusion of either carotid arteries were found. DSC MRI revealed abnormalities in 14 patients in dynamic parameters of perfusion images. These included four cases with elevated hemispheric gray matter CBV ipsilateral to the stenosis, above 2 SD of the level determined in control subjects. These four patients showed large variation in the degree of stenosis. We also found three cases with ipsilateral gray matter CBV below 2 SD of the control value, two of these with >70% stenosis. Gray matter OER ipsilateral to the stenosis was above 2 SD of the control range in eight CS patients, three of these showing also high CBV. Use of the present approach to determine OER for the assessment of hemodynamic adaptations in CS patients is discussed in the light of documented hemodynamic adaptations to carotid stenosis.

Water diffusion in a rat glioma during ganciclovir-thymidine kinase gene therapy-induced programmed cell death in vivo: Correlation with cell density

PURPOSE

To study the characteristics of diffusion magnetic resonance imaging (MRI) contrast in a rat brain BT4C glioma during progression of ganciclovir (GCV)-thymidine kinase gene therapy-induced programmed cell death (PCD) in vivo.

MATERIALS AND METHODS

The trace of the diffusion tensor (Dav = 1/3TraceD), T2, and spin density were determined by MRI and the apparent diffusion coefficient (ADC) of water by diffusion nuclear MR (NMR) spectroscopy using largely varying b values and diffusion times (tD) at 4.7 T. Cell count and apoptotic cells were quantified by histological means.

RESULTS

Decline in cell count was strongly associated with increase in both Dav and T2. Spin density ratio between tumor and contralateral parietal cortex increased with a very similar time course as Dav and T2, indicating net water gain into the eradicating tumor. Diffusion spectroscopy showed a nonmonoexponential signal decay at all tD values ranging from 14-192 msec. During PCD, the ADC of the component yielding fast diffusion coefficient (D1), as acquired with tD > or = 47 msec, increased with kinetics similar to those of Dav (tD = 4.8 msec). The fractional size of D1 increased by 10% to 15% throughout the entire tD range. Apparent water residence time of the slow diffusion component, D2, shortened from a value of 38.3 +/- 1.7 msec on day 0 to 33.4 +/- 0.5 msec by day 8.

CONCLUSION

The present results show that reduced cell density and increased water content, leading to altered water microenvironment, are associated with increased water diffusion coefficient in eradicating gliomas as a result of PCD.

Diffusion abnormalities of the globi pallidi in manganese neurotoxicity

Manganese is an essential trace metal required for normal central nervous system function, which is toxic when in excess amounts in serum. Manganese neurotoxicity has been demonstrated in patients with chronic liver/biliary failure where an inability to excrete manganese via the biliary system causes increased serum levels, and in patients on total parenteral nutrition (TPN), occupational/inhalational exposure, or other source of excess exogenous manganese. Manganese has been well described in the literature to deposit selectively in the globi pallidi and to induce focal neurotoxicity. We present a case of a 53-year-old woman who presented for a brain MR 3 weeks after liver transplant due to progressively decreasing level of consciousness. The patient had severe liver failure by liver function tests and bilirubin levels, and had also been receiving TPN since the transplant. The MR demonstrated symmetric hyperintensity on T1-weighted images in the globi pallidi. Apparent diffusion coefficient (ADC) map indicated restricted diffusion in the globi pallidi bilaterally. The patient eventually succumbed to systemic aspergillosis 3 days after the MR. The serum manganese level was 195 mcg/l (micrograms per liter) on postmortem exam (over 20 times the upper limits of normal). The patient was presumed to have suffered from manganese neurotoxicity since elevated serum manganese levels have been shown in the literature to correlate with hyperintensity on T1-weighted images, neurotoxicity symptoms, and focal concentration of manganese in the globi pallidi. Neuropathologic sectioning of the globi pallidi at autopsy was also consistent with manganese neurotoxicity.

Effects of tracer arrival time on flow estimates in MR perfusion-weighted imaging

A common technique for calculating cerebral blood flow (CBF) and mean transit time (MTT) is to track a bolus of contrast agent using perfusion-weighted MRI (PWI) and to deconvolve the change in concentration with an arterial input function (AIF) using singular value decomposition (SVD). This method has been shown to often overestimate the volume of tissue that infarcts and in cases of severe vasculopathy to produce CBF maps that are inconsistent with clinical presentation. This study examines the effects of tracer arrival time differences between tissue and a user-selected global AIF on flow estimates. CBF and MTT were calculated in both numerically simulated and clinically acquired PWI data where the AIF and tissue signals were shifted backward and forward in time with respect to one another. Results show that when the AIF leads the tissue, CBF is underestimated independent of extent of delay, but dependent on MTT. When the AIF lags the tissue, flow may be over- or underestimated depending on MTT and extent of timing differences. These conditions may occur in practice due to the application of a user-selected AIF that is not the "true AIF" and therefore caution must be taken in interpreting CBF and MTT estimates.

Imaging the ischaemic penumbra

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

The existence of biexponential signal decay in magnetic resonance diffusion-weighted imaging appears to be independent of compartmentalization

It is generally believed that the apparent diffusion coefficient (ADC) changes measured by diffusion-weighted imaging (DWI) in brain pathologies are related to alterations in the water compartments. The aim of this study was to elucidate the role of compartmentalization in DWI via biexponential analysis of the signal decay due to diffusion. DWI experiments were performed on mouse brain over an extended range of b-values (up to 10,000 mm(-2) s) under intact, global ischemic, and cold-injury conditions. DWI was additionally applied to centrifuged human erythrocyte samples with a negligible extracellular space. Biexponential signal decay was found to occur in the cortex of the intact mouse brain. During global ischemia, in addition to a drop in the ADC in both components, a shift from the volume fraction of the rapidly diffusing component to the slowly diffusing one was observed. In cold injury, the biexponential signal decay was still present despite the electron-microscopically validated disintegration of the membranes. The biexponential function was also applicable for fitting of the data obtained on erythrocyte samples. The results suggest that compartmentalization is not an essential feature of biexponential decay in diffusion experiments.

Advances in Penumbra Imaging with MR

The concept of the ischaemic penumbra as critically hypoperfused and functionally impaired, but potentially viable brain, was introduced over 25 years ago. Recent studies have used a combination of perfusion-weighted magnetic resonance imaging (PWI) and diffusion-weighted imaging (DWI) to delineate the putative penumbra. PWI provides semiquantitative cerebral blood flow imaging and DWI is an index of the largely irreversible ischaemic core. PWI > DWI mismatch is an operational definition of the penumbra that was introduced in the late 1990s. This definition has been modified in recent years with the recognition that the PWI boundary includes a region of benign oligaemia and that a portion of the DWI core is potentially salvageable with rapid reperfusion. An MRI penumbral signature is present in the majority of patients within 6 h of stroke onset, often but not invariably associated with proximal arterial occlusion on magnetic resonance angiography, and is strictly time dependent. It has been postulated that penumbral imaging using MRI can provide a physiological 'tissue clock' and be used to predict benefit from thrombolytic therapy beyond the established 3-hour window. This has been suggested by pilot studies, but confirmation will rely on ongoing, prospective, randomized trials. The presence and extent of the penumbra may also predict the opportunity for tissue salvage with neuroprotection strategies. DWI and PWI parameters are being used in proof-of-principle stroke trials. Such trials can be performed with 100-200 patients randomized between treated and control groups and provide a biological signal of efficacy with only 10% of the sample size required for a Phase III study.

A comparison of images generated from diffusion-weighted and diffusion-tensor imaging data in hyper-acute stroke

PURPOSE

To compare isotropic (combined diffusion-weighted image [CMB], apparent diffusion coefficient [ADC], TRACE, exponential ADC [eADC], and isotropically-weighted diffusion image [isoDWI]) and anisotropic (relative anisotropy [RA], fractional anisotropy [FA], and volume ratio [VR]) diffusion images collected with fast magnetic resonance (MR) diffusion-weighted (DWI) and diffusion-tensor (DTI) acquisition strategies (each less than one minute) in hyper-acute stroke.

MATERIALS AND METHODS

Twenty-one patients suffering from ischemic stroke-imaged within six hours of symptom onset using both DWI and DTI-were analyzed. Regions of interest were placed in the ischemic lesion and in normal contralateral tissue and the percent difference in image intensity was calculated for all nine generated images.

RESULTS

The average absolute percent changes for the isotropic strategies were all > 38%, with isoDWI found to have a difference of 50.7% +/- 7.9% (mean +/- standard error, P < 0.001). The ADC maps had the most significant difference (-42.4% +/- 2.0%, P < 0.001, coefficient of variation = 0.22). No anisotropic images had significant differences.

CONCLUSION

Anisotropic maps do not consistently show changes in the first six hours of ischemic stroke; therefore, isotropic maps, such as those obtained using DWI, are more appropriate for detecting hyper-acute stroke. Anisotropic images, however, may be useful to differentiate hyper-acute stroke from acute and sub-acute stroke.

Diffusion-Weighted MRI Findings in Patients with Capsular Warning Syndrome

BACKGROUND AND PURPOSE

The 'capsular warning syndrome' (CWS) of recurrent stereotyped episodes of motor or sensory dysfunction is clinically well recognized, and is associated with a high risk of imminent lacunar infarction with permanent deficits resembling those of CWS. However, the pathophysiology of CWS has not been well characterized. We report a clinicoanatomic correlation with MR imaging studies in the acute and chronic phases in patients with CWS.

MATERIAL AND METHODS

Between April 1997 and March 2001, we prospectively studied 8 patients, mean age 73.3 years, presenting with 4-17 motor or sensorimotor transient ischemic attacks (TIAs; duration 2-90 min) up to 3 days after onset of the first episode. Four patients were free of symptoms between the attacks and had no residua, whereas 4 patients developed a pure motor or sensorimotor stroke within 1-3 days after symptom onset. Diffusion-weighted echoplanar MRI (DWI) and T(2)-weighted MRI studies were performed within 1 week after symptom onset and were repeated 1-2 months later.

RESULTS

Seven of the 8 patients had an appropriate lesion on DWI in the acute phase. DWI abnormalities in the 3 patients with TIAs were 4-10 mm in diameter and confined to the lateral thalamus or medial globus pallidus without involving the internal capsule, whereas 4 patients who developed a stroke had abnormalities localized to the putamen extending to corona radiata (3 patients), or the pontomesencephalic junction (1 patient). All 6 patients who underwent follow-up MRI had an infarct on T(2)-weighted images corresponding to, but usually smaller than, the acute phase DWI abnormality.

CONCLUSIONS

Small infarcts in the basal ganglia or the pons, close to central motor pathways, appear to be the primary lesion in CWS. The pathophysiology of CWS is complex, and may involve hemodynamic mechanisms in penetrating arterial territories, as well as molecular mechanisms, such as peri-infarct depolarizations affecting adjacent motor pathways.

Evidence-based neuroimaging in acute ischemic stroke

The imaging work-up of patients with acute neurologic deficits should begin with noncontrast CT to exclude intracerebral hemorrhage. Based on positive results from the NINDS t-PA trial, the overriding objectives of imaging in the selection of patients for t-PA treatment are the detection of hemorrhage and rapid evaluation (speed of imaging). Despite its limited sensitivity for the identification of an ischemic stroke lesion, CT has multiple advantages over MR imaging in the initial diagnostic work-up. Advanced MR techniques promise to provide anatomic, physiologic, and vascular information in a single examination, and the ability to increase treatment specificity and improve outcome. Clinical outcome data are lacking; therefore, the routine use of screening MR imaging before t-PA therapy is not supported. Rigorous validation and correlation to clinical outcomes will be required.

Effect of incomplete (spontaneous and postthrombolytic) recanalization after middle cerebral artery occlusion: a magnetic resonance imaging study

BACKGROUND AND PURPOSE

Early reperfusion is one of the best predictors of good outcome after acute middle cerebral artery (MCA) occlusion. The purpose of this study was to analyze the frequency and relevance of incomplete recanalization for tissue and clinical outcome.

METHODS

From a larger acute stroke database (Kompetenznetzwerk Schlaganfall B5), all patients (n=82) with MCA main stem occlusion (excluding carotid T-occlusions) were selected. These patients had received a multiparametric stroke MRI protocol including diffusion- and perfusion-weighted imaging (DWI, PWI) and MR angiography (MRA) within 6 hours after symptom onset, at day 1 and after 1 week. Recanalization status was determined with MRA on day 1 (according to Thrombolysis In Myocardial Infarction flow grades) and used to group patients into those with persistent occlusion (0) or minimal (1), partial (2), or complete (3) recanalization.

RESULTS

Incomplete recanalization according to MRI criteria was found in 39 patients (grade 1: n=20; grade 2: n=19), complete recanalization in 10, and persistent occlusion in 33. There was no statistically significant difference in any of the clinical (National Institutes of Health Stroke Scale score) or MRI baseline parameters (DWI lesion, PWI deficit, mismatch volume, mismatch ratio). However, lesion growth was smaller in patients with recanalization (even in patients with only minimal recanalization) and outcome was related to the degree of recanalization (mean modified Rankin score at 90 days: 3.36, 2.70, 1.79, and 1.44 for the groups with no, minimal, partial, and complete recanalization, respectively). Both incomplete and complete recanalization was more frequent in patients receiving thrombolysis.

CONCLUSIONS

Incomplete recanalization on day 1 is a frequent MR finding after MCA main stem occlusion, indicating a more favorable clinical course than persistent occlusion. MR indicators of early recanalization could be useful surrogates of efficacy in thrombolytic trials.

Magnetic resonance imaging in experimental models of brain disorders

This review gives an overview of the application of magnetic resonance imaging (MRI) in experimental models of brain disorders. MRI is a noninvasive and versatile imaging modality that allows longitudinal and three-dimensional assessment of tissue morphology, metabolism, physiology, and function. MRI can be sensitized to proton density, T1, T2, susceptibility contrast, magnetization transfer, diffusion, perfusion, and flow. The combination of different MRI approaches (e.g., diffusion-weighted MRI, perfusion MRI, functional MRI, cell-specific MRI, and molecular MRI) allows in vivo multiparametric assessment of the pathophysiology, recovery mechanisms, and treatment strategies in experimental models of stroke, brain tumors, multiple sclerosis, neurodegenerative diseases, traumatic brain injury, epilepsy, and other brain disorders. This report reviews established MRI methods as well as promising developments in MRI research that have advanced and continue to improve our understanding of neurologic diseases and that are believed to contribute to the development of recovery improving strategies.

Acute Ischemic Cerebrovascular Syndrome

Background— Existing diagnostic classification systems for cerebrovascular disease are based primarily on clinical impression of temporal features, clinical syndrome, inferred localization, or ischemic mechanism. Diagnostic certainty of the ischemic pathology based on supportive or refuting laboratory or radiological evidence has been of secondary importance.

Summary of Comment— Acute ischemic cerebrovascular syndrome (AICS) describes a spectrum of clinical presentations that share a similar underlying pathophysiology: cerebral ischemia. Diagnostic criteria for AICS incorporate prior classification systems and currently available information provided by neuroimaging and laboratory data to define 4 categories ranging from “definite AICS” to “not AICS,” which define the degree of diagnostic certainty.

Conclusions— Clinical trials testing new treatments for acute ischemic stroke or secondary stroke prevention should limit enrollment to patients with “definite” AICS whenever feasible.

Cilostazol reduces brain lesion induced by focal cerebral ischemia in rats—an MRI study

To investigate the effects of cilostazol on the hemispheric ischemic lesion, we monitored the apparent diffusion coefficient (ADC) and T2 images by MRI techniques in comparison with histology at the terminal of and after 24-h reperfusion following 2-h occlusion of middle cerebral artery (MCA). The ADC values of tissue water and T2-weighted images were quantified by high field magnetic resonance. No significant difference was observed by ADC image among vehicle and cilostazol treatment groups when measured during MCA occlusion. Oral treatment with cilostazol 30 mg/kg two times at 5 min and 4 h significantly suppressed the hemispheric lesion area and volumes when detected by ADC, T2 images and histology, but 3 and 10 mg/kg cilostazol were without effect. Cilostazol (30 mg/kg) significantly reduced the increased cerebral water content at the ischemic hemisphere compared with vehicle group. In line with these results, the neurological deteriorations were much improved in the cilostazol-treated group. Taken together, it is concluded that post-treatment with cilostazol exerts a potent protective effect against cerebral infarct size by reducing the cytotoxic edema.

MRI-guided therapy in acute stroke

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

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

BACKGROUND

The ability to quickly and efficiently identify the ischemic penumbra in the acute stroke clinical setting is an important goal for stroke researchers and clinicians. Early and accurate identification of potentially salvageable versus irreversibly infarcted brain tissue may enable selection of the most appropriate candidates for early stroke therapies and identify patients who may still benefit from late recanalization or neuroprotective treatment. Recent advances in magnetic resonance imaging of the ischemic penumbra have been driven by serial MRI studies characterizing the natural evolution of cerebral infarction as well as the brain's response to reperfusion.

SUMMARY OF COMMENT

Based on these studies, various models for imaging the penumbra with MRI have been proposed, including the pioneering diffusion-perfusion mismatch model and later multivariate approaches. Each model has its own unique advantages and disadvantages.

CONCLUSIONS

There now are sufficient data to support paradigm shifts in a variety of central tenets regarding MRI and the ischemic penumbra. These include the insights that diffusion-perfusion mismatch does not optimally define the penumbra; that early diffusion lesions are in part reversible and often include both irreversibly infarcted tissue and penumbra; that the visible zone of perfusion abnormality overestimates the penumbra by including regions of benign oligemia; that MRI is a very practical method for acute stroke imaging; and that therapeutic salvage of the ischemic penumbra has been demonstrated in humans using diffusion-perfusion MRI.

Perfusion thresholds in acute stroke thrombolysis

BACKGROUND AND PURPOSE

Perfusion-weighted MRI has been shown to be useful in the early identification of cerebral tissue at risk of infarction during acute ischemia. Identification of threshold perfusion measures that predict infarction may assist in the selection of patients for thrombolysis.

METHODS

Mean transit time (MTT), regional cerebral blood flow (rCBF), and regional cerebral blood volume (rCBV) maps were generated in 35 acute stroke patients (17 treated with tissue plasminogen activator and 18 control patients) imaged within 6 hours from symptom onset. Day 90 outcome infarcts (T2-weighted MRI) were superimposed on acute MTT, rCBF, and rCBV maps. Perfusion-weighted MRI measures were then calculated for 2 regions: infarcted and salvaged tissue.

RESULTS

MTT was prolonged by 22% in infarcted regions relative to salvaged tissue (P<0.001). rCBF was 10% lower in infarcted tissue than in salvaged regions (P<0.01). rCBV did not differ significantly between infarcted and salvaged regions. When reperfusion occurred, tissue with more severely prolonged MTT was salvaged from infarction relative to patients with persistent hypoperfusion (P<0.05). In contrast, rCBF in salvaged regions did not differ between patients with and without reperfusion. In reperfused patients, an inverse correlation (R=0.93, P<0.001) was found between time of initial MRI scan and MTT delay in salvaged tissue.

CONCLUSIONS

Both increases in MTT and decreases in rCBF predict infarction. Differences in MTT also predict salvage in more severely hypoperfused tissue after reperfusion, suggesting that it is the most clinically useful quantitative perfusion measure. Perfusion thresholds for infarction need to be assessed in the context of symptom duration.

Behavioural recovery correlated with MRI in a rat experimental stroke model

PRIMARY OBJECTIVE

To characterize a necrotic lesion using MRI and motor recovery using behavioural methods.

RESEARCH DESIGN

Stroke model based on two steps: (1) development of a lesion using MR-imaging parameters and (2) behavioural recovery.

METHODS AND PROCEDURES

Seventy male Sprague-Dawley rats were used. A focal lesion of the right sensorimotor cortex was induced photochemically.

MAIN OUTCOMES AND RESULTS

The maximum volume of oedema and the lesion damage was reached by approximately 6 hours. In the lesion area, the apparent diffusion coefficient (ADC) increased from 6 hours, then decreased from 24 hours. All animals spontaneously recovered motor function by day 10, despite the continued presence of the cortical lesion.

CONCLUSIONS

The results show that this model mimics a core lesion, as well as the late phase in a human stroke episode. This model might be used for longitudinal study of the basic mechanisms of motor recovery.

Newer MR imaging techniques for head and neck

Dynamic and functional imaging techniques are being developed to improve the evaluation of various pathologic processes of the head and neck region. These techniques include dynamic contrast-enhanced MR imaging for evaluating soft tissue masses and cervical lymph nodes, the use of ultrasmall superparamagnetic iron oxide contrast agent, and functional techniques such as in vivo and in vitro MR spectroscopy of head and neck cancer and lymph nodes and apparent diffusion coefficient mapping of parotid glands. These techniques can help to differentiate nonmalignant tissue from malignant tumors and lymph nodes and can aid in differentiating residual malignancies from postradiation changes. From methodological development, they are making the critical transition to preclinical and clinical validating methods and eventually to widespread clinical tools.

Diffusion-weighted MRI in acute spinal cord ischaemia

Acute spinal cord ischaemia is often undetectable with conventional MRI. Diffusion-weighted MRI (DWI) has been difficult to use in the spine because of susceptibility artefacts. We assessed the diagnostic value of echoplanar DWI for early confirmation of spinal cord ischaemia. We performed conventional MRI and DWI in two men and three women, aged 54-75 years with clinically suspected acute spinal cord ischaemia. Imaging was performed 9-46 h after the onset of symptoms, and 2-9 days later to assess the extent of ischaemic signal change. Spatial resolution of DWI within the spine using standard equipment was poor, but in all patients, early DWI revealed areas of high signal indicating decreased diffusion, confirmed by measurement of apparent diffusion coefficients. Follow-up MRI showed high signal on T2-weighted images and contrast enhancement at the expected levels. Neurological deficits corresponded with radiological findings in four patients: various syndromes, including isolated bilateral weakness or sensory change and combined deficits, were found. Echoplanar DWI may be helpful for confirmation of spinal cord ischaemia in the acute stage, but follow-up T2-weighted images have superior spatial resolution and correlation with clinical findings and lesion extent.

Trial design and reporting standards for intra-arterial cerebral thrombolysis for acute ischemic stroke

BACKGROUND AND PURPOSE

The National Institutes of Health (NIH) estimates that stroke costs now exceed 45 billion dollars per year. Stroke is the third leading cause of death and one of the leading causes of adult disability in North America, Europe, and Asia. A number of well-designed randomized stroke trials and case series have now been reported in the literature to evaluate the safety and efficacy of thrombolytic therapy for the treatment of acute ischemic stroke. These stroke trials have included intravenous studies, intra-arterial studies, and combinations of both, as well as use of mechanical devices for removal of thromboemboli and of neuroprotectant drugs, alone or in combination with thrombolytic therapy. At this time, the only therapy demonstrated to improve outcomes from an acute stroke is thrombolysis of the clot responsible for the ischemic event. There is room for improvement in stroke lysis studies. Divergent criteria, with disparate reporting standards and definitions, have made direct comparisons between stroke trials difficult to compare and contrast in terms of overall patient outcomes and efficacy of treatment. There is a need for more uniform definitions of multiple variables such as collateral flow, degree of recanalization, assessment of perfusion, and infarct size. In addition, there are multiple unanswered questions that require further investigation, in particular, questions as to which patients are best treated with thrombolysis. One of the most important predictors of clinical success is time to treatment, with early treatment of <3 hours for intravenous tissue plasminogen activator and <6 hours for intra-arterial thrombolysis demonstrating significant improvement in terms of 90-day clinical outcome and reduced cerebral hemorrhage. It is possible that improved imaging that identifies the ischemic penumbra and distinguishes it from irreversibly infarcted tissue will more accurately select patients for therapy than duration of symptoms. There are additional problems in the assessment of patients eligible for thrombolysis. These include being able to predict whether a particular site of occlusion can be successfully revascularized, predict an individual patient's prognosis and outcome after revascularization, and in particular, to predict the development of intracerebral hemorrhage, with and without clinical deterioration. It is not clear to assume that achieving immediate flow restoration due to thrombolytic therapy implies clinical success and improved outcome. There is no simple correlation between recanalization and observed clinical benefit in all ischemic stroke patients, because other interactive variables, such as collateral circulation, the ischemic penumbra, lesion location and extent, time to treatment, and hemorrhagic conversion, are all interrelated to outcome.

METHODS

This article was written under the auspices of the Technology Assessment Committees for both the American Society of Interventional and Therapeutic Neuroradiology and the Society of Interventional Radiology. The purpose of this document is to provide guidance for the ongoing study design of trials of intra-arterial cerebral thrombolysis in acute ischemic stroke. It serves as a background for the intra-arterial thrombolytic trials in North America and Europe, discusses limitations of thrombolytic therapy, defines predictors for success, and offers the rationale for the different considerations that might be important during the design of a clinical trial for intra-arterial thrombolysis in acute stroke. Included in this guidance document are suggestions for uniform reporting standards for such trials. These definitions and standards are mainly intended for research trials; however, they should also be helpful in clinical practice and applicable to all publications. This article serves to standardize reporting terminology and includes pretreatment assessment, neurologic evaluation with the NIH Stroke Scale score, imaging evaluation, occlusion sites, perfusion grades, follow-up imaging studies, and neurologic assessments. Moreover, previously used and established definitions for patient selection, outcome assessment, and data analysis are provided, with some possible variations on specific end points. This document is therefore targeted to help an investigator to critically review the scales and scores used previously in stroke trials. This article also seeks to standardize patient selection for treatment based on neurologic condition at presentation, baseline imaging studies, and utilization of standardized inclusion/exclusion criteria. It defines outcomes from therapy in phase I, II, and III studies. Statistical approaches are presented for analyzing outcomes from prospective, randomized trials with both primary and secondary variable analysis. A discussion on techniques for angiography, intra-arterial thrombolysis, anticoagulation, adjuvant therapy, and patient management after therapy is given, as well as recommendations for posttreatment evaluation, duration of follow-up, and reporting of disability outcomes. Imaging assessment before and after treatment is given. In the past, noncontrast CT brain scans were used as the initial screening examination of choice to exclude cerebral hemorrhage. However, it is now possible to quantify the volume of early infarct by using contiguous, discrete (nonhelical) images of 5 mm. In addition, CT angiography by helical scanning and 100 mL of intravenous contrast agent can be used expeditiously to obtain excellent vascular anatomy, define the occlusion site, obtain 2D and 3D reformatted vascular images, grade collateral blood flow, and perform tissue-perfusion studies to define transit times of a contrast bolus through specific tissue beds and regions of interest in the brain. Dynamic CT perfusion scans to assess the whole dynamics of a contrast agent transit curve can now be routinely obtained at many hospitals involved in these studies. The rationale, current status of this technology, and potential use in future clinical trials are given. Many hospitals are also performing MR brain studies at baseline in addition to, or instead of, CT scans. MRI has a high sensitivity and specificity for the diagnosis of ischemic stroke in the first several hours from symptom onset, identifies arterial occlusions, and characterizes ischemic pathology noninvasively. Case series have demonstrated and characterized the early detection of intraparenchymal hemorrhage and subarachnoid hemorrhage by MRI. Echo planar images, used for diffusion MRI and, in particular, perfusion MRI are inherently sensitive for the susceptibility changes caused by intraparenchymal blood products. Consequently, MRI has replaced CT to rule out acute hemorrhage in some centers. The rationale and the potential uses of MR scanning are provided. In addition to established criteria, technology is continuously evolving, and imaging techniques have been introduced that offer new insights into the pathophysiology of acute ischemic stroke. For example, a better patient stratification might be possible if CT and/or MRI brain scans are used not only as exclusion criteria but also to provide individual inclusion and exclusion criteria based on tissue physiology. Imaging techniques might also be used as a surrogate outcome measure in future thrombolytic trials. The context of a controlled study is the best environment to validate emerging imaging and treatment techniques. The final section details reporting standards for complications and adverse outcomes; defines serious adverse events, adverse events, and unanticipated adverse events; and describes severity of complications and their relation to treatment groups. Recommendations are made regarding comparing treatment groups, randomization and blinding, intention-to-treat analysis, quality-of-life analysis, and efficacy analysis. This document concludes with an analysis of general costs associated with therapy, a discussion regarding entry criteria, outcome measures, and the variability of assessment of the different stroke scales currently used in the literature is also featured.

CONCLUSIONS

In summary, this article serves to provide a more uniform set of criteria for clinical trials and reporting outcomes used in designing stroke trials involving intra-arterial thrombolytic agents, either alone or in combination with other therapies. It is anticipated that by having a more uniform set of reporting standards, more meaningful analysis of the data and the literature will be able to be achieved.

Metabolic counterpart of decreased apparent diffusion coefficient during hyperacute ischemic stroke: a brain proton magnetic resonance spectroscopic imaging study

BACKGROUND AND PURPOSE

Recent studies have shown that the brain ischemic area defined by the map of decreased apparent diffusion coefficient (ADC) obtained by diffusion-weighted imaging (DWI) during the first hours of ischemic stroke includes a significant part of ischemic penumbra. We hypothesize that the misjudgment of the final infarct size by ADC mapping may be related to a restricted ability of DWI to capture variations in the intensity of cellular suffering. In an attempt to characterize metabolically the hypoperfused brain parenchyma, we studied the relationship between ADC values and brain metabolic parameters measured by proton MR spectroscopic imaging (SI).

METHODS

Six patients with hyperacute ischemic stroke were explored within the first 7 hours after onset with the use of a MR protocol including T2*-weighted MRI, DWI, SI, perfusion-weighted imaging, and MR angiography.

RESULTS

This study demonstrates, for the first time, a wide gradient of ischemia-related metabolic anomalies within the abnormal area delineated by DWI during hyperacute ischemic stroke. In the narrow range of decreased mean ADC values (0.60 to 0.40 x 10(-9) m2 x s(-1)), a 33% decrease in mean ADC is associated with a 122% increase in lactate/N-acetyl aspartate ratio. Mean ADC values never fall below 0.40 x 10(-9) m2 x s(-1) within the severely affected ischemic tissue, while SI still detects a large metabolic heterogeneity inside areas showing similar decreased mean ADC values close to this threshold.

CONCLUSIONS

Our results indicate that the region of very low mean ADC values observed during hyperacute ischemic stroke contains areas of various tissue damage intensity characterized by SI in relation to different stages of cellular metabolic injury. This observation may explain why ADC mapping does not reliably predict final infarct size.

Hypoperfusion and Its Augmentation in Patients with Brain Ischemia

Control of hypertension is a well-established goal of the primary and secondary prevention of stroke. However, management of blood pressure in the setting of acute brain ischemia is complicated by the possible effect of blood pressure changes on cerebral perfusion. In acute stroke, patients may have an ischemic penumbra of brain tissue, which has impaired perfusion but which is not irreversibly damaged. The ischemic penumbra may be salvaged with reperfusion. Lowering of blood pressure in this setting, however, would hasten the progression of the penumbra to infarction. With the exception of patients treated with thrombolytic agents, blood pressure reduction is not recommended in acute ischemic stroke for this reason. Preliminary studies suggest that there may be a role for interventions to elevate blood pressure as a treatment for acute stroke patients. Despite interest in induced hypertension as a treatment of stroke dating back to the 1950s, this practice has not achieved widespread use owing to concerns about potential adverse effects such as intracerebral hemorrhage, cerebral edema, and myocardial ischemia. It is commonly used, however, to treat patients with threatened cerebral ischemia due to vasospasm after subarachnoid hemorrhage. Until future studies clarify the effectiveness of induced hypertension in stroke treatment, maintaining adequate blood pressure and fluid volume is recommended for patients with acute ischemic stroke, particularly if the neurologic deficits are fluctuating or the patient has persistent large-vessel occlusive disease.

Early ischemic lesion recurrence within a week after acute ischemic stroke

Previous observations suggested that multiple ischemic lesions on diffusion-weighted imaging (DWI) are common in acute stroke patients. We hypothesized that a source of these multiple lesions was the recurrence of ischemic lesions within a week after a clinically symptomatic stroke. We analyzed 99 acute ischemic stroke patients scanned within 6 hours of onset and at subsequent times within the first week. Ischemic lesion recurrence was defined as any new lesion separate from the index lesion. Recurrent lesions occurring outside initial perfusion deficit were termed 'distant lesion recurrence'. We estimated the hazard ratio (HR) of recurrence associated with clinical and imaging characteristics using log-rank test. Any lesion recurrence was found in 34%, with distant lesion recurrence in 15%, while clinical recurrence was evident in 2%. Initial multiple DWI lesions were associated with any lesion recurrence (HR, 2.83; 95% confidence interval [CI], 1.65-10.29; p = 0.002) and with distant lesion recurrence (HR, 5.99; 95% CI, 4.05-64.07; p < 0.0001). Large-artery atherosclerosis was the most frequent stroke subtype associated with any lesion recurrence (p = 0.026). These results may indicate a prolonged state of increased ischemic risk over the first week and suggest DWI as a possible surrogate measure for recurrent stroke.

Tracer arrival timing-insensitive technique for estimating flow in MR perfusion-weighted imaging using singular value decomposition with a block-circulant deconvolution matrix

Relative cerebral blood flow (CBF) and tissue mean transit time (MTT) estimates from bolus-tracking MR perfusion-weighted imaging (PWI) have been shown to be sensitive to delay and dispersion when using singular value decomposition (SVD) with a single measured arterial input function. This study proposes a technique that is made time-shift insensitive by the use of a block-circulant matrix for deconvolution with (oSVD) and without (cSVD) minimization of oscillation of the derived residue function. The performances of these methods are compared with standard SVD (sSVD) in both numerical simulations and in clinically acquired data. An additional index of disturbed hemodynamics (oDelay) is proposed that represents the tracer arrival time difference between the AIF and tissue signal. Results show that PWI estimates from sSVD are weighted by tracer arrival time differences, while those from oSVD and cSVD are not. oSVD also provides estimates that are less sensitive to blood volume compared to cSVD. Using PWI data that can be routinely collected clinically, oSVD shows promise in providing tracer arrival timing-insensitive flow estimates and hence a more specific indicator of ischemic injury. Shift maps can continue to provide a sensitive reflection of disturbed hemodynamics.