Magnetic resonance imaging of acute stroke

Spectroscopic assessment of alterations in macromolecule and small-molecule metabolites in human brain after stroke

BACKGROUND AND PURPOSE

We sought to measure the temporal evolution and spatial distribution of lesion macromolecules and small molecules (lactate, N-acetyl compounds, creatine, and choline) in stroke patients by using short echo time in vivo proton MR spectroscopy.

METHODS

Single-voxel spectra with TE=22 ms were obtained with and without inversion recovery suppression of small-molecule resonances from 30 examinations of 24 patients 3 to 214 days after stroke. Subtraction of the suppressed from the unsuppressed spectra yielded metabolite spectra without overlap from macromolecules. Two-dimensional spectroscopic images were acquired with macromolecule and small-molecule suppression from 5 additional patients.

RESULTS

Macromolecule signals were elevated in lesions relative to normal brain and tended to increase in the subacute period, even as lactate peaks declined. Regions of increased lactate, increased macromolecule signal at 1.3 ppm, and decreased N-acetyl compounds were closely correlated in the 2D spectroscopic images.

CONCLUSIONS

Short echo time spectra can be acquired in vivo in a manner that improves signal-to-noise ratio over long echo experiments and resolves overlapping macromolecule and small-molecule signals. The prominent macromolecule signals seen in the subacute period in association with persistently elevated lactate may represent mobile lipids in macrophages or other cells.

MR perfusion imaging in Moyamoya Syndrome: potential implications for clinical evaluation of occlusive cerebrovascular disease

BACKGROUND AND PURPOSE

Ischemic symptoms in patients with moyamoya syndrome (MMS) are usually due to hemodynamically mediated perfusion failure, and identification of abnormal tissue perfusion in these patients is therefore clinically important. Although dynamic susceptibility contrast (DSC) MRI can be used to study tissue perfusion, there are potential technical problems in MMS. This study investigates the scope and limitations of perfusion MRI in the clinical evaluation of such patients.

METHODS

Thirteen patients with bilateral MMS were studied with the use of structural, diffusion, and perfusion MRI. The DSC MRI data were analyzed both visually and by a quantitative regional analysis, and the relationship between perfusion status and clinical symptoms was investigated.

RESULTS

Extensive bilateral DSC MRI abnormalities were observed in all the patients. There was a very heterogeneous distribution of bolus arrival time. The areas of abnormality included the major arterial border zones in all cases, although these usually appeared normal on structural and diffusion MRI. Only the most clinically unstable patients had peak width (defined as time to peak minus bolus arrival time) >5 seconds on the quantitative regional analysis. Several technical limitations of perfusion quantification in MMS are described, as well as the implications of these limitations in patients with other forms of occlusive large-vessel disease.

CONCLUSIONS

The technical limitations of DSC MRI described in this study are important for the accurate interpretation of perfusion MRI in MMS. Despite these limitations, these preliminary findings suggest that the use of quantitative regional analysis of summary parameters may provide clinically useful information in patients with MMS.

Assessment of brain tissue viability in acute ischemic stroke by BOLD MRI

The introduction of new neuroprotective treatment strategies for acute stroke patients has provided a requirement for neuroimaging methods capable of identifying salvageable tissue in acute stroke patients. Substantial positron emission tomography evidence points to the fact that a peri-infarct zone with blood flow of 20-45% of normal, metabolic rate of oxygen of >35% of normal and oxygen extraction ratio (OER) of >0.7 are indices of tissue at risk of infarction, yet with potential for recovery. The sensitivity of T(2) to blood oxygen level dependent (BOLD) effects allows the mismatch between oxygen delivery and consumption in the brain to be imaged. Previous evidence from animal models of cerebral hypoperfusion and ischemic stroke strongly suggest that T(2) BOLD MRI highlights viable and salvageable brain regions. The Hahn-echo T(2) and diffusion show distinct flow thresholds in the rat brain so that the former parameter probes areas with high OER and the latter genuine ischemia. In the flow-compromised tissue showing negative T(2) BOLD, substantial residual perfusion is evident as revealed by bolus-tracking perfusion MRI, in agreement with the idea that tissue metabolic viability must be preserved for expression of BOLD. It is concluded that BOLD MRI may have potential for the assessment of tissue viability in acute ischemic stroke.

Time course of changes in diffusion-weighted magnetic resonance imaging in a case of neonatal encephalopathy with defined onset and duration of hypoxic-ischemic insult

The onset and duration of hypoxic-ischemic (HI) insults rarely can be determined precisely in perinatal asphyxia. The need to establish the timing of HI insults will be critical for the successful application of evolving neuroprotective therapies that may be administered to the asphyxiated newborn. Diffusion-weighted magnetic resonance imaging has emerged as an imaging technique that can be used to identify HI brain injury before the detection of abnormalities by conventional magnetic resonance imaging. This case illustrates the early changes in diffusion-weighted and conventional magnetic resonance imaging studies and in quantitative values of the apparent diffusion coefficient in a unique case of neonatal asphyxia in which the onset and duration of the HI insult were known.hypoxia-ischemia, newborn brain, perinatal asphyxia, diffusion-weighted imaging, proton magnetic resonance spectroscopy.

Time course of cerebral infarction in the middle cerebral arterial territory: deep watershed versus territorial subtypes on diffusion-weighted MR images

PURPOSE

To examine possible differences between the evolution of cerebral watershed infarction (WI) and that of territorial thromboembolic infarction (TI) by using diffusion-weighted (DW) and T2-weighted magnetic resonance (MR) images and apparent diffusion coefficient (ADC) maps.

MATERIALS AND METHODS

Fourteen patients with TI and nine with WI underwent MR imaging from the acute to chronic infarction stages. ADC maps were derived from DW images. Lesion-to-normal tissue signal intensity ratios on ADC maps (rADC), echo-planar T2-weighted images, and DW images were calculated. Lesion volumes at acute or early subacute infarction stages were measured on DW images, and final lesion volumes were estimated on fluid-attenuated inversion-recovery images.

RESULTS

Analysis of variance revealed a significant difference in temporal evolution patterns of rADC between WI and TI (P <.001). rADC pseudonormalization following TI began about 10 days after symptom onset, but that following WI did not occur until about 1 month after symptom onset. The Pearson correlation coefficient between final and initial infarct volumes was 0.9899 for both infarction subtypes, indicating that the initial ischemic injury volume measured at the acute or early subacute stage predicted the final lesion volume fairly well.

CONCLUSION

The evolution time of ADC is faster for TI than for WI. This difference, which likely originates from the different pathophysiologic and hemodynamic features of the two infarction types, might account for the relatively large range of ADC values reported for the time course of ischemic strokes.

The Future of Imaging in Drug Discovery

The number of new chemical entities being registered by drug companies each year is declining, while at the same time, the number of new compounds, and thereby potential therapeutics, is increasing at an exponential rate. The need to demonstrate the safety, efficacy, and the “value” of these new compounds to a sophisticated pharmaceutical market, driven in turn by the forces of healthcare economics, make drug development difficult, resulting in a very lengthy and complex series of steps in the development of a drug. Many aspects of clinical pharmacology are more art than science, and detecting pharmacological effects at the level of living integrated systems is difficult. These challenges are most evident when developing new therapeutics for neuropsychiatric illnesses. We may at last be entering a postmonoamine era, exemplified by compounds such as NK-1 antagonists and metatropic glutamate receptor agonists. Such developments hold significant promise for the treatment of severe mental illness, while at the same time being confronted with completely unknown clinical pharmacologies. Functional imaging may not only be useful for the development of new CNS compounds, but it may in fact be essential for helping to define their clinical pharmacology. Several examples will be addressed that highlight the utility of functional imaging in the development of potentially new CNS drugs.

A serial MR study of cerebral blood flow changes and lesion development following endothelin-1-induced ischemia in rats

The vasoconstrictive peptide endothelin-1 (ET-1) has been used previously to transiently occlude the middle cerebral artery (MCA) in rats. However, the duration of the resulting reduction in cerebral blood flow (CBF) and the reperfusion characteristics are poorly understood. In this study perfusion and T(2)-weighted MRI were used together with histology to characterize the cerebral perfusion dynamics and lesion development following ET-1 injection. Twenty-two rats received an intracerebral injection of ET-1 adjacent to the MCA. CBF was reduced to 30-50% of control levels, and a significant reduction persisted for 16 h in the cortex and 7 h in the striatum. The lesion size measured by T(2)-weighted imaging at 48 h correlated with the final infarct size measured by histology at 7 d. The sustained reduction in CBF and the gradual development of the ischemic lesion resemble human stroke evolution, suggesting that this model may be useful for evaluating therapeutic agents, particularly when treatment is delayed.

CT and MRI in the diagnosis of acute stroke and their role in thrombolysis

Thrombolysis is an effective but potential deleterious therapy and should therefore be limited to patients with acute intracerebral vessel occlusion and salvageable tissue. MRI currently develops towards the new diagnostic standard for the selection of stroke patients eligible for acute thrombolytic treatment and acute stroke studies. Diffusion- and perfusion-weighed MRI provides diagnostic information not available from the neurological assessments or from CCT and conventional spin-echo MRI. As high-speed DWI and PWI protocols become standardized, a 15-minute integrated stroke protocol of employing echo-planar imaging (EPI) can be outinely performed in the setting of acute clinical stroke. The combination of these MR techniques is suitable to define tissue at risk of infarction that is potentially salvageable brain tissue (an estimate of the ischemic penumbra) and may respond to early recanalization even beyond 3 hours after stroke onset. The extension of the therapeutic window for thrombolytic therapy towards 6 hours in a subpopulation of acute stroke patients might open the way for the successful reperfusion therapy in more stroke patients.

Characterisation of gene expression changes following permanent MCAO in the rat using subtractive hybridisation

Failure of several putative neuroprotectants in large multicentred clinical trials has re-focussed attention on the predictability of pre-clinical animal models of stroke. Model characterisation and relationship to heterogeneous patient sub-groups remains of paramount importance. Information gained from magnetic resonance imaging (MRI) signatures indicates that the Zea Longa model of rat middle cerebral artery occlusion may be more representative of slowly evolving infarcts. Understanding the molecular changes over several hours following cerebral ischaemia will allow detailed characterisation of the adaptive response to brain injury. Using a fully characterised model of Zea Longa middle cerebral artery occlusion we have used the representational difference analysis (RDA) subtractive hybridisation method to identify transcripts that accumulate in the ischaemic cortex. Along with a number of established ischaemia-induced gene products (including MCP-1, TIMP-1, hsp 70) we were also able to identify nine genes which have not previously been shown to accumulate following focal ischaemia (including SOCS-3, GADD45gamma, Xin).

Cerebral T1rho relaxation time increases immediately upon global ischemia in the rat independently of blood glucose and anoxic depolarization

Time-dependent changes of T1 in the rotating frame (T1rho), diffusion, T2, and magnetization transfer contrast on cardiac arrest-induced global ischemia in rat were investigated. T1rho, as acquired with spin lock amplitudes >0.6 G, started to increase 10-20 sec after cardiac arrest followed by an increase within 3-4 min to a level that was 6-8% greater than in normal brain. The ischemic T1rho response coincided with the drop of water diffusion coefficient in normoglycemic animals. However, unlike the rate of diffusion, the kinetics of T1rho were not affected by either preischemic hypoglycemia or hyperglycemia. Similar to diffusion, the kinetics of anoxic depolarization were dependent on preischemic blood glucose levels. Ischemia caused a reduction in the Hahn spin echo T2 as a result of blood oxygenation level-dependent (BOLD) effect; maximal negative BOLD seen by 40 sec. In the animals injected with an ironoxide particle contrast agent, AMI-227, prior to the insult, both T1rho and T2 immediately increased in concert on induction of ischemia. In contrast to the T1rho and diffusion changes, a much slower change in magnetization transfer contrast was evident over the first 20 min of ischemia. These data demonstrate that T1rho immediately increases following ischemia and that the pathophysiological mechanisms affecting this relaxation time may not directly involve magnetization transfer. The mechanisms prolonging T1rho differ from those affecting water diffusion with respect to their sensitivities to glucose and are apparently independent of membrane depolarization.

Prediction of tissue survival after middle cerebral artery occlusion based on changes in the apparent diffusion of water

OBJECT

In this study the authors tested the hypothesis that the estimate of the apparent diffusion coefficient (ADC) of water is a reliable pathophysiological index of the viability of ischemic brain tissue.

METHODS

Cerebral blood flow (CBF) and the cerebral metabolic rates of oxygen and glucose (CMRO2 and CMRglc, respectively) were measured using positron emission tomography (PET) scanning before and after permanent middle cerebral artery occlusion (MCAO) or reperfusion in pigs. The ADC value, which was measured using diffusion-weighted magnetic resonance (DW MR) imaging was compared with physiological variables obtained by PET scanning and with histological findings. After both permanent MCAO and reperfusion, the decrease in the ADC was significantly correlated with decrease in the CMRO2 and CMRglc. The infarction coincided with a CMRO2 threshold of 50% of the value measured on the contralateral side. Thus, an ADC value of 80% or 75% of the contralateral value reflected the CMRO2 threshold after permanent MCAO or reperfusion, respectively. On DW MR images, lesions with ADC values above 80% of the contralateral value are potentially reversible until 6 hours after MCAO, whereas lesions with ADC values below 75% of the contralateral value are irreversible as early as 2 hours after MCAO.

CONCLUSIONS

The ADC of water provides a reliable pathophysiological index for tailoring therapy to the condition of individual stroke patients in clinical practice.

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. Diffusion-weighted magnetic resonance imaging (DWI) is a powerful tool for evaluation of acute cerebral ischemia. We studied the effects of a novel, broad-spectrum inhibitor of several cation channels, LOE 908 MS, on acute ischemic lesion development with DWI and on cerebral infarct size using 2,3,5-triphenyltetrazolium chloride (TTC) staining postmortem. Eighteen male Sprague-Dawley rats underwent middle cerebral artery occlusion (MCAO) and were randomly and blindly assigned to either LOE 908 MS (1 mg/kg bolus 30 min after MCAO and continuous i.v. infusion of 10 mg/kg for 4 h thereafter) or vehicle. Whole-brain DWI was done before initiation of treatment and repeated every 30 min for the next 3.5 h. The animals were reperfused in the magnetic resonance imaging (MRI) scanner 90 min after MCAO. At 24 h, the animals were killed, and the brains were cut into six 2-mm-thick slices and stained with 2% TTC. Percent hemispheric lesion volume (%HLV) was calculated for each animal. Physiological parameters, body weight, and premature mortality (3 in the placebo group and 1 in the treated group) did not differ between the groups. No hypotension, abnormal behavior, or other adverse effects were seen. Pretreatment, the DWI-derived %HLV did not differ between the groups (19.8 +/- 6.2 in the control group and 17.9 +/- 7.9 in the treated group), whereas at 4 h after MCAO, it was significantly smaller in the treated group (21.8 +/- 15.4 vs 40.4 +/- 15.5, p = 0.03). Postmortem, TTC-derived %HLV was significantly attenuated in the LOE 908 MS group (21.3 +/- 11.9 vs 50.1 +/- 10.7, p = 0.0001) and the neurological scores at 24 h were significantly better among the treated rats (2.1 +/- 1.5 vs 4.0 +/- 1.0, p < 0.02). LOE 908 MS significantly improved neurological outcome and reduced infarct size without observable effects in rats as demonstrated in vivo by DWI and confirmed postmortem by TTC staining. Blocking several distinct cation channels by LOE 908 MS showed significant neuroprotection.

Regional variations in the apparent diffusion coefficient and the intracellular distribution of water in rat brain during acute focal ischemia

BACKGROUND AND PURPOSE

The apparent diffusion coefficient of water (ADC) rapidly drops in ischemic tissue after cerebral artery occlusion. This acute drop is thought to be caused by the loss of extracellular fluid and the gain of intracellular fluid. To test the latter possibility, changes in ADC and the size of several cellular compartments were assessed in 3 regions of rat brain at the end of 90 minutes of focal cerebral ischemia.

METHODS

One middle cerebral artery was permanently occluded in 8 Sprague-Dawley rats; sham occlusions were performed in 2 other rats. ADC maps were generated 90 minutes later, and the brains were immediately perfusion fixed. Three regions of interest (ROIs) were defined on the basis of ADC range. Various neuronal, astrocytic, and capillary compartments in each ROI were quantified with light and electron microscopy.

RESULTS

At the end of 90 minutes of ischemia, mean ADC was normal in the cortex of sham-operated rats and the contralateral cortex of ischemic rats (ROI-a), 25% lower in the ipsilateral frontoparietal cortex (ROI-b), and 45% lower in the ischemic lateral caudoputamen (ROI-c). At this time, the frequency of swollen astrocytic cell bodies and volume of swollen dendrites and astrocytic processes in neuropil were ROI-a<ROI-b<ROI-c. In ROI-b and ROI-c, 40% and 60% of the neurons, respectively, were shrunken; the shrunken neurons were approximately 25% smaller in ROI-c than in ROI-b. In these areas, many capillary endothelial cells, pericytes, and perivascular foot processes were swollen.

CONCLUSIONS

The initial lowering of ADC during focal ischemia probably is the result of not only the acute loss of extracellular fluid and concomitant swelling of various cellular compartments but also concurrent neuronal shrinkage.

Delayed rt-PA treatment in a rat embolic stroke model: diagnosis and prognosis of ischemic injury and hemorrhagic transformation with magnetic resonance imaging

The authors characterized effects of late recombinant tissue plasminogen activator (rt-PA) administration in a rat embolic stroke model with magnetic resonance imaging (MRI), to assess potential MRI correlates, or predictors, or both, of rt-PA-induced hemorrhage. Diffusion-, perfusion-, and postcontrast T1-weighted MRI were performed between 4 and 9 hours and at 24 hours after embolic stroke in spontaneously hypertensive rats. Treatment with either rt-PA or saline was started 6 hours after stroke. A spectrophotometric hemoglobin assay quantified hemorrhage severity. Before treatment, relative cerebral blood flow index (rCBFi) and apparent diffusion coefficient (ADC) in the ischemic territory were 30% +/- 23% and 60% +/- 5% (of contralateral), respectively, which increased to 45% +/- 39% and 68% +/- 4% 2 hours after rt-PA. After 24 hours, rCBFi and ADC were 27% +/- 27% and 59 +/- 5%. Hemorrhage volume after 24 hours was significantly greater in rt-PA-treated animals than in controls (8.7 +/- 3.7 microL vs. 5.1 +/- 2.4 microL, P < 0.05). Before rt-PA administration, clear postcontrast T1-weighted signal intensity enhancement was evident in areas of subsequent bleeding. These areas had lower rCBFi levels than regions without hemorrhage (23% +/- 22% vs. 36% +/- 29%, P < 0.05). In conclusion, late thrombolytic therapy does not necessarily lead to successful reperfusion. Hemorrhage emerged in areas with relatively low perfusion levels and early blood-brain barrier damage. Magnetic resonance imaging may be useful for quantifying effects of thrombolytic therapy and predicting risks of hemorrhagic transformation.

Diffusion-weighted mr in cerebral venous thrombosis

The diagnosis of cerebral venous thrombosis is often difficult both clinically and radiologically and until now there is no method available to predict if brain lesions, detected clinically and using conventional brain imaging methods, may lead to full recovery, as expected in vasogenic edema or ischemic infarcts and even a hematoma. New fast neuroimaging techniques such as diffusion-weighted imaging (DWI) are sensitive to different reasons of changes in local tissular water concentration thus giving further insight into the pathophysiological mechanism as well as prognosis of cerebral venous thrombosis. We report the cases of 18 consecutive patients with a diagnosis of cerebral venous thrombosis based on clinical and imaging criteria. All patients underwent magnetic resonance imaging (MRI) of the brain, which comprised isotropic diffusion-weighted MR. Diffusion-weighted MRI showed positive findings in 17/18 cases. In 7 cases the clot could be directly visualized as an area of hyperintensity in the affected vein on DWI. In 7 cases DWI showed areas of signal loss corresponding to hematomas. In 6 cases DWI showed changes in signal intensity that were more subtle. In 4 cases of superficial venous thrombosis, there were areas of decreased ADC values (0.65-0.79 x 10(-3) mm(2)/s) whereas in 2 cases of deep venous thrombosis, increased DWI intensities could be found that corresponded to both an increase and a decrease in ADC, corresponding to a coexistence of cytotoxic and vasogenic edemas. Diffusion-weighted MRI can demonstrate directly the presence of an intravenous clot in a select number of patients. It can also demonstrate early ischemic changes, and can differentiate conventional T2-weighted MR areas of cytotoxic from vasogenic edema.

Imaging of acute stroke

The utility of diagnostic imaging during the critical first few hours after stroke onset has many important applications. First and foremost, imaging technologies that can reliably detect and quantify the location of acute stroke will greatly enhance the clinician's ability to accurately diagnose individual stroke patients. Secondly, if imaging technology could provide information about the likely severity of the ischemic injury, patient prognosis and management would be enhanced. The possibility of potentially distinguishing severely injured and likely irreversible ischemic brain tissue from ischemic tissue likely not yet irreversibly injured may soon be attainable. The ability of imaging technology to reliably distinguish the status of focally ischemic brain will presumably dramatically impact upon patient management. This information, along with the data about the severity and extent of blood flow and tissue perfusion abnormalities, will help acute stroke care evolve beyond rigid time windows to individualized, pathophysiologically based treatment decisions. Not only will decisions to treat or not be made based upon imaging-derived status, but also the most appropriate type of therapy to be employed, i.e. thrombolysis, neuroprotection, therapy to reduce secondary reperfusion-related injury or combinations of these modalities. In this brief and necessarily incomplete overview of acute stroke imaging, the focus will be on new developments in CT and MRI.

Stroke genomics: approaches to identify, validate, and understand ischemic stroke gene expression

Sequencing of the human genome is nearing completion and biologists, molecular biologists, and bioinformatics specialists have teamed up to develop global genomic technologies to help decipher the complex nature of pathophysiologic gene function. This review will focus on differential gene expression in ischemic stroke. It will discuss inheritance in the broader stroke population, how experimental models of spontaneous stroke might be applied to humans to identify chromosomal loci of increased risk and ischemic sensitivity, and also how the gene expression induced by stroke is related to the poststroke processes of brain injury, repair, and recovery. In addition, we discuss and summarise the literature of experimental stroke genomics and compare several approaches of differential gene expression analyzes. These include a comparison of representational difference analysis we have provided using an experimental stroke model that is representative of stroke evolution observed most often in man, and a summary of available data on stroke differential gene expression. Issues regarding validation of potential genes as stroke targets, the verification of message translation to protein products, the relevance of the expression of neuroprotective and neurodestructive genes and their specific timings, and the emerging problems of handling novel genes that may be discovered during differential gene expression analyses will also be addressed.

Rapid three-dimensional diffusion MRI facilitates the study of acute stroke in mice

MRI studies using mouse brain models of ischemia are becoming a valuable tool for understanding the mechanism of stroke, since transgenic models are now available. However, the small size of the mouse brain and the surgical complexity of creating ischemia in mice make it technically challenging to obtain high-quality MRI data. Therefore, there are few reports of MRI studies in murine cerebral ischemia. In this project a newly developed rapid 3D diffusion-weighted imaging (DWI) technique was applied to study experimental stroke in a mouse model of reversible middle cerebral artery occlusion (MCAO). Ischemic volumes were successfully delineated using this 3D whole-brain imaging technique with high spatial (0.34 x 0.5 x 1.0 mm(3) before zero-filling) and temporal (7 min) resolution. The 3D observation revealed the characteristic evolution of stroke after transient MCAO. There was a temporarily high diffusion constant in the cortex during early reperfusion, followed by a secondary energy failure in the cortex and caudate-putamen at 6 and 21 h of reperfusion. Magn Reson Med 46:183-188, 2001.

Neuroprotective effect of aminoguanidine on transient focal ischaemia in the rat brain

Using serial magnetic resonance imaging we have evaluated the effectiveness of aminoguanidine (AG) as a neuroprotective agent in a rat model of transient middle cerebral artery occlusion (MCAO). Because aminoguanidine's neuroprotective properties have primarily been ascribed to its action as iNOS inhibitor, we also performed a biochemical analysis of nitric oxide metabolites and NOS isoforms in our model of ischaemia. Daily injections of AG (100 mg/kg) or saline, were started at 6 h after the occlusion and the effects of this treatment on lesion progression monitored by T(2)-weighted MRI at 6 (pre-treatment scan), 24 and 72 h. Measurements of lesion volumes showed that between 6 and 72 h post-MCAO, lesion growth was slower in AG-treated rats than in control rats. This difference was most pronounced between 24 and 72 h post-MCAO when AG halted the lesion volume expansion observed in control rats. Measurements of plasma NOx (nitrite plus nitrate) at 0, 24, 48 and 72 h after MCAO, showed that NO levels did not differ significantly between the AG- and saline-treated groups at any time-point. Moreover, NOS activity assays revealed that no iNOS activity was present in any of the brains tested and that constitutive neuronal NOS activity was similar across the two hemispheres between both groups. The absence of iNOS protein in the ischaemic and contralateral hemispheres at 48 and 72 h after MCAO (control group only) was confirmed by Western blot analysis. These results suggest that AG treatment reduces the rate of growth of ischaemic lesions, perhaps preserving the functioning of perifocal neurons. Our observations contradict suggestions that high levels of NO generated by iNOS are partially responsible for exacerbating the neuronal damage in the postischaemic phase of MCAO. Although this does not rule out a role for AG as a neuroprotective agent via its ability to inhibit iNOS, these findings indicate that neuroprotective actions of AG may also be mediated via other cellular targets.

Perfusion thresholds in human cerebral ischemia: historical perspective and therapeutic implications

After middle cerebral artery occlusion (MCAO) in the laboratory animal, the ischemic penumbra has been documented as a severely hypoperfused, functionally impaired, but still viable cortex which can regain its function and escape infarction if it is reperfused before a certain time has elapsed. The penumbra surrounds the ischemic core of already irreversibly damaged tissue, and is progressively recruited into the core with increasing MCAO duration. In the animal, the threshold of cerebral blood flow (CBF) below which neuronal function is impaired and the tissue is at risk of infarction is around 22 ml/100 g/min (approximately 40% of normal) in the awake or lightly anesthetized monkey, and around 30--35 ml/100 g/min in the cat and the rat. The threshold of CBF below which the tissue becomes irreversibly damaged and will progress to infarction depends on the duration of ischemia, and is around 10 ml/100 g/min for 1--2 h (approximately 20% of normal) and around 18 ml/100 g/min for permanent ischemia in the monkey. Mildly reduced CBF down to the 40% threshold (termed 'oligemia') is normally well tolerated, and the affected tissue is not at risk of infarction under uncomplicated conditions (in the animal, however, selective neuronal death may occur even with only mildly reduced CBF values, but this sequela of stroke seems an exceptional encounter in man). Classic studies with carotid artery clamping in man have provided estimates for the penumbra threshold at around 20 ml/ 100 g/min for the whole brain, but only recently have imaging studies allowed to document the existence of the penumbra in acute stroke and given estimates of local CBF thresholds. With PET, the penumbra is characterized by a reduced CBF, an increased oxygen extraction fraction, and a relatively preserved oxygen consumption (CMRO(2)). In a series of PET studies performed 5--18 h after stroke onset, we have determined the threshold for penumbra to be around 20 ml/100 g/min, and documented that the extent of neurological recovery is proportional to the volume of penumbra that eventually escaped infarction. Within this time interval, the thresholds for irreversible damage were around 8 ml/ 100 g/min for CBF and around 0.9 ml/100 g/min for CMRO(2). Recent studies with diffusion-weighted and perfusion MR have reported similar relative thresholds for CBF of about 50 and 18% for penumbra and core, respectively. Although it is likely that the threshold for irreversibility will be lower with shorter duration since clinical onset, this has not been documented thus far. Because saving the penumbra will improve clinical outcome, it should constitute the main target of acute stroke therapy. We found evidence of penumbra in about one third of the cases studied between 5 and 18 h after onset, and as late as 16 h after symptom onset in occasional patients, suggesting the therapeutic window may be protracted in at least a fraction of the cases; similar experience has recently accrued from diffusion-weighted MR and perfusion MR. In the remaining patients, there was evidence of early extensive damage or early spontaneous reperfusion, which would make them inappropriate candidates for neuroprotective therapy. Recent evidence from PET studies of relative perfusion performed within 3 h of onset suggests that early thrombolysis indeed saves the tissue with CBF below a critical threshold of 12 ml/ 100 g/min, with a correlation between the volume of such tissue escaping infarction and subsequent neurological recovery. Thus, mapping the penumbra in the individual patient with physiologic imaging should allow to formulate a pathophysiological diagnosis, and in turn to design a rational management of the stroke patient and to increase the sensitivity of drug trials by appropriate patient selection.

MRI analysis of the changes in apparent water diffusion coefficient, T(2) relaxation time, and cerebral blood flow and volume in the temporal evolution of cerebral infarction following permanent middle cerebral artery occlusion in rats

Detailed knowledge of similarities and differences between animal models and human stroke is decisive for selecting clinically effective drugs based on efficacy data obtained preclinically. Differences in the temporal evolution of stroke pathologies between animal models and man have been reported. In view of the importance of this issue for the development of neuroprotective treatments, the temporal evolution of stroke pathologies in the rat permanent middle cerebral artery occlusion (pMCAO) model has been evaluated with magnetic resonance imaging modalities under experimental conditions matching as close as possible those used in humans. Changes in the ipsilateral and contralateral cortex and striatum of cerebral blood flow (CBF) and volume (CBV), apparent diffusion coefficient (ADC), and spin-spin relaxation time (T(2)), as well as total cortical and striatal infarct volumes, calculated from CBF, ADC, and T(2) maps, were determined starting 1 h up to 216 h post-pMCAO. The temporal evolution of the MRI parameters in this rat model was similar to that observed in humans. In particular, the ADC values were decreased for more than 3 days and returned back to baseline between 4 to 8 days, to increase by day 9 only. Thus the stroke pathology in this rat model develops at a similar pace as in stroke patients arguing against a fundamental difference in the mechanisms involved. The infarct volumes however develop differently in this rat model as they invariably increase over the first 48 h, while in humans the evolution of infarct volume is slower and more heterogeneous.

Relevance of experimental ischemia in cats for stroke management: a comparative reevaluation

Repeat studies in animal models of acute focal ischemia can be compared to incidental studies in the course of ischemic stroke in order to shed light on the development of changes causing ischemic infarcts or recovery of critically perfused tissue. Positron emission tomography (PET) studies of regional cerebral blood flow, cerebral metabolic rate for oxygen, oxygen extraction fraction (OEF), cerebral metabolic rate of glucose and flumazenil (FMZ) binding in the cat middle cerebral artery occlusion (MCAO) model and in patients with acute ischemic hemispheric stroke were reviewed. After permanent MCAO, the development of "misery-perfused" penumbral tissue and its centrifugal conversion into necrosis could be demonstrated, resembling focal pathophysiological changes in patients with ischemic attacks. In the experimental model and in vascular insults in humans, a chance of recovery existed if collateral perfusion developed spontaneously within the first hours. In transient MCAO, reperfusion was only effective in preventing infarction when it was initiated as long as misery perfusion persisted; in these cases tissue was salvaged and large infarcts did not develop. In the other instances when oxygen metabolism broke down, and an increased OEF was no longer seen, reperfusion even at levels above preocclusion had no effect, and large space-occupying infarcts developed. These experimental findings are comparable to the variable outcome after thrombolytic therapy; if reperfusion is achieved within the therapeutic window of tissue viability, large infarcts are prevented and complete or partial recovery can be achieved. In the experimental model of focal ischemia and in human stroke, FMZ can be utilized as a marker of neuronal integrity. If FMZ binding in the cortex is decreased below 4 times the mean value of white matter in the acute stage, permanent infarcts were observed on late CT/MRI; this irreversible damage could not be prevented by thrombolytic therapy. These results demonstrated that PET studies in suitable ischemia models in cats can help to explain various courses and diverging outcomes of acute ischemic stroke. Comparable findings from experimental ischemia and human stroke may affect the selection of appropriate therapeutic strategies.

Diffusion MRI in ischemic stroke compared to pathologically verified infarction

BACKGROUND

Diffusion MRI abnormality correlates with pathology in animal ischemic stroke models. A combined retrospective and prospective analysis of consecutive patients over a 3-year period who had a clinical diagnosis of probable new ischemic stroke, underwent diffusion MRI, and were later studied at autopsy was performed.

METHODS

Inclusion criteria for the retrospective analysis were 1) symptom onset within 14 days of presentation, 2) diffusion MRI within 28 days of symptom onset, and 3) autopsy within 16 weeks of symptom onset. Patients with suspected further infarcts between MRI and autopsy were excluded. The locations of all areas of MRI abnormality were identified by a blinded neuroradiologist, and recent infarcts were identified by review of pathologic records and microscopic slides.

RESULTS

Eleven patients were identified who fulfilled inclusion criteria, with 25 discrete pathologic infarcts. Diffusion MRI abnormality corresponded to pathologically verified infarction in 23 cases, was present in two locations where no pathologic infarct was identified, and was absent in two locations where an infarct was present at autopsy. In two cases, despite clinical suspicion of acute ischemic stroke, no MRI abnormality or pathologic infarct was found. The sensitivity and specificity of diffusion MRI were 88.5% (95% CI, 69.9% to 97.6%) and 96.6% (95% CI, 91.5% to 99.1%). Accuracy was 95.1% (95% CI, 90.2% to 98%). Three further patients who died during the course of the retrospective analysis were studied prospectively, and are described separately.

CONCLUSIONS

These findings suggest high accuracy of diffusion MRI for detection of ischemic infarction compared with pathologic examination.

A model for multiparametric mri tissue characterization in experimental cerebral ischemia with histological validation in rat: part 1

BACKGROUND AND PURPOSE

After stroke, brain tissue undergoes time-dependent heterogeneous histopathological change. These tissue alterations have MRI characteristics that allow segmentation of ischemic from nonischemic tissue. Moreover, MRI segmentation generates different zones within the lesion that may reflect heterogeneity of tissue damage.

METHODS

A vector tissue signature model is presented that uses multiparametric MRI for segmentation and characterization of tissue. An objective (unsupervised) computer segmentation algorithm was incorporated into this model with the use of a modified version of the Iterative Self-Organizing Data Analysis Technique (ISODATA). The ability of the model to characterize ischemic tissue after permanent middle cerebral ischemia occlusion in the rat was tested. Multiparametric ISODATA measurements of the ischemic tissue were compared with quantitative histological characterization of the tissue from 4 hours to 1 week after stroke.

RESULTS

The ISODATA segmentation of tissue identified a gradation of cerebral tissue damage at all time points after stroke. The histological scoring of ischemic tissue from 4 hours to 1 week after stroke on all the animals was significantly correlated with ISODATA segmentation (r=0.78, P<0.001; n=20) when a multiparametric (T2-, T1-, diffusion-weighted imaging) data set was used, less correlated (r=0.70, P<0.01; n=20) when a T2- and T1-weighted data set was used, and not correlated (r=-0.12, P>0.47; n=20) when only a diffusion-weighted imaging data set was used.

CONCLUSIONS

Our data indicate that an integrated set of MRI parameters can distinguish and stage ischemic tissue damage in an objective manner.

Multiparametric MRI tissue characterization in clinical stroke with correlation to clinical outcome: part 2

BACKGROUND AND PURPOSE

Multiparametric MRI generates different zones within the lesion that may reflect heterogeneity of tissue damage in cerebral ischemia. This study presents the application of a novel model of tissue characterization based on an angular separation between tissues obtained with the use of an objective (unsupervised) computer segmentation algorithm implementing a modified version of the Iterative Self-Organizing Data Analysis Technique (ISODATA). We test the utility of this model to identify ischemic tissue in clinical stroke.

METHODS

MR parameters diffusion-, T2-, and T1-weighted imaging (DWI, T2WI, and T1WI, respectively) were obtained from 10 patients at 3 time points (30 studies) after stroke: acute (</=12 hours), subacute (3 to 5 days), and chronic (3 months). The National Institutes of Health Stroke Scale (NIHSS) was measured, and volumes were obtained from the ISODATA, DWI, and T2WI maps on patients at each time point.

RESULTS

The acute (</=12 hours) multiparametric ISODATA volume was significantly correlated with the acute (</=12 hours) DWI (r=0.96, P<0.05; n=10) and chronic (3 months) T2WI volume (r=0.69, P<0.05; n=10). The ISODATA-defined tissue regions exhibited MR indices consistent with ischemic and/or infarcted tissue at each time point. The acute (</=12 hours) multiparametric ISODATA volumes were significantly correlated (r=0.82, P<0.009; n=10) with the final NIHSS score. In comparison, the acute (</=12 hours) DWI volumes were less correlated (r=0.77, P<0.05; n=10) and T2WI volume (</=12h) exhibited a marginal correlation (r=0.66, P<0.05; n=10) with the final NIHSS score.

CONCLUSIONS

The integrated ISODATA approach to tissue segmentation and classification discriminated abnormal from normal tissue at each time point. The ISODATA volume was significantly correlated with the current MR standards used in the clinical setting and the 3-month clinical status of the patient.

Predicting tissue outcome in acute human cerebral ischemia using combined diffusion- and perfusion-weighted MR imaging

BACKGROUND AND PURPOSE

Tissue signatures from acute MR imaging of the brain may be able to categorize physiological status and thereby assist clinical decision making. We designed and analyzed statistical algorithms to evaluate the risk of infarction for each voxel of tissue using acute human functional MRI.

METHODS

Diffusion-weighted MR images (DWI) and perfusion-weighted MR images (PWI) from acute stroke patients scanned within 12 hours of symptom onset were retrospectively studied and used to develop thresholding and generalized linear model (GLM) algorithms predicting tissue outcome as determined by follow-up MRI. The performances of the algorithms were evaluated for each patient by using receiver operating characteristic curves.

RESULTS

At their optimal operating points, thresholding algorithms combining DWI and PWI provided 66% sensitivity and 83% specificity, and GLM algorithms combining DWI and PWI predicted with 66% sensitivity and 84% specificity voxels that proceeded to infarct. Thresholding algorithms that combined DWI and PWI provided significant improvement to algorithms that utilized DWI alone (P=0.02) but no significant improvement over algorithms utilizing PWI alone (P=0.21). GLM algorithms that combined DWI and PWI showed significant improvement over algorithms that used only DWI (P=0.02) or PWI (P=0.04). The performances of thresholding and GLM algorithms were comparable (P>0.2).

CONCLUSIONS

Algorithms that combine acute DWI and PWI can assess the risk of infarction with higher specificity and sensitivity than algorithms that use DWI or PWI individually. Methods for quantitatively assessing the risk of infarction on a voxel-by-voxel basis show promise as techniques for investigating the natural spatial evolution of ischemic damage in humans.

Evaluation of a reduced MR imaging sequencing protocol in adult patients with stroke

PURPOSE

To compare a reduced (three-sequence) magnetic resonance (MR) imaging protocol with a full (eight- to 10-sequence) MR imaging protocol in adults suspected of having stroke.

MATERIALS AND METHODS

Six neuroradiologists interpreted a consecutive sample of 265 MR images in patients suspected of having stroke. Each read reduced-protocol images in a discrete series of 40 patients (one read images in only 15) and corresponding full-protocol images 1 month later (reduced/full protocol). Five of the readers each read images in 10 additional cases, five each as full/full and reduced/reduced protocol controls. kappa values between full and reduced protocols, reader assessment of protocol adequacy, confidence level, and need for additional sequences or examinations were evaluated.

RESULTS

In the reduced/full protocol, the kappa value for detecting ischemia was 0.797; and that for detecting any clinically important abnormality, 0.635. Statistically similar kappa values were found with the full/full control design (kappa = 0.802 and 0.715, respectively). The full protocol was judged more adequate than the reduced protocol (2.0 of 5.0 points vs 1.6, P <.001) and generated greater diagnostic confidence (8.6 of 10.0 points vs 8.9, P =.01), less need for additional sequences (2.7 of 6.0 points vs 1.5, P <.001), and more requests for additional examinations (28.4% vs 36.3%).

CONCLUSION

Disagreement between interpretations of reduced- and full-protocol images might be attributable to baseline-level intraobserver inconsistency, as demonstrated in control designs. A greater number of sequences did not lead to greater consistency.

Use of spin echo T(2) BOLD in assessment of cerebral misery perfusion at 1.5 T

Inadequate blood supply relative to metabolic demand, a haemodynamic condition termed as misery perfusion, often occurs in conjunction with acute ischaemic stroke. Misery perfusion results in adaptive changes in cerebral physiology including increased cerebral blood volume (CBV) and oxygen extraction ratio (OER) to secure substrate supply for the brain. It has been suggested that the presence of misery perfusion may be an indication of reversible ischaemia, thus detection of this condition may have clinical impact in acute stroke imaging. The ability of single spin echo T(2) to detect misery perfusion in the rat brain at 1.5 T owing to its sensitivity to blood oxygenation level dependent (BOLD) contrast was studied both theoretically and experimentally. Based on the known physiology of misery perfusion, tissue morphometry and blood relaxation data, T(2) behaviour in misery perfusion was simulated. The interpretation of these computations was experimentally assessed by quantifying T(2) in a rat model for cerebral misery perfusion. CBF was quantified with the H(2) clearance method. A drop of CBF from 58+/-8 to 17+/-3 ml/100 g/min in the parieto-frontal cortex caused shortening of T(2) from 66.9+/-0.4 to 64.6+/-0.5 ms. Under these conditions, no change in diffusion MRI was detected. In contrast, the cortex with CBF of 42+/-7 ml/100 g/min showed no change in T(2). Computer simulations accurately predicted these T(2) responses. The present study shows that the acute drop of CBF by 70% causes a negative BOLD that is readily detectable by T(2) MRI at 1.5 T. Thus BOLD may serve as an index of misery perfusion thus revealing viable tissue with increased OER.

A survey of neuroimaging research in European neurological departments

In the international neurological literature, neuroimaging research plays an important role. Neuroimaging techniques are also of steadily increasing importance for clinical diagnosis and treatment monitoring. Therefore, neuroimaging research activities were surveyed by a questionnaire, which was completed by 100 neurological centres across Europe. It showed that most groups use magnetic resonance imaging (MRI), fMRI, computerized tomography (CT) and single photon emission computerized tomography (SPECT). Positron emission tomography (PET) and ultrasound are also employed by nearly half of the centres. Neuroimaging research involves co-operation amongst typically five to 10 disciplines. Cerebrovascular disease, dementia, cognitive disorders, epilepsy, movement disorders, brain tumours and multiple sclerosis are frequently being studied. Many groups rely on small budgets, have few full-time scientists and limited access to expensive resources. There is little exchange of scientists amongst laboratories. It was felt that funding and co-operation needed improvement in order to maintain a high standard in neuroimaging research.

Changes in apparent diffusion coefficients of metabolites in rat brain after middle cerebral artery occlusion measured by proton magnetic resonance spectroscopy

Diffusion-weighted proton MR spectroscopy and imaging have been applied to a rat brain model of unilateral middle cerebral artery occlusion between 1 and 4 hr post occlusion. Similar apparent diffusion coefficients (ADC) of most metabolites were observed within each hemisphere. In the ischemic ipsilateral hemisphere, the ADCs were (0.083--0.116). 10(-3) mm(2)/sec for lactate (Lac), alanine (Ala), gamma-amino butyric acid (GABA), N-acetyl aspartate (NAA), glutamine (Gln), glutamate (Glu), total creatine (tCr), choline-containing compounds (Cho), and myo-inositol (Ins), in the contralateral hemisphere (0.138--0.158). 10(-3) mm(2)/sec for NAA, Glu, tCr, Cho, and Ins. Higher ADCs was determined for taurine (Tau) in the ipsilateral (0.144. 10(-3) mm(2)/sec) and contralateral (0.198. 10(-3) mm(2)/sec) hemisphere. In the ischemic hemisphere, a relative ADC decrease to 65--75% was observed for NAA, Glu, tCr, Cho, Ins and Tau, which was similar to the decrease of the water ADC (to 67%). The results suggest a common cause of the observed ADC changes and provide a broader experimental basis to evaluate theories of water and metabolite diffusion. Magn Reson Med 45:383-389, 2001.

Comparison of metabolite levels and water diffusion between cortical and subcortical strokes as monitored by MRI and MRS

Labelle M, Khiat A, Durocher A, et al. Comparison of metabolite levels and water diffusion between cortical and subcortical strokes as monitored by MRI and MRS. Invest Radiol 2001;36:155-163.

RATIONALE AND OBJECTIVES

Proton magnetic resonance spectroscopy (MRS) and functional imaging techniques are increasingly recognized as useful tools for the characterization of strokes. The aim of this study was to compare cortical and subcortical (lacunar) strokes by MRS and diffusion-weighted imaging (DWI) experiments as a function of time.

METHODS

Single-voxel MRS, DWI, and perfusion-weighted imaging data were recorded on patients with cortical (n = 7) or subcortical (n = 7) strokes in the acute, subacute, and chronic periods. Magnetic resonance spectra were acquired in three regions: hyperintense DWI area, adjacent area with normal DWI intensity, and contralateral area. Neurological deficits were estimated by the National Institutes of Health Stroke Scale.

RESULTS

Decreases in N-acetylaspartate, choline-containing compounds, and creatine/phosphocreatine signal intensity as well as the presence of lactate were observed at all times in the hyperintense DWI area of all lesions. Small decreases were measured in the subacute and chronic phases for the adjacent area of cortical strokes but not for the adjacent area of subcortical strokes. The existence of a surrounding affected area in subcortical strokes is deduced from a combination of MRS and DWI results, possibly corresponding to the ischemic penumbra. Differences were found between the two types of lesion, especially an increased time variability of apparent diffusion coefficients in subcortical strokes.

CONCLUSIONS

Magnetic resonance spectroscopy provides evidence for the existence of affected tissue outside the hyperintense DWI regions in subcortical strokes. Cortical and subcortical strokes display different DWI and MRS characteristics.

Understanding and managing ischemic stroke

Transient or permanent focal brain injury following acute thromboembolic occlusion develops from a complex cascade of pathophysiological events. The processes of excitotoxicity, peri-infarct depolarisation, inflammation, and apoptosis within the ischemic penumbra are proposed. While the translation of therapeutic agents from the animal models to human clinical trials have been disappointing, there remains an atmosphere of optimism as a result of the development of new diagnostic and therapeutic approaches, which include physiological, as opposed to pharmacological, intervention. This article provides an insight into the understanding of cerebral ischemia, together with current and future treatment strategies.Key words: cerebral ischemia, stroke, pathophysiology.

Thrombolysis in acute ischaemic stroke

In conclusion, thrombolysis with rtPA given within six hours of the onset of stroke in carefully selected patients is a safe therapy. However, efficacy has only been demonstrated within three hours after stroke onset. At present, only 6%-12% of all stroke patients are likely to be eligible for thrombolysis. Improved methods for investigating acute stroke, particularly magnetic resonance techniques, may improve the appropriate targeting of this treatment to those patients most likely to benefit. What is certain is that any increasing use of thrombolysis will have major effects on stroke services. The emphasis will have to be on early assessment and referral, if only to reach an imaging facility for a treatment decision to be made.

Imaging in cerebrovascular disease

Various functional imaging modalities can be applied in acute ischaemic stroke to identify functionally impaired, but morphologically preserved tissue (i.e. the penumbra), and to distinguish it from irreversibly damaged tissue. Flow thresholds for irreversible tissue destruction resulting in functional impairment, as determined by positron emission tomography, perfusion and diffusion-weighted magnetic resonance imaging, single-photon computed tomography and xenon computed tomography, were comparable and ranged between 5 and 12 ml/100 g per min for the lower and 14 and 22 ml/100 g per min for the upper limit of penumbra. These imaging modalities help to select patients for thrombolytic therapy and provide evidence for the effect of this treatment on critically perfused tissue. They can also serve as surrogate markers in the evaluation of therapeutic efficacy. Further progress in interventional neuroradiology has been achieved with intra-arterial thrombolysis, which has become a treatment option beyond the 3-h therapeutic window in acute ischaemic stroke. Angioplasty and stenting of stenosis of arteries that supply the brain with blood have reached a point in their development at which a randomized trial to compare these treatments with vascular surgery is warranted.

Perfusion-based segmentation of the human brain using similarity mapping

In this work, a method for segmenting human brain MR scans on the basis of perfusion is described. This technique uses a measure of similarity between the time-intensity curves obtained with dynamic susceptibility contrast-enhanced MRI and a modeled curve of reference to isolate a tissue of interest, such as white or gray matter. The aim of this study was to validate the method by performing segmentation of white and gray matter in six controls. The relative regional blood volume gray-to-white matter ratio was used as a criterion to assess the quality of segmentation. On average, this ratio was 2.1 +/- 0.2, which is in good agreement with the literature, thus suggesting reliable segmentation. In the case of abnormal perfusion, time-intensity curves are different in shape than that of normal tissue. Therefore, this approach might allow the segmentation of pathological regions, and combined with an indicator-dilution analysis might offer new possibilities for characterizing a brain pathology. Magn Reson Med 45:261-268, 2001.

Functional MRI and its applications to the clinical neurosciences

Functional magnetic resonance imaging (fMRI) is an emerging methodology for studying regional brain function in vivo at relatively high spatial and temporal resolution. Because MRI methods are comparatively inexpensive and entirely noninvasive, fMRI has rapidly become one of the most popular approaches for brain mapping in cognitive and systems neuroscience. There has also been great interest in using fMRI to assist in clinical diagnosis and management, with promising demonstrations of feasibility in a number of applications. Both resting and task-specific regional brain activity can be measured, primarily utilizing alterations in regional cerebral blood flow (CBF) as a surrogate marker for neural function. This article reviews the biophysical and physiological bases of fMRI and its applications to the clinical neurosciences, with particular attention to potential challenges of fMRI under pathophysiological conditions. Carefully controlled prospective evaluation of clinical fMRI in its various potential applications will be required for fMRI to be validated as a clinically useful tool. Because the technology for fMRI is widely available, its impact could be substantial.

Reproducibility of measurements of cerebral infarct volume on CT scans

BACKGROUND AND PURPOSE

Infarct volume is increasingly used as an outcome measure in clinical trials of therapies for acute ischemic stroke. We tested which of 5 different methods to measure infarct size or volume on CT scans has the highest reproducibility.

METHODS

Infarct volume and total intracranial volume were measured with Leica Q500 MCP image analysis software, or with a caliper, on 38 CT scans of patients who participated in the Tirilazad Efficacy Stroke Study II (TESS II). The scans were performed 8 days (+/-2 days) after the onset of symptoms. The 5 methods tested were based on (1) semiautomated pixel thresholding, (2) manual tracing of the perimeter, (3) a stereological counting grid, (4) measurement of the 3 largest diameters, and (5) the single largest diameter. The measurements were performed independently by 2 observers; the first observer performed all measurements twice.

RESULTS

The single largest diameter did not correlate well with infarct volume. Of the other methods, manual tracing of the perimeter of the infarct had the lowest intraobserver and interobserver variability: coefficients of variation were 8.6% and 14.1%, respectively. For total intracranial volume, manual tracing also provided the highest reproducibility: intraobserver and interobserver coefficients of variation were 3.3% and 4.9%, respectively.

CONCLUSIONS

Manual tracing of the perimeter is the most reproducible method for measuring the volumes of the infarct and the total intracranial space in multicenter trials of therapies for acute ischemic stroke.

Magnetic resonance prediction of outcome after thrombolytic treatment

Treatment of clinical stroke with recombinant tissue plasminogen activator (rt-PA) carries the risk of hemorrhagic complications. Hence, predictors of therapeutic outcome with respect to (a) reperfusion and (b) tissue recovery would be very useful to identify potentially salvageable brain tissue. Magnetic resonance (MR) parameters, especially the apparent diffusion coefficient of water (ADC), perfusion-weighted imaging (PWI) and T(2) relaxometry are thought to provide this information. We evaluated the prognostic implications of ADC, PWI and T(2) relaxometry immediately before initiation of thrombolytic treatment in a model of clot embolism in rats. Animals (n = 14) were treated with intraarterial rt-PA (10 mg/kg) at 90 min after embolism. MR imaging was repeatedly performed at 4.7 T before and up to 5.5 h after embolism. ADC was calculated from diffusion-weighted images (b-values: 30, 765, 1500 s/mm(2)), arterial spin tagging was used for PWI, and quantitative T(2) relaxometry was performed with a Carr-Purcell-Meiboom-Gill (CPMG) sequence. A reperfusion index was calculated to assess the quality of thrombolytic recanalization. The decline of ADC at the end of the experiment to below 80% of control was defined as unfavorable outcome. The probability of tissue injury at the end of the experiments increased with the severity of ADC changes before the initiation of treatment (probability of unfavorable outcome: 21%, 44%, 65% for ADC values of 80-90%, 70-80% and <70% of control, respectively). Pretreatment PWI or T(2) relaxometry also correlated with outcome but-alone or in combination with pretreatment ADC maps-did not improve injury prediction over that obtained by ADC alone. Outcome was influenced positively by successful reperfusion the quality of which, however, could not be predicted by pre-treatment MR characteristics. The data demonstrate that ADC mapping performed before the initiation of thrombolytic treatment provides reliable risk assessment of impeding brain injury but due to uncertainties of postischemic reperfusion does not allow precise outcome prediction in individual experiments.

Penumbral probability thresholds of cortical flumazenil binding and blood flow predicting tissue outcome in patients with cerebral ischaemia

Active treatment of acute ischaemic stroke can only be successful as long as tissue in the area of ischaemic compromise is still viable. Therefore, the identification of the area of irreversible damage, and its distinction from the penumbral zone, may improve the estimation of the potential efficacy of various therapeutic strategies. Ten patients (seven male, three female, aged 52-75 years) with acute ischaemic stroke, in whom MRI delineated an infarct involving the cortex 3 weeks after the attack, were studied by [(11)C]flumazenil (FMZ) PET to assess their neuronal integrity, and regional cerebral blood flow (CBF) was measured by H(2)(15)O PET 2-12 h (median interval 6 h) after onset of symptoms. Cortical volumes of interest (3 mm radius) were placed on co-registered CBF, FMZ and on late MRI scans. Using initial CBF and FMZ binding data from volumes of interest finally located within or outside the cortical infarct, cumulative probability curves were computed to predict eventual infarction or non-infarction. Positive (at least 95% chance of infarct) and negative (at least 95% chance of non-infarct) prediction limits for CBF (4.8 and 14.1 ml/100 g/min, respectively) and for FMZ binding (3.4 and 5.5 times the mean of normal white matter, respectively) were determined to define the penumbral range. Using the lower FMZ binding threshold of 3.4 for irreversible tissue damage and the upper CBF value of 14.1 ml/ 100 g/min for the threshold of critical perfusion at or above which tissue will likely be preserved, various cortical subcompartments were identified: of the final cortical infarct (median size 25.7 cm(3)) a major portion comprising, on average, 55.1% showed FMZ binding critically decreased, thus predicting necrosis. In 20.5% of the final infarct, on average, CBF was in the penumbral range (<14.1 ml/100 g/min) and FMZ binding was above the critical threshold of irreversible damage. Only 12.9% of the final infarct exhibited neuronal integrity and CBF values above the penumbral range. Therefore, most of the final infarct is irreversibly damaged already at the time of the initial evaluation, when studied several hours after stroke onset. A much smaller portion is still viable but suffers from insufficient blood supply: this tissue may be salvaged by effective reperfusion. Only an even smaller compartment is viable and sufficiently perfused, but eventually becomes necrotic, mainly owing to delayed mechanisms, and may benefit from neuroprotective or other measures targeted at secondary damage. Therefore, early reperfusion is crucial in acute ischaemic stroke.

Use of diffusion and perfusion magnetic resonance imaging as a tool in acute stroke clinical trials

In light of the slow progress in developing effective therapies for ischemic stroke, magnetic resonance imaging techniques have emerged as new tools in stroke clinical trials. Rapid imaging with magnetic resonance imaging, diffusion weighted imaging, perfusion imaging and angiography are being incorporated into phase II and phase III stroke trials to optimize patient selection based on positive imaging diagnosis of the ischemic pathophysiology specifically related to a drug's mechanism of action and as a direct biomarker of the effect of a treatment's effect on the brain.

Perfusion mapping using computed tomography allows accurate prediction of cerebral infarction in experimental brain ischemia

BACKGROUND AND PURPOSE

We have developed a dynamic CT method to measure absolute cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). In this study we evaluated the ability of CT-derived functional maps to detect infarction in a rabbit model of focal cerebral ischemia.

METHODS

Sequential dynamic CT studies were performed at 2 different slices in 5 control rabbits and another 8 after induction of focal cerebral ischemia. The size of critically ischemic tissue was correlated to size of infarction measured by postmortem 2,3,5-triphenyltetrazolium chloride staining. In the control rabbits, short-term variability of the parameters was assessed by ANOVA analysis.

RESULTS

In 7 of 8 animals of the ischemia group, cerebral infarction was visible on 2,3, 5-triphenyltetrazolium chloride staining, constituting 16.7+/-10.6% of the ipsilateral hemisphere. Good agreement of CBF functional maps with tissue specimens was found with respect to size and location of infarction. Best prediction of infarction was found for thresholds of CBF <10 mL/100 g per minute (mean size, 17.5+/-13.4%; r=0.95) and MTT >6 seconds (mean size, 15.6+/-13.5%; r=0.85), with regression slopes close to unity. CBV maps were less predictive of occurrence of infarction, especially in cases of small infarction. The short-term variability of CBF, CBV, and MTT in the control group was 10.9%, 15.2%, and 19.9%, respectively.

CONCLUSIONS

Functional CT measurements of absolute CBF and MTT early after onset of ischemia allow prediction of the size and location of cerebral infarction with good accuracy.

Recent advances in the diagnosis and treatment of stroke

Stroke is defined as an abnormality in brain function resulting from disruption of cerebral circulation. It is the third leading cause of death in the United States and the primary cause of long-term disability. The economic burden of stroke will only increase as the population ages, making prevention and treatment of stroke one of the most important public health issues of the upcoming millennium. New therapies for the treatment of acute stroke, especially thrombolysis, have turned what was once considered an inevitable deficit into a potentially treatable illness. It is increasingly important for all physicians to be able to identify symptoms of cerebral ischemia. Neurons have a very limited tolerance for ischemia, making the rapid evaluation and diagnosis of stroke critical. This is particularly relevant for the ophthalmologist, who may be the first physician to see individuals presenting with visual deficits. Trials are underway to look specifically at central retinal artery and basilar artery ischemia and their response to thrombolytic therapy. This review will focus on description of recent advances in treatment and diagnosis of stroke, including thrombolytic trials and the expanding role of neuroimaging.