Diffusion-weighted magnetic resonance imaging of acute focal cerebral ischemia: comparison of signal intensity with changes in brain water and Na+,K(+)-ATPase activity

Diffusion-weighted imaging as a problem-solving tool in the evaluation of patients with acute strokelike syndromes

This article addresses syndromes that clinically and/or radiologically resemble acute stroke. These syndromes generally fall into four categories. (1) Patients with acute neurological deficits with nonischemic lesions and no acute abnormality on diffusion-weighted images. These patients may have peripheral vertigo, migraines, seizures, dementia, functional disorders, amyloid angiopathy, or metabolic disorders. When these patients present, we can confidently predict that they are not undergoing infarction. (2) Patients with ischemic lesions with reversible clinical deficits. Nearly 50% of patients with transient ischemic attacks have lesions with restricted diffusion. Patients with transient global amnesia may have punctate lesions with restricted diffusion in the medial hippocampus, parahippocampal gyms, and corpus callosum. (3) Vasogenic edema syndromes that may mimic acute infarction clinically and on conventional imaging. These include eclampsia/hypertensive encephalopathy, other posterior leukoencephalopathies, human immunodeficiency virus encephalopathy, hyperperfusion syndrome following carotid endarterectomy, venous sinus thrombosis, acute demyelination, and neoplasm. These syndromes demonstrate elevated diffusion rather than the restricted diffusion associated with acute ischemic stroke. (4) Entities in which restricted diffusion may resemble acute infarction. These include pyogenic infections, herpes virus encephalitis, Creutzfeldt-Jakob disease, diffuse axonal injury, tumors with dense cell packing, and rare acute demyelinative lesions.

Carotenoid oxidative degradation products inhibit Na+-K+-ATPase

This study investigates the biological significance of carotenoid oxidation products using inhibition of Na+-K+-ATPase activity as an index. Beta-carotene was completely oxidized by hypochlorous acid and the oxidation products were analyzed by capillary gas-liquid chromatography and high performance liquid chromatography. The Na+-K+-ATPase activity was assayed in the presence of these oxidized carotenoids and was rapidly and potently inhibited. This was demonstrated for a mixture of beta-carotene oxidative breakdown products, beta-Apo-10'-carotenal and retinal. Most of the beta-carotene oxidation products were identified as aldehydic. The concentration of the oxidized carotenoid mixture that inhibited Na+-K+-ATPase activity by 50% (IC50) was equivalent to 10 microM non-degraded beta-carotene, whereas the IC50 for 4-hydroxy-2-nonenal, a major lipid peroxidation product, was 120 microM. Carotenoid oxidation products are more potent inhibitors of Na+-K+-ATPase than 4-hydroxy-2-nonenal. Enzyme activity was only partially restored with hydroxylamine and/or beta-mercaptoethanol. Thus, in vitro binding of carotenoid oxidation products results in strong enzyme inhibition. These data indicate the potential toxicity of oxidative carotenoid metabolites and their activity on key enzyme regulators and signal modulators.

Effects of deferoxamine, a chelator of free iron, on NA(+), K(+)-ATPase activity of cortical brain cell membrane during early reperfusion after hypoxia-ischemia in newborn lambs

Free iron chelation after hypoxia-ischemia can reduce free radical-induced damage to brain cell membranes and preserve electrical brain activity. We investigated whether chelation of free iron with deferoxamine (DFO) preserved cortical cell membrane activity of Na(+),K(+)-ATPase and electrocortical brain activity (ECBA) of newborn lambs during early reperfusion after severe hypoxia-ischemia. Hypoxia was induced in 16 lambs by decreasing the fraction of inspired oxygen to 0.07 for 30 min, followed by a 5-min period of hypotension (mean arterial blood pressure <35 mm Hg). ECBA (in microvolts) was measured using a cerebral function monitor. Immediately after hypoxia and additional ischemia, eight lambs received DFO (2.5 mg/kg, i.v.), and seven lambs received a placebo (PLAC). Two lambs underwent sham operation. One hundred eighty minutes after completion of hypoxia and ischemia, the brains were obtained and frozen. Na(+),K(+)-ATPase activity was measured in the P(2) fraction of cortical tissue. Na(+),K(+)-ATPase activity was 35.1 +/- 7.4, 42.0 +/- 7.6, and 40.7 +/- 1.4 micromol inorganic phosphate/mg protein per hour in PLAC-treated, DFO-treated, and sham-operated lambs, respectively (p < 0.05: DFO versus PLAC). ECBA was 11.2 +/- 6.1, 14.8 +/- 4.8, and 17.5+/-.0.5 microV in PLAC-treated, DFO-treated, and sham-operated lambs, respectively (p = 0.06: DFO versus PLAC). Na(+),K(+)-ATPase activity correlated with ECBA at 180 min of reperfusion (r = 0.85, p < 0.001). We conclude that Na(+),K(+)-ATPase activity of cortical brain tissue was higher in DFO-treated lambs compared with PLAC-treated animals during the early reperfusion phase after severe hypoxia-ischemia, suggesting a reduction of free radical formation by DFO. Furthermore, a positive relationship was found between Na(+),K(+)-ATPase activity and ECBA.

Diffusion-weighted imaging in neonatal cerebral infarction: clinical utility and follow-up

We describe the clinical utility of echo-planar diffusion-weighted imaging in neonatal cerebral infarction. Eight full-term neonates aged 1 to 8 days referred for neonatal seizures were studied. Patients were followed for a mean of 17 months with detailed neurologic examinations at regular intervals. Head computed tomography (CT) and conventional magnetic resonance (MRI) and diffusion-weighted images were obtained. Percent lesion contrast was evaluated for 19 lesions on T2-weighted and diffusion-weighted images. Follow-up conventional MRIs were obtained in seven patients. The findings on diffusion-weighted imaging were correlated with CT and conventional MRI findings as well as with short-term neurodevelopmental outcome. Four patients had focal cerebral infarctions. Four patients had diffuse injury consistent with hypoxic-ischemic encephalopathy. Percent lesion contrast of all 19 lesions was significantly higher on diffusion-weighted images when compared with T2-weighted images. In five patients, there were lesions visualized only with diffusion-weighted imaging. In all patients, there was increased lesion conspicuity and better definition of lesion extent on the diffusion-weighted images compared with the CT and T2-weighted MR images. In seven of eight patients follow-up imaging confirmed prior infarctions. Short-term neurologic outcome correlated with the extent of injury seen on the initial diffusion-weighted imaging scans for all patients. Diffusion-weighted imaging is useful in the evaluation of acute ischemic brain injury and seizure etiology in neonates. In the acute setting, diffusion-weighted imaging provides information not available on CT and conventional MRI. This information correlates with short-term clinical outcome.

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

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

Effect of carnosine on rats under experimental brain ischemia

The effect of dietary carnosine on the behavioral and biochemical characteristics of rats under experimental ischemia was studied. Carnosine was shown to improve the animals orientation and learning in "Open Field" and "T-Maze" tests, and this effect was accompanied with an increase in glutamate binding to N-methyl-D-aspartate (NMDA) receptors in brain synaptosomes. Long-term brain ischemia induced by both sides' occlusion of common carotid arteries resulted in 55% mortality of experimental rats, and those who survived were characterized by partial suppression of orientation in T-maze. In the group of rats treated with carnosine, mortality after ischemic attack was decreased (from 55% to 17%) and most of the learning parameters were kept at the pre-ischemic level. Monoamine oxidase B (MAO B) activity in brain of the carnosine treated rats was not changed by ischemia significantly (compared to that of ischemic untreated rats) but NMDA binding to brain synaptosomal membranes being increased by ischemic attack was significantly suppressed and reached the level characteristic of normal brain. The suggestion was made that carnosine possesses a dual effect on NMDA receptors resulting in increase in their amount after long-term treatment but decrease the capacity to bind NMDA after ischemic attack.

Diffusion weighted imaging and magnetic resonance spectroscopy in a low flow ischaemia model due to endothelin induced vasospasm

The aim of this MR study was to determine if vasospasm induced by application of endothelin-1 (ET-1) in the rat brain would model the abnormalities attributed to vasospasm described in patients with subarachnoid haemorrhage (SAH) with reversible neurological deficits. Following application of ET-1 in concentrations of 10(-4) M or 10(-6) M to the middle cerebral artery, there was an immediate drop in pH, an increase in the inorganic phosphate (P(i)) to phosphocreatine (PCr) ratio and elevated lactate. There was gradual recovery to control in the 10(-6) M group, but in the 10(-4) M group there was a loss of approximately 10% in the absolute signal intensities of PCr and adenosine triphosphate (ATP). In a second similarly treated group of animals, the area of the hemisphere with a low apparent diffusion coefficient (ADC) was 27 +/- 6% at 30 min and remained at about 20-21% at 90 min and beyond. Together these data suggest that the regions with persistently low ADC were metabolically compromised, with incomplete recovery of PCr and ATP, and represent irreversibly damaged tissue. This raises the possibility that MR spectroscopy and imaging could be a sensitive indicator of tissue viability. This is a potentially useful model of low flow as seen in clinical vasospasm following SAH.

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

BACKGROUND AND PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Magnetic resonance imaging indexes of therapeutic efficacy of recombinant tissue plasminogen activator treatment of rat at 1 and 4 hours after embolic stroke

With use of magnetic resonance imaging (MRI), the effects of early and delayed treatment of embolic stroke in rat with recombinant tissue plasminogen activator (rt-PA) were investigated. Rats with embolic stroke were treated with rt-PA at 1 (n = 9) or 4 (n = 7) hours after stroke onset or were untreated (n = 15). Diffusion-weighted imaging, perfusion-weighted imaging, and T2-weighted imaging were performed before and after embolization from 1 hour to 7 days. No significant differences were detected in the relative areas with low cerebral blood flow (CBF), apparent diffusion coefficient of water (ADCw), and T2 between the 4-hour treated group and the untreated group. Significant decreases in the average relative areas with low CBF were detected in the 1-hour treated group from 4 to 48 hours after embolization as compared with the untreated group. The increase in T2 in the 1-hour treated group was significantly lower than in the untreated and 4-hour treated groups. A significant increase in ADCw was detected in the 1-hour treated group at 3 and 24 hours after embolization as compared with the untreated and 4-hour treated groups. Secondary embolization was detected by both MRI and laser scanning confocal microscopy. The data suggest that MRI can detect the efficacy of rt-PA treatment and secondary ischemic damage.

Diffusion-weighted imaging identifies a subset of lacunar infarction associated with embolic source

BACKGROUND AND PURPOSE

Small infarcts in the territory of penetrator arteries were described as causing a number of distinct clinical syndromes. The vascular pathophysiology underlying such infarcts is difficult to ascertain without careful pathological study. However, the occurrence of multiple, small infarcts, linked closely in time but dispersed widely in the brain, raises the possibility of an embolic mechanism. The current study determines the frequency and clinical characteristics of patients with well-defined lacunar syndromes and the diffusion-weighted imaging (DWI) evidence of multiple acute lesions.

METHODS

Sixty-two consecutive patients who presented to the emergency room with a clinically well-defined lacunar syndrome were studied by DWI within the first 3 days of admission.

RESULTS

DWI showed multiple regions of increased signal intensity in 10 patients (16%). A hemispheric or brain stem lesion in a penetrator territory that accounted for the clinical syndrome ("index lesion") was found in all. DWI-hyperintense lesions other than the index lesion ("subsidiary infarctions") were punctate and lay within leptomeningeal artery territories in the majority. As opposed to patients with a single lacunar infarction, patients with a subsidiary infarction more frequently (P<0.05) harbored an identifiable cause of stroke.

CONCLUSIONS

Almost 1 of every 6 patients presenting with a classic lacunar syndrome has multiple infarctions demonstrated on DWI. This DWI finding usually indicates an identifiable cause of stroke and therefore may influence clinical decisions regarding the extent of etiologic investigations and treatment for secondary prevention.

The nature of penumbral depolarizations following focal cerebral ischemia in the rat

It has been previously suggested that the transient ischemic depolarizations (IDs), thought involved in the gradual expansion of ischemic injury in the first hours following middle cerebral artery occlusion (MCAo), are akin to spreading depression (SD). However, previous studies indicate that the characteristics of these events are heterogeneous (unlike those of SDs). We therefore sought to determine whether different types of IDs exist or not. Using four cortical microelectrodes, we compared the spatial and the temporal characteristics of IDs that occur following intraluminal MCAo in halothane-anesthetized rats to those of electrically induced SDs. An average 4.6+/-3.2 series of events, sequentially affecting the four electrodes, were recorded in 5 h following the induction of ischemia. The distribution of ID duration disclosed two types: short IDs (<7 min, 53% of all events) and long IDs (>7 min; 9% of all events). Most long IDs occurred within the first 30 min and as the initial electrophysiological event. Later on and often restricted to a single or reduced number of recording sites, intermittent IDs were of reduced amplitude or even replaced entirely by suppressed electrocorticographic activity (38% of all events). While the amplitude, duration and spreading characteristics were similar between short IDs and SDs provoked in the cortex of non-ischemic rats, those of long IDs were markedly different. Our results indicate that two types of IDs exist and confirm that most IDs (short ones) are similar in nature to SDs. Long IDs may represent a penumbral anoxic depolarization (AD), reversed by an improvement of perfusion, in the early stages of ischemia. Furthermore, we show that intermittent blockade of depolarization waves occurs and that its incidence increases with time. This blockade may reflect adaptive mechanisms which take place to prevent further depolarizations, the nature of which remains to be determined. The present description of electrophysiological abnormalities might have implications for anti-depolarization therapy in focal cerebral ischemia and to interpret the results of non-invasive techniques which enable the imaging of depolarized areas following stroke.

Reversal of acute apparent diffusion coefficient abnormalities and delayed neuronal death following transient focal cerebral ischemia in rats

Twenty-two rats were subjected to 8, 15, 30, or 60 minutes of temporary middle cerebral artery occlusion (n = 5 per group) or sham occlusion (n = 2) in the magnetic resonance imaging unit. Diffusion-, perfusion-, and T2-weighted imaging were acquired before and during occlusion, and after reperfusion. A coregistration method was used to correlate the acute changes of the average apparent diffusion coefficient (ADCav) with the histology after 72 hours at the same topographic sites. The initially reduced ADCav values recovered completely in both the lateral caudoputamen and upper frontoparietal cortex in the 8-, 15-, and 30-minute groups, partially in the cortex, and not at all in the caudoputamen in the 60-minute group. The histology showed that the caudoputamen was either normal or had mild neuronal injury in the 8-minute group and invariably had some degree of neuronal death in the 15-, 30-, and 60-minute groups, whereas the cortex was either normal or had varying degrees of neuronal injury in all groups. No histological abnormalities were seen in the sham-operated rats. Our data suggest that acute ADCav reversal does not always predict tissue recovery from ischemic injury and that temporary focal ischemia for even 8-minute duration can cause delayed neuronal death that is more severe in the caudoputamen where the initial ADCav decline was greater than in the cortex.

Normal diffusion-weighted MRI during stroke-like deficits

BACKGROUND

Diffusion-weighted MRI (DWI) represents a major advance in the early diagnosis of acute ischemic stroke. When abnormal in patients with stroke-like deficit, DWI usually establishes the presence and location of ischemic brain injury. However, this is not always the case.

OBJECTIVE

To investigate patients with stroke-like deficits occurring without DWI abnormalities in brain regions clinically suspected to be responsible.

METHODS

We identified 27 of 782 consecutive patients scanned when stroke-like neurologic deficits were still present and who had normal DWI in the brain region(s) clinically implicated. Based on all the clinical and radiologic data, we attempted to arrive at a pathophysiologic diagnosis in each.

RESULTS

Best final diagnosis was a stroke mimic in 37% and a cerebral ischemic event in 63%. Stroke mimics (10 patients) included migraine, seizures, functional disorder, transient global amnesia, and brain tumor. The remaining patients were considered to have had cerebral ischemic events: lacunar syndrome (7 patients; 3 with infarcts demonstrated subsequently) and hemispheric cortical syndrome (10 patients; 5 with TIA, 2 with prolonged reversible deficits, 3 with infarction on follow-up imaging). In each of the latter three patients, the regions destined to infarct showed decreased perfusion on the initial hemodynamically weighted MRI (HWI).

CONCLUSIONS

Normal DWI in patients with stroke-like deficits should stimulate a search for nonischemic cause of symptoms. However, more than one-half of such patients have an ischemic cause as the best clinical diagnosis. Small brainstem lacunar infarctions may escape detection. Concomitant HWI can identify some patients with brain ischemia that is symptomatic but not yet to the stage of causing DWI abnormality.

Recent advances in imaging of cerebrovascular disease

The recent advances made in CT and MR imaging have led to increased accuracy in making a number of diagnoses in the emergency room setting. Increasingly, radiologists are asked to perform these studies and accurately interpret the findings, which often have a dramatic impact on triaging and treatment of the patient. Future trials need to address further the relative merits of each of the techniques outlined previously in specific settings. In addition, given the increasing number of means of obtaining diagnostic information, cost effectiveness studies are needed to better formulate an appropriate algorithm for each diagnosis.

Focal cerebral ischaemia induces a decrease in activity and a shift in ouabain affinity of Na+, K+-ATPase isoforms without modifications in mRNA and protein expression

In a mouse model of focal cerebral ischaemia, we observed after 1 h of ischaemia, that the total Na+, K+-ATPase activity was decreased by 39.4%, and then did not vary significantly up to 6 h post-occlusion. In the sham group, the dose-response curves for ouabain disclosed three inhibitory sites of low (LA), high (HA) and very high (VHA) affinity. In ischaemic animals, we detected the presence of only two inhibitory sites for ouabain. After 1 h of permanent occlusion, the first site exhibited a low affinity while the second site presented an affinity intermediate between those of HA and VHA sites, which evolved after 3 h and 6 h of occlusion towards that of the VHA site. The presence of only two ouabain sites for Na+, K+-ATPase after ischaemia could result from a change in ouabain affinity of both HA and VHA sites (alpha2 and alpha3 isoforms, respectively) to form a unique component. Irrespective of the duration of ischaemia, the smaller activity of this second site accounted entirely for the loss in total activity. Surprisingly, no modifications in protein and mRNA expression of any alpha or beta isoforms of the enzyme were observed, thus suggesting that ischaemia could induce intrinsic modifications of the Na+, K+-ATPase.

Magnetic resonance imaging during cerebral hypoxia-ischemia: T2 increases in 2-week-old but not 4-week-old rats

We investigated whether the changes detectable with magnetic resonance imaging techniques during and after an episode of cerebral hypoxia-ischemia differ in immature and older brain. Diffusion weighted (DW) and T2-weighted (T2W) images were repeatedly acquired before, during, and after an episode of cerebral hypoxia-ischemia (unilateral carotid artery occlusion plus hypoxia) in 2- and 4-wk-old rats lightly anesthetized with isoflurane. Areas of increased brightness were detected in DW images from both 2- and 4-wk-old rats by 10-20 min after the start of hypoxia. These hyperintense areas increased during hypoxia, comprising 60.8+/-4.9% and 30.5+/-2.7% of the brain image at the level of the thalamus in 2-wk-old and 4-wk-old animals, respectively (p < 0.003). Hyperintense areas (e.g. 27.0+/-8.3%) also appeared in T2W images during hypoxia-ischemia in 2-wk-old animals, but these did not occur in 4-wk-old animals (p < 0.02). This observation was reflected in T2, which increased during hypoxia-ischemia in the 2-wk-old but not the 4-wk-old group. By 60 min after the termination of hypoxia-ischemia in either age group, areas of hyperintensity resolved and then reappeared 24 h later on both DW and T2W images. Thus, irrespective of age, magnetic resonance imaging changes during transient hypoxia-ischemia generally recover with a delayed or secondary increase in DW and T2W hyperintensity hours later. Immature brain differs from older brain primarily with respect to some combination of hypoxic/ischemic cellular or biochemical changes, that are detectable as increases in T2 within 2-wk-old but not 4-wk-old animals.

Na/K-ATPase under oxidative stress: molecular mechanisms of injury

1. The authors compare oxidative injury to brain and kidney Na/K-ATPase using in vitro and in vivo approaches. The substrate dependence of dog kidney Na/K-ATPase was examined both before and after partial hydrogen peroxide modification. A computer simulation model was used for calculating kinetic parameters. 2. The substrate dependence curve for the unmodified endogenous enzyme displayed a typical curve with an intermediate plateau, adequately described by the sum of hyperbolic and sigmoidal components. 3. The modified enzyme demonstrated a dependent curve that closely approximates normal hyperbola. The estimated ATP K(m) value for the endogenous enzyme was about 85 microM; the Kh was equal to 800 microM. The maximal number of protomers interacting was 8. Following oxidative modification, the enzyme substrate dependence curve did not show a significant change in the maximal protomer rate Vm, while the K(m) was increased slightly and interprotomer interaction was abolished. 4. Na/K-ATPase from an ischemic gerbil brain showed a 22% decrease in specific activity. The maximal rate of ATP hydrolysis by an enzyme protomer changed slightly. but the sigmoidal component, characterizing the enzyme's ability to form oligomers was abolished completely. The K(m) value was almost unchanged, but the Hill coefficient fell to 1. These data show that Na/K-ATPase molecules isolated from the ischemic brain have lost the ability to interact with one another. 5. We suggest that the most important consequence of oxidative modification is Na/K-ATPase oligomeric structure formation and subsequent hydrolysis rate suppression.

Carnosine: an endogenous neuroprotector in the ischemic brain

1. The biological effects of carnosine, a natural hydrophilic neuropeptide, on the reactive oxygen species (ROS) pathological generation are reviewed. 2. We describe direct antioxidant action observed in the in vitro experiments. 3. Carnosine was found to effect metabolism indirectly. These effects are reflected in ROS turnover regulation and lipid peroxidation (LPO) processes. 4. During brain ischemia carnosine acts as a neuroprotector, contributing to better cerebral blood flow restoration, electroencephalography (EEG) normalization, decreased lactate accumulation, and enzymatic protection against ROS. 5. The data presented demonstrate that carnosine is a specific regulator of essential metabolic pathways in neurons supporting brain homeostasis under unfavorable conditions.

Usefulness of diffusion-weighted MRI with echo-planar technique in the evaluation of cellularity in gliomas

The purpose of this study was to evaluate the utility of diffusion-weighted magnetic resonance imaging (MRI) with echo-planar imaging (EPI) technique in depicting the tumor cellularity and grading of gliomas. Twenty consecutive patients (13 men and 7 women, ranging in age from 13 to 69 years) with histologically proven gliomas were examined using a 1.5 T superconducting imager. Tumor cellularity, analyzed with National Institutes of Health Image 1.60 software on a Macintosh computer, was compared with the minimum apparent diffusion coefficient (ADC) and the signal intensity on the T2-weighted images. The relationship of the minimum ADC to the tumor grade was also evaluated. Tumor cellularity correlated well with the minimum ADC value of the gliomas (P = 0.007), but not with the signal intensity on the T2-weighted images. The minimum ADC of the high-grade gliomas was significantly higher than that of the low-grade gliomas. Diffusion-weighted MRI with EPI is a useful technique for assessing the tumor cellularity and grading of gliomas. This information is not obtained with conventional MRI and is useful for the diagnosis and characterization of gliomas.

Diffusion-weighted MR imaging: diagnostic accuracy in patients imaged within 6 hours of stroke symptom onset

PURPOSE

To evaluate the diagnostic accuracy of diffusion-weighted magnetic resonance (MR) imaging performed within 6 hours of the onset of stroke symptoms.

MATERIALS AND METHODS

The authors reviewed the patient records and images from all patients hospitalized in a 10-month period in whom diffusion-weighted imaging was performed within 6 hours of the onset of strokelike symptoms (n = 22). Analyses included comparison of the initial interpretation of the diffusion-weighted images with the final clinical diagnosis; blinded reviews of computed tomographic (CT) scans and conventional and diffusion-weighted images; and determination of lesion contrast-to-noise ratios (CNRs).

RESULTS

Diffusion-weighted images indicated stroke in 14 patients, all of whom had a final diagnosis of acute stroke. Diffusion-weighted images were negative in eight patients, all of whom had a final clinical diagnosis other than stroke (100% sensitivity, 100% specificity, chi 2 = 23.00, P < .0001). Blinded reviews yielded 100% sensitivity and 86% specificity for diffusion-weighted MR imaging (chi 2 = 15.43, P < .0005); 18% sensitivity and 100% specificity for conventional MR imaging (chi 2 = 2.85, P > .2); and 45% sensitivity and 100% specificity for CT (chi 2 = 4.40, P > .10). Lesion percentage CNRs were 77% for diffusion-weighted imaging, 5.5% for CT, 9.8% for T2-weighted MR imaging, and 3.1% for proton-density-weighted MR imaging (P < .002 for diffusion-weighted imaging vs others).

CONCLUSION

Diffusion-weighted MR imaging is highly accurate for diagnosing stroke within 6 hours of symptom onset and is superior to CT and conventional MR imaging.

Diffusion-weighted magnetic resonance imaging confirms marked neuroprotective efficacy of albumin therapy in focal cerebral ischemia

BACKGROUND AND PURPOSE

We have recently shown high-dose human serum albumin therapy to confer marked histological protection in experimental middle cerebral artery occlusion (MCAo). We have now used diffusion-weighted magnetic resonance imaging (DWI) in conjunction with morphological methods to expand our understanding of this therapeutic approach.

METHODS

Physiologically controlled Sprague-Dawley rats received 2-hour MCAo by the modified intraluminal suture method. Treated rats received 25% human serum albumin solution (1% by body weight) immediately after the MCA was reopened. Vehicle-treated rats received saline. Computer-based image averaging was used to analyze DWI data obtained 24 hours after MCAo and light-microscopic histopathology obtained at 3 days. In a matched series, plasma osmolality and colloid oncotic pressure, as well as brain water content, were determined.

RESULTS

Albumin therapy, which lowered the hematocrit on average by 37% and raised plasma colloid oncotic pressure by 56%, improved the neurological score throughout the 3-day survival period. Within the ischemic focus, the apparent diffusion coefficient (ADC) computed from DWI data declined by 40% in vehicle-treated rats but was preserved at near-normal levels (8% decline) in albumin-treated rats (P<0.001). Albumin also led to higher ADC values within unlesioned brain regions. Histology revealed large consistent cortical and subcortical infarcts in vehicle-treated rats, while albumin therapy reduced infarct volume at these sites, on average, by 84% and 33%, respectively. Total infarct volume was reduced by 66% and brain swelling was virtually eliminated by albumin treatment. Microscopically, while infarcted regions of vehicle-treated rats had the typical changes of pannecrosis, infarcted zones of albumin-treated brains showed persistence of vascular endothelium and prominent microglial activation, suggesting that albumin therapy may help to preserve the neuropil within zones of residual infarction.

CONCLUSIONS

These findings confirm the striking neuroprotective efficacy of albumin therapy in focal cerebral ischemia and reveal that this effect is associated with DWI normalization and a mitigation of pannecrotic changes within zones of residual injury.

Diffusion-, T2-, and perfusion-weighted nuclear magnetic resonance imaging of middle cerebral artery embolic stroke and recombinant tissue plasminogen activator intervention in the rat

Thrombolysis of embolic stroke in the rat was measured using diffusion (DWI)-, T2 (T2WI)-, and perfusion (PWI)-weighted magnetic resonance imaging (MRI). An embolus was placed at the origin of the middle cerebral artery (MCA) by injection of an autologous single blood clot via an intraluminal catheter placed in the intracranial segment of internal carotid artery. Rats were treated with a recombinant tissue plasminogen activator (rt-PA) 1 hour after embolization (n = 9) or were not treated (n = 15). Diffusion-weighted imaging, T2WI, and PWI were performed before, during, and after embolization from 1 hour to 7 days. After embolization in both rt-PA-treated and control animals, the apparent diffusion coefficient of water (ADCw) and cerebral blood flow (CBF) in the ischemic region significantly declined from the preischemic control values (P < 0.001). However, mean CBF and ADCw in the rt-PA-treated group was elevated early after administration of rt-PA compared with the untreated control group, and significant differences between the two groups were detected in CBF (24 hours after embolization, P < 0.05) and ADCw (3, 4, and 24 hours after embolization, P < 0.05). T2 values maximized at 24 (control group, P < 0.001) or 48 hours (treated group, P < 0.01) after embolization. The increase in T2 in the control group was significantly higher at 24 hours and 168 hours than in the rt-PA-treated group (P < 0.05). Significant correlations (r > or = 0.80, P < 0.05) were found between lesion volume measured 1 week after embolization and CBF and ADCw obtained 1 hour after injection of rt-PA. Within a coronal section of brain, MRI cluster analysis, which combines ADCw and T2 data maps, indicated a significant reduction (P < 0.05) in the lesion 24 hours after thrombolysis compared with nontreated animals. These data demonstrate that the values for CBF and ADCw obtained 1 hour after injection of rt-PA correlate with histologic outcome in the tissue, and that the beneficial effect of thrombolysis of an intracranial embolus by means of rt-PA is reflected in an increase of CBF and ADCw, a reduction in the increase of T2, and a reduction of the ischemic lesion size measured using MRI cluster analysis.

Magnetic resonance imaging of acute stroke

In the investigation of ischemic stroke, conventional structural magnetic resonance (MR) techniques (e.g., T1-weighted imaging, T2-weighted imaging, and proton density-weighted imaging) are valuable for the assessment of infarct extent and location beyond the first 12 to 24 hours after onset, and can be combined with MR angiography to noninvasively assess the intracranial and extracranial vasculature. However, during the critical first 6 to 12 hours, the probable period of greatest therapeutic opportunity, these methods do not adequately assess the extent and severity of ischemia. Recent developments in functional MR imaging are showing great promise for the detection of developing focal cerebral ischemic lesions within the first hours. These include (1) diffusion-weighted imaging, which provides physiologic information about the self-diffusion of water, thereby detecting one of the first elements in the pathophysiologic cascade leading to ischemic injury; and (2) perfusion imaging. The detection of acute intraparenchymal hemorrhagic stroke by susceptibility weighted MR has also been reported. In combination with MR angiography, these methods may allow the detection of the site, extent, mechanism, and tissue viability of acute stroke lesions in one imaging study. Imaging of cerebral metabolites with MR spectroscopy along with diffusion-weighted imaging and perfusion imaging may also provide new insights into ischemic stroke pathophysiology. In light of these advances in structural and functional MR, their potential uses in the study of the cerebral ischemic pathophysiology and in clinical practice are described, along with their advantages and limitations.

A reproducible model of middle cerebral artery occlusion in mice: hemodynamic, biochemical, and magnetic resonance imaging

A reproducible model of thread occlusion of the middle cerebral artery (MCA) was established in C57 Black/6J mice by matching the diameter of the thread to the weight of the animals. For this purpose, threads of different diameter (80 to 260 microns) were inserted into the MCA of animals of different weights (18 to 33 g), and the success of vascular occlusion was evaluated by imaging the ischemic territory on serial brain sections with carbon black. Successful occlusion of the MCA resulted in a linear relationship between body weight and thread diameter (r = 0.46, P < 0.01), allowing precise selection of the appropriate thread size. Laser-Doppler measurements of CBF, neurological scoring, and 2,3,5-triphenyltetrazolium chloride staining confirmed that matching of animal weight and suture diameter produced consistent cerebral infarction. Three hours after MCA occlusion, imaging of ATP, tissue pH, and cerebral protein synthesis allowed differentiation between the central infarct core, in which ATP was depleted, and a peripheral penumbra with reduced protein synthesis and tissue acidosis but preserved ATP content. Perfusion deficits and ischemic tissue alterations could also be detected by perfusion- and diffusion-weighted magnetic resonance imaging, demonstrating the feasibility of dynamic evaluations of infarct evolution. The use of multiparametric imaging techniques in this improved MCA occlusion model opens the way for advanced pathophysiological studies of stroke in gene-manipulated animals.

The influence of preischemic hyperglycemia on acute changes in brain water ADCw following focal ischemia in rats

The effect of preischemic hyperglycemia on the acute decline of brain apparent diffusion coefficient of water (ADCw) following cerebral ischemia was studied in a rat model of middle cerebral artery occlusion (MCAO). ADCw was measured by NMR with a newly developed spin-echo line-scan protocol that provides for an ADCw calculation every 15 s at a spatial resolution of 3.4 microl/pixel. A remote controlled occluding device was used to initiate ischemia from outside the magnet, allowing for continuous monitoring of ADCw before, during and after MCAO. Preischemic hyperglycemia (25-30 mM) was achieved via i.v. infusion of 50% glucose. The decline in ADCw following ischemia was analyzed to obtain three-time constants: the time from onset of ischemia to initial significant ADCw decline below baseline level (i.e., 20% of maximal decline, T0.20), the time to decline by 50% (T0.50), and the time to decline by 95% (T0.95). Mean (+/-S.D.) values for T0.20, T0.50, T0.95 were: 39.6+/-7.2, 54. 0+/-7.8, 105.0+/-15.0 s for the normoglycemic group (n=7), and 49. 2+/-33.0, 116.4+/-2.4, 351.0+/-189.0 s for the hyperglycemic group (n=6), respectively. Hyperglycemia significantly prolongs T0.50 and T0.95 but does not affect T0.20. The temporal profiles of ADCw decline following ischemia under normo- and hyperglycemia are distinctively different from the known time course of membrane depolarization under similar experimental conditions, suggesting that mechanisms other than membrane depolarization and cell swelling may contribute to changes in ADCw in cerebral ischemia.

Measurements of (Na+,K+)ATPase after in vitro hypoxia and reoxygenation are affected by methods of membrane preparation

(Na+,K+ )ATPase activity was evaluated in membranes from rat hippocampal slices after in vitro hypoxia and reoxygenation. Membranes were prepared with two different methods, one using an isotonic medium and another using a hypotonic one. The changes that were found after hypoxia went into opposite directions in the two cases. Membranes prepared in a hypotonic medium are probably more suitable for these measurements. Using these membranes, hypoxia results in a slight decrease of (Na+,K+)ATPase activity and in a further decrease after reoxygenation. We also found that expressing (Na+,K+)ATPase activity as a percent of total ATPase activity is appropriate for membranes prepared under hypotonic conditions and can unveil (by reducing variability between experiments) significant changes that may be masked in small samples like ours.

The effects of a butanediol treatment on acute focal cerebral ischemia assessed by quantitative diffusion and T2 MR imaging

Increased water T2 values indicates the presence of vasogenic edema. Decreased apparent diffusion coefficient (ADC) maps reveal ischemic areas displaying cytotoxic edema. ADC and T2 abnormalities spread through the middle cerebral artery (MCA) territory up to 24 h after middle cerebral artery occlusion (MCAO). Also, it was found that ADC and T2 contours closely match at 3.5 and 24 h. Since butanediol reduces vasogenic edema and improves energy status in various models of ischemia, we used these two techniques to investigate putative improvements in cytotoxic and vasogenic edema after permanent MCAO performed on rats. Rats were given no treatment (n = 8), or a treatment with 25 mmol/kg intraperitoneal (i.p.) butanediol (n = 5), 30 min before and 2.5 h after MCAO. Quantitative ADC and T2 maps of brain water were obtained, from which the volumes presenting abnormalities were calculated at various time points up to 24 h. Effects of butanediol on the ADC and T2 values in these areas were determined. Butanediol reduced neither the ADC volume nor the initial ADC decline. However, it reduced T2 volumes by 32% at 3.5 h and 15% at 24 h (p < 0.05), and reduced T2 increase in the striatum at 3.5 h post-MCAO. Therefore, our results show for the first time that a pharmacological agent such as butanediol can delay the development of vasogenic edema but does not limit the development of vasogenic edema but does not limit the development of cytotoxic edema. ADC imaging detects areas of severe metabolic disturbance but not moderately ischemic peripheral areas where butanediol is presumed to be more efficacious.

Changes in ouabain affinity of Na+, K+-ATPase during focal cerebral ischaemia in the mouse

We investigated the effect of focal cerebral ischaemia on the activity and the affinity of the ouabain sites of Na+,K+-ATPase in the mouse. The Na+,K+-ATPase activity was decreased by 38% as early as 30 min following ischaemia. In the sham group, the dose-response curves for ouabain disclosed three inhibitory states which contribute, respectively, 24.9 +/- 6.7%, 39.1 +/- 7.5% and 36.0% of the total activity (low affinity, LA; high affinity, HA and very high affinity, VHA, respectively). Their computed IC50 values are, respectively: 1.3 X 10(-3) M, 4.5 X 10(-6) M and 2.9 X 10(-9) M. Surprisingly, in ischaemic cortices, only two sites for ouabain were detected. The first site exhibits a LA (IC50 = 2.0 X 10[-4] M) but its relative contribution to the total activity (46.1 +/- 5.2%) is twice that noted for the LA site in non-ischaemic tissues. The second site presents an affinity intermediate between those of HA and VHA sites of the sham group (IC50 = 1.7 X 10[-7] M) and contributes 53.9% to the total activity. Loss in the specific activity of the second site explains that of the total activity. The most likely explanation in the presence of only two ouabain sites of Na+,K+-ATPase following ischaemia may be a change in ouabain affinity of alpha2 and/or alpha3 isoforms, as the presence of all three alpha isoforms has been observed by Western blotting. These results suggest that ischaemia induces intrinsic modifications in Na+,K+-ATPase which result from perturbations in membrane integrity and/or association of the alpha isoforms of this enzyme.

Diffusion-weighted imaging discriminates between cytotoxic and vasogenic edema in a patient with eclampsia

BACKGROUND

The pathophysiology of eclampsia remains unclear. While the majority of patients develop reversible T2 hyperintense signal abnormalities on MR scans and reversible neurological deficits, some patients do develop infarctions (permanent T2 hyperintense abnormalities) and permanent neurological impairment. Routine MRI cannot prospectively differentiate between these two patient groups. Echo-planar diffusion-weighted imaging, however, is a new technique that clearly differentiates between cytotoxic and vasogenic edema.

CASE DESCRIPTION

A 30-year-old woman developed symptoms consistent with eclampsia 24 hours after delivering premature twins. An MRI demonstrated extensive, diffuse T2 hyperintense signal abnormalities involving subcortical white matter and adjacent gray matter with a posterior predominance, consistent with either infarction or hypertensive ischemic encephalopathy. Diffusion-weighted images demonstrated increased diffusion, consistent with vasogenic edema and hypertensive ischemic encephalopathy.

CONCLUSIONS

Unlike routine MRI, diffusion-weighted imaging reliably differentiates between vasogenic edema and cytotoxic edema. Consequently, in eclamptic patients diffusion-weighted imaging can afford clear differentiation between hypertensive ischemic encephalopathy and infarction, two very different entities with very different treatment protocols. Diffusion-weighted imaging should be performed in all eclamptic patients and should greatly affect their management.

Improved perfusion with rt-PA and hirulog in a rabbit model of embolic stroke

We conducted a study using diffusion-weighted (DWI) and perfusion-weighted (PWI) magnetic resonance imaging (MRI) to evaluate the efficacy of thrombolysis in an embolic stroke model with recombinant tissue plasminogen activator (rt-PA) and hirulog, a novel direct-acting antithrombin. DWI can identify areas of ischemia minutes from stroke onset, while PWI identifies regions of impaired blood flow. Right internal carotid arteries of 36 rabbits were embolized using aged heterologous thrombi. Baseline DWI and PWI scans were obtained to confirm successful embolization. Four animals with no observable DWI lesion on the initial scan were excluded; therefore, a total of 32 animals were randomized to one of three treatment groups: rt-PA (n = 11), rt-PA plus hirulog (n = 11), or placebo (n = 10). Treatment was begun 1 h after stroke induction. Intravenous doses were as follows: rt-PA, 5 mg/kg over 0.5 h with 20% of the total dose given as a bolus; hirulog, 1 mg/kg bolus followed by 5 mg/kg over 1 h. MRI was performed at 2, 3, and 5 h following embolization. Six hours after embolization, brains were harvested, examined for hemorrhage, then prepared for histologic analysis. The rt-PA decreased fibrinogen levels by 73%, and hirulog prolonged the aPTT to four times the control value. Posttreatment areas of diffusion abnormality and perfusion delay were expressed as a ratio of baseline values. Significantly improved perfusion was seen in the rt-PA plus hirulog group compared with placebo (normalized ratios of the perfusion delay areas were as follows: placebo, 1.58, 0.47-3.59; rt-PA, 1.12, 0.04-3.95; rt-PA and hirulog, 0.40, 0.02-1.08; p < 0.05). Comparison of diffusion abnormality ratios measured at 5 h showed trends favoring reduced lesion size in both groups given rt-PA (normalized ratios of diffusion abnormality areas were as follows: placebo, 3.69, 0.39-15.71; rt-PA, 2.57, 0.74-5.00; rt-PA and hirulog, 1.95, 0.33-6.80; p = 0.32). Significant cerebral hemorrhage was observed in one placebo, two rt-PA, and three rt-PA plus hirulog treated animals. One fatal systemic hemorrhage was observed in each of the rt-PA groups. We conclude that rt-PA plus hirulog improves cerebral perfusion but does not necessarily reduce cerebral injury. DWI and PWI are useful methods for monitoring thrombolysis.

Biochemical and physiological evidence that carnosine is an endogenous neuroprotector against free radicals

1. Carnosine, anserine, and homocarnosine are endogenous dipeptides concentrated in brain and muscle whose biological functions remain in doubt. 2. We have tested the hypothesis that these compounds function as endogenous protective substances against molecular and cellular damage from free radicals, using two isolated enzyme systems and two models of ischemic brain injury. Carnosine and homocarnosine are both effective in activating brain Na, K-ATPase measured under optimal conditions and in reducing the loss of its activity caused by incubation with hydrogen peroxide. 3. In contrast, all three endogenous dipeptides cause a reduction in the activity of brain tyrosine hydroxylase, an enzyme activated by free radicals. In hippocampal brain slices subjected to ischemia, carnosine increased the time to loss of excitability. 4. In in vivo experiments on rats under experimental hypobaric hypoxia, carnosine increased the time to loss of ability to stand and breath and decreased the time to recovery. 5. These actions are explicable by effects of carnosine and related compounds which neutralize free radicals, particularly hydroxyl radicals. In all experiments the effective concentration of carnosine was comparable to or lower than those found in brain. These observations provide further support for the conclusion that protection against free radical damage is a major role of carnosine, anserine, and homocarnosine.

Spreading of vasogenic edema and cytotoxic edema assessed by quantitative diffusion and T2 magnetic resonance imaging

BACKGROUND AND PURPOSE

The apparent diffusion coefficient (ADC) of water should be sensitive to the cytotoxic edema triggered by energy failure during ischemia. Elevated values of T2. the nuclear MR transverse relaxation time of water, seen on T2 nuclear MR images detect vasogenic edema and infarcted areas. The temporal and spatial changes in ADC and T2 abnormalities after occlusion of the middle cerebral artery (MCAO) were therefore estimated by these two quantitative techniques.

METHODS

Permanent MCAO was performed on rats. Quantitative ADC and T2 maps of brain water were obtained, from which the ischemic volumes were calculated at various times up to 48 hours after MCAO.

RESULTS

The areas of decreased ADC represented 36 +/- 7% of the final infarct volume (24 hours) at 0.5 hours and 64 +/- 4% at 5 hours after MCAO, suggesting that there is recruitment of peripheral areas with disturbed energy metabolism and cytotoxic edema. The ADC and T2 contours closely matched at 3.5, 24, and 48 hours after MCAO.

CONCLUSIONS

T2 imaging can assess ischemic insults as well as ADC imaging, but only 3.5 hours after the onset of ischemia. Assessment of edematous swelling (approximately 24.5% of total infarcted volume) demonstrates that ADC and therefore T2 imaging detect all the tissue that will become infarcted approximately 7 hours after occlusion. The spread of ADC and T2 abnormalities would therefore stop at approximately 7 hours, and any further increase in volume observed on the images would be mainly due to edematous swelling.

Delayed progression of cytotoxic oedema in focal cerebral ischemia after treatment with a torasemide derivative: a diffusion-weighted magnetic resonance imaging study

Diffusion-weighted magnetic resonance imaging (MRI) was used to assess the effect of an astrocytic Na+2Cl-K+ cotransporter inhibitor, a novel torasemide derivative, on the time course and spatial evolution of a focal cerebral ischemia in the rat. The drug (1 mg/ kg, i.p.) was injected 30 min before middle cerebral artery occlusion and diffusion-weighted images were acquired at various times thereafter. The results showed that the drug reduced the size of the hyperintensity during the first hours, but did not affect the time constant of growth or the final size. The temporary reduction of the cytotoxic oedema induced by the torasemide derivative, demonstrates an antioedematous activity.

Developmental changes in NMDA-induced cell swelling and its transition to necrosis measured with 1H magnetic resonance imaging, impedance and histology

The vulnerability of the rat brain to intracerebrally injected N-methyl-D-aspartate (NMDA) drastically changes with age. We evaluated the developmental changes in the early and late responses to NMDA using 1H magnetic resonance imaging (MRI), cortical impedance and histology. NMDA, injected in the striatum of rats at postnatal days (P) 4, 7, 10, 14 and 21, induced a significant age-dependent reduction in the apparent diffusion coefficient (ADC) of tissue water in the striatum and the cerebral cortex monitored 1 h later using diffusion-weighted MRI. The reduction in ADC amounted 65% at P4 with lower values thereafter and was about 30% at P21. NMDA similarly induced a reduction in the cortical extracellular space (by 50% at P7 and 10% at P16) as measured with impedance recordings. The progressive decrease in the effect of NMDA with brain development was also indicated by a decrease in the volume of tissue in which the changes in ADC occurred (50 mm3 at P4 and 8 mm3 at P21). The diffusion of extracellular tracer molecules Mn2+ or [3H]-(R)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) injected in the striatum and monitored with T1-weighted MRI and autoradiography respectively showed a similar age dependence with the diffusion volume being twofold larger in P7 than in P21 brain. Thus restriction in diffusion during brain development may contribute to the decrease in NMDA-induced injury with age. The volume of tissue necrosis and gliosis, measured with T2-weighted MRI and histology 5 days after NMDA injection, was similar to that outlined by the ADC reduction detected soon after the insult at P4, P7 and P21. However, at P10 and P14 only 50% of the tissue showing a hyperintense signal in DW images displayed necrosis and gliosis 5 days later. This study shows that during development the early response to NMDA in terms of cytotoxic cell swelling (indicated both with impedance recordings and diffusion-weighted MRI) decreases with age. In addition, with maturation only part of the brain tissue acutely affected by NMDA does proceed into necrosis and gliosis, indicating an increased capacity of cells in the developing rat brain to survive NMDA-induced cell swelling.

Imaging focal reperfusion injury following global ischemia with diffusion-weighted magnetic resonance imaging and 1H-magnetic resonance spectroscopy

The purpose of the study was to determine whether diffusion-weighted magnetic resonance imaging (DWI) could identify focal lesions that develop in ischemia-sensitive cerebral tissues during reperfusion following global brain ischemia. Localized 1H-Magnetic Resonance Spectroscopy (1H-MRS) measurements were also obtained to determine whether abnormal spectroscopic markers were associated with focal lesions and to define time correlations between DWI and metabolic changes. Brain diffusion-weighted magnetic resonance imaging measurements were made in a cat model of repetitive global cerebral ischemia and reperfusion. Five animals were exposed to three episodes of 10 min vascular occlusions at hourly intervals. Three animals were evaluated as controls. DWI, T2WI, and 1H-MRS data were acquired for up to 12 h. Transient focal DWI hyperintensity was detected in the hippocampus, basal ganglia, and cortical watershed areas. These focal abnormalities usually appeared during the final reperfusion and eventually spread to encompass all of the gray matter. Spectroscopic measurements demonstrated the expected elevation of the lactate signal intensity during vessel occlusion, which returned to normal during early reperfusion. A subsequent rise in the lactate signal occurred approximately 3-4 h after the beginning of the third reperfusion. This late lactate elevation occurred after focal hyperintensities were identified by DWI. No significant signal changes were seen in spectroscopic metabolites other than lactate. The study illustrates that DWI and 1H-MRS are sensitive to focal cerebral lesions that occur during reperfusion following global cerebral ischemia.

Resuscitation from cardiac arrest in cats: influence of epinephrine dosage on brain recovery

The quality of brain recovery after cardiac arrest depends crucially on the speed of cardiac resuscitation because the low cerebral perfusion pressure during the resuscitation procedure facilitates the development of no-reflow. To accelerate return of spontaneous circulation, high dose epinephrine has been recommended but the effect on the dynamics of early brain recovery is still unknown. We, therefore, studied the dynamics of brain resuscitation after cardiopulmonary resuscitation (CPR) with standard and high dose epinephrine using non-invasive NMR techniques. Fifteen min cardiac arrest was induced in normothermic cats by ventricular fibrillation. CPR was performed using an inflatable pneumatic vest for cyclic chest compression. With the beginning of CPR the standard dose group received 0.02 mg/kg epinephrine (n = 6) and the high dose group received 0.2 mg/kg (n = 8). Brain recovery was monitored by magnetic resonance imaging of the apparent diffusion coefficient (ADC) of water for 3 h. Although high dose epinephrine treatment led to a significantly higher blood pressure during early reperfusion, rapidly changing heterogeneities of early brain recovery were observed in both groups. High dose epinephrine thus does not improve the quality of post-cardiac arrest brain recovery during the first 3 h of reperfusion.

Diffusion-weighted NMR imaging: application to experimental focal cerebral ischemia

Changes in diffusion NMR imaging are believed to be based on intra/extracellular water homeostasis and will therefore reflect early disturbances of ion and water homeostasis after the onset of an ischemic event. Diffusion-weighted NMR imaging (DWI) thus has the potential to be a sensitive tool for the observation of stroke evolution. The present state of information extracted from diffusion-weighted NMR imaging for the understanding of cerebral focal ischemia in experimental research has been compiled in this review. The emphasis was set on three essential aspects of the technique in relation to focal ischemia. Firstly, the sensitivity of diffusion-weighted imaging for ischemic alterations is described. A comparison with conventional NMR imaging using relaxation time changes is included. Secondly, the comparison of the diffusion-weighted imaging with invasive techniques is discussed. Here, interpretation of the physiological, metabolic and hemodynamic alterations reflected in the observed diffusion changes is presented. The importance of regionally resolved information for a meaningful assignment of DWI changes to pathophysiological alterations is demonstrated for the differentiation between ischemic core and penumbra from DWI and quantitative diffusion coefficient data. The time dependence of correlations with physiological, biochemical and hemodynamic variables as a further important aspect is stressed. Thirdly, the potential of the technique for the assessment of development and effectiveness of new therapeutical strategies against stroke is demonstrated.

Evolution of regional changes in apparent diffusion coefficient during focal ischemia of rat brain: the relationship of quantitative diffusion NMR imaging to reduction in cerebral blood flow and metabolic disturbances

Middle cerebral artery occlusion was performed in rats while the animals were inside the nuclear magnetic resonance (NMR) tomograph. Successful occlusion was confirmed by the collapse of amplitude on an electrocorticogram. The ultrafast NMR imaging technique UFLARE was used to measure the apparent diffusion coefficient (ADC) immediately after the induction of cerebral ischemia. ADC values of normal cortex and caudate-putamen were 726 +/- 22 x 10(-6) mm2/s and 659 +/- 17 x 10(-6) mm2/s, respectively. Within minutes of occlusion, a large territory with reduced ADC became visible in the ipsilateral hemisphere. Over the 2 h observation period, this area grew continuously. Quantitative analysis of the ADC reduction in this region showed a gradual ADC decrease from the periphery to the core, the lowest ADC value amounting to about 60% of control. Two hours after the onset of occlusion, the regional distribution of ATP and tissue pH were determined with bioluminescence and fluorescence techniques, respectively. There was a depletion of ATP in the core of the ischemic territory (32 +/- 20% of the hemisphere) and an area of tissue acidosis (57 +/- 19% of the hemisphere) spreading beyond that of ATP depletion. Regional CBF (rCBF) was measured autoradiographically with the iodo[14C]antipyrine method. CBF gradually decreased from the periphery to the ischemic core, where it declined to values as low as 5 ml 100 g-1. When reductions in CBF and in ADC were matched to the corresponding areas of energy breakdown and of tissue acidosis, the region of energy depletion corresponded to a threshold in rCBF of 18 +/- 14 ml 100 g-1 min-1 and to an ADC reduction to 77 +/- 3% of control. Tissue acidosis corresponded to a flow value below 31 +/- 11 ml 100 g-1 min-1 and to an ADC value below 90 +/- 4% of control. Thus, the quantification of ADC in the ischemic territory allows the distinction between a core region with total breakdown of energy metabolism and a corona with normal energy balance but severe tissue acidosis.

The effect of post-asphyxial reoxygenation with 21% vs. 100% oxygen on Na+,K(+)-ATPase activity in striatum of newborn piglets

To compare the effect of 21% vs. 100% oxygen during post-asphyxial reoxygenation on brain cell membrane function in the striatum, 20 anesthetized, ventilated newborn piglets were studied: group 1 (normoxia, n = 5), group 2 (asphyxia, no reoxygenation, n = 5), group 3 (asphyxia followed by reoxygenation with 21% O2, n = 5), and group 4 (asphyxia followed by reoxygenation with 100% O2, n = 5). Asphyxia was induced by a stepwise reduction in FiO2 at 20 min intervals from 21% to 14%, 11%, and 8%. Following a total 60 min of asphyxia, piglets in groups 3 and 4 were recovered for 2 h with either 21% or 100% O2. Na+,K(+)-ATPase activity (mumol Pi/mg protein/h) in striatal cell membranes was 31 +/- 1, 22 +/- 2, 32 +/- 2 and 26 +/- 1 in groups 1, 2, 3 and 4, respectively. Na+,K(+)-ATPase activities in groups 2 and 4 were significantly lower than in groups 1 and 3 (p < 0.01). Piglets recovered post-asphyxia for 2 h with 21% O2 had restoration of Na+,K(+)-ATPase activity to baseline levels, while those treated with 100% O2 during recovery had persistent Na+,K(+)-ATPase inhibition of 16%. This could result from increased free radical production during reoxygenation with 100% O2 which could contribute to post-asphyxial cellular injury in the striatum.

Correlation of early reduction in the apparent diffusion coefficient of water with blood flow reduction during middle cerebral artery occlusion in rats

To determine the relationship between reductions in the apparent diffusion coefficient of water (ADC) and in cerebral blood flow (CBF) during focal ischemia, we used diffusion-weighted magnetic resonance (D-MR) imaging and autoradiographic CBF analysis to examine rats subjected to 30 or 90 min of permanent middle cerebral artery (MCA) occlusion. In the 30-min occlusion group (n = 10), the area with substantially reduced ADC (15% or more below the contralateral level [ADC15]) corresponded best to the area with CBF below 25 ml/100 g/min and was significantly smaller than the area with CBF below 50 ml/100 g/min (CBF50), a level associated with reduced protein synthesis and delayed necrosis (40 +/- 13% versus 74 +/- 8% of the ischemic hemisphere; P < 0.0001). In the 90-min occlusion group (n = 6), the ADC15 area corresponded best to the CBF30 to CBF35 area and was again significantly smaller than the CBF50 area (54 +/- 13% versus 73 +/- 20%, P < 0.05). Thus, the area of substantially reduced ADC at 30 and 90 min represents only 53% and 74%, respectively, of the tissue at risk for infarction. These findings indicate a potential limitation in using early D-MR imaging to predict stroke outcome.

Correlation of rapid changes in the average water diffusion constant and the concentrations of lactate and ATP breakdown products during global ischemia in cat brain

Rapid changes in the average water diffusion constant, Dav = 1/3[Dxx+Dyy+Dzz], and in the concentrations of lactate and purine nucleotides and nucleosides were measured upon global ischemia (cardiac arrest) in cat brain, at a combined time resolution of 36 s (n = 7). At this time resolution, the normalized time curves of 1 - Dav and the increase in ATP breakdown product did not coincide, with the changes in Dav being most rapid. The normalized curves of 1 - Dav and the lactate increase coincided for the first 2-2.5 min after which the change in Dav was more rapid. After this time point, an excellent correlation was found between the drop in Dav and the decrease in energy utilization rate, which was calculated from the measured time curves of lactate formation and ATP breakdown, and from the time curve for phosphocreatine use reported in the literature. These results are in agreement with the expected biphasic changes in ion and water homeostasis during ischemia and with the model of diffusional changes being a consequence of a water shift from interstitial to intracellular space.