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

Physiology of Glymphatic Solute Transport and Waste Clearance from the Brain

This review focuses on the physiology of glymphatic solute transport and waste clearance, using evidence from experimental animal models as well as from human studies. Specific topics addressed include the biophysical characteristics of fluid and solute transport in the central nervous system, glymphatic-lymphatic coupling, as well as the role of cerebrospinal fluid movement for brain waste clearance. We also discuss the current understanding of mechanisms underlying increased waste clearance during sleep.

Disparate volumetric fluid shifts across cerebral tissue compartments with two different anesthetics

Background

Large differences in glymphatic system transport—similar in magnitude to those of the sleep/wake cycle—have been observed during anesthesia with dexmedetomidine supplemented with low dose isoflurane (DEXM-I) in comparison to isoflurane (ISO). However, the biophysical and bioenergetic tissue status underlying glymphatic transport differences between anesthetics remains undefined. To further understand biophysical characteristics underlying these differences we investigated volume status across cerebral tissue compartments, water diffusivity, and T2* values in rats anesthetized with DEXM-I in comparison to ISO.

Methods

Using a crossover study design, a group of 12 Sprague Dawley female rats underwent repetitive magnetic resonance imaging (MRI) under ISO and DEXM-I. Physiological parameters were continuously measured. MRI included a proton density weighted (PDW) scan to investigate cerebrospinal fluid (CSF) and parenchymal volumetric changes, a multigradient echo scan (MGE) to calculate T2* maps as a measure of ‘bioenergetics’, and a diffusion scan to quantify the apparent diffusion coefficient (ADC).

Results

The heart rate was lower with DEXM-I in comparison to ISO, but all other physiological variables were similar across scans and groups. The PDW images revealed a 1% parenchymal volume increase with ISO compared to DEXM-I comprising multiple focal tissue areas scattered across the forebrain. In contrast, with DEXM-I the CSF compartment was enlarged by ~ 6% in comparison to ISO at the level of the basal cisterns and peri-arterial conduits which are main CSF influx routes for glymphatic transport. The T2* maps showed brain-wide increases in T2* in ISO compared to DEXM-I rats. Diffusion-weighted images yielded no significant differences in ADCs across the two anesthesia groups.

Conclusions

We demonstrated CSF volume expansion with DEXM-I (in comparison to ISO) and parenchymal (GM) expansion with ISO (in comparison to DEXM-I), which may explain the differences in glymphatic transport. The T2* changes in ISO are suggestive of an increased bioenergetic state associated with excess cellular firing/bursting when compared to DEXM-I.

Microstructural Integrity of Salvaged Penumbra after Mechanical Thrombectomy

BACKGROUND AND PURPOSE

There are sparse data on the microstructural integrity of salvaged penumbral tissue after mechanical thrombectomy of large-vessel occlusions. The aim of the study was to analyze possible microstructural alteration in the penumbra and their association with clinical symptoms as well as angiographic reperfusion success in patients undergoing mechanical thrombectomy.

MATERIALS AND METHODS

All patients who underwent mechanical thrombectomy for large-vessel occlusions in the anterior circulation and who received an admission CT perfusion together with postinterventional DTIs were included (n = 65). Angiographic reperfusion success by means of modified Thrombolysis in Cerebral Infarction (mTICI) scale and clinical outcome were recorded. Microstructural integrity was assessed by DTI evaluating the mean diffusivity index within the salvaged gray matter of the former penumbra.

RESULTS

The mean diffusivity index was higher in completely recanalized patients (mTICI 3: -0.001 ± 0.034 versus mTICI <3: -0.030 ± 0.055, P = .03). There was a positive correlation between the mean diffusivity index and NIHSS score improvement (r = 0.49, P = .003) and the mean diffusivity index was associated with midterm functional outcome (r = -0.37, P = .04) after adjustment for confounders. In mediation analysis, the mean diffusivity index and infarction growth mediated the association between reperfusion success and clinical outcomes.

CONCLUSIONS

The macroscopic salvaged penumbra included areas of microstructural integrity changes, most likely related to the initial hypoperfusion. These abnormalities were found early after mechanical thrombectomy, were dependent on angiographic results, and correlated with the clinical outcome. When confirmed, these findings prompt the evaluation of therapies for protection of the penumbral tissue integrity.

Apparent diffusion coefficient changes in human brain during sleep - Does it inform on the existence of a glymphatic system?

The role of sleep in brain physiology is poorly understood. Recently rodent studies have shown that the glymphatic system clears waste products from brain more efficiently during sleep compared to wakefulness due to the expansion of the interstitial fluid space facilitating entry of cerebrospinal fluid (CSF) into the brain. Here, we studied water diffusivity in the brain during sleep and awake conditions, hypothesizing that an increase in water diffusivity during sleep would occur concomitantly with an expansion of CSF volume - an effect that we predicted based on preclinical findings would be most prominent in cerebellum. We used MRI to measure slow and fast components of the apparent diffusion coefficient (ADC) of water in the brain in 50 healthy participants, in 30 of whom we compared awake versus sleep conditions and in 20 of whom we compared rested-wakefulness versus wakefulness following one night of sleep-deprivation. Sleep compared to wakefulness was associated with increases in slow-ADC in cerebellum and left temporal pole and with decreases in fast-ADC in thalamus, insula, parahippocampus and striatal regions, and the density of sleep arousals was inversely associated with ADC changes. The CSF volume was also increased during sleep and was associated with sleep-induced changes in ADCs in cerebellum. There were no differences in ADCs with wakefulness following sleep deprivation compared to rested-wakefulness. Although we hypothesized increases in ADC with sleep, our findings uncovered both increases in slow ADC (mostly in cerebellum) as well as decreases in fast ADC, which could reflect the distinct biological significance of fast- and slow-ADC values in relation to sleep. While preliminary, our findings suggest a more complex sleep-related glymphatic function in the human brain compared to rodents. On the other hand, our findings of sleep-induced changes in CSF volume provide preliminary evidence that is consistent with a glymphatic transport process in the human brain.

Early Functional Connectome Integrity and 1-Year Recovery in Comatose Survivors of Cardiac Arrest

Purpose To assess whether early brain functional connectivity is associated with functional recovery 1 year after cardiac arrest (CA). Materials and Methods Enrolled in this prospective multicenter cohort were 46 patients who were comatose after CA. Principal outcome was cerebral performance category at 12 months, with favorable outcome (FO) defined as cerebral performance category 1 or 2. All participants underwent multiparametric structural and functional magnetic resonance (MR) imaging less than 4 weeks after CA. Within- and between-network connectivity was measured in dorsal attention network (DAN), default-mode network (DMN), salience network (SN), and executive control network (ECN) by using seed-based analysis of resting-state functional MR imaging data. Structural changes identified with fluid-attenuated inversion recovery and diffusion-weighted imaging sequences were analyzed by using validated morphologic scales. The association between connectivity measures, structural changes, and the principal outcome was explored with multivariable modeling. Results Patients underwent MR imaging a mean 12.6 days ± 5.6 (standard deviation) after CA. At 12 months, 11 patients had an FO. Patients with FO had higher within-DMN connectivity and greater anticorrelation between SN and DMN and between SN and ECN compared with patients with unfavorable outcome, an effect that was maintained after multivariable adjustment. Anticorrelation of SN-DMN predicted outcomes with higher accuracy than fluid-attenuated inversion recovery or diffusion-weighted imaging scores (area under the receiver operating characteristic curves, respectively, 0.88, 0.74, and 0.71). Conclusion MR imaging-based measures of cerebral functional network connectivity obtained in the acute phase of CA were independently associated with FO at 1 year, warranting validation as early markers of long-term recovery potential in patients with anoxic-ischemic encephalopathy. ^© RSNA, 2017.

The continuum of spreading depolarizations in acute cortical lesion development: Examining Leão's legacy

A modern understanding of how cerebral cortical lesions develop after acute brain injury is based on Aristides Leão's historic discoveries of spreading depression and asphyxial/anoxic depolarization. Treated as separate entities for decades, we now appreciate that these events define a continuum of spreading mass depolarizations, a concept that is central to understanding their pathologic effects. Within minutes of acute severe ischemia, the onset of persistent depolarization triggers the breakdown of ion homeostasis and development of cytotoxic edema. These persistent changes are diagnosed as diffusion restriction in magnetic resonance imaging and define the ischemic core. In delayed lesion growth, transient spreading depolarizations arise spontaneously in the ischemic penumbra and induce further persistent depolarization and excitotoxic damage, progressively expanding the ischemic core. The causal role of these waves in lesion development has been proven by real-time monitoring of electrophysiology, blood flow, and cytotoxic edema. The spreading depolarization continuum further applies to other models of acute cortical lesions, suggesting that it is a universal principle of cortical lesion development. These pathophysiologic concepts establish a working hypothesis for translation to human disease, where complex patterns of depolarizations are observed in acute brain injury and appear to mediate and signal ongoing secondary damage.

Microstructural and functional correlates of glutamate concentration in the posterior cingulate cortex

Glutamate is the major excitatory neurotransmitter in the human brain and has a central role in both intrinsic and stimulus-induced activity. We conducted a study in a cohort of healthy, male volunteers in which glutamate levels were measured in the posterior cingulate cortex (PCC) using 1H magnetic resonance spectroscopy at 3T. The advantages of simultaneous electroencephalography and magnetic resonance imaging (EEG-MRI) were exploited and the subjects were measured in the same session and under the same physiological conditions. Diffusion tensor imaging (DTI), functional MRI (fMRI) and EEG were measured in order to investigate the functional and microstructural correlates of glutamate. The concentration of glutamate (institute units) was calculated and those values were tested for correlation with the metrics of resting state fMRI, DTI, and EEG electrical sources. Our results showed that the concentration of glutamate in the PCC had a significant negative correlation with the tissue mean diffusivity in the same area. The analysis of resting state networks did not show any relationship between the concentration of glutamate and the intrinsic activity of the resting state networks. The concentration of glutamate showed a positive correlation with the electrical generators of α-1 frequency and a negative correlation with the generators of α-2 and β-1 electrical generators. © 2017 Wiley Periodicals, Inc.

Translational MR Neuroimaging of Stroke and Recovery

Multiparametric magnetic resonance imaging (MRI) has become a critical clinical tool for diagnosing focal ischemic stroke severity, staging treatment, and predicting outcome. Imaging during the acute phase focuses on tissue viability in the stroke vicinity, while imaging during recovery requires the evaluation of distributed structural and functional connectivity. Preclinical MRI of experimental stroke models provides validation of non-invasive biomarkers in terms of cellular and molecular mechanisms, while also providing a translational platform for evaluation of prospective therapies. This brief review of translational stroke imaging discusses the acute to chronic imaging transition, the principles underlying common MRI methods employed in stroke research, and the experimental results obtained by clinical and preclinical imaging to determine tissue viability, vascular remodeling, structural connectivity of major white matter tracts, and functional connectivity using task-based and resting-state fMRI during the stroke recovery process.

The macrosphere model-an embolic stroke model for studying the pathophysiology of focal cerebral ischemia in a translational approach

The main challenge of stroke research is to translate promising experimental findings from the bench to the bedside. Many suggestions have been made how to achieve this goal, identifying the need for appropriate experimental animal models as one key issue. We here discuss the macrosphere model of focal cerebral ischemia in the rat, which closely resembles the pathophysiology of human stroke both in its acute and chronic phase. Key pathophysiological processes such as brain edema, cortical spreading depolarizations (CSD), neuroinflammation, and stem cell-mediated regeneration are observed in this stroke model, following characteristic temporo-spatial patterns. Non-invasive in vivo imaging allows studying the macrosphere model from the very onset of ischemia up to late remodeling processes in an intraindividual and longitudinal fashion. Such a design of pre-clinical stroke studies provides the basis for a successful translation into the clinic.

Reduced brain edema and infarct volume in aquaporin-4 deficient mice after transient focal cerebral ischemia

Aquaporin-4 (AQP4) is a water channel expressed in astrocyte end-feet lining the blood-brain barrier. AQP4 deletion in mice is associated with improved outcomes in global cerebral ischemia produced by transient carotid artery occlusion, and focal cerebral ischemia produced by permanent middle cerebral artery occlusion (MCAO). Here, we investigated the consequences of 1-h transient MCAO produced by intraluminal suture blockade followed by 23 h of reperfusion. In nine AQP4(+/+) and nine AQP4(-/-) mice, infarct volume was significantly reduced by an average of 39 ± 4% at 24h in AQP4(-/-) mice, cerebral hemispheric edema was reduced by 23 ± 3%, and Evans Blue extravasation was reduced by 31 ± 2% (mean ± SEM). Diffusion-weighted magnetic resonance imaging showed greatest reduction in apparent diffusion coefficient around the occlusion site after reperfusion, with remarkably lesser reduction in AQP4(-/-) mice. The reduced infarct volume in AQP4(-/-) mice following transient MCAO supports the potential utility of therapeutic AQP4 inhibition in stroke.

Serial magnetic resonance imaging in experimental primate stroke: Validation of MRI for pre-clinical cerebroprotective trials

Precise assessment of stroke outcome is critical for pre-clinical testing of cerebroprotective strategies. Differences in stroke volume measured by various magnetic resonance imaging (MRI) techniques are documented in humans, but not well described in experimental primate stroke. This study characterizes changes in stroke volume using serial MRI scans in a baboon model of reperfused cerebral ischemia. The location/area of hyperintensity on MRI corresponded with the TTC-stained infarct region. T2-weighted fast spin echo (T2W FSE), fluid attenuated inversion recovery (FLAIR), and diffusion weighted imaging (DWI) showed a decrease in infarct volume between 72 h and nine days post-ischemia (p = ns, p = 0.029, and p = 0.006). T2W FSE and FLAIR demonstrated an increase in infarct volume from 24 h to nine days post-ischemia, while DWI displayed a decrease over the same period. While early T2W FSE, FLAIR, and DWI all correlated with late infarct volume (p < 0.001), 72 h T2W FSE was the best direct measure (2.39% +/- 1.40% unity deviation). Serial MRI in a nonhuman primate model of focal cerebral ischemia recapitulates findings in clinical stroke. MRI at 72 h accurately predicts late infarct volume.

Preventing flow-metabolism uncoupling acutely reduces axonal injury after traumatic brain injury

We have previously presented evidence that the development of secondary traumatic axonal injury is related to the degree of local cerebral blood flow (LCBF) and flow-metabolism uncoupling. We have now tested the hypothesis that augmenting LCBF in the acute stages after brain injury prevents further axonal injury. Data were acquired from rats with or without acetazolamide (ACZ) that was administered immediately following controlled cortical impact injury to increase cortical LCBF. Local cerebral metabolic rate for glucose (LCMRglc) and LCBF measurements were obtained 3 h post-trauma in the same rat via ¹⁸F-fluorodeoxyglucose and ¹⁴C-iodoantipyrine co-registered autoradiographic images, and compared to the density of damaged axonal profiles in adjacent sections, and in additional groups at 24 h used to assess different populations of injured axons stereologically. ACZ treatment significantly and globally elevated LCBF twofold above untreated-injured rats at 3 h (p<0.05), but did not significantly affect LCMRglc. As a result, ipsilateral LCMRglc:LCBF ratios were reduced by twofold to sham-control levels, and the density of β-APP-stained axons at 24 h was significantly reduced in most brain regions compared to the untreated-injured group (p<0.01). Furthermore, early LCBF augmentation prevented the injury-associated increase in the number of stained axons from 3-24 h. Additional robust stereological analysis of impaired axonal transport and neurofilament compaction in the corpus callosum and cingulum underlying the injury core confirmed the amelioration of β-APP axon density, and showed a trend, but no significant effect, on RMO14-positive axons. These data underline the importance of maintaining flow-metabolism coupling immediately after injury in order to prevent further axonal injury, in at least one population of injured axons.

Inhaled nitric oxide improves outcomes after successful cardiopulmonary resuscitation in mice

BACKGROUND

Sudden cardiac arrest (CA) is a leading cause of death worldwide. Breathing nitric oxide (NO) reduces ischemia/reperfusion injury in animal models and in patients. The objective of this study was to learn whether inhaled NO improves outcomes after CA and cardiopulmonary resuscitation (CPR).

METHODS AND RESULTS

Adult male mice were subjected to potassium-induced CA for 7.5 minutes whereupon CPR was performed with chest compression and mechanical ventilation. One hour after CPR, mice were extubated and breathed air alone or air supplemented with 40 ppm NO for 23 hours. Mice that were subjected to CA/CPR and breathed air exhibited a poor 10-day survival rate (4 of 13), depressed neurological and left ventricular function, and increased caspase-3 activation and inflammatory cytokine induction in the brain. Magnetic resonance imaging revealed brain regions with marked water diffusion abnormality 24 hours after CA/CPR in mice that breathed air. Breathing air supplemented with NO for 23 hours starting 1 hour after CPR attenuated neurological and left ventricular dysfunction 4 days after CA/CPR and markedly improved 10-day survival rate (11 of 13; P=0.003 versus mice breathing air). The protective effects of inhaled NO on the outcome after CA/CPR were associated with reduced water diffusion abnormality, caspase-3 activation, and cytokine induction in the brain and increased serum nitrate/nitrite levels. Deficiency of the α1 subunit of soluble guanylate cyclase, a primary target of NO, abrogated the ability of inhaled NO to improve outcomes after CA/CPR.

CONCLUSIONS

These results suggest that NO inhalation after CA and successful CPR improves outcome via soluble guanylate cyclase-dependent mechanisms.

Potassium-selective microelectrode revealed difference in threshold potassium concentration for cortical spreading depression in female and male rat brain

It has been found with potassium-selective microelectrodes that the threshold extracellular concentration of potassium [K(+)](e) for eliciting fast spreading depression from the site of administration 1.5mm away is 80% lower in the rat female cortex (8.0 ± 0.6 mM) than in the male cortex (14.4 ± 0.4mM). The rate of the first slow phase of [K(+)](e) rise before reaching the threshold and speed of SD propagation were similar for both sexes as well as the rise of the second active fast phase of the [K(+)](e) increase. Lower [K(+)](e) threshold in females can facilitate the rapid K(+) release from the nerve tissue cells that may cause a cerebrovascular vasodilatation and the attack of migraine pain.

Neuroimaging study designs, computational analyses and data provenance using the LONI pipeline

Modern computational neuroscience employs diverse software tools and multidisciplinary expertise to analyze heterogeneous brain data. The classical problems of gathering meaningful data, fitting specific models, and discovering appropriate analysis and visualization tools give way to a new class of computational challenges—management of large and incongruous data, integration and interoperability of computational resources, and data provenance. We designed, implemented and validated a new paradigm for addressing these challenges in the neuroimaging field. Our solution is based on the LONI Pipeline environment [3], [4], a graphical workflow environment for constructing and executing complex data processing protocols. We developed study-design, database and visual language programming functionalities within the LONI Pipeline that enable the construction of complete, elaborate and robust graphical workflows for analyzing neuroimaging and other data. These workflows facilitate open sharing and communication of data and metadata, concrete processing protocols, result validation, and study replication among different investigators and research groups. The LONI Pipeline features include distributed grid-enabled infrastructure, virtualized execution environment, efficient integration, data provenance, validation and distribution of new computational tools, automated data format conversion, and an intuitive graphical user interface. We demonstrate the new LONI Pipeline features using large scale neuroimaging studies based on data from the International Consortium for Brain Mapping [5] and the Alzheimer's Disease Neuroimaging Initiative [6]. User guides, forums, instructions and downloads of the LONI Pipeline environment are available at http://pipeline.loni.ucla.edu.

Diffusion tensor MRI for the assessment of cerebral ischemia/reperfusion injury in the penumbra of non-human primate stroke model

OBJECTIVES

The purpose of this study is to use diffusion tensor imaging (DTI) parameters to evaluate cerebral ischemia/reperfusion injury in the infarct core (IC) and ischemic penumbra (IP) in a rhesus transient middle cerebral artery occlusion (MCAO) model.

METHODS

Seven rhesus monkeys were used to construct the MCAO model. The temporal evolution of the relative apparent diffusion coefficient (rADC) and the relative fractional anisotropy (rFA) in the IC area, infarct growth area (IG), and reversible penumbra area (RP) were investigated.

RESULTS

The rADC increased in the three areas in the early stage of reperfusion (1 hour after the reperfusion). However, the rate of rADC improvement was significantly slower in IG than in IC and RP. Different temporal evolutions of rFA were observed in the three areas in the following stage of reperfusion (3-24 hours after the reperfusion), which continued to decline in IG but slightly elevated in IC and RP.

DISCUSSION

These findings suggest that the evolution of DTI parameters can help in the assessment of cerebral ischemia/reperfusion injury in the penumbra and predict the growth of the infarction area after stroke.

Quantified T1 as an adjunct to apparent diffusion coefficient for early infarct detection: a high-field magnetic resonance study in a rat stroke model

BACKGROUND

Thrombolytic treatment for acute stroke has focused attention on accurate identification of injured vs. salvageable brain tissue, particularly if reperfusion occurs. However, our knowledge of differences in acute magnetic resonance imaging changes between transient and permanent ischemia and how they reflect permanently damaged tissue remain incomplete.

AIMS AND/OR HYPOTHESIS

Magnetic resonance imaging characteristics vary widely following ischemia and, at acute times, T1, T2 or apparent diffusion coefficient quantification may differentiate viable tissue from that destined to infarct.

METHODS

High-resolution magnetic resonance imaging was performed at 9.4 T following permanent or transient (90 min) middle cerebral artery occlusion in spontaneously hypertensive male rats or Wistar rats. Within 30 min, quantified maps of the apparent diffusion coefficient, T1, and T2 were performed and measures determined for sequences in the infarct and compared with that in the contralateral region. Lesion area for each magnetic resonance imaging sequence (T1, T2, apparent diffusion coefficient, and perfusion maps) was delineated for different time points using quantitative threshold measures and compared with final histological damage.

RESULTS

Early extensive changes in T1 following both transient and permanent middle cerebral artery occlusion provided a sensitive early indicator of the final infarct area. Following reperfusion, small but measurable early T2 changes indicative of early development of vasogenic edema occurred in the transient but not permanent groups. In transient middle cerebral artery occlusion, at 70 min apparent diffusion coefficient decreased (P<0.001) and then pseudonormalized at 150 min. In permanent middle cerebral artery occlusion, apparent diffusion coefficient declined over time. Lesion area detected using T1 maps exceeded that with T2 and apparent diffusion coefficient at 70 and 150 min in both groups (P<0.001).

CONCLUSIONS

The results indicate that, independent of reperfusion, quantified T1 is superior for detecting early ischemic changes that are not necessarily detected with T2 or apparent diffusion coefficient.

Diffusion-weighted MRI of hepatic tumor in rats: comparison between in vivo and postmortem imaging acquisitions

PURPOSE

To determine the feasibility of in vivo diffusion-weighted imaging (DWI) to distinguish between normal liver, viable tumor and necrosis compared to postmortem DWI in a rat model with vascular-targeting treatment.

MATERIALS AND METHODS

Fifteen rats with liver implantation of 30 rhabdomyosarcomas were treated with combretastatin A-4-phosphate (CA4P) at 10 mg/kg. Two days after treatment, T2-weighted imaging, precontrast T1-weighted imaging, postcontrast T1-weighted imaging, and DWI were performed in vivo and postmortem with a 1.5T scanner. Apparent diffusion coefficients (ADCs) calculated from DWIs with b values of 0, 50, and 100 seconds/mm2 (ADClow), 500, 750, and 1000 seconds/mm2 (ADChigh), 0, 500, and 1000 seconds/mm2 (ADC3b), and 0-1000 seconds/mm2 (ADC10b) for tumor, liver, therapeutic necrosis, and phantoms were compared and validated with ex vivo microangiographic and histopathologic findings.

RESULTS

Except ADClow between tumor and necrosis, in vivo ADCs successfully differentiated liver, viable tumor, and necrosis (P<0.05). Compared to in vivo outcomes, postmortem ADCs significantly dropped in tumor and liver (P<0.05) except ADChigh of tumor, but not in necrosis and phantoms. Compared to ADClow, ADChigh was less affected by vital status.

CONCLUSION

Advantageous over postmortem DWI, in vivo DWI provides a noninvasive easy-performing tool for distinguishing between liver, viable tumor, and necrosis. ADClow and ADChigh better reflect tissue perfusion and water diffusion, respectively.

Focal reversible deactivation of cerebral metabolism affects water diffusion

The underlying biophysical mechanisms which affect cerebral diffusion contrast remain poorly understood. We hypothesized that cerebral metabolism may affect cerebral diffusion contrast. The purpose of this study was to develop the methodology to reversibly deactivate cerebral metabolism and measure the effect on the diffusion MRI signal. We developed an MRI-compatible cortical cooling system to reversibly deactivate cortical metabolism in rhesus monkey area V1 and used MR thermometry to calculate three-dimensional temperature maps of the brain to define the extent of deactivated brain in vivo. Significant changes in the apparent diffusion coefficient (ADC) were only observed during those experiments in which the cortex was cooled below the metabolic cutoff temperature of 20 degrees C. ADC decreases (12-20%) were observed during cortical cooling in regions where the temperature did not change. The normalized in vivo ADC as function of temperature was measured and found to be equivalent to the normalized ADC of free water at temperatures above 20 degrees C, but was significantly decreased below 20 degrees C (20-25% decrease). No changes in fractional anisotropy were observed. In future experiments, we will apply this methodology to quantify the effect of reversible deactivation on neural activity as measured by the hemodynamic response and compare water diffusion changes with hemodynamic changes.

Imaging experimental cerebral malaria in vivo: significant role of ischemic brain edema

The first in vivo magnetic resonance study of experimental cerebral malaria is presented. Cerebral involvement is a lethal complication of malaria. To explore the brain of susceptible mice infected with Plasmodium berghei ANKA, multimodal magnetic resonance techniques were applied (imaging, diffusion, perfusion, angiography, spectroscopy). They reveal vascular damage including blood-brain barrier disruption and hemorrhages attributable to inflammatory processes. We provide the first in vivo demonstration for blood-brain barrier breakdown in cerebral malaria. Major edema formation as well as reduced brain perfusion was detected and is accompanied by an ischemic metabolic profile with reduction of high-energy phosphates and elevated brain lactate. In addition, angiography supplies compelling evidence for major hemodynamics dysfunction. Actually, edema further worsens ischemia by compressing cerebral arteries, which subsequently leads to a collapse of the blood flow that ultimately represents the cause of death. These findings demonstrate the coexistence of inflammatory and ischemic lesions and prove the preponderant role of edema in the fatal outcome of experimental cerebral malaria. They improve our understanding of the pathogenesis of cerebral malaria and may provide the necessary noninvasive surrogate markers for quantitative monitoring of treatment.

Is anoxic depolarisation associated with an ADC threshold? A Markov chain Monte Carlo analysis

A Bayesian nonlinear hierarchical random coefficients model was used in a reanalysis of a previously published longitudinal study of the extracellular direct current (DC)-potential and apparent diffusion coefficient (ADC) responses to focal ischaemia. The main purpose was to examine the data for evidence of an ADC threshold for anoxic depolarisation. A Markov chain Monte Carlo simulation approach was adopted. The Metropolis algorithm was used to generate three parallel Markov chains and thus obtain a sampled posterior probability distribution for each of the DC-potential and ADC model parameters, together with a number of derived parameters. The latter were used in a subsequent threshold analysis. The analysis provided no evidence indicating a consistent and reproducible ADC threshold for anoxic depolarisation.

Evolution of apparent diffusion coefficient and transverse relaxation time (T2) in the subchronic stage of global cerebral oligemia in different rat models

Using magnetic resonance imaging techniques, we examined the time course of apparent diffusion coefficient (ADC), T2, and T2* relaxation times in 1-year-old rats after different forms of cerebral oligemia had been induced by (1) transient systemic hypotension, (2) permanent bilateral carotid artery occlusion (BCCAO), and (3) combined hypotension and BCCAO over a time period of 14 days after the oligemic event. These groups were compared with a group of sham-operated adult rats (controls, 4) to rule out a drift of the parameters over time. The animals were examined in a 2.35 T scanner. ADC, T2, and T2* were measured in both hemispheres of rat parietotemporal cerebral cortex, thalamic nuclei, and hippocampus 1 day before as well as on days 1, 3, 7, and 14 after sham operation and in different models of oligemia, respectively. Hypotension alone had no significant effect on MRI parameters in rat brain. After BCCAO, an increase in T2* was observed. If a permanent BCCAO was combined with transient hypotension, however, 84% of 1-year-old animals died within 14 days after surgery. In the surviving animals, significant changes in ADC, T2, and T2* were observed in the hippocampus and parietotemporal cerebral cortex. ADC showed a decrease on day 1 after oligemia, and an increase on days 3, 7, and 14. The T2* and T2 values were markedly increased on days 7 and 14 after surgery. In conclusion, only severe oligemia combining BCCAO and hypotension induces significant changes in tissue integrity (as shown by ADC) and in blood oxygenation levels in the subchronic period, whereas no significant changes were detected if permanent BCCAO or transient hypotension was applied separately.

Cell-permeant calcium buffer induced neuroprotection after cortical devascularization

An excitotoxic cascade resulting in a significant intracellular calcium load is thought to be a primary mechanism leading to neuronal death after ischemia. One way to protect neurons from injury is through the use of cell-permeant calcium buffers. These molecules have been reported to be neuroprotective via their ability to increase the cell's overall Ca(2+) buffering load as well as by attenuating neurotransmitter release. However, their efficacy when given after injury has yet to be determined. We used diffusion-weighted magnetic resonance imaging (DWI), histological, and immunohistochemical methods to determine the neuroprotective efficacy of 2-aminophenol-N, N, O-triacetic acid acetoxymethyl ester (APTRA-AM) after focal cerebral ischemia. Injured animals were given two injections of APTRA-AM at 1 and 12 h after injury. Animals were imaged prior to injury and then at 12, 24, 48 h and 3 and 7 days after injury. After 7 days the animals were euthanized for correlative cresyl violet histology and immunohistochemistry. Injury resulted in a decrease in the apparent diffusion coefficient (ADC) of the injured area within the first 12 h of injury, which returned to normal by 7 days. In contrast, animals injected with APTRA-AM showed no significant change in the ADC at any time point studied. Tissue analysis showed that APTRA-AM significantly reduced the infarct size by 85% and extent of inflammatory cell infiltration by 94%. The results clearly demonstrate significant neuroprotection by APTRA-AM when given after injury.

Improvements in the intraluminal thread technique to induce focal cerebral ischaemia in rabbits

Although the intraluminal thread technique has been used to induce focal cerebral ischaemia in rabbits, its success rate is not high. We, therefore, attempted to improve the stability and reproducibility of this method by using thread tips of appropriate diameter as determined from the anatomical characteristics of the carotid and cerebral arteries of New Zealand white rabbits. Following intraarterial injection of casting material, we tested threads of four different tip diameters to determine the optimal thread tip that could occlude the middle cerebral artery (MCA). 2,3,5-Triphenyltertrazolium chloride (TTC) staining showed that, consistent with the intraluminal diameter of the MCA from the arterial casts ( 0.50 +/- 0.06 mm), thread with 0.51-0.55 mm tip diameter was optimal for the occlusion of the MCA. Ability to induce focal cerebral ischaemia was also dependent on variations in the anatomy of the internal carotid artery (ICA), especially the origin of the occipital artery. Our results suggest that use of appropriately sized thread and accurate manipulation of its tip significantly improves the stability and reproducibility of this model.

Middle cerebral artery occlusion during MR-imaging: investigation of the hyperacute phase of stroke using a new in-bore occlusion model in rats

Magnetic resonance imaging (MRI) provides insights into the dynamics of focal cerebral ischemia. Usually, experimental stroke is induced outside the magnet bore, preventing investigators from acquiring pre-ischemic images for later pixel-by-pixel comparisons and from studying the earliest changes in the hyperacute phase of ischemia. Herein, we introduce a new and easy to apply in-bore occlusion protocol based on the intraarterial embolization of ceramic macrospheres. PE-50 tubing, filled with saline and six macrospheres (0.315-0.355 mm in diameter), was placed into the internal carotid artery (ICA) of anesthetized Sprague-Dawley rats. The animals were transferred into an MRI scanner (7.0 T) and baseline diffusion-weighted imaging (DWI) and T2-imaging was performed. Then the macrospheres were injected into the internal artery to occlude the MCA. Post-ischemic DWI and T2-imaging was started immediately thereafter. The apparent diffusion coefficient (ADC) (a marker for cytotoxic brain edema) and T2-relaxation time (a marker for vasogenic brain edema) were determined in the ischemic lesions and compared to the unaffected hemisphere. ADC significantly declined within the first 5-10 min after stroke onset. T2-relaxation time increased as early as at the first T2-imaging time-point (20-35 min after embolization). After 150 min of ischemia, the lesions covered 18.0 +/- 7.4% of the hemispheres. The model failed in one out of nine animals (11%). This model allows MR-imaging from the initial minutes after permanent middle cerebral artery (MCA) occlusion. It does not permit reperfusion. This technique might provide information about the pathophysiological processes in the hyperacute phase of stroke.

Temporal relation between the ADC and DC potential responses to transient focal ischemia in the rat: a Markov chain Monte Carlo simulation analysis

Markov chain Monte Carlo simulation was used in a reanalysis of the longitudinal data obtained by Harris et al. (J Cereb Blood Flow Metab 20:28-36) in a study of the direct current (DC) potential and apparent diffusion coefficient (ADC) responses to focal ischemia. The main purpose was to provide a formal analysis of the temporal relationship between the ADC and DC responses, to explore the possible involvement of a common latent (driving) process. A Bayesian nonlinear hierarchical random coefficients model was adopted. DC and ADC transition parameter posterior probability distributions were generated using three parallel Markov chains created using the Metropolis algorithm. Particular attention was paid to the within-subject differences between the DC and ADC time course characteristics. The results show that the DC response is biphasic, whereas the ADC exhibits monophasic behavior, and that the two DC components are each distinguishable from the ADC response in their time dependencies. The DC and ADC changes are not, therefore, driven by a common latent process. This work demonstrates a general analytical approach to the multivariate, longitudinal data-processing problem that commonly arises in stroke and other biomedical research.

Cyclooxygenase-2 mRNA expression is associated with c-fos mRNA expression and transient water ADC reduction detected with diffusion MRI during acute focal ischemia in rats

Cyclooxygenase-2 (COX-2) plays an important role in the development of injury during cerebral ischemia and inhibition of its activity can reduce infarct size. COX-2 expression during acute ischemia is caused by activation of post-synaptic glutamate receptors, which occurs during spreading depression and ischemic depolarization. Both of these phenomena cause a reduction in the apparent diffusion coefficient of water (ADC), which can be detected with diffusion-weighted magnetic resonance imaging. The reduction is believed to be caused by cellular swelling that occurs as cells depolarize. The goal of this work was to determine the spatial relationship between cyclooxygenase-2 mRNA (cox-2) expression, c-fos mRNA expression and ADC reduction during acute focal cerebral ischemia. Adult rats were subjected to either 30- or 60-min permanent occlusion of the middle cerebral artery. A 2-Tesla scanner was used to acquire diffusion-weighted echo-planar images throughout the ischemic period, which were used to calculate ADC maps. Cox-2 and c-fos mRNA were detected with (35)S in situ hybridization. The results indicate that, for rats subjected to 60-min ischemia, cox-2 was observed in superficial layers of cortex, where transient ADC reduction and c-fos expression were observed. The same was true for most rats subjected to 30-min ischemia. However, in a small number of rats of the 30-min group, cox-2 mRNA expression was observed in regions exhibiting transient and persistent ADC reduction with no c-fos expression. The results suggest that cox-2 mRNA expression during acute MCA occlusion is caused by either or both spreading depression and transient ischemic depolarization.

In vivo excitotoxicity induced by ouabain, a Na+/K+-ATPase inhibitor

The susceptibility of immature rat brain to neurotoxicity of N-methyl-D-aspartate (NMDA) has provided a widely used paradigm to study excitotoxicity relevant to acute neurodegenerative diseases such as cerebral ischemia. In this study, excitotoxicity was induced via injection of ouabain (1 mM/0.5 microL), a Na+/K+ -ATPase-inhibitor, into neonatal rat brain and compared with NMDA injection. The aim of the study was to induce excitotoxicity secondary to cellular membrane depolarization, thereby more closely mimicking the pathophysiologic processes of ischemia-induced brain injury where NMDA-receptor overstimulation by glutamate follows, not precedes, membrane depolarization. Na+/K+ -ATPase-inhibition caused an acute, 40% +/- 8% decrease of the apparent diffusion coefficient (ADC) of water, as measured using diffusion-weighted magnetic resonance imaging (MRI), and resulted in infarctlike lesions as measured using T2-weighted MRI and histology up to 2 weeks later. Localized one- and two-dimensional 1H-magnetic resonance spectroscopy (MRS) demonstrated that the early excitotoxic diffusion changes were not accompanied by an overall metabolic disturbance. Furthermore, 31P-MRS demonstrated that energy depletion is not a prerequisite for ADC decrease or excitotoxic cell death. Treatment with the NMDA-antagonist MK-801 (1 mg/kg) attenuated the volume of tissue exhibiting a decreased ADC (P < 0.005), demonstrating that the ouabain-induced injury is indeed excitotoxic in nature. The authors argue that, compared with NMDA-injection, ouabain-induced excitotoxicity elicits more appropriate glutamate-receptor overstimulation and is better suited to detect relevant neuroprotection in that it is more sensitive to attenuation of synaptic glutamate levels.

Temporal profile of changes in brain tissue extracellular space and extracellular ion (Na(+), K(+)) concentrations after cerebral ischemia and the effects of mild cerebral hypothermia

Cerebral ischemic cellular swelling occurs primarily in astrocytes. This water influx into the intracellular space is believed to result from osmotic water movement after disruption of membrane ionic homeostasis. However, cellular swelling occurs earlier than expected after ischemia and new ionic and water channels have been discovered. This study examined the temporal profile of the water and ionic movement across the cell membrane after global ischemia by measuring the changes in extracellular space (ECS), extracellular K(+) and Na(+) ion concentrations ([K(+)](e) and [Na(+)](e)) using a high resolution tissue impedance probe and ion selective micropipettes in the rat cortex. The effect of mild cerebral hypothermia (31.5 +/- 2.6 degrees C brain temperature) on these parameters was also examined. The ECS started to decrease at 34 +/- 8 sec after global ischemia and reached half the maximum change at 61 +/- 17 sec. [K(+)](e) started to increase initially at 33 +/- 11 sec (phase 1) and then increased rapidly at 62 +/- 25 sec (phase 2), and [Na(+)](e) started to decrease at 88 +/- 27 sec after ischemia. With mild hypothermia, the ECS started to decrease at 75 +/- 35 sec after ischemia and reached half the maximum change at 123 +/- 44 sec, [K(+)](e) started to increase initially at 80 +/- 24 sec (phase 1) and then increased rapidly at 120 +/- 32 sec (phase 2), and [Na(+)](e) started to decrease at 172 +/- 70 sec. The present study shows that ischemic cellular swelling (decreased ECS) occurs concomitantly with the phase 1 increase of [K(+)](e) but precedes the disruption of ionic membrane homeostasis (phase 2). Mild hypothermia prolongs the onset of these phenomena but does not affect the magnitude of the changes in ECS and ion concentrations.

Transgenic mice expressing human copper-zinc superoxide dismutase exhibit attenuated apparent diffusion coefficient reduction during reperfusion following focal cerebral ischemia

Since ADC reduction reflects intracellular edema which is an early indicator of ischemic cellular metabolic stress, we hypothesized that a decrease in ADC as determined by diffusion weighted MR imaging could be attenuated by SOD expression in transgenic mice during reperfusion following focal cerebral ischemia. Diffusion weighted imaging (DWI) was performed to evaluate apparent diffusion coefficient (ADC) reduction by constructing ADC maps with a color scale to localize ADC change in transgenic (Tg) mice expressing human CuZn superoxide dismutase (SOD) and wild type (Wt) mice during 1 h middle cerebral artery occlusion (MCAO) and 1 h reperfusion. Heat shock protein (hsp) 70-kDa mRNA analysis was evaluated as a marker of sublethal cell stress by in situ hybridization after 4 h reperfusion for comparison with Nissl staining of adjacent sections to assess infarction. Sequential ADC maps were prepared in Tg mice with sufficient temporal and spatial resolution to permit comparison with Wt mice. Tg mice showed substantial recovery of the ADC lesion after reperfusion, while Wt mice showed no recovery. There was no difference between Tg and Wt mice in the size or distribution of the ADC lesion during ischemia. The area with strong expression of hsp70 mRNA in the ischemic hemisphere was substantially larger in the Tg mice. Nissl staining showed less damage of brain tissue in Tg mice than Wt mice especially in the cortex after 4 h reperfusion following 1 h MCAO. Results demonstrate that antioxidant effects of human CuZn-SOD reduce cellular edema due to oxidative stress during reperfusion but not during ischemia after 1 h MCAO. Hsp70 could be one of the proteins that mediates protection by SOD against oxidative stress.

Cerebral ischemia and trauma-different etiologies yet similar mechanisms: neuroprotective opportunities

Cerebral ischemia leads to brain damage caused by pathogenetic mechanisms that are also activated by neurotrauma. These mechanisms include among others excitotoxicity, over production of free radicals, inflammation and apoptosis. Furthermore, cerebral ischemia and trauma both trigger similar auto-protective mechanisms including the production of heat shock proteins, anti-inflammatory cytokines and endogenous antioxidants. Neuroprotective therapy aims at minimizing the activation of toxic pathways and at enhancing the activity of endogenous neuroprotective mechanisms. The similarities in the damage-producing and endogenous auto-protective mechanisms may imply that neuroprotective compounds found to be active against one of these conditions may indeed be also protective in the other. This review summarizes the pathogenetic events of ischemic and traumatic brain injury and reviews the neuroprotective strategies employed thus far in each of these conditions with a special emphasize on their clinical relevance and on future directions in the field of neuronal protection.

Transient hypoxia-ischemia in rats: changes in diffusion-sensitive MR imaging findings, extracellular space, and Na+-K+ -adenosine triphosphatase and cytochrome oxidase activity

PURPOSE

To investigate the correlation between diffusion-weighted (DW) magnetic resonance (MR) image changes with alterations in extracellular volume and changes in cytochrome oxidase and Na(+)-K(+)-adenosine triphosphatase (ATPase) activity at various times during and after cerebral hypoxia-ischemia in neonatal and juvenile rats.

MATERIALS AND METHODS

One- and 4-week-old rats were randomly assigned to control or transient cerebral hypoxia-ischemia (ie, right carotid artery occlusion plus exposure to 8% oxygen) groups. Hypoxic-ischemic changes compared with normal ipsilateral brain tissue on DW images and the apparent diffusion coefficient of water were measured during and at 1 and 24 hours after hypoxia-ischemia ended. Hypoxic-ischemic changes in extracellular space and ipsilateral versus contralateral differences in Na(+)-K(+)-ATPase and cytochrome oxidase activity were measured.

RESULTS

Hyperintensities on DW images obtained during hypoxia-ischemia correlated well (P <.05) with extracellular space reductions, which occurred 15 minutes earlier in the brains of 4-week-old rats than in the brains of 1-week-old rats. Similarly, within 1 hour after hypoxia-ischemia ended, DW image and extracellular space changes normalized. In contrast, Na(+)-K(+)-ATPase and cytochrome oxidase activity decreased in some regions during hypoxia-ischemia and remained reduced 1 hour after the end of hypoxia-ischemia. Twenty-four hours after signal intensity normalization, hyperintense areas reappeared on DW images, and Na(+)-K(+)-ATPase and cytochrome oxidase activity remained decreased.

CONCLUSION

Signal intensity alterations with diffusion-sensitive MR imaging during and after transient hypoxia-ischemia are closely associated with a corresponding shrinkage and reexpansion of the extracellular space, irrespective of age. Mechanisms other than Na(+)-K(+)-ATPase changes may induce the early cell volume changes detected with diffusion-sensitive MR imaging.

MRI measurement of blood-brain barrier permeability following spontaneous reperfusion in the starch microsphere model of ischemia

Quantification of the acute increases in blood-brain barrier (BBB) permeability that occur subsequent to experimental ischemic injury has been limited to single time-point, invasive methodologies. Although permeability can be qualitatively assessed to visualise regional changes during sequential studies on the same animal using contrast-enhanced magnetic resonance imaging (MRI), quantitative information on the magnitude of change is required to compare barrier function during sequential studies on the same animal or between different animals. Recently, improvements in MRI tracer kinetic models and in MR hardware design mean that an estimate of permeability in vivo can now be obtained with acceptable accuracy and precision. We report here the use of such methods to study acute changes following spontaneous reperfusion in an animal model of ischemia. We have obtained estimates of BBB permeability following spontaneous reperfusion, subsequent to forebrain ischemia by unilateral carotid injection of starch microspheres in the rat. T2*-weighted and diffusion-trace imaging were used to monitor the initial reduction in CBF and the time-course of ischemia, respectively. Following reperfusion, an intraveneous bolus of dimeglumine gadopentetate (Gd-DTPA) and horseradish peroxidase (HRP) was given during a continuous acquisition of T1 maps with a 48 s temporal resolution. Permeability maps were constructed using a 4-compartment model; K(trans), the permeability-surface area product of the capillary walls was estimated to be 9.2 +/- 0.6 x 10(-4) min(-1) in the cortex. Visualisation of the regional extent of HRP extravasation on histological sections following termination of the experiment demonstrated very little correspondence to the region of Gd-DTPA leakage. Quantitative MRI assessment of BBB permeability following ischemia-reperfusion is consistent with published values obtained by invasive methods. Differences between Gd-DTPA-enhancement and HRP may reflect differences in the molecular size of the tracers.

Application of magnetic resonance to animal models of cerebral ischemia

The present review has been compiled to highlight the role of magnetic resonance imaging (MRI) and MR spectroscopy (MRS) for the investigation of cerebral ischemia in the animal experimental field of basic research. We have focused on stroke investigations analyzing the pathomechanisms of the disease evolution and on new advances in both nuclear MR (NMR) methodology or genetic engineering of transgenic animals for the study of complex molecular relationships and causes of the disease. Furthermore, we have tried to include metabolic and genetic aspects, as well as the application of functional imaging, for the investigation of the disturbance or restitution of functional brain activation under pathological conditions as relates to controlled animal experiments.

Complementary emerging techniques: high-resolution PET and MRI

Noninvasive imaging technologies provide a unique window on the anatomy, physiology and function of living organisms. Imaging systems and methods have been developed for the study of small animal model systems that offer exciting new possibilities in neuroscience. Advances in magnetic resonance microscopy and positron emission tomography, and their applications in brain imaging, have provided many benefits to neurobiology, ranging from detailed in vivo neuroanatomy to the measurement of specific molecular targets.

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.

Possible control of intermittent cerebral ischemia by monitoring of direct-current potentials

OBJECT

Neurosurgically induced temporary occlusion of intracranial arteries carries the risk of cerebral ischemic damage. Because negative shifts in the cortical direct-current (DC) potential indicate tissue depolarization and, thus, critical ischemic stress, the authors hypothesized that recordings of these potentials could help to determine the optimal duration and frequency of induced intermittent focal ischemia to prevent brain injury. The investigators related the results of DC recordings both to simultaneously recorded decreases in extracellular Ca++ concentration ([Ca++]o), which reflect Ca++ entry into cells, and to histological outcome.

METHODS

In cats anesthetized with halothane the effects of intermittent brief (10 minutes long, six times [6 x 10-min group]) and prolonged (20 minutes long, three times [3 x 20-min group]) episodes of middle cerebral artery occlusions were compared with those of a single continuous episode (1 x 60-min group). Laser Doppler flow probes and ion-selective microelectrodes were used to measure cerebral blood flow, DC potentials, and [Ca++]o in cortical tissues of ectosylvian gyri. Negative shifts in DC potential were evaluated in the three groups during the entire 60-minute-long period of ischemia and were smallest in the 6 x 10-min group, larger in the 3 x 20-min group, and largest in the 1 x 60-min group. Accordingly, infarct volumes were smallest in the 6 x 10-min group, intermediate in the 3 x 20-min group, and largest in the 1 x 60-min group. Decreases in ischemic [Ca++]o were significantly greater in the 1 x 60-min group than in the two groups in which there were repetitive occlusions, and recovery of [Ca++]o after reperfusion normalized only in the 1 x 60-min group.

CONCLUSIONS

The DC potential may provide a reliable measure to optimize intermittent ischemia and to achieve minimal ischemic brain injury during temporary neurosurgical occlusion of cerebral arteries.

Vasospasm after subarachnoid hemorrhage: interest in diffusion-weighted MR imaging

BACKGROUND AND PURPOSE

Vasospasm secondary to subarachnoid hemorrhage (SAH) is responsible for severe ischemic complications. Although effective, angioplasty must be performed at a very early stage to produce any clinical recovery. Diagnostic investigations to assess arterial narrowing (transcranial Doppler, angiography) or cerebral perfusion (xenon CT, single-photon emission CT) do not provide evidence of the extent of parenchymal ischemia. In stroke, diffusion-weighted MR imaging (DWI) appears to be the most sensitive procedure to detect cerebral ischemia. We studied asymptomatic vasospasm in patients with aneurysmal SAH to assess whether DWI provides predictive markers of silent ischemic lesions and/or progression toward symptomatic ischemia.

METHODS

Seven asymptomatic vasospasm patients (average blood velocity rates >120 cm/s), 3 patients with symptomatic vasospasm, and 4 patients with SAH but without vasospasm were studied at regular intervals by DWI, and their apparent diffusion coefficients (ADCs) were calculated.

RESULTS

All patients with vasospasm including those without symptoms presented abnormalities on DWI with a reduction of the ADC prevalently in the white matter. No such abnormalities were observed in patients without vasospasm. The abnormalities on DWI resolved completely in 4 of the 7 patients, with no parenchymal lesion. Resolution was partial in 3 patients whose white matter still presented residual round, focal ischemic lesions.

CONCLUSIONS

Being able to correlate abnormalities on DWI with parenchymal involvement in asymptomatic patients would be of considerable clinical significance. It is hoped that larger studies will be undertaken to determine whether the ADC has a reversibility threshold, because this would facilitate patient management.

The effects of microscopic tissue parameters on the diffusion weighted magnetic resonance imaging experiment

This review examines the way in which microscopic tissue parameters can affect MR experiments which are sensitive to diffusion. The interaction between the intra- and extravascular as well as that between the intra- and extracellular spaces is examined. Susceptibility gradients due to the presence of deoxyhemoglobin can cause diffusion-induced signal losses which are significant in functional magnetic resonance experiments, particularly at higher main magnetic field strengths. This is also true of the fast response that manifests itself as an early negative signal change in functional magnetic resonance experiments. The fields surrounding paramagnetic vessels are described and the way in which diffusion in these fields contributes to functional signal changes is examined. Flow in the capillary bed can be a confounding factor in experiments which aim to examine the diffusion characteristics of extravascular water. It is potentially also a method for assessing capillary perfusion. The intravoxel incoherent motion experiment is described in terms of how significantly this effect can influence diffusion attenuation curves from water. The major models for describing water diffusion in tissue are presented, as are the main experimental results that have contributed to an understanding of the mechanisms of diffusion contrast. The widely accepted view that changes in the diffusion characteristics are caused by a shift of water to the intracellular space and a concomitant change in extracellular tortuosity is examined critically. More recent experiments that indicate that a reduction in the intracellular diffusion may occur simultaneously with the cell swelling are described and their compatibility with existing models discussed.

Relation of apparent diffusion coefficient changes and metabolic disturbances after 1 hour of focal cerebral ischemia and at different reperfusion phases in rats

Changes in apparent diffusion coefficients (ADC) were compared with alterations of adenosine triphosphate (ATP) concentration and pH in different phases of transient focal cerebral ischemia to study the ADC threshold for breakdown of energy metabolism and tissue acidosis during ischemia and reperfusion. Male Wistar rats underwent 1 hour of middle cerebral artery occlusion without recirculation (n = 3) or with 1 hour (n = 4) or 10 hours of reperfusion (n=5) inside the magnet, using a remotely controlled thread occlusion model. ADC maps were calculated from diffusion-weighted images and normalized to the preischemic value to obtain relative ADC maps. Hemispheric lesion volume (HLV) was determined on the last relative ADC maps at different relative ADC thresholds and was compared to the HLV measured by ATP depletion and by tissue acidosis. The HLVs, defined by ATP depletion and tissue acidosis, were 26.0% +/- 10.6% and 38.1% +/- 6.5% at the end of ischemia, 3.3% +/- 2.4% and 4.8% +/- 3.5% after 1 hour of reperfusion, and 11.2% +/- 4.7% and 10.9% +/- 5.2% after 10 hours of recirculation, respectively. The relative ADC thresholds for energy failure were consistently approximately 77% of the control value in the three different groups. The threshold for tissue acidosis was higher at the end of ischemia (86% of control) but was similar to the results obtained for ATP depletion after 1 hour (78% of control) and 10 hours (76% of control) of recirculation. These results indicate that the described relative ADC threshold of approximately 77% of control provides a good estimate for the breakdown of energy metabolism not only during middle cerebral artery occlusion but also at the early phase of reperfusion, when recovery of energy metabolism is expected to occur, or some hours later, when development of secondary energy failure was described.

Cerebrovascular reactivity following focal brain ischemia in the rat: a functional magnetic resonance imaging study

An essential goal of stroke research is to identify potentially salvageable regions of brain that may respond to therapy. However, current imaging methods are inadequate for this purpose. We therefore used dynamic magnetic resonance imaging of vascular reactivity following focal occlusion in the rat to determine whether measurement of perfusion reserve would help resolve this problem. We used the increase in blood-oxygen-level-dependent (BOLD) signal that occurs in normal brain following a CO2 challenge, to map vascular reactivity over the brain at 30-min intervals for 3.5 h after complete (CO) or partial (PO) focal ischemia. We assessed the regional correspondence between reactivity changes and areas of lowered apparent diffusion coefficient (ADC) and initial perfusion deficit. The area of lowered ADC was significantly smaller in the PO group compared to the CO group despite similar areas of perfusion deficit (P < 0.05). We identified four distinct areas within hypoperfused brain: a core area with low/absent reactivity and low ADC; borderzone areas with normal reactivity and either reduced ADC (CO group) or normal ADC (PO group); and an area with normal ADC and reduced/absent reactivity. In all ischemic regions, the BOLD peak arrival time in the brain was delayed or absent. There was a negative correlation between BOLD peak latency time and ADC (r = -0.42, P < 0.001), although latency alone did not differentiate individual ischemic regions. In conclusion, combining perfusion, ADC, and vascular reactivity mapping of the ischemic brain enables improved discrimination of core and borderzone regions.

Comparison of diffusion, blood oxygenation, and blood volume changes during global ischemia in rats

Rapid diffusion, blood oxygenation, and blood volume weighted echo planar imaging was used to monitor global cerebral ischemia by cardiac arrest in rats. Serial CBV measurements used intravascular iron oxide contrast media (iron dextran). ADC dropped by 5% within 20 sec of cardiac arrest, then continued to decay slowly until a larger rapid drop after 2 min. After iron oxide injection, the initial 5% drop was not observed. The transverse relaxation rate (R(2), R(*)(2) no iron injection) increased rapidly after cardiac arrest, peaking at about 30 sec, then declining towards baseline. The CBV dropped by about 50% within 20 sec. The initial 5% ADC drop may be a vascular artifact. The rapidity of the CBV-weighted signal drop suggests a flow-mediated contribution to the iron oxide contrast mechanism. Magn Reson Med 45:10-16, 2001.

The role of neuroimaging in selecting treatments for patients with acute stroke

The armamentarium available for treating acute stroke patients is growing as clinical trials show benefit of intravenous thrombolysis, intra-arterial clot lysis, and antiplatelet agents. Patients with dissection or severe atheromatous stenosis of major cerebrovascular vessels are commonly treated with anticoagulation to prevent recurrent artery-to-artery embolus or arterial thrombosis. These advances in acute stroke treatments demand an accurate means to quickly identify those patients most likely to benefit (or not benefit) from a specific therapy. Fortunately, advances in imaging cerebrovascular lesions, decreased brain perfusion, and even ischemic tissue injury now make it possible to consider tailoring therapy to the individual patient's cerebrovascular problem. Experience and controlled clinical data in this endeavor is meager. Here we describe the ability of various emergency neuroimaging tools to provide information on the state of brain blood flow, metabolism, and vascular anatomy. Most importantly, we present the rationale and limited available evidence relevant to how the neuroimaging information might be used to select optimal treatments for individual patients.

Effect of a neuronal sodium channel blocker on magnetic resonance derived indices of brain water content during global cerebral ischemia

Diffusion-weighted magnetic resonance imaging (DWI) with calculation of the apparent diffusion coefficient (ADC) of water is a widely used noninvasive method to measure movement of water from the extracellular to the intracellular compartment during cerebral ischemia. Lamotrigine, a neuronal Na(+) channel blocker, has been shown to attenuate the increase in extracellular concentrations of excitatory amino acids (EAA) during ischemia and to improve neurological and histological outcome. Because of its proven ability to reduce EAA levels during ischemia, lamotrigine should also minimize excitotoxic-induced increases in intracellular water content and therefore attenuate changes in the ADC. In this study, we sought to determine the effect of lamotrigine on intra- and extracellular water shifts during transient global cerebral ischemia. Fifteen New Zealand white rabbits were anesthetized and randomized to one of three groups: a control group, a lamotrigine-treated group, or a sham group. After being positioned in the bore of the magnet, a 12-min 50-s period of global cerebral ischemia was induced by inflating a neck tourniquet. During ischemia and early reperfusion there was a similar and significant decrease of the ADC in both the lamotrigine and control group. The ADC in the sham ischemia group remained at baseline throughout the experiment. Lamotrigine-mediated blockade of voltage-gated sodium channels did not prevent the intracellular movement of water during 12 min 50 s of global ischemia, as measured by the ADC, suggesting that the ADC decline may not be mediated by voltage-gated sodium influx and glutamate release.

Mild hypothermia decreases the incidence of transient ADC reduction detected with diffusion MRI and expression of c-fos and hsp70 mRNA during acute focal ischemia in rats

The effects of mild hypothermia on the apparent diffusion coefficient of water (ADC) and expression of c-fos and hsp70 mRNA were examined during acute focal cerebral ischemia. Young adult rats were subjected to 60-min middle cerebral artery occlusion under either normothermia (37.5 degrees C) or hypothermia (33 degrees C). Diffusion-weighted echo-planar magnetic resonance imaging was used to monitor changes in ADC throughout the ischemic period. Perfusion MRI with dysprosium contrast was used at the end of the ischemic period to verify that the occlusion was successful. C-fos and hsp70 mRNA expression were examined with in situ hybridization at the end of the ischemic period. The results indicate that the size of the region that exhibited reduced ADC was smaller during hypothermia than during normothermia. Hypothermia also decreased the frequency of occurrence of transient ADC reductions, especially in dorsal aspects of cortex. Expression of both c-fos and hsp70 mRNA were markedly reduced by hypothermia. Transient ADC reduction and c-fos expression are associated with spreading depression, which is believed to contribute to lesion expansion during acute focal ischemia. The results suggest that part of the neuroprotective effect of hypothermia may be due to a reduced incidence of spreading depression.

Magnetic resonance imaging in acute stroke: clinical perspective

The efficacy of intravenous (i.v.) and intra-arterial (i.a.) thrombolysis for hyperacute stroke has made "brain attack" a treatable emergency. The addition of ultrafast magnetic resonance imaging (MRI) to acute stroke investigation has both increased our knowledge of acute stroke pathophysiology and brought a tool to study how to best select patients for thrombolytic therapy. MR offers the three essential components: vascular lesion identification, delineation of injured brain tissue, and map of ischemic brain. MR angiography can demonstrate the site of major cerebral artery occlusion, providing a means to screen for i.a. thrombolysis. Diffusion-weighted imaging (DWI) is capable of showing acute ischemic injury within minutes of symptom onset. Perfusion-weighted imaging (PWI) shows the total area of acute ischemia, more accurately reflecting the extent of neurological dysfunction. Combining DWI and PWI immediately gives information that bears on how much tissue is injured (DWI) and how much tissue is functionally inactive but still viable (ischemic on PWI but still normal on DWI). A number of important questions remain, but current knowledge of natural history of stroke with MRI has provided a framework for comparing new therapeutic interventions. Ideally, patient treatment in the future will be tailored not to a fixed time window but to the physiological state of the ischemic tissue as defined by MRI.

Functional differentiation of multiple perilesional zones after focal cerebral ischemia

Transient and permanent focal cerebral ischemia results in a series of typical pathophysiologic events. These consequences evolve in time and space and are not limited to the lesion itself, but they can be observed in perilesional (penumbra) and widespread ipsi- and sometimes contralateral remote areas (diaschisis). The extent of these areas is variable depending on factors such as the type of ischemia, the model, and the functional modality investigated. This review describes some typical alterations attributable to focal cerebral ischemia using the following classification scheme to separate different lesioned and perilesional areas: (1) The lesion core is the brain area with irreversible ischemic damage. (2) The penumbra is a brain region that suffers from ischemia, but in which the ischemic damage is potentially, or at least partially, reversible. (3) Remote brain areas are brain areas that are not directly affected by ischemia. With respect to the etiology, several broad categories of remote changes may be differentiated: (3a) remote changes caused by brain edema; (3b) remote changes caused by waves of spreading depression; (3c) remote changes in projection areas; and (3d) remote changes because of reactive plasticity and systemic effects. The various perilesional areas are not necessarily homogeneous; but a broad differentiation of separate topographic perilesional areas according to their functional state and sequelae allows segregation into several signaling cascades, and may help to understand the functional consequences and adaptive processes after focal brain ischemia.