Effect of bilateral carotid occlusion on cerebral hemodynamics and perivascular innervation: An experimental rat model.

We aimed to investigate the effect of chronic cerebral hypoperfusion on cerebral hemodynamics and perivascular nerve density in a rat model. Bilateral common carotid artery (CCA) ligation (n = 24) or sham-operation (n = 24) was performed with a 1-week interval. A subgroup (ligated n = 6; sham-operated n = 3) underwent magnetic resonance imaging (MRI) before the procedures and 2 and 4 weeks after the second procedure. After termination, carotids were harvested for assessment of complete ligation and nerve density in cerebral arteries that were stained for the general neural marker PGP 9.5 and sympathetic marker TH by computerized image analysis. Five rats were excluded because of incomplete ligation. MRI-based tortuosity of the posterior communicating artery (Pcom), first part of the posterior cerebral artery (P1) and basilar artery was observed in the ligated group, as well as an increased volume (p = 0.05) and relative signal intensity in the basilar artery (p = 0.04; sham-group unchanged). Immunohistochemical analysis revealed that compared to sham-operated rats, ligated rats had increased diameters of all intracircular segments and the extracircular part of the internal carotid artery (p < 0.05). Ligated rats showed a higher general nerve density compared to controls in P1 (10%, IQR:8.7-10.5 vs. 6.6%, IQR:5.5-7.4, p = 0.003) and Pcom segments (6.4%, IQR:5.8-6.5 vs. 3.2%, IQR:2.4-4.3, p = 0.003) and higher sympathetic nerve density in Pcom segments (3.7%, IQR:2.8-4.8 vs. 1.7%, IQR:1.3-2.2, p = 0.02). Bilateral CCA occlusion resulted in redistribution of blood flow to posteriorly located cerebral arteries with remarkable changes in morphology and perivascular nerve density, suggesting a functional role for perivascular nerves in cerebral autoregulation.

Improved estimation of MR relaxation parameters using complex-valued data.

PURPOSE: In MR image analysis, T1 , T2 , and T2* maps are generally calculated using magnitude MR data. Without knowledge of the underlying noise variance, parameter estimates at low signal to noise ratio (SNR) are usually biased. This leads to confounds in studies that compare parameters across SNRs and or across scanners. This article compares several estimation techniques which use real or complex-valued MR data to achieve unbiased estimation of MR relaxation parameters without the need for additional preprocessing. THEORY AND METHODS: Several existing and new techniques to estimate relaxation parameters using complex-valued data were compared with widely used magnitude-based techniques. Their bias, variance and processing times were studied using simulations covering various aspects of parameter variations. Validation on noise-degraded experimental measurements was also performed. RESULTS: Simulations and experiments demonstrated the superior performance of techniques based on complex-valued data, even in comparison with magnitude-based techniques that account for Rician noise characteristics. This was achieved with minor modifications to data modeling and at computational costs either comparable to or higher ( ≈two fold) than magnitude-based estimators. Theoretical analysis shows that estimators based on complex-valued data are statistically efficient. CONCLUSION: The estimation techniques that use complex-valued data provide minimum variance unbiased estimates of parametric maps and markedly outperform commonly used magnitude-based estimators under most conditions. They additionally provide phase maps and field maps, which are unavailable with magnitude-based methods. Magn Reson Med 77:385-397, 2017. © 2016 Wiley Periodicals, Inc.

Effects of long-term methylphenidate treatment in adolescent and adult rats on hippocampal shape, functional connectivity and adult neurogenesis.

Methylphenidate (MPH) is a widely prescribed stimulant drug for the treatment of attention deficit hyperactivity disorder (ADHD) in children and adolescents. Its use in this age group raises concerns regarding the potential interference with ongoing neurodevelopmental processes. Particularly the hippocampus is a highly plastic brain region that continues to develop postnatally and is involved in cognition and emotional behavior, functions known to be affected by MPH. In this study, we assessed whether hippocampal structure and function were affected by chronic oral MPH treatment and whether its effects were different in adolescent or adult rats. Using behavioral testing, resting-state functional MRI, post-mortem structural magnetic resonance imaging (MRI), and immunohistochemistry, we assessed MPH's effects on recognition memory, depressive-like behavior, topological features of functional connectivity networks, hippocampal shape and markers for hippocampal neurogenesis and proliferation. Object recognition memory was transiently impaired in adolescent treated rats, while in animals treated during adulthood, increased depressive-like behavior was observed. Neurogenesis was increased in adolescent treated rats, whereas cell proliferation was decreased following adult treatment. Adolescent treated rats showed inward shape deformations adjacent to ventral parahippocampal regions known to be involved in recognition memory, whereas such deformations were not observed in adult treated animals. Irrespective of the age of treatment, MPH affected topological features of ventral hippocampal functional networks. Thus, chronic oral treatment with a therapeutically relevant dose of MPH preferentially affected the ventral part of the hippocampus and induced contrasting effects in adolescent and adult rats. The differences in behavior were paralleled by opposite effects on adult neurogenesis and granule cell proliferation.

Longitudinal assessment of blood-brain barrier leakage during epileptogenesis in rats. A quantitative MRI study.

The blood-brain barrier (BBB) plays an important role in the homeostasis of the brain. BBB dysfunction has been implicated in the pathophysiology of various neurological disorders, including epilepsy in which it may contribute to disease progression. Precise understanding of BBB dynamics during epileptogenesis may be of importance for the assessment of future therapies, including BBB leakage blocking-agents. Longitudinal changes in BBB integrity can be studied with in vivo magnetic resonance imaging (MRI) in combination with paramagnetic contrast agents. Although this approach has shown to be suitable to detect major BBB leakage during the acute phase in experimental epilepsy models, so far no studies have provided information on dynamics of the extent of BBB leakage towards later phases. Therefore a sensitive and quantitative approach was used in the present study, involving fast T1 mapping (dynamic approach) during a steady-state infusion of gadobutrol, as well as pre- and post-contrast T1-weighted MRI (post-pre approach). This was applied in an experimental epilepsy model in which previous MRI studies failed to detect BBB leakage during epileptogenesis. Adult male Sprague-Dawley rats were injected with kainic acid to induce status epilepticus (SE). MRI experiments were performed before SE (control) and during the acute (1 day) and chronic epileptic phases (6 weeks after SE). BBB leakage was quantified by fast T1 mapping (Look-Locker gradient echo MRI) with a time resolution of 48 s from 5 min before up to 45 min after 20 min step-down infusion of 0.2M gadobutrol. In addition, T1-weighted MRI was acquired before and 45 min after infusion. MRI data were compared to post-mortem microscopic analysis using the BBB tracer fluorescein. Our MRI data showed BBB leakage, which was evident at 1 day and 6 weeks after SE in the hippocampus, entorhinal cortex, amygdala and piriform cortex. These findings were confirmed by microscopic analysis of fluorescein leakage. Furthermore, our MRI data revealed non-uniform BBB leakage throughout epileptogenesis. This study demonstrates BBB leakage in specific brain regions during epileptogenesis, which can be quantified using MRI. Therefore, MRI may be a valuable tool for experimental or clinical studies to elucidate the role of the BBB in epileptogenesis.

Potentials of magnesium treatment in subarachnoid haemorrhage.

Subarachnoid hemorrhage from a ruptured aneurysm is a subset of stroke. The young age (median 55 years) and poor outcome (50% of patients die; 30% of survivors remain dependent) explain why in the population the loss of productive life years from aneurysmal subarachnoid hemorrhage (SAH) is as large as that from brain infarcts, the most common type of stroke. Ischemia plays an important role in the pathophysiological process after SAH. A period of global cerebral ischemia firstly occurs in the acute phase, immediately after rupture of the aneurysm, due to acute vasoconstriction and elevated intracranial pressure, which leads to a drop in perfusion pressure. This is quite distinct from the secondly, delayed cerebral ischemia (DCI), which is focal or multi-focal. DCI usually occurs between 4 and 10 days after the initial bleeding, has a gradual onset and is multi-focal, and is an important cause of death and dependency after SAH. The interval between the bleeding and the onset of ischemia provides an opportunity for preventive treatment. Magnesium is readily available, inexpensive and has a well-established clinical profile in obstetrical and cardiovascular practice. It is beneficial in the treatment of eclampsia, a disease with a pathophysiology comparable to DCI after subarachnoid hemorrhage. Neuroprotective mechanisms of magnesium include inhibition of the release of excitatory amino-acids and blockade of the NMDA-glutamate receptor. Magnesium is also a non-competitive antagonist of voltage dependent calcium channels, has cerebrovascular dilatory activity and is an important co-factor of cellular ATPases, including the Na/K-ATPase. Magnesium can reverse delayed cerebral vasospasm and reduces the extent of acute ischemic cerebral lesions after experimental subarachnoid hemorrhage in rats. In this article we discuss the neuroprotective potency of magnesium in SAH by describing the pathophysiology of ischaemia after SAH and the many ways magnesium may interfere with this.

Functional magnetic resonance imaging of reorganization in rat brain after stroke.

Functional recovery after stroke has been associated with brain plasticity; however, the exact relationship is unknown. We performed behavioral tests, functional MRI, and histology in a rat stroke model to assess the correlation between temporal changes in sensorimotor function, brain activation patterns, cerebral ischemic damage, and cerebrovascular reactivity. Unilateral stroke induced a large ipsilateral infarct and acute dysfunction of the contralateral forelimb, which significantly recovered at later stages. Forelimb impairment was accompanied by loss of stimulus-induced activation in the ipsilesional sensorimotor cortex; however, local tissue and perfusion were only moderately affected and cerebrovascular reactivity was preserved in this area. At 3 days after stroke, extensive activation-induced responses were detected in the contralesional hemisphere. After 14 days, we found reduced involvement of the contralesional hemisphere, and significant responses in the infarction periphery. Our data suggest that limb dysfunction is related to loss of brain activation in the ipsilesional sensorimotor cortex and that restoration of function is associated with biphasic recruitment of peri- and contralesional functional fields in the brain.

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

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

Diffusion NMR spectroscopy.

MR offers unique tools for measuring molecular diffusion. This review focuses on the use of diffusion-weighted MR spectroscopy (DW-MRS) to non-invasively quantitate the translational displacement of endogenous metabolites in intact mammalian tissues. Most of the metabolites that are observed by in vivo MRS are predominantly located in the intracellular compartment. DW-MRS is of fundamental interest because it enables one to probe the in situ status of the intracellular space from the diffusion characteristics of the metabolites, while at the same time providing information on the intrinsic diffusion properties of the metabolites themselves. Alternative techniques require the introduction of exogenous probe molecules, which involves invasive procedures, and are also unable to measure molecular diffusion in and throughout intact tissues. The length scale of the process(es) probed by MR is in the micrometer range which is of the same order as the dimensions of many intracellular entities. DW-MRS has been used to estimate the dimensions of the cellular elements that restrict intracellular metabolite diffusion in muscle and nerve tissue. In addition, it has been shown that DW-MRS can provide novel information on the cellular response to pathophysiological changes in relation to a range of disorders, including ischemia and excitotoxicity of the brain and cancer.

In vivo glucose detection by homonuclear spectral editing.

A frequency-selective multiple-quantum-coherence spectral editing pulse sequence, Ssel-MQC, was implemented for the detection of the betaH1-glucose resonance at 4.63 ppm in rat brain in vivo. Unwanted signal suppression and glucose coherence transfer pathway selection were performed with magnetic field gradients. To optimize sensitivity, the sequence was executed with surface coil signal reception and adiabatic RF pulse transmission. The glucose editing capabilities of Ssel-MQC were first evaluated in vitro. Ssel-MQC achieved excellent water suppression (suppression factor >10(5)), at the expense of an approximately 60% loss of the glucose signal due to incomplete coherence transfer pathway selection. Next, the sequence was used for in vivo glucose detection in normal rat brain during D-glucose infusion and in the brain of diabetic rats prior to and following insulin infusion.

Correlation between tissue depolarizations and damage in focal ischemic rat brain.

Ischemia-induced depolarizations may play a key role in the development of cerebral ischemic injury. Our goal was to assess the relationship between tissue depolarizations and tissue damage in focal ischemia. We performed multi-electrode cortical direct current (DC) potential recording and, subsequently, diffusion-weighted and T(2)-weighted magnetic resonance imaging (MRI) in rats after i) cortical application of KCl, and ii) permanent and transient middle cerebral artery (MCA)-occlusion in rats. Cortical KCl application induced 10.0+/-2.2 transient negative DC potential shifts per h on the ipsilateral hemisphere (i.e. cortical spreading depressions) (n=4). During 6 h of permanent MCA-occlusion (n=9) 1-10 DC potential shifts were observed, dependent on the brain location. Anoxic depolarization developed in the ischemic core. Outside ischemic areas DC potential shifts resembled cortical spreading depressions. Depolarizations in cortical ischemic borderzones were also transient, but generally long-lasting. Reperfusion induced 1 (n=5) or 3 h (n=6) after MCA-occlusion resulted in repolarization in 2.9+/-1.5 min. Ischemic lesion volumes after 7 h, calculated from diffusion-weighted and T(2)-weighted MR images, correlated significantly with total depolarization time in cortical perifocal zones (R=0.741, p<0.05), but not with the number of depolarizations. The extent of ischemic damage, as measured from alterations in the water diffusion coefficient and T(2), was also significantly related to the total time of depolarization (R=0.762 and 0.738, respectively, p<0.01). We conclude that early ischemic tissue injury is related to the total duration of tissue depolarization and not to the frequency of depolarizations.

Spatial assessment of the dynamics of lactate formation in focal ischemic rat brain.

Early identification of the potentially salvageable penumbra is critical for the determination of therapeutic intervention strategies in acute focal cerebral ischemia. This study differentiates the ischemic penumbra from the core on the basis of the dynamics of lactate formation. This was tested in a rat model of focal cerebral ischemia by infusion of [1-13C]-glucose, using lactate-edited magnetic resonance spectroscopic imaging techniques. The authors detected essentially no enrichment of lactate with 13C-label from the infused 13C-glucose in the ischemic core. However, in borderzone areas, 13C was incorporated into lactate, which could point toward compromised but potentially viable tissue. The authors' findings suggest that this combination of 13C-glucose infusion with the proposed magnetic resonance methods may aid in differentiating the penumbra from the core in cerebral ischemia.

Changes in the diffusion of water and intracellular metabolites after excitotoxic injury and global ischemia in neonatal rat brain.

The reduction of the apparent diffusion coefficient (ADC) of brain tissue water in acute cerebral ischemia, as measured by diffusion-weighted magnetic resonance imaging, is generally associated with the development of cytotoxic edema. However, the underlying mechanism is still unknown. Our aim was to elucidate diffusion changes in the intracellular environment in cytotoxic edematous tissue. The ADC of intracellular metabolites was measured by use of diffusion-weighted 1H-magnetic resonance spectroscopy after (1) unilateral N-methyl-D-aspartate (NMDA) injection and (2) cardiac arrest-induced global ischemia in neonatal rat brain. The distinct water ADC drop early after global ischemia was accompanied by a significant reduction of the ADC of all measured metabolites (P < 0.01, n = 8). In the first hours after excitotoxic injury, the ADC of water and the metabolites taurine and N-acetylaspartate dropped significantly (P < 0.05, n = 8). At 24 and 72 hours after NMDA injection brain metabolite levels were diminished and metabolite ADC approached contralateral values. Administration of the NMDA-antagonist MK-801 1.5 hours after NMDA injection completely normalized the water ADC but not the metabolite ADC after 1 to 2 hours (n = 8). No damage was detected 72 hours later and, water and metabolite ADC had normal values (n = 8). The contribution of brain temperature changes (calculated from the chemical shift between the water and N-acetylaspartate signals) and tissue deoxygenation to ischemia-induced intracellular ADC changes was minor. These data lend support to previous suggestions that the ischemia-induced brain water ADC drop may partly be caused by reduced diffusional displacement of intracellular water, possibly involving early alterations in intracellular tortuosity, cytoplasmic streaming, or intracellular molecular interactions.

Suppression of cortical spreading depressions after magnesium treatment in the rat.

The aim of this study was to investigate whether the neuroprotective properties of magnesium in cerebral ischaemia involve suppression of repetitive tissue depolarizations. Cortical spreading depressions (CSDs), evoked by cortical KCl application, and cardiac arrest-induced anoxic depolarization (AD) were measured by extracellular DC recording on intact rat brain. At 90 min after onset of CSDs saline, MK-801 (3 mg/kg) or MgSO4 (90 mg/kg) was given i.v. Latency time to AD was measured after 4 h. The frequency of CSDs was significantly reduced in animals treated with MgSO4 or MK-801. AD was significantly delayed by MgSO4 but not by MK-801. Our results suggest that suppression of depolarization by magnesium may play a role in its neuroprotective properties in cerebral ischaemia.

Dynamics of cerebral tissue injury and perfusion after temporary hypoxia-ischemia in the rat: evidence for region-specific sensitivity and delayed damage.

BACKGROUND AND PURPOSE: Selective regional sensitivity and delayed damage in cerebral ischemia provide opportunities for directed and late therapy for stroke. Our aim was to characterize the spatial and temporal profile of ischemia-induced changes in cerebral perfusion and tissue status, with the use of noninvasive MRI techniques, to gain more insight in region-specific vulnerability and delayed damage. METHODS: Rats underwent 20 minutes of unilateral cerebral hypoxia-ischemia (HI). We performed combined repetitive quantitative diffusion-weighted, T2-weighted, and dynamic susceptibility contrast-enhanced MRI from before HI to 5 hours after HI. Data were correlated with parallel blood oxygenation level-dependent MRI and laser-Doppler flowmetry. Finally, MRI and histology were done 24 and 72 hours after HI. RESULTS: Severe hypoperfusion during HI caused acute reductions of the apparent diffusion coefficient (ADC) of tissue water in the ipsilateral hemisphere. Reperfusion resulted in dynamic perfusion alterations that varied spatially. The ADC recovered completely within 1 hour in the hippocampus (from 0.68 +/- 0.07 to 0.83 +/- 0.09 x 10[-3] mm2/s), cortex (from 0.56 +/- 0.06 to 0.77 +/- 0.07 x 10[-3] mm2/s), and caudate putamen (from 0.58 +/- 0.06 to 0.75 +/- 0.06 x 10[-3] mm2/s) but only partially or not at all in the thalamus (from 0.65 +/- 0.07 to 0.68 +/- 0.12 x 10[-3] mm2/s) and substantia nigra (from 0.80 +/- 0.08 to 0.76 +/- 0.10 x 10[-3] mm2/s). Secondary ADC reductions, accompanied by significant T2 elevations and histological damage, were observed after 24 hours. Initial and secondary ADC decreases were observed invariably in the hippocampus, cortex, and caudate putamen and in approximately 70% of the animals in the thalamus and substantia nigra. CONCLUSIONS: Region-specific responses and delayed ischemic damage after transient HI were demonstrated by MRI. Acute reperfusion-induced normalization of ADCs appeared to poorly predict ultimate tissue recovery since secondary, irreversible damage developed eventually.

Cerebral ischemia and white matter edema in experimental hydrocephalus: a combined in vivo MRI and MRS study.

T2 and diffusion weighted MRI, as well as 31P and 1H MRS were performed in kaolin-induced hydrocephalic rats. Extracellular white matter edema was detected in the early stages of progressive hydrocephalus. Phosphocreatine (PCr)/inorganic phosphate (Pi) ratios in hydrocephalic animals were decreased compared to controls, and lactate was detected during the acute and chronic stages of hydrocephalus. These MR spectroscopic results are indicative of a compromised energy metabolism and suggest the occurrence of cerebral ischemia in experimental hydrocephalus.

Regional assessment of tissue oxygenation and the temporal evolution of hemodynamic parameters and water diffusion during acute focal ischemia in rat brain.

We assessed the temporal and spatial correlation between perfusion deficits and tissue damage in the first hours of focal cerebral ischemia in the rat. Repetitive dynamic susceptibility contrast-enhanced ('bolus track') and diffusion-weighted (DW) MRI, performed from ca. 0.5 up to 6 h after intraluminal middle cerebral artery occlusion (MCA-O), allowed the determination of the time course of various hemodynamic parameters and ischemic tissue damage in specific brain regions. In addition, blood oxygenation level dependent (BOLD) MRI combined with a respiratory challenge provided complementary information on brain hemodynamics. Within the territory of reduced blood flow, the degree of the hemodynamic disturbances was heterogeneous. Interestingly, the spatial pattern of perfusion deficiencies remained essentially the same from ca. 0.5 to 6 h post-MCA-O. However, the area and the extent of ischemic tissue damage, as expressed by reductions in the apparent diffusion coefficient (ADC) of tissue water, tended to progress with increasing occlusion time. Different ADC profiles correlated with different degrees of hemodynamic disturbances. In the ischemic core, which showed severely compromized perfusion, the ADC dropped significantly within 1 h. In perifocal areas, ADC reductions were delayed and less pronounced. Data from the bolus track and BOLD MRI experiments revealed the existence of residual flow, particularly in perifocal regions. Our data point to a time-dependent change in the relationship between ADC reductions and hemodynamic alterations and, therefore, agree with the concept of a progressively increasing perfusion threshold for ischemic tissue damage as a function of time of ischemia.

Biexponential diffusion attenuation in various states of brain tissue: implications for diffusion-weighted imaging.

Diffusion-weighted single voxel experiments conducted at b-values up to 1 x 10(4) smm-2 yielded biexponential signal attenuation curves for both normal and ischemic brain. The relative fractions of the rapidly and slowly decaying components (f1, f2) are f1 = 0.80 +/- 0.02, f2 = 0.17 +/- 0.02 in healthy adult rat brain and f1 = 0.90 +/- 0.02, f2 = 0.11 +/- 0.01 in normal neonatal rat brain, whereas the corresponding values for the postmortem situation are f1 = 0.69 +/- 0.02, f2 = 0.33 +/- 0.02. It is demonstrated that the changes in f1 and f2 occur simultaneously to those in the extracellular and intracellular space fractions (fex, f(in)) during: (i) cell swelling after total circulatory arrest, and (ii) the recovery from N-methyl-D-aspartate induced excitotoxic brain edema evoked by MK-801, as measured by changes in the electrical impedance. Possible reasons for the discrepancy between the estimated magnitude components and the physiological values are presented and evaluated. Implications of the biexponential signal attenuation curves for diffusion-weighted imaging experiments are discussed.

Diffusion of metabolites in normal and ischemic rat brain measured by localized 1H MRS.

The apparent diffusion coefficient (ADC) of choline-containing compounds (Cho), creatine and phosphocreatine (Cre), N-acetyl-aspartate (NAA), lactate, and water was measured in normal rat brain, and in the ischemic and contralateral region of rat brain approximately 3 and 24 h after induction of focal cerebral ischemia. After 3 h of ischemia, the ADC of Cre and NAA in the ischemic region had significantly decreased by 29% and 19%, respectively (P < 0.05). Lactate ADC was also obtained in the ischemic region. After 24 h of focal ischemia, no ADC values could be measured for NAA, Cre and Cho in the ischemic region because their concentrations had become too low. The ADCs of lactate and water in the ischemic volume were virtually identical at 3 and 24 h after occlusion. The experiments suggest that the ADC decrease of water after induction of ischemia is partly caused by changes in the diffusion characteristics of the intracellular compartment.

Dynamic changes in water ADC, energy metabolism, extracellular space volume, and tortuosity in neonatal rat brain during global ischemia.

To obtain a better understanding of the mechanisms underlying early changes in the brain water apparent diffusion coefficient (ADC) observed in cerebral ischemia, dynamic changes in the ADC of water and in the energy status were measured at postnatal day 8 or 9 in neonatal rat brains after cardiac arrest using 1H MRS/MRI and 31P MRS, respectively. The time courses of the MR parameters were compared with changes in the extracellular space (ECS) volume fraction (alpha) and tortuosity (lambda), determined from concentration-time profiles of tetramethylammonium applied by iontophoresis. The data show a decrease of the ADC of tissue water after induction of global ischemia of which the time course strongly correlates with the time course of the decrease in the ECS volume fraction and the increase in ECS tortuosity. This indicates that cell swelling is an important cause for the ADC decrease of water.

Status of the neonatal rat brain after NMDA-induced excitotoxic injury as measured by MRI, MRS and metabolic imaging.

Intrastriatal injection of the excitotoxin N-methyl-D-aspartate (NMDA) in neonatal rat brain resulted in an acute ipsilateral decrease of the apparent diffusion coefficient (ADC) of brain tissue water, as measured with diffusion-weighted MRI. The early diffusion changes were accompanied by only mild changes in the overall metabolic status as measured by in vivo 1H MRS and 31P MRS and metabolic imaging of brain sections. Minimal decreases in the high-energy phosphate levels and a small hemispheric acidosis were observed in the first 6 h after NMDA administration. In addition, there was very modest lactate accumulation. Twenty-four hours after the induction of the excitotoxic injury the tissue energy status was still only moderately affected, whereas an overall decrease of 1H MRS-detected brain metabolites was found. Treatment with the non-competitive NMDA-antagonist MK-801 given within 90 min after NMDA injection rapidly reversed the NMDA-induced changes in the entire ipsilateral hemisphere. The effect of the competitive NMDA-antagonist D-CPPene was restricted to the cortical areas and was accomplished on a slower time scale. Our results indicate that; (i) early excitotoxicity in the neonatal rat brain does not lead to profound changes in the metabolic status; and (ii) brain tissue water ADC changes are not necessarily associated with a metabolic energy failure.