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

The apparent diffusion coefficient color Map for evaluating a large ischemic core

INTRODUCTION

Our previous work demonstrated that evaluating large ischemic cores using the apparent diffusion coefficient (ADC) could predict EVT outcomes, with the most frequent ADC (peak ADC) ≥520×10-6 mm2/s associated with better clinical results. Since the degree of ADC reduction reflects the severity of ischemic stress, this study aimed to assess the utility of an ADC color map in visualizing this stress.

PATIENTS AND METHODS

This retrospective cohort study included consecutive patients with a low Alberta Stroke Program Early Computed Tomography Score (ASPECTS) using diffusion-weighted imaging (DWI) who underwent successful EVT recanalization between April 2014 and March 2023. To create a visual representation of ischemic stress, we assigned different colors to diffusion-weighted image (DWI) lesions based on their ADC values: ≥520×10-6 mm2/s, 520-440×10-6 mm2/s, and <440×10-6 mm2/s. We compared patients with peak ADC ≥520×10-6 mm2/s to those with lower peak ADC to identify factors associated with the higher value.

RESULTS

A total of 78 patients were enrolled, with 34 having a peak ADC ≥520×10-6 mm2/s. The optimal ratio for discriminating peak ADC ≥520×10-6 mm2/s was found to be 60 % for the volume of the lesion with ADC ≥520×10-6 mm2/s (ADC520) relative to the total DWI lesion volume. This ratio demonstrated a sensitivity of 86 % and a specificity of 82 %.

DISCUSSION AND CONCLUSION

The ADC color map effectively portrays the depth of ischemic stress. A large ischemic core with an ADC520/DWI ratio >60 % may be salvageable with EVT. This approach offers a visual means for assessing EVT suitability in acute large ischemic stroke.

Amide proton transfer MRI at 9.4 T for differentiating tissue acidosis in a rodent model of ischemic stroke

PURPOSE

Differentiating ischemic brain damage is critical for decision making in acute stroke treatment for better outcomes. We examined the sensitivity of amide proton transfer (APT) MRI, a pH-weighted imaging technique, to achieve this differentiation.

METHODS

In a rat stroke model, the ischemic core, oligemia, and the infarct-growth region (IGR) were identified by tracking the progression of the lesions. APT MRI signals were measured alongside ADC, T1, and T2 maps to evaluate their sensitivity in distinguishing ischemic tissues. Additionally, stroke under hyperglycemic conditions was studied.

RESULTS

The APT signal in the IGR decreased by about 10% shortly after stroke onset, and further decreased to 35% at 5 h, indicating a progression from mild to severe acidosis as the lesion evolved into infarction. Although ADC, T1, and T2 contrasts can only detect significant differences between the IGR and oligemia for a portion of the stroke duration, APT contrast consistently differentiates between them at all time points. However, the contrast to variation ratio at 1 h is only about 20% of the contrast to variation ratio between the core and normal tissues, indicating limited sensitivity. In the ischemic core, the APT signal decreases to about 45% and 33% of normal tissue level at 1 h for the normoglycemic and hyperglycemic groups, respectively, confirming more severe acidosis under hyperglycemia.

CONCLUSION

The sensitivity of APT MRI is high in detecting severe acidosis of the ischemic core but is much lower in detecting mild acidosis, which may affect the accuracy of differentiation between the IGR and oligemia.

Dual contrast CEST MRI for pH-weighted imaging in stroke

PURPOSE

pH enhanced (pHenh ) CEST imaging combines the pH sensitivity from amide and guanidino signals, but the saturation parameters have not been optimized. We propose pHdual as a variant of pHenh that suppresses background signal variations, while enhancing pH sensitivity and potential for imaging ischemic brain injury of stroke.

METHODS

Simulation and in vivo rodent stroke experiments of pHenh MRI were performed with varied RF saturation powers for both amide and guanidino protons to optimize the contrast between lesion/normal tissues, while simultaneously minimizing signal variations across different types of normal tissues. In acute stroke, contrast and volume ratio measured by pHdual imaging were compared with an amide-CEST approach, and perfusion and diffusion MRI.

RESULTS

Simulation experiments indicated that amide and guanidino CEST signals exhibit unique sensitivities across different pH ranges, with pHenh producing greater sensitivity over a broader pH regime. The pHenh data of rodent stroke brain demonstrated that the lesion/normal tissue contrast was maximized for an RF saturation power pair of 0.5 μT at 2.0 ppm and 1.0 μT at 3.6 ppm, whereas an optimal contrast-to-variation ratio (CVR) was obtained with a 0.7 μT saturation at 2.0 ppm and 0.8 μT at 3.6 ppm. In acute stroke, CVR optimized pHenh (i.e., pHdual ) achieved a higher sensitivity than the three-point amide-CEST approach, and distinct patterns of lesion tissue compared to diffusion and perfusion MRI.

CONCLUSION

pHdual MRI improves the sensitivity of pH-weighted imaging and will be a valuable tool for assessing tissue viability in stroke.

The Introduction of an MR-Conditional Prototype for Cardiopulmonary Bypass Support: Technical Aspects and System Requirements

INTRODUCTION

The use of cardiopulmonary bypass (CBP; also known as a heart-lung machine) in newborns with complex congenital heart defects may result in brain damage. Magnetic resonance imaging (MRI) assessments cannot be performed safely because the metal components used to construct CBP devices may elicit adverse effects on patients when they are placed in a magnetic field. Thus, this project aimed to develop a prototype MR-conditional circulatory support system that could be used to perform cerebral perfusion studies in animal models.

METHODS

The circulatory support device includes a roller pump with two rollers. The ferromagnetic and most of the metal components of the roller pump were modified or replaced, and the drive was exchanged by an air-pressure motor. All materials used to develop the prototype device were tested in the magnetic field according to the American Society for Testing and Materials (ASTM) Standard F2503-13. The technical performance parameters, including runtime/durability as well as achievable speed and pulsation behavior, were evaluated and compared to standard requirements. The behavior of the prototype device was compared with a commercially available pump.

RESULTS

The MRI-conditional pump system produced no image artifacts and could be safely operated in the presence of the magnetic field. The system exhibited minor performance-related differences when compared to a standard CPB pump; feature testing revealed that the prototype meets the requirements (i.e., operability, controllability, and flow range) needed to proceed with the planned animal studies.

CONCLUSION

This MR-conditional prototype is suitable to perform an open-heart surgery in an animal model to assess brain perfusion in an MR environment.

Need for a Paradigm Shift in the Treatment of Ischemic Stroke: The Blood-Brain Barrier

Blood-brain barrier (BBB) integrity is essential to maintaining brain health. Aging-related alterations could lead to chronic progressive leakiness of the BBB, which is directly correlated with cerebrovascular diseases. Indeed, the BBB breakdown during acute ischemic stroke is critical. It remains unclear, however, whether BBB dysfunction is one of the first events that leads to brain disease or a down-stream consequence. This review will focus on the BBB dysfunction associated with cerebrovascular disease. An added difficulty is its association with the deleterious or reparative effect, which depends on the stroke phase. We will first outline the BBB structure and function. Then, we will focus on the spatiotemporal chronic, slow, and progressive BBB alteration related to ischemic stroke. Finally, we will propose a new perspective on preventive therapeutic strategies associated with brain aging based on targeting specific components of the BBB. Understanding BBB age-evolutions will be beneficial for new drug development and the identification of the best performance window times. This could have a direct impact on clinical translation and personalised medicine.

Investigated regional apparent diffusion coefficient values of the morphologically normal feline brain

OBJECTIVES

Diffusion-weighted imaging (DWI) MRI is increasingly available in veterinary medicine for investigation of the brain. However, apparent diffusion coefficient (ADC) values have only been reported in a small number of cats or in research settings. The aim of this study was to investigate the ADC values of different anatomical regions of the morphologically normal brain in a feline patient population. Additionally, we aimed to assess the possible influence on the ADC values of different patient-related factors, such as sex, body weight, age, imaging of the left and right side of the cerebral hemispheres and white vs grey matter regions.

METHODS

This retrospective study included cats undergoing an MRI (3T) examination with DWI sequences of the head at the Vetsuisse Faculty of the University Zurich between 2015 and 2021. Only cats with morphologically normal brains were included. On the ADC maps, 10 regions of interest (ROIs) were manually drawn on the following anatomical regions: caudate nucleus; internal capsule (two locations); piriform lobe; thalamus; hippocampus; cortex cerebri (two locations); cerebellar hemisphere; and one ROI in the centre of the cerebellar vermis. Except for the ROI at the cerebellar vermis, each ROI was drawn in the left and right hemisphere. The ADC values were calculated by the software and recorded.

RESULTS

A total of 129 cats were included in this study. The ADC varied in the different ROIs, with the highest mean ADC value in the hippocampus and the lowest in the cerebellar hemisphere. ADC was significantly lower in the white cerebral matter compared with the grey matter. ADC values were not influenced by age, with the exception of the hippocampus and the cingulate gyrus.

CONCLUSION AND RELEVANCE

ADC values of different anatomical regions of the morphologically normal feline brain in a patient population of 129 cats in a clinical setting are reported for the first time.

ADC Level is Related to DWI Reversal in Patients Undergoing Mechanical Thrombectomy: A Retrospective Cohort Study

BACKGROUND AND PURPOSE

In patients with ischemic stroke, DWI lesions can occasionally be reversed by reperfusion therapy. This study aimed to ascertain the relationship between ADC levels and DWI reversal in patients with acute ischemic stroke who underwent recanalization treatment.

MATERIALS AND METHODS

We conducted a retrospective cohort study in patients with acute ischemic stroke who underwent endovascular mechanical thrombectomy with successful recanalization between April 2017 and March 2021. DWI reversal was assessed through follow-up MR imaging approximately 24 hours after treatment.

RESULTS

In total, 118 patients were included. DWI reversal was confirmed in 42 patients. The ADC level in patients with reversal was significantly higher than that in patients without reversal. Eighty-three percent of patients with DWI reversal areas had mean ADC levels of ≥520 × 10^-6 mm²/s, and 71% of patients without DWI reversal areas had mean ADC levels of <520 × 10^-6 mm²/s. The mean ADC threshold was 520 × 10^-6 mm²/s with a sensitivity and specificity of 71% and 83%, respectively. In multivariate analysis, the mean ADC level (OR, 1.023; 95% CI, 1.013-1.033; P < .0001) was independently associated with DWI reversal. Patients with DWI reversal areas had earlier neurologic improvement (NIHSS at 7 days) than patients without reversal areas (P < .0001).

CONCLUSIONS

In acute ischemic stroke, the ADC value is independently associated with DWI reversal. Lesions with a mean ADC of ≥520 × 10^-6 mm²/s are salvageable by mechanical thrombectomy, and DWI reversal areas regain neurologic function. The ADC value is easily assessed and is a useful tool to predict viable lesions.

Migraine Aura, Transient Ischemic Attacks, Stroke, and Dying of the Brain Share the Same Key Pathophysiological Process in Neurons Driven by Gibbs–Donnan Forces, Namely Spreading Depolarization

Neuronal cytotoxic edema is the morphological correlate of the near-complete neuronal battery breakdown called spreading depolarization, or conversely, spreading depolarization is the electrophysiological correlate of the initial, still reversible phase of neuronal cytotoxic edema. Cytotoxic edema and spreading depolarization are thus different modalities of the same process, which represents a metastable universal reference state in the gray matter of the brain close to Gibbs–Donnan equilibrium. Different but merging sections of the spreading-depolarization continuum from short duration waves to intermediate duration waves to terminal waves occur in a plethora of clinical conditions, including migraine aura, ischemic stroke, traumatic brain injury, aneurysmal subarachnoid hemorrhage (aSAH) and delayed cerebral ischemia (DCI), spontaneous intracerebral hemorrhage, subdural hematoma, development of brain death, and the dying process during cardio circulatory arrest. Thus, spreading depolarization represents a prime and simultaneously the most neglected pathophysiological process in acute neurology. Aristides Leão postulated as early as the 1940s that the pathophysiological process in neurons underlying migraine aura is of the same nature as the pathophysiological process in neurons that occurs in response to cerebral circulatory arrest, because he assumed that spreading depolarization occurs in both conditions. With this in mind, it is not surprising that patients with migraine with aura have about a twofold increased risk of stroke, as some spreading depolarizations leading to the patient percept of migraine aura could be caused by cerebral ischemia. However, it is in the nature of spreading depolarization that it can have different etiologies and not all spreading depolarizations arise because of ischemia. Spreading depolarization is observed as a negative direct current (DC) shift and associated with different changes in spontaneous brain activity in the alternating current (AC) band of the electrocorticogram. These are non-spreading depression and spreading activity depression and epileptiform activity. The same spreading depolarization wave may be associated with different activity changes in adjacent brain regions. Here, we review the basal mechanism underlying spreading depolarization and the associated activity changes. Using original recordings in animals and patients, we illustrate that the associated changes in spontaneous activity are by no means trivial, but pose unsolved mechanistic puzzles and require proper scientific analysis.

Uric Acid and Gluconic Acid as Predictors of Hyperglycemia and Cytotoxic Injury after Stroke

Hyperglycemia is a feature of worse brain injury after acute ischemic stroke, but the underlying metabolic changes and the link to cytotoxic brain injury are not fully understood. In this observational study, we applied regression and machine learning classification analyses to identify metabolites associated with hyperglycemia and a neuroimaging proxy for cytotoxic brain injury. Metabolomics and lipidomics were carried out using liquid chromatography-tandem mass spectrometry in admission plasma samples from 381 patients presenting with an acute stroke. Glucose was measured by a central clinical laboratory, and a subgroup of patients (n = 201) had apparent diffusion coefficient (ADC) imaging quantified on magnetic resonance imaging (MRI) to estimate cytotoxic injury. Uric acid was the leading metabolite in univariate analysis of both hyperglycemia (OR 19.6, 95% CI 8.6-44.7, P = 1.44 × 10-12) and ADC (OR 5.3, 95% CI 2.2-13.0, P = 2.42 × 10-4). To further prioritize model features and account for non-linear correlation structure, a random forest machine learning algorithm was applied to separately model hyperglycemia and ADC. The statistical techniques used have identified uric acid and gluconic acids as leading candidate markers common to all models (R2 = 68%, P = 2.2 × 10-10 for uric acid; R2 = 15%, P = 8.09 × 10-10 for gluconic acid). Both uric acid and gluconic acid were associated with hyperglycemia and cytotoxic brain injury. Both metabolites are linked to oxidative stress, which highlights two candidate targets for limiting brain injury after stroke.

Characterizing spatiotemporal progression and prediction of infarct lesion volumes in experimental acute ischemia using quantitative perfusion and diffusion imaging

PURPOSE

This study was conducted to explore the diagnostic value of arterial spin labeling (ASL) combined with diffusion weighted imaging (DWI) in characterizing the spatiotemporal progression of infarct lesions in a rabbit middle cerebral artery occlusion (MCAO) model and predicting the acute cerebral infarction (ACI) volume.

MATERIALS AND METHODS

Forty-two male rabbits (2.9 ± 0.2 kg body weight) were used in this experimental study. Animals were initially anesthetized by intravenous injection of uratan. There were seven experimental groups with six rabbits in each group. The apparent diffusion coefficient (ADC) and cerebral blood flow (CBF) thresholds were established in the control group (n = 6), which were sacrificed at 12 h, stained for infarct volume, and imaged at each time point.

RESULTS

The normal ADC and CBF were estimated as 0.90 ± 0.03 × 10-3 mm2/s and 0.68 ± 0.06 mL g-1 min-1, respectively. The viability thresholds of ADC and CBF yielding the lesion volumes (LVs) at 3 h, which best approximated the 2,3,5-triphenyltetrazolium chloride (TTC) infarct volumes at 12 h, were 0.52 ± 0.02 × 10-3 mm2/s (42.2 ± 3% reduction) and 0.33 ± 0.09 mL g-1 min-1 (51.0 ± 11% reduction), respectively. The temporal evolution of the ADC- and CBF-defined LVs showed a significant perfusion/diffusion mismatch up to 1 h (p = 0.001).

CONCLUSION

ADC values and ACI volumes were positively correlated, while CBF was negatively correlated, which is supposed to be a reference for predicting ACI volume.

Pathophysiology of Blood-Brain Barrier Permeability Throughout the Different Stages of Ischemic Stroke and Its Implication on Hemorrhagic Transformation and Recovery

The blood-brain barrier (BBB) is a dynamic interface responsible for maintaining the central nervous system homeostasis. Its unique characteristics allow protecting the brain from unwanted compounds, but its impairment is involved in a vast number of pathological conditions. Disruption of the BBB and increase in its permeability are key in the development of several neurological diseases and have been extensively studied in stroke. Ischemic stroke is the most prevalent type of stroke and is characterized by a myriad of pathological events triggered by an arterial occlusion that can eventually lead to fatal outcomes such as hemorrhagic transformation (HT). BBB permeability seems to follow a multiphasic pattern throughout the different stroke stages that have been associated with distinct biological substrates. In the hyperacute stage, sudden hypoxia damages the BBB, leading to cytotoxic edema and increased permeability; in the acute stage, the neuroinflammatory response aggravates the BBB injury, leading to higher permeability and a consequent risk of HT that can be motivated by reperfusion therapy; in the subacute stage (1-3 weeks), repair mechanisms take place, especially neoangiogenesis. Immature vessels show leaky BBB, but this permeability has been associated with improved clinical recovery. In the chronic stage (>6 weeks), an increase of BBB restoration factors leads the barrier to start decreasing its permeability. Nonetheless, permeability will persist to some degree several weeks after injury. Understanding the mechanisms behind BBB dysregulation and HT pathophysiology could potentially help guide acute stroke care decisions and the development of new therapeutic targets; however, effective translation into clinical practice is still lacking. In this review, we will address the different pathological and physiological repair mechanisms involved in BBB permeability through the different stages of ischemic stroke and their role in the development of HT and stroke recovery.

Paradoxical association of symptomatic cerebral edema with local hypoperfusion caused by the 'watershed shift' after revascularization surgery for adult moyamoya disease: a case report

Superficial temporal artery–middle cerebral artery anastomosis is generally considered as an effective method in improving damage associated with intracerebral occlusions in moyamoya disease. Hemodynamic changes caused by revascularization are the cause of many postoperative complications. Of the 186 consecutive surgeries for moyamoya disease at our hospital from 2015, we herein presented one case of adult-onset moyamoya disease that manifested symptomatic local cerebral edema and local hypoperfusion caused by the ‘watershed shift’. A 67-year-old woman presented with limb numbness on the right side and underwent superficial temporal artery–middle cerebral artery anastomosis, resulting in neurological dysfunction and the formation of a reversible high-signal lesion at left frontotemporal lobes on T2-weighted images along with a decrease in perfusion values on ¹²³I N-isopropyl-p-iodoamphetamine single-photon emission computed tomography, while the anastomotic vessel was patent on magnetic resonance angiography. This phenomenon of hypoperfusion area (left frontotemporal lobe) remote to anastomotic site (left temporal lobe area) led to the diagnosis of the ‘watershed shift’ phenomenon. In light of the hypoperfusion induced by ‘watershed shift’, the patient was treated with fluid replacement. With the gradual recovery of perfusion, the patient presented significantly improvement both on the magnetic resonance imaging findings and neurological symptoms. In conclusion, regional cerebral edema with hypoperfusion, possibly due to cerebral ischemia and the ‘watershed shift’ phenomenon, may be another novel entity that needs to be considered as a potential complication after extracranial–intracranial bypass for moyamoya disease.

Change in CSF Dynamics Responsible for ICP Elevation After Ischemic Stroke in Rats: a New Mechanism for Unexplained END?

It has been proposed that intracranial pressure (ICP) elevation and collateral failure are responsible for unexplained early neurological deterioration (END) in stroke. The study's aims were to investigate whether cerebral spinal fluid (CSF) dynamics, rather than edema, are responsible for elevation of ICP after ischemic stroke. Permanent middle cerebral artery occlusion (pMCAO) was induced with an intraluminal filament. At 24 h after stroke, baseline ICP was measured and CSF dynamics were probed via a steady-state infusion method. Diffusion-weighted imaging (DWI) and T2-weighted magnetic resonance imaging were performed to define cerebral ischemic damage and the volume of brain swelling. We found that the pMCAO group exhibited a significant increase in CSF outflow resistance (2.27 ± 0.15 mmHg μL^-1 min) compared with the sham group (0.93 ± 0.06 mmHg μL^-1 min, p = 0.002). There was no correlation between mean ICP at 24 h post-pMCAO and edema (r² = - 0.03, p = 0.5) or infarct volumes (r² = 0.09, p = 0.5). However, for the first time, we found a significant correlation between the baseline ICP at 24 h post-stroke and the value of CSF outflow resistance. Results show that CSF outflow resistance, rather than edema, was the mechanism responsible for ICP elevation following ischemic stroke. This challenges current concepts and suggests the possibility that intracranial hypertension may be occurring undetected in a much wider range of stroke patients than is currently considered to be the case. In addition, this further supports the hypothesis that unexplained early neurological deterioration is the result of elevated ICP, leading to reduced collateral flow and cerebral perfusion.

Estimation of microvascular capillary physical parameters using MRI assuming a pseudo liquid drop as model of fluid exchange on the cellular level

Aim

One of the most important microvasculatures' geometrical variables is number of pores per capillary length that can be evaluated using MRI. The transportation of blood from inner to outer parts of the capillary is studied by the pores and the relationship among capillary wall thickness, size and the number of pores is examined.

Background

Characterization of capillary space may obtain much valuable information on the performance of tissues as well as the angiogenesis.

Methods

To estimate the number of pores, a new pseudo-liquid drop model along with appropriate quantitative physiological purposes has been investigated toward indicating a package of data on the capillary space. This model has utilized the MRI perfusion, diffusion and relaxivity parameters such as cerebral blood volume (CBV), apparent diffusion coefficient (ADC), ΔR ₂ and Δ R 2 * values. To verify the model, a special protocol was designed and tested on various regions of eight male Wistar rats.

Results

The maximum number of pores per capillary length in the various conditions such as recovery, core, normal-recovery, and normal-core were found to be 183 ± 146, 176 ± 160, 275 ± 166, and 283 ± 143, respectively. This ratio in the normal regions was more than that of the damaged ones. The number of pores increased with increasing mean radius of the capillary and decreasing the thickness of the wall in the capillary space.

Conclusion

Determination of the number of capillary pore may most likely help to evaluate angiogenesis in the tissues and treatment planning of abnormal ones.

Neuroprotective effects of TRPA1 channels in the cerebral endothelium following ischemic stroke

Hypoxia and ischemia are linked to oxidative stress, which can activate the oxidant-sensitive transient receptor potential ankyrin 1 (TRPA1) channel in cerebral artery endothelial cells, leading to vasodilation. We hypothesized that TRPA1 channels in endothelial cells are activated by hypoxia-derived reactive oxygen species, leading to cerebral artery dilation and reduced ischemic damage. Using isolated cerebral arteries expressing a Ca²⁺ biosensor in endothelial cells, we show that 4-hydroxynonenal and hypoxia increased TRPA1 activity, detected as TRPA1 sparklets. TRPA1 activity during hypoxia was blocked by antioxidants and by TRPA1 antagonism. Hypoxia caused dilation of cerebral arteries, which was disrupted by antioxidants, TRPA1 blockade and by endothelial cell-specific Trpa1 deletion (Trpa1 ecKO mice). Loss of TRPA1 channels in endothelial cells increased cerebral infarcts, whereas TRPA1 activation with cinnamaldehyde reduced infarct in wildtype, but not Trpa1 ecKO, mice. These data suggest that endothelial TRPA1 channels are sensors of hypoxia leading to vasodilation, thereby reducing ischemic damage.

A novel voxel-wise lesion segmentation technique on 3.0-T diffusion MRI of hyperacute focal cerebral ischemia at 1 h after permanent MCAO in rats

To assess hyperacute focal cerebral ischemia in rats on 3.0-Tesla diffusion-weighted imaging (DWI), we developed a novel voxel-wise lesion segmentation technique that overcomes intra- and inter-subject variation in apparent diffusion coefficient (ADC) distribution. Our novel technique involves the following: (1) intensity normalization including determination of the optimal type of region of interest (ROI) and its intra- and inter-subject validation, (2) verification of focal cerebral ischemic lesions at 1 h with gross and high-magnification light microscopy of hematoxylin-eosin (H&E) pathology, (3) voxel-wise segmentation on ADC with various thresholds, and (4) calculation of dice indices (DIs) to compare focal cerebral ischemic lesions at 1 h defined by ADC and matching H&E pathology. The best coefficient of variation was the mode of the left hemisphere after normalization using whole left hemispheric ROI, which showed lower intra- (2.54 ± 0.72%) and inter-subject (2.67 ± 0.70%) values than the original. Focal ischemic lesion at 1 h after middle cerebral artery occlusion (MCAO) was confirmed on both gross and microscopic H&E pathology. The 83 relative threshold of normalized ADC showed the highest mean DI (DI = 0.820 ± 0.075). We could evaluate hyperacute ischemic lesions at 1 h more reliably on 3-Tesla DWI in rat brains.

Spreading depolarization is not an epiphenomenon but the principal mechanism of the cytotoxic edema in various gray matter structures of the brain during stroke

Spreading depolarization (SD) is a phenomenon of various cerebral gray matter structures that only occurs under pathological conditions. In the present paper, we summarize the evidence from several decades of research that SD and cytotoxic edema in these structures are largely overlapping terms. SD/cytotoxic edema is a toxic state that - albeit initially reversible - leads eventually to cellular death when it is persistent. Both hemorrhagic and ischemic stroke are among the most prominent causes of SD/cytotoxic edema. SD/cytotoxic edema is the principal mechanism that mediates neuronal death in these conditions. This applies to gray matter structures in both the ischemic core and the penumbra. SD/cytotoxic edema is often a single terminal event in the core whereas, in the penumbra, a cluster of repetitive prolonged SDs is typical. SD/cytotoxic edema also propagates widely into healthy surrounding tissue as short-lasting, relatively harmless events so that regional electrocorticographic monitoring affords even remote detection of ischemic zones. Ischemia cannot only cause SD/cytotoxic edema but it can also be its consequence through inverse neurovascular coupling. Under this condition, ischemia does not start simultaneously in different regions but spreads in the tissue driven by SD/cytotoxic edema-induced microvascular constriction (= spreading ischemia). Spreading ischemia prolongs SD/cytotoxic edema. Thus, it increases the likelihood for the transition from SD/cytotoxic edema into cellular death. Vasogenic edema is the other major type of cerebral edema with relevance to ischemic stroke. It results from opening of the blood-brain barrier. SD/cytotoxic edema and vasogenic edema are distinct processes with important mutual interactions. This article is part of the Special Issue entitled 'Cerebral Ischemia'.

Magnetic Resonance Imaging Protocol for Stroke Onset Time Estimation in Permanent Cerebral Ischemia

MRI provides a sensitive and specific imaging tool to detect acute ischemic stroke by means of a reduced diffusion coefficient of brain water. In a rat model of ischemic stroke, differences in quantitative T₁ and T₂ MRI relaxation times (qT₁ and qT₂) between the ischemic lesion (delineated by low diffusion) and the contralateral non-ischemic hemisphere increase with time from stroke onset. The time dependency of MRI relaxation time differences is heuristically described by a linear function and thus provides a simple estimate of stroke onset time. Additionally, the volumes of abnormal qT₁ and qT₂ within the ischemic lesion increase linearly with time providing a complementary method for stroke timing. A (semi)automated computer routine based on the quantified diffusion coefficient is presented to delineate acute ischemic stroke tissue in rat ischemia. This routine also determines hemispheric differences in qT₁ and qT₂ relaxation times and the location and volume of abnormal qT₁ and qT₂ voxels within the lesion. Uncertainties associated with onset time estimates of qT₁ and qT₂ MRI data vary from ± 25 min to ± 47 min for the first 5 hours of stroke. The most accurate onset time estimates can be obtained by quantifying the volume of overlapping abnormal qT₁ and qT₂ lesion volumes, termed 'V^overlap' (± 25 min) or by quantifying hemispheric differences in qT₂ relaxation times only (± 28 min). Overall, qT₂ derived parameters outperform those from qT₁. The current MRI protocol is tested in the hyperacute phase of a permanent focal ischemia model, which may not be applicable to transient focal brain ischemia.

Hyperglycemia is associated with more severe cytotoxic injury after stroke

Hyperglycemia is a common complication after ischemic stroke, but its link to worse outcome is not well understood. We hypothesized that hyperglycemia may reflect an impaired metabolic response that is associated with worse cytotoxic brain injury. We performed retrospective analysis of magnetic resonance imaging from a cohort of acute ischemic stroke patients prospectively collected from 2006 to 2010 with baseline demographic and laboratory data as well as three-month outcomes. The severity of cytotoxic injury was quantified in vivo using apparent diffusion coefficient imaging by measuring the signal intensity within the stroke relative to the normal signal intensity of the contralateral hemisphere. Both hyperglycemia and lower apparent diffusion coefficient signal were associated with worse outcome after ischemic stroke (OR 0.239, p = 0.017; OR 1.11, p < 0.0001, respectively). Hyperglycemia was also associated with lower apparent diffusion coefficient (r = -0.32, p < 0.001). In multivariate analysis, apparent diffusion coefficient but not hyperglycemia was associated with outcome, suggesting that cytotoxicity may mediate the effect of hyperglycemia. For interventions designed to target hyperglycemia in acute ischemic stroke, a concomitant effect on the evolution of apparent diffusion coefficient may provide insight into whether hyperglycemia leads to or reflects worse cytotoxic injury.

Stroke Onset Time Determination Using MRI Relaxation Times without Non-Ischaemic Reference in A Rat Stroke Model

BACKGROUND

Objective timing of stroke in emergency departments is expected to improve patient stratification. Magnetic resonance imaging (MRI) relaxations times, T₂ and T_1ρ , in abnormal diffusion delineated ischaemic tissue were used as proxies of stroke time in a rat model.

METHODS

Both 'non-ischaemic reference'-dependent and -independent estimators were generated. Apparent diffusion coefficient (ADC), T₂ and T_1ρ , were sequentially quantified for up to 6 hours of stroke in rats (n = 8) at 4.7T. The ischaemic lesion was identified as a contiguous collection of voxels with low ADC. T₂ and T_1ρ in the ischaemic lesion and in the contralateral non-ischaemic brain tissue were determined. Differences in mean MRI relaxation times between ischaemic and non-ischaemic volumes were used to create reference-dependent estimator. For the reference-independent procedure, only the parameters associated with log-logistic fits to the T₂ and T_1ρ distributions within the ADC-delineated lesions were used for the onset time estimation.

RESULT

The reference-independent estimators from T₂ and T_1ρ data provided stroke onset time with precisions of ±32 and ±27 minutes, respectively. The reference-dependent estimators yielded respective precisions of ±47 and ±54 minutes.

CONCLUSIONS

A 'non-ischaemic anatomical reference'-independent estimator for stroke onset time from relaxometric MRI data is shown to yield greater timing precision than previously obtained through reference-dependent procedures.

Sustained diffusion reversal with in-bore reperfusion in monkey stroke models: Confirmed by prospective magnetic resonance imaging

Although early diffusion lesion reversal after recanalization treatment of acute ischaemic stroke has been observed in clinical settings, the reversibility of lesions observed by diffusion-weighted imaging remains controversial. Here, we present consistent observations of sustained diffusion lesion reversal after transient middle cerebral artery occlusion in a monkey stroke model. Seven rhesus macaques were subjected to endovascular transient middle cerebral artery occlusion with in-bore reperfusion confirmed by repeated prospective diffusion-weighted imaging. Early diffusion lesion reversal was defined as lesion reversal at 3 h after reperfusion. Sustained diffusion lesion reversal was defined as the difference between the ADC-derived pre-reperfusion maximal ischemic lesion volume (ADC_{D-P Match}) and the lesion on 4-week follow-up FLAIR magnetic resonance imaging. Diffusion lesions were spatiotemporally assessed using a 3-D voxel-based quantitative technique. The ADC_{D-P Match} was 9.7 ± 6.0% (mean ± SD) and the final infarct was 1.2-6.0% of the volume of the ipsilateral hemisphere. Early diffusion lesion reversal and sustained diffusion lesion reversal were observed in all seven animals, and the calculated percentages compared with their ADC_{D-P Match} ranged from 8.3 to 51.9% (mean ± SD, 26.9 ± 15.3%) and 41.7-77.8% (mean ± SD, 65.4 ± 12.2%), respectively. Substantial sustained diffusion lesion reversal and early reversal were observed in all animals in this monkey model of transient focal cerebral ischaemia.

Automated Ischemic Lesion Segmentation in MRI Mouse Brain Data after Transient Middle Cerebral Artery Occlusion

Magnetic resonance imaging (MRI) has become increasingly important in ischemic stroke experiments in mice, especially because it enables longitudinal studies. Still, quantitative analysis of MRI data remains challenging mainly because segmentation of mouse brain lesions in MRI data heavily relies on time-consuming manual tracing and thresholding techniques. Therefore, in the present study, a fully automated approach was developed to analyze longitudinal MRI data for quantification of ischemic lesion volume progression in the mouse brain. We present a level-set-based lesion segmentation algorithm that is built using a minimal set of assumptions and requires only one MRI sequence (T2) as input. To validate our algorithm we used a heterogeneous data set consisting of 121 mouse brain scans of various age groups and time points after infarct induction and obtained using different MRI hardware and acquisition parameters. We evaluated the volumetric accuracy and regional overlap of ischemic lesions segmented by our automated method against the ground truth obtained in a semi-automated fashion that includes a highly time-consuming manual correction step. Our method shows good agreement with human observations and is accurate on heterogeneous data, whilst requiring much shorter average execution time. The algorithm developed here was compiled into a toolbox and made publically available, as well as all the data sets.

Preserved Collateral Blood Flow in the Endovascular M2CAO Model Allows for Clinically Relevant Profiling of Injury Progression in Acute Ischemic Stroke

Interventional treatment regimens have increased the demand for accurate understanding of the progression of injury in acute ischemic stroke. However, conventional animal models severely inhibit collateral blood flow and mimic the malignant infarction profile not suitable for treatment. The aim of this study was to provide a clinically relevant profile of the emergence and course of ischemic injury in cases suitable for acute intervention, and was achieved by employing a M2 occlusion model (M2CAO) that more accurately simulates middle cerebral artery (MCA) occlusion in humans. Twenty-five Sprague-Dawley rats were subjected to Short (90 min), Intermediate (180 min) or Extended (600 min) transient M2CAO and examined longitudinally with interleaved diffusion-, T2- and arterial spin labeling perfusion-weighted magnetic resonance imaging before and after reperfusion. We identified a rapid emergence of cytotoxic edema within tissue regions undergoing infarction, progressing in several distinct phases in the form of subsequent moderation and then reversal at 230 min (p < 0.0001). We identified also the early emergence of vasogenic edema, which increased consistently before and after reperfusion (p < 0.0001). The perfusion of the penumbra correlated more strongly to the perfusion of adjacent tissue regions than did the perfusion of regions undergoing infarction (p = 0.0088). This was interpreted as an effect of preserved collateral blood flow during M2CAO. Accordingly, we observed only limited recruitment of penumbra regions to the infarction core. However, a gradual increase in infarction size was still occurring as late as 10 hours after M2CAO. Our results indicate that patients suffering MCA branch occlusion stand to benefit from interventional therapy for an extended time period after the emergence of ischemic injury.

Comparison of stroke volume evolution on diffusion-weighted imaging and fluid-attenuated inversion recovery following endovascular thrombectomy

Background To compare the evolution of the infarct lesion volume on both diffusion-weighted imaging and fluid-attenuated inversion recovery in the first five days after endovascular thrombectomy. Methods We included 109 patients from the CRISP and DEFUSE 2 studies. Stroke lesion volumes obtained on diffusion-weighted imaging and fluid-attenuated inversion recovery images both early post-procedure (median 18 h after symptom onset) and day 5, were compared using median, interquartile range, and correlation plots. Patients were dichotomized based on the time after symptom onset of their post procedure images (≥18 h vs. <18 h), and the degree of reperfusion (on Tmax>6 s; ≥ 90% vs. < 90%). Results Early post-procedure, median infarct lesion volume was 19 ml [(IQR) 7-43] on fluid-attenuated inversion recovery, and 23 ml [11-64] on diffusion-weighted imaging. On day 5, median infarct lesion volume was 52 ml [20-118] on fluid-attenuated inversion recovery, and 37 ml [16-91] on diffusion-weighted imaging. Infarct lesion volume on early post-procedure diffusion-weighted imaging, compared to fluid-attenuated inversion recovery, correlated better with day 5 diffusion-weighted imaging and fluid-attenuated inversion recovery lesions (r = 0.88 and 0.88 vs. 0.78 and 0.77; p < 0.0001). Median lesion growth was significantly smaller on diffusion-weighted imaging when the early post-procedure scan was obtained ≥18 h post stroke onset (5 ml [-1-13]), compared to <18 h (13 ml [2-47]; p = 0.03), but was not significantly different on fluid-attenuated inversion recovery (≥18 h: 26 ml [12-57]; <18 h: 21 ml [5-57]; p = 0.65). In the <90% reperfused group, the median infarct growth was significantly larger for diffusion-weighted imaging and fluid-attenuated inversion recovery (diffusion-weighted imaging: 23 ml [8-57], fluid-attenuated inversion recovery: 41 ml [13-104]) compared to ≥90% (diffusion-weighted imaging: 6 ml [2-24]; p = 0.003, fluid-attenuated inversion recovery: 19 ml [8-46]; p = 0.001). Conclusions Early post-procedure lesion volume on diffusion-weighted imaging is a better estimate of day 5 infarct volume than fluid-attenuated inversion recovery. However, both early post-procedure diffusion-weighted imaging and fluid-attenuated inversion recovery underestimate day 5 diffusion-weighted imaging and fluid-attenuated inversion recovery lesion volumes, especially in patients who do not reperfuse.

Stroke onset time estimation from multispectral quantitative magnetic resonance imaging in a rat model of focal permanent cerebral ischemia

Background

Quantitative T2 relaxation magnetic resonance imaging allows estimation of stroke onset time.

Aims

We aimed to examine the accuracy of quantitative T1 and quantitative T2 relaxation times alone and in combination to provide estimates of stroke onset time in a rat model of permanent focal cerebral ischemia and map the spatial distribution of elevated quantitative T1 and quantitative T2 to assess tissue status.

Methods

Permanent middle cerebral artery occlusion was induced in Wistar rats. Animals were scanned at 9.4T for quantitative T1, quantitative T2, and Trace of Diffusion Tensor (Dav) up to 4 h post-middle cerebral artery occlusion. Time courses of differentials of quantitative T1 and quantitative T2 in ischemic and non-ischemic contralateral brain tissue (ΔT1, ΔT2) and volumes of tissue with elevated T1 and T2 relaxation times ( f1, f2) were determined. TTC staining was used to highlight permanent ischemic damage.

Results

ΔT1, ΔT2, f1, f2, and the volume of tissue with both elevated quantitative T1 and quantitative T2 (VOverlap) increased with time post-middle cerebral artery occlusion allowing stroke onset time to be estimated. VOverlap provided the most accurate estimate with an uncertainty of ±25 min. At all times-points regions with elevated relaxation times were smaller than areas with Dav defined ischemia.

Conclusions

Stroke onset time can be determined by quantitative T1 and quantitative T2 relaxation times and tissue volumes. Combining quantitative T1 and quantitative T2 provides the most accurate estimate and potentially identifies irreversibly damaged brain tissue.

The central role of aquaporins in the pathophysiology of ischemic stroke

Stroke is a complex and devastating neurological condition with limited treatment options. Brain edema is a serious complication of stroke. Early edema formation can significantly contribute to infarct formation and thus represents a promising target. Aquaporin (AQP) water channels contribute to water homeostasis by regulating water transport and are implicated in several disease pathways. At least 7 AQP subtypes have been identified in the rodent brain and the use of transgenic mice has greatly aided our understanding of their functions. AQP4, the most abundant channel in the brain, is up-regulated around the peri-infarct border in transient cerebral ischemia and AQP4 knockout mice demonstrate significantly reduced cerebral edema and improved neurological outcome. In models of vasogenic edema, brain swelling is more pronounced in AQP4-null mice than wild-type providing strong evidence of the dual role of AQP4 in the formation and resolution of both vasogenic and cytotoxic edema. AQP4 is co-localized with inwardly rectifying K(+)-channels (Kir4.1) and glial K(+) uptake is attenuated in AQP4 knockout mice compared to wild-type, indicating some form of functional interaction. AQP4-null mice also exhibit a reduction in calcium signaling, suggesting that this channel may also be involved in triggering pathological downstream signaling events. Associations with the gap junction protein Cx43 possibly recapitulate its role in edema dissipation within the astroglial syncytium. Other roles ascribed to AQP4 include facilitation of astrocyte migration, glial scar formation, modulation of inflammation and signaling functions. Treatment of ischemic cerebral edema is based on the various mechanisms in which fluid content in different brain compartments can be modified. The identification of modulators and inhibitors of AQP4 offer new therapeutic avenues in the hope of reducing the extent of morbidity and mortality in stroke.

Multimodal MR imaging of acute and subacute experimental traumatic brain injury: Time course and correlation with cerebral energy metabolites

BACKGROUND

Traumatic brain injury (TBI) is one of the leading causes of death and permanent disability world-wide. The predominant cause of death after TBI is brain edema which can be quantified by non-invasive diffusion-weighted magnetic resonance imaging (DWI).

PURPOSE

To provide a better understanding of the early onset, time course, spatial development, and type of brain edema after TBI and to correlate MRI data and the cerebral energy state reflected by the metabolite adenosine triphosphate (ATP).

MATERIAL AND METHODS

The spontaneous development of lateral fluid percussion-induced TBI was investigated in the acute (6 h), subacute (48 h), and chronic (7 days) phase in rats by MRI of quantitative T2 and apparent diffusion coefficient (ADC) mapping as well as perfusion was combined with ATP-specific bioluminescence imaging and histology.

RESULTS

An induced TBI led to moderate to mild brain damages, reflected by transient, pronounced development of vasogenic edema and perfusion reduction. Heterogeneous ADC patterns indicated a parallel, but mixed expression of vasogenic and cytotoxic edema. Cortical ATP levels were reduced in the acute and subacute phase by 13% and 27%, respectively, but were completely normalized at 7 days after injury.

CONCLUSION

The partial ATP reduction was interpreted to be partially caused by a loss of neurons in parallel with transient dilution of the regional ATP concentration by pronounced vasogenic edema. The normalization of energy metabolism after 7 days was likely due to infiltrating glia and not to recovery. The MRI combined with metabolite measurement further improves the understanding and evaluation of brain damages after TBI.

Dynamic perfusion and diffusion MRI of cortical spreading depolarization in photothrombotic ischemia

Cortical spreading depolarization (CSD) is known to exacerbate ischemic damage, as the number of CSDs correlates with the final infarct volumes and suppressing CSDs improves functional outcomes. To investigate the role of CSD in ischemic damage, we developed a novel rat model of photothrombotic ischemia using a miniature implantable optic fiber that allows lesion induction inside the magnetic resonance imaging (MRI) scanner. We were able to precisely control the location and the size of the ischemic lesion, and continuously monitor dynamic perfusion and diffusion MRI signal changes at high temporal resolution before, during and after the onset of focal ischemia. Our model showed that apparent diffusion coefficient (ADC) and cerebral blood flow (CBF) in the ischemic core dropped immediately after lesion onset by 20±6 and 41±23%, respectively, and continually declined over the next 5h. Meanwhile, CSDs were observed in all animals (n=36) and displayed either a transient decrease of ADC by 17±3% or an increase of CBF by 104±15%. All CSDs were initiated from the rim of the ischemic core, propagated outward, and confined to the ipsilesional cortex. Additionally, we demonstrated that by controlling the size of perfusion-diffusion mismatch (which approximates the penumbra) in our model, the number of CSDs correlated with the mismatch area rather than the final infarct volume. This study introduces a novel platform to study CSDs in real-time with high reproducibility using MRI.

Multiparametric magnetic resonance imaging of acute experimental brain ischaemia

Ischaemia is a condition in which blood flow either drops to zero or proceeds at severely decreased levels that cannot supply sufficient oxidizable substrates to maintain energy metabolism in vivo. Brain, a highly oxidative organ, is particularly susceptible to ischaemia. Ischaemia leads to loss of consciousness in seconds and, if prolonged, permanent tissue damage is inevitable. Ischaemia primarily results in a collapse of cerebral energy state, followed by a series of subtle changes in anaerobic metabolism, ion and water homeostasis that eventually initiate destructive internal and external processes in brain tissue. (31)P and (1)H NMR spectroscopy were initially used to evaluate anaerobic metabolism in brain. However, since the early 1990s (1)H Magnetic Resonance Imaging (MRI), exploiting the nuclear magnetism of tissue water, has become the key method for assessment of ischaemic brain tissue. This article summarises multi-parametric (1)H MRI work that has exploited diffusion, relaxation and magnetisation transfer as 'contrasts' to image ischaemic brain in preclinical models for the first few hours, with a view to assessing evolution of ischaemia and tissue viability in a non-invasive manner.

Impact of aging on spreading depolarizations induced by focal brain ischemia in rats

Spreading depolarization (SD) contributes to the ischemic damage of the penumbra. Although age is the largest predictor of stroke, no studies have examined age dependence of SD appearance. We characterized the electrophysiological and hemodynamic changes in young (6 weeks old, n = 7), middle-aged (9 months old, n = 6), and old (2 years old, n = 7) male Wistar rats during 30 minutes of middle cerebral artery occlusion (MCAO), utilizing multimodal imaging through a closed cranial window over the ischemic cortex: membrane potential changes (with a voltage-sensitive dye), cerebral blood volume (green light reflectance), and cerebral blood flow (CBF, laser-speckle imaging) were observed. The initial CBF drop was similar in all groups, with a significant further reduction during ischemia in old rats (p < 0.01). Age reduced the total number of SDs (p < 0.05) but increased the size of ischemic area displaying prolonged SD (p < 0.01). The growth of area undergoing prolonged SDs positively correlated with the growth of ischemic core area (p < 0.01) during MCAO. Prolonged SDs and associated hypoperfusion likely compromise cortical tissue exposed to even a short focal ischemia in aged rats.

Decreased infarct volume and intracranial hemorrhage associated with intra-arterial nonionic iso-osmolar contrast material in an MCA occlusion/reperfusion model

BACKGROUND AND PURPOSE

Infarct volume and intracranial hemorrhage after reperfusion with nonionic low-osmolar and iso-osmolar iodinated IRCM has not been previously compared. We postulated that iso-osmolar and low-osmolar iodinated contrast media exert varied effects on cerebral infarct after intra-arterial injection. We compared infarct volume and hemorrhagic changes following intra-arterial infusion of iodixanol, iopamidol, or normal saline in a rat MCA occlusion/reperfusion model.

MATERIALS AND METHODS

Infarct was induced in 30 rats by a previously validated method of MCA suture occlusion. Reperfusion was performed after 5 hours with either iodixanol (n = 9), iopamidol (n = 12), or saline (n = 9). MR images were obtained at both 6 and 24 hours after ischemia, followed by sacrifice. Infarct volume was measured with T2WI and DWI by semiautomatic segmentation. Incidence and area of hemorrhage were measured on brain sections postmortem.

RESULTS

T2WI mean infarct volumes were 242 ± 89, 324 ± 70, and 345 ± 92 mm(3) at 6 hours, and 341 ± 147,470 ± 91, and 462 ± 71 mm(3) at 24 hours in the iodixanol, iopamidol, and saline groups, respectively. Differences in infarct volume among groups were significant at 6 hours (P < .03) and 24 hours (P < .05). In the iodixanol, iopamidol, and saline groups, mean areas for cortical intracranial hemorrhage were 0.8, 18.2, and 25.7 mm(2); and 28, 31, and 56.7 mm(2), respectively, for deep intracranial hemorrhage. The differences in intracranial hemorrhage area among groups were statistically significant for cortical intracranial hemorrhage (P < .01).

CONCLUSIONS

Intra-arterial infusion of nonionic iso-osmolar iodixanol showed reduced infarct volume and reduced cortical intracranial hemorrhage areas in comparison with nonionic low-osmolar iopamidol and saline. Our results may be relevant in the setting of intra-arterial therapy for acute stroke in humans, warranting further investigation.

Effects of noninvasive facial nerve stimulation in the dog middle cerebral artery occlusion model of ischemic stroke

BACKGROUND AND PURPOSE

Facial nerve stimulation has been proposed as a new treatment of ischemic stroke because autonomic components of the nerve dilate cerebral arteries and increase cerebral blood flow when activated. A noninvasive facial nerve stimulator device based on pulsed magnetic stimulation was tested in a dog middle cerebral artery occlusion model.

METHODS

We used an ischemic stroke dog model involving injection of autologous blood clot into the internal carotid artery that reliably embolizes to the middle cerebral artery. Thirty minutes after middle cerebral artery occlusion, the geniculate ganglion region of the facial nerve was stimulated for 5 minutes. Brain perfusion was measured using gadolinium-enhanced contrast MRI, and ATP and total phosphate levels were measured using 31P spectroscopy. Separately, a dog model of brain hemorrhage involving puncture of the intracranial internal carotid artery served as an initial examination of facial nerve stimulation safety.

RESULTS

Facial nerve stimulation caused a significant improvement in perfusion in the hemisphere affected by ischemic stroke and a reduction in ischemic core volume in comparison to sham stimulation control. The ATP/total phosphate ratio showed a large decrease poststroke in the control group versus a normal level in the stimulation group. The same stimulation administered to dogs with brain hemorrhage did not cause hematoma enlargement.

CONCLUSIONS

These results support the development and evaluation of a noninvasive facial nerve stimulator device as a treatment of ischemic stroke.

Experimental models of brain ischemia: a review of techniques, magnetic resonance imaging, and investigational cell-based therapies

Stroke continues to be a significant cause of death and disability worldwide. Although major advances have been made in the past decades in prevention, treatment, and rehabilitation, enormous challenges remain in the way of translating new therapeutic approaches from bench to bedside. Thrombolysis, while routinely used for ischemic stroke, is only a viable option within a narrow time window. Recently, progress in stem cell biology has opened up avenues to therapeutic strategies aimed at supporting and replacing neural cells in infarcted areas. Realistic experimental animal models are crucial to understand the mechanisms of neuronal survival following ischemic brain injury and to develop therapeutic interventions. Current studies on experimental stroke therapies evaluate the efficiency of neuroprotective agents and cell-based approaches using primarily rodent models of permanent or transient focal cerebral ischemia. In parallel, advancements in imaging techniques permit better mapping of the spatial-temporal evolution of the lesioned cortex and its functional responses. This review provides a condensed conceptual review of the state of the art of this field, from models and magnetic resonance imaging techniques through to stem cell therapies.

Present status and future challenges of electroencephalography- and magnetic resonance imaging-based monitoring in preclinical models of focal cerebral ischemia

Animal models are useful tools for better understanding the mechanisms underlying neurological deterioration after an ischemic insult as well as subsequent evolution of changes and recovery of functions. In response to the updated requirements for preclinical investigations of stroke to include relevant functional measurement techniques and biomarker endpoints, we here review the state of knowledge on application of some translational electrophysiological and neuroimaging methods, and in particular, electroencephalography monitoring and magnetic resonance imaging in rodent models of ischemic stroke. This may lead to improvement of diagnostic methods and identification of new therapeutic targets, which would considerably advance the translational value of preclinical stroke research.

Diurnal microstructural variations in healthy adult brain revealed by diffusion tensor imaging

Biorhythm is a fundamental property of human physiology. Changes in the extracellular space induced by cell swelling in response to the neural activity enable the in vivo characterization of cerebral microstructure by measuring the water diffusivity using diffusion tensor imaging (DTI). To study the diurnal microstructural alterations of human brain, fifteen right-handed healthy adult subjects were recruited for DTI studies in two repeated sessions (8∶30 AM and 8∶30 PM) within a 24-hour interval. Fractional anisotropy (FA), apparent diffusion coefficient (ADC), axial (λ_//) and radial diffusivity (λ_⊥) were compared pixel by pixel between the sessions for each subject. Significant increased morning measurements in FA, ADC, λ_// and λ_⊥ were seen in a wide range of brain areas involving frontal, parietal, temporal and occipital lobes. Prominent evening dominant λ_⊥ (18.58%) was detected in the right inferior temporal and ventral fusiform gyri. AM-PM variation of λ_⊥ was substantially left side hemisphere dominant (p<0.05), while no hemispheric preference was observed for the same analysis for ADC (p = 0.77), λ_// (p = 0.08) or FA (p = 0.25). The percentage change of ADC, λ_//, λ_⊥, and FA were 1.59%, 2.15%, 1.20% and 2.84%, respectively, for brain areas without diurnal diffusivity contrast. Microstructural variations may function as the substrates of the phasic neural activities in correspondence to the environment adaptation in a light-dark cycle. This research provided a baseline for researches in neuroscience, sleep medicine, psychological and psychiatric disorders, and necessitates that diurnal effect should be taken into account in following up studies using diffusion tensor quantities.

Identification of hyperacute ischemic stroke with a more homogenous nature

Previous reports revealed that middle cerebral artery occlusion (MCAO) models in rats were very diverse in nature, and experimental stroke of a more homogenous nature had not been previously documented. This paper aims to present our novel observations of experimental stroke in rats with similar MRI characteristics after MCAO. Immediately after MCAO, 19 rats were placed into a 4.7 T MRI scanner, and diffusion weighted imaging (DWI) of axial and coronal planes was repeated every 10 minutes up to post-occlusion 115 minutes. Apparent diffusion coefficient (ADC) values of the ischemic lesions were calculated and compared to those of the unaffected contra-lateral hemispheres. Successful MCAO was defined when the whole left MCA territory showed ADC abnormality on DWI. Percentage of hemispheric lesion volume (% HLV), relative ADC value (rADC), and relative DWI signal intensity (rDWI) were serially evaluated for quantitative analysis of ADC-derived lesion characteristics. Successful MCA territorial infarction was induced in nine rats (9/19, 47.4%). In quantitative analysis of ADC-derived lesion characteristics, lesion volumes of seven rats (group 1) were very similar, but larger than those of the other two rats (group 2): % HLV of initial MRI = 45.4 ± 2.5 / 19.1 ± 6.6. rADCs and rDWIs of group 1 showed similar patterns of temporal change, which was different from those of group 2. Using prospective diffusion MRI after MCAO in rats, we identified territorial hyperacute ischemic lesions with similar MRI characteristics. This observation would contribute to the establishment of more homogenous rodent models for ischemic stroke translational research.

Quantitative temporal profiles of penumbra and infarction during permanent middle cerebral artery occlusion in rats

The basic premise of neuroprotection in acute stroke is the presence of salvageable tissue, but the spatiotemporal volume profiles of the penumbra and infarction remain poorly defined in preclinical animal models of acute stroke used to evaluate therapies for clinical application. Our aim was to define these profiles using magnetic resonance imaging (MRI) quantitative cerebral blood flow (CBF) and apparent diffusion coefficient (ADC) for dual-parameter voxel analysis in the rat suture permanent middle cerebral artery occlusion (pMCAO) model. Eleven male Sprague Dawley rats were subjected to pMCAO with MRI measurements of quantitative CBF and ADC at baseline, over the first 4 h (n=9) and at 7, 14, and 21 days (n=4). Voxel analysis of CBF and ADC was used to characterize brain tissue ischemic transitions. Penumbra, core, and hyperemic infarction volumes were significantly elevated (P<0.05) and unchanged over the first 4 h of pMCAO while the total lesion volume progressively rose. At 7, 14, and 21 days, tissue compartment transitions reflected infarction, tissue cavitation, and selective ischemic neuronal necrosis. Anatomical distribution of penumbra and core revealed marked heterogeneity with penumbra scattered within core and penumbra persisting even after 4 h of permanent MCAO.

Translational Perspectives on Perfusion–Diffusion Mismatch in Ischemic Stroke

Magnetic resonance imaging has tremendous potential to illuminate ischemic stroke pathophysiology and guide rational treatment decisions. Clinical applications to date have been largely limited to trials. However, recent analyses of the major clinical studies have led to refinements in selection criteria and improved understanding of the potential implications for the risk vs. benefit of thrombolytic therapy. In parallel, preclinical studies have provided complementary information on the evolution of stroke that is difficult to obtain in humans due to the requirement for continuous or repeated imaging and pathological verification. We review the clinical and preclinical advances that have led to perfusion–diffusion mismatch being applied in phase 3 randomized trials and, potentially, future routine clinical practice.

Apparent diffusion coefficient threshold for delineation of ischemic core

BACKGROUND

MRI-based selection of patients for acute stroke interventions requires rapid accurate estimation of the infarct core on diffusion-weighted MRI. Typically used manual methods to delineate restricted diffusion lesions are subjective and time consuming. These limitations would be overcome by a fully automated method that can rapidly and objectively delineate the ischemic core. An automated method would require predefined criteria to identify the ischemic core.

AIM

The aim of this study is to determine apparent diffusion coefficient-based criteria that can be implemented in a fully automated software solution for identification of the ischemic core.

METHODS

Imaging data from patients enrolled in the Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution (DEFUSE) study who had early revascularization following intravenous thrombolysis were included. The patients' baseline restricted diffusion and 30-day T2 -weighted fluid-attenuated inversion recovery lesions were manually delineated after coregistration. Parts of the restricted diffusion lesion that corresponded with 30-day infarct were considered ischemic core, whereas parts that corresponded with normal brain parenchyma at 30 days were considered noncore. The optimal apparent diffusion coefficient threshold to discriminate core from noncore voxels was determined by voxel-based receiver operating characteristics analysis using the Youden index.

RESULTS

51,045 diffusion positive voxels from 14 patients who met eligibility criteria were analyzed. The mean DWI lesion volume was 24 (± 23) ml. Of this, 18 (± 22) ml was ischemic core and 3 (± 5) ml was noncore. The remainder corresponded to preexisting gliosis, cerebrospinal fluid, or was lost to postinfarct atrophy. The apparent diffusion coefficient of core was lower than that of noncore voxels (P < 0.0001). The optimal threshold for identification of ischemic core was an apparent diffusion coefficient ≤ 620 × 10(-6) mm(2) /s (sensitivity 69% and specificity 78%).

CONCLUSIONS

Our data suggest that the ischemic core can be identified with an absolute apparent diffusion coefficient threshold. This threshold can be implemented in image analysis software for fully automated segmentation of the ischemic core.

Magnetic resonance imaging of perfusion-diffusion mismatch in rodent and non-human primate stroke models

Stroke is a leading cause of death and long-term disability. Non-invasive magnetic resonance imaging (MRI) has been widely used for the early detection of ischemic stroke and the longitudinal monitoring of novel treatment strategies. Recent advances in MRI techniques have enabled improved sensitivity and specificity to detecting ischemic brain injury and monitoring functional recovery. This review describes recent progresses in the development and application of multimodal MRI and image analysis techniques to study experimental stroke in rats and non-human primates.

Penumbra detection using PWI/DWI mismatch MRI in a rat stroke model with and without comorbidity: comparison of methods

Perfusion-diffusion (perfusion-weighted imaging (PWI)/diffusion-weighted imaging (DWI)) mismatch is used to identify penumbra in acute stroke. However, limitations in penumbra detection with mismatch are recognized, with a lack of consensus on thresholds, quantification and validation of mismatch. We determined perfusion and diffusion thresholds from final infarct in the clinically relevant spontaneously hypertensive stroke-prone (SHRSP) rat and its normotensive control strain, Wistar-Kyoto (WKY) and compared three methods for penumbra calculation. After permanent middle cerebral artery occlusion (MCAO) (WKY n=12, SHRSP n=15), diffusion-weighted (DWI) and perfusion-weighted (PWI) images were obtained for 4 hours post stroke and final infarct determined at 24 hours on T(2) scans. The PWI/DWI mismatch was calculated from volumetric assessment (perfusion deficit volume minus apparent diffusion coefficient (ADC)-defined lesion volume) or spatial assessment of mismatch area on each coronal slice. The ADC-derived lesion growth provided the third, retrospective measure of penumbra. At 1 hour after MCAO, volumetric mismatch detected smaller volumes of penumbra in both strains (SHRSP: 31 ± 50 mm(3), WKY: 22 ± 59 mm(3), mean ± s.d.) compared with spatial assessment (SHRSP: 36 ± 15 mm(3), WKY: 43 ± 43 mm(3)) and ADC lesion expansion (SHRSP: 41 ± 45 mm(3), WKY: 65 ± 41 mm(3)), although these differences were not statistically significant. Spatial assessment appears most informative, using both diffusion and perfusion data, eliminating the influence of negative mismatch and allowing the anatomical location of penumbra to be assessed at given time points after stroke.

Dynamic functional cerebral blood volume responses to normobaric hyperoxia in acute ischemic stroke

Studies suggest that neuroprotective effects of normobaric oxygen (NBO) therapy in acute stroke are partly mediated by hemodynamic alterations. We investigated cerebral hemodynamic effects of repeated NBO exposures. Serial magnetic resonance imaging (MRI) was performed in Wistar rats subjected to focal ischemic stroke. Normobaric oxygen-induced functional cerebral blood volume (fCBV) responses were analyzed. All rats had diffusion-weighted MRI (DWI) lesions within larger perfusion deficits, with DWI lesion expansion after 3 hours. Functional cerebral blood volume responses to NBO were spatially and temporally heterogeneous. Contralateral healthy tissue responded consistently with vasoconstriction that increased with time. No significant responses were evident in the acute DWI lesion. In hypoperfused regions surrounding the acute DWI lesion, tissue that remained viable until the end of the experiment showed relative preservation of mean fCBV at early time points, with some rats showing increased fCBV (vasodilation); however, these regions later exhibited significantly decreased fCBV (vasoconstriction). Tissue that became DWI abnormal by study-end initially showed marginal fCBV changes that later became moderate fCBV reductions. Our results suggest that a reverse-steal hemodynamic effect may occur in peripheral ischemic zones during NBO treatment of focal stroke. In addition, CBV responses to NBO challenge may have potential as an imaging marker to distinguish ischemic core from salvageable tissues.

Neuroimaging, biochemical and cellular evidence of protection by mycophenolate mofetil on middle cerebral artery occlusion induced injury in rats

Stroke is a major cause of mortality and disability worldwide. Presently, recombinant tissue plasminogen activator is the only approved drug for the management of acute ischemic stroke. However, it has limitations like narrow therapeutic window and increased risk of intracranial hemorrhage. In previous studies, immunosuppressive agents such as cyclosporine A and tacrolimus have shown neuroprotection by improving neurological functions and infarct volume in models of ischemic stroke. Therefore, the present study was designed to evaluate the effect of mycophenolate mofetil (MMF) on the cerebral ischemic injury in the middle cerebral artery occlusion (MCAo) model in rats. MCAo was carried out in male Wistar rats by inserting an intraluminal thread. One hour after MCAo, the animals were treated with MMF (50, 100, 200mg/kg, i.p.). Reperfusion was done after 2h of occlusion. Thirty minutes after reperfusion, animals were subjected to diffusion-weighted magnetic resonance imaging for assessment of neuroprotective effect of MMF. Twenty four hours after MCAo, motor performance was assessed and the animals were euthanized for estimation of brain malondialdehyde, glutathione, myeloperoxidase and nitric oxide levels. The effect of MMF on apoptosis was also evaluated. MMF significantly attenuated the percent infarct area, apparent diffusion coefficient and signal intensity as compared to a vehicle treated group. Treatment with MMF prevented the motor impairment and significantly reversed the changes in levels of malondialdehyde, glutathione, myeloperoxidase and nitric oxide. MMF treatment significantly reduced the apoptosis. Data of the present study indicate neuroprotective effect of MMF in the experimental model of ischemic stroke.

Magnetic resonance diffusion-perfusion mismatch in acute ischemic stroke: An update

The concept of magnetic resonance perfusion-diffusion mismatch (PDM) provides a practical and approximate measure of the tissue at risk and has been increasingly applied for the evaluation of hyperacute and acute stroke in animals and patients. Recent studies demonstrated that PDM does not optimally define the ischemic penumbra; because early abnormality on diffusion-weighted imaging overestimates the infarct core by including part of the penumbra, and the abnormality on perfusion weighted imaging overestimates the penumbra by including regions of benign oligemia. To overcome these limitations, many efforts have been made to optimize conventional PDM. Various alternatives beyond the PDM concept are under investigation in order to better define the penumbra. The PDM theory has been applied in ischemic stroke for at least three purposes: to be used as a practical selection tool for stroke treatment; to test the hypothesis that patients with PDM pattern will benefit from treatment, while those without mismatch pattern will not; to be a surrogate measure for stroke outcome. The main patterns of PDM and its relation with clinical outcomes were also briefly reviewed. The conclusion was that patients with PDM documented more reperfusion, reduced infarct growth and better clinical outcomes compared to patients without PDM, but it was not yet clear that thrombolytic therapy is beneficial when patients were selected on PDM. Studies based on a larger cohort are currently under investigation to further validate the PDM hypothesis.

DWI of the brain: postmortal DWI of the brain in comparison with in vivo data

PURPOSE

Changes in water diffusion can be quantified by diffusion-weighted MR imaging. However, there are only few reports about changes in post mortem brain. The aim of this study was to investigate the temporal pattern of the apparent diffusion coefficient (ADC) in the brain after death, to compare the values to in vivo brain and to assess the value of ex vivo DWI as a forensic tool.

MATERIAL AND METHODS

The study was approved by the local Ethics Committee, and informed consent was obtained from all relatives and the control subjects. Twenty-one corpses, died of natural cause, were examined (13 males, 8 females; age: 70.5±8.7 y, weight 74±18 kg). Diffusion-Weighted Imaging (DWI) was performed with b-values of 0 and 1000 s/mm(2) at 1.5 T. Scans were repeated in intervals of 1 h. ADC-maps were calculated in thalamus, cerebrum and cerebellum. The obtained values were statistically compared to healthy volunteers (n=3) and to literature data.

RESULTS

The ADC in the three regions decreased characteristically during the examination time. In the cerebrum there was a significant difference between ex vivo and in vivo ADC (p<0.001) as well as in the other regions (thalamus: p<0.001, cerebellum: p=0.045).

CONCLUSION

DWI of the postmortal brain can be added to the MRI methods for a post mortem imaging.

Optical coherence microscopy for deep tissue imaging of the cerebral cortex with intrinsic contrast

In vivo optical microscopic imaging techniques have recently emerged as important tools for the study of neurobiological development and pathophysiology. In particular, two-photon microscopy has proved to be a robust and highly flexible method for in vivo imaging in highly scattering tissue. However, two-photon imaging typically requires extrinsic dyes or contrast agents, and imaging depths are limited to a few hundred microns. Here we demonstrate Optical Coherence Microscopy (OCM) for in vivo imaging of neuronal cell bodies and cortical myelination up to depths of ~1.3 mm in the rat neocortex. Imaging does not require the administration of exogenous dyes or contrast agents, and is achieved through intrinsic scattering contrast and image processing alone. Furthermore, using OCM we demonstrate in vivo, quantitative measurements of optical properties (index of refraction and attenuation coefficient) in the cortex, and correlate these properties with laminar cellular architecture determined from the images. Lastly, we show that OCM enables direct visualization of cellular changes during cell depolarization and may therefore provide novel optical markers of cell viability.

Multimodal MRI of experimental stroke

Stroke is the fourth leading cause of death and the leading cause of long-term disability in USA. Brain imaging data from experimental stroke models and stroke patients have shown that there is often a gradual progression of potentially reversible ischemic injury toward infarction. Reestablishing tissue perfusion and/or treating with neuroprotective drugs in a timely fashion are expected to salvage some ischemic tissues. Diffusion-weighted imaging based on magnetic resonance imaging (MRI) in which contrast is based on water motion can detect ischemic injury within minutes after onsets, whereas computed tomography and other imaging modalities fail to detect stroke injury for at least a few hours. Along with quantitative perfusion imaging, the perfusion-diffusion mismatch which approximates the ischemic penumbra could be imaged noninvasively. This review describes recent progresses in the development and application of multimodal MRI and image analysis techniques to study ischemic tissue at risk in experimental stroke in rats.