Development of a high resolution MRI intracranial atherosclerosis imaging phantom.

BACKGROUND AND PURPOSE: Currently, there is neither a standard protocol for vessel wall MR imaging of intracranial atherosclerotic disease (ICAD) nor a gold standard phantom to compare MR sequences. In this study, a plaque phantom is developed and characterized that provides a platform for establishing a uniform imaging approach for ICAD. MATERIALS AND METHODS: A patient specific injection mold was 3D printed to construct a geometrically accurate ICAD phantom. Polyvinyl alcohol hydrogel was infused into the core shell mold to form the stenotic artery. The ICAD phantom incorporated materials mimicking a stenotic vessel and plaque components, including fibrous cap and lipid core. Two phantoms were scanned using high resolution cone beam CT and compared with four different 3 T MRI systems across eight different sites over a period of 18 months. Inter-phantom variability was assessed by lumen dimensions and contrast to noise ratio (CNR). RESULTS: Quantitative evaluation of the minimum lumen radius in the stenosis showed that the radius was on average 0.80 mm (95% CI 0.77 to 0.82 mm) in model 1 and 0.77 mm (95% CI 0.74 to 0.81 mm) in model 2. The highest CNRs were observed for comparisons between lipid and vessel wall. To evaluate manufacturing reproducibility, the CNR variability between the two models had an average absolute difference of 4.31 (95% CI 3.82 to 5.78). Variation in CNR between the images from the same scanner separated by 7 months was 2.5-6.2, showing reproducible phantom durability. CONCLUSIONS: A plaque phantom composed of a stenotic vessel wall and plaque components was successfully constructed for multicenter high resolution MRI standardization.

In Vivo Molecular MRI of ICAM-1 Expression on Endothelium and Leukocytes from Subacute to Chronic Stages After Experimental Stroke.

Molecular MRI allows in vivo detection of vascular cell adhesion molecules expressed on inflamed endothelium, which enables detection of specific targets for anti-neuroinflammatory treatment. We explored to what extent MR contrast agent targeted to intercellular adhesion molecule-1 (ICAM-1) could detect endothelial- and leukocyte-associated ICAM-1 expression at different stages after experimental stroke. Furthermore, we assessed potential interfering effects of ICAM-1-targeted contrast agent on post-stroke lesion growth. Micron-sized particles of iron oxide (MPIO) functionalized with control IgG (IgG-MPIO) or anti-ICAM-1 antibody (αICAM-1-MPIO) were administrated at 1, 2, 3, 7, and 21 days after unilateral transient middle cerebral artery occlusion in mice, followed by in vivo MRI and postmortem immunohistochemistry. αICAM-1-MPIO induced significant contrast effects in the lesion core on post-stroke days 1, 2, and 3, and in the lesion borderzone and contralesional tissue on post-stroke day 2. αICAM-1-MPIO were confined to ICAM-1-positive vessels and occasionally co-localized with leukocytes. On post-stroke day 21, abundant leukocyte-associated αICAM-1-MPIO was immunohistochemically detected in the lesion core. However, MRI-based detection of αICAM-1-MPIO-labeled leukocytes was confounded by pre-contrast MRI hypointensities, presumably caused by phagocytosed blood remains. IgG-MPIO did not induce significant MRI contrast effects at 1 h after injection. Lesion development was not affected by injection of αICAM-1-MPIO or IgG-MPIO. αICAM-1-MPIO are suitable for in vivo MRI of ICAM-1 expression on vascular endothelium and leukocytes at different stages after stroke. Development of clinically applicable MPIO may offer unique opportunities for MRI-based diagnosis of neuroinflammation and identification of anti-inflammatory targets in acute stroke patients.

In situ tissue engineering: endothelial growth patterns as a function of flow diverter design.

BACKGROUND: Vascular remodeling in response to implantation of a tissue engineering scaffold such as a flow diverter (FD) leads to the cure of intracranial aneurysms. We hypothesize that the vascular response is dependent on FD design, and CD34+ progenitor cells play an important role in the endothelialization of the implant. METHODS: Sixteen rabbit aneurysms were randomly treated with two different single-layer braided FDs made of cobalt-chrome alloys. The FD-48 and FD-72 devices had 48 and 72 wires, respectively. Aneurysm occlusion rate was assessed during the final digital subtraction angiogram at 10, 20, 30, and 60 days (n=2 per device per time point). Implanted vessels were analyzed with scanning electron microscopy for tissue coverage, endothelialization, and immuno-gold labeling for CD34+ cells. RESULTS: Complete aneurysm occlusion rates were similar between the devices; however, complete or near complete occlusion was more frequently observed in aneurysms with neck ≤4.2 mm (p=0.008). Total tissue coverage at 10 days over the surface of the FD-48 and FD-72 devices was 56.4±11.6% and 76.6±3.6%, respectively. Endothelial cell growth over the surface was time-dependent for the FD-72 device (Spearman's r=0.86, p=0.013) but not for the FD-48 device (Spearman's r=-0.59, p=0.094). The endothelialization score was marginally correlated with the distance from the aneurysm neck for the FD-48 device (Spearman's r=1, p=0.083) but not for the FD-72 device (Spearman's r=0.8, p=0.33). CD34+ cells were present along the entirety of both devices at all time points. CONCLUSIONS: This study gives preliminary evidence that temporal and spatial endothelialization is dependent on FD design. Circulating CD34+ progenitor cells contribute to endothelialization throughout the healing process.

Quantitative assessment of device-clot interaction for stent retriever thrombectomy.

PURPOSE: Rapid revascularization in emergent large vessel occlusion with endovascular embolectomy has proven clinical benefit. We sought to measure device-clot interaction as a potential mechanism for efficient embolectomy. METHODS: Two different radiopaque clot models were injected to create a middle cerebral artery occlusion in a patient-specific vascular phantom. A radiopaque stent retriever was deployed within the clot by unsheathing the device or a combination of unsheathing followed by pushing the device (n=8/group). High-resolution cone beam CT was performed immediately after device deployment and repeated after 5 min. An image processing pipeline was created to quantitatively evaluate the volume of clot that integrates with the stent, termed the clot integration factor (CIF). RESULTS: The CIF was significantly different for the two deployment variations when the device engaged the hard clot (p=0.041), but not the soft clot (p=0.764). In the hard clot, CIF increased significantly between post-deployment and final imaging datasets when using the pushing technique (p=0.019), but not when using the unsheathing technique (p=0.067). When we investigated the effect of time on CIF in the different clot models disregarding the technique, the CIF was significantly increased in the final dataset relative to the post-deployment dataset in both clot models (p=0.004-0.007). CONCLUSIONS: This study demonstrates in an in vitro system the benefit of pushing the Trevo stent during device delivery in hard clot to enhance integration. Regardless of delivery technique, clot-device integration increased in both clot models by waiting 5 min.

Grading of Regional Apposition after Flow-Diverter Treatment (GRAFT): a comparative evaluation of VasoCT and intravascular OCT.

BACKGROUND: Poor vessel wall apposition of flow diverter (FD) stents poses risks for stroke-related complications when treating intracranial aneurysms, necessitating long-term surveillance imaging. To facilitate quantitative evaluation of deployed devices, a novel algorithm is presented that generates intuitive two-dimensional representations of wall apposition from either high-resolution contrast-enhanced cone-beam CT (VasoCT) or intravascular optical coherence tomography (OCT) images. METHODS: VasoCT and OCT images were obtained after FD implant (n=8 aneurysms) in an experimental sidewall aneurysm model in canines. Surface models of the vessel wall and FD device were extracted, and the distance between them was presented on a two-dimensional flattened map. Maps and cross-sections at potential locations of malapposition detected on VasoCT-based maps were compared. The performance of OCT-based apposition detection was evaluated on manually labeled cross-sections using logistic regression against a thresholded (≥0.25 mm) apposition measure. RESULTS: VasoCT and OCT acquisitions yielded similar Grading of Regional Apposition after Flow-Diverter Treatment (GRAFT) apposition maps. GRAFT maps from VasoCT highlighted 16 potential locations of malapposition, of which two were found to represent malapposed device struts. Logistic regression showed that OCT could detect malapposition with a sensitivity of 98% and a specificity of 81%. CONCLUSIONS: GRAFT delivered quantitative and visually convenient representations of potential FD malapposition and occasional acute thrombus formation. A powerful combination for future neuroendovascular applications is foreseen with the superior resolution delivered by intravascular OCT.

Imaging Inflammation in Cerebrovascular Disease.

Imaging inflammation in large intracranial artery pathology may play an important role in the diagnosis of and risk stratification for a variety of cerebrovascular diseases. Looking beyond the lumen has already generated widespread excitement in the stroke community, and the potential to unveil molecular processes in the vessel wall is a natural evolution to develop a more comprehensive understanding of the pathogenesis of diseases, such as ICAD and brain aneurysms.

Altered contralateral sensorimotor system organization after experimental hemispherectomy: a structural and functional connectivity study.

Hemispherectomy is often followed by remarkable recovery of cognitive and motor functions. This reflects plastic capacities of the remaining hemisphere, involving large-scale structural and functional adaptations. Better understanding of these adaptations may (1) provide new insights in the neuronal configuration and rewiring that underlies sensorimotor outcome restoration, and (2) guide development of rehabilitation strategies to enhance recovery after hemispheric lesioning. We assessed brain structure and function in a hemispherectomy model. With MRI we mapped changes in white matter structural integrity and gray matter functional connectivity in eight hemispherectomized rats, compared with 12 controls. Behavioral testing involved sensorimotor performance scoring. Diffusion tensor imaging and resting-state functional magnetic resonance imaging were acquired 7 and 49 days post surgery. Hemispherectomy caused significant sensorimotor deficits that largely recovered within 2 weeks. During the recovery period, fractional anisotropy was maintained and white matter volume and axial diffusivity increased in the contralateral cerebral peduncle, suggestive of preserved or improved white matter integrity despite overall reduced white matter volume. This was accompanied by functional adaptations in the contralateral sensorimotor network. The observed white matter modifications and reorganization of functional network regions may provide handles for rehabilitation strategies improving functional recovery following large lesions.

Stress-induced alterations in large-scale functional networks of the rodent brain.

Stress-related psychopathology is associated with altered functioning of large-scale brain networks. Animal research into chronic stress, one of the most prominent environmental risk factors for development of psychopathology, has revealed molecular and cellular mechanisms potentially contributing to human mental disease. However, so far, these studies have not addressed the system-level changes in extended brain networks, thought to critically contribute to mental disorders. We here tested the effects of chronic stress exposure (10 days immobilization) on the structural integrity and functional connectivity patterns in the brain, using high-resolution structural MRI, diffusion kurtosis imaging, and resting-state functional MRI, while confirming the expected changes in neuronal dendritic morphology using Golgi-staining. Stress effectiveness was confirmed by a significantly lower body weight and increased adrenal weight. In line with previous research, stressed animals displayed neuronal dendritic hypertrophy in the amygdala and hypotrophy in the hippocampal and medial prefrontal cortex. Using independent component analysis of resting-state fMRI data, we identified ten functional connectivity networks in the rodent brain. Chronic stress appeared to increase connectivity within the somatosensory, visual, and default mode networks. Moreover, chronic stress exposure was associated with an increased volume and diffusivity of the lateral ventricles, whereas no other volumetric changes were observed. This study shows that chronic stress exposure in rodents induces alterations in functional network connectivity strength which partly resemble those observed in stress-related psychopathology. Moreover, these functional consequences of stress seem to be more prominent than the effects on gross volumetric change, indicating their significance for future research.

Experimental focal neocortical epilepsy is associated with reduced white matter volume growth: results from multiparametric MRI analysis.

Focal epilepsy has recently been associated with remote white matter damage, including reduced white matter volume. Longitudinal assessment of these white matter changes, in relation to functional mechanisms and consequences, may be ideally done by in vivo neuroimaging in well-controlled experimental animal models. We assessed whether advanced machine learning algorithm models could accurately detect volumetric changes in white matter from multiparametric MR images, longitudinally collected in a neocortical focal epilepsy rat model. We measured classification accuracy in two supervised segmentation models: i.e. the generalized linear model and the nonlinear random forest model-by comparing computed white matter probabilities with actual neuroanatomically identified white matter. We found excellent overall discriminatory power for both models. However, the random forest model demonstrated a superior goodness-of-fit calibration plot that was close to the ideal calibration line. Based on this model, we measured that total white matter volume increased in young adult control and epileptic rats over a period of 10 weeks, but the average white matter volume was significantly lower in the focal epilepsy group. Changes in gray matter volume were not significantly different between control and epileptic rats. Our results (1) indicate that recurrent spontaneous seizures have an adverse effect on global white matter growth and (2) show that individual whole brain white matter volume can be accurately determined using a combination of multiparametric MRI and supervised segmentation models, offering a powerful tool to assess white matter volume changes in preclinical studies of neurological disease.

Effects of transient unilateral functional brain disruption on global neural network status in rats: a methods paper.

Permanent focal brain damage can have critical effects on the function of nearby as well as remote brain regions. However, the effects of transient disturbances on global brain function are largely unknown. Our goal was to develop an experimental in vivo model to map the impact of transient functional brain impairment on large-scale neural networks in the absence of structural damage. We describe a new rat model of transient functional hemispheric disruption using unilateral focal anesthesia by intracarotid pentobarbital injection. The brain's functional status was assessed with resting-state fMRI (rs-fMRI) and electroencephalography (EEG). We performed network analysis to identify and quantify highly connected network hubs, i.e., "rich-club organization," in pre- and postbarbital functional networks. Perfusion MRI data demonstrated that the catheterized carotid artery predominantly supplied the ipsilateral hemisphere, allowing for selective hemispheric brain silencing. The prebarbital baseline network displayed strong functional connectivity (FC) within and between hemispheres. Following pentobarbital injection, the disrupted hemisphere revealed increased intrahemispheric FC with concomitant decrease of interhemispheric connectivity. The bilateral functional network was characterized by a strong positive rich-club effect, which was not affected by ipsilateral disruption. Nevertheless, the rich-club value was significantly decreased in the ipsilateral hemisphere and to a lesser extent contralaterally. Loss of interhemispheric EEG synchronization supported the rs-fMRI findings. Our data support the concept that densely connected rich-club regions play a central role in global brain communication, and show that network hub configurations can be significantly affected by focal temporary functional hemispheric disruption without structural neuronal damage. Further studies with this rat model will provide essential additional insights into network reorganization patterns in response to transient functional brain disruption.

Long-term oral methylphenidate treatment in adolescent and adult rats: differential effects on brain morphology and function.

Methylphenidate is a widely prescribed psychostimulant for treatment of attention deficit hyperactivity disorder (ADHD) in children and adolescents, which raises questions regarding its potential interference with the developing brain. In the present study, we investigated effects of 3 weeks oral methylphenidate (5 mg/kg) vs vehicle treatment on brain structure and function in adolescent (post-natal day [P]25) and adult (P65) rats. Following a 1-week washout period, we used multimodal magnetic resonance imaging (MRI) to assess effects of age and treatment on independent component analysis-based functional connectivity (resting-state functional MRI), D-amphetamine-induced neural activation responses (pharmacological MRI), gray and white matter tissue volumes and cortical thickness (postmortem structural MRI), and white matter structural integrity (postmortem diffusion tensor imaging (DTI)). Many age-related differences were found, including cortical thinning, white matter development, larger dopamine-mediated activation responses and increased striatal functional connectivity. Methylphenidate reduced anterior cingulate cortical network strength in both adolescents and adults. In contrast to clinical observations from ADHD patient studies, methylphenidate did not increase white matter tissue volume or cortical thickness in rat. Nevertheless, DTI-based fractional anisotropy was higher in the anterior part of the corpus callosum following adolescent treatment. Furthermore, methylphenidate differentially affected adolescents and adults as evidenced by reduced striatal volume and myelination upon adolescent treatment, although we did not observe adverse treatment effects on striatal functional activity. Our findings of small but significant age-dependent effects of psychostimulant treatment in the striatum of healthy rats highlights the importance of further research in children and adolescents that are exposed to methylphenidate.

Functional and structural neural network characterization of serotonin transporter knockout rats.

Brain serotonin homeostasis is crucially maintained by the serotonin transporter (5-HTT), and its down-regulation has been linked to increased vulnerability for anxiety- and depression-related behavior. Studies in 5-HTT knockout (5-HTT(-/-)) rodents have associated inherited reduced functional expression of 5-HTT with increased sensitivity to adverse as well as rewarding environmental stimuli, and in particular cocaine hyperresponsivity. 5-HTT down-regulation may affect normal neuronal wiring of implicated corticolimbic cerebral structures. To further our understanding of its contribution to potential alterations in basal functional and structural properties of neural network configurations, we applied resting-state functional MRI (fMRI), pharmacological MRI of cocaine-induced activation, and diffusion tensor imaging (DTI) in 5-HTT(-/-) rats and wild-type controls (5-HTT(+/+)). We found that baseline functional connectivity values and cocaine-induced neural activity within the corticolimbic network was not significantly altered in 5-HTT(-/-) versus 5-HTT(+/+) rats. Similarly, DTI revealed mostly intact white matter structural integrity, except for a reduced fractional anisotropy in the genu of the corpus callosum of 5-HTT(-/-) rats. At the macroscopic level, analyses of complex graphs constructed from either functional connectivity values or structural DTI-based tractography results revealed that key properties of brain network organization were essentially similar between 5-HTT(+/+) and 5-HTT(-/-) rats. The individual tests for differences between 5-HTT(+/+) and 5-HTT(-/-) rats were capable of detecting significant effects ranging from 5.8% (fractional anisotropy) to 26.1% (pharmacological MRI) and 29.3% (functional connectivity). Tentatively, lower fractional anisotropy in the genu of the corpus callosum could indicate a reduced capacity for information integration across hemispheres in 5-HTT(-/-) rats. Overall, the comparison of 5-HTT(-/-) and wild-type rats suggests mostly limited effects of 5-HTT genotype on MRI-based measures of brain morphology and function.

MRI of ICAM-1 upregulation after stroke: the importance of choosing the appropriate target-specific particulate contrast agent.

PURPOSE: Magnetic resonance imaging (MRI) with targeted contrast agents provides a promising means for diagnosis and treatment monitoring after cerebrovascular injury. Our goal was to demonstrate the feasibility of this approach to detect the neuroinflammatory biomarker intercellular adhesion molecule-1 (ICAM-1) after stroke and to establish a most efficient imaging procedure. PROCEDURES: We compared two types of ICAM-1-functionalized contrast agent: T 1-shortening gadolinium chelate-containing liposomes and T2(*)-shortening micron-sized iron oxide particles (MPIO). Binding efficacy and MRI contrast effects were tested in cell cultures and a mouse stroke model. RESULTS: Both ICAM-1-targeted agents bound effectively to activated cerebrovascular cells in vitro, generating significant MRI contrast-enhancing effects. Direct in vivo MRI-based detection after stroke was only achieved with ICAM-1-targeted MPIO, although both contrast agents showed similar target-specific vascular accumulation. CONCLUSIONS: Our study demonstrates the potential of in vivo MRI of post-stroke ICAM-1 upregulation and signifies target-specific MPIO as most suitable contrast agent for molecular MRI of cerebrovascular inflammation.

Functional MRI and diffusion tensor imaging of brain reorganization after experimental stroke.

The potential of the adult brain to reorganize after ischemic injury is critical for functional recovery and provides a significant target for therapeutic strategies to promote brain repair. Despite the accumulating evidence of brain plasticity, the interaction and significance of morphological and physiological modifications in post-stroke brain tissue remain mostly unclear. Neuroimaging techniques such as functional MRI (fMRI) and diffusion tensor imaging (DTI) enable in vivo assessment of the spatial and temporal pattern of functional and structural changes inside and outside ischemic lesion areas. This can contribute to the elucidation of critical aspects in post-stroke brain remodeling. Task/stimulus-related fMRI, resting-state fMRI, or pharmacological MRI enables direct or indirect measurement of neuronal activation, functional connectivity, or neurotransmitter system responses, respectively. DTI allows estimation of the structural integrity and connectivity of white matter tracts. Together, these MRI methods provide an unprecedented means to (a) measure longitudinal changes in tissue structure and function close by and remote from ischemic lesion areas, (b) evaluate the organizational profile of neural networks after stroke, and (c) identify degenerative and restorative processes that affect post-stroke functional outcome. Besides, the availability of MRI in clinical institutions as well as research laboratories provides an optimal basis for translational research on stroke recovery. This review gives an overview of the current status and perspectives of fMRI and DTI applications to study brain reorganization in experimental stroke models.

Extent of bilateral neuronal network reorganization and functional recovery in relation to stroke severity.

Remodeling of neuronal structures and networks is believed to significantly contribute to (partial) restoration of functions after stroke. However, it has been unclear to what extent the brain reorganizes and how this correlates with functional recovery in relation to stroke severity. We applied serial resting-state functional MRI and diffusion tensor imaging together with behavioral testing to relate longitudinal modifications in functional and structural connectivity of the sensorimotor neuronal network to changes in sensorimotor function after unilateral stroke in rats. We found that gradual improvement of functions is associated with wide-ranging changes in functional and structural connectivity within bilateral neuronal networks, particularly after large stroke. Both after medium and large stroke, brain reorganization eventually leads to (partial) normalization of neuronal signal synchronization within the affected sensorimotor cortical network (intraregional signal coherence), as well as between the affected and unaffected sensorimotor cortices (interhemispheric functional connectivity). Furthermore, the bilateral network configuration shifts from subacutely increased "small-worldness," possibly reflective of initial excessive neuronal clustering and wiring, toward a baseline small-world topology, optimal for global information transfer and local processing, at chronic stages. Cortical network remodeling was accompanied by recovery of initially disrupted structural integrity in corticospinal tract regions, which correlated positively with retrieval of sensorimotor functions. Our study demonstrates that the degree of functional recovery after stroke is associated with the extent of preservation or restoration of ipsilesional corticospinal tracts in combination with reinstatement of interhemispheric neuronal signal synchronization and normalization of small-world cortical network organization.

Alterations in the cholinergic system after frontal cortical infarction in rat brain: pharmacological magnetic resonance imaging of muscarinic receptor responsiveness and stereological analysis of cholinergic forebrain neurons.

Vascular cognitive impairment has been related to dysfunction of the central cholinergic system. Studies exploring the putative relationship between vascular cognitive impairment and cholinergic dysfunction have largely been aimed at symptomatic cholinergic treatment rather than focusing on etiological and pathological factors. The present study characterizes chronic responses of the cholinergic system to focal cerebral infarction. Two separate experiments investigated changes in receptor responsiveness versus changes in cell number after photothrombotic infarction of the frontal cortex in rat brain. First, we conducted pharmacological magnetic resonance imaging (phMRI) together with pilocarpine injection to assess relative cerebral blood volume (CBV) responses related to cholinergic muscarinic receptor activation. PhMRI was conducted at 1 and 3 weeks after photothrombotic infarction of either the left or right frontal cortex. Second, stereological assessment was performed on choline acetyltransferase (ChAT)-immunostained sections to determine cholinergic cell body count in several basal forebrain nuclei at 4 weeks after infarction. Significant reductions in relative CBV responses were observed both inside the ischemic area at 1 and 3 weeks, and in areas distant from the lesion at 3 weeks after right-sided frontal cortical infarction. In contrast, cholinergic cell number remained unchanged. These results demonstrate that cholinergic receptor responsiveness may be significantly altered following cerebral infarction, while projecting cholinergic cells are preserved.

MRI of bilateral sensorimotor network activation in response to direct intracortical stimulation in rats after unilateral stroke.

Reinstatement of perilesional activation and connectivity may underlie functional recovery after stroke. To measure activation responsiveness in perilesional cortex in relation to white matter integrity, we performed functional functional magnetic resonance imaging during stimulation of the contralesional cortex, together with diffusion tensor imaging, 3 and 28 days after stroke in rats. Despite disturbed sensorimotor function and abnormal callosal appearance at day 3, activation amplitudes were preserved in the perilesional sensorimotor cortex, although time-to-peak was significantly delayed. This indicates that in spite of dysfunction, perilesional cortical tissue can be activated subacutely after stroke, while delay of the hemodynamic activation response suggests impaired neurovascular coupling.

Temporal scaling properties and spatial synchronization of spontaneous blood oxygenation level-dependent (BOLD) signal fluctuations in rat sensorimotor network at different levels of isoflurane anesthesia.

Spontaneous fluctuations in the blood oxygenation level-dependent (BOLD) MRI signal during the resting state are increasingly being studied in healthy and diseased brain in humans and animal models. Yet, the relationship between functional brain status and the characteristics of spontaneous BOLD fluctuations remains poorly understood. In order to obtain more insights into this relationship and, in particular, the effects of anesthesia thereupon, we investigated the spatial and temporal correlations of spontaneous BOLD fluctuations in somatosensory and motor regions of rat brain at different inhalation levels of the frequently applied anesthetic isoflurane. We found that the temporal scaling, characterized by the Hurst exponent (H), showed persistent behavior (H > 0.5) at 0.5-1.0% isoflurane. Furthermore, low-pass-filtered spontaneous BOLD oscillations were correlated significantly in bilateral somatosensory and bilateral motor cortices, reflective of interhemispheric functional connectivity. Under 2.9% isoflurane anesthesia, the temporal scaling characteristics approached those of Gaussian white noise (H = 0.5), the relative amplitude of BOLD low-frequency fluctuations declined, and cross-correlations of these oscillations between functionally connected regions decreased significantly. Loss of interhemispheric functional connectivity at 2.9% isoflurane anesthesia was stronger between bilateral motor regions than between bilateral somatosensory regions, which points to distinct effects of anesthesia on differentially organized neuronal networks. Although we cannot completely rule out a possible contribution from hemodynamic signals with a non-neuronal origin, our results emphasize that spatiotemporal characteristics of spontaneous BOLD fluctuations are related to the brain's specific functional status and network organization, and demonstrate that these are largely preserved under light to mild anesthesia with isoflurane.

Correspondence between altered functional and structural connectivity in the contralesional sensorimotor cortex after unilateral stroke in rats: a combined resting-state functional MRI and manganese-enhanced MRI study.

This study shows a significant correlation between functional connectivity, as measured with resting-state functional magnetic resonance imaging (MRI), and neuroanatomical connectivity, as measured with manganese-enhanced MRI, in rats at 10 weeks after unilateral stroke and in age-matched controls. Reduced interhemispheric functional connectivity between the contralesional primary motor cortex (M1) and ipsilesional sensorimotor cortical regions was accompanied by a decrease in transcallosal manganese transfer from contralesional M1 to the ipsilesional sensorimotor cortex after a large unilateral stroke. Increased intrahemispheric functional connectivity in the contralesional sensorimotor cortex was associated with locally enhanced neuroanatomical tracer uptake, which underlines the strong link between functional and structural reorganization of neuronal networks after stroke.

Recovery of sensorimotor function after experimental stroke correlates with restoration of resting-state interhemispheric functional connectivity.

Despite the success of functional imaging to map changes in brain activation patterns after stroke, spatiotemporal dynamics of cerebral reorganization in correlation with behavioral recovery remain incompletely characterized. Here, we applied resting-state functional magnetic resonance imaging (rs-fMRI) together with behavioral testing to longitudinally assess functional connectivity within neuronal networks, in relation to changes in associated function after unilateral stroke in rats. Our specific goals were (1) to identify temporal alterations in functional connectivity within the bilateral cortical sensorimotor system and (2) to elucidate the relationship between those alterations and changes in sensorimotor function. Our study revealed considerable loss of functional connectivity between ipsilesional and contralesional primary sensorimotor cortex regions, alongside significant sensorimotor function deficits in the first days after stroke. The interhemispheric functional connectivity restored in the following weeks, but remained significantly reduced up to 10 weeks after stroke in animals with lesions that comprised subcortical and cortical tissue, whereas transcallosal neuroanatomical connections were preserved. Intrahemispheric functional connectivity between primary somatosensory and motor cortex areas was preserved in the lesion border zone and moderately enhanced contralesionally. The temporal pattern of changes in functional connectivity between bilateral primary motor and somatosensory cortices correlated significantly with the evolution of sensorimotor function scores. Our study (1) demonstrates that poststroke loss and recovery of sensorimotor function is associated with acute deterioration and subsequent retrieval of interhemispheric functional connectivity within the sensorimotor system and (2) underscores the potential of rs-fMRI to assess spatiotemporal characteristics of functional brain reorganization that may underlie behavioral recovery after brain injury.