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.

1H/13C MR spectroscopic imaging of regionally specific metabolic alterations after experimental stroke.

Loss of function and subsequent spontaneous recovery after ischaemic stroke are associated with functional and structural alterations in brain tissue. Acute functional tissue damage involves distortion of key metabolic processes, such as oxidative glycolysis and neurotransmitter metabolism. Nevertheless, initially perturbed metabolism may be restored at later stages, e.g. in perilesional areas, which could play a key role in post-stroke recovery of brain function. The pattern of metabolic recovery in relation to ischaemic tissue damage, however, is basically unknown. The goal of our study was to reveal changes in glycolysis and glutamatergic neurotransmitter metabolism that could underlie post-stroke changes in functional status. We performed in vivo (1)H/(13)C magnetic resonance spectroscopic imaging (MRSI) during (13)C-labelled glucose infusion, and MRI, at 24 h (n = 6) and 3 weeks (n = 8) after stroke in a rat model to characterize alterations in baseline metabolite levels, glutamate (Glu) and glutamine (Gln) turnover, and active lactate (Lac) formation in areas with different degrees of ischaemic injury. Inside the lesion, we detected significant reductions in baseline metabolite levels, ongoing Lac formation and seriously diminished Glu and Gln turnover at both time points, indicative of irreversible functional tissue damage. In perilesional areas, significant decrease of N-acetyl aspartate (NAA) levels, and Glu and Gln turnover indicated neuronal dysfunction at 24 h. After 3 weeks, when animals showed significant neurological improvement, anaerobic glycolysis had ceased, NAA levels were normalized, Glu turnover was maintained and Gln turnover had recovered. These findings point out that early metabolic impairment in the lesion borderzone can be restored over time. Alterations in brain metabolism in perilesional areas probably contribute significantly to changes in functional status in stroke subjects, and may provide a gateway for therapeutic strategies directed at improvement of functional recovery after stroke.

Longitudinal in vivo MRI of alterations in perilesional tissue after transient ischemic stroke in rats.

Spontaneous restoration of function after stroke is associated with remodelling of functional neuronal networks in and around the ischemic lesion. However, the spatiotemporal profile of structural alterations in (peri)lesional tissue in relation to post-stroke recovery of neuronal function remains largely to be elucidated. We performed neurological testing in combination with in vivo serial T(2)-weighted magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) to assess functional recovery in relation to longitudinal changes in tissue integrity from 3 h up to 9 weeks after experimental unilateral stroke in rats (n=7). Subsequently, to evaluate perilesional neuronal connectivity, we conducted manganese-enhanced MRI after MnCl(2) injection in cortical tissue at the boundary of the lesion at 10 weeks post-stroke (n=5). All animals showed significant improvement of neurological function over time. Normalization of tissue T(2) and fractional diffusion anisotropy (FA) after significant subacute change was observed in cortical and subcortical lesion borderzones between 3 and 9 weeks post-stroke. Progressive FA increase above baseline levels was detected in perilesional white matter areas (n=4). In these animals particularly, significant manganese enhancement appeared within the neuronal network around the chronic lesion, including areas that were part of the lesion at day 3 post-stroke. This longitudinal multi-parametric MRI study suggests that resolution of early ischemic damage and reorganization of white matter in perilesional tissue is chronically accompanied by preservation or restoration of neuronal connectivity, which may significantly contribute to post-stroke functional recovery.

Manganese-enhanced MRI of brain plasticity in relation to functional recovery after experimental stroke.

Restoration of function after stroke may be associated with structural remodeling of neuronal connections outside the infarcted area. However, the spatiotemporal profile of poststroke alterations in neuroanatomical connectivity in relation to functional recovery is still largely unknown. We performed in vivo magnetic resonance imaging (MRI)-based neuronal tract tracing with manganese in combination with immunohistochemical detection of the neuronal tracer wheat-germ agglutinin horseradish peroxidase (WGA-HRP), to assess changes in intra- and interhemispheric sensorimotor network connections from 2 to 10 weeks after unilateral stroke in rats. In addition, functional recovery was measured by repetitive behavioral testing. Four days after tracer injection in perilesional sensorimotor cortex, manganese enhancement and WGA-HRP staining were decreased in subcortical areas of the ipsilateral sensorimotor network at 2 weeks after stroke, which was restored at later time points. At 4 to 10 weeks after stroke, we detected significantly increased manganese enhancement in the contralateral hemisphere. Behaviorally, sensorimotor functions were initially disturbed but subsequently recovered and plateaued 17 days after stroke. This study shows that manganese-enhanced MRI can provide unique in vivo information on the spatiotemporal pattern of neuroanatomical plasticity after stroke. Our data suggest that the plateau stage of functional recovery is associated with restoration of ipsilateral sensorimotor pathways and enhanced interhemispheric connectivity.

Changes in neuronal connectivity after stroke in rats as studied by serial manganese-enhanced MRI.

Loss of function and subsequent spontaneous recovery after stroke have been associated with physiological and anatomical alterations in neuronal networks in the brain. However, the spatiotemporal pattern of such changes has been incompletely characterized. Manganese-enhanced MRI (MEMRI) provides a unique tool for in vivo investigation of neuronal connectivity. In this study, we measured manganese-induced changes in longitudinal relaxation rate, R(1), to assess the spatiotemporal pattern of manganese distribution after focal injection into the intact sensorimotor cortex in control rats (n=10), and in rats at 2 weeks after 90-min unilateral occlusion of the middle cerebral artery (n=10). MEMRI data were compared with results from conventional tract tracing with wheat-germ agglutinin horseradish peroxidase (WGA-HRP). Distinct areas of the sensorimotor pathway were clearly visualized with MEMRI. At 2 weeks after stroke, manganese-induced changes in R(1) were significantly delayed and diminished in the ipsilateral caudate putamen, thalamus and substantia nigra. Loss of connectivity between areas of the sensorimotor network was also identified from reduced WGA-HRP staining in these areas on post-mortem brain sections. This study demonstrates that MEMRI enables in vivo assessment of spatiotemporal alterations in neuronal connectivity after stroke, which may lead to improved insights in mechanisms underlying functional loss and recovery after stroke.