Ralph Weber

A fully noninvasive and robust experimental protocol for longitudinal fMRI studies in the rat

Functional magnetic resonance imaging (fMRI) is a unique tool to study brain activity and plasticity changes. Combination of blood-oxygen level-dependent (BOLD) fMRI and electrical forepaw stimulation has been used as a standard model to study the somatosensory pathway and brain rehabilitation in rats. The majority of fMRI studies have been performed in animals anesthetized with alpha-chloralose as functional-metabolic coupling is best preserved under this anesthesia. However, alpha-chloralose is not suitable for survival procedures due to side effects, limiting its use to single time point studies of the same animal. We therefore developed a new, totally noninvasive fMRI protocol, using sedation with the alpha2-adrenoreceptor agonist medetomidine in combination with transcutaneous monitoring of blood gases. The continuous subcutaneous administration of medetomidine resulted in stable physiological conditions over a long time and all animals tolerated the repetitive fMRI experiments well. A robust and reproducible, significant BOLD signal increase was observed upon forepaw stimulation in the contralateral primary somatosensory cortex in two consecutive medetomidine sessions in all rats, which was similar to the BOLD signal increase observed in the same animals under alpha-chloralose during a third independent session. Activation in the secondary somatosensory cortex was observed less frequently under both medetomidine and alpha-chloralose. No head motion artifacts or nonspecific brain activation was present. Sedation was quickly reversed by the administration of the antagonist atipamezole after the fMRI experiment. These results demonstrate that longitudinal fMRI studies can be performed safely under sedation with medetomidine to study functional recovery processes upon therapeutical treatment.

Early Prediction of Functional Recovery after Experimental Stroke: Functional Magnetic Resonance Imaging, Electrophysiology, and Behavioral Testing in Rats

Therapeutic success of treatment of cerebral diseases must be assessed in terms of functional outcome. In experimental stroke studies, this has been limited to behavioral studies combined with morphological evaluations and single time point functional magnetic resonance imaging (fMRI) measurements but lacking the access to understanding underlying mechanisms for alterations in brain activation. Using a recently developed blood oxygenation level-dependent fMRI protocol to study longitudinal and intraindividual profiles of functional brain activation in the somatosensory system, we have demonstrated activation reemergence in the original representation field as the basic principle of functional recovery from experimental stroke. No plastic reorganization has been observed at any time point during 7 weeks after stroke induction. Applying combined recording of fMRI and somatosensory evoked potentials, we observed a tight coupling of electrical brain activity and hemodynamic response at all times, indicating persistent preservation of neurovascular coupling. Identification of functional brain recovery mechanisms has important implications for the understanding of brain plasticity after cerebral lesions, whereas preservation of neurovascular coupling is important for the clinical translation of fMRI.

Temporal profile of T2-Weighted MRI Distinguishes between Pannecrosis and Selective Neuronal Death after Transient Focal Cerebral Ischemia in the Rat:

Transient middle cerebral artery occlusion (MCAO) by an intraluminal thread leads to primarily subcortical infarctions with little sensorimotor impairment in the Wistar rat strain. We investigated the course of infarct development in this lesion type for 10 weeks using magnetic resonance imaging (MRI) along with histological characterization. MCAO was induced in male Wistar rats (260 to 300 g) for 60 mins. Animals received follow-up T1- and T2-weighted MRI from day 1 until week 10. Separate groups of animals were analyzed histologically after 2, 6, and 10 weeks. Histology included immunohistochemistry for neuronal and astrocytic markers as well as hematoxylin eosin and luxol fast blue-cresyl violet staining. In contrast to lesions involving the cortex, exclusively subcortical infarctions were characterized by a complete resolution of initially increased T1 and T2 relaxation times by 10 weeks. Between 2 and 10 weeks, neuronal death and gliosis as well as a dense inflammatory infiltrate were evident in these lesions, without damage to fiber tracts or development of cystic cavities. Exclusively subcortical lesions in Wistar rats are characterized by normalization of T1 and T2 relaxation times, which might, however, not be mistaken for tissue recovery. Despite this MRI normalization, selective neuronal death and gliosis develop. Although MRI at individual time points might therefore be ambiguous, the temporal profile of relaxation time changes over the chronic time period allows discrimination of the lesion development into selective neuronal death or pannecrosis.

Present status of Magnetic resonance imaging and spectroscopy in animal stroke models

Magnetic resonance imaging (MRI) is based on a wide variety of physical parameters, which, in principle, can all influence the image contrast conditions. As these diverse variables are validated by independent physiological, metabolic, hemodynamic, and histological techniques, a physiological MRI evolves. This imaging modality has been successfully applied to experimental stroke studies, covering a broad range of raised questions. In the present review, we present an overview of possible physiological criteria to be studied by in vivo MRI and magnetic resonance spectroscopy, and critically analyze the present limits and future potential of the imaging technique for experimental stroke investigations. The documented applications cover the spectrum from morphological-structural details of the lesion to hemodynamic and metabolic alterations, inflammatory reaction, evaluation of thrombolytic treatment, studies on recovery of functional brain activation by functional MRI, and, finally, the most recent applications of exploring stem cells for regenerative therapy.

MRI detection of macrophage activity after experimental stroke in rats: New indicators for late appearance of vascular degradation?

Focal cerebral ischemia was induced in rats and followed with high‐resolution MRI methods for a chronic period of 10 weeks. Two weeks after stroke induction and at the end of the observation period, conventional histological analysis was combined with immunohistochemical staining for macrophages and with Prussian blue staining for the detection of ferric iron. In the late chronic phase, a patchy hypointensity was observed in the ischemic caudoputamen exclusively on T  2* ‐weighted (T  2* W) images, with no change in quantitative T1 and T2 relaxation time maps. This characteristic MRI pattern is different from hemorrhagic transformations (HTs) at earlier time points (2 weeks post stroke induction), which became apparent on images of all three imaging sequences. The exclusive T  2* ‐sensitive hypointensity colocalized with iron‐positive macrophages in the lesion territory at this time. These iron‐containing macrophages were found predominantly around blood vessels in the ischemic tissue, and interpreted as the result of a phagocytotic incorporation of red blood cells leaking from slowly degrading vessels. The present investigation demonstrates the sensitivity of heavily T  2* W 3D MRI for observing the inflammatory response in the chronic phase after stroke, without prior systemic labeling of the blood‐borne macrophages by iron oxide nanoparticles. Magn Reson Med 54:59–66, 2005.

Improved Stem Cell MR Detectability in Animal Models by Modification of the Inhalation Gas

In vivo monitoring of cells labeled with paramagnetic iron oxide particles by magnetic resonance imaging (MRI) is complicated by intrinsic contrast of blood vessels. Distinction between T2* effects caused by blood vessels from those caused by labeled cells was so far only possible after carefully following the location of hypointense regions through subsequent slices of T2*-weighted 3-D MRI datasets, a procedure that is time consuming and not always reliable in the case of smaller blood vessels. Here, we demonstrate that the modification of the inhalation gas mixture from the routinely used composition 35% O2 and 65% N2O to a mixture containing 95% O2 and 5% CO2 results in a contrast suppression of most small blood vessels and reduces the intrinsic T2* effect of large vessels dramatically in an animal model. This change in protocol of physiological conditions was well tolerated by all studied animals, even over prolonged experimental times. The changed inhalation gas mixture thus provides a more reliable identification method for small clusters of iron oxide labeled cells in vivo.

Subcortical lesions after transient thread occlusion in the rat: T2‐weighted magnetic resonance imaging findings without corresponding sensorimotor deficits

To investigate infarct evolution and functional consequences of exclusive subcortical or cortico‐subcortical strokes, transient middle cerebral artery occlusion (MCAO) was conducted in Wistar rats.

Aktuelle Aspekte der Sekundärprophylaxe des ischämischen Schlaganfalls

Secondary prevention including lifestyle modulation and medical interventions remain the basic principle in our therapeutic challenge to reduce the risk of recurrent subsequent ischemic stroke. The substantial number of randomized clinical trials published in the past 2 years was broadened our evidence-based therapeutic armament in the field of secondary prevention of ischemic stroke. An update of current knowledge in secondary stroke prevention is presented in this review on the basis of the 2007 revised guidelines of the German Neurological Society and the German Stroke Society. Special emphasis is given to medical and nonmedical modulation of cardiovascular risk factors (treatment of hypertension, hypercholesterolemia, and diabetes mellitus), prophylactic vitamin supplementation, and the use of platelet inhibitors and treatment of symptomatic intracranial stenosis.

Current aspects of secondary prevention of ischemic stroke

Secondary prevention including lifestyle modulation and medical interventions remain the basic principle in our therapeutic challenge to reduce the risk of recurrent subsequent ischemic stroke. The substantial number of randomized clinical trials published in the past 2 years was broadened our evidence-based therapeutic armament in the field of secondary prevention of ischemic stroke. An update of current knowledge in secondary stroke prevention is presented in this review on the basis of the 2007 revised guidelines of the German Neurological Society and the German Stroke Society. Special emphasis is given to medical and nonmedical modulation of cardiovascular risk factors (treatment of hypertension, hypercholesterolemia, and diabetes mellitus), prophylactic vitamin supplementation, and the use of platelet inhibitors and treatment of symptomatic intracranial stenosis.

Aktuelle Aspekte der Sekundärprophylaxe des ischämischen Schlaganfalls@@@Current aspects of secondary prevention of ischemic stroke

Secondary prevention including lifestyle modulation and medical interventions remain the basic principle in our therapeutic challenge to reduce the risk of recurrent subsequent ischemic stroke. The substantial number of randomized clinical trials published in the past 2 years was broadened our evidence-based therapeutic armament in the field of secondary prevention of ischemic stroke. An update of current knowledge in secondary stroke prevention is presented in this review on the basis of the 2007 revised guidelines of the German Neurological Society and the German Stroke Society. Special emphasis is given to medical and nonmedical modulation of cardiovascular risk factors (treatment of hypertension, hypercholesterolemia, and diabetes mellitus), prophylactic vitamin supplementation, and the use of platelet inhibitors and treatment of symptomatic intracranial stenosis.