Imaging of inflammation in the peripheral and central nervous system by magnetic resonance imaging

Immunoexcitotoxicity as a central mechanism in chronic traumatic encephalopathy-A unifying hypothesis

Some individuals suffering from mild traumatic brain injuries, especially repetitive mild concussions, are thought to develop a slowly progressive encephalopathy characterized by a number of the neuropathological elements shared with various neurodegenerative diseases. A central pathological mechanism explaining the development of progressive neurodegeneration in this subset of individuals has not been elucidated. Yet, a large number of studies indicate that a process called immunoexcitotoxicity may be playing a central role in many neurodegenerative diseases including chronic traumatic encephalopathy (CTE). The term immunoexcitotoxicity was first coined by the lead author to explain the evolving pathological and neurodevelopmental changes in autism and the Gulf War Syndrome, but it can be applied to a number of neurodegenerative disorders. The interaction between immune receptors within the central nervous system (CNS) and excitatory glutamate receptors trigger a series of events, such as extensive reactive oxygen species/reactive nitrogen species generation, accumulation of lipid peroxidation products, and prostaglandin activation, which then leads to dendritic retraction, synaptic injury, damage to microtubules, and mitochondrial suppression. In this paper, we discuss the mechanism of immunoexcitotoxicity and its link to each of the pathophysiological and neurochemical events previously described with CTE, with special emphasis on the observed accumulation of hyperphosphorylated tau.

Cellular markers of neuroinflammation and neurogenesis after ischemic brain injury in the long-term survival rat model

MRI was employed to follow the neurodegenerative foci and the localization of inflammatory cells by magnetically labeled CD4+ or CD8+ lymphocytes in the ischemia/reperfusion long-lived rats (9 and 13 months after 10 min of cardiac arrest). MRI of ischemic rats showed: (1) blood-brain barrier (BBB) leakage in the area of the dorsal hippocampus and brainstem-hindbrain level in basal cerebellum, (2) unlike anti-CD8 magnetic antibodies anti-CD4 ultra small paramagnetic iron oxide particles (USPIO) antibodies revealed hypointense areas in the brainstem-interbrain region and caudoputamen not found in animals that were not injected with USPIO antibodies, and (3) dilation in the retrosplenial area. Immunocytochemistry revealed microglial activation in the hippocampus and striatum, with indications of activation in thalamic lateral dorsal nuclei and the subventricular zone. In the CA1 and CA3 regions, it was noted that OX42- and ED1-positive granules appear in neuronal somata. Immunostaining of lymphocytes with TCR confirmed the T-cell presence in ischemic brain parenchyma of the hippocampus and striatum. The above observations thus point to a persistent dysfunction of BBB that in long-term may still lead to infiltration of T cells that are predominantly of helper (CD4+) type. Such inflammatory processes are backed by microglial activity even up to 1 year after ischemia/reperfusion. Moreover, in these animals an augmented expression of neurogenesis markers and neuroblast migration was also revealed in the subventricular zone. Thus, a balance of degenerative processes and inflammatory surveillance with neurogenesis could determine the long-term outcome of global ischemia survival or the previously proposed formation of amyloid plaques and Alzheimer's-type dementia.

Inflammation in epilepsy: clinical observations

Over the past decade, an increasing number of observations indicate that activation of inflammatory processes occurs in variety of focal epilepsies. Understanding the feature and consequences of neuroinflammation, including the contribution to development and perpetuation of seizures, as well as to mood or cognitive dysfunction, is a major requisite for delineating its role in epilepsy. The present article discusses the most recent observations supporting the involvement of the inflammatory response in human focal epilepsy. It also evaluates emerging evidence concerning the possibility to identify epilepsy-associated inflammatory biomarkers in cerebrospinal fluid and serum, as well as the potential application of neuroimaging approaches to study the inflammatory reactions in chronic epilepsy patients in vivo, aiming to improve the recognition of appropriate patient populations who might benefit from antiinflammatory or immunomodulatory therapies.

Immune-mediated epilepsies

A pathogenic role of immunity in epilepsies has long been suggested based on observations of the efficacy of immune-modulating treatments and, more recently, by the finding of inflammation markers including autoantibodies in individuals with a number of epileptic disorders. Clinical and experimental data suggest that both innate and adaptive immunity may be involved in epilepsy. Innate immunity represents an immediate, nonspecific host response against pathogens via activation of resident brain immune cells and inflammatory mediators. These are hypothesized to contribute to seizures and epileptogenesis. Adaptive immunity employs activation of antigen-specific B and T lymphocytes or antibodies in the context of viral infections and autoimmune disorders. In this article we critically review the evidence for pathogenic roles of adaptive immune responses in several types of epilepsy, and discuss potential mechanisms and therapeutic targets. We highlight future directions for preclinical and clinical research that are required for improved diagnosis and treatment of immune-mediated epilepsies.

In vivo imaging of inflammation in the peripheral nervous system by (19)F MRI

Visualization of neuroinflammation is still a major task in neuroscience and neurology since inflammatory processes play a central pathophysiological role in many disorders of the nervous system but are not yet covered by conventional imaging techniques. Recently, (19)F magnetic resonance imaging (MRI) was introduced as a new cellular imaging technology. In the present study, we established (19)F high field MRI for cell tracking in the peripheral nervous system (PNS) of rats using dedicated MR coils. To mimic focal neuroinflammation, lysolecithin was locally injected into the left sciatic nerve inducing demyelination followed by severe infiltration of monocytes/macrophages from the circulation. Systemic administration of perfluorocarbons (PFC) led to a fluorine signal along the proximal stretch of the affected sciatic nerves in in vivo(19)F MRI which was not seen on the right healthy side. The preferential in vivo uptake of PFC by circulating mononuclear cells was confirmed by density gradient centrifugation of the blood. Removal of nerves with consecutive ex vivo(19)F MRI and additional (19)F spectroscopy for quantification corroborated the localization of the (19)F marker within the injured nerves (1.07×10(18)±1.00×10(18) mean detectable fluorine spins) while contralateral naive nerves did not exhibit any detectable fluorine signal. Histological assessment confirmed the presence of numerous ED1-positive macrophages within the nerve lesions. Control experiments showed that intraneural application of saline led to an inflammatory reaction restricted to the perineurium which could also be detected by (19)F MRI. In conclusion, we show that (19)F MRI is a promising new technology to visualize hematogenous macrophage responses in the nervous system.

Superparamagnetic iron oxide nanoparticles: promises for diagnosis and treatment of multiple sclerosis

Smart superparamagnetic iron oxide nanoparticles (SPIONs) are the most promising candidate for theragnosis (i.e., diagnosis and treatment) of multiple sclerosis. A deep understanding of the dynamics of the in vivo neuropathology of multiple sclerosis can be achieved by improving the efficiency of various medical techniques (e.g., positron emission tomography and magnetic resonance imaging) using multimodal SPIONs. In this Review, recent advances and challenges in the development of smart SPIONs for theragnostic applications are comprehensively described. In addition, critical outlines of emerging developments are provided from the points of view of both clinicians and nanotechnologists.

Detection of amyloid plaques targeted by USPIO-Aβ1-42 in Alzheimer's disease transgenic mice using magnetic resonance microimaging

Amyloid plaques are one of the pathological hallmarks of Alzheimer's disease (AD). The visualization of amyloid plaques in the brain is important to monitor AD progression and to evaluate the efficacy of therapeutic interventions. Our group has developed several contrast agents to detect amyloid plaques in vivo using magnetic resonance microimaging (μMRI) in AD transgenic mice, where we used intra-carotid mannitol to enhance blood-brain barrier (BBB) permeability. In the present study, we used ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles, chemically coupled with Aβ1-42 peptide to detect amyloid deposition along with mannitol for in vivo μMRI by femoral intravenous injection. A 3D gradient multi-echo sequence was used for imaging with a 100μm isotropic resolution. The amyloid plaques detected by T2*-weighted μMRI were confirmed with matched histological sections. Furthermore, two different quantitative analyses were used. The region of interest-based quantitative measurement of T2* values showed contrast-injected APP/PS1 mice had significantly reduced T2* values compared to wild-type mice. In addition, the scans were examined with voxel-based morphometry (VBM) using statistical parametric mapping (SPM) for comparison of contrast-injected AD transgenic and wild-type mice. The regional differences seen in VBM comparing USPIO-Aβ1-42 injected APP/PS1 and wild-type mice correlated with the amyloid plaque distribution histologically, contrasting with no differences between the two groups of mice without contrast agent injection in regions of the brain with amyloid deposition. Our results demonstrated that both approaches were able to identify the differences between AD transgenic mice and wild-type mice, after injected with USPIO-Aβ1-42. The feasibility of using less invasive intravenous femoral injections for amyloid plaque detection in AD transgenic mice facilitates using this method for longitudinal studies in the pathogenesis of AD.

Experimental stroke research: the contributions of in vivo MRI

Stroke is a disease that develops from the very acute time point of first symptoms during the next several hours and further to a chronic time period of days or even weeks. During this evolution process, a whole series of pathophysiological events takes place. Therefore, the disease is characterized by a continuously changing pathophysiological pattern. In consequence, as the disease develops over time, different imaging modalities must be chosen to accurately describe the status of stroke. In the present chapter, we have divided the evolution of stroke into various dominant steps of the cascade of events, with corresponding time windows. Choice of MRI variables for depiction of the most important aspects during these time windows are presented and their information content is discussed for diagnosis and for investigations into a better understanding of the underlying mechanisms for the disease as well as the relevance of these imaging tools in success assessments for therapeutic strategies.

New approaches to neuroimaging of central nervous system inflammation

PURPOSE OF REVIEW

Inflammation is an important component not only in autoimmune but also in ischemic/degenerative disorders of the central nervous system (CNS). We here review magnetic resonance imaging (MRI)-based techniques to visualize neuroinflammation in vivo.

RECENT FINDINGS

Iron oxide particles such as superparamagnetic iron oxide (SPIO) and ultrasmall SPIO (USPIO) are phagocytosed by hematogeneous macrophages upon systemic application into the circulation and allow in-vivo tracking of infiltration to the CNS due to their paramagnetic effect by MRI in experimental CNS disorders, and also in multiple sclerosis and stroke. Thereby, the size and application scheme of the iron particles is critical for interpretation of the MRI data which in addition to neuroinflammation involves passive diffusion and intravascular trapping. Targeting of inflammatory, activation-dependent enzymes such as myeloperoxidase or immune function molecules by MR contrast agents represents a molecular approach to visualize critical steps of lesion development caused by neuroinflammation. Clinical studies with Gd-DTPA in conjunction with experimental investigations employing more sensitive MR contrast agents such as gadofluorine revealed that breakdown of the blood-brain barrier and SPIO/USPIO-related macrophage infiltration occur mostly independently.

SUMMARY

Cellular and targeted molecular MRI provides important insights into the dynamics of neuroinflammation.

Nanotechnology for treatment of stroke and spinal cord injury

The use of nanotechnology in cell therapy and tissue engineering offers promising future perspectives for brain and spinal cord injury treatment. Stem cells have been shown to selectively target injured brain and spinal cord tissue and improve functional recovery. To allow cell detection, superparamagnetic iron-oxide nanoparticles can be used to label transplanted cells. MRI is then a suitable method for the in vivo tracking of grafted cells in the host organism. CNS, and particularly spinal cord, injury is accompanied by tissue damage and the formation of physical and biochemical barriers that prevent axons from regenerating. One aspect of nanomedicine is the development of biologically compatible nanofiber scaffolds that mimic the structure of the extracellular matrix and can serve as a permissive bridge for axonal regeneration or as a drug-delivery system. The incorporation of biologically active epitopes and/or the utilization of these scaffolds as stem cell carriers may further enhance their therapeutic efficacy.

In vivo morphological changes in animal models of amyotrophic lateral sclerosis and Alzheimer's-like disease: MRI approach

Magnetic resonance imaging (MRI) is the only noninvasive technique that provides structural information on both cell loss and metabolic changes. After reviewing all the results obtained in clinical studies, reliable biomarkers in neurological diseases are still lacking. Diffusional MRI, MR spectroscopy, and the assessment of regional atrophy are promising approaches, but they cannot be simultaneously used on a single patient. Thus, for further research progress, reliable animal models are needed. To this aim, we have used the clinical MRI to assess neurodegenerative processes in the hSOD-1(G93A) ALS rat model and in the trimethyltin (TMT)-treated model of Alzheimer's-like disease. T2-weighted (T2W) hyperintensive neurodegenerative foci were found in the brainstem of the ALS rat with apparent lateral ventricle dilation (T1W-hypointensity vs. T2W-hyperintensity). Degenerative processes in these areas were also confirmed by confocal images of GFAP-positive astrogliosis. MRI after i.v.i. of magnetic anti-CD4 antibodies indicated an accumulation of inflammatory cells near dilated ventricles. TMT-treated rats also revealed the dilation of lateral ventricles. Expected deterioration in the hippocampus was not observed by clinical MRI, but immunocytochemistry could reveal significant redistribution of macro- and microglia in this structure. In both models, Gd-DTPA contrast revealed a compromised blood brain barrier that may serve as the passage for inflammatory immune cells in the vicinity of dilated lateral ventricles. Moreover, in both models the midbrain region of the dorsal hippocampus was the target of BBB compromise, thus revealing a potentially vulnerable point that can be the primary target of neurodegeneration in the central nervous system.

Early assessment of pulmonary inflammation by 19F MRI in vivo

BACKGROUND

Emulsified perfluorocarbons (PFCs) are preferentially phagocytized by monocytes/macrophages and are readily detected by (19)F MRI. This study tests the hypothesis that (19)F MRI can be used to quantitate pulmonary inflammation by tracking of infiltrating PFC-loaded monocytes.

METHODS AND RESULTS

Pneumonia was induced in mice by intratracheal instillation of lipopolysaccharides (LPS) followed by intravenous injection of PFCs. Whereas regular (1)H MRI provided no evidence of lung injury 24 hours after LPS, the concurrent (19)F images clearly show PFC accumulation in both pulmonary lobes. Imaging at 48 hours after LPS revealed signals in (1)H images at the same location as the 24-hour (19)F signals. Thus, progressive pneumonia was first predicted by (19)F MRI early after PFC administration. Without LPS, at no time were (19)F signals observed within the lung. Histology and fluorescence-activated cell sorting (FACS) combined with (19)F MRI confirmed the presence of infiltrating PFC-loaded monocytes/macrophages after LPS challenge. Additional experiments with graded doses of LPS demonstrated that (19)F signal intensity strongly correlated with both LPS dose and pathological markers of lung inflammation. In separate studies, dexamethasone and CGS21680 (adenosine 2A receptor agonist) were used to demonstrate the ability of (19)F MRI to monitor anti-inflammatory therapies.

CONCLUSIONS

PFCs serve as a contrast agent for the prognostic and quantitative assessment of pulmonary inflammation by in vivo (19)F MRI, which is characterized by a high degree of specificity due to the lack of any (19)F background. Because PFCs are biochemically inert, this approach may also be suitable for human applications.

Radioisotopic imaging of neuroinflammation

Inflammatory responses are closely associated with many neurologic disorders and influence their outcome. In vivo imaging can document events accompanying neuroinflammation, such as changes in blood flow, vascular permeability, tightness of the blood-to-brain barrier, local metabolic activity, and expression of specific molecular targets. Here, we briefly review current methods for imaging neuroinflammation, with special emphasis on nuclear imaging techniques.

Anomalous venous blood flow and iron deposition in multiple sclerosis

Multiple sclerosis (MS) is primarily an autoimmune disorder of unknown origin. This review focuses iron overload and oxidative stress as surrounding cause that leads to immunomodulation in chronic MS. Iron overload has been demonstrated in MS lesions, as a feature common with other neurodegenerative disorders. However, the recent description of chronic cerebrospinal venous insufficiency (CCSVI) associated to MS, with significant anomalies in cerebral venous outflow hemodynamics, permit to propose a parallel with chronic venous disorders (CVDs) in the mechanism of iron deposition. Abnormal cerebral venous reflux is peculiar to MS, and was not found in a miscellaneous of patients affected by other neurodegenerative disorders characterized by iron stores, such as Parkinson's, Alzheimer's, amyotrophic lateral sclerosis. Several recently published studies support the hypothesis that MS progresses along the venous vasculature. The peculiarity of CCSVI-related cerebral venous blood flow disturbances, together with the histology of the perivenous spaces and recent findings from advanced magnetic resonance imaging techniques, support the hypothesis that iron deposits in MS are a consequence of altered cerebral venous return and chronic insufficient venous drainage.

Diffusion tensor imaging to assess axonal regeneration in peripheral nerves

Development of outcome measures to assess ongoing nerve regeneration in the living animal that can be translated to human can provide extremely useful tools for monitoring the effects of therapeutic interventions to promote nerve regeneration. Diffusion tensor imaging (DTI), a magnetic resonance based technique, provides image contrast for nerve tracts and can be applied serially on the same subject with potential to monitor nerve fiber content. In this study, we examined the use of ex vivo high-resolution DTI for imaging intact and regenerating peripheral nerves in mice and correlated the MRI findings with electrophysiology and histology. DTI was done on sciatic nerves with crush, without crush, and after complete transection in different mouse strains. DTI measures, including fractional anisotropy (FA), parallel diffusivity, and perpendicular diffusivity were acquired and compared in segments of uninjured and crushed/transected nerves and correlated with morphometry. A comparison of axon regeneration after sciatic nerve crush showed a comparable pattern of regeneration in different mice strains. FA values were significantly lower in completely denervated nerve segments compared to uninjured sciatic nerve and this signal was restored toward normal in regenerating nerve segments (crushed nerves). Histology data indicate that the FA values and the parallel diffusivity showed a positive correlation with the total number of regenerating axons. These studies suggest that DTI is a sensitive measure of axon regeneration in mouse models and provide basis for further development of imaging technology for application to living animals and humans.

Spatial diversity of blood-brain barrier alteration and macrophage invasion in experimental autoimmune encephalomyelitis: a comparative MRI study

Inflammation plays a central role in the development of numerous disorders of the central nervous system (CNS) such as multiple sclerosis (MS). For a long time it was assumed that recruitment of macrophages into the CNS and breakdown of the blood-brain barrier (BBB) are closely linked. In the present study we challenge this concept. We used small superparamagnetic iron oxide particles (SPIO)-enhanced T2-weighted (T2-w) magnetic resonance imaging (MRI) on a routine 1.5 T MRI unit to follow macrophage infiltration in experimental autoimmune encephalomyelitis (EAE), the animal model of MS. After an initial SPIO-enhanced MRI, gadofluorine M (Gf), an experimental contrast agent which proved to be more sensitive in detecting BBB leakage than gadolinium (Gd)-DTPA (Bendszus, M., Ladewig, G., Jestaedt, L., Misselwitz, B., Solymosi, L., Toyka, K.V., Stoll, G., Gadofluorine-M enhancement allows more sensitive detection of inflammatory CNS lesions than T2-w imaging: a quantitative MRI study. Brain 2008; 1-12), was applied to the same animals followed by a second scan. Areas with SPIO-induced signal loss on T2-w MRI indicative of recent macrophage infiltration were matched with areas showing Gf enhancement as a measure of BBB disturbance. Overall 87 EAE lesions showed iron-related signal loss, while 57 lesions showed Gf enhancement. By direct comparison we could detect concomitant SPIO-induced signal loss and Gf enhancement only in a small minority of lesions. In conclusion, our findings show macrophage infiltration in the CNS during EAE in areas with a closed BBB for humoral factors. This holds true despite the use of a more sensitive MR contrast agent for BBB disruption than Gd-DTPA. Our experimental observations may have implications for disease monitoring in MS patients by MRI which guides treatment decisions.

The role of the blood-CNS barrier in CNS disorders and their treatment

The physical barrier between blood and the CNS (the blood-brain barrier, the blood-spinal cord barrier and the blood-CSF barrier) protects the CNS from both toxic and pathogenic agents in the blood. It is now clear that disruption of the blood-CNS barrier plays a key role in a number of CNS disorders, particularly those associated with neurodegeneration. Such disruption is inevitably accompanied by inflammatory change, as immune cells and immune mediators gain access to the brain or spinal cord. The blood-CNS barrier also presents a major obstacle for potential CNS medicines. Robust methods to assess CNS permeation are therefore essential for CNS drug discovery, particularly when brain pharmacokinetics are taken into account and especially when such measures are linked to neurochemical, physiological, behavioural or neuroimaging readouts of drug action. Drug candidates can be successfully designed to cross the blood-CNS barrier, but for those that can't there is the possibility of entry with a delivery system that facilitates the movement of drug candidate across the blood-CNS barrier.

Imaging microglial activation during neuroinflammation and Alzheimer's disease

Microglial activation is an important pathogenic component of neurodegenerative disease processes. This state of increased inflammation is associated not only with neurotoxic consequences but also neuroprotective effects, e.g., phagocytosis and clearance of amyloid in Alzheimer's disease. In addition, activation of microglia appears to be one of the major mechanisms of amyloid clearance following active or passive immunotherapy. Imaging techniques may provide a minimally invasive tool to elucidate the complexities and dynamics of microglial function and dysfunction in aging and neurodegenerative diseases. Imaging microglia in vivo in live subjects by confocal or two/multiphoton microscopy offers the advantage of studying these cells over time in their native environment. Imaging microglia in human subjects by positron emission tomography scanning with translocator protein-18 kDa ligands can offer a measure of the inflammatory process and a means of detecting progression of disease and efficacy of therapeutics over time.

Blood-brain barrier breakdown-inducing astrocytic transformation: novel targets for the prevention of epilepsy

Epileptogenesis is common following brain insults such as trauma, ischemia and infection. However, the mechanisms underlying injury-related epileptogenesis remain unknown. Recent studies demonstrated impaired integrity of the blood-brain barrier (BBB) during epileptogenesis. Here we review accumulating experimental evidence supporting the potential involvement of primary BBB lesion in epileptogenesis. Data from animal experiments demonstrate that primary breakdown of the BBB prone animals to develop focal neocortical epilepsy that is followed by neuronal loss and impaired functions. The extravasation of albumin from the circulation into the brain neuropil was found to be sufficient for the induction of epileptogenesis. Albumin binds to transforming growth factor beta receptor 2 (TGFbetaR2) in astrocytes and induces rapid transcriptional modifications, astrocytic transformation and dysfunction. We highlight a novel cascade of events which is initiated by increased BBB permeability, eventually leading to neuronal dysfunction, epilepsy and cell loss. We review potential mechanisms and existing experimental evidence for the important role of astrocytes and the TGFbeta pathway in epileptogenesis. Finally, we review evidence from human clinical data supporting the involvement of BBB lesion in epilepsy. We propose that primary vascular injury, and specifically BBB breakdown and repair, are key elements in altered interactions within the neurovascular unit and thus may serve as new therapeutic targets.

Dynamic MRI using iron oxide nanoparticles to assess early vascular effects of antiangiogenic versus corticosteroid treatment in a glioma model

The vascular effects of antiangiogenic treatment may pose problems for evaluating brain tumor response based on contrast-enhanced magnetic resonance imaging (MRI). We used serial dynamic contrast-enhanced MRI at 12 T to assess vascular responses to antiangiogenic versus steroid therapy. Athymic rats with intracerebral U87MG human glioma (n=17) underwent susceptibility-weighted perfusion MRI with ferumoxytol, a solely intravascular ultrasmall superparamagnetic iron oxide (USPIO) nanoparticle, followed by T1-weighted dynamic gadodiamide-enhanced MRI to measure vascular permeability. Rats were imaged before and after 24, 48, and 72 h of treatment with the antiangiogenic agent bevacizumab or the corticosteroid dexamethasone. Contrast agent extravasation was seen rapidly after gadodiamide, but not with ferumoxytol administration. Bevacizumab significantly decreased the blood volume and decreased permeability in tumors as determined by increased time-to-peak enhancement. A single dose of 45 mg/kg bevacizumab resulted in changes analogous to dexamethasone given in an extremely high dose (12 mg/kg per day), and was significantly more effective than dexamethasone at 2 mg/kg per day. We conclude that dynamic perfusion MRI measurements with ferumoxytol USPIO to assess cerebral blood volume, along with dynamic gadodiamide-enhanced MR to assess vascular permeability, hold promise in more accurately detecting therapeutic responses to antiangiogenic therapy.

Magnetic resonance imaging of the peripheral nervous system

The diagnostic work up of patients with peripheral neuropathy largely depends on clinical and electrophysiological investigations. In contrast to disorders of the CNS, magnetic resonance imaging (MRI) has not been widely used as a diagnostic tool in the PNS except for detection of nerve compressing mass lesions. Normal nerves appear isointense to the surrounding tissue on T1- and T2-weighted (w) MRIs, but upon injury the nerves become hyperintense and thus visible on T2-w MRI. These signal alterations can be exploited to diagnose nerve damage in vivo and to follow regeneration. In patients with peripheral nerve disorders, MRI has been especially useful in detecting focal intrinsic and extrinsic nerve lesions and may reveal treatable conditions even in the absence of gross electrophysiological alterations. This clinical review provides practical guidelines on the performance of nerve imaging by MRI and will focus on focal lesions exemplified by case presentations.

Non-invasive visualization of CNS inflammation with nuclear and optical imaging

Inflammation is crucially involved in many diseases of the CNS. Immune cells may attack the CNS, as in multiple sclerosis, and therefore be responsible for primary damage. Immune cells may also be activated by injury to the CNS, as for example in stroke or brain trauma, secondarily enhancing lesion growth. In general, CNS inflammation involves a complex interplay of pro- and anti-inflammatory cells and molecules. The blood-brain barrier loses its integrity, plasma proteins leak into the CNS parenchyma, followed by invasion of blood-borne immune cells, and activation of resident microglial cells and astrocytes. However, inflammation not only exacerbates CNS disease, it is also indispensable in containment and resolution of tissue damage, as well as repair and regeneration. The time course and the contribution of inflammatory processes to the pathophysiology of the disease depend on several factors including the type of injury and the time point after injury, and can exhibit a high individual variability. Imaging technologies that enable specific visualization of these inflammatory processes non-invasively are therefore highly desirable. They provide powerful tools to further evaluate the contribution of specific processes to the pathophysiology of CNS disease. Moreover, these technologies may be valuable in detecting and assessing disease progression, in stratifying patients for therapy, and in monitoring therapy. Among the existing non-invasive imaging methods to visualize neuroinflammation in the CNS, we here review the current status of nuclear and optical imaging techniques, with particular emphasis on the sensitivity, specificity, as well as the limitations of these approaches.