Beyond blood brain barrier breakdown - in vivo detection of occult neuroinflammatory foci by magnetic nanoparticles in high field MRI

Quantitative MRI of Gd-DOTA Accumulation in the Mouse Brain After Intraperitoneal Administration: Validation by Mass Spectrometry

BACKGROUND

In mice, intraperitoneal (ip) contrast agent (CA) administration is convenient for mapping microvascular parameters over a long-time window. However, continuous quantitative MRI of CA accumulation in brain over hours is still missing.

PURPOSE

To validate a quantitative time-resolved MRI technique for mapping the CA kinetics in brain upon ip administration.

STUDY TYPE

Prospective, animal model.

SPECIMEN

25 C57Bl/6JRj mice underwent MRI.

FIELD STRENGTH/SEQUENCE

7-T, gradient echo sequence.

ASSESSMENT

Gd-DOTA concentration was monitored by MRI (25 s/repetition) over 135 minutes with (N = 15) and without (N = 10) ip mannitol challenge (5 g/kg). After the final repetition, the brains were sampled to quantify gadolinium by mass spectrometry (MS). Upon manual brain segmentation, the average gadolinium concentration was compared with the MS quantification in transcardially perfused (N = 20) and unperfused (N = 5) mice. Precontrast T1-maps were acquired in 8 of 25 mice.

STATISTICAL TESTS

One-tailed Spearman and Pearson correlation between gadolinium quantification by MRI and by MS, D'Agostino-Pearson test for normal distribution, Bland-Altman analysis to evaluate the agreement between MRI and MS. Significance was set at P-value <0.05.

RESULTS

MRI showed that ip administered CA reached the blood compartment (>5 mM) within 10 minutes and accumulated continuously for 2 hours in cerebrospinal fluid (>1 mM) and in brain tissue. The MRI-derived concentration maps showed interindividual differences in CA accumulation (from 0.47 to 0.81 mM at 2 hours) with a consistent distribution resembling the pathways of the glymphatic system. The average in-vivo brain concentration 2 hours post-CA administration correlated significantly (r = 0.8206) with the brain gadolinium quantification by MS for N = 21 paired observations available.

DATA CONCLUSION

The presented experimental and imaging protocol may be convenient for monitoring the spatiotemporal pattern of CA uptake and clearance in the mouse brain over 2 hours. The quantification of the CA from the MRI signal in brain is corroborated by MS.

EVIDENCE LEVEL

N/A TECHNICAL EFFICACY: Stage 1.

Magnetic labeling of primary murine monocytes using very small superparamagnetic iron oxide nanoparticles

Due to their very small size, nanoparticles can interact with all cells in the central nervous system. One of the most promising nanoparticle subgroups are very small superparamagnetic iron oxide nanoparticles (VSOP) that are citrate coated for electrostatic stabilization. To determine their influence on murine blood-derived monocytes, which easily enter the injured central nervous system, we applied VSOP and carboxydextran-coated superparamagnetic iron oxide nanoparticles (Resovist). We assessed their impact on the viability, cytokine, and chemokine secretion, as well as iron uptake of murine blood-derived monocytes. We found that (1) the monocytes accumulated VSOP and Resovist, (2) this uptake seemed to be nanoparticle- and time-dependent, (3) the decrease of monocytes viability was treatment-related, (4) VSOP and Resovist incubation did not alter cytokine homeostasis, and (5) overall a 6-hour treatment with 0.75 mM VSOP-R1 was probably sufficient to effectively label monocytes for future experiments. Since homeostasis is not altered, it is safe to label blood-derived monocles with VSOP. VSOP labeled monocytes can be used to study injured central nervous system sites further, for example with drug-carrying VSOP.

Recent advancements in nanoparticle-mediated approaches for restoration of multiple sclerosis

Multiple Sclerosis (MS) is an autoimmune disease with complicated immunopathology which necessitates considering multifactorial aspects for its management. Nano-sized pharmaceutical carriers named nanoparticles (NPs) can support impressive management of disease not only in early detection and prognosis level but also in a therapeutic manner. The most prominent initiator of MS is the domination of cellular immunity to humoral immunity and increment of inflammatory cytokines. The administration of several platforms of NPs for MS management holds great promise so far. The efforts for MS management through in vitro and in vivo (experimental animal models) evaluations, pave a new way to a highly efficient therapeutic means and aiding its translation to the clinic in the near future.

Ginsenosides in central nervous system diseases: Pharmacological actions, mechanisms, and therapeutics

The nervous system is one of the most complex physiological systems, and central nervous system diseases (CNSDs) are serious diseases that affect human health. Ginseng (Panax L.), the root of Panax species, are famous Chinese herbs that have been used for various diseases in China, Japan, and Korea since ancient times, and remain a popular natural medicine used worldwide in modern times. Ginsenosides are the main active components of ginseng, and increasing evidence has demonstrated that ginsenosides can prevent CNSDs, including neurodegenerative diseases, memory and cognitive impairment, cerebral ischemia injury, depression, brain glioma, multiple sclerosis, which has been confirmed in numerous studies. Therefore, this review summarizes the potential pathways by which ginsenosides affect the pathogenesis of CNSDs mainly including antioxidant effects, anti-inflammatory effects, anti-apoptotic effects, and nerve protection, which provides novel ideas for the treatment of CNSDs.

Contribution of Tissue Inflammation and Blood-Brain Barrier Disruption to Brain Softening in a Mouse Model of Multiple Sclerosis

Neuroinflammatory processes occurring during multiple sclerosis cause disseminated softening of brain tissue, as quantified by in vivo magnetic resonance elastography (MRE). However, inflammation-mediated tissue alterations underlying the mechanical integrity of the brain remain unclear. We previously showed that blood-brain barrier (BBB) disruption visualized by MRI using gadolinium-based contrast agent (GBCA) does not correlate with tissue softening in active experimental autoimmune encephalomyelitis (EAE). However, it is unknown how confined BBB changes and other inflammatory processes may determine local elasticity changes. Therefore, we aim to elucidate which inflammatory hallmarks are determinant for local viscoelastic changes observed in EAE brains. Hence, novel multifrequency MRE was applied in combination with GBCA-based MRI or very small superparamagnetic iron oxide particles (VSOPs) in female SJL mice with induced adoptive transfer EAE (n = 21). VSOPs were doped with europium (Eu-VSOPs) to facilitate the post-mortem analysis. Accumulation of Eu-VSOPs, which was previously demonstrated to be sensitive to immune cell infiltration and ECM remodeling, was also found to be independent of GBCA enhancement. Following registration to a reference brain atlas, viscoelastic properties of the whole brain and areas visualized by either Gd or VSOP were quantified. MRE revealed marked disseminated softening across the whole brain in mice with established EAE (baseline: 3.1 ± 0.1 m/s vs. EAE: 2.9 ± 0.2 m/s, p < 0.0001). A similar degree of softening was observed in sites of GBCA enhancement i.e., mainly within cerebral cortex and brain stem (baseline: 3.3 ± 0.4 m/s vs. EAE: 3.0 ± 0.5 m/s, p = 0.018). However, locations in which only Eu-VSOP accumulated, mainly in fiber tracts (baseline: 3.0 ± 0.4 m/s vs. EAE: 2.6 ± 0.5 m/s, p = 0.023), softening was more pronounced when compared to non-hypointense areas (percent change of stiffness for Eu-VSOP accumulation: -16.81 ± 16.49% vs. for non-hypointense regions: -5.85 ± 3.81%, p = 0.048). Our findings suggest that multifrequency MRE is sensitive to differentiate between local inflammatory processes with a strong immune cell infiltrate that lead to VSOP accumulation, from disseminated inflammation and BBB leakage visualized by GBCA. These pathological events visualized by Eu-VSOP MRI and MRE may include gliosis, macrophage infiltration, alterations of endothelial matrix components, and/or extracellular matrix remodeling. MRE may therefore represent a promising imaging tool for non-invasive clinical assessment of different pathological aspects of neuroinflammation.

MR Elastography-Based Assessment of Matrix Remodeling at Lesion Sites Associated With Clinical Severity in a Model of Multiple Sclerosis

Magnetic resonance imaging (MRI) with gadolinium based contrast agents (GBCA) is routinely used in the clinic to visualize lesions in multiple sclerosis (MS). Although GBCA reveal endothelial permeability, they fail to expose other aspects of lesion formation such as the magnitude of inflammation or tissue changes occurring at sites of blood-brain barrier (BBB) disruption. Moreover, evidence pointing to potential side effects of GBCA has been increasing. Thus, there is an urgent need to develop GBCA-independent imaging tools to monitor pathology in MS. Using MR-elastography (MRE), we previously demonstrated in both MS and the animal model experimental autoimmune encephalomyelitis (EAE) that inflammation was associated with a reduction of brain stiffness. Now, using the relapsing-remitting EAE model, we show that the cerebellum—a region with predominant inflammation in this model—is especially prone to loss of stiffness. We also demonstrate that, contrary to GBCA-MRI, reduction of brain stiffness correlates with clinical disability and is associated with enhanced expression of the extracellular matrix protein fibronectin (FN). Further, we show that FN is largely expressed by activated astrocytes at acute lesions, and reflects the magnitude of tissue remodeling at sites of BBB breakdown. Therefore, MRE could emerge as a safe tool suitable to monitor disease activity in MS.

Imaging in mice and men: Pathophysiological insights into multiple sclerosis from conventional and advanced MRI techniques

Magnetic resonance imaging (MRI) is the most important tool for diagnosing multiple sclerosis (MS). However, MRI is still unable to precisely quantify the specific pathophysiological processes that underlie imaging findings in MS. Because autopsy and biopsy samples of MS patients are rare and biased towards a chronic burnt-out end or fulminant acute early stage, the only available methods to identify human disease pathology are to apply MRI techniques in combination with subsequent histopathological examination to small animal models of MS and to transfer these insights to MS patients. This review summarizes the existing combined imaging and histopathological studies performed in MS mouse models and humans with MS (in vivo and ex vivo), to promote a better understanding of the pathophysiology that underlies conventional MRI, diffusion tensor and magnetization transfer imaging findings in MS patients. Moreover, it provides a critical view on imaging capabilities and results in MS patients and mouse models and for future studies recommends how to combine those particular MR sequences and parameters whose underlying pathophysiological basis could be partly clarified. Further combined longitudinal in vivo imaging and histopathological studies on rationally selected, appropriate mouse models are required.

Iron oxide nanoparticles as multimodal imaging tools

In medicine, obtaining simply a resolute and accurate image of an organ of interest is a real challenge.

Superparamagnetic iron oxide nanocolloids in MRI studies of neuroinflammation

Iron oxide (IO) nanocolloids are being increasingly used to image cellular contribution to neuroinflammation using MRI, as these particles are capable of labeling circulating cells with phagocytic activity, allowing to assess cell trafficking from the blood to neuroinflammation sites. The use of IOs relies on the natural phagocytic properties of immune cells, allowing their labeling either in vitro or directly in vivo, following intravenous injection. Despite concerns on the specificity of the latter approach, the widespread availability and relatively low cost of these techniques, coupled to a sensitivity that allows to reach single cell detection, have promoted their use in several preclinical and clinical studies. In this review, we discuss the results of currently available preclinical and clinical IO-enhanced MRI studies of immune cell trafficking in neuroinflammation, examining the specificity of the existing findings, in view of the different possible mechanisms underlying IO accumulation in the brain. From this standpoint, we assess the implications of the temporal and spatial differences in the enhancement pattern of IOs, compared to gadolinium-based contrast agents, a clinically established MRI marker blood-brain barrier breakdown. While concerns on the specificity of cell labeling obtained using the in-vivo labeling approach still need to be fully addressed, these techniques have indeed proved able to provide additional information on neuroinflammatory phenomena, as compared to conventional Gadolinium-enhanced MRI.

Synthesis of europium-doped VSOP, customized enhancer solution and improved microscopy fluorescence methodology for unambiguous histological detection

Background

Intrinsic iron in biological tissues frequently precludes unambiguous the identification of iron oxide nanoparticles when iron-based detection methods are used. Here we report the full methodology for synthesizing very small iron oxide nanoparticles (VSOP) doped with europium (Eu) in their iron oxide core (Eu-VSOP) and their unambiguous qualitative and quantitative detection by fluorescence.

Methods and results

The resulting Eu-VSOP contained 0.7 to 2.7% Eu relative to iron, which was sufficient for fluorescent detection while not altering other important particle parameters such as size, surface charge, or relaxivity. A customized enhancer solution with high buffer capacity and nearly neutral pH was developed to provide an antenna system that allowed fluorescent detection of Eu-VSOP in cells and histologic tissue slices as well as in solutions even under acidic conditions as frequently obtained from dissolved organic material. This enhancer solution allowed detection of Eu-VSOP using a standard fluorescence spectrophotometer and a fluorescence microscope equipped with a custom filter set with an excitation wavelength (λ_ex) of 338 nm and an emission wavelength (λ_em) of 616 nm.

Conclusion

The fluorescent detection of Eu-doped very small iron oxide nanoparticles (Eu-VSOP) provides a straightforward tool to unambiguously characterize VSOP biodistribution and toxicology at tissue, and cellular levels, providing a sensitive analytical tool to detect Eu-doped IONP in dissolved organ tissue and biological fluids with fluorescence instruments.

Electronic supplementary material

The online version of this article (doi:10.1186/s12951-017-0301-6) contains supplementary material, which is available to authorized users.

Biocompatibility of very small superparamagnetic iron oxide nanoparticles in murine organotypic hippocampal slice cultures and the role of microglia

Superparamagnetic iron oxide nanoparticles (SPIO) are applied as contrast media for magnetic resonance imaging (MRI) and treatment of neurologic diseases despite the fact that important information concerning their local interactions is still lacking. Due to their small size, SPIO have great potential for magnetically labeling different cell populations, facilitating their MRI tracking in vivo. Before SPIO are applied, however, their effect on cell viability and tissue homoeostasis should be studied thoroughly. We have previously published data showing how citrate-coated very small superparamagnetic iron oxide particles (VSOP) affect primary microglia and neuron cell cultures as well as neuron-glia cocultures. To extend our knowledge of VSOP interactions on the three-dimensional multicellular level, we further examined the influence of two types of coated VSOP (R1 and R2) on murine organotypic hippocampal slice cultures. Our data show that 1) VSOP can penetrate deep tissue layers, 2) long-term VSOP-R2 treatment alters cell viability within the dentate gyrus, 3) during short-term incubation VSOP-R1 and VSOP-R2 comparably modify hippocampal cell viability, 4) VSOP treatment does not affect cytokine homeostasis, 5) microglial depletion decreases VSOP uptake, and 6) microglial depletion plus VSOP treatment increases hippocampal cell death during short-term incubation. These results are in line with our previous findings in cell coculture experiments regarding microglial protection of neurite branching. Thus, we have not only clarified the interaction between VSOP, slice culture, and microglia to a degree but also demonstrated that our model is a promising approach for screening nanoparticles to exclude potential cytotoxic effects.

Iron Oxide as an MRI Contrast Agent for Cell Tracking

Iron oxide contrast agents have been combined with magnetic resonance imaging for cell tracking. In this review, we discuss coating properties and provide an overview of ex vivo and in vivo labeling of different cell types, including stem cells, red blood cells, and monocytes/macrophages. Furthermore, we provide examples of applications of cell tracking with iron contrast agents in stroke, multiple sclerosis, cancer, arteriovenous malformations, and aortic and cerebral aneurysms. Attempts at quantifying iron oxide concentrations and other vascular properties are examined. We advise on designing studies using iron contrast agents including methods for validation.

New findings about iron oxide nanoparticles and their different effects on murine primary brain cells

The physicochemical properties of superparamagnetic iron oxide nanoparticles (SPIOs) enable their application in the diagnostics and therapy of central nervous system diseases. However, since crucial information regarding side effects of particle–cell interactions within the central nervous system is still lacking, we investigated the influence of novel very small iron oxide particles or the clinically approved ferucarbotran or ferumoxytol on the vitality and morphology of brain cells. We exposed primary cell cultures of microglia and hippocampal neurons, as well as neuron–glia cocultures to varying concentrations of SPIOs for 6 and/or 24 hours, respectively. Here, we show that SPIO accumulation by microglia and subsequent morphological alterations strongly depend on the respective nanoparticle type. Microglial viability was severely compromised by high SPIO concentrations, except in the case of ferumoxytol. While ferumoxytol did not cause immediate microglial death, it induced severe morphological alterations and increased degeneration of primary neurons. Additionally, primary neurons clearly degenerated after very small iron oxide particle and ferucarbotran exposure. In neuron–glia cocultures, SPIOs rather stimulated the outgrowth of neuronal processes in a concentration- and particle-dependent manner. We conclude that the influence of SPIOs on brain cells not only depends on the particle type but also on the physiological system they are applied to.

Understanding disease processes in multiple sclerosis through magnetic resonance imaging studies in animal models

There are exciting new advances in multiple sclerosis (MS) resulting in a growing understanding of both the complexity of the disorder and the relative involvement of grey matter, white matter and inflammation. Increasing need for preclinical imaging is anticipated, as animal models provide insights into the pathophysiology of the disease. Magnetic resonance (MR) is the key imaging tool used to diagnose and to monitor disease progression in MS, and thus will be a cornerstone for future research. Although gadolinium-enhancing and T₂ lesions on MRI have been useful for detecting MS pathology, they are not correlative of disability. Therefore, new MRI methods are needed. Such methods require validation in animal models. The increasing necessity for MRI of animal models makes it critical and timely to understand what research has been conducted in this area and what potential there is for use of MRI in preclinical models of MS. Here, we provide a review of MRI and magnetic resonance spectroscopy (MRS) studies that have been carried out in animal models of MS that focus on pathology. We compare the MRI phenotypes of animals and patients and provide advice on how best to use animal MR studies to increase our understanding of the linkages between MR and pathology in patients. This review describes how MRI studies of animal models have been, and will continue to be, used in the ongoing effort to understand MS.

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MRI studies of pathology in various animal models of MS are reviewed.

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MRI phenotypes in animal models of MS and MS patients are compared.

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Animal MRI can increase understanding of links between MR and pathology in patients.

Nanomedicine in autoimmunity

The application of nanotechnology to the diagnosis and therapy of human diseases is already a reality and is causing a real revolution in how we design new therapies and vaccines. In this review we focus on the applications of nanotechnology in the field of autoimmunity. First, we review scenarios in which iron oxide nanoparticles have been used in the diagnosis of autoimmune diseases, mostly through magnetic resonance imaging (MRI), both in animal models and patients. Second, we discuss the potential of nanoparticles as an immunotherapeutic platform for autoimmune diseases, for now exclusively in pre-clinical models. Finally, we discuss the potential of this field to generate the 'perfect drug' with the capacity to report on its therapeutic efficacy in real time, that is, the birth of theranostics in autoimmunity.

Iron oxide magnetic nanoparticles highlight early involvement of the choroid plexus in central nervous system inflammation

Neuroinflammation during multiple sclerosis involves immune cell infiltration and disruption of the BBB (blood–brain barrier). Both processes can be visualized by MRI (magnetic resonance imaging), in multiple sclerosis patients and in the animal model EAE (experimental autoimmune encephalomyelitis). We previously showed that VSOPs (very small superparamagnetic iron oxide particles) reveal CNS (central nervous system) lesions in EAE which are not detectable by conventional contrast agents in MRI. We hypothesized that VSOP may help detect early, subtle inflammatory events that would otherwise remain imperceptible. To investigate the capacity of VSOP to reveal early events in CNS inflammation, we induced EAE in SJL mice using encephalitogenic T-cells, and administered VSOP prior to onset of clinical symptoms. In parallel, we administered VSOP to mice at peak disease, and to unmanipulated controls. We examined the distribution of VSOP in the CNS by MRI and histology. Prior to disease onset, in asymptomatic mice, VSOP accumulated in the choroid plexus and in spinal cord meninges in the absence of overt inflammation. However, VSOP was undetectable in the CNS of non-immunized control mice. At peak disease, VSOP was broadly distributed; we observed particles in perivascular inflammatory lesions with apparently preserved glia limitans. Moreover, at peak disease, VSOP was prominent in the choroid plexus and was seen in elongated endothelial structures, co-localized with phagocytes, and diffusely disseminated in the parenchyma, suggesting multiple entry mechanisms of VSOP into the CNS. Thus, using VSOP we were able to discriminate between inflammatory events occurring in established EAE and, importantly, we identified CNS alterations that appear to precede immune cell infiltration and clinical onset.

Visualizing brain inflammation with a shingled-leg radio-frequency head probe for 19F/1H MRI

Magnetic resonance imaging (MRI) provides the opportunity of tracking cells in vivo. Major challenges in dissecting cells from the recipient tissue and signal sensitivity constraints albeit exist. In this study, we aimed to tackle these limitations in order to study inflammation in autoimmune encephalomyelitis. We constructed a very small dual-tunable radio frequency (RF) birdcage probe tailored for ¹⁹F (fluorine) and ¹H (proton) MR mouse neuroimaging. The novel design eliminated the need for extra electrical components on the probe structure and afforded a uniform -field as well as good SNR. We employed fluorescently-tagged ¹⁹F nanoparticles and could study the dynamics of inflammatory cells between CNS and lymphatic system during development of encephalomyelitis, even within regions of the brain that are otherwise not easily visualized by conventional probes. ¹⁹F/¹H MR Neuroimaging will allow us to study the nature of immune cell infiltration during brain inflammation over an extensive period of time.

Neuroinflammatory imaging biomarkers: relevance to multiple sclerosis and its therapy

Magnetic resonance imaging is an established tool in the management of multiple sclerosis (MS). Loss of blood brain barrier integrity assessed by gadolinium (Gd) enhancement is the current standard marker of MS activity. To explore the complex cascade of the inflammatory events, other magnetic resonance imaging, but also positron emission tomographic markers reviewed in this article are being developed to address active neuroinflammation with increased sensitivity and specificity. Alternative magnetic resonance contrast agents, positron emission tomographic tracers and imaging techniques could be more sensitive than Gd to early blood brain barrier alteration, and they could assess the inflammatory cell recruitment and/or the associated edema accumulation. These markers of active neuroinflammation, although some of them are limited to experimental studies, could find great relevance to complete Gd information and thereby increase our understanding of acute lesion pathophysiology and its noninvasive follow-up, especially to monitor treatment efficacy. Furthermore, such accurate markers of inflammation combined with those of neurodegeneration hold promise to provide a more complete picture of MS, which will be of great benefit for future therapeutic strategies.

Blood-brain barrier alterations in the cerebral cortex in experimental autoimmune encephalomyelitis

The pathophysiology of cerebral cortical lesions in multiple sclerosis (MS) is not understood. We investigated cerebral cortex microvessels during immune-mediated demyelination in the MS model chronic murine experimental autoimmune encephalomyelitis (EAE) by immunolocalization of the endothelial cell tight junction (TJ) integral proteins claudin-5 and occludin, a structural protein of caveolae, caveolin-1, and the blood-brain barrier-specific endothelial transporter, Glut 1. In EAE-affected mice, there were areas of extensive subpial demyelination and well-demarcated lesions that extended to deeper cortical layers. Activation of microglia and absence of perivascular inflammatory infiltrates were common in these areas. Microvascular endothelial cells showed increased expression of caveolin-1 and a coincident loss of both claudin-5 and occludin normal junctional staining patterns. At a very early disease stage, claudin-5 molecules tended to cluster and form vacuoles that were also Glut 1 positive; the initially preserved occludin pattern became diffusely cytoplasmic at more advanced stages. Possible internalization of claudin-5 on TJ dismantling was suggested by its coexpression with the autophagosomal marker MAP1LC3A. Loss of TJ integrity was confirmed by fluorescein isothiocyanate-dextran experiments that showed leakage of the tracer into the perivascular neuropil. These observations indicate that, in the cerebral cortex of EAE-affected mice, there is a microvascular disease that differentially targets claudin-5 and occludin during ongoing demyelination despite only minimal inflammation.

Magnetic particle imaging: introduction to imaging and hardware realization

Magnetic Particle Imaging (MPI) is a recently invented tomographic imaging method that quantitatively measures the spatial distribution of a tracer based on magnetic nanoparticles. The new modality promises a high sensitivity and high spatial as well as temporal resolution. There is a high potential of MPI to improve interventional and image-guided surgical procedures because, today, established medical imaging modalities typically excel in only one or two of these important imaging properties. MPI makes use of the non-linear magnetization characteristics of the magnetic nanoparticles. For this purpose, two magnetic fields are created and superimposed, a static selection field and an oscillatory drive field. If superparamagnetic iron-oxide nanoparticles (SPIOs) are subjected to the oscillatory magnetic field, the particles will react with a non-linear magnetization response, which can be measured with an appropriate pick-up coil arrangement. Due to the non-linearity of the particle magnetization, the received signal consists of the fundamental excitation frequency as well as of harmonics. After separation of the fundamental signal, the nanoparticle concentration can be reconstructed quantitatively based on the harmonics. The spatial coding is realized with the static selection field that produces a field-free point, which is moved through the field of view by the drive fields. This article focuses on the frequency-based image reconstruction approach and the corresponding imaging devices while alternative concepts like x-space MPI and field-free line imaging are described as well. The status quo in hardware realization is summarized in an overview of MPI scanners.

Identification of cellular infiltrates during early stages of brain inflammation with magnetic resonance microscopy

A comprehensive view of brain inflammation during the pathogenesis of autoimmune encephalomyelitis can be achieved with the aid of high resolution non-invasive imaging techniques such as microscopic magnetic resonance imaging (μMRI). In this study we demonstrate the benefits of cryogenically-cooled RF coils to produce μMRI in vivo, with sufficient detail to reveal brain pathology in the experimental autoimmune encephalomyelitis (EAE) model. We could visualize inflammatory infiltrates in detail within various regions of the brain, already at an early phase of EAE. Importantly, this pathology could be seen clearly even without the use of contrast agents, and showed excellent correspondence with conventional histology. The cryogenically-cooled coil enabled the acquisition of high resolution images within short scan times: an important practical consideration in conducting animal experiments. The detail of the cellular infiltrates visualized by in vivo μMRI allows the opportunity to follow neuroinflammatory processes even during the early stages of disease progression. Thus μMRI will not only complement conventional histological examination but will also enable longitudinal studies on the kinetics and dynamics of immune cell infiltration.

Magnetic nanoparticles in magnetic resonance imaging and diagnostics

Magnetic nanoparticles are useful as contrast agents for magnetic resonance imaging (MRI). Paramagnetic contrast agents have been used for a long time, but more recently superparamagnetic iron oxide nanoparticles (SPIOs) have been discovered to influence MRI contrast as well. In contrast to paramagnetic contrast agents, SPIOs can be functionalized and size-tailored in order to adapt to various kinds of soft tissues. Although both types of contrast agents have a inducible magnetization, their mechanisms of influence on spin-spin and spin-lattice relaxation of protons are different. A special emphasis on the basic magnetism of nanoparticles and their structures as well as on the principle of nuclear magnetic resonance is made. Examples of different contrast-enhanced magnetic resonance images are given. The potential use of magnetic nanoparticles as diagnostic tracers is explored. Additionally, SPIOs can be used in diagnostic magnetic resonance, since the spin relaxation time of water protons differs, whether magnetic nanoparticles are bound to a target or not.

Spontaneous ocular and neurologic deficits in transgenic mouse models of multiple sclerosis and noninvasive investigative modalities: a review

Multiple sclerosis (MS) is an autoimmune, inflammatory, neurodegenerative, demyelinating disease of the central nervous system, predominantly involving myelinated neurons of the brain, spinal cord, and optic nerve. Optic neuritis is frequently associated with MS and often precedes other neurologic deficits associated with MS. A large number of patients experience visual defects and have abnormalities concomitant with neurologic abnormalities. Transgenic mice manifesting spontaneous neurologic and ocular disease are unique models that have revolutionized the study of MS. Spontaneous experimental autoimmune encephalomyelitis (sEAE) presents with spontaneous onset of demyelination, without the need of an injectable immunogen. This review highlights the various models of sEAE, their disease characteristics, and applicability for future research. The study of optic neuropathy and neurologic manifestations of demyelination in sEAE will expand our understanding of the pathophysiological mechanisms underlying MS. Early and precise diagnosis of MS with different noninvasive methods has opened new avenues in managing symptoms, reducing morbidity, and limiting disease burden. This review discusses the spectrum of available noninvasive techniques, such as electrophysiological and behavioral assessment, optical coherence tomography, scanning laser polarimetry, confocal scanning laser ophthalmoscopy, pupillometry, magnetic resonance imaging, positron emission tomography, gait, and cardiovascular monitoring, and their clinical relevance.

Maintaining brain health by monitoring inflammatory processes: a mechanism to promote successful aging

Maintaining brain health promotes successful aging. The main determinants of brain health are the preservation of cognitive function and remaining free from structural and metabolic abnormalities, including loss of neuronal synapses, atrophy, small vessel disease and focal amyloid deposits visible by neuroimaging. Promising studies indicate that these determinants are to some extent modifiable, even among adults seventy years and older. Converging animal and human evidence further suggests that inflammation is a shared mechanism, contributing to both cognitive decline and abnormalities in brain structure and metabolism. Thus, inflammation may provide a target for intervention. Specifically, circulating inflammatory markers have been associated with declines in cognitive function and worsening of brain structural and metabolic characteristics. Additionally, it has been proposed that older brains are characterized by a sensitization to neuroinflammatory responses, even in the absence of overt disease. This increased propensity to central inflammation may contribute to poor brain health and premature brain aging. Still unknown is whether and how peripheral inflammatory factors directly contribute to decline of brain health. Human research is limited by the challenges of directly measuring neuroinflammation in vivo. This review assesses the role that inflammation may play in the brain changes that often accompany aging, focusing on relationships between peripheral inflammatory markers and brain health among well-functioning, community-dwelling adults seventy years and older. We propose that monitoring and maintaining lower levels of systemic and central inflammation among older adults could help preserve brain health and support successful aging. Hence, we also identify plausible ways and novel experimental study designs of maintaining brain health late in age through interventions that target the immune system.

Nanoparticle-mediated brain-specific drug delivery, imaging, and diagnosis

Central nervous system (CNS) diseases represent the largest and fastest-growing area of unmet medical need. Nanotechnology plays a unique instrumental role in the revolutionary development of brain-specific drug delivery, imaging, and diagnosis. With the aid of nanoparticles of high specificity and multifunctionality, such as dendrimers and quantum dots, therapeutics, imaging agents, and diagnostic molecules can be delivered to the brain across the blood-brain barrier (BBB), enabling considerable progress in the understanding, diagnosis, and treatment of CNS diseases. Nanoparticles used in the CNS for drug delivery, imaging, and diagnosis are reviewed, as well as their administration routes, toxicity, and routes to cross the BBB. Future directions and major challenges are outlined.

Robust uptake of magnetic nanoparticles (MNPs) by central nervous system (CNS) microglia: implications for particle uptake in mixed neural cell populations

Magnetic nanoparticles (MNPs) are important contrast agents used to monitor a range of neuropathological processes; microglial cells significantly contribute to MNP uptake in sites of pathology. Microglial activation occurs following most CNS pathologies but it is not known if such activation alters MNP uptake, intracellular processing and toxicity. We assessed these parameters in microglial cultures with and without experimental ‘activation’. Microglia showed rapid and extensive MNP uptake under basal conditions with no changes found following activation; significant microglial toxicity was observed at higher particle concentrations. Based on our findings, we suggest that avid MNP uptake by endogenous CNS microglia could significantly limit uptake by other cellular subtypes in mixed neural cell populations.