Using magnetic resonance imaging in animal models to guide drug development in multiple sclerosis

The Value of Various Post-Processing Modalities of Diffusion Weighted Imaging in the Detection of Multiple Sclerosis

Diffusion tensor imaging (DTI) showed its adequacy in evaluating the normal-appearing white matter (NAWM) and lesions in the brain that are difficult to evaluate with routine clinical magnetic resonance imaging (MRI) in multiple sclerosis (MS). Recently, MRI systems have been developed with regard to software and hardware, leading to different proposed diffusion analysis methods such as diffusion tensor imaging, q-space imaging, diffusional kurtosis imaging, neurite orientation dispersion and density imaging, and axonal diameter measurement. These methods have the ability to better detect in vivo microstructural changes in the brain than DTI. These different analysis modalities could provide supplementary inputs for MS disease characterization and help in monitoring the disease’s progression as well as treatment efficacy. This paper reviews some of the recent diffusion MRI methods used for the assessment of MS in vivo.

Assessment of the Characteristics of Different Kinds of MS Lesions Using Multi-Parametric MRI

Background: Studying different pathological aspects of lesions in multiple sclerosis (MS) patients could be useful to modify the diagnosis and treatment of this neurological disorder. Magnetic resonance imaging (MRI) modalities have the potential to investigate variations in brain tissue because of inflammatory and neurodegenerative processes in various types of MS-related lesions. Objectives: This study was done to investigate the quantitative changes in MRI-based parameters, like perfusion and magnetization transfer ratio (MTR) of different types of brain lesions, to demonstrate the ability of MRI to detect structural and pathological differences in MS lesions. Methods: Quantitative MRI modalities were performed on 18 patients with five different kinds of lesions (T1 holes, acute and chronic white matter (WM), and acute and chronic gray matter (GM) lesions) using a 3 T MRI scanner. The following protocols were used to characterize the pathology of lesions: (I) fluid-attenuated inversion recovery (FLAIR); (II) pre- and post-contrast T1-weighted; (III) dynamic contrast-enhanced (DCE); and (IV) MTR imaging. Quantitative comparison of Ktrans, cerebral blood volume (CBV), cerebral blood flow (CBF), and MTR was done to find the best parameter to distinguish different lesions. Finally, a multivariate classifier was applied to introduce the best parameter to indicate differences in lesions. Results: Five lesions were characterized by perfusion and MTR parameters. The pathological changes were measured, including: (I) the highest value of parameters in both acute WM and GM lesions; (II) the lowest value of four parameters in both chronic WM and GM lesions; (III) MTR had the highest rank among parameters using the classifier. Conclusions: The degree of pathological alterations due to inflammatory and neurodegenerative processes in MS-related lesions was indicated through the used parameters in different kinds of lesions. Inflammation was the dominant process in acute lesions, while neurodegeneration and tissue loss were observed mostly in chronic lesions. Both inflammation and neurodegeneration were detected in T1 holes. Perfusion parameters and MTR were reasonable parameters to describe differences in brain lesions. Thus, it could be confirmed that magnetization transfer imaging (MTI) and DCE-MRI are high-sensitivity methods to detect microstructural changes in the brain and subtle changes in the blood-brain-barrier. Classification of the parameters indicated that MTR was the best biomarker than others to show variations in lesions pathology.

Multicenter Evaluation of Geometric Accuracy of MRI Protocols Used in Experimental Stroke

It has recently been suggested that multicenter preclinical stroke studies should be carried out to improve translation from bench to bedside, but the accuracy of magnetic resonance imaging (MRI) scanners routinely used in experimental stroke has not yet been evaluated. We aimed to assess and compare geometric accuracy of preclinical scanners and examine the longitudinal stability of one scanner using a simple quality assurance (QA) protocol. Six 7 Tesla animal scanners across six different preclinical imaging centers throughout Europe were used to scan a small structural phantom and estimate linear scaling errors in all orthogonal directions and volumetric errors. Between-scanner imaging consisted of a standard sequence and each center's preferred sequence for the assessment of infarct size in rat models of stroke. The standard sequence was also used to evaluate the drift in accuracy of the worst performing scanner over a period of six months following basic gradient calibration. Scaling and volumetric errors using the standard sequence were less variable than corresponding errors using different stroke sequences. The errors for one scanner, estimated using the standard sequence, were very high (above 4% scaling errors for each orthogonal direction, 18.73% volumetric error). Calibration of the gradient coils in this system reduced scaling errors to within ±1.0%; these remained stable during the subsequent 6-month assessment. In conclusion, despite decades of use in experimental studies, preclinical MRI still suffers from poor and variable geometric accuracy, influenced by the use of miscalibrated systems and various types of sequences for the same purpose. For effective pooling of data in multicenter studies, centers should adopt standardized procedures for system QA and in vivo imaging.

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.

Multicontrast MRI Quantification of Focal Inflammation and Degeneration in Multiple Sclerosis

INTRODUCTION

Local microstructural pathology in multiple sclerosis patients might influence their clinical performance. This study applied multicontrast MRI to quantify inflammation and neurodegeneration in MS lesions. We explored the impact of MRI-based lesion pathology in cognition and disability.

METHODS

36 relapsing-remitting MS subjects and 18 healthy controls underwent neurological, cognitive, behavioural examinations and 3 T MRI including (i) fluid attenuated inversion recovery, double inversion recovery, and magnetization-prepared gradient echo for lesion count; (ii) T1, T2, and T2(*) relaxometry and magnetisation transfer imaging for lesion tissue characterization. Lesions were classified according to the extent of inflammation/neurodegeneration. A generalized linear model assessed the contribution of lesion groups to clinical performances.

RESULTS

Four lesion groups were identified and characterized by (1) absence of significant alterations, (2) prevalent inflammation, (3) concomitant inflammation and microdegeneration, and (4) prevalent tissue loss. Groups 1, 3, 4 correlated with general disability (Adj-R (2) = 0.6; P = 0.0005), executive function (Adj-R (2) = 0.5; P = 0.004), verbal memory (Adj-R (2) = 0.4; P = 0.02), and attention (Adj-R (2) = 0.5; P = 0.002).

CONCLUSION

Multicontrast MRI provides a new approach to infer in vivo histopathology of plaques. Our results support evidence that neurodegeneration is the major determinant of patients' disability and cognitive dysfunction.

Functional Magnetic Resonance Imaging of Rats with Experimental Autoimmune Encephalomyelitis Reveals Brain Cortex Remodeling

Cortical reorganization occurring in multiple sclerosis (MS) patients is thought to play a key role in limiting the effect of structural tissue damage. Conversely, its exhaustion may contribute to the irreversible disability that accumulates with disease progression. Several aspects of MS-related cortical reorganization, including the overall functional effect and likely modulation by therapies, still remain to be elucidated. The aim of this work was to assess the extent of functional cortical reorganization and its brain structural/pathological correlates in Dark Agouti rats with experimental autoimmune encephalomyelitis (EAE), a widely accepted preclinical model of chronic MS. Morphological and functional MRI (fMRI) were performed before disease induction and during the relapsing and chronic phases of EAE. During somatosensory stimulation of the right forepaw, fMRI demonstrated that cortical reorganization occurs in both relapsing and chronic phases of EAE with increased activated volume and decreased laterality index versus baseline values. Voxel-based morphometry demonstrated gray matter (GM) atrophy in the cerebral cortex, and both GM and white matter atrophy were assessed by ex vivo pathology of the sensorimotor cortex and corpus callosum. Neuroinflammation persisted in the relapsing and chronic phases, with dendritic spine density in the layer IV sensory neurons inversely correlating with the number of cluster of differentiation 45-positive inflammatory lesions. Our work provides an innovative experimental platform that may be pivotal for the comprehension of key mechanisms responsible for the accumulation of irreversible brain damage and for the development of innovative therapies to reduce disability in EAE/MS.

SIGNIFICANCE STATEMENT

Since the early 2000s, functional MRI (fMRI) has demonstrated profound modifications in the recruitment of cortical areas during motor, cognitive, and sensory tasks in multiple sclerosis (MS) patients. Experimental autoimmune encephalomyelitis (EAE) represents a reliable model of the chronic-progressive variant of MS. fMRI studies in EAE have not been performed extensively up to now. This paper reports fMRI studies in a rat model of MS with somatosensory stimulation of the forepaw. We demonstrated modifications in the recruitment of cortical areas consistent with data from MS patients. To the best of our knowledge, this is the first report of cortical remodeling in a preclinical in vivo model of MS.

Magnetic nanoparticles for oligodendrocyte precursor cell transplantation therapies: progress and challenges

Oligodendrocyte precursor cells (OPCs) have shown high promise as a transplant population to promote regeneration in the central nervous system, specifically, for the production of myelin - the protective sheath around nerve fibers. While clinical trials for these cells have commenced in some areas, there are currently key barriers to the translation of neural cell therapies. These include the ability to (a) image transplant populations in vivo; (b) genetically engineer transplant cells to augment their repair potential; and (c) safely target cells to sites of pathology. Here, we review the evidence that magnetic nanoparticles (MNPs) are a 'multifunctional nanoplatform' that can aid in safely addressing these translational challenges in neural cell/OPC therapy: by facilitating real-time and post-mortem assessment of transplant cell biodistribution, and biomolecule delivery to transplant cells, as well as non-invasive 'magnetic cell targeting' to injury sites by application of high gradient fields. We identify key issues relating to the standardization and reporting of physicochemical and biological data in the field; we consider that it will be essential to systematically address these issues in order to fully evaluate the utility of the MNP platform for neural cell transplantation, and to develop efficacious neurocompatible particles for translational applications.

Magnetic nanoparticles for oligodendrocyte precursor cell transplantation therapies: progress and challenges

Oligodendrocyte precursor cells (OPCs) have shown high promise as a transplant population to promote regeneration in the central nervous system, specifically, for the production of myelin – the protective sheath around nerve fibers. While clinical trials for these cells have commenced in some areas, there are currently key barriers to the translation of neural cell therapies. These include the ability to (a) image transplant populations in vivo; (b) genetically engineer transplant cells to augment their repair potential; and (c) safely target cells to sites of pathology. Here, we review the evidence that magnetic nanoparticles (MNPs) are a ‘multifunctional nanoplatform’ that can aid in safely addressing these translational challenges in neural cell/OPC therapy: by facilitating real-time and post-mortem assessment of transplant cell biodistribution, and biomolecule delivery to transplant cells, as well as non-invasive ‘magnetic cell targeting’ to injury sites by application of high gradient fields. We identify key issues relating to the standardization and reporting of physicochemical and biological data in the field; we consider that it will be essential to systematically address these issues in order to fully evaluate the utility of the MNP platform for neural cell transplantation, and to develop efficacious neurocompatible particles for translational applications.

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.