MR microscopy of transgenic mice that spontaneously acquire experimental allergic encephalomyelitis

Chronic experimental autoimmune encephalomyelitis is an excellent model to study neuroaxonal degeneration in multiple sclerosis

Animal models of multiple sclerosis (MS), specifically experimental autoimmune encephalomyelitis (EAE), have been used extensively to develop anti-inflammatory treatments. However, the similarity between MS and one particular EAE model does not end at inflammation. MS and chronic EAE induced in C57BL/6 mice using myelin oligodendrocyte glycoprotein (MOG) peptide 35-55 share many neuropathologies. Beyond both having white matter lesions in spinal cord, both also have widespread neuropathology in the cerebral cortex, hippocampus, thalamus, striatum, cerebellum, and retina/optic nerve. In this review, we compare neuropathologies in each of these structures in MS with chronic EAE in C57BL/6 mice, and find evidence that this EAE model is well suited to study neuroaxonal degeneration in MS.

Magnetic resonance imaging investigations reveal that PM2.5 exposure triggers visual dysfunction in mice

OBJECTIVES

To investigate how PM_2.5 exposure affects the microstructure, metabolites or functions of the visual system.

METHODS

C57BL/6J mice were randomly assigned to groups exposed to the filtered air (the control group) or the concentrated ambient PM_2.5 (the PM_2.5 group). Visual evoked potentials (VEP), electroretinograms (ERG), diffusion tensor imaging (DTI), proton magnetic resonance spectroscopy (¹H-MRS) and resting-state functional MRI (rsfMRI) were performed. Parameters were obtained and compared between the two groups, including latencies and amplitudes of the P1 wave, N1 wave and P2 wave from VEP, latencies and amplitudes of the a wave and b wave from ERG, fractional anisotropy (FA), mean diffusion (MD), axial diffusivity (AD) and radial diffusivity (RD) from DTI, visual cortex (VC) metabolites from ¹H-MRS, and regional homogeneity (ReHo) from rsfMRI.

RESULTS

Compared with the values of the control group, the PM_2.5 group showed a prolonged N1 latency (43.11 ± 7.94 ms vs. 38.75 ± 4.60 ms) and lowered P1 amplitude (5.62 ± 4.38 μV vs. 8.56 ± 5.92 μV) on VEP (all p < 0.05). On ERG, the amplitude of the a wave was lowered (- 91.39 ± 56.29 μV vs. - 138.68 ± 89.05 μV), the amplitude of the b wave was lowered (194.38 ± 126.27 μV vs. 284.72 ± 170.99 μV), and the latency of the b wave was prolonged (37.78 ± 10.72 ms vs. 33.01 ± 4.34 ms) than the values of the control group (all p < 0.05). DTI indicated FA increase in the bilateral piriform cortex (Pir), FA decrease in the bilateral somatosensory cortex (S) and the bilateral striatum (Stri), AD decrease in the bilateral VC, the right S and the bilateral Pir, MD decrease in the bilateral Pir, and RD decrease in the bilateral Pir in the PM_2.5 mice (all p < 0.05, Alphasim corrected). ¹H-MRS showed Glutamate (Glu) increase and Phosphocholine (PCh) increase in the VC of the PM_2.5 group than those of the control group (PCh 1.63 ± 0.25 vs. 1.50 ± 0.25; PCh/total creatine(tCr) 0.19 ± 0.03 vs. 0.18 ± 0.03; Glu 10.46 ± 1.50 vs. 9.60 ± 1.19; Glu/tcr 1.23 ± 0.11 vs. 1.12 ± 0.11) (all p < 0.05). rsfMRI showed higher ReHo in the PM_2.5 mice in the left superior colliculus, the left motor cortex, the hippocampus, the periaqueductal gray and the right mesencephalic reticular formation (all p < 0.01, AlphaSim corrected).

CONCLUSIONS

This study revealed that PM_2.5 exposure triggered visual dysfunction, and altered microstructure, metabolite and function in the retina and visual brain areas along the visual system.

Sex chromosome contributions to sex differences in multiple sclerosis susceptibility and progression

BACKGROUND

Why are women more susceptible to multiple sclerosis, but men have worse disability progression? Sex differences in disease may be due to sex hormones, sex chromosomes, or both.

OBJECTIVE

Determine whether differences in sex chromosomes can contribute to sex differences in multiple sclerosis using experimental autoimmune encephalomyelitis.

METHODS

Sex chromosome transgenic mice, which permit the study of sex chromosomes not confounded by differences in sex hormones, were used to examine an effect of sex chromosomes on autoimmunity and neurodegeneration, focusing on X chromosome genes.

RESULTS

T-lymphocyte DNA methylation studies of the X chromosome gene Foxp3 suggested that maternal versus paternal imprinting of X chromosome genes may underlie sex differences in autoimmunity. Bone marrow chimeras with the same immune system but different sex chromosomes in the central nervous system suggested that differential expression of the X chromosome gene Toll-like receptor 7 in neurons may contribute to sex differences in neurodegeneration.

CONCLUSION

Mapping the transcriptome and methylome in T lymphocytes and neurons in females versus males could reveal mechanisms underlying sex differences in autoimmunity and neurodegeneration.

Diverse application of MRI for mouse phenotyping

Small animal models, particularly mouse models, of human diseases are becoming an indispensable tool for biomedical research. Studies in animal models have provided important insights into the etiology of diseases and accelerated the development of therapeutic strategies. Detailed phenotypic characterization is essential, both for the development of such animal models and mechanistic studies into disease pathogenesis and testing the efficacy of experimental therapeutics. MRI is a versatile and noninvasive imaging modality with excellent penetration depth, tissue coverage, and soft tissue contrast. MRI, being a multi-modal imaging modality, together with proven imaging protocols and availability of good contrast agents, is ideally suited for phenotyping mutant mouse models. Here we describe the applications of MRI for phenotyping structural birth defects involving the brain, heart, and kidney in mice. The versatility of MRI and its ease of use are well suited to meet the rapidly increasing demands for mouse phenotyping in the coming age of functional genomics. Birth Defects Research 109:758-770, 2017. © 2017 Wiley Periodicals, Inc.

Phenotyping the central nervous system of the embryonic mouse by magnetic resonance microscopy

Genetic mouse models of neurodevelopmental disorders are being massively generated, but technologies for their high-throughput phenotyping are missing. The potential of high-resolution magnetic resonance imaging (MRI) for structural phenotyping has been demonstrated before. However, application to the embryonic mouse central nervous system has been limited by the insufficient anatomical detail. Here we present a method that combines staining of live embryos with a contrast agent together with MR microscopy after fixation, to provide unprecedented anatomical detail at relevant embryonic stages. By using this method we have phenotyped the embryonic forebrain of Robo1/2(-/-) double mutant mice enabling us to identify most of the well-known anatomical defects in these mutants, as well as novel more subtle alterations. We thus demonstrate the potential of this methodology for a fast and reliable screening of subtle structural abnormalities in the developing mouse brain, as those associated to defects in disease-susceptibility genes of neurologic and psychiatric relevance.

Multimodal imaging of human cerebellum - merging X-ray phase microtomography, magnetic resonance microscopy and histology

Imaging modalities including magnetic resonance imaging and X-ray computed tomography are established methods in daily clinical diagnosis of human brain. Clinical equipment does not provide sufficient spatial resolution to obtain morphological information on the cellular level, essential for applying minimally or non-invasive surgical interventions. Therefore, generic data with lateral sub-micrometer resolution have been generated from histological slices post mortem. Sub-cellular spatial resolution, lost in the third dimension as a result of sectioning, is obtained using magnetic resonance microscopy and micro computed tomography. We demonstrate that for human cerebellum grating-based X-ray phase tomography shows complementary contrast to magnetic resonance microscopy and histology. In this study, the contrast-to-noise values of magnetic resonance microscopy and phase tomography were comparable whereas the spatial resolution in phase tomography is an order of magnitude better. The registered data with their complementary information permit the distinct segmentation of tissues within the human cerebellum.

Susceptibility-weighted imaging in the experimental autoimmune encephalomyelitis model of multiple sclerosis indicates elevated deoxyhemoglobin, iron deposition and demyelination

BACKGROUND

Susceptibility-weighted imaging (SWI) is an iron-sensitive magnetic resonance imaging (MRI) method that has shown iron-related lesions in multiple sclerosis (MS) patients. The contribution of deoxyhemoglobin to the signals seen in SWI has not been well characterized in MS.

OBJECTIVES

To determine if SWI lesions (seen as focal hypointensities) exist in the experimental autoimmune encephalomyelitis (EAE) animal model of MS, and to determine whether the lesions relate to iron deposits, inflammation, demyelination, and/or deoxyhemoglobin in the vasculature.

METHODS

We performed SWI on the lumbar spinal cord and cerebellum of EAE and control mice (both complete Freund's adjuvant/pertussis toxin (CFA/PTX)-immunized and naive). We also performed SWI on mice before and after perfusion (to remove blood from vessels). SWI lesions were counted and their locations were compared to histology for iron, myelin and inflammation.

RESULTS

SWI lesions were found to exist in the EAE model. Many lesions seen by SWI were not present after perfusion, especially at the grey/white matter boundary of the lumbar spinal cord and in the cerebellum, indicating that these lesion signals were associated with deoxyhemoglobin present in the lumen of vessels. We also observed SWI lesions in the white matter of the lumbar spinal cord that corresponded to iron deposition, inflammation and demyelination. In the cerebellum, SWI lesions were present in white matter tracts, where we found histological evidence of inflammatory perivascular cuffs.

CONCLUSIONS

SWI lesions exist in EAE mice. Many lesions seen in SWI were a result of deoxyhemoglobin in the blood, and so may indicate areas of hypoxia. A smaller number of SWI lesions coincided with parenchymal iron, demyelination, and/or inflammation.

Cortical atrophy in experimental autoimmune encephalomyelitis: in vivo imaging

There are strong correlations between cortical atrophy observed by MRI and clinical disability and disease duration in multiple sclerosis (MS). The objective of this study was to evaluate the progression of cortical atrophy over time in vivo in experimental autoimmune encephalomyelitis (EAE), the most commonly used animal model for MS. Volumetric changes in brains of EAE mice and matched healthy controls were quantified by collecting high-resolution T2-weighted magnetic resonance images in vivo and labeling anatomical structures on the images. In vivo scanning permitted us to evaluate brain structure volumes in individual animals over time and we observed that though brain atrophy progressed differently in each individual animal, all mice with EAE demonstrated significant atrophy in whole brain, cerebral cortex, and whole cerebellum compared to normal controls. Furthermore, we found a strong correlation between cerebellar atrophy and cumulative disease score in mice with EAE. Ex vivo MRI showed a significant decrease in brain and cerebellar volume and a trend that did not reach significance in cerebral cortex volume in mice with EAE compared to controls. Cross modality correlations revealed a significant association between neuronal loss on neuropathology and in vivo atrophy of the cerebral cortex by neuroimaging. These results demonstrate that longitudinal in vivo imaging is more sensitive to changes that occur in neurodegenerative disease models than cross-sectional ex vivo imaging. This is the first report of progressive cortical atrophy in vivo in a mouse model of MS.

Spinal cord - MR of rodent models

Different MR techniques, such as relaxation times, diffusion, perfusion, and spectroscopy have been employed to study rodent spinal cord. In this chapter, a description of these methods is given, along with examples of normal metrics that can be derived from the MR acquisitions, as well as examples of applications to pathology.

MRI in rodent models of brain disorders

Magnetic resonance imaging (MRI) is a well-established tool in clinical practice and research on human neurological disorders. Translational MRI research utilizing rodent models of central nervous system (CNS) diseases is becoming popular with the increased availability of dedicated small animal MRI systems. Projects utilizing this technology typically fall into one of two categories: 1) true "pre-clinical" studies involving the use of MRI as a noninvasive disease monitoring tool which serves as a biomarker for selected aspects of the disease and 2) studies investigating the pathomechanism of known human MRI findings in CNS disease models. Most small animal MRI systems operate at 4.7-11.7 Tesla field strengths. Although the higher field strength clearly results in a higher signal-to-noise ratio, which enables higher resolution acquisition, a variety of artifacts and limitations related to the specific absorption rate represent significant challenges in these experiments. In addition to standard T1-, T2-, and T2*-weighted MRI methods, all of the currently available advanced MRI techniques have been utilized in experimental animals, including diffusion, perfusion, and susceptibility weighted imaging, functional magnetic resonance imaging, chemical shift imaging, heteronuclear imaging, and (1)H or (31)P MR spectroscopy. Selected MRI techniques are also exclusively utilized in experimental research, including manganese-enhanced MRI, and cell-specific/molecular imaging techniques utilizing negative contrast materials. In this review, we describe technical and practical aspects of small animal MRI and provide examples of different MRI techniques in anatomical imaging and tract tracing as well as several models of neurological disorders, including inflammatory, neurodegenerative, vascular, and traumatic brain and spinal cord injury models, and neoplastic diseases.

Diffusion tensor imaging of white matter pathology in the mouse brain

Diffusion tensor imaging has been increasingly used for studying white matter pathology in rodent models of neurological diseases. Here, applications of diffusion tensor imaging in detecting major and subtle white matter pathology in the mouse CNS are reviewed, followed by several technical details that may be helpful in designing studies that involve diffusion tensor imaging of rodent brain and spinal cord.

Echo planar diffusion tensor imaging of the mouse spinal cord at thoracic and lumbar levels: A feasibility study

Diffusion tensor imaging is increasingly used for probing spinal cord (SC) pathologies, especially in mouse models of human diseases. However, diffusion tensor imaging series requires a long acquisition time and mouse experiments rarely use rapid imaging techniques such as echo planar imaging. A recent preliminary study demonstrated the feasibility and robustness of the echo planar imaging sequence for mouse cervical SC diffusion tensor imaging investigations. The feasibility of echo planar imaging at thoracic and lumbar levels, however, remained unknown due to bulk motion, field inhomogeneities, and off-centering of the SC in the axial plane. In the present study, the feasibility and the robustness of an echo planar imaging-based diffusion tensor imaging sequence for mouse thoracic and lumbar SC investigations is demonstrated. Quantitative and accurate diffusion tensor imaging metrics, as well as high spatially resolved images, have been obtained. This successful demonstration may open new perspectives in the field of mouse SC imaging. Echo planar imaging is used in several imaging modalities, such as relaxometry or perfusion, and may prove to be very attractive for multimodal MR investigations to acquire a more detailed characterization of the SC tissue.

White-matter abnormalities in brain during early abstinence from methamphetamine abuse

BACKGROUND

Previous studies revealed microstructural abnormalities in prefrontal white matter and corpus callosum of long-term abstinent chronic methamphetamine abusers. In view of the importance of the early abstinence period in treatment retention, we compared 23 methamphetamine-dependent subjects abstinent from methamphetamine for 7-13 days with 18 healthy comparison subjects. As certain metabolic changes in the brain first manifest after early abstinence from methamphetamine, it is also possible that microstructural white-matter abnormalities are not yet present during early abstinence.

METHODS

Using diffusion tensor imaging at 1.5 T, fractional anisotropy (FA) was measured in prefrontal white matter at four inferior-superior levels parallel to the anterior commissure-posterior commissure (AC-PC) plane. We also sampled FA in the corpus callosum at the midline and at eight bilateral, fiber-tract sites in other regions implicated in effects of methamphetamine.

RESULTS

The methamphetamine group exhibited lower FA in right prefrontal white matter above the AC-PC plane (11.9% lower; p = 0.007), in midline genu corpus callosum (3.9%; p = 0.019), in left and right midcaudal superior corona radiata (11.0% in both hemispheres, p's = 0.020 and 0.016, respectively), and in right perforant fibers (7.3%; p = 0.025). FA in left midcaudal superior corona radiata was correlated with depressive and generalized psychiatric symptoms within the methamphetamine group.

CONCLUSIONS

The findings support the idea that methamphetamine abuse produces microstructural abnormalities in white matter underlying and interconnecting prefrontal cortices and hippocampal formation. These effects are already present during the first weeks of abstinence from methamphetamine and are linked to psychiatric symptoms assessed during this period.

Short-scan-time multi-slice diffusion MRI of the mouse cervical spinal cord using echo planar imaging

Mouse spinal cord (SC) diffusion-weighted imaging (DWI) provides important information on tissue morphology and structural changes that may occur during pathologies such as multiple sclerosis or SC injury. The acquisition scheme of the commonly used DWI techniques is based on conventional spin-echo encoding, which is time-consuming. The purpose of this work was to investigate whether the use of echo planar imaging (EPI) would provide good-quality diffusion MR images of mouse SC, as well as accurate measurements of diffusion-derived metrics, and thus enable diffusion tensor imaging (DTI) and highly resolved DWI within reasonable scan times. A four-shot diffusion-weighted spin-echo EPI (SE-EPI) sequence was evaluated at 11.75 T on a group of healthy mice (n = 10). SE-EPI-derived apparent diffusion coefficients of gray and white matter were compared with those obtained using a conventional spin-echo sequence (c-SE) to validate the accuracy of the method. To take advantage of the reduction in acquisition time offered by the EPI sequence, multi-slice DTI acquisitions were performed covering the cervical segments (six slices, six diffusion-encoding directions, three b values) within 30 min (vs 2 h for c-SE). From these measurements, fractional anisotropy and mean diffusivities were calculated, and fiber tracking along the C1 to C6 cervical segments was performed. In addition, high-resolution images (74 x 94 microm(2)) were acquired within 5 min per direction. Clear delineation of gray and white matter and identical apparent diffusion coefficient values were obtained, with a threefold reduction in acquisition time compared with c-SE. While overcoming the difficulties associated with high spatially and temporally resolved DTI measurements, the present SE-EPI approach permitted identification of reliable quantitative parameters with a reproducibility compatible with the detection of pathologies. The SE-EPI method may be particularly valuable when multiple sets of images from the SC are needed, in cases of rapidly evolving conditions, to decrease the duration of anesthesia or to improve MR exploration by including additional MR measurements.

Identifying white-matter fiber bundles in DTI data using an automated proximity-based fiber-clustering method

We present a method for clustering diffusion tensor imaging (DTI) integral curves into anatomically plausible bundles. An expert rater evaluated the anatomical accuracy of the bundles. We also evaluated the method by applying an experimental cross-subject labeling method to the clustering results. We first employ a sampling and culling strategy for generating DTI integral curves and then constrain the curves so that they terminate in gray matter. We then employ a clustering method based on a proximity measure calculated between every pair of curves. We interactively selected a proximity threshold to achieve visually optimal clustering in models from four DTI datasets. An expert rater then assigned a confidence rating about bundle presence and accuracy for each of 12 target fiber bundles of varying calibers and type in each dataset. We then created a fiber bundle template to cluster and label the fiber bundles automatically in new datasets. According to expert evaluation, the automated proximity-based clustering and labeling algorithm consistently yields anatomically plausible fiber bundles on large and coherent clusters. This work has the potential to provide an automatic and robust way to find and study neural fiber bundles within DTI.

Quantitative tractography metrics of white matter integrity in diffusion-tensor MRI

We present new quantitative diffusion-tensor imaging (DTI) tractography-based metrics for assessing cerebral white matter integrity. These metrics extend prior work in this area. Tractography models of cerebral white matter were produced from each subject's DTI data. The models are a set of curves (e.g., "streamtubes") derived from DTI data that represent the underlying topography of the cerebral white matter. Nine metrics were calculated in whole brain tractography models and in three "tracts-of-interest": transcallosal fibers and the left and right cingulum bundles. The metrics included the number of streamtubes and several other based on the summed length of streamtubes, including some that were weighted by scalar anisotropy metrics and normalized for estimated intracranial volume. We then tested whether patients with subcortical ischemic vascular disease (i.e., vascular cognitive impairment or VCI) vs. healthy controls (HC) differed on the metrics. The metrics were significantly lower in the VCI group in whole brain and in transcallosal fibers but not in the left or right cingulum bundles. The metrics correlated significantly with cognitive functions known to be impacted by white matter abnormalities (e.g., processing speed) but not with those more strongly impacted by cortical disease (e.g., naming). These new metrics help bridge the gap between DTI tractography and scalar analytical methods and provide a potential means for examining group differences in white matter integrity in specific tracts-of-interest.

Sampling DTI fibers in the human brain based on DWI forward modeling

We present a forward-modeling-based sampling of diffusion-tensor imaging (DTI) integral curves. This work has the potential to generate accurate brain neural fiber models that fit the data well with an economic number of curves. DTI integral curves are integrated from the first eigenvector field of the DTI field. Usually the seed points are generated randomly or from a regular grid in the data volume. The resulting set of integral curves is dense around the long and skinny neural fiber structures and sparse around the short and fat structures. There is currently a lack of quantitative indication of how well various models fit the data. We build a forward model that simulates diffusion-weighted images (DWIs) from the DTI integral curves based on a multi-tensor model. We employ the sum of the difference between the simulated DWIs and the acquired DWIs as the goal function and optimize the placement of the DTI integral curves with a greedy algorithm and a simulated annealing algorithm. The results show that with the same number of curves, the optimized set of DTI integral curves fit better to the data than randomly seeded integral curves.

Quantitative cortical mapping of fractional anisotropy in developing rat brains

Cortical development is associated with a series of events that involve axon and dendrite growth and synaptic formation. Although these developmental processes have been investigated in detail with histology, three-dimensional and quantitative imaging methods for rodent brains may be useful for genetic and pharmacological studies in which cortical developmental abnormalities are suspected. It has been shown that diffusion tensor imaging (DTI) can delineate the columnar organization of the fetal and early neonatal cortex based on a high degree of diffusion anisotropy along the columnar structures. This anisotropy is known to decrease during brain development. In this study, we applied DTI to developing rat brains at five developmental stages, postnatal days 0, 3, 7, 11 and 19, and used diffusion anisotropy as an index to characterize the structural change. Statistical analysis reveals four distinctive cortical areas that demonstrate a characteristic time course of anisotropy loss. This method may provide a means to delineate specific cortical areas and a quantitative method to detect abnormalities in cortical development in rodent pathological models.

Neuroimaging of demyelination and remyelination models

Small-animal magnetic resonance imaging is becoming an increasingly utilized noninvasive tool in the study of animal models of MS including the most commonly used autoimmune, viral, and toxic models. Because most MS models are induced in rodents with brains and spinal cords of a smaller magnitude than humans, small-animal MRI must accomplish much higher resolution acquisition in order to generate useful data. In this review, we discuss key aspects and important differences between high field strength experimental and human MRI. We describe the role of conventional imaging sequences including T1, T2, and proton density-weighted imaging, and we discuss the studies aimed at analyzing blood-brain barrier (BBB) permeability and acute inflammation utilizing gadolinium-enhanced MRI. Advanced MRI methods, including diffusion-weighted and magnetization transfer imaging in monitoring demyelination, axonal damage, and remyelination, and studies utilizing in vivo T1 and T2 relaxometry, provide insight into the pathology of demyelinating diseases at previously unprecedented details. The technical challenges of small voxel in vivo MR spectroscopy and the biologically relevant information obtained by analysis of MR spectra in demyelinating models is also discussed. Novel cell-specific and molecular imaging techniques are becoming more readily available in the study of experimental MS models. As a growing number of tissue restorative and remyelinating strategies emerge in the coming years, noninvasive monitoring of remyelination will be an important challenge in small-animal imaging. High field strength small-animal experimental MRI will continue to evolve and interact with the development of new human MR imaging and experimental NMR techniques.

Longitudinal magnetic resonance imaging of spinal cord injury in mouse: changes in signal patterns associated with the inflammatory response

Contusion-type spinal cord injury (SCI) in mice was followed longitudinally using in vivo magnetic resonance (MR) imaging along with neurobehavioral tests performed on postinjury Days 1, 7, 14 and 28. Magnetic resonance images were acquired from seven injured wild-type mice using a 9.4-T scanner and presented in sagittal and axial views to reflect the current state of the injured cord neuropathology on each day. The data were analyzed individually to gain more insights on the neuroinflammatory response unique to the mouse, to characterize the spatiotemporal evolution of the lesion and to quantify the changes in lesion volume and length with time. The MR intensity patterns on Day 1 showed acute injuries as focal in one group of three mice and as diffuse in the remaining group of four mice. The focal injuries appeared as a region of hypointensity with well-defined boundaries. These injuries first enlarged on Day 7, but then shrunk slightly by Days 14 and 28. In contrast, the diffuse injuries were initially obscure on Day 1, mainly because of loss of contrast between gray and white matters. On Day 7, lesions expanded asymptotically in both rostral and caudal directions with respect to the epicenter, and maintained its size on Days 14 and 28. Previous studies based on postmortem histological analysis have reported lesions behaving more like in the focal group. However, this new injury with diffuse characteristics may have important implications for SCI research carried out with mice. Unique experiments on genetically engineered mice with altered neuroinflammatory response should help clarify the origin of these differences in the lesion formation.

Postmortem delay does not change regional diffusion anisotropy characteristics in mouse spinal cord white matter

It has been demonstrated previously that water diffusion anisotropy in vivo is equivalent to that observed ex vivo after perfusion fixation in the mouse brain. This finding supports the practice of ex vivo diffusion tensor imaging (DTI) measurement on perfusion-fixed tissues. However, the validity of extrapolating ex vivo DTI measurements from immersion-fixed autopsy specimens to the in vivo state is questionable because of variable postmortem delays often encountered before fixation. In this study, we investigated the effect of postmortem delay on the water diffusion anisotropy of ventrolateral spinal cord white matter from mice. Mouse spinal cords, each from the same animal, were examined using DTI in vivo, in situ after death before fixation, and ex vivo immersion fixed 10 h after death. Our results suggest that diffusion anisotropy in mouse spinal cord is preserved up to 10 h after death. Regional characteristics of diffusion anisotropy in mouse spinal cord white matter are equivalent in vivo, in situ after death (up to 10 h before fixation), and ex vivo 15 weeks after immersion fixation.

MRI of mouse models of neurological disorders

MRI has contributed to significant advances in the understanding of neurological diseases in humans. It has also been used to evaluate the spectrum of mouse models spanning from developmental abnormalities during embryogenesis, evaluation of transgenic and knockout models, through various neurological diseases such as stroke, tumors, degenerative and inflammatory diseases. The MRI techniques used clinically are technically more challenging in the mouse because of the size of the brain; however, mouse imaging provides researchers with the ability to explore cellular and molecular imaging that one day may translate into clinical practice. This article presents an overview of the use of MRI in mouse models of a variety of neurological disorders and a brief review of cellular imaging using magnetically tagged cells in the mouse central nervous system.

Two-dimensional and three-dimensional time-lapse microscopic magnetic resonance imaging of Xenopus gastrulation movements using intrinsic tissue-specific contrast

The amphibian embryo undergoes radical tissue transformations during blastula and gastrula stages, but live observation of internal morphogenetic events by optical microscopy is not feasible due to the opacity of the early embryo. Here, we report on the use of microscopic magnetic resonance imaging (MRI) to directly follow morphogenetic movements during blastula and gastrula stages of the Xenopus laevis embryo. We compare three different MRI modalities that take advantage of the intrinsic contrast present in embryonic tissues: three-dimensional (3D) fat-imaging, 3D water-imaging, and 2D high-speed high-resolution imaging of early embryonic stages. We show that the features revealed by the intrinsic contrast correlate with the histological structure of the embryo. Using this tissue specific intrinsic contrast, the main embryonic tissues and internal tissue movements as well as archenteron invagination can be differentiated without cell labeling. We present 2D and 3D time-lapse sequences of early Xenopus embryonic development, spanning the stages from early blastula to the end of gastrula, which show the complex internal rearrangements of gastrulation in essentially real-time.

Identification of fibers at risk for degeneration by diffusion tractography in patients at high risk for MS after a clinically isolated syndrome

PURPOSE

Focal inflammatory/demyelinating lesions are thought to be the source of Wallerian degeneration or other injury to local, transiting fiber tracts in the brain or spinal cord in multiple sclerosis (MS). A methodology is established to isolate connections between focal demyelinating lesions and intersecting fibers to permit explicit analyses of the pathology of secondary fiber injury distant from the focal lesion.

MATERIALS AND METHODS

A strategy is described and feasibility demonstrated in three patients with a clinically isolated syndrome and positive MRI (at high risk for MS). The strategy utilizes streamtube diffusion tractography to identify neuronal fibers that intersect a focal lesion and pass through a region of interest, in this case the corpus callosum, where distal (to focal lesion) interrogation can be accomplished.

RESULTS

A sizeable fraction of the normal appearing white matter (NAWM) in the early stages of disease can be defined in the corpus callosum, which is distinctive in that this tissue connects to distant demyelinating lesions.

CONCLUSION

The new class of tissue called fibers-at-risk for degeneration (FAR) can be identified and interrogated by a variety of quantitative MRI methodologies to better understand neuronal degeneration in MS.

Polarized light imaging of white matter architecture

Polarized light imaging (PLI) is a method to image fiber orientation in gross histological brain sections based on the birefringent properties of the myelin sheaths. The method uses the transmission of polarized light to quantitatively estimate the fiber orientation and inclination angles at every point of the imaged section. Multiple sections can be assembled into a 3D volume, from which the 3D extent of fiber tracts can be extracted. This article describes the physical principles of PLI and describes two major applications of the method: the imaging of white matter orientation of the rat brain and the generation of fiber orientation maps of the human brain in white and gray matter. The strengths and weaknesses of the method are set out.

Multimodal, multidimensional models of mouse brain

Naturally occurring mutants and genetically manipulated strains of mice are widely used to model a variety of human diseases. Atlases are an invaluable aid in understanding the impact of such manipulations by providing a standard for comparison and to facilitate the integration of anatomic, genetic, and physiologic observations from multiple subjects and experiments. We have developed digital atlases of the C57BL/6J mouse brain (adult and neonate) as comprehensive frameworks for storing and accessing the myriad types of information about the mouse brain. Along with raw and annotated images, these contain database management systems and a set of tools for comparing information from different techniques and different animals. Each atlas establishes a canonical representation of the mouse brain and provides the tools for the manipulation and analysis of new data. We describe both these atlases and discuss how they may be put to use in organizing and analyzing data from mouse models of epilepsy.

Principles of diffusion tensor imaging and its applications to basic neuroscience research

The brain contains more than 100 billion neurons that communicate with each other via axons for the formation of complex neural networks. The structural mapping of such networks during health and disease states is essential for understanding brain function. However, our understanding of brain structural connectivity is surprisingly limited, due in part to the lack of noninvasive methodologies to study axonal anatomy. Diffusion tensor imaging (DTI) is a recently developed MRI technique that can measure macroscopic axonal organization in nervous system tissues. In this article, the principles of DTI methodologies are explained, and several applications introduced, including visualization of axonal tracts in myelin and axonal injuries as well as human brain and mouse embryonic development. The strengths and limitations of DTI and key areas for future research and development are also discussed.

Differential sensitivity of in vivo and ex vivo diffusion tensor imaging to evolving optic nerve injury in mice with retinal ischemia

Decreased axial (lambda(||)) and increased radial (lambda( perpendicular)) diffusivity have been shown to reflect axonal and myelin injury respectively. In the present study, evolving white matter injury within the optic nerves of mice with retinal ischemia was examined by in vivo and ex vivo measurements of lambda(||) and lambda( perpendicular). The results show that at 3 days after retinal ischemia, a 33% decrease in vivo and a 38% decrease ex vivo in lambda(||) without change in lambda( perpendicular) was observed in the injured optic nerve compared to the control, suggestive of axonal damage without myelin injury. At 14 days, both in vivo and ex vivo measured lambda( perpendicular) increased significantly to 220-240% of the control level in the injured optic nerve suggestive of myelin damage. In contrast, the axonal injury that was clearly detected in vivo as a significantly decreased lambda(||) (33% decrease) was not as clearly detected by ex vivo lambda(||) (17% decrease). The current findings suggest that ex vivo lambda( perpendicular) is comparable to in vivo lambda( perpendicular) in detecting myelin injury. However, the structural changes resulting from axonal damage causing the decreased in vivo lambda(||) may not be preserved ex vivo in the fixed tissues. Despite the accurate depiction of the pathology using lambda(||) and lambda( perpendicular) in vivo, the use of ex vivo lambda(||) to extrapolate the status of axonal injury in vivo would require further investigation.

Cerebellar cortical atrophy in experimental autoimmune encephalomyelitis

Brain atrophy measured by MRI is an important correlate with clinical disability and disease duration in multiple sclerosis (MS). Unfortunately, neuropathologic mechanisms which lead to this grey matter atrophy remain unknown. The objective of this study was to determine whether brain atrophy occurs in the mouse model, experimental autoimmune encephalomyelitis (EAE). Postmortem high-resolution T2-weighted magnetic resonance microscopy (MRM) images from 32 mouse brains (21 EAE and 11 control) were collected. A minimum deformation atlas was constructed and a deformable atlas approach was used to quantify volumetric changes in neuroanatomical structures. A significant decrease in the mean cerebellar cortex volume in mice with late EAE (48-56 days after disease induction) as compared to normal strain, gender, and age-matched controls was observed. There was a direct correlation between cerebellar cortical atrophy and disease duration. At an early time point in disease, 15 days after disease induction, cerebellar white matter lesions were detected by both histology and MRM. These data demonstrate that myelin-specific autoimmune responses can lead to grey matter atrophy in an otherwise normal CNS. The model described herein can now be used to investigate neuropathologic mechanisms that lead to the development of gray matter atrophy in this setting.

Functional pathway-defined MRI diffusion measures reveal increased transverse diffusivity of water in multiple sclerosis

The diffusion properties of water are sensitive to microscopic changes in the white matter of multiple sclerosis (MS) patients. Typical MRI measures of disease burden in MS demonstrate modest to poor correlation with disability. Functional MRI and DTI-based fiber tracking were used to define the interhemispheric white matter pathway connecting bilateral supplementary motor areas (SMA) in 16 MS patients sand 16 control subjects. Fractional anisotropy (FA), mean diffusivity (MD), longitudinal (lambda(1)) and transverse diffusivity (lambda(2)) were measured along this pathway in all subjects. Mean FA was 0.587 +/- 0.032 for patients and 0.608 +/- 0.020 for controls (P < 0.02). Mean MD was (0.821 +/- 0.055) x 10(-3) mm(2) s(-1) for patients and (0.770 +/- 0.020) x 10(-3) mm(2) s(-1) for controls (P < 0.004). Mean lambda(1) values were (1.462 +/- 0.099) x 10(-3) mm(2) s(-1) for patients and (1.400 +/- 0.034) x 10(-3) mm(2) s(-1) for controls (P < 0.02). Mean lambda(2) values were (0.500 +/- 0.047) x 10(-3) mm(2) s(-1) for patients and (0.454 +/- 0.027) x 10(-3) mm(2) s(-1) for controls (P < 0.001). In addition, the correlation between the Multiple Sclerosis Functional Composite (MSFC) and transverse diffusivity was -0.341 (P < 0.05). The component test of the MSFC most related to the SMA pathway studied with our MRI method (Nine-hole Peg Test) showed significant correlation with transverse diffusivity (r = 0.392, P < 0.02), indicating that probing functional pathways with MRI measures can lead to a better reflection of disease status.

Time course of acute phase in mouse spinal cord injury monitored by ex vivo quantitative MRI

During the acute phase of spinal cord injury (SCI), major alterations of white and grey matter are a key issue, which determine the neurological outcome. The present study with ex vivo quantitative high-field magnetic resonance microimaging (MRI) was intended in order to identify sensitive parameters of tissue disruption in a well-controlled mouse model of ischemic SCI. MR imaging evidenced changes as early as the second hour after the lesion in the dorsal horns, which appear swollen. After 4 h, alterations of the white matter of dorsal and lateral funiculi were reflected by a progressive loss of white/grey matter contrast with further ventral extension by the 24th hour. Diffusion tensor imaging and multi-exponential T2 measurements permitted to quantify these physicochemical, time-related, alterations during the 24-h period. This characterization of spatial and temporal evolution of SCI will contribute to better define both the most appropriate targets for future therapies and more accurate therapeutic windows. Upcoming directions include the use of these parameters on in vivo animal models and their application to clinics. Indeed, magnetic resonance techniques appear now as a major non-invasive translation tool in CNS pathologies based on the development of more appropriate pre-clinical models.

Myelin deficiencies visualized in vivo: Visually evoked potentials and T2-weighted magnetic resonance images of shiverer mutant and wild-type mice

Visually evoked potentials (VEPs) and micro magnetic resonance imaging (micro MRI) are widely used as noninvasive techniques for diagnosis of central nervous system (CNS) diseases, especially myelin diseases, such as multiple sclerosis. Here we use these techniques in tandem to validate the in vivo data in mouse models. We used the shiverer mutant mouse, which has little or no CNS myelin, as a test model. These data show that shiverer (MBP(shi)/MBP(shi)) has a VEP latency that is 30% longer than that of its wild-type sibling. Surprisingly, the heterozygous (MBP(shi)/+) mouse, with apparently normal myelin, nevertheless has a 7% increase in its VEP latency vs. wild type. The micro MRIs of the same animals show that myelinated white matter is hypointense compared with gray matter as a result of the shorter T2 in myelinated regions of the CNS. T2-weighted images of wild-type and heterozygous shiverer mice show regions of hypointensity corresponding to the major myelinated tracts, including the optic nerve and the optic tract of the CNS, whereas shiverer mice have no regions of low intensity and therefore no detectable myelinated areas. In shiverer mice, micro MRI can discern hypomyelination throughout the brain, including the optic tract, and these changes correlate with longer VEP latencies. In addition, VEPs can also detect changes in the molecular make up of myelin that are not discernible with histology or micro MR. These data show the potential of using micro MRI in combination with VEPs to follow changes in both the quality and the quantity of myelin in vivo. These combined methods would be useful for longitudinal studies and therapy testing.

Improved detectability of experimental allergic encephalomyelitis in excised swine spinal cords by high b-value q-space DWI

Experimental allergic encephalomyelitis (EAE) is the primary experimental model of multiple sclerosis (MS), which involves both inflammation and demyelination and is known to be species-dependent. Spinal cord abnormalities were found in more than 80% of postmortem specimens of MS patients. In the present study, T1, T2 and high b-value q-space diffusion-weighted magnetic resonance imaging (MRI) were used, for the first time, to characterize the EAE model in excised swine spinal cords. The MR images were compared with histological staining and clinical scoring. Although all spinal cords were excised from swine with severe or very severe (clinical score between 3 to 5 on a scale of 5) motor impairments, T1- and T2-weighted MRI revealed white matter (WM) abnormalities in only five of the ten EAE diseased spinal cords studied, while high b-value q-space diffusion weighted MRI (q-space DWI) detected WM abnormalities in all diseased spinal cords studied. Interestingly, high b-value q-space DWI was able to detect abnormalities in the normal appearing white matter (NAWM) even in spinal cords where no plaques were identified by the T1- and T2-weighted MR images. Good anatomical correlation was observed between the high b-value q-space MR images and histology. The extent of DWI abnormalities paralleled the clinical scoring and correlated with histology. In addition, areas classified as NAWM by the T1- and T2-weighted MR images that showed abnormalities in the q-space DWI were also found to have abnormal histology. This improved detection level of the EAE model by high b-value q-space DWI over conventional T1-, and T2-weighted MRI is briefly discussed.

Statistical diffusion tensor histology reveals regional dysmyelination effects in the shiverer mouse mutant

Shiverer is an important model of central nervous system dysmyelination characterized by a deletion in the gene encoding myelin basic protein with relevance to human dysmyelinating and demyelinating diseases. Perfusion fixed brains from shiverer mutant (C3Fe.SWV Mbp(shi)/Mbp(shi)n = 6) and background control (C3HeB.FeJ, n = 6) mice were compared using contrast enhanced volumetric diffusion tensor magnetic resonance microscopy with a nominal isotropic spatial resolution of 80 mum. Images were accurately coregistered using non-linear warping allowing voxel-wise statistical parametric mapping of tensor invariant differences between control and shiverer groups. Highly significant differences in the tensor trace and both the axial and radial diffusivity were observed within the major white matter tracts and in the thalamus, midbrain, brainstem and cerebellar white matter, consistent with a high density of myelinated axons within these regions. The fractional anisotropy was found to be much less sensitive than the trace and eigenvalues to dysmyelination and associated microanatomic changes.

Magnetic resonance imaging, microscopy, and spectroscopy of the central nervous system in experimental animals

Over the last two decades, microscopic resolution in vivo magnetic resonance imaging (MRI) techniques have been developed and extensively used in the study of animal models of human diseases. Standard MRI methods are frequently used in clinical studies and in the general clinical practice of human neurological diseases. This generates a need for similar studies in experimental animal research. Because small rodents are the most commonly used species as animal models of neurological diseases, the MRI techniques need to be able to provide microscopic resolution and high signal-to-noise ratio images in relatively short time. Small animal MRI systems use very high field-strength magnets, which results in higher signal to noise ratio; however, the contrast characteristics of live tissue are different at these field strengths. In addition to standard MRI techniques, several new applications have been implemented in experimental animals, including diffusion and perfusion studies, MR angiography, functional MRI studies, MRI tractography, proton and phosphorous spectroscopy, cellular and molecular imaging using novel contrast methods. Here we give an overview of how to establish a small animal imaging facility with the goal of CNS imaging. We describe the basic physical processes leading to MR signal generation, highlighting the differences between standard clinical MRI and small animal MRI. Finally, typical findings in the most common neurological disease categories and novel MRI/magnetic resonance spectroscopy methods used in their study are also described.

Magnetic resonance microscopy: recent advances and applications

Magnetic resonance microscopy is receiving increased attention as more researchers in the biological sciences are turning to non-invasive imaging to characterize development, perturbations, phenotypes and pathologies in model organisms ranging from amphibian embryos to adult rodents and even plants. The limits of spatial resolution are being explored as hardware improvements address the need for increased sensitivity. Recent developments include in vivo cell tracking, restricted diffusion imaging, functional magnetic resonance microscopy and three-dimensional mouse atlases. Important applications are also being developed outside biology in the fields of fluid mechanics, geology and chemistry.

Magnetic resonance imaging of mouse spinal cord

The feasibility of performing high-resolution in vivo MRI on mouse spinal cord (SC) at 9.4 T magnetic field strength is demonstrated. The MR properties of the cord tissue were measured and the characteristics of water diffusion in the SC were quantified. The data indicate that the differences in the proton density (PD) and transverse relaxation time between gray matter (GM) and white matter (WM) dominate the contrast seen on the mouse SC images at 9.4 T. However, on heavily T(2)-weighted images these differences result in a reversal of contrast. The diffusion of water in the cord is anisotropic, but the WM exhibits greater anisotropy and principal diffusivity than the GM. The quantitative data presented here should establish a standard for comparing similar measurements obtained from the SCs of genetically engineered mouse or mouse models of SC injury (SCI).

In Vivo Imaging of Autoimmune Disease in Model Systems

Autoimmune diseases are characterized by infiltration of the target tissue with specific immune cells that ultimately leads to the destruction of normal tissue and the associated disease. There is a need for imaging tools that allow the monitoring of ongoing inflammatory disease as well as the response to therapy. We discuss new magnetic resonance imaging-based technologies that have been used to monitor inflammation and disease progression in animal models of type 1 diabetes, multiple sclerosis, and rheumatoid arthritis. Therapeutic strategies for these diseases include the transfer of immune cells, such as dendritic cells, with the aim of preventing or halting the disease course. We discuss several new MRI labeling techniques developed to allow tracking of immune cells in vivo. These include direct ex vivo labeling techniques as well as the genetic modification of cells to allow them to produce their own contrast agents. This is an area of intense recent research and can be expanded to other conditions such as cancer.

A multimodal, multidimensional atlas of the C57BL/6J mouse brain

Strains of mice, through breeding or the disruption of normal genetic pathways, are widely used to model human diseases. Atlases are an invaluable aid in understanding the impact of such manipulations by providing a standard for comparison. We have developed a digital atlas of the adult C57BL/6J mouse brain as a comprehensive framework for storing and accessing the myriad types of information about the mouse brain. Our implementation was constructed using several different imaging techniques: magnetic resonance microscopy, blockface imaging, classical histology and immunohistochemistry. Along with raw and annotated images, it contains database management systems and a set of tools for comparing information from different techniques. The framework allows facile correlation of results from different animals, investigators or laboratories by establishing a canonical representation of the mouse brain and providing the tools for the insertion of independent data into the same space as the atlas. This tool will aid in managing the increasingly complex and voluminous amounts of information about the mammalian brain. It provides a framework that encompasses genetic information in the context of anatomical imaging and holds tremendous promise for producing new insights into the relationship between genotype and phenotype. We describe a suite of tools that enables the independent entry of other types of data, facile retrieval of information and straightforward display of images. Thus, the atlas becomes a framework for managing complex genetic and epigenetic information about the mouse brain. The atlas and associated tools may be accessed at http://www.loni.ucla.edu/MAP.

Nuclear magnetic resonance microimaging of mouse spinal cord in vivo

The spinal cord is the site of traumatic injuries, the devastating consequences of which constitute a public health problem in our societies. So far, there is no efficient repair therapeutic approach, and this is mainly due to the great difficulty for elaborating predictive experimental models of this pathology. Up to now, most pathophysiological studies were based on postmortem evaluation of the quantity and extent of the lesions, and their comparison in-between human and rodent specimen. Recent progresses of magnetic resonance imaging provide new tools to examine in vivo rodent central nervous system, and eventually to monitor the progression of lesions. However, up to now, mice spinal cord has been inaccessible to such studies, due to specific physiological characteristics and to the small size of the cord. In this study, the first diffusion-weighted images depicting the mouse thoracic spinal cord in vivo are shown. Motion-related artifacts are significantly reduced by respiratory gating using a dedicated sensor. By changing the direction of diffusion-sensitizing gradients, different contrasts were obtained that are compared with ex vivo MRI and histological preparations. In addition, preliminary results obtained on pathological cords are presented.

Predictability of FTY720 efficacy in experimental autoimmune encephalomyelitis by in vivo macrophage tracking: Clinical implications for ultrasmall superparamagnetic iron oxide-enhanced magnetic resonance imaging

PURPOSE

To examine the efficacy of FTY720 as a new agent to reduce inflammatory activity in an animal model of multiple sclerosis (MS) by in vivo macrophage tracking.

MATERIAL AND METHODS

FTY720 was used for treatment of rats in a model of chronic relapsing experimental autoimmune encephalomyelitis (EAE) at an oral dose of 0.3 mg/kg/day. Magnetic resonance imaging (MRI) based on in vivo tracking of macrophages labeled with ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles, immunohistological staining (IHC), and neurological readouts was used to study the burden of disease in treated and untreated animals.

RESULTS

While untreated animals showed severe paralysis of the hind paws, intense accumulation of macrophages in brain tissue, and areas of blood-brain barrier (BBB) disruption, FTY720-treated animals displayed no signs of inflammatory activity or neurological impairment. These observations were made for both acute phase and first relapse.

CONCLUSION

Tracking of macrophages by MRI provides direct evidence of the immunomodulatory efficacy of FTY720 in the EAE model and correlates well with neurological symptoms and histology.

Relative indices of water diffusion anisotropy are equivalent in live and formalin-fixed mouse brains

Formalin fixation of tissue is a common laboratory practice. A direct comparison of diffusion tensor imaging (DTI) parameters from mouse brains before (in vivo) and after (ex vivo) formalin fixation is reported herein. Five diffusion indices were examined in a cohort of seven mice: relative anisotropy (RA), directional correlation (DC), trace (Tr(D)), trace-normalized axial diffusivity (D(axially)), and radial diffusivity (D(radially)). Seven regions of interest (ROIs), including five in white matter and two in gray matter, were selected for examination. Consistent with previous findings, a significant decrease of Tr(D) was observed for all ROIs after fixation. However, water diffusion anisotropy, as defined by the indices RA, DC, D(axially), and D(radially), remained unchanged after fixation. Thus, fixation does not appear to alter diffusion anisotropy in the mouse brain. This finding supports the utility of diffusion anisotropy analysis of fixed tissue. The combination of DTI measurements and standard histology may shed light on the microstructural determinants of diffusion anisotropy in normal and disease states.

High-resolution magnetic resonance imaging at 2 Tesla: potential for atherosclerotic lesions exploration in the apolipoprotein E knockout mouse

INTRODUCTION

The aim of the present study was to evaluate the potential of high-resolution MRI at 2 Tesla (T) for direct noninvasive imaging of the aortic wall in a mouse model of atherosclerosis.

MATERIAL AND METHODS

A specific mouse antenna was developed and sequence parameters were adjusted. T(1)- and T2-weighted images of abdominal aorta were obtained at 2 T with a spatial resolution of 86 x 86 x 800 microm3 in vivo. With a dedicated small coil, ex vivo MRI of the aorta was performed with a spatial resolution of 54 x 54 x 520 microm3.

RESULTS

In vivo, the aortic wall was clearly defined on T(2)-weighted images in 15 of 16 mice: along the aorta the lumen circumference ranged from 1.07 to 3.61 mm and mean wall thickness from 0.11 to 0.67 mm. In vivo measurements of plaque distribution were confirmed by ex vivo MR imaging and by histology, with a good correlation with histology regarding lumen circumference (r = 0.94) and wall thickness (r = 0.97).

CONCLUSION

Magnetic resonance imaging at 2 T to analyze in vivo atherosclerotic lesions in mice is possible with a spatial resolution of 86 x 86 x 800 microm3 and thus can be used for noninvasive follow-up in evaluation of new drugs.

Diffusion tensor imaging in spinal cord: methods and applications - a review

The spinal cord is a clinically eloquent site within the central nervous system, containing important sensorimotor tracts confined within a small cross-sectional area. Damage to the spinal cord may be caused by a wide range of pathologies, and can result in profound functional disability. Characterization of the structural integrity of the spinal cord can be assessed using diffusion tensor imaging methods. Development and application of this technique may improve our understanding of the nature and evolution of structural damage in spinal cord disease. Possible developments include improved detection of ischaemic lesions, clarification of the relationship between clinical disability and structural damage to the cord and monitoring of anti-inflammatory or neuroprotective therapies. In this review current technical aspects, clinical applications and the suggested future development of spinal cord diffusion imaging are discussed.

High b-value q-space analyzed diffusion-weighted MRS and MRI in neuronal tissues - a technical review

This review deals with high b-value q-space diffusion-weighted MRI (DW-MRI) of neuronal tissues. It is well documented that at sufficiently high b-values (and high q-values) neuronal water signal decay in diffusion experiments is not mono-exponential. This implies the existence of more than one apparent diffusing component or evidence for restriction. The assignment of the different apparent diffusing components to real physical entities is not straightforward. However, the apparent slow diffusing component that was found to be restricted to a compartment of a few microns, if originating mainly from a specific pool and if assigned correctly, may potentially be used to obtain more specific MR images with regard to specific pathologies of the CNS. This review examines the utility of analyzing high b-value diffusion MRS and MRI data using the q-space approach introduced by Callaghan and by Cory and Garroway. This approach provides displacement probability maps that emphasize, at long diffusion times, the characteristics of the apparent slow diffusing component. Examples from excised spinal cord, where the experimental conditions for which the q-space analysis of MR diffusion data was developed can be met or approached will be presented. Then examples from human MS patients, where q-space requirement for the short gradient pulse is clearly violated, are presented. In the excised spinal cord studies, this approach was used to study spinal cord maturation and trauma, and was found to be more sensitive than other conventional methods in following spinal cord degeneration in an experimental model of vascular dementia (VaD). High b-value q-space DWI was also recently used to study healthy and MS diseased human brains. This approach was found to be very sensitive to the disease load in MS, compared with other conventional MRI methods, especially in the normal appearing white matter (NAWM) of MS brains. Finally, the potential diagnostic capacity embedded in high b-value q-space analyzed diffusion MR images is discussed. The potentials and caveats of this approach are outlined and experimental data are presented that show the effect of violating the short gradient pulse (SGP) approximation on the extracted parameters from the q-space analysis.

Diffusion-tensor MRI: theory, experimental design and data analysis - a technical review

This article treats the theoretical underpinnings of diffusion-tensor magnetic resonance imaging (DT-MRI), as well as experimental design and data analysis issues. We review the mathematical model underlying DT-MRI, discuss the quantitative parameters that are derived from the measured effective diffusion tensor, and describe artifacts that arise in typical DT-MRI acquisitions. We also discuss difficulties in identifying appropriate models to describe water diffusion in heterogeneous tissues, as well as in interpreting experimental data obtained in such issues. Finally, we describe new statistical methods that have been developed to analyse DT-MRI data, and their potential uses in clinical and multi-site studies.

The basis of anisotropic water diffusion in the nervous system - a technical review

Anisotropic water diffusion in neural fibres such as nerve, white matter in spinal cord, or white matter in brain forms the basis for the utilization of diffusion tensor imaging (DTI) to track fibre pathways. The fact that water diffusion is sensitive to the underlying tissue microstructure provides a unique method of assessing the orientation and integrity of these neural fibres, which may be useful in assessing a number of neurological disorders. The purpose of this review is to characterize the relationship of nuclear magnetic resonance measurements of water diffusion and its anisotropy (i.e. directional dependence) with the underlying microstructure of neural fibres. The emphasis of the review will be on model neurological systems both in vitro and in vivo. A systematic discussion of the possible sources of anisotropy and their evaluation will be presented followed by an overview of various studies of restricted diffusion and compartmentation as they relate to anisotropy. Pertinent pathological models, developmental studies and theoretical analyses provide further insight into the basis of anisotropic diffusion and its potential utility in the nervous system.

MR microscopy and high resolution small animal MRI: applications in neuroscience research

The application of magnetic resonance (MR) imaging in the study of human disease using small animals has steadily evolved over the past two decades and strongly established the fields of "small animal MR imaging" and "MR microscopy." An increasing number of neuroscience related investigations now implement MR microscopy in their experiments. Research areas of growth pertaining to MR microscopy studies are focused on (1). phenotyping of genetically engineered mice models of human neurological diseases and (2). rodent brain atlases. MR microscopy can be performed in vitro on tissue specimens, ex vivo on brain slice preparations and in vivo (typically on rodents). Like most new imaging technologies, MR microscopy is technologically demanding and requires broad expertise. Uniform guidelines or "standards" of a given MR microscopy experiment are non-existent. The main focus therefore of this review will be on biological applications of MR microscopy and the experimental requirements. We also take a critical look at the biological information that small animal (rodent) MR imaging has provided in neuroscience research.

Imaging inflammation: direct visualization of perivascular cuffing in EAE by magnetic resonance microscopy

PURPOSE

To determine if the architectural features revealed by magnetic resonance microscopy (MRM) allow one to detect microscopic abnormalities associated with neuroinflammation in fixed brain sections from animals with experimental allergic encephalomyelitis (EAE), an animal model for multiple sclerosis (MS).

MATERIALS AND METHODS

Imaging was performed at the Center for In Vivo Microscopy (CIVM) using a 9.4-Tesla, 89-mm bore, superconducting magnet with actively shielded gradients capable of 850 mT/m. A number of MR contrasts and spatial resolutions were explored.

RESULTS

The assessment of EAE brain showed that it is possible to visualize perivascular cuffing in vitro by MRM on three-dimensional T1 proton stains.

CONCLUSION

Inflammatory cell infiltration is a prerequisite for the development of lesions in EAE and MS. Thus, the ability to directly detect individual perivascular cuffs of inflammation may provide a useful means of monitoring the time course of inflammatory events, as conventional histopathological scoring of perivascular cuffs is utilized, but in the absence of sectioning and staining.

First application of extremely high-resolution magnetic resonance imaging to study microscopic features of normal and LHON human optic nerve

PURPOSE

To apply new methods in magnetic resonance imaging (MRI) in resolving the histoarchitecture of the human optic nerve obtained from normal individuals and a Leber's hereditary optic neuropathy (LHON) case.

DESIGN

Small case series--clinicopathologic correlation.

METHOD

Three optic nerves were obtained from two normal subjects, aged 69 and 70, and a LHON/3460 patient, aged 75. The posterior pole of the eye with attached optic nerves was fixed in buffered paraformaldehyde and placed into a 10-mm quartz tube. Images were acquired in a Bruker AMX500 12 Tesla microimaging system. The three-dimensional data were acquired with 512 x 256 x 256 points, yielding a final isotopic resolution of 30 microm.

RESULTS

The sclera, choroids, and retina were easily distinguished. The nerve fiber layer was seen to enter the optic disc and traverse the lamina cribrosa (LC). The resolution of the image of the optic nerve head was such that the LC was visualized as multiple stacked plates. The fibers emerged from glial columns in the LC as distinct fascicles and could be made out to change appearance as they became myelinated and expanded in the more posterior nerve. The ophthalmic artery and vein were visualized, as were the optic nerve arachnoid and dural sheaths. In the Leber's case, the LC plates seemed collapsed or compressed. The axonal bundles were atrophic and the pial-collagen septae markedly thickened. The entire nerve had shrunk, creating space under the arachnoid, down and around the central ophthalmic artery and vein.

CONCLUSIONS

These results demonstrate the feasibility of using extremely high-resolution magnetic resonance imaging (microMRI) to examine the three-dimensional (30 microm) images of the human optic nerve. Several atrophic lesions, normally visible only by histopathologic examination, were visualized in the Leber's optic nerve. microMRI may eventually permit the in vivo visualization of lesions in or about the optic nerve.