Detection of multiple pathways in the spinal cord using q-ball imaging

Quantitative and Computational Spinal Imaging in Neurodegenerative Conditions and Acquired Spinal Disorders: Academic Advances and Clinical Prospects

Introduction: Quantitative spinal cord imaging has facilitated the objective appraisal of spinal cord pathology in a range of neurological conditions both in the academic and clinical setting. Diverse methodological approaches have been implemented, encompassing a range of morphometric, diffusivity, susceptibility, magnetization transfer, and spectroscopy techniques. Advances have been fueled both by new MRI platforms and acquisition protocols as well as novel analysis pipelines. The quantitative evaluation of specific spinal tracts and grey matter indices has the potential to be used in diagnostic and monitoring applications. The comprehensive characterization of spinal disease burden in pre-symptomatic cohorts, in carriers of specific genetic mutations, and in conditions primarily associated with cerebral disease, has contributed important academic insights. Methods: A narrative review was conducted to examine the clinical and academic role of quantitative spinal cord imaging in a range of neurodegenerative and acquired spinal cord disorders, including hereditary spastic paraparesis, hereditary ataxias, motor neuron diseases, Huntington's disease, and post-infectious or vascular disorders. Results: The clinical utility of specific methods, sample size considerations, academic role of spinal imaging, key radiological findings, and relevant clinical correlates are presented in each disease group. Conclusions: Quantitative spinal cord imaging studies have demonstrated the feasibility to reliably appraise structural, microstructural, diffusivity, and metabolic spinal cord alterations. Despite the notable academic advances, novel acquisition protocols and analysis pipelines are yet to be implemented in the clinical setting.

Investigation of endophenotype potential of decreased fractional anisotropy in pediatric bipolar disorder patients and unrelated offspring of bipolar disorder patients

BACKGROUND

Bipolar disorder (BD) is a severe psychiatric disorder associated with structural and functional brain abnormalities, some of which have been found in unaffected relatives as well. In this study, we examined the potential role of decreased fractional anisotropy (FA) as a BD endophenotype, in adolescents at high risk for BD.

METHODS

We included 15 offspring of patients with BD, 16 pediatric BD patients, and 16 matched controls. Diffusion weighted scans were obtained on a 3T scanner using an echo-planar sequence. Scans were segmented using FreeSurfer.

RESULTS

Our results showed significantly decreased FA in six brain areas of offspring group; left superior temporal gyrus (LSTG; P < .0001), left transverse temporal gyrus (LTTG; P = .002), left banks of the superior temporal sulcus (LBSTS; P = .002), left anterior cingulum (LAC; P = .003), right temporal pole (RTP; P = .004) and left frontal pole (LFP; P = .017). On analysis, LSTG, LAC, and RTP demonstrated a potential to be an endophenotype when comparing all three groups. FA values in three regions, LBSTS, LTTG, and LFP were increased only in controls.

CONCLUSION

Our findings point at decreased FA as a possible endophenotype for BD, as they were found in children of patients with BD. Most of these areas were previously found to have morphological and functional changes in adult and pediatric BD, and are thought to play important roles in affected domains of functioning. Prospective follow up studies should be performed to detect reliability of decreased FA as an endophenotype and effects of treatment on FA.

MRI diffusion tensor imaging scalar values in dogs with intervertebral disc herniation: A comparison between manual and semiautomated region of interest methods

Magnetic resonance imaging (MRI) diffusion tensor imaging (DTI) measures have been described as methods for quantifying spinal cord injury and predicting outcome in dogs with intervertebral disc herniation (IVDH); however, studies comparing methods for selecting regions of interest (ROIs) are currently lacking. The aims of this retrospective, methods comparison, observational study were to compare DTI measurements acquired using manual (mROI) versus semiautomated ROI (sROI) methods and to compare DTI measurements with patient outcomes. Magnetic resonance imaging scans that included DTI pulse sequences were retrieved for 65 dogs with confirmed IVDH. Regions of interest were placed at one vertebral length cranial and caudal to the region of spinal cord compression (RSCC) using the mROI and sROI methods. Scalar values based on the mROI and sROI methods were compared. There was a significant difference for all DTI measures (P < 0.0001), where fractional anisotropy was higher (95% confidence interval [CI]: 0.15, 0.19) and mean diffusivity (MD; CI: -0.41, -0.35), axial diffusivity (AD; CI: -0.47, -0.36) and radial diffusivity (RD; CI: -0.36, -0.27) were lower for the mROI than for the sROI. For both the mROI and sROI, MD, AD, and RD were significantly lower (p < 0.05) at the RSCC in paraplegic dogs that did not regain motor function. The findings indicated that DTI methods for quantifying SCI using open source software and ROI were feasible for use in dogs with IVDH; however, values based on sROI methods differed from values based on mROI methods. Some DTI measures based on both the mROI and sROI methods were predictive of poor patient outcome.

In vivo restricted diffusion imaging (RDI) is sensitive to differences in axonal density in typical children and adults

The ability to dissociate axonal density in vivo from other microstructural properties is important for the diagnosis and treatment of neurologic disease, and new methods to do so are being developed. We investigated one such method-restricted diffusion imaging (RDI)-to see whether it can more accurately replicate histological axonal density patterns in the corpus callosum (CC) of adults and children compared to diffusion tensor imaging (DTI), neurite orientation dispersion and density imaging (NODDI), and generalized q-sampling imaging (GQI) methods. To do so, we compared known axonal density patterns defined by histology to diffusion-weighted imaging (DWI) scans of 840 healthy 20- to 40-year-old adults, and to DWI scans of 129 typically developing 7-month-old to 18-year-old children and adolescents. Contrast analyses were used to compare pattern similarities between the in vivo metric and previously published histological density models. We found that RDI was effective at mapping axonal density of small (Cohen's d = 2.60) and large fiber sizes (Cohen's d = 2.84) in adults. The same pattern was observed in the developing sample (Cohen's d = 3.09 and 3.78, respectively). Other metrics, notably NODDI's intracellular volume fraction in adults and GQI generalized fractional anisotropy in children, were also sensitive metrics. In conclusion, the study showed that the novel RDI metric is sensitive to density of small and large axons in adults and children, with both single- and multi-shell acquisition DWI data. Its effectiveness and availability to be used on standard as well as advanced DWI acquisitions makes it a promising method in clinical settings.

Atlas-Based Quantification of DTI Measures in a Typically Developing Pediatric Spinal Cord

BACKGROUND AND PURPOSE

Multi-parametric MRI, provides a variety of biomarkers sensitive to white matter integrity, However, spinal cord MRI data in pediatrics is rare compared to adults. The purpose of this work was 3-fold: 1) to develop a processing pipeline for atlas-based generation of the typically developing pediatric spinal cord WM tracts, 2) to derive atlas-based normative values of the DTI indices for various WM pathways, and 3) to investigate age-related changes in the obtained normative DTI indices along the extracted tracts.

MATERIALS AND METHODS

DTI scans of 30 typically developing subjects (age range, 6-16 years) were acquired on a 3T MR imaging scanner. The data were registered to the PAM50 template in the Spinal Cord Toolbox. Next, the DTI indices for various WM regions were extracted at a single section centered at the C3 vertebral body in all the 30 subjects. Finally, an ANOVA test was performed to examine the effects of the following: 1) laterality, 2) functionality, and 3) age, with DTI-derived indices in 34 extracted WM regions.

RESULTS

A postprocessing pipeline was developed and validated to delineate pediatric spinal cord WM tracts. The results of ANOVA on fractional anisotropy values showed no effect for laterality (P = .72) but an effect for functionality (P < .001) when comparing the 30 primary WM labels. There was a significant (P < .05) effect of age and maturity of the left spinothalamic tract on mean diffusivity, radial diffusivity, and axial diffusivity values.

CONCLUSIONS

The proposed automated pipeline in this study incorporates unique postprocessing steps followed by template registration and quantification of DTI metrics using atlas-based regions. This method eliminates the need for manual ROI analysis of WM tracts and, therefore, increases the accuracy and speed of the measurements.

White matter microstructure varies with post-traumatic stress severity following medical trauma

The prefrontal cortex, amygdala, hippocampus, and hypothalamus are important components of the neural network that mediates the healthy learning, expression, and regulation of emotion. These brain regions are connected by white matter pathways that include the cingulum bundle, uncinate fasciculus, and fornix/stria terminalis. Individuals with trauma and stress-related disorders show dysfunction of the cognitive-affective processes supported by the brain regions these white matter tracts connect. Therefore, variability in the microstructure of these white matter pathways may play an important role in the cognitive-affective dysfunction related to post-traumatic stress disorder. Thus, the current study used diffusion weighted imaging to assess the white matter microstructure of the cingulum bundle, uncinate fasciculus, and fornix/stria terminalis acutely (< 1 month) following trauma. Further, we assessed both acute (i.e., < 1 month) and subacute (i.e., 3 months post-trauma) post-traumatic stress symptom severity. White matter microstructure (assessed < 1 month post-trauma) of the uncinate fasciculus and fornix/stria terminalis varied with acute post-traumatic stress severity (assessed < 1 month post-trauma). Further, white matter microstructure (assessed < 1 month post-trauma) of the cingulum bundle and fornix/stria terminalis varied with subacute post-traumatic stress severity (assessed 3 months post-trauma). The current results suggest white matter architecture of the prefrontal cortex - amygdala network plays an important role in the development of trauma and stress-related disorders.

Probing Tissue Microarchitecture of the Baby Brain via Spherical Mean Spectrum Imaging

During the first years of life, the human brain undergoes dynamic spatially-heterogeneous changes, invo- lving differentiation of neuronal types, dendritic arbori- zation, axonal ingrowth, outgrowth and retraction, synaptogenesis, and myelination. To better quantify these changes, this article presents a method for probing tissue microarchitecture by characterizing water diffusion in a spectrum of length scales, factoring out the effects of intra-voxel orientation heterogeneity. Our method is based on the spherical means of the diffusion signal, computed over gradient directions for a set of diffusion weightings (i.e., b -values). We decompose the spherical mean profile at each voxel into a spherical mean spectrum (SMS), which essentially encodes the fractions of spin packets undergoing fine- to coarse-scale diffusion proce- sses, characterizing restricted and hindered diffusion stemming respectively from intra- and extra-cellular water compartments. From the SMS, multiple orientation distribution invariant indices can be computed, allowing for example the quantification of neurite density, microscopic fractional anisotropy ( μ FA), per-axon axial/radial diffusivity, and free/restricted isotropic diffusivity. We show that these indices can be computed for the developing brain for greater sensitivity and specificity to development related changes in tissue microstructure. Also, we demonstrate that our method, called spherical mean spectrum imaging (SMSI), is fast, accurate, and can overcome the biases associated with other state-of-the-art microstructure models.

HARDI-ZOOMit protocol improves specificity to microstructural changes in presymptomatic myelopathy

Diffusion magnetic resonance imaging (dMRI) proved promising in patients with non-myelopathic degenerative cervical cord compression (NMDCCC), i.e., without clinically manifested myelopathy. Aim of the study is to present a fast multi-shell HARDI-ZOOMit dMRI protocol and validate its usability to detect microstructural myelopathy in NMDCCC patients. In 7 young healthy volunteers, 13 age-comparable healthy controls, 18 patients with mild NMDCCC and 15 patients with severe NMDCCC, the protocol provided higher signal-to-noise ratio, enhanced visualization of white/gray matter structures in microstructural maps, improved dMRI metric reproducibility, preserved sensitivity (SE = 87.88%) and increased specificity (SP = 92.31%) of control-patient group differences when compared to DTI-RESOLVE protocol (SE = 87.88%, SP = 76.92%). Of the 56 tested microstructural parameters, HARDI-ZOOMit yielded significant patient-control differences in 19 parameters, whereas in DTI-RESOLVE data, differences were observed in 10 parameters, with mostly lower robustness. Novel marker the white-gray matter diffusivity gradient demonstrated the highest separation. HARDI-ZOOMit protocol detected larger number of crossing fibers (5–15% of voxels) with physiologically plausible orientations than DTI-RESOLVE protocol (0–8% of voxels). Crossings were detected in areas of dorsal horns and anterior white commissure. HARDI-ZOOMit protocol proved to be a sensitive and practical tool for clinical quantitative spinal cord imaging.

Missing the forest because of the trees: slower alternations during binocular rivalry are associated with lower levels of visual detail during ongoing thought

Conscious awareness of the world fluctuates, either through variation in how vividly we perceive the environment, or when our attentional focus shifts away from information in the external environment towards information that we generate via imagination. Our study combined individual differences in experience sampling, psychophysical reports of perception and neuroimaging descriptions of structural connectivity to better understand these changes in conscious awareness. In particular, we examined (i) whether aspects of ongoing thought—indexed via multi-dimensional experience sampling during a sustained attention task—are associated with the white matter fibre organization of the cortex as reflected by their relative degree of anisotropic diffusion and (ii) whether these neurocognitive descriptions of ongoing experience are related to a more constrained measure of visual consciousness through analysis of bistable perception during binocular rivalry. Individuals with greater fractional anisotropy in right hemisphere white matter regions involving the inferior fronto-occipital fasciculus, the superior longitudinal fasciculus and the cortico-spinal tract, described their ongoing thoughts as lacking external details. Subsequent analysis indicated that the combination of low fractional anisotropy in these right hemisphere regions, with reports of thoughts with high levels of external details, was associated with the shortest periods of dominance during binocular rivalry. Since variation in binocular rivalry reflects differences between bottom-up and top-down influences on vision, our study suggests that reports of ongoing thoughts with vivid external details may occur when conscious precedence is given to bottom-up representation of perceptual information.

The mechanisms of the protective effect of education in cognitive aging

Frequent cases of pathological brain aging are an important scientific and social problem. Some people have the ability to compensate for the initial manifestations of pathological aging and delay the development of the clinical phenomena of the disease. The concept of “cognitive reserve” allows us to study the possibilities of increasing brain stability in conditions of pathological aging. The identification of the dominant form of thinking, converging or diverging, reveals possible neurophysiological mechanisms of the cognitive reserve. Understanding the mechanisms of formation of individual cognitive styles actualizes the contribution of the “Education” factor to the development of the cognitive reserve. The research material was scientific publications on the topic of work. It was revealed that the “education” factor can realize its protective effect on the pathological aging of the brain due to the formation of an adaptive form of thinking. The data presented in this review make it possible to substantiate the medical and social significance of educational programs for people of mature and older age.

A Joint Modelling Approach to Analyze Risky Decisions by Means of Diffusion Tensor Imaging and Behavioural Data

Understanding dependencies between brain functioning and cognition is a challenging task which might require more than applying standard statistical models to neural and behavioural measures to be accomplished. Recent developments in computational modelling have demonstrated the advantage to formally account for reciprocal relations between mathematical models of cognition and brain functional, or structural, characteristics to relate neural and cognitive parameters on a model-based perspective. This would allow to account for both neural and behavioural data simultaneously by providing a joint probabilistic model for the two sources of information. In the present work we proposed an architecture for jointly modelling the reciprocal relation between behavioural and neural information in the context of risky decision-making. More precisely, we offered a way to relate Diffusion Tensor Imaging data to cognitive parameters of a computational model accounting for behavioural outcomes in the popular Balloon Analogue Risk Task (BART). Results show that the proposed architecture has the potential to account for individual differences in task performances and brain structural features by letting individual-level parameters to be modelled by a joint distribution connecting both sources of information. Such a joint modelling framework can offer interesting insights in the development of computational models able to investigate correspondence between decision-making and brain structural connectivity.

Spinal Cord Imaging in Amyotrophic Lateral Sclerosis: Historical Concepts-Novel Techniques

Amyotrophic lateral sclerosis (ALS) is the most common adult onset motor neuron disease with no effective disease modifying therapies at present. Spinal cord degeneration is a hallmark feature of ALS, highlighted in the earliest descriptions of the disease by Lockhart Clarke and Jean-Martin Charcot. The anterior horns and corticospinal tracts are invariably affected in ALS, but up to recently it has been notoriously challenging to detect and characterize spinal pathology in vivo. With recent technological advances, spinal imaging now offers unique opportunities to appraise lower motor neuron degeneration, sensory involvement, metabolic alterations, and interneuron pathology in ALS. Quantitative spinal imaging in ALS has now been used in cross-sectional and longitudinal study designs, applied to presymptomatic mutation carriers, and utilized in machine learning applications. Despite its enormous clinical and academic potential, a number of physiological, technological, and methodological challenges limit the routine use of computational spinal imaging in ALS. In this review, we provide a comprehensive overview of emerging spinal cord imaging methods and discuss their advantages, drawbacks, and biomarker potential in clinical applications, clinical trial settings, monitoring, and prognostic roles.

Fiber tractography of the optic radiations: impact of diffusion model, voxel shape and orientation

BACKGROUND

Reliable visualization of the optic radiations (OR) is of major importance in tumor surgery close to the OR to prevent permanent visual field deficits. Diffusion tensor imaging (DTI) based fiber tractography (FT) has become a standard tool to visualize major white matter tracts and to support the prevention of postoperative deficits. Nevertheless, FT of the OR is notoriously challenging due to its high neuroanatomical complexity.

METHODS

To improve FT of the OR we analyzed the effect of a more complex diffusion model and the effect of different voxel shapes and orientations. MRI data of 21 healthy subjects was acquired using isometric and anisometric voxel sizes and standard and adapted slice angulation. FT was performed using the DTI based approach and an orientation distribution function (ODF) based approach. Results were visually inspected, and fiber tract volumes were compared.

RESULTS

DTI based FT led to poor results, failing to reconstruct plausible tracts at all in up to 26.11 % of all cases. The ODF based approach resulted in more compound and solid tracts showing also significantly larger tract volumes. Voxel shape or orientation did not influence DTI but ODF based FT. Isometric or anisometric voxels with standard slice orientation revealed highest tract volumes. Adapted orientation in combination with anisometric voxels led to significantly smaller tract volumes.

CONCLUSIONS

Plausible tractography of the OR can be achieved using ODF based fiber tracking within a clinically feasible timeframe. Voxel shape and orientation seem to be of minor importance and might be kept to isometric voxel for flexible application of FT.

Microstructural imaging in the spinal cord and validation strategies

In vivo histology using magnetic resonance imaging (MRI) is a newly emerging research field that aims to non-invasively characterize tissue microstructure. The implications of in vivo histology are many, from discovering novel biomarkers to studying human development, to providing tools for disease diagnosis and monitoring the effects of novel treatments on tissue. This review focuses on quantitative MRI (qMRI) techniques that are used to map spinal cord microstructure. Opening with a rationale for non-invasive imaging of the spinal cord, this article continues with a brief overview of the existing MRI techniques for axon and myelin imaging, followed by the specific challenges and potential solutions for acquiring and processing such data. The final part of this review focuses on histological validation, with suggested tissue preparation, acquisition and processing protocols for large-scale microscopy.

Postmortem diffusion MRI of the entire human spinal cord at microscopic resolution

The human spinal cord is a central nervous system structure that plays an important role in normal motor and sensory function, and can be affected by many debilitating neurologic diseases. Due to its clinical importance, the spinal cord is frequently the subject of imaging research. Common methods for visualizing spinal cord anatomy and pathology include histology and magnetic resonance imaging (MRI), both of which have unique benefits and drawbacks. Postmortem microscopic resolution MRI of fixed specimens, sometimes referred to as magnetic resonance microscopy (MRM), combines many of the benefits inherent to both techniques. However, the elongated shape of the human spinal cord, along with hardware and scan time limitations, have restricted previous microscopic resolution MRI studies (both in vivo and ex vivo) to small sections of the cord. Here we present the first MRM dataset of the entire postmortem human spinal cord. These data include 50 μm isotropic resolution anatomic image data and 100 μm isotropic resolution diffusion data, made possible by a 280 h long multi-segment acquisition and automated image segment composition. We demonstrate the use of these data for spinal cord lesion detection, automated volumetric gray matter segmentation, and quantitative spinal cord morphometry including estimates of cross sectional dimensions and gray matter fraction throughout the length of the cord.

Application and evaluation of NODDI in the cervical spinal cord of multiple sclerosis patients

INTRODUCTION

There is a need to develop imaging methods sensitive to axonal injury in multiple sclerosis (MS), given the prominent impact of axonal pathology on disability and outcome. Advanced multi-compartmental diffusion models offer novel indices sensitive to white matter microstructure. One such model, neurite orientation dispersion and density imaging (NODDI), is sensitive to neurite morphology, providing indices of apparent volume fractions of axons (vin), isotropic water (viso) and the dispersion of fibers about a central axis (orientation dispersion index, ODI). NODDI has yet to be studied for its sensitivity to spinal cord pathology. Here, we investigate the feasibility and utility of NODDI in the cervical spinal cord of MS patients.

METHODS

NODDI was applied in the cervical spinal cord in a cohort of 8 controls and 6 MS patients. Statistical analyses were performed to test the sensitivity of NODDI-derived indices to pathology in MS (both lesion and normal appearing white matter NAWM). Diffusion kurtosis imaging (DKI) and diffusion tensor imaging (DTI) analysis were also performed to compare with NODDI.

RESULTS

A decrease in NODDI-derived vin was observed at the site of the lesion (p < 0.01), whereas a global increase in ODI was seen throughout white matter (p < 0.001). DKI-derived mean kurtosis (MK) and radial kurtosis (RK) and DTI-derived fractional anisotropy (FA) and radial diffusivity (RD) were all significantly different in MS patients (p < 0.02), however NODDI provided higher contrast between NAWM and lesion in all MS patients.

CONCLUSION

NODDI provides unique contrast that is not available with DKI or DTI, enabling improved characterization of the spinal cord in MS.

Diffusion MRI of the spinal cord: from structural studies to pathology

Diffusion MRI is extensively used to study brain microarchitecture and pathologies, and water diffusion appears highly anisotropic in the white matter (WM) of the spinal cord (SC). Despite these facts, the use of diffusion MRI to study the SC, which has increased in recent years, is much less common than that in the brain. In the present review, after a brief outline of early studies of diffusion MRI (DWI) and diffusion tensor MRI (DTI) of the SC, we provide a short survey on DTI and on diffusion MRI methods beyond the tensor that have been used to study SC microstructure and pathologies. After introducing the porous view of WM and describing the q-space approach and q-space diffusion MRI (QSI), we describe other methodologies that can be applied to study the SC. Selected applications of the use of DTI, QSI, and other more advanced diffusion MRI methods to study SC microstructure and pathologies are presented, with some emphasis on the use of less conventional diffusion methodologies. Because of length constraints, we concentrate on structural studies and on a few selected pathologies. Examples of the use of diffusion MRI to study dysmyelination, demyelination as in experimental autoimmune encephalomyelitis and multiple sclerosis, amyotrophic lateral sclerosis, and traumatic SC injury are presented. We conclude with a brief summary and a discussion of challenges and future directions for diffusion MRI of the SC. Copyright © 2016 John Wiley & Sons, Ltd.

SCT: Spinal Cord Toolbox, an open-source software for processing spinal cord MRI data

For the past 25 years, the field of neuroimaging has witnessed the development of several software packages for processing multi-parametric magnetic resonance imaging (mpMRI) to study the brain. These software packages are now routinely used by researchers and clinicians, and have contributed to important breakthroughs for the understanding of brain anatomy and function. However, no software package exists to process mpMRI data of the spinal cord. Despite the numerous clinical needs for such advanced mpMRI protocols (multiple sclerosis, spinal cord injury, cervical spondylotic myelopathy, etc.), researchers have been developing specific tools that, while necessary, do not provide an integrative framework that is compatible with most usages and that is capable of reaching the community at large. This hinders cross-validation and the possibility to perform multi-center studies. In this study we introduce the Spinal Cord Toolbox (SCT), a comprehensive software dedicated to the processing of spinal cord MRI data. SCT builds on previously-validated methods and includes state-of-the-art MRI templates and atlases of the spinal cord, algorithms to segment and register new data to the templates, and motion correction methods for diffusion and functional time series. SCT is tailored towards standardization and automation of the processing pipeline, versatility, modularity, and it follows guidelines of software development and distribution. Preliminary applications of SCT cover a variety of studies, from cross-sectional area measures in large databases of patients, to the precise quantification of mpMRI metrics in specific spinal pathways. We anticipate that SCT will bring together the spinal cord neuroimaging community by establishing standard templates and analysis procedures.

Segmentation of the human spinal cord

Segmenting the spinal cord contour is a necessary step for quantifying spinal cord atrophy in various diseases. Delineating gray matter (GM) and white matter (WM) is also useful for quantifying GM atrophy or for extracting multiparametric MRI metrics into specific WM tracts. Spinal cord segmentation in clinical research is not as developed as brain segmentation, however with the substantial improvement of MR sequences adapted to spinal cord MR investigations, the field of spinal cord MR segmentation has advanced greatly within the last decade. Segmentation techniques with variable accuracy and degree of complexity have been developed and reported in the literature. In this paper, we review some of the existing methods for cord and WM/GM segmentation, including intensity-based, surface-based, and image-based methods. We also provide recommendations for validating spinal cord segmentation techniques, as it is important to understand the intrinsic characteristics of the methods and to evaluate their performance and limitations. Lastly, we illustrate some applications in the healthy and pathological spinal cord. One conclusion of this review is that robust and automatic segmentation is clinically relevant, as it would allow for longitudinal and group studies free from user bias as well as reproducible multicentric studies in large populations, thereby helping to further our understanding of the spinal cord pathophysiology and to develop new criteria for early detection of subclinical evolution for prognosis prediction and for patient management. Another conclusion is that at the present time, no single method adequately segments the cord and its substructure in all the cases encountered (abnormal intensities, loss of contrast, deformation of the cord, etc.). A combination of different approaches is thus advised for future developments, along with the introduction of probabilistic shape models. Maturation of standardized frameworks, multiplatform availability, inclusion in large suite and data sharing would also ultimately benefit to the community.

In vivo mapping of human spinal cord microstructure at 300mT/m

The ability to characterize white matter microstructure non-invasively has important applications for the diagnosis and follow-up of several neurological diseases. There exists a family of diffusion MRI techniques, such as AxCaliber, that provide indices of axon microstructure, such as axon diameter and density. However, to obtain accurate measurements of axons with small diameters (<5μm), these techniques require strong gradients, i.e. an order of magnitude higher than the 40-80mT/m currently available in clinical systems. In this study we acquired AxCaliber diffusion data at a variety of different q-values and diffusion times in the spinal cord of five healthy subjects using a 300mT/m whole body gradient system. Acquisition and processing were optimized using state-of-the-art methods (e.g., 64-channel coil, template-based analysis). Results consistently show an average axon diameter of 4.5+/-1.1μm in the spinal cord white matter. Diameters ranged from 3.0μm (gracilis) to 5.9μm (spinocerebellar tracts). Values were similar across laterality (left-right), but statistically different across spinal cord pathways (p<10(-5)). The observed trends are similar to those observed in animal histology. This study shows, for the first time, in vivo mapping of axon diameter in the spinal cord at 300mT/m, thus creating opportunities for applications in spinal cord diseases.

Structural abnormalities in bipolar euthymia: a multicontrast molecular diffusion imaging study

BACKGROUND

Evidence from decades of magnetic resonance imaging (MRI) research in bipolar disorder has been summarized in meta-analyses of various MRI modalities. Notably, although structural MRI studies suggest gray matter reductions are restricted to specific cortical regions, functional MRI has also shown involvement of subcortical dysfunction. Such disparity in results is open to discussion and requires further exploration with additional MRI modalities.

METHODS

We applied whole-brain high angular resolution molecular diffusion imaging to compare different properties of the water diffusion process in brain tissues, using different contrasts. Specifically, we looked at fractional anisotropy, mean diffusivity, probability of return to the origin, and generalized fractional anisotropy in a sample of 40 euthymic patients with bipolar disorder and 40 well-matched healthy control subjects.

RESULTS

Convergent abnormalities were detected by contrasts in various tissue types. Apart from alterations in white matter (in corpus callosum, cingulum bundle, corona radiata, and superior fronto-occipital fasciculus) and cortical gray matter (in medial frontal cortex, left insula, Heschl's gyrus, and cerebellum), three of the contrasts (fractional anisotropy, mean diffusivity, and generalized fractional anisotropy) revealed abnormalities in subcortical structures, including the hippocampus, the thalamus and the caudate nucleus.

CONCLUSIONS

Our findings point to a wider pattern of axonal pathology in bipolar disorder than previously thought. Although findings related to cortical gray matter are consistent with structural meta-analyses, subcortical abnormalities suggest a cytoarchitectonic basis for previously reported subcortical dysfunction. Diffusion results could be interpreted in terms of loss of tissue volume and/or altered membrane permeability, agreeing with both hypotheses of mitochondrial malfunction and neuroinflammation.

Advanced MRI strategies for assessing spinal cord injury

Advanced magnetic resonance (MR) approaches permit the noninvasive quantification of macromolecular, functional, and physiological properties of biological tissues. In this chapter, we review the application of advanced MR techniques to the spinal cord. Macromolecular properties of the spinal cord can be studied using magnetization transfer (MT) MR, diffusion tensor imaging (DTI), Q-space diffusion spectroscopy, and selective detection of myelin water. The functional and metabolic status of the spinal cord can be studied using functional MRI (fMRI), perfusion imaging, and magnetic resonance spectroscopy (MRS). Finally, we consider the outlook for advanced MR studies in persons in whom metal hardware has been implanted to stabilize the cord. In spite of the spinal cord's diminutive size, its location deep within the body, and constant motion, recent work shows that the spinal cord can be studied using these advanced MR approaches.

The current state-of-the-art of spinal cord imaging: applications

A first-ever spinal cord imaging meeting was sponsored by the International Spinal Research Trust and the Wings for Life Foundation with the aim of identifying the current state-of-the-art of spinal cord imaging, the current greatest challenges, and greatest needs for future development. This meeting was attended by a small group of invited experts spanning all aspects of spinal cord imaging from basic research to clinical practice. The greatest current challenges for spinal cord imaging were identified as arising from the imaging environment itself; difficult imaging environment created by the bone surrounding the spinal canal, physiological motion of the cord and adjacent tissues, and small crosssectional dimensions of the spinal cord, exacerbated by metallic implants often present in injured patients. Challenges were also identified as a result of a lack of "critical mass" of researchers taking on the development of spinal cord imaging, affecting both the rate of progress in the field, and the demand for equipment and software to manufacturers to produce the necessary tools. Here we define the current state-of-the-art of spinal cord imaging, discuss the underlying theory and challenges, and present the evidence for the current and potential power of these methods. In two review papers (part I and part II), we propose that the challenges can be overcome with advances in methods, improving availability and effectiveness of methods, and linking existing researchers to create the necessary scientific and clinical network to advance the rate of progress and impact of the research.

Diffusion tensor imaging of the mouse brainstem and cervical spinal cord

Concurrent and/or progressive degeneration of upper and lower motor neurons (LMNs) causes neurological symptoms and dysfunctions in motor neuron diseases (MNDs) such as amyotrophic lateral sclerosis (ALS). Although brain lesions are readily detected, magnetic resonance imaging of the brainstem and cervical spinal cord lesions resulting from damage to LMNs has proven to be difficult. With the development of mouse models of MNDs, a noninvasive neuroimaging modality capable of detecting lesions resulting from axonal and neuronal injury in mouse brainstem and cervical spinal cord could improve our understanding of the underlying mechanism of MNDs and aid in the development of effective treatments. Here we present a protocol that allows the concomitant acquisition of high-quality in vivo full-diffusion tensor magnetic resonance images from the mouse brainstem and cervical spinal cord using the actively decoupled, anatomically shaped pair of coils--the surface-receive coil and the minimized volume-transmit coil. To improve the data quality, we used a custom-made nose cone to monitor respiratory motion for synchronizing data acquisition and assuring physiological stability of mice under examination. The protocol allows the acquisition of in vivo diffusion tensor imaging of the mouse brainstem and cervical spinal cord at 117 μm × 117 μm in-plane resolution with a 500-μm slice thickness in 1 h on a 4.7-T horizontal small animal imaging scanner equipped with an actively shielded gradient coil capable of pulsed gradient strengths up to 18 G cm(−1) with a gradient rise time of ≤295 μs.

The rocky road to translation in spinal cord repair

Over the past 2 decades, the biological understanding of the mechanisms underlying structural and functional repair of the injured central nervous system has strongly increased. This has resulted in the development of multiple experimental treatment strategies with the collective aim of enhancing and surpassing the limited spontaneous recovery occurring in animal models and ultimately humans suffering from spinal cord or brain injuries. Several of these experimental treatments have revealed beneficial effects in animal models of spinal cord injury. With the exception of neurorehabilitative therapies, however, therapeutic interventions that enhance recovery are currently absent within the clinical realm of spinal cord injury. The present review surveys the prospects and challenges in experimental and clinical spinal cord repair. Major shortcomings in experimental research center on the difficulty of closely modeling human traumatic spinal cord injury in animals, the small number of investigations done on cervical spinal injury and tetraplegia, and the differences in lesion models, species, and functional outcome parameters used between laboratories. The main challenges in the clinical field of spinal cord repair are associated with the standardization and sensitivity of functional outcome measures, the definition of the inclusion/exclusion criteria for patient recruitment in trials, and the accuracy and reliability of an early diagnosis to predict subsequent neurological outcome. Research and clinical networks were recently created with the goal of optimizing animal studies and human trials. Promising clinical trials are currently in progress. The time has come to translate the biologic-mechanistic knowledge from basic science into efficacious treatments able to improve the conditions of humans suffering from spinal cord injury.

Neuroimaging in amyotrophic lateral sclerosis

The catastrophic system failure in amyotrophic lateral sclerosis is characterized by progressive neurodegeneration within the corticospinal tracts, brainstem nuclei and spinal cord anterior horns, with an extra-motor pathology that has overlap with frontotemporal dementia. The development of computed tomography and, even more so, MRI has brought insights into neurological disease, previously only available through post-mortem study. Although largely research-based, radionuclide imaging has continued to provide mechanistic insights into neurodegenerative disorders. The evolution of MRI to use advanced sequences highly sensitive to cortical and white matter structure, parenchymal metabolites and blood flow, many of which are now applicable to the spinal cord as well as the brain, make it a uniquely valuable tool for the study of a multisystem disorder such as amyotrophic lateral sclerosis. This comprehensive review considers the full range of neuroimaging techniques applied to amyotrophic lateral sclerosis over the last 25 years, the biomarkers they have revealed and future developments.

Spinal cord markers in ALS: diagnostic and biomarker considerations

Despite considerable involvement of the spinal cord in amyotrophic lateral sclerosis (ALS), current biomarker research is primarily centred on brain imaging and CSF proteomics. In clinical practice, spinal cord imaging in ALS is performed primarily to rule out alternative conditions in the diagnostic phase of the disease. Quantitative spinal cord imaging has traditionally been regarded as challenging, as it requires high spatial resolution while minimizing partial volume effects, physiological motion and susceptibility distortions. In recent years however, as acquisition and post-processing methods have been perfected, a number of exciting and promising quantitative spinal imaging and electrophysiology techniques have been developed. We performed a systematic review of the trends, methodologies, limitations and conclusions of recent spinal cord studies in ALS to explore the diagnostic and prognostic potential of spinal markers. Novel corrective techniques for quantitative spinal cord imaging are systematically reviewed. Recent findings demonstrate that imaging techniques previously used in brain imaging, such as diffusion tensor, functional and metabolic imaging can now be successfully applied to the human spinal cord. Optimized electrophysiological approaches make the non-invasive assessment of corticospinal pathways possible, and multimodal spinal techniques are likely to increase the specificity and sensitivity of proposed spinal markers. In conclusion, spinal cord imaging is an emerging area of ALS biomarker research. Novel quantitative spinal modalities have already been successfully used in ALS animal models and have the potential for development into sensitive ALS biomarkers in humans.

Advanced MRI strategies for assessing spinal cord injury

Advanced magnetic resonance (MR) approaches permit the noninvasive quantification of macromolecular, functional, and physiological properties of biological tissues. In this chapter, we review the application of advanced MR techniques to the spinal cord. Macromolecular properties of the spinal cord can be studied using magnetization transfer (MT) MR, diffusion tensor imaging (DTI), Q-space diffusion spectroscopy, and selective detection of myelin water. The functional and metabolic status of the spinal cord can be studied using functional MRI (fMRI), perfusion imaging, and magnetic resonance spectroscopy (MRS). Finally, we consider the outlook for advanced MR studies in persons in whom metal hardware has been implanted to stabilize the cord. In spite of the spinal cord's diminutive size, its location deep within the body, and constant motion, recent work shows that the spinal cord can be studied using these advanced MR approaches.

Quality assessment of high angular resolution diffusion imaging data using bootstrap on Q-ball reconstruction

PURPOSE

To develop a bootstrap method to assess the quality of High Angular Resolution Diffusion Imaging (HARDI) data using Q-Ball imaging (QBI) reconstruction.

MATERIALS AND METHODS

HARDI data were re-shuffled using regular bootstrap with jackknife sampling. For each bootstrap dataset, the diffusion orientation distribution function (ODF) was estimated voxel-wise using QBI reconstruction based on spherical harmonics functions. The reproducibility of the ODF was assessed using the Jensen-Shannon divergence (JSD) and the angular confidence interval was derived for the first and the second ODF maxima. The sensitivity of the bootstrap method was evaluated on a human subject by adding synthetic noise to the data, by acquiring a map of image signal-to-noise ratio (SNR) and by varying the echo time and the b-value.

RESULTS

The JSD was directly linked to the image SNR. The impact of echo times and b-values was reflected by both the JSD and the angular confidence interval, proving the usefulness of the bootstrap method to evaluate specific features of HARDI data.

CONCLUSION

The bootstrap method can effectively assess the quality of HARDI data and can be used to evaluate new hardware and pulse sequences, perform multifiber probabilistic tractography, and provide reliability metrics to support clinical studies.

32-channel RF coil optimized for brain and cervical spinal cord at 3 T

Diffusion and functional magnetic resonance imaging of the spinal cord remain challenging due to the small cross-sectional size of the cord and susceptibility-related distortions. Although partially addressable through parallel imaging, few highly parallel array coils have been implemented for the cervical cord. Here, we developed a 32-channel coil that fully covers the brain and c-spine and characterized its performance in comparison with a commercially available head/neck/spine array. Image and temporal signal-to-noise ratio were, respectively, increased by 2× and 1.8× in the cervical cord. Averaged g-factors at 4× acceleration were lowered by 22% in the brain and by 39% in the spinal cord, enabling 1-mm isotropic R = 4 multi-echo magnetization prepared gradient echo of the full brain and c-spine in 3:20 min. Diffusion imaging of the cord at 0.6 × 0.6 × 5 mm(3) resolution and tractography of the full brain and c-spine at 1.7-mm isotropic resolution were feasible without noticeable distortion. Improvements of this nature potentially enhance numerous basic and clinical research studies focused on spinal and supraspinal regions.

Wallerian degeneration after spinal cord lesions in cats detected with diffusion tensor imaging

One goal of in vivo neuroimaging is the detection of neurodegenerative processes and anatomical reorganizations after spinal cord (SC) injury. Non-invasive examination of white matter fibers in the living SC can be conducted using magnetic resonance diffusion-weighted imaging. However, this technique is challenging at the spinal level due to the small cross-sectional size of the cord and the presence of physiological motion and susceptibility artifacts. In this study, we acquired in vivo high angular resolution diffusion imaging (HARDI) data at 3T in cats submitted to partial SC injury. Cats were imaged before, 3 and 21 days after injury. Spatial resolution was enhanced to 1.5 × 1.5 × 1 mm(3) using super-resolution technique and distortions were corrected using the reversed gradient method. Tractography-derived regions of interest were generated in the dorsal, ventral, right and left quadrants, to evaluate diffusion tensor imaging (DTI) and Q-Ball imaging metrics with regards to their sensitivity in detecting primary and secondary lesions. A three-way ANOVA tested the effect of session (intact, D3, D21), cross-sectional region (left, right, dorsal and ventral) and rostrocaudal location. Significant effect of session was found for FA (P<0.001), GFA (P<0.05) and radial diffusivity (P<0.001). Post-hoc paired T-test corrected for multiple comparisons showed significant changes at the lesion epicenter (P<0.005). More interestingly, significant changes were also found several centimeters from the lesion epicenter at both 3 and 21 days. This decrease was specific to the type of fibers, i.e., rostrally to the lesion on the dorsal aspect of the cord and caudally to the lesion ipsilaterally, suggesting the detection of Wallerian degeneration.

Distribution of collateral fibers in the monkey cervical spinal cord detected with diffusion-weighted magnetic resonance imaging

Diffusion anisotropy monitored with diffusion-weighted magnetic resonance imaging (DWMRI) is a sensitive marker to monitor developmental or pathological microstructural changes in spinal cord. The white matter is often treated as a unidirectional axonal bundle but collateral fibers branching off the main spinal pathways contradicts this assumption and affects the diffusion anisotropy. It is the aim of this study to investigate to what extent collateral fibers are apparent in diffusion tensor data and if collaterals can be detected as individual fiber directions using crossing fiber detection techniques. We calculate the diffusion tensor and the persistent angular structure (PAS), a multi-fiber reconstruction technique, from high quality post mortem data of a perfusion-fixed vervet monkey cervical spinal cord sample and simulated crossing fiber data. Our results show that (i) cylindrical geometry in the white matter of the spinal cord is an invalid assumption due to collateral fibers. We also demonstrate that (ii) collateral fibers can be resolved as distinct peaks in the water diffusion propagator in white matter using multi-fiber models. Finally, we show that (iii) crossing fibers are mainly located laterally and increase towards the cervical enlargement.

Demyelination and degeneration in the injured human spinal cord detected with diffusion and magnetization transfer MRI

Characterizing demyelination/degeneration of spinal pathways in traumatic spinal cord injured (SCI) patients is crucial for assessing the prognosis of functional rehabilitation. Novel techniques based on diffusion-weighted (DW) magnetic resonance imaging (MRI) and magnetization transfer (MT) imaging provide sensitive and specific markers of white matter pathology. In this paper we combined for the first time high angular resolution diffusion-weighted imaging (HARDI), MT imaging and atrophy measurements to evaluate the cervical spinal cord of fourteen SCI patients and age-matched controls. We used high in-plane resolution to delineate dorsal and ventrolateral pathways. Significant differences were detected between patients and controls in the normal-appearing white matter for fractional anisotropy (FA, p<0.0001), axial diffusivity (p<0.05), radial diffusivity (p<0.05), generalized fractional anisotropy (GFA, p<0.0001), magnetization transfer ratio (MTR, p<0.0001) and cord area (p<0.05). No significant difference was detected in mean diffusivity (p=0.41), T1-weighted (p=0.76) and T2-weighted (p=0.09) signals. MRI metrics were remarkably well correlated with clinical disability (Pearson's correlations, FA: p<0.01, GFA: p<0.01, radial diffusivity: p=0.01, MTR: p=0.04 and atrophy: p<0.01). Stepwise linear regressions showed that measures of MTR in the dorsal spinal cord predicted the sensory disability whereas measures of MTR in the ventro-lateral spinal cord predicted the motor disability (ASIA score). However, diffusion metrics were not specific to the sensorimotor scores. Due to the specificity of axial and radial diffusivity and MT measurements, results suggest the detection of demyelination and degeneration in SCI patients. Combining HARDI with MT imaging is a promising approach to gain specificity in characterizing spinal cord pathways in traumatic injury.

3-Tesla Study of the Spinal Cord White Matter

Spinal cord as soon as brain, can be affected by dysmyelinating and demyelinating diseases, as Multiple Sclerosis (MS), Acute Disseminated Encephalomyelitis (ADEM), Neuromyelis Optica (NMO) and Transverse Myelitis. Investigation of the spinal cord with a high field strength MR system is hampered by the inhomogeneous magnetic field, physiological movements and the small size of the anatomical area. We describe normal and pathological neuroradiological findings in spinal cord white matter and the parameters of optimized sequences for use with the 3T MR systems.

Re-expression of Locomotor Function After Partial Spinal Cord Injury

After a complete spinal section, quadruped mammals (cats, rats, and mice) can generally regain hindlimb locomotion on a treadmill because the spinal cord below the lesion can express locomotion through a neural circuitry termed the central pattern generator (CPG). In this review, we propose that the spinal CPG also plays a crucial role in the locomotor recovery after incomplete spinal cord injury.