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

Congenital and Hereditary Diseases of the Spinal Cord

Congenital anomalies of the spinal cord can pose a diagnostic dilemma to the radiologist. Several classification systems of these anomalies exist. Antenatal ultrasound and fetal magnetic resonance imaging is playing an increasingly important role in the early diagnosis and management of patients. Understanding the underlying anatomy as well as embryology of these disorders can be valuable in correctly identifying the type of spinal cord dysraphic defect. Hereditary spinal cord diseases are rare but can be devastating. When the onset is in adulthood, delay in diagnosis is common. Although the spine findings are nonspecific, some imaging features combined with brain imaging findings can be distinctive. Sometimes, the radiologist may be the first to raise the possibility of these disorders.

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.

Topologically preserving straightening of spinal cord MRI

PURPOSE

To propose a robust and accurate method for straightening magnetic resonance (MR) images of the spinal cord, based on spinal cord segmentation, that preserves spinal cord topology and that works for any MRI contrast, in a context of spinal cord template-based analysis.

MATERIALS AND METHODS

The spinal cord curvature was computed using an iterative Non-Uniform Rational B-Spline (NURBS) approximation. Forward and inverse deformation fields for straightening were computed by solving analytically the straightening equations for each image voxel. Computational speed-up was accomplished by solving all voxel equation systems as one single system. Straightening accuracy (mean and maximum distance from straight line), computational time, and robustness to spinal cord length was evaluated using the proposed and the standard straightening method (label-based spline deformation) on 3T T₂ - and T₁ -weighted images from 57 healthy subjects and 33 patients with spinal cord compression due to degenerative cervical myelopathy (DCM).

RESULTS

The proposed algorithm was more accurate, more robust, and faster than the standard method (mean distance = 0.80 vs. 0.83 mm, maximum distance = 1.49 vs. 1.78 mm, time = 71 vs. 174 sec for the healthy population and mean distance = 0.65 vs. 0.68 mm, maximum distance = 1.28 vs. 1.55 mm, time = 32 vs. 60 sec for the DCM population).

CONCLUSION

A novel image straightening method that enables template-based analysis of quantitative spinal cord MRI data is introduced. This algorithm works for any MRI contrast and was validated on healthy and patient populations. The presented method is implemented in the Spinal Cord Toolbox, an open-source software for processing spinal cord MRI data.

LEVEL OF EVIDENCE

1 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2017;46:1209-1219.

Investigating resting-state functional connectivity in the cervical spinal cord at 3T

The study of spontaneous fluctuations in the blood-oxygen-level-dependent (BOLD) signal has recently been extended from the brain to the spinal cord. Two ultra-high field functional magnetic resonance imaging (fMRI) studies in humans have provided evidence for reproducible resting-state connectivity between the dorsal horns as well as between the ventral horns, and a study in non-human primates has shown that these resting-state signals are impacted by spinal cord injury. As these studies were carried out at ultra-high field strengths using region-of-interest (ROI) based analyses, we investigated whether such resting-state signals could also be observed at the clinically more prevalent field strength of 3 T. In a reanalysis of a sample of 20 healthy human participants who underwent a resting-state fMRI acquisition of the cervical spinal cord, we were able to observe significant dorsal horn connectivity as well as ventral horn connectivity, but no consistent effects for connectivity between dorsal and ventral horns, thus replicating the human 7 T results. These effects were not only observable when averaging along the acquired length of the spinal cord, but also when we examined each of the acquired spinal segments separately, which showed similar patterns of connectivity. Finally, we investigated the robustness of these resting-state signals against variations in the analysis pipeline by varying the type of ROI creation, temporal filtering, nuisance regression and connectivity metric. We observed that – apart from the effects of band-pass filtering – ventral horn connectivity showed excellent robustness, whereas dorsal horn connectivity showed moderate robustness. Together, our results provide evidence that spinal cord resting-state connectivity is a robust and spatially consistent phenomenon that could be a valuable tool for investigating the effects of pathology, disease progression, and treatment response in neurological conditions with a spinal component, such as spinal cord injury.

MRI monitoring of spinal cord changes in patients with multiple sclerosis

PURPOSE OF REVIEW

We analyze recent data on technical aspects, clinical indications, and imaging features of spinal cord MRI in multiple sclerosis, and on the value of this examination for assessing the type and extension of spinal cord damage, and for predicting prognosis in patients with this disease.

RECENT FINDINGS

Spinal cord MRI on patients with multiple sclerosis is technically challenging and a standardized protocol that optimizes the accuracy of this examination is essential, particularly as recent studies have shown its value for diagnostic and prognostic purposes. Several recent studies have proven the potential value of new, quantitative spinal cord magnetic resonance metrics for assessing the type and degree of spinal cord damage. Although these measures can bring new insights into the understanding of the disease, there is not enough evidence to support their use outside the research scenario.

SUMMARY

Neurologists and neuroradiologists should be aware of the added value of conventional spinal cord MRI in the initial diagnosis and monitoring of multiple sclerosis. The use of advanced quantitative magnetic resonance techniques, which better assess the degree of irreversible tissue damage within the spinal cord, is mainly restricted to clinical research and cannot yet be incorporated into the daily clinical practice.

Reduced Field-of-View Diffusion-Weighted Imaging of the Lumbosacral Enlargement: A Pilot In Vivo Study of the Healthy Spinal Cord at 3T

Diffusion tensor imaging (DTI) has recently started to be adopted into clinical investigations of spinal cord (SC) diseases. However, DTI applications to the lower SC are limited due to a number of technical challenges, related mainly to the even smaller size of the SC structure at this level, its position relative to the receiver coil elements and the effects of motion during data acquisition. Developing methods to overcome these problems would offer new means to gain further insights into microstructural changes of neurological conditions involving the lower SC, and in turn could help explain symptoms such as bladder and sexual dysfunction. In this work, the feasibility of obtaining grey and white matter (GM/WM) DTI indices such as axial/radial/mean diffusivity (AD/RD/MD) and fractional anisotropy (FA) within the lumbosacral enlargement (LSE) was investigated using a reduced field-of-view (rFOV) single-shot echo-planar imaging (ss-EPI) acquisition in 14 healthy participants using a clinical 3T MR system. The scan-rescan reproducibility of the measurements was assessed by calculating the percentage coefficient of variation (%COV). Mean FA was higher in WM compared to GM (0.58 and 0.4 in WM and GM respectively), AD and MD were higher in WM compared to GM (1.66 μm2ms-1 and 0.94 μm2ms-1 in WM and 1.2 μm2ms-1 and 0.82 μm2ms-1 in GM for AD and MD respectively) and RD was lower in WM compared to GM (0.58 μm2ms-1 and 0.63 μm2ms-1 respectively). The scan-rescan %COV was lower than 10% in all cases with the highest values observed for FA and the lowest for MD. This pilot study demonstrates that it is possible to obtain reliable tissue-specific estimation of DTI indices within the LSE using a rFOV ss-EPI acquisition. The DTI acquisition and analysis protocol presented here is clinically feasible and may be used in future investigations of neurological conditions implicating the lower SC.

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.

Advancements in Imaging Technology: Do They (or Will They) Equate to Advancements in Our Knowledge of Recovery in Whiplash?

Synopsis It is generally accepted that up to 50% of those with a whiplash injury following a motor vehicle collision will fail to fully recover. Twenty-five percent of these patients will demonstrate a markedly complex clinical picture that includes severe pain-related disability, sensory and motor disturbances, and psychological distress. A number of psychosocial factors have shown prognostic value for recovery following whiplash from a motor vehicle collision. To date, no management approach (eg, physical therapies, education, psychological interventions, or interdisciplinary strategies) for acute whiplash has positively influenced recovery rates. For many of the probable pathoanatomical lesions (eg, fracture, ligamentous rupture, disc injury), there remains a lack of available clinical tests for identifying their presence. Fractures, particularly at the craniovertebral and cervicothoracic junctions, may be radiographically occult. While high-resolution computed tomography scans can detect fractures, there remains a lack of prevalence data for fractures in this population. Conventional magnetic resonance imaging has not consistently revealed lesions in patients with acute or chronic whiplash, a "failure" that may be due to limitations in the resolution of available devices and the use of standard sequences. The technological evolution of imaging techniques and sequences eventually might provide greater resolution to reveal currently elusive anatomical lesions (or, perhaps more importantly, temporal changes in physiological responses to assumed lesions) in those patients at risk of poor recovery. Preliminary findings from 2 prospective cohort studies in 2 different countries suggest that this is so, as evidenced by changes to the structure of skeletal muscles in those who do not fully recover. In this clinical commentary, we will briefly introduce the available imaging decision rules and the current knowledge underlying the pathomechanics and pathophysiology of whiplash. We will then acknowledge known prognostic factors underlying functional recovery. Last, we will highlight emerging evidence regarding the pathobiology of muscle degeneration/regeneration, as well as advancements in neuroimaging and musculoskeletal imaging techniques (eg, functional magnetic resonance imaging, magnetization transfer imaging, spectroscopy, diffusion-weighted imaging) that may be used as noninvasive and objective complements to known prognostic factors associated with whiplash recovery, in particular, poor functional recovery. J Orthop Sports Phys Ther 2016;46(10):861-872. doi:10.2519/jospt.2016.6735.

Denoising spinal cord fMRI data: Approaches to acquisition and analysis

Functional magnetic resonance imaging (fMRI) of the human spinal cord is a difficult endeavour due to the cord's small cross-sectional diameter, signal drop-out as well as image distortion due to magnetic field inhomogeneity, and the confounding influence of physiological noise from cardiac and respiratory sources. Nevertheless, there is great interest in spinal fMRI due to the spinal cord's role as the principal sensorimotor interface between the brain and the body and its involvement in a variety of sensory and motor pathologies. In this review, we give an overview of the various methods that have been used to address the technical challenges in spinal fMRI, with a focus on reducing the impact of physiological noise. We start out by describing acquisition methods that have been tailored to the special needs of spinal fMRI and aim to increase the signal-to-noise ratio and reduce distortion in obtained images. Following this, we concentrate on image processing and analysis approaches that address the detrimental effects of noise. While these include variations of standard pre-processing methods such as motion correction and spatial filtering, the main focus lies on denoising techniques that can be applied to task-based as well as resting-state data sets. We review both model-based approaches that rely on externally acquired respiratory and cardiac signals as well as data-driven approaches that estimate and correct for noise using the data themselves. We conclude with an outlook on techniques that have been successfully applied for noise reduction in brain imaging and whose use might be beneficial for fMRI of the human spinal cord.

Fully-integrated framework for the segmentation and registration of the spinal cord white and gray matter

The spinal cord white and gray matter can be affected by various pathologies such as multiple sclerosis, amyotrophic lateral sclerosis or trauma. Being able to precisely segment the white and gray matter could help with MR image analysis and hence be useful in further understanding these pathologies, and helping with diagnosis/prognosis and drug development. Up to date, white/gray matter segmentation has mostly been done manually, which is time consuming, induces a bias related to the rater and prevents large-scale multi-center studies. Recently, few methods have been proposed to automatically segment the spinal cord white and gray matter. However, no single method exists that combines the following criteria: (i) fully automatic, (ii) works on various MRI contrasts, (iii) robust towards pathology and (iv) freely available and open source. In this study we propose a multi-atlas based method for the segmentation of the spinal cord white and gray matter that addresses the previous limitations. Moreover, to study the spinal cord morphology, atlas-based approaches are increasingly used. These approaches rely on the registration of a spinal cord template to an MR image, however the registration usually doesn't take into account the spinal cord internal structure and thus lacks accuracy. In this study, we propose a new template registration framework that integrates the white and gray matter segmentation to account for the specific gray matter shape of each individual subject. Validation of segmentation was performed in 24 healthy subjects using T₂^*-weighted images, in 8 healthy subjects using diffusion weighted images (exhibiting inverted white-to-gray matter contrast compared to T₂*-weighted), and in 5 patients with spinal cord injury. The template registration was validated in 24 subjects using T₂*-weighted data. Results of automatic segmentation on T₂*-weighted images was in close correspondence with the manual segmentation (Dice coefficient in the white/gray matter of 0.91/0.71 respectively). Similarly, good results were obtained in data with inverted contrast (diffusion-weighted image) and in patients. When compared to the classical template registration framework, the proposed framework that accounts for gray matter shape significantly improved the quality of the registration (comparing Dice coefficient in gray matter: p=9.5×10^-6). While further validation is needed to show the benefits of the new registration framework in large cohorts and in a variety of patients, this study provides a fully-integrated tool for quantitative assessment of white/gray matter morphometry and template-based analysis. All the proposed methods are implemented in the Spinal Cord Toolbox (SCT), an open-source software for processing spinal cord multi-parametric MRI data.

Functional magnetic resonance imaging of the cervical spinal cord during thermal stimulation across consecutive runs

The spinal cord is the first site of nociceptive processing in the central nervous system and has a role in the development and perpetuation of clinical pain states. Advancements in functional magnetic resonance imaging are providing a means to non-invasively measure spinal cord function, and functional magnetic resonance imaging may provide an objective method to study spinal cord nociceptive processing in humans. In this study, we tested the validity and reliability of functional magnetic resonance imaging using a selective field-of-view gradient-echo echo-planar-imaging sequence to detect activity induced blood oxygenation level-dependent signal changes in the cervical spinal cord of healthy volunteers during warm and painful thermal stimulation across consecutive runs. At the group and subject level, the activity was localized more to the dorsal hemicord, the spatial extent and magnitude of the activity was greater for the painful stimulus than the warm stimulus, and the spatial extent and magnitude of the activity exceeded that of a control analysis. Furthermore, the spatial extent of the activity for the painful stimuli increased across the runs likely reflecting sensitization. Overall, the spatial localization of the activity varied considerably across the runs, but despite this variability, a machine-learning algorithm was able to successfully decode the stimuli in the spinal cord based on the distributed pattern of the activity. In conclusion, we were able to successfully detect and characterize cervical spinal cord activity during thermal stimulation at the group and subject level.

Organization of the intrinsic functional network in the cervical spinal cord: A resting state functional MRI study

Resting state functional magnetic resonance imaging (rsfMRI) has been extensively applied to investigate the organization of functional networks in the brain. As an essential part of the central nervous system (CNS), the spinal cord has not been well explored about its intrinsic functional network. In this study, we aim to thoroughly investigate the characteristics of the intrinsic functional network in the spinal cord using rsfMRI. Functional connectivity and graph theory analysis were employed to evaluate the organization of the functional network, including its topology and network communication properties. Furthermore, the reproducibility of rsfMRI analysis on the spinal cord was also examined by intra-class correlation (ICC). Comprehensive evaluation of the intrinsic functional organization presented a non-uniform distribution of topological characteristics of the functional network, in which the upper levels (C2 and C3 vertebral levels) of the cervical spinal cord showed high levels of connectivity. The present results revealed the significance of the upper cervical cord in the intrinsic functional network of the human cervical spinal cord. In addition, this study demonstrated the efficiency of the cervical spinal cord functional network and the reproducibility of rsfMRI analysis on the spinal cord was also confirmed. As knowledge expansion of intrinsic functional network from the brain to the spinal cord, this study shed light on the organization of the spinal cord functional network in both normal development and clinical disorders.

Abdomen and spinal cord segmentation with augmented active shape models

Active shape models (ASMs) have been widely used for extracting human anatomies in medical images given their capability for shape regularization of topology preservation. However, sensitivity to model initialization and local correspondence search often undermines their performances, especially around highly variable contexts in computed-tomography (CT) and magnetic resonance (MR) images. In this study, we propose an augmented ASM (AASM) by integrating the multiatlas label fusion (MALF) and level set (LS) techniques into the traditional ASM framework. Using AASM, landmark updates are optimized globally via a region-based LS evolution applied on the probability map generated from MALF. This augmentation effectively extends the searching range of correspondent landmarks while reducing sensitivity to the image contexts and improves the segmentation robustness. We propose the AASM framework as a two-dimensional segmentation technique targeting structures with one axis of regularity. We apply AASM approach to abdomen CT and spinal cord (SC) MR segmentation challenges. On 20 CT scans, the AASM segmentation of the whole abdominal wall enables the subcutaneous/visceral fat measurement, with high correlation to the measurement derived from manual segmentation. On 28 3T MR scans, AASM yields better performances than other state-of-the-art approaches in segmenting white/gray matter in SC.

Comparison of Diffusion MRI Acquisition Protocols for the In Vivo Characterization of the Mouse Spinal Cord: Variability Analysis and Application to an Amyotrophic Lateral Sclerosis Model

Diffusion-weighted Magnetic Resonance Imaging (dMRI) has relevant applications in the microstructural characterization of the spinal cord, especially in neurodegenerative diseases. Animal models have a pivotal role in the study of such diseases; however, in vivo spinal dMRI of small animals entails additional challenges that require a systematical investigation of acquisition parameters. The purpose of this study is to compare three acquisition protocols and identify the scanning parameters allowing a robust estimation of the main diffusion quantities and a good sensitivity to neurodegeneration in the mouse spinal cord. For all the protocols, the signal-to-noise and contrast-to noise ratios and the mean value and variability of Diffusion Tensor metrics were evaluated in healthy controls. For the estimation of fractional anisotropy less variability was provided by protocols with more diffusion directions, for the estimation of mean, axial and radial diffusivity by protocols with fewer diffusion directions and higher diffusion weighting. Intermediate features (12 directions, b = 1200 s/mm²) provided the overall minimum inter- and intra-subject variability in most cases. In order to test the diagnostic sensitivity of the protocols, 7 G93A-SOD1 mice (model of amyotrophic lateral sclerosis) at 10 and 17 weeks of age were scanned and the derived diffusion parameters compared with those estimated in age-matched healthy animals. The protocols with an intermediate or high number of diffusion directions provided the best differentiation between the two groups at week 17, whereas only few local significant differences were highlighted at week 10. According to our results, a dMRI protocol with an intermediate number of diffusion gradient directions and a relatively high diffusion weighting is optimal for spinal cord imaging. Further work is needed to confirm these results and for a finer tuning of acquisition parameters. Nevertheless, our findings could be important for the optimization of acquisition protocols for preclinical and clinical dMRI studies on the spinal cord.

Gray matter segmentation of the spinal cord with active contours in MR images

OBJECTIVE

Fully or partially automated spinal cord gray matter segmentation techniques for spinal cord gray matter segmentation will allow for pivotal spinal cord gray matter measurements in the study of various neurological disorders. The objective of this work was multi-fold: (1) to develop a gray matter segmentation technique that uses registration methods with an existing delineation of the cord edge along with Morphological Geodesic Active Contour (MGAC) models; (2) to assess the accuracy and reproducibility of the newly developed technique on 2D PSIR T1 weighted images; (3) to test how the algorithm performs on different resolutions and other contrasts; (4) to demonstrate how the algorithm can be extended to 3D scans; and (5) to show the clinical potential for multiple sclerosis patients.

METHODS

The MGAC algorithm was developed using a publicly available implementation of a morphological geodesic active contour model and the spinal cord segmentation tool of the software Jim (Xinapse Systems) for initial estimate of the cord boundary. The MGAC algorithm was demonstrated on 2D PSIR images of the C2/C3 level with two different resolutions, 2D T2* weighted images of the C2/C3 level, and a 3D PSIR image. These images were acquired from 45 healthy controls and 58 multiple sclerosis patients selected for the absence of evident lesions at the C2/C3 level. Accuracy was assessed though visual assessment, Hausdorff distances, and Dice similarity coefficients. Reproducibility was assessed through interclass correlation coefficients. Validity was assessed through comparison of segmented gray matter areas in images with different resolution for both manual and MGAC segmentations.

RESULTS

Between MGAC and manual segmentations in healthy controls, the mean Dice similarity coefficient was 0.88 (0.82-0.93) and the mean Hausdorff distance was 0.61 (0.46-0.76) mm. The interclass correlation coefficient from test and retest scans of healthy controls was 0.88. The percent change between the manual segmentations from high and low-resolution images was 25%, while the percent change between the MGAC segmentations from high and low resolution images was 13%. Between MGAC and manual segmentations in MS patients, the average Dice similarity coefficient was 0.86 (0.8-0.92) and the average Hausdorff distance was 0.83 (0.29-1.37) mm.

CONCLUSION

We demonstrate that an automatic segmentation technique, based on a morphometric geodesic active contours algorithm, can provide accurate and precise spinal cord gray matter segmentations on 2D PSIR images. We have also shown how this automated technique can potentially be extended to other imaging protocols.

Association of asymptomatic spinal cord lesions and atrophy with disability 5 years after a clinically isolated syndrome

Background:

Spinal cord pathology is an important substrate for long-term disability in multiple sclerosis (MS).

Objective:

To investigate longitudinal changes in spinal cord lesions and atrophy in patients with a non-spinal clinically isolated syndrome (CIS), and how they relate to the development of disability.

Methods:

In all, 131 patients with a non-spinal CIS had brain and spinal cord imaging at the time of CIS and approximately 5 years later (median: 5.2 years, range: 3.0–7.9 years). Brain magnetic resonance imaging (MRI) measures consisted of T2-hyperintense and T1-hypointense lesion loads plus brain atrophy. Spinal cord MRI measures consisted of lesion number and the upper cervical cord cross-sectional area (UCCA). Disability was measured using the Expanded Disability Status Scale (EDSS). Multiple linear regression was used to identify independent predictors of disability after 5 years.

Results:

During follow-up, 93 (71%) patients were diagnosed with MS. Baseline spinal cord lesion number, change in cord lesion number and change in UCCA were independently associated with EDSS ( R2 = 0.53) at follow-up. Including brain T2 lesion load and brain atrophy only modestly increased the predictive power of the model ( R2 = 0.64).

Conclusion:

Asymptomatic spinal cord lesions and spinal cord atrophy contribute to the development of MS-related disability over the first 5 years after a non-spinal CIS.

Tract-specific and age-related variations of the spinal cord microstructure: a multi-parametric MRI study using diffusion tensor imaging (DTI) and inhomogeneous magnetization transfer (ihMT)

Being able to finely characterize the spinal cord (SC) microstructure and its alterations is a key point when investigating neural damage mechanisms encountered in different central nervous system (CNS) pathologies, such as multiple sclerosis, amyotrophic lateral sclerosis or myelopathy. Based on novel methods, including inhomogeneous magnetization transfer (ihMT) and dedicated SC probabilistic atlas post-processing, the present study focuses on the in vivo characterization of the healthy SC tissue in terms of regional microstructure differences between (i) upper and lower cervical vertebral levels and (ii) sensory and motor tracts, as well as differences attributed to normal aging. Forty-eight healthy volunteers aged from 20 to 70 years old were included in the study and scanned at 3 T using axial high-resolution T2 *-w imaging, diffusion tensor imaging (DTI) and ihMT, at two vertebral levels (C2 and C5). A processing pipeline with minimal user intervention, SC segmentation and spatial normalization into a reference space was implemented in order to assess quantitative morphological and structural parameters (cross-sectional areas, scalar DTI and MT/ihMT metrics) in specific white and gray matter regions of interest. The multi-parametric MRI metrics collected allowed upper and lower cervical levels to be distinguished, with higher ihMT ratio (ihMTR), higher axial diffusivity (λ∥ ) and lower radial diffusivity (λ⊥ ) at C2 compared with C5. Significant differences were also observed between white matter fascicles, with higher ihMTR and lower λ∥ in motor tracts compared with posterior sensory tracts. Finally, aging was found to be associated with significant metric alterations (decreased ihMTR and λ∥ ). The methodology proposed here, which can be easily transferred to the clinic, provides new insights for SC characterization. It bears great potential to study focal and diffuse SC damage in neurodegenerative and demyelinating diseases. Copyright © 2016 John Wiley & Sons, Ltd.

ZOOM or Non-ZOOM? Assessing Spinal Cord Diffusion Tensor Imaging Protocols for Multi-Centre Studies

The purpose of this study was to develop and evaluate two spinal cord (SC) diffusion tensor imaging (DTI) protocols, implemented at multiple sites (using scanners from two different manufacturers), one available on any clinical scanner, and one using more advanced options currently available in the research setting, and to use an automated processing method for unbiased quantification. DTI parameters are sensitive to changes in the diseased SC. However, imaging the cord can be technically challenging due to various factors including its small size, patient-related and physiological motion, and field inhomogeneities. Rapid acquisition sequences such as Echo Planar Imaging (EPI) are desirable but may suffer from image distortions. We present a multi-centre comparison of two acquisition protocols implemented on scanners from two different vendors (Siemens and Philips), one using a reduced field-of-view (rFOV) EPI sequence, and one only using options available on standard clinical scanners such as outer volume suppression (OVS). Automatic analysis was performed with the Spinal Cord Toolbox for unbiased and reproducible quantification of DTI metrics in the white matter. Images acquired using the rFOV sequence appear less distorted than those acquired using OVS alone. SC DTI parameter values obtained using both sequences at all sites were consistent with previous measurements made at 3T. For the same scanner manufacturer, DTI parameter inter-site SDs were smaller for the rFOV sequence compared to the OVS sequence. The higher inter-site reproducibility (for the same manufacturer and acquisition details, i.e. ZOOM data acquired at the two Philips sites) of rFOV compared to the OVS sequence supports the idea that making research options such as rFOV more widely available would improve accuracy of measurements obtained in multi-centre clinical trials. Future multi-centre studies should also aim to match the rFOV technique and signal-to-noise ratios in all sequences from different manufacturers/sites in order to avoid any bias in measured DTI parameters and ensure similar sensitivity to pathological changes.

Advances in MRI-based computational neuroanatomy: from morphometry to in-vivo histology

PURPOSE OF REVIEW

Current computational neuroanatomy based on MRI focuses on morphological measures of the brain. We present recent methodological developments in quantitative MRI (qMRI) that provide standardized measures of the brain, which go beyond morphology. We show how biophysical modelling of qMRI data can provide quantitative histological measures of brain tissue, leading to the emerging field of in-vivo histology using MRI (hMRI).

RECENT FINDINGS

qMRI has greatly improved the sensitivity and specificity of computational neuroanatomy studies. qMRI metrics can also be used as direct indicators of the mechanisms driving observed morphological findings. For hMRI, biophysical models of the MRI signal are being developed to directly access histological information such as cortical myelination, axonal diameters or axonal g-ratio in white matter. Emerging results indicate promising prospects for the combined study of brain microstructure and function.

SUMMARY

Non-invasive brain tissue characterization using qMRI or hMRI has significant implications for both research and clinics. Both approaches improve comparability across sites and time points, facilitating multicentre/longitudinal studies and standardized diagnostics. hMRI is expected to shed new light on the relationship between brain microstructure, function and behaviour, both in health and disease, and become an indispensable addition to computational neuroanatomy.

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.

Translating state-of-the-art spinal cord MRI techniques to clinical use: A systematic review of clinical studies utilizing DTI, MT, MWF, MRS, and fMRI

BACKGROUND

A recent meeting of international imaging experts sponsored by the International Spinal Research Trust (ISRT) and the Wings for Life Foundation identified 5 state-of-the-art MRI techniques with potential to transform the field of spinal cord imaging by elucidating elements of the microstructure and function: diffusion tensor imaging (DTI), magnetization transfer (MT), myelin water fraction (MWF), MR spectroscopy (MRS), and functional MRI (fMRI). However, the progress toward clinical translation of these techniques has not been established.

METHODS

A systematic review of the English literature was conducted using MEDLINE, MEDLINE-in-Progress, Embase, and Cochrane databases to identify all human studies that investigated utility, in terms of diagnosis, correlation with disability, and prediction of outcomes, of these promising techniques in pathologies affecting the spinal cord. Data regarding study design, subject characteristics, MRI methods, clinical measures of impairment, and analysis techniques were extracted and tabulated to identify trends and commonalities. The studies were assessed for risk of bias, and the overall quality of evidence was assessed for each specific finding using the Grading of Recommendations Assessment, Development and Evaluation (GRADE) framework.

RESULTS

A total of 6597 unique citations were identified in the database search, and after full-text review of 274 articles, a total of 104 relevant studies were identified for final inclusion (97% from the initial database search). Among these, 69 studies utilized DTI and 25 used MT, with both techniques showing an increased number of publications in recent years. The review also identified 1 MWF study, 11 MRS studies, and 8 fMRI studies. Most of the studies were exploratory in nature, lacking a priori hypotheses and showing a high (72%) or moderately high (20%) risk of bias, due to issues with study design, acquisition techniques, and analysis methods. The acquisitions for each technique varied widely across studies, rendering direct comparisons of metrics invalid. The DTI metric fractional anisotropy (FA) had the strongest evidence of utility, with moderate quality evidence for its use as a biomarker showing correlation with disability in several clinical pathologies, and a low level of evidence that it identifies tissue injury (in terms of group differences) compared with healthy controls. However, insufficient evidence exists to determine its utility as a sensitive and specific diagnostic test or as a tool to predict clinical outcomes. Very low quality evidence suggests that other metrics also show group differences compared with controls, including DTI metrics mean diffusivity (MD) and radial diffusivity (RD), the diffusional kurtosis imaging (DKI) metric mean kurtosis (MK), MT metrics MT ratio (MTR) and MT cerebrospinal fluid ratio (MTCSF), and the MRS metric of N-acetylaspartate (NAA) concentration, although these results were somewhat inconsistent.

CONCLUSIONS

State-of-the-art spinal cord MRI techniques are emerging with great potential to improve the diagnosis and management of various spinal pathologies, but the current body of evidence has only showed limited clinical utility to date. Among these imaging tools DTI is the most mature, but further work is necessary to standardize and validate its use before it will be adopted in the clinical realm. Large, well-designed studies with a priori hypotheses, standardized acquisition methods, detailed clinical data collection, and robust automated analysis techniques are needed to fully demonstrate the potential of these rapidly evolving techniques.

Assessing Nociception by fMRI of the Human Spinal Cord: A Systematic Review

OBJECTIVE

To assess the use of fMRI of the spinal cord in measuring noxious stimulation.

METHODS

The Scopus, Medline, EMBASE, and Web of Science databases were searched, along with the reference lists of included articles. Two independent reviewers screened abstracts, full-text articles, and extracted data. Original research was included if fMRI of the human spinal cord was used to measure responses to noxious stimulation.

RESULTS

Of the 192 abstracts screened, 19 met the search criteria and were divided according to their focus: investigating pain responses (n = 6), methodology (n = 6), spinal cord injury (n = 2), or cognition–pain interactions (n = 5). All but one study appear to have observed activity in ipsilateral and dorsal gray matter regions in response to noxious stimuli, although contralateral or ventral activity was also widely observed.

CONCLUSIONS

Although nociception can be investigated using spinal fMRI, establishing reliability, standardizing methodology, and reporting of results will greatly advance this field.

Differential fMRI Activation Patterns to Noxious Heat and Tactile Stimuli in the Primate Spinal Cord

Mesoscale local functional organizations of the primate spinal cord are largely unknown. Using high-resolution fMRI at 9.4 T, we identified distinct interhorn and intersegment fMRI activation patterns to tactile versus nociceptive heat stimulation of digits in lightly anesthetized monkeys. Within a spinal segment, 8 Hz vibrotactile stimuli elicited predominantly fMRI activations in the middle part of ipsilateral dorsal horn (iDH), along with significantly weaker activations in ipsilateral (iVH) and contralateral (cVH) ventral horns. In contrast, nociceptive heat stimuli evoked widespread strong activations in the superficial part of iDH, as well as in iVH and contralateral dorsal (cDH) horns. As controls, only weak signal fluctuations were detected in the white matter. The iDH responded most strongly to both tactile and heat stimuli, whereas the cVH and cDH responded selectively to tactile versus nociceptive heat, respectively. Across spinal segments, iDH activations were detected in three consecutive segments in both tactile and heat conditions. Heat responses, however, were more extensive along the cord, with strong activations in iVH and cDH in two consecutive segments. Subsequent subunit B of cholera toxin tracer histology confirmed that the spinal segments showing fMRI activations indeed received afferent inputs from the stimulated digits. Comparisons of the fMRI signal time courses in early somatosensory area 3b and iDH revealed very similar hemodynamic stimulus-response functions. In summary, we identified with fMRI distinct segmental networks for the processing of tactile and nociceptive heat stimuli in the cervical spinal cord of nonhuman primates. Significance statement: This is the first fMRI demonstration of distinct intrasegmental and intersegmental nociceptive heat and touch processing circuits in the spinal cord of nonhuman primates. This study provides novel insights into the local functional organizations of the primate spinal cord for pain and touch, information that will be valuable for designing and optimizing therapeutic interventions for chronic pain management.

The Brain and Spinal Injury Center score: a novel, simple, and reproducible method for assessing the severity of acute cervical spinal cord injury with axial T2-weighted MRI findings

OBJECT

Previous studies that have evaluated the prognostic value of abnormal changes in signals on T2-weighted MRI scans of an injured spinal cord have focused on the longitudinal extent of this signal abnormality in the sagittal plane. Although the transverse extent of injury and the degree of spared spinal cord white matter have been shown to be important for predicting outcomes in preclinical animal models of spinal cord injury (SCI), surprisingly little is known about the prognostic value of altered T2 relaxivity in humans in the axial plane.

METHODS

The authors undertook a retrospective chart review of 60 patients who met the inclusion criteria of this study and presented to the authors’ Level I trauma center with an acute blunt traumatic cervical SCI. Within 48 hours of admission, all patients underwent MRI examination, which included axial and sagittal T2 images. Neurological symptoms, evaluated with the grades according to the American Spinal Injury Association (ASIA) Impairment Scale (AIS), at the time of admission and at hospital discharge were correlated with MRI findings. Five distinct patterns of intramedullary spinal cord T2 signal abnormality were defined in the axial plane at the injury epicenter. These patterns were assigned ordinal values ranging from 0 to 4, referred to as the Brain and Spinal Injury Center (BASIC) scores, which encompassed the spectrum of SCI severity.

RESULTS

The BASIC score strongly correlated with neurological symptoms at the time of both hospital admission and discharge. It also distinguished patients initially presenting with complete injury who improved by at least one AIS grade by the time of discharge from those whose injury did not improve. The authors’ proposed score was rapid to apply and showed excellent interrater reliability.

CONCLUSIONS

The authors describe a novel 5-point ordinal MRI score for classifying acute SCIs on the basis of axial T2-weighted imaging. The proposed BASIC score stratifies the SCIs according to the extent of transverse T2 signal abnormality during the acute phase of the injury. The new score improves on current MRI-based prognostic descriptions for SCI by reflecting functionally and anatomically significant patterns of intramedullary T2 signal abnormality in the axial plane.

Neural correlates of temporal summation of second pain in the human brainstem and spinal cord

Temporal summation of second pain (TSSP) occurs when painful stimuli are presented repetitively (≥ 0.33 Hz) and results from a C-fibre evoked enhancement (or "wind-up") of the dorsal horn neurons. Based on electrophysiological studies in intact animals, windup is considered a purely central phenomenon. With advancements in functional MRI (fMRI), we can now probe the central mechanisms of this pain response in humans. The aim of this study is to characterize the fMRI responses in the healthy human brainstem and spinal cord that correspond to TSSP. Functional MRI of healthy female adults (N = 15) was conducted while brief, repetitive heat pain stimuli were applied to the right thenar eminence (C6 dermatome), and TSSP (0.33 Hz) and control (0.17 Hz) heat pain paradigms were employed. The stimulus intensity was adjusted to each participant's heat pain sensitivity. Data were analyzed by means of a general linear model, and region-of-interest analyses. As predicted, participants demonstrated significant behavioural summation of pain in the TSSP condition. FMRI results identified enhanced activity in the spinal cord dorsal horn at C6 in response to the TSSP condition. Additionally, multiple areas of the brainstem (RVM and PAG) showed greater responses with the TSSP condition. These results suggest that, in humans, increased pain perception in the TSSP condition is reflected by greater responses in the dorsal horn and in regions known to play a role in the descending modulation of pain, which may modulate the spinal cord response.

White matter atlas of the human spinal cord with estimation of partial volume effect

Template-based analysis has proven to be an efficient, objective and reproducible way of extracting relevant information from multi-parametric MRI data. Using common atlases, it is possible to quantify MRI metrics within specific regions without the need for manual segmentation. This method is therefore free from user-bias and amenable to group studies. While template-based analysis is common procedure for the brain, there is currently no atlas of the white matter (WM) spinal pathways. The goals of this study were: (i) to create an atlas of the white matter tracts compatible with the MNI-Poly-AMU template and (ii) to propose methods to quantify metrics within the atlas that account for partial volume effect. The WM atlas was generated by: (i) digitalizing an existing WM atlas from a well-known source (Gray's Anatomy), (ii) registering this atlas to the MNI-Poly-AMU template at the corresponding slice (C4 vertebral level), (iii) propagating the atlas throughout all slices of the template (C1 to T6) using regularized diffeomorphic transformations and (iv) computing partial volume values for each voxel and each tract. Several approaches were implemented and validated to quantify metrics within the atlas, including weighted-average and Gaussian mixture models. Proof-of-concept application was done in five subjects for quantifying magnetization transfer ratio (MTR) in each tract of the atlas. The resulting WM atlas showed consistent topological organization and smooth transitions along the rostro-caudal axis. The median MTR across tracts was 26.2. Significant differences were detected across tracts, vertebral levels and subjects, but not across laterality (right-left). Among the different tested approaches to extract metrics, the maximum a posteriori showed highest performance with respect to noise, inter-tract variability, tract size and partial volume effect. This new WM atlas of the human spinal cord overcomes the biases associated with manual delineation and partial volume effect. Combined with multi-parametric data, the atlas can be applied to study demyelination and degeneration in diseases such as multiple sclerosis and will facilitate the conduction of longitudinal and multi-center studies.

Spinal cord MRI in multiple sclerosis--diagnostic, prognostic and clinical value

Multiple sclerosis (MS) is an inflammatory disorder of the CNS that affects both the brain and the spinal cord. MRI studies in MS focus more often on the brain than on the spinal cord, owing to the technical challenges in imaging this smaller, mobile structure. However, spinal cord abnormalities at disease onset have important implications for diagnosis and prognosis. Furthermore, later in the disease course, in progressive MS, myelopathy becomes the primary characteristic of the clinical presentation, and extensive spinal cord pathology--including atrophy, diffuse abnormalities and numerous focal lesions--is common. Recent spinal cord imaging studies have employed increasingly sophisticated techniques to improve detection and quantification of spinal cord lesions, and to elucidate their relationship with physical disability. Quantitative MRI measures of cord size and tissue integrity could be more sensitive to the axonal loss and other pathological processes in the spinal cord than is conventional MRI, putting quantitative MRI in a key role to elucidate the association between disability and spinal cord abnormalities seen in people with MS. In this Review, we summarize the most recent MS spinal cord imaging studies and discuss the new insights they have provided into the mechanisms of neurological impairment. Finally, we suggest directions for further and future research.

A reliable spatially normalized template of the human spinal cord--Applications to automated white matter/gray matter segmentation and tensor-based morphometry (TBM) mapping of gray matter alterations occurring with age

Recently, a T2*-weighted template and probabilistic atlas of the white and gray matter (WM, GM) of the spinal cord (SC) have been reported. Such template can be used as tissue-priors for automated WM/GM segmentation but can also provide a common reference and normalized space for group studies. Here, a new template has been created (AMU40), and accuracy of automatic template-based WM/GM segmentation was quantified. The feasibility of tensor-based morphometry (TBM) for studying voxel-wise morphological differences of SC between young and elderly healthy volunteers was also investigated. Sixty-five healthy subjects were divided into young (n=40, age<40years old, mean age 28±5years old) and elderly (n=25, age>50years old, mean age 57±5years old) groups and scanned at 3T using an axial high-resolution T2*-weighted sequence. Inhomogeneity correction and affine intensity normalization of the SC and cerebrospinal fluid (CSF) signal intensities across slices were performed prior to both construction of the AMU40 template and WM/GM template-based segmentation. The segmentation was achieved using non-linear spatial normalization of T2*-w MR images to the AMU40 template. Validation of WM/GM segmentations was performed with a leave-one-out procedure by calculating DICE similarity coefficients between manual and automated WM/GM masks. SC morphological differences between young and elderly healthy volunteers were assessed using the same non-linear spatial normalization of the subjects' MRI to a common template, derivation of the Jacobian determinant maps from the warping fields, and a TBM analysis. Results demonstrated robust WM/GM automated segmentation, with mean DICE values greater than 0.8. Concerning the TBM analysis, an anterior GM atrophy was highlighted in elderly volunteers, demonstrating thereby, for the first time, the feasibility of studying local structural alterations in the SC using tensor-based morphometry. This holds great promise for studies of morphological impairment occurring in several central nervous system pathologies.

Longitudinal assessment of spinal cord injuries in nonhuman primates with quantitative magnetization transfer

PURPOSE

This study aimed to evaluate the reproducibility and specificity of quantitative magnetization transfer (qMT) imaging for monitoring spinal cord injuries (SCIs).

METHODS

MRI scans were performed in anesthetized monkeys at 9.4T, before and serially after a unilateral lesion of the cervical spinal cord. A two-pool fitting model was used to derive qMT parameters.

RESULTS

qMT measures were reproducible across normal subjects, with an average pool size ratio (PSR) of 0.086 ± 0.003 (mean ± SD) for gray matter, and 0.120 ± 0.005 for white matter, respectively. Compared with normal gray matter, the PSR of abnormal tissues rostral and caudal to the injury site decreased by 19.5% (P < 0.05), while the PSR of the cyst-like volume decreased drastically weeks after SCI. Strong correlations in cyst-like regions were observed between PSR and other MRI measures including longitudinal relaxation rate (R1 ), apparent diffusion coefficient and fractional anisotropy (FA). Decreased PSR and FA values correlated well with demyelination in abnormal tissues.

CONCLUSION

The qMT parameters provide robust and specific information about the molecular and cellular changes produced by SCI. PSR detected demyelination and loss of macromolecules in abnormal tissue regions rostral and caudal to the cyst/lesion sites.

Spinal cord-midbrain functional connectivity is related to perceived pain intensity: a combined spino-cortical FMRI study

The dynamic interaction between ascending spinocortical nociceptive signaling and the descending control of the dorsal horn (DH) by brain regions such as the periaqueductal gray matter (PAG) plays a critical role in acute and chronic pain. To noninvasively investigate the processing of nociceptive stimuli in humans, previous fMRI studies either focused exclusively on the brain or, more recently, on the spinal cord. However, to relate neuronal responses in the brain to responses in the spinal cord and to assess the functional interplay between both sites in normal and aberrant conditions, fMRI of both regions within one experiment is necessary. Employing a new MRI acquisition protocol with two separate slice stacks, individually adapted resolutions and parameter settings that are dynamically updated to the optimized settings for the respective region we assessed neuronal activity in the spinal cord and in the brain within one measurement at 3 T. Using a parametric pain paradigm with thermal stimulation to the left radial forearm, we observed BOLD responses in the ipsilateral DH of the spinal segment C6 and corresponding neuronal responses in typical pain-processing brain regions. Based on correlations of adjusted time series, we are able to reveal functional connectivity between the spinal C6-DH and the thalamus, primary somatosensory cortex, bilateral insula, bilateral striatum, and key structures of the descending pain-modulatory system such as the PAG, the hypothalamus, and the amygdala. Importantly, the individual strength of the spinal-PAG coupling predicted individual pain ratings highlighting the functional relevance of this system during physiological pain signaling.

Injury alters intrinsic functional connectivity within the primate spinal cord

Recent demonstrations of correlated low-frequency MRI signal variations between subregions of the spinal cord at rest in humans, similar to those found in the brain, suggest that such resting-state functional connectivity constitutes a common feature of the intrinsic organization of the entire central nervous system. We report our detection of functional connectivity within the spinal cords of anesthetized squirrel monkeys at rest and show that the strength of connectivity within these networks is altered by the effects of injuries. By quantifying the low-frequency MRI signal correlations between different horns within spinal cord gray matter, we found distinct functional connectivity relationships between the different sensory and motor horns, a pattern that was similar to activation patterns evoked by nociceptive heat or tactile stimulation of digits. All horns within a single spinal segment were functionally connected, with the strongest connectivity occurring between ipsilateral dorsal and ventral horns. Each horn was strongly connected to the same horn on neighboring segments, but this connectivity reduced drastically along the spinal cord. Unilateral injury to the spinal cord significantly weakened the strength of the intrasegment horn-to-horn connectivity only on the injury side and in slices below the lesion. These findings suggest resting-state functional connectivity may be a useful biomarker of functional integrity in injured and recovering spinal cords.

Protection and Repair After Spinal Cord Injury: Accomplishments and Future Directions

It was an honor for me to present the 2014 G. Heiner Sell Memorial Lecture at the annual American Spinal Injury Association (ASIA) meeting in San Antonio. For this purpose, I provided a comprehensive review of the scope of research targeting discovery and translational and clinical investigations into spinal cord injury (SCI) research. Indeed, these are exciting times in the area of spinal cord research and clinical initiatives. Many laboratories and clinical programs throughout the world are publishing data related to the pathophysiology of SCI and new strategies for protecting and promoting recovery in both animal models and humans. For this lecture, several topics were discussed including neuroprotective and reparative strategies, neurorehabilitation, quality of life issues, and future directions. In the area of neuroprotection, pathophysiological events that may be targeted with therapeutic strategies, including pharmacological and targeted temperature management were reviewed. For reparative approaches, the importance of both intrinsic and extrinsic mechanisms of axonal regeneration was highlighted. Various cell therapies currently being tested in preclinical and clinical arenas were reviewed as well as ongoing US Food and Drug Administration approved trials for SCI patients. Neurorehabilitation is an evolving research field with locomotive training strategies, electrical stimulation, and brain-machine interface programs targeting various types of SCI. The importance of testing combination approaches including neuroprotective, reparative, and rehabilitative strategies to maximize recovery mechanisms was therefore emphasized. Finally, quality of life issues that affect thousands of individuals living with paralysis were also presented. Future directions and specific obstacles that require attention as we continue to move the SCI field forward were discussed.

Evidence for early neurodegeneration in the cervical cord of patients with primary progressive multiple sclerosis

Spinal neurodegeneration is an important determinant of disability progression in patients with primary progressive multiple sclerosis. Advanced imaging techniques, such as single-voxel (1)H-magnetic resonance spectroscopy and q-space imaging, have increased pathological specificity for neurodegeneration, but are challenging to implement in the spinal cord and have yet to be applied in early primary progressive multiple sclerosis. By combining these imaging techniques with new clinical measures, which reflect spinal cord pathology more closely than conventional clinical tests, we explored the potential for spinal magnetic resonance spectroscopy and q-space imaging to detect early spinal neurodegeneration that may be responsible for clinical disability. Data from 21 patients with primary progressive multiple sclerosis within 6 years of disease onset, and 24 control subjects were analysed. Patients were clinically assessed on grip strength, vibration perception thresholds and postural stability, in addition to the Expanded Disability Status Scale, Nine Hole Peg Test, Timed 25-Foot Walk Test, Multiple Sclerosis Walking Scale-12, and Modified Ashworth Scale. All subjects underwent magnetic resonance spectroscopy and q-space imaging of the cervical cord and conventional brain and spinal magnetic resonance imaging at 3 T. Multivariate analyses and multiple regression models were used to assess the differences in imaging measures between groups and the relationship between magnetic resonance imaging measures and clinical scores, correcting for age, gender, spinal cord cross-sectional area, brain T2 lesion volume, and brain white matter and grey matter volume fractions. Although patients did not show significant cord atrophy when compared with healthy controls, they had significantly lower total N-acetyl-aspartate (mean 4.01 versus 5.31 mmol/l, P = 0.020) and glutamate-glutamine (mean 4.65 versus 5.93 mmol/l, P = 0.043) than controls. Patients showed an increase in q-space imaging-derived indices of perpendicular diffusivity in both the whole cord and major columns compared with controls (P < 0.05 for all indices). Lower total N-acetyl-aspartate was associated with higher disability, as assessed by the Expanded Disability Status Scale (coefficient = -0.41, 0.01 < P < 0.05), Modified Ashworth Scale (coefficient = -3.78, 0.01 < P < 0.05), vibration perception thresholds (coefficient = -4.37, P = 0.021) and postural sway (P < 0.001). Lower glutamate-glutamine predicted increased postural sway (P = 0.017). Increased perpendicular diffusivity in the whole cord and columns was associated with increased scores on the Modified Ashworth Scale, vibration perception thresholds and postural sway (P < 0.05 in all cases). These imaging findings indicate reduced structural integrity of neurons, demyelination, and abnormalities in the glutamatergic pathways in the cervical cord of early primary progressive multiple sclerosis, in the absence of extensive spinal cord atrophy. The observed relationship between imaging measures and disability suggests that early spinal neurodegeneration may underlie clinical impairment, and should be targeted in future clinical trials with neuroprotective agents to prevent the development of progressive disability.

Correlation of in vivo and ex vivo (1)H-MRI with histology in two severities of mouse spinal cord injury

Spinal cord injury (SCI) is a debilitating neuropathology with no effective treatment. Magnetic resonance imaging (MRI) technology is the only method used to assess the impact of an injury on the structure and function of the human spinal cord. Moreover, in pre-clinical SCI research, MRI is a non-invasive method with great translational potential since it provides relevant longitudinal assessment of anatomical and structural alterations induced by an injury. It is only recently that MRI techniques have been effectively used for the follow-up of SCI in rodents. However, the vast majority of these studies have been carried out on rats and when conducted in mice, the contusion injury model was predominantly chosen. Due to the remarkable potential of transgenic mice for studying the pathophysiology of SCI, we examined the use of both in and ex vivo¹H-MRI (9.4 T) in two severities of the mouse SCI (hemisection and over-hemisection) and documented their correlation with histological assessments. We demonstrated that a clear distinction between the two injury severities is possible using in and ex vivo¹H-MRI and that ex vivo MR images closely correlate with histology. Moreover, tissue modifications at a remote location from the lesion epicenter were identified by conventional ex vivo MRI analysis. Therefore, in vivo MRI has the potential to accurately identify in mice the progression of tissue alterations induced by SCI and is successfully implemented by ex vivo MRI examination. This combination of in and ex vivo MRI follow-up associated with histopathological assessment provides a valuable approach for further studies intended to evaluate therapeutic strategies on SCI.

Neurite orientation dispersion and density imaging of the healthy cervical spinal cord in vivo

Here we present the application of neurite orientation dispersion and density imaging (NODDI) to the healthy spinal cord in vivo. NODDI provides maps such as the intra-neurite tissue volume fraction (vin), the orientation dispersion index (ODI) and the isotropic volume fraction (viso), and here we investigate their potential for spinal cord imaging. We scanned five healthy volunteers, four of whom twice, on a 3T MRI system with a ZOOM-EPI sequence. In accordance to the published NODDI protocol, multiple b-shells were acquired at cervical level and both NODDI and diffusion tensor imaging (DTI) metrics were obtained and analysed to: i) characterise differences in grey and white matter (GM/WM); ii) assess the scan-rescan reproducibility of NODDI; iii) investigate the relationship between NODDI and DTI; and iv) compare the quality of fit of NODDI and DTI. Our results demonstrated that: i) anatomical features can be identified in NODDI maps, such as clear contrast between GM and WM in ODI; ii) the variabilities of vin and ODI are comparable to that of DTI and are driven by biological differences between subjects for ODI, have similar contribution from measurement errors and biological variation for vin, whereas viso shows higher variability, driven by measurement errors; iii) NODDI identifies potential sources contributing to DTI indices, as in the brain; and iv) NODDI outperforms DTI in terms of quality of fit. In conclusion, this work shows that NODDI is a useful model for in vivo diffusion MRI of the spinal cord, providing metrics closely related to tissue microstructure, in line with findings in the brain.

Characterizing the location of spinal and vertebral levels in the human cervical spinal cord

BACKGROUND AND PURPOSE

Advanced MR imaging techniques are critical to understanding the pathophysiology of conditions involving the spinal cord. We provide a novel, quantitative solution to map vertebral and spinal cord levels accounting for anatomic variability within the human spinal cord. For the first time, we report a population distribution of the segmental anatomy of the cervical spinal cord that has direct implications for the interpretation of advanced imaging studies most often conducted across groups of subjects.

MATERIALS AND METHODS

Twenty healthy volunteers underwent a T2-weighted, 3T MRI of the cervical spinal cord. Two experts marked the C3-C8 cervical nerve rootlets, C3-C7 vertebral bodies, and pontomedullary junction. A semiautomated algorithm was used to locate the centerline of the spinal cord and measure rostral-caudal distances from a fixed point in the brain stem, the pontomedullary junction, to each of the spinal rootlets and vertebral bodies. Distances to each location were compared across subjects. Six volunteers had 2 additional scans in neck flexion and extension to measure the effects of patient positioning in the scanner.

RESULTS

We demonstrated that substantial variation exists in the rostral-caudal position of spinal cord segments among individuals and that prior methods of predicting spinal segments are imprecise. We also show that neck flexion or extension has little effect on the relative location of vertebral-versus-spinal levels.

CONCLUSIONS

Accounting for spinal level variation is lacking in existing imaging studies. Future studies should account for this variation for accurate interpretation of the neuroanatomic origin of acquired MR signals.

2D phase-sensitive inversion recovery imaging to measure in vivo spinal cord gray and white matter areas in clinically feasible acquisition times

PURPOSE

To present and assess a procedure for measurement of spinal cord total cross-sectional areas (TCA) and gray matter (GM) areas based on phase-sensitive inversion recovery imaging (PSIR). In vivo assessment of spinal cord GM and white matter (WM) could become pivotal to study various neurological diseases, but it is challenging because of insufficient GM/WM contrast provided by conventional magnetic resonance imaging (MRI).

MATERIALS AND METHODS

We acquired 2D PSIR images at 3T at each disc level of the spinal axis in 10 healthy subjects and measured TCA, cord diameters, WM and GM areas, and GM area/TCA ratios. Second, we investigated 32 healthy subjects at four selected levels (C2-C3, C3-C4, T8-T9, T9-T10, total acquisition time <8 min) and generated normative reference values of TCA and GM areas. We assessed test-retest, intra- and interoperator reliability of the acquisition strategy, and measurement steps.

RESULTS

The measurement procedure based on 2D PSIR imaging allowed TCA and GM area assessments along the entire spinal cord axis. The tests we performed revealed high test-retest/intraoperator reliability (mean coefficient of variation [COV] at C2-C3: TCA = 0.41%, GM area = 2.75%) and interoperator reliability of the measurements (mean COV on the 4 levels: TCA = 0.44%, GM area = 4.20%; mean intraclass correlation coefficient: TCA = 0.998, GM area = 0.906).

CONCLUSION

2D PSIR allows reliable in vivo assessment of spinal cord TCA, GM, and WM areas in clinically feasible acquisition times. The area measurements presented here are in agreement with previous MRI and postmortem studies.

Multiparametric MRI reveals dynamic changes in molecular signatures of injured spinal cord in monkeys

PURPOSE

To monitor the spontaneous recovery of cervical spinal cord injury (SCI) using longitudinal multiparametric MRI methods.

METHODS

Quantitative MRI imaging including diffusion tensor imaging, magnetization transfer (MT), and chemical exchange saturation transfer (CEST) were conducted in anesthetized squirrel monkeys at 9.4T. The structural, cellular, and molecular features of the spinal cord were examined before and at different time points after a dorsal column lesion in each monkey.

RESULTS

Images with MT contrast enhanced visualization of the gray and white matter boundaries and the lesion and permitted differentiation of core and rim compartments within an abnormal volume (AV). In the early weeks after SCI, both core and rim exhibited low cellular density and low protein content, with high levels of exchanging hydroxyl, amine, and amide protons, as evidenced by increased apparent diffusion coefficient, decreased fractional anisotropy, decreased MT ratio, decreased nuclear Overhauser effect, and large CEST effects. Over time, cellular density and fiber density increased, whereas amide, amine, and hydroxyl levels dropped significantly, but at differing rates. Histology confirmed the nature of the AV to be a cyst.

CONCLUSION

Multiparametric MRI offers a novel method to quantify the spontaneous changes in structure and cellular and molecular compositions of SC during spontaneous recovery from injury.

Framework for integrated MRI average of the spinal cord white and gray matter: the MNI-Poly-AMU template

The field of spinal cord MRI is lacking a common template, as existing for the brain, which would allow extraction of multi-parametric data (diffusion-weighted, magnetization transfer, etc.) without user bias, thereby facilitating group analysis and multi-center studies. This paper describes a framework to produce an unbiased average anatomical template of the human spinal cord. The template was created by co-registering T2-weighted images (N = 16 healthy volunteers) using a series of pre-processing steps followed by non-linear registration. A white and gray matter probabilistic template was then merged to the average anatomical template, yielding the MNI-Poly-AMU template, which currently covers vertebral levels C1 to T6. New subjects can be registered to the template using a dedicated image processing pipeline. Validation was conducted on 16 additional subjects by comparing an automatic template-based segmentation and manual segmentation, yielding a median Dice coefficient of 0.89. The registration pipeline is rapid (~15 min), automatic after one C2/C3 landmark manual identification, and robust, thereby reducing subjective variability and bias associated with manual segmentation. The template can notably be used for measurements of spinal cord cross-sectional area, voxel-based morphometry, identification of anatomical features (e.g., vertebral levels, white and gray matter location) and unbiased extraction of multi-parametric data.

The use of the lumbosacral enlargement as an intrinsic imaging biomarker: feasibility of grey matter and white matter cross-sectional area measurements using MRI at 3T

Histopathological studies have demonstrated the involvement of spinal cord grey matter (GM) and white matter (WM) in several diseases and recent research has suggested the use of magnetic resonance imaging (MRI) as a promising tool for in vivo assessment of the upper spinal cord. However, many neurological conditions would benefit from quantitative assessment of tissue integrity at different levels and relatively little work has been done, mainly due to technical challenges associated with imaging the lower spinal cord. In this study, the value of the lumbosacral enlargement (LSE) as an intrinsic imaging biomarker was determined by exploring the feasibility of obtaining within it reliable GM and WM cross-sectional area (CSA) measurements by means of a commercially available MRI system at 3 tesla (T). 10 healthy volunteers (mean age 27.5 years, 6 female) gave written informed consent and high resolution images of the LSE were acquired and analysed using an optimised MRI acquisition and analysis protocol. GM and WM mean CSA measurements were obtained from a 15 mm section at the level of the LSE and the reproducibility of the measurements was determined by means of scan-rescan, intra- and inter-observer assessments. Mean (±SD) LSE cross-sectional area (LSE-CSA) was 62.3 (±4.1) mm2 and mean (±SD) LSE grey matter cross-sectional area (LSE-GM-CSA) was 19.8 (±3.3) mm2. The mean scan-rescan, intra- and inter-observer % coefficient of variation (COV) for measuring the LSE-CSA were 2%, 2% and 2.5%, respectively and for measuring the LSE-GM-CSA were 7.8%, 8% and 8.6%, respectively. This study has shown that the LSE can be used reliably as an intrinsic imaging biomarker. The method presented here can be potentially extended to study the LSE in the diseased state and could provide a solid foundation for subsequent multi-parametric MRI investigations.

Multiple sclerosis: improved identification of disease-relevant changes in gray and white matter by using susceptibility-based MR imaging

PURPOSE

To evaluate the potential of quantitative susceptibility (QS) and R2* mapping as surrogate biomarkers of clinically relevant, age-adjusted demyelination and iron deposition in multiple sclerosis (MS).

MATERIALS AND METHODS

All study participants gave written informed consent, and the study was approved by the institutional review board. Quantitative maps of the magnetic resonance imaging susceptibility parameters (R2* and QS) were computed for 25 patients with either clinically isolated syndrome (CIS) or relapsing-remitting MS, as well as for 15 age- and sex-matched control subjects imaged at 7 T. The candidate MR imaging biomarkers were correlated with Extended Disability Status Scale (EDSS), time since CIS diagnosis, time since MS diagnosis, and age.

RESULTS

QS maps aided identification of significant, voxel-level increases in iron deposition in subcortical gray matter (GM) of patients with MS compared with control subjects. These voxel-level increases were not observed on R2* maps. Region-of-interest analysis of mean R2* and QS in subcortical GM demonstrated that R2* (R ≥ 0.39, P < .01) and QS (R ≥ 0.44, P < .01) were strongly correlated with EDSS. In white matter (WM), the volume of total WM damage (defined by a z score of less than -2.0 criterion, indicating demyelination) on QS maps correlated significantly with EDSS (R = 0.46, P = .02). Voxelwise QS also supported a significant contribution of age to demyelination in patients with MS, suggesting that age-adjusted clinical scores may provide more robust measures of MS disease severity compared with non-age-adjusted scores.

CONCLUSION

Using QS and R2* mapping, evidence of both significant increases in iron deposition in subcortical GM and myelin degeneration along the WM skeleton of patients with MS was identified. Both effects correlated strongly with EDSS.

Robust, accurate and fast automatic segmentation of the spinal cord

Spinal cord segmentation provides measures of atrophy and facilitates group analysis via inter-subject correspondence. Automatizing this procedure enables studies with large throughput and minimizes user bias. Although several automatic segmentation methods exist, they are often restricted in terms of image contrast and field-of-view. This paper presents a new automatic segmentation method (PropSeg) optimized for robustness, accuracy and speed. The algorithm is based on the propagation of a deformable model and is divided into three parts: firstly, an initialization step detects the spinal cord position and orientation using a circular Hough transform on multiple axial slices rostral and caudal to the starting plane and builds an initial elliptical tubular mesh. Secondly, a low-resolution deformable model is propagated along the spinal cord. To deal with highly variable contrast levels between the spinal cord and the cerebrospinal fluid, the deformation is coupled with a local contrast-to-noise adaptation at each iteration. Thirdly, a refinement process and a global deformation are applied on the propagated mesh to provide an accurate segmentation of the spinal cord. Validation was performed in 15 healthy subjects and two patients with spinal cord injury, using T1- and T2-weighted images of the entire spinal cord and on multiecho T2*-weighted images. Our method was compared against manual segmentation and against an active surface method. Results show high precision for all the MR sequences. Dice coefficients were 0.9 for the T1- and T2-weighted cohorts and 0.86 for the T2*-weighted images. The proposed method runs in less than 1min on a normal computer and can be used to quantify morphological features such as cross-sectional area along the whole spinal cord.

Rapid, high-resolution quantitative magnetization transfer MRI of the human spinal cord

Quantitative magnetization transfer (qMT) imaging can provide indices describing the interactions between free water protons and immobile macromolecular protons. These indices include the macromolecular proton fraction (MPF), which has been shown to correlate with myelin content in white matter. Because of the long scan times required for high-resolution spinal cord imaging, qMT studies of the human spinal cord have not found wide-spread application. Herein, we investigated whether these limitations could be overcome by utilizing only a single MT-weighted acquisition and a reference measurement, as was recently proposed in the brain. High-resolution, in vivo qMT data were obtained at 3.0T in the spinal cords of healthy volunteers and patients with relapsing remitting multiple sclerosis (MS). Low- and high-resolution acquisitions (low/high resolution=1×1×5mm(3)/0.65×0.65×5mm(3)) with clinically acceptable scan times (12min/7min) were evaluated. We also evaluated the reliability over time and the sensitivity of the model to the assumptions made in the single-point method, both in disease and healthy tissues. Our findings suggest that the single point qMT technique can provide maps of the MPF in the spinal cord in vivo with excellent grey/white matter contrast, can be reliably obtained within reasonable scan times, and are sensitive to MS pathology. Consistent with previous qMT studies in the brain, the observed MPF values were higher in healthy white matter (0.16±0.01) than in grey matter (0.13±0.01) and in MS lesions (0.09±0.01). The single point qMT technique applied at high resolution provides an improved method for obtaining qMT in the human spinal cord and may offer a reliable outcome measure for evaluating spinal cord disease.