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

Characterizing multiple sclerosis disease progression using a combined structural and functional connectivity metric

PURPOSE

A combined resting state functional connectivity MRI (fcMRI) and diffusion tensor imaging (DTI) metric called structural and functional connectivity index (SFCI) was recently proposed for tracking disease status and progression in multiple sclerosis (MS). The metric combines fcMRI and transverse diffusivity (TD) along different functional pathways involved in principle symptomatic domains of MS. In a longitudinal study of patients with MS receiving the same MS therapy, initial worsening of transcallosal (TC) motor and frontoparietal (FP) cognitive networks, as measured by fcMRI and DTI over the first year was followed by stabilization in the second year of follow-up. In this study we have (i) probed relationships between individual and composite neurological measures of MS with SFCI and its individual components along TC motor and FP cognitive pathways and (ii) compared sensitivity of SFCI to treatment-induced longitudinal changes with each individual imaging measure.

METHODS

Twenty five patients with MS (15 female, age 42 ± 8 y) participated in this study and were scanned at 3 T whole body MRI scanner with diffusion tensor imaging (DTI) and resting-state functional connectivity MRI (fcMRI) scan protocol at baseline and 6, 12, 18 and 24 months after starting fingolimod. fcMRI and TD along TC and FP pathways were combined to form structural and functional connectivity index (SFCI) at each time point. Correlations between individual/combined neurological measures and individual imaging components/SFCI at baseline and were evaluated and compared. In addition, efficacies of individual and combined imaging metrics in tracking network integrity were compared.

RESULTS

Individual TD along the TC pathway was significantly inversely correlated with all individual/composite neurological scores. There were moderate correlations of TC and FP components of SFCI with most of the neurological scores, and the pathway-combined SFCI was significantly correlated with all neurological scores. Trend-level increases of both TC and FP fcMRI were observed during the second year of follow-up, both TC and FP TD increased significantly in the first year and then stabilized during the second year. A trend toward a decrease in combined imaging metrics along TC and FP were observed during the first year, followed by a trend toward an increase in these metrics during the second year, while a significant decrease in SFCI during the first year followed by a significant increase during the second year was observed.

CONCLUSIONS

SFCI was more effective in tracking network integrity/disease progression than individual pathway-specific components, which supports its use as an imaging marker for MS disease status and progression.

Evaluation of a connectivity-based imaging metric that reflects functional decline in Multiple Sclerosis

Cognitive impairment is a common symptom in individuals with Multiple Sclerosis (MS), but meaningful, reliable biomarkers relating to cognitive decline have been elusive, making evaluation of the impact of therapeutics on cognitive function difficult. Here, we combine pathway-based MRI measures of structural and functional connectivity to construct a metric of functional decline in MS. The Structural and Functional Connectivity Index (SFCI) is proposed as a simple, z-scored metric of structural and functional connectivity, where changes in the metric have a simple statistical interpretation and may be suitable for use in clinical trials. Using data collected at six time points from a 2-year longitudinal study of 20 participants with MS and 9 age- and sex-matched healthy controls, we probe two common symptomatic domains, motor and cognitive function, by measuring structural and functional connectivity in the transcallosal motor pathway and posterior cingulum bundle. The SFCI is significantly lower in participants with MS compared to controls (p = 0.009) and shows a significant decrease over time in MS (p = 0.012). The change in SFCI over two years performed favorably compared to measures of brain parenchymal fraction and lesion volume, relating to follow-up measures of processing speed (r = 0.60, p = 0.005), verbal fluency (r = 0.57, p = 0.009), and score on the Multiple Sclerosis Functional Composite (r = 0.67, p = 0.003). These initial results show that the SFCI is a suitable metric for longitudinal evaluation of functional decline in MS.

Changes in structural and functional connectivity during two years of fingolimod therapy for multiple sclerosis

BACKGROUND

Fingolimod, an oral drug, has been reported to reduce relapse rate in multiple sclerosis (MS). However disease progression may still occur in spite of control of inflammation. Functional imbalances within and between cerebral networks associated with disruption of structural and functional network integrity, have been reported in MS. An effective therapy is expected to stabilize such functional network integrity.

OBJECTIVE

The purpose of this study was to investigate changes in structural and resting-state functional connectivity of motor and cognitive networks, and associated changes in neurologic scores in MS, during 2 years of fingolimod therapy.

METHODS

Twenty five subjects with MS were recruited for this study. Subjects were scanned with diffusion tensor imaging (DTI) and resting-state functional connectivity MRI (fcMRI) scan protocol at 3 T with 6-month interval over a period of 2 years. Neurologic performance scores of motor and cognitive performances were also obtained.

RESULTS

DTI measures worsened during the 1st year and then stabilized; any trend of stabilization of fcMRI was delayed until the 2nd year. While motor performance did not change, cognitive performance showed improvement. Several baseline DTI measures correlated with relevant neurologic scores.

CONCLUSION

Initial worsening of motor and cognitive network was reported after 1 year of treatment, but seems DTI and fcMRI measures seem to stabilize after around one year fingolimod therapy.

Examining the Effects of Anabolic-Androgenic Steroids on Repetitive Mild Traumatic Brain Injury (RmTBI) Outcomes in Adolescent Rats

Background: Repetitive mild traumatic brain injury (RmTBI) is increasingly common in adolescents. Anabolic–androgenic steroid (AAS) consumption among younger professional athletes is a significant risk factor for impaired neurodevelopment. Given the increased rates and overlapping symptomology of RmTBI and AAS use, we sought to investigate the behavioural and neuropathological outcomes associated with the AAS Metandienone (Met) and RmTBI on rats. Methods: Rats received either Met or placebo and were then administered RmTBIs or sham injuries, followed by a behavioural test battery. Post-mortem MRI was conducted to examine markers of brain integrity and qRT-PCR assessed mRNA expression of markers for neurodevelopment, neuroinflammation, stress responses, and repair processes. Results: Although AAS and RmTBI did not produce cumulative deficits, AAS use was associated with detrimental outcomes including changes to depression, aggression, and memory; prefrontal cortex (PFC) atrophy and amygdala (AMYG) enlargement; damaged white matter integrity in the corpus callosum; and altered mRNA expression in the PFC and AMYG. RmTBI affected general activity and contributed to PFC atrophy. Conclusions: Findings corroborate previous results indicating that RmTBI negatively impacts neurodevelopment but also demonstrates that AAS results in significant neuropathological insult to the developing brain.

Hair cortisol concentration, cognitive, behavioral, and motor impairment in multiple sclerosis

Multiple sclerosis (MS) is an autoimmune neurodegenerative disease that is characterized by the demyelinated inflammatory processes that occur within the central nervous system. Hypothalamus-pituitary-adrenal axis (HPA axis) dysfunctions have been associated with the triggering or increase in MS symptoms. We thus aimed at evaluating motor and behavioral functions, planning skills, processing speed, and their relationship with stress through measuring hair cortisol concentration from patients with MS. The sample was composed of 40 volunteers that were clinically diagnosed with MS, along with 33 healthy adults. Evaluations included: Clinical Evaluation Form, Mini-Mental State Exam, Hamilton Depression Rating Scale, Multiple Sclerosis Functional Composite Measure, Expanded Disability Status Scale, Berg Balance Scale, Perceived Stress Scale, Zoo Map task, and a hair sample to analyze cortisol levels in the last 30 days. MS patients showed highly elevated hair cortisol levels in comparison to the control group (p = 0.048). All groups presented some degree of depressive and anxiety symptoms, aside from considerable perceived stress levels. The MS group presented deficits in gait, balance, manual skills and processing speed, and this was particularly so in individuals with moderate impairments when compared to control group (p < 0.001). Individuals with MS spent less time planning on ZooMap1 (p = 0.024) and made more mistakes (p < 0.001). No correlation was found between hair cortisol and the symptoms we assessed. However, depressive symptoms and anxiety were related to perceived stress, and higher hair cortisol suggests a change in levels in the HPA axis in MS. Nevertheless, future studies will be necessary to further understand how basal hair cortisol is related to MS symptoms.

Quantitative quality assurance in a multicenter HARDI clinical trial at 3T

A phantom-based quality assurance (QA) protocol was developed for a multicenter clinical trial including high angular resolution diffusion imaging (HARDI). A total of 27 3T MR scanners from 2 major manufacturers, GE (Discovery and Signa scanners) and Siemens (Trio and Skyra scanners), were included in this trial. With this protocol, agar phantoms doped to mimic relaxation properties of brain tissue are scanned on a monthly basis, and quantitative procedures are used to detect spiking and to evaluate eddy current and Nyquist ghosting artifacts. In this study, simulations were used to determine alarm thresholds for minimal acceptable signal-to-noise ratio (SNR). Our results showed that spiking artifact was the most frequently observed type of artifact. Overall, Trio scanners exhibited less eddy current distortion than GE scanners, which in turn showed less distortion than Skyra scanners. This difference was mainly caused by the different sequences used on these scanners. The SNR for phantom scans was closely correlated with the SNR from volunteers. Nearly all of the phantom measurements with artifact-free images were above the alarm threshold, suggesting that the scanners are stable longitudinally. Software upgrades and hardware replacement sometimes affected SNR substantially but sometimes did not. In light of these results, it is important to monitor longitudinal SNR with phantom QA to help interpret potential effects on in vivo measurements. Our phantom QA procedure for HARDI scans was successful in tracking scanner performance and detecting unwanted artifacts.

Modern Methods for Interrogating the Human Connectome

OBJECTIVES

Connectionist theories of brain function took hold with the seminal contributions of Norman Geschwind a half century ago. Modern neuroimaging techniques have expanded the scientific interest in the study of brain connectivity to include the intact as well as disordered brain.

METHODS

In this review, we describe the most common techniques used to measure functional and structural connectivity, including resting state functional MRI, diffusion MRI, and electroencephalography and magnetoencephalography coherence. We also review the most common analytical approaches used for examining brain interconnectivity associated with these various imaging methods.

RESULTS

This review presents a critical analysis of the assumptions, as well as methodological limitations, of each imaging and analysis approach.

CONCLUSIONS

The overall goal of this review is to provide the reader with an introduction to evaluating the scientific methods underlying investigations that probe the human connectome.

Effects of a home-based step training programme on balance, stepping, cognition and functional performance in people with multiple sclerosis – a randomized controlled trial

Background:

Stepping impairments increase fall risk in people with MS. No studies have evaluated step training for reducing fall risk in this population.

Objectives:

To determine if step training can improve physical and neuropsychological measures associated with falls in MS.

Methods:

50 PwMS with moderate disability participated in a randomized controlled trial in which intervention group participants ( n = 28) performed step training for 12 weeks while controls ( n = 22) continued usual physical activity. The primary outcomes were choice stepping reaction time (CSRT) and Stroop stepping test (SST) time. Secondary outcomes included balance test (postural sway, CSRT components), gait speed and cognitive tests, nine-hole peg test (9-HPT) and MS functional composite (MSFC) score.

Results:

44 participants completed the study and no adverse events were reported. Compared with the control group, the intervention group performed significantly better at retest in CSRT and SST times, and tests of sway with eyes open, 9-HPT, single and dual task gait speed and MSFC score. There was a non-significant trend for fewer falls in the intervention group.

Conclusions:

The findings indicate that the step training programme is feasible, safe and effective in improving stepping, standing balance, coordination and functional performance in people with MS.

Associations between Proprioceptive Neural Pathway Structural Connectivity and Balance in People with Multiple Sclerosis

Mobility and balance impairments are a hallmark of multiple sclerosis (MS), affecting nearly half of patients at presentation and resulting in decreased activity and participation, falls, injuries, and reduced quality of life. A growing body of work suggests that balance impairments in people with mild MS are primarily the result of deficits in proprioception, the ability to determine body position in space in the absence of vision. A better understanding of the pathophysiology of balance disturbances in MS is needed to develop evidence-based rehabilitation approaches. The purpose of the current study was to (1) map the cortical proprioceptive pathway in vivo using diffusion-weighted imaging and (2) assess associations between proprioceptive pathway white matter microstructural integrity and performance on clinical and behavioral balance tasks. We hypothesized that people with MS (PwMS) would have reduced integrity of cerebral proprioceptive pathways, and that reduced white matter microstructure within these tracts would be strongly related to proprioceptive-based balance deficits. We found poorer balance control on proprioceptive-based tasks and reduced white matter microstructural integrity of the cortical proprioceptive tracts in PwMS compared with age-matched healthy controls (HC). Microstructural integrity of this pathway in the right hemisphere was also strongly associated with proprioceptive-based balance control in PwMS and controls. Conversely, while white matter integrity of the right hemisphere’s proprioceptive pathway was significantly correlated with overall balance performance in HC, there was no such relationship in PwMS. These results augment existing literature suggesting that balance control in PwMS may become more dependent upon (1) cerebellar-regulated proprioceptive control, (2) the vestibular system, and/or (3) the visual system.

Diffusion tensor imaging of neurofibromatosis bright objects in children with neurofibromatosis type 1

Neurofibromatosis bright objects (NBOs) are poorly understood. This article aimed to investigate: 1) differences in fractional anisotropy (FA) between NBOs based in gray matter (GM) and white matter (WM), and 2) the relationship between NBOs and the affected white matter tracts. Fourteen NF1 patients were included in this study. Apparent diffusion coefficient (ADC), FA, radial diffusivity (RD) and eigenvalues were used to compare NBOs and matching contralateral normal-appearing sites (NAS). Diffusion tensor imaging scalars were also compared with age-matched healthy controls. Fiber tractography was performed to assess NBO-induced changes in WM trajectories. ADC values were higher for GM and WM NBOs than for NAS and controls. FA values were lower in GM and WM NBOs compared with controls. In all regions, eigenvalues were higher in NBOs than in NAS and controls. Only three out of 18 NOBs appeared to disrupt WM tracts. ADC, λ2 and RD values of WM NBOs were higher in symptomatic compared to asymptomatic patients. Increased ADC, RD and eigenvalues and decreased FA values in NBOs can be explained by myelin and axonal damage. Increased ADC values and RD in WM NBOs correlated with the presence of symptoms. Tract integrity predominated in our study.

Bayesian scalar-on-image regression with application to association between intracranial DTI and cognitive outcomes

Diffusion tensor imaging (DTI) measures water diffusion within white matter, allowing for in vivo quantification of brain pathways. These pathways often subserve specific functions, and impairment of those functions is often associated with imaging abnormalities. As a method for predicting clinical disability from DTI images, we propose a hierarchical Bayesian "scalar-on-image" regression procedure. Our procedure introduces a latent binary map that estimates the locations of predictive voxels and penalizes the magnitude of effect sizes in these voxels, thereby resolving the ill-posed nature of the problem. By inducing a spatial prior structure, the procedure yields a sparse association map that also maintains spatial continuity of predictive regions. The method is demonstrated on a simulation study and on a study of association between fractional anisotropy and cognitive disability in a cross-sectional sample of 135 multiple sclerosis patients.

White matter integrity, language, and childhood onset schizophrenia

BACKGROUND

The heterogeneity of symptoms and cognitive deficits in schizophrenia can be explained by abnormal connectivity between brain regions. Childhood-onset schizophrenia (COS) is a particularly severe form of schizophrenia, with an onset during a key time period for both cerebral pruning and myelination.

METHODS

Diffusion tensor images were acquired from 18 children and adolescents with COS and 25 controls. The COS group was divided into two sub-groups-one with linguistic impairment (LI) and the other without (NLI). The fractional anisotropy (FA), axial (AD), and radial diffusivity (RD) data from the two COS sub-groups were compared to each other and to the controls using tract-based spatial statistics (TBSS) analyses, which is a voxel-based method used to identify regions of white matter abnormalities.

RESULTS

TBSS identified several regions in the left hemisphere where the LI group had increased AD and RD relative to the NLI and the control groups. These areas primarily localized to linguistic tracts: left superior longitudinal fasciculus and left inferior longitudinal fasciculus/inferior fronto-occipital fasciculus. Regions of increased RD overlapped regions of increased AD, with the former showing more pronounced effects.

CONCLUSIONS

Studies of adult-onset schizophrenia typically identify areas of higher RD but unchanged AD; however, normal development studies have shown that while RD decreases are pronounced over this age range, smaller decreases in AD can also be detected. The observed increases in both RD and AD suggest that developmental disturbances affecting the structural connectivity of these pathways are more severe in COS accompanied by severe linguistic impairments.

Correlation of diffusion and metabolic alterations in different clinical forms of multiple sclerosis

Diffusion tensor imaging (DTI) and MR spectroscopic imaging (MRSI) provide greater sensitivity than conventional MRI to detect diffuse alterations in normal appearing white matter (NAWM) of Multiple Sclerosis (MS) patients with different clinical forms. Therefore, the goal of this study is to combine DTI and MRSI measurements to analyze the relation between diffusion and metabolic markers, T2-weighted lesion load (T2-LL) and the patients clinical status. The sensitivity and specificity of both methods were then compared in terms of MS clinical forms differentiation. MR examination was performed on 71 MS patients (27 relapsing remitting (RR), 26 secondary progressive (SP) and 18 primary progressive (PP)) and 24 control subjects. DTI and MRSI measurements were obtained from two identical regions of interest selected in left and right centrum semioval (CSO) WM. DTI metrics and metabolic contents were significantly altered in MS patients with the exception of N-acetyl-aspartate (NAA) and NAA/Choline (Cho) ratio in RR patients. Significant correlations were observed between diffusion and metabolic measures to various degrees in every MS patients group. Most DTI metrics were significantly correlated with the T2-LL while only NAA/Cr ratio was correlated in RR patients. A comparison analysis of MR methods efficiency demonstrated a better sensitivity/specificity of DTI over MRSI. Nevertheless, NAA/Cr ratio could distinguish all MS and SP patients groups from controls, while NAA/Cho ratio differentiated PP patients from controls. This study demonstrated that diffusivity changes related to microstructural alterations were correlated with metabolic changes and provided a better sensitivity to detect early changes, particularly in RR patients who are more subject to inflammatory processes. In contrast, the better specificity of metabolic ratios to detect axonal damage and demyelination may provide a better index for identification of PP patients.

Challenges to clinical trials in multiple sclerosis: outcome measures in the era of disease-modifying drugs

PURPOSE OF REVIEW

This review summarizes standard and evolving outcome measures in multiple sclerosis (MS) clinical trials.

RECENT FINDINGS

Progress in the development of MS treatments has led to an increasing number of clinical trials and a need for sensitive, timely, and clinically relevant outcome measures. Relapse rate and the Expanded Disability Status Scale remain the standard clinical outcome measures, but the MS Functional Composite continues to gain additional validation as a meaningful outcome measure. The uncertain relationship between MRI outcome measures and clinical disability has been a persistent challenge in MS clinical trials, but there is increasing evidence supporting a correlation between MRI changes and disability in relapsing-remitting MS patients. Additionally, new imaging techniques are being developed to further increase the sensitivity of MRI as a tool in MS clinical trials. Optical coherence tomography is another outcome measure gaining influence in clinical trials. Some of the greatest challenges remain in the subset of primary progressive MS clinical trials in which brain atrophy appears to be the most promising imaging outcome measure, but the optimal clinical measures and study durations are still uncertain.

SUMMARY

Progress in MS clinical trials requires critical evaluation of existing and future outcome measures and their relationships to one another.

Brain axonal and myelin evaluation in heart failure

Although gray matter injury appears in heart failure (HF) patients, the presence, extent, and nature of axonal injury impacting on cardiovascular regulation and other functions is unclear. We performed diffusion tensor imaging (3.0-Tesla magnetic resonance imaging scanner) in 16 HF and 26 control subjects, and assessed whole-brain water diffusion parallel (axial diffusivity; axonal status) and perpendicular (radial diffusivity; myelin changes) to fibers. Regions with increased axial diffusivity only, indicating impaired axonal integrity, emerged in cardiovascular, hedonic, and pain regulatory areas, including basal forebrain, hypothalamic and limbic projections through the medial forebrain bundle and raphe magnus projections to the medulla and cerebellum. Other fiber paths between sites implicated in cognition, including limbic, basal-ganglia, thalamic, internal capsule, and corpus callosum were also altered. Sites with increased radial diffusivity only, indicating myelin breakdown, appeared in the corpus callosum, cingulate, and temporal, parietal, occipital, and frontal regions. Both higher axial and radial diffusivity, indicating loss of tissue integrity, appeared in parietal and occipital lobes, limbic regions, insula, internal capsule, cerebellum, and dorsolateral medulla. Axons and myelin are altered in HF, likely resulting from ischemic/hypoxic processes acting chronically and sub-acutely, respectively. The alterations would contribute to the multiple autonomic and neuropsychological symptoms found in HF.

Penalized functional regression analysis of white-matter tract profiles in multiple sclerosis

Diffusion tensor imaging (DTI) enables noninvasive parcellation of cerebral white matter into its component fiber bundles or tracts. These tracts often subserve specific functions, and damage to the tracts can therefore result in characteristic forms of disability. Attempts to quantify the extent of tract-specific damage have been limited in part by substantial spatial variation of imaging properties from one end of a tract to the other, variation that can be compounded by the effects of disease. Here, we develop a "penalized functional regression" procedure to analyze spatially normalized tract profiles, which powerfully characterize such spatial variation. The central idea is to identify and emphasize portions of a tract that are more relevant to a clinical outcome score, such as case status or degree of disability. The procedure also yields a "tract abnormality score" for each tract and MRI index studied. Importantly, the weighting function used in this procedure is constrained to be smooth, and the statistical associations are estimated using generalized linear models. We test the method on data from a cross-sectional MRI and functional study of 115 multiple-sclerosis cases and 42 healthy volunteers, considering a range of quantitative MRI indices, white-matter tracts, and clinical outcome scores, and using training and testing sets to validate the results. We show that attention to spatial variation yields up to 15% (mean across all tracts and MRI indices: 6.4%) improvement in the ability to discriminate multiple sclerosis cases from healthy volunteers. Our results confirm that comprehensive analysis of white-matter tract-specific imaging data improves with knowledge and characterization of the normal spatial variation.

Microstructural brain abnormalities in patients with obsessive-compulsive disorder: diffusion-tensor MR imaging study at 3.0 T

PURPOSE

To use diffusion-tensor (DT) magnetic resonance (MR) imaging to explore the integrity and connectivity of brain white matter in patients with obsessive-compulsive disorder (OCD) and to correlate DT parameters with clinical measures.

MATERIALS AND METHODS

This study was approved by the local ethical committee, and written informed consent was obtained from all participants. DT imaging was performed by using a 3.0-T MR imager in 23 patients with OCD and 23 healthy control subjects matched for age, sex, education level, and handedness. By using voxel-based analysis, fractional anisotropy (FA) and axial and radial diffusivities were compared between patients and control subjects with a two-sample t test and were tested for correlation with symptom severity, as measured by using the Yale-Brown Obsessive-Compulsive Scale and obsessive-compulsive subscale scores, and with illness duration, as measured by using simple regression in statistical parametric mapping program.

RESULTS

Compared with control subjects, OCD patients demonstrated significantly increased FA in the genu and body of corpus callosum and white matter of right superior frontal gyrus and corpus callosum; no areas of significantly decreased FA were found. For areas of increased FA, axial diffusivity was higher than that in control subjects, while radial diffusivity was not significantly different. The FA values in the white matter of left middle temporal gyrus in OCD patients correlated positively with clinical measures (r = 0.542, P < .001).

CONCLUSION

OCD is associated with axonal microstructural abnormalities within the white matter, which may indicate impaired axonal integrity and increased connectivity. The positive correlation between DT abnormalities and symptom severity suggests that DT imaging may be of clinical value in measuring and following disability in OCD patients.

Advanced MRI in multiple sclerosis: current status and future challenges

MRI has rapidly become a leading research tool in the study of multiple sclerosis (MS). Conventional imaging is useful in diagnosis and management of the inflammatory stages of MS but has limitations in describing the degree of tissue injury and cause of progressive disability seen in later stages. Advanced MRI techniques hold promise for filling this void. These imaging tools hold great promise to increase understanding of MS pathogenesis and provide greater insight into the efficacy of new MS therapies.

Neurocircuitry of emotion and cognition in alcoholism: contributions from white matter fiber tractography

Chronic alcoholism is characterized by impaired control over emotionally motivated actions towards alcohol use. Neuropathologically, it is associated with widespread brain structural compromise marked by gray matter shrinkage, ventricular enlargement, and white matter degradation. The extent to which cortical damage itself or cortical disconnection by white matter fiber pathway disruption contribute to deficits in emotion, cognition, and behavior can be investigated with in vivo structural neuroimaging and diffusion tensor imaging (DTI)-based quantitative fiber tracking. Tractography in alcoholism has revealed abnormalities in selective white matter fiber bundles involving limbic fiber tracts (fornix and cingulum) that connect cortico-limbic-striatal nodes of emotion and reward circuits. Studies documenting brain-behavior relationships support the role of alcoholism-related white matter fiber degradation as a substrate of clinical impairment. An understanding of the role of cortico-limbic fiber degradation in emotional dysregulation in alcoholism is now emerging.

Rostral brain axonal injury in congenital central hypoventilation syndrome

Brain injury underlying the state-related loss of ventilatory drive, autonomic, cognitive, and affective deficits in congenital central hypoventilation syndrome (CCHS) patients appears throughout the brain, as demonstrated by magnetic resonance (MR) T2 relaxometry and mean diffusivity studies. However, neither MR measure is optimal to describe types of axonal injury essential for assessing neural interactions responsible for CCHS characteristics. To evaluate axonal integrity and partition the nature of tissue damage (axonal vs. myelin injury) in CCHS, we measured water diffusion parallel (axial diffusivity) and perpendicular (radial diffusivity) to rostral brain fibers, indicative of axonal and myelin changes, respectively, with diffusion tensor imaging (DTI). We performed DTI in 12 CCHS (age 18.5 + or - 4.9 years, 7 male) and 30 control (17.7 + or - 4.6 years, 18 male) subjects, using a 3.0-Tesla MR imaging scanner. Axial and radial diffusivity maps were calculated, spatially normalized, smoothed, and compared between groups (analysis of covariance; covariates, age and gender). Significantly increased radial diffusivity, primarily indicative of myelin injury, emerged in fibers of the corona radiata, internal capsule, corpus callosum, hippocampus through the fornix, cingulum bundle, and temporal and parietal lobes. Increased axial diffusivity, suggestive of axonal injury, appeared in fibers of the internal capsule, thalamus, corona radiata, and occipital and temporal lobes. Multiple brain regions showed both higher axial and radial diffusivity, indicative of loss of tissue integrity with a combination of myelin and axonal injury, including basal ganglia, bed nucleus, and limbic, occipital, and temporal areas. The processes underlying injury are unclear, but likely stem from both hypoxic and developmental processes.

Measuring myelin repair and axonal loss with diffusion tensor imaging

BACKGROUND AND PURPOSE

DTI is an MR imaging measure of brain tissue integrity and provides an attractive metric for use in neuroprotection clinical trials. The purpose of our study was to use DTI to evaluate the longitudinal changes in brain tissue integrity in a group of patients with MS.

MATERIALS AND METHODS

Twenty-one patients with MS starting natalizumab were imaged serially for 12 months. Gadolinium-enhancing lesions and 20 regions of interest from normal-appearing white and gray matter brain tissue were followed longitudinally. Average values within each region of interest were derived for FA, λ(∥), λ(⊥), and MD. New T1 black holes were identified at 12 months. Analysis was performed by using mixed-model regression analysis with slope (ie, DTI change per month) as the dependent variable.

RESULTS

During 1 year, FA increased in gadolinium-enhancing lesions but decreased in NABT (P < .0001 for both). Changes in FA within gadolinium-enhancing lesions were driven by decreased λ(⊥) (P < .001), and within NABT, by decreased λ(∥) (P < .0001). A higher λ(⊥) within gadolinium-enhancing lesions at baseline predicted conversion to T1 black holes at 12 months. MD was unchanged in both gadolinium-enhancing lesions and NABT.

CONCLUSIONS

We observed changes in DTI measures during 1 year in both gadolinium-enhancing lesions and NABT. The DTI results may represent possible remyelination within acute lesions and chronic axonal degeneration in NAWM. These results support the use of DTI as a measure of tissue integrity for studies of neuroprotective therapies.

MRI of the corpus callosum in multiple sclerosis: association with disability

Inflammatory demyelination and axon damage in the corpus callosum are prominent features of multiple sclerosis (MS) and may partially account for impaired performance on complex tasks. The objective of this article was to characterize quantitative callosal MRI abnormalities and their association with disability. In 69 participants with MS and 29 healthy volunteers, lesional and extralesional callosal MRI indices were estimated via diffusion tensor tractography. expanded disability status scale (EDSS) and MS functional composite (MSFC) scores were recorded in 53 of the participants with MS. All tested callosal MRI indices were diffusely abnormal in MS. EDSS score was correlated only with age (r = 0.51). Scores on the overall MSFC and its paced serial auditory addition test (PASAT) and 9-hole peg test components were correlated with callosal fractional anisotropy (r = 0.27, 0.35, and 0.31, respectively) and perpendicular diffusivity (r = -0.29, -0.30, and -0.31) but not with overall callosal volume or callosal lesion volume; the PASAT score was more weakly correlated with callosal magnetization-transfer ratio (r = 0.21). Anterior callosal abnormalities were associated with impaired PASAT performance and posterior abnormalities with slow performance on the 9-hole peg test. In conclusion, abnormalities in the corpus callosum can be assessed with quantitative MRI and are associated with cognitive and complex upper-extremity dysfunction in MS.

Confirming white matter fMRI activation in the corpus callosum: co-localization with DTI tractography

Recently, functional magnetic resonance imaging (fMRI) activation has been detected in white matter, despite the widely-held belief that fMRI activation is restricted to gray matter. The objective of the current study was to determine whether the regions of white matter fMRI activation were structurally connected to the functional network in gray matter. To do this, we used fMRI-guided tractography to evaluate whether tracts connecting regions of gray matter fMRI activation were co-localized with white matter fMRI activation. An established interhemispheric transfer task was employed to elicit activation in the corpus callosum. Diffusion tensor imaging (DTI) tractography was used to determine the existence of tracts that connected regions of gray matter fMRI activation to regions of activation in the corpus callosum. Corpus callosum activation was detected in the majority of participants. While there was individual variability in the location of corpus callosum activation, activation was commonly observed in the callosal mid-body, isthmus/splenium, or both. Despite the variability, gray matter fMRI-guided tractography identified tracts that were co-localized with corpus callosum fMRI activation in all instances. In addition, callosal activation had tracts to bilateral gray matter fMRI activation for 7/8 participants. The results confirmed that the activated regions of the corpus callosum were structurally connected to the functional network of gray matter regions involved in the task. These findings are an important step towards establishing the functional significance of white matter fMRI, and provide the foundation for future work combining white matter fMRI and DTI tractography to study brain connectivity.

Neurocircuitry of emotion and cognition in alcoholism: contributions from white matter fiber tractography

Chronic alcoholism is characterized by impaired control over emotionally motivated actions towards alcohol use. Neuropathologically, it is associated with widespread brain structural compromise marked by gray matter shrinkage, ventricular enlargement, and white matter degradation. The extent to which cortical damage itself or cortical disconnection by white matter fiber pathway disruption contribute to deficits in emotion, cognition, and behavior can be investigated with in vivo structural neuroimaging and diffusion tensor imaging (DTI)-based quantitative fiber tracking. Tractography in alcoholism has revealed abnormalities in selective white matter fiber bundles involving limbic fiber tracts (fornix and cingulum) that connect cortico-limbic-striatal nodes of emotion and reward circuits. Studies documenting brain-behavior relationships support the role of alcoholism-related white matter fiber degradation as a substrate of clinical impairment. An understanding of the role of cortico-limbic fiber degradation in emotional dysregulation in alcoholism is now emerging.

Altered white matter microstructure in the corpus callosum in Huntington's disease: implications for cortical "disconnection"

The corpus callosum (CC) is the major conduit for information transfer between the cerebral hemispheres and plays an integral role in relaying sensory, motor and cognitive information between homologous cortical regions. The majority of fibers that make up the CC arise from large pyramidal neurons in layers III and V, which project contra-laterally. These neurons degenerate in Huntington's disease (HD) in a topographically and temporally selective way. Since any focus of cortical degeneration could be expected to secondarily de-afferent homologous regions of cortex, we hypothesized that regionally selective cortical degeneration would be reflected in regionally selective degeneration of the CC. We used conventional T1-weighted, diffusion tensor imaging (DTI), and a modified corpus callosum segmentation scheme to examine the CC in healthy controls, huntingtin gene-carriers and symptomatic HD subjects. We measured mid-sagittal callosal cross-sectional thickness and several DTI parameters, including fractional anisotropy (FA), which reflects the degree of white matter organization, radial diffusivity, a suggested index of myelin integrity, and axial diffusivity, a suggested index of axonal damage of the CC. We found a topologically selective pattern of alterations in these measures in pre-manifest subjects that were more extensive in early symptomatic HD subjects and that correlated with performance on distinct cognitive measures, suggesting an important role for disrupted inter-hemispheric transfer in the clinical symptoms of HD. Our findings provide evidence for early degeneration of commissural pyramidal neurons in the neocortex, loss of cortico-cortical connectivity, and functional compromise of associative cortical processing.

Abnormal connectivity of the sensorimotor network in patients with MS: a multicenter fMRI study

In this multicenter study, we used dynamic causal modeling to characterize the abnormalities of effective connectivity of the sensorimotor network in 61 patients with multiple sclerosis (MS) compared with 74 age-matched healthy subjects. We also investigated the correlation of such abnormalities with findings derived from structural MRI. In a subgroup of subjects, diffusion tensor (DT) MRI metrics of the corpus callosum and the left corticospinal tract (CST) were also assessed. MS patients showed increased effective connectivity relative to controls between: (a) the left primary SMC and the left dorsal premotor cortex (PMd), (b) the left PMd and the supplementary motor areas (SMA), (c) the left secondary sensorimotor cortex (SII) and the SMA, (d) the right SII and the SMA, (e) the left SII and the right SII, and (f) the right SMC and the SMA. MS patients had relatively reduced effective connectivity between the left SMC and the right cerebellum. No interaction was found between disease group and center. Coefficients of altered connectivity were weakly correlated with brain T2 LV, but moderately correlated with DT MRI-measured damage of the left CST. In conclusion, large multicenter fMRI studies of effective connectivity changes in diseased people are feasible and can facilitate studies with sample size large enough for robust outcomes. Increased effective connectivity in the patients for the simple motor task suggests local network modulation contributing to enhanced long-distance effective connectivity in MS patients. This extends and generalizes previous evidence that enhancement of effective connectivity may provide an important compensatory mechanism in MS.

Transverse diffusivity of cerebral parenchyma predicts visual tracking performance in relapsing-remitting multiple sclerosis

This study investigated the relation between cerebral damage related to multiple sclerosis (MS) and cognitive decline as determined by two classical mental tracking tests. Cerebral damage in 15 relapsing-remitting MS patients was measured by diffusion tensor imaging (DTI). Fractional anisotropy, longitudinal and transverse diffusivity were defined in the cerebral parenchyma. Cognitive performance of the MS patients was assessed with the oral response format of the Symbol Digit Modalities Test (SDMT) and the Paced Auditory Serial Addition Test (PASAT). A significant correlation was found between performance on the SDMT and the fractional anisotropy in the brain. This correlation was predominantly induced by transverse diffusivity. Transverse diffusivity refers to the diffusion across fibers rather than along the fibers and is believed to be a specific marker for axonal loss and demyelination associated with MS. No significant association between DTI-measures and PASAT performance was found and this negative finding was mainly attributed to psychometric qualities. These results indicate that diffusivity along the non-principal diffusion direction, a possible signature of MS-related white matter pathology, contributes to information processing speed as measured with the SDMT, a task that requires close visual tracking and a widely used clinical marker for cognitive decline in MS.

Imaging structural and functional connectivity: towards a unified definition of human brain organization?

PURPOSE OF REVIEW

Diffusion tractography and functional/effective connectivity MRI provide a better understanding of the structural and functional human brain connectivity. This review will underline the major recent methodological developments and their exceptional respective contributions to physiological and pathophysiological studies in vivo. We will also emphasize the benefits provided by computational models of complex networks such as graph theory.

RECENT FINDINGS

Imaging structural and functional brain connectivity has revealed the complex brain organization into large-scale networks. Such an organization not only permits the complex information segregation and integration during high cognitive processes but also determines the clinical consequences of alterations encountered in development, ageing, or neurological diseases. Recently, it has also been demonstrated that human brain networks shared topological properties with the so-called 'small-world' mathematical model, allowing a maximal efficiency with a minimal energy and wiring cost.

SUMMARY

Separately, magnetic resonance tractography and functional MRI connectivity have both brought new insights into brain organization and the impact of injuries. The small-world topology of structural and functional human brain networks offers a common framework to merge structural and functional imaging as well as dynamical data from electrophysiology that might allow a comprehensive definition of the brain organization and plasticity.

New imaging techniques in the diagnosis of multiple sclerosis

BACKGROUND

Multiple sclerosis (MS) is a chronic disabling disorder histopathologically characterized by inflammation, demyelination and axonal loss. Conventional MRI has made most contributions to the diagnosis of MS. However, it is not sufficiently sensitive and specific to reveal the extent and severity of the damage in the disease. Other nuclear magnetic resonance (NMR) techniques including magnetic resonance spectroscopy, magnetization transfer imaging, diffusion weighted and diffusion tensor imaging, and functional MRI have provided additional information that improves the diagnosis and understanding of MS. Optical techniques including optical coherence tomography (OCT) and coherent anti-Stokes Raman scattering (CARS) microscopy have shown promise in diagnosis and mechanistic study of myelin diseases.

OBJECTIVE

To review new imaging techniques and their potential in diagnosis of MS.

METHOD

The principles of three imaging techniques (MRI, OCT and CARS) and their applications to MS studies are described. Their advantages and disadvantages are compared.

CONCLUSION

Conventional MRI remains a critical tool in the diagnosis of MS. Alternative NMR/MRI techniques have improved specificity for the detection of lesions and provided more quantitative information about MS. Optical techniques including OCT and CARS microscopy are opening up new ways for diagnosis and mechanistic study of myelin diseases.

Diffusion tensor imaging demonstrates brainstem and cerebellar abnormalities in congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) patients show reduced breathing drive during sleep, decreased hypoxic and hypercapnic ventilatory responses, and autonomic and affective deficits, suggesting both brainstem and forebrain injuries. Forebrain damage was previously described in CCHS, but methodological limitations precluded detection of brainstem injury, a concern because genetic mutations in CCHS target brainstem autonomic nuclei. To assess brainstem and cerebellar areas, we used diffusion tensor imaging-based measures, namely axial diffusivity, reflecting water diffusion parallel to fibers, and sensitive to axonal injury, and radial diffusivity, measuring diffusion perpendicular to fibers, and indicative of myelin injury. Diffusion tensor imaging was performed in 12 CCHS and 26 controls, and axial and radial diffusivity maps were compared between groups using analysis of covariance (covariates; age and gender). Increased axial diffusivity in CCHS appeared within the lateral medulla and clusters with injury extended from the dorsal midbrain through the periaqueductal gray, raphé, and superior cerebellar decussation, ventrally to the basal-pons. Cerebellar cortex and deep nuclei, and the superior and inferior cerebellar peduncles showed increased radial diffusivity. Midbrain, pontine, and lateral medullary structures, and the cerebellum and its fiber systems are injured in CCHS, likely contributing to the characteristics found in the syndrome.

Resting state sensorimotor functional connectivity in multiple sclerosis inversely correlates with transcallosal motor pathway transverse diffusivity

Recent studies indicate that functional connectivity using low-frequency BOLD fluctuations (LFBFs) is reduced between the bilateral primary sensorimotor regions in multiple sclerosis. In addition, it has been shown that pathway-dependent measures of the transverse diffusivity of water in white matter correlate with related clinical measures of functional deficit in multiple sclerosis. Taken together, these methods suggest that MRI methods can be used to probe both functional connectivity and anatomic connectivity in subjects with known white matter impairment. We report the results of a study comparing anatomic connectivity of the transcallosal motor pathway, as measured with diffusion tensor imaging (DTI) and functional connectivity of the bilateral primary sensorimotor cortices (SMC), as measured with LFBFs in the resting state. High angular resolution diffusion imaging was combined with functional MRI to define the transcallosal white matter pathway connecting the bilateral primary SMC. Maps were generated from the probabilistic tracking employed and these maps were used to calculate the mean pathway diffusion measures fractional anisotropy FA, mean diffusivity MD, longitudinal diffusivity lambda(1), and transverse diffusivity lambda(2). These were compared with LFBF-based functional connectivity measures (F(c)) obtained at rest in a cohort of 11 multiple sclerosis patients and approximately 10 age- and gender-matched control subjects. The correlation between FA and F(c) for MS patients was r = -0.63, P < 0.04. The correlation between all subjects lambda(2) and F(c) was r = 0.42, P < 0.05. The correlation between all subjects lambda(2) and F(c) was r = -0.50, P < 0.02. None of the control subject correlations were significant, nor were FA, lambda(1), or MD significantly correlated with F(c) for MS patients. This constitutes the first in vivo observation of a correlation between measures of anatomic connectivity and functional connectivity using spontaneous LFBFs.

Diffusion tensor MRI in multiple sclerosis

During the last 10 years, thanks to the development of sophisticated acquisition schemes and the application of novel image analysis and postprocessing, diffusion tensor (DT) magnetic resonance imaging (MRI) has increasingly been applied to the study of multiple sclerosis (MS). DT MRI proved to be able to detect and quantify tissue damage within and outside T2-visible MS lesions. In addition, DT MRI has been shown to be sensitive to the evolution of MS damage over short-term periods of time, and therefore holds promise to provide us with in vivo correlates of MS clinical severity, as well as predictors of long-term disease evolution. Recent developments of the technique, such as DT tractography, are likely to improve dramatically our understanding of the mechanisms associated to the accumulation of MS disability. Unresolved issues to be addressed include the definition of the actual features underlying diffusion changes in MS and the potential of DT MRI in the differential diagnosis between MS and other demyelinating conditions. The best acquisition and postprocessing strategies for DT MRI studies of MS also remain a matter of debate. Moreover, the precision and accuracy of DT MRI scans in detecting longitudinal, MS-related changes need to be further investigated. This is a pivotal issue for a future application of DT MRI to the monitoring of MS evolution in large-scale clinical trials and, possibly, in individual patients.

Structure of WM bundles constituting the working memory system in early multiple sclerosis: a quantitative DTI tractography study

Working memory impairment is frequently observed in patients with early multiple sclerosis (MS). MRI and functional MRI studies have shown that working memory impairment is mostly due to diffuse white matter (WM) damage affecting the connectivity between distant cortical areas. However, working memory deficits in early MS patients can be either completely or partly masked by compensatory functional plasticity. It seems likely that concomitantly with the WM bundle injury resulting from pathological processes, the functional plasticity present in early MS patients may be accompanied by reactive structural WM plasticity. This structural plasticity may effectively compensate for connectivity disturbances and/or contribute to functional brain reorganization. The diffusion characteristics of WM bundles involved in working memory were assessed here by performing quantitative diffusion tensor imaging (DTI) tractography on 24 patients with early relapsing-remitting MS and 15 healthy control subjects. The DTI tractography findings showed that WM connections constituting the executive system of working memory were structurally impaired (the fractional anisotropy was lower than normal and the mean diffusivity, higher than normal). A significantly larger number of connections between the left and right thalami was concurrently observed in the MS patients than in the control subjects, which suggests that the WM is endowed with reactive structural plasticity.

Volumetric assessment of tumor infiltration of adjacent white matter based on anatomic MRI and diffusion tensor tractography

RATIONALE AND OBJECTIVES

To perform a retrospective, quantitative assessment of the anatomic relationship between intra-axial, supratentorial, primary brain tumors, and adjacent white matter fiber tracts based on anatomic and diffusion tensor magnetic resonance imaging (MRI). We hypothesized that white matter infiltration may be common among different types of tumor.

MATERIAL AND METHODS

Preoperative, anatomic (T1- and T2-weighted), and LINESCAN diffusion tensor MRI were obtained in 12 patients harboring supratentorial gliomas (World Health Organization [WHO] Grades II and III). The two imaging modalities were rigidly registered. The tumors were manually segmented from the T1- and T2-weighted MRI, and their volume calculated. A three-dimensional tractography was performed in each case. A second segmentation and volume measurement was performed on the tumor regions intersecting adjacent white matter fiber tracts. Statistical methods included summary statistics to examine the fraction of tumor volume infiltrating adjacent white matter.

RESULTS

There were five patients with low-grade oligodendroglioma (WHO Grade II), one with low-grade mixed oligoastrocytoma (WHO Grade II), one with ganglioglioma, two with low-grade astrocytoma (WHO Grade II), and three with anaplastic astrocytoma (WHO Grade III). We identified white matter tracts infiltrated by tumor in all 12 cases. The median tumor volume (+/- standard deviation) in our patient population was 42.5 +/- 28.9 mL. The median tumor volume (+/- standard deviation) infiltrating white matter fiber tracts was 5.2 +/- 9.9 mL. The median percentage of tumor volume infiltrating white matter fiber tracts was 21.4% +/- 9.7%.

CONCLUSIONS

The information provided by diffusion tensor imaging combined with anatomic MRI might be useful for neurosurgical planning and intraoperative guidance. Our results confirm previous reports that extensive white matter infiltration by primary brain tumors is a common occurrence. However, prospective, large population studies are required to definitively clarify this issue, and how infiltration relates to histologic tumor type, tumor size, and location.