Is the Relationship between Cortical and White Matter Pathologic Changes in Multiple Sclerosis Spatially Specific? A Multimodal 7-T and 3-T MR Imaging Study with Surface and Tract-based Analysis

Quantitative mapping of the brain's structural connectivity using diffusion MRI tractography: A review

Diffusion magnetic resonance imaging (dMRI) tractography is an advanced imaging technique that enables in vivo reconstruction of the brain's white matter connections at macro scale. It provides an important tool for quantitative mapping of the brain's structural connectivity using measures of connectivity or tissue microstructure. Over the last two decades, the study of brain connectivity using dMRI tractography has played a prominent role in the neuroimaging research landscape. In this paper, we provide a high-level overview of how tractography is used to enable quantitative analysis of the brain's structural connectivity in health and disease. We focus on two types of quantitative analyses of tractography, including: 1) tract-specific analysis that refers to research that is typically hypothesis-driven and studies particular anatomical fiber tracts, and 2) connectome-based analysis that refers to research that is more data-driven and generally studies the structural connectivity of the entire brain. We first provide a review of methodology involved in three main processing steps that are common across most approaches for quantitative analysis of tractography, including methods for tractography correction, segmentation and quantification. For each step, we aim to describe methodological choices, their popularity, and potential pros and cons. We then review studies that have used quantitative tractography approaches to study the brain's white matter, focusing on applications in neurodevelopment, aging, neurological disorders, mental disorders, and neurosurgery. We conclude that, while there have been considerable advancements in methodological technologies and breadth of applications, there nevertheless remains no consensus about the "best" methodology in quantitative analysis of tractography, and researchers should remain cautious when interpreting results in research and clinical applications.

Disease correlates of rim lesions on quantitative susceptibility mapping in multiple sclerosis

Quantitative susceptibility mapping (QSM), an imaging technique sensitive to brain iron, has been used to detect paramagnetic rims of iron-laden active microglia and macrophages in a subset of multiple sclerosis (MS) lesions, known as rim+ lesions, that are consistent with chronic active lesions. Because of the potential impact of rim+ lesions on disease progression and tissue damage, investigating their influence on disability and neurodegeneration is critical to establish the impact of these lesions on the disease course. This study aimed to explore the relationship between chronic active rim+ lesions, identified as having a hyperintense rim on QSM, and both clinical disability and imaging measures of neurodegeneration in patients with MS. The patient cohort was composed of 159 relapsing-remitting multiple sclerosis patients. The Expanded Disability Status Scale (EDSS) and Brief International Cognitive Assessment for Multiple Sclerosis, which includes both the Symbol Digit Modalities Test and California Verbal Learning Test-II, were used to assess clinical disability. Cortical thickness and thalamic volume were evaluated as imaging measures of neurodegeneration. A total of 4469 MS lesions were identified, of which 171 QSM rim+ (3.8%) lesions were identified among 57 patients (35.8%). In a multivariate regression model, as the overall total lesion burden increased, patients with at least one rim+ lesion on QSM performed worse on both physical disability and cognitive assessments, specifically the Symbol Digit Modalities Test (p = 0.010), California Verbal Learning Test-II (p = 0.030), and EDSS (p = 0.001). In a separate univariate regression model, controlling for age (p < 0.001) and having at least one rim+ lesion was related to more cortical thinning (p = 0.03) in younger patients (< 45 years). Lower thalamic volume was associated with older patients (p = 0.038) and larger total lesion burden (p < 0.001); however, the association did not remain significant with rim+ lesions (p = 0.10). Our findings demonstrate a novel observation that chronic active lesions, as identified on QSM, modify the impact of lesion burden on clinical disability in MS patients. These results support further exploration of rim+ lesions for therapeutic targeting in MS to reduce disability and subsequent neurodegeneration.

Voxel-based analysis of gray matter relaxation rates shows different correlation patterns for cognitive impairment and physical disability in relapsing-remitting multiple sclerosis

BACKGROUND

Regional analyses of markers of microstructural gray matter (GM) changes, including relaxation rates, have shown inconsistent correlations with physical and cognitive impairment in MS.

OBJECTIVE

To assess voxelwise the correlation of the R1 and R2 relaxation rates with the physical and cognitive impairment in MS.

METHODS

GM R1 and R2 relaxation rate maps were obtained in 241 relapsing-remitting MS patients by relaxometric segmentation of MRI studies. Correlations with the Expanded Disability Status Scale (EDSS) and the percentage of impaired cognitive test (Brief Repeatable Battery and Stroop Test, available in 186 patients) were assessed voxelwise, including voxel GM content as nuisance covariate to remove the effect of atrophy on the correlations.

RESULTS

Extensive clusters of inverse correlation between EDSS and R2 were detected throughout the brain, while inverse correlations with R1 were mostly limited to perirolandic and supramarginal cortices. Cognitive impairment correlated negatively with R1, and to a lesser extent with R2, in the middle frontal, mesial temporal, midcingulate and medial parieto-occipital cortices.

CONCLUSION

In relapsing-remitting MS patients, GM microstructural changes correlate diffusely with physical disability, independent of atrophy, with a preferential role of the sensorimotor cortices. Neuronal damage in the limbic system and dorsolateral prefrontal cortices correlates with cognitive dysfunction.

Diffusion tensor imaging tractography reveals altered fornix in all diagnostic subtypes of multiple sclerosis

INTRODUCTION

Diffusion tensor imaging (DTI) has shown abnormalities of the fornix and other limbic white matter tracts in multiple sclerosis (MS), mainly focusing on relapsing-remitting MS.

METHODS

The goal here was to evaluate the fornix, cingulum, and uncinate fasciculus with DTI tractography at 1.7 mm isotropic resolution in three MS subgroups (11 relapsing-remitting (RRMS), nine secondary progressive (SPMS), eight primary progressive (PPMS)) versus 11 controls, and assess correlations with cognitive and clinical scores.

RESULTS

The MS group overall showed extensive diffusion abnormalities of the fornix with less volume, lower fractional anisotropy (FA), and higher mean and radial diffusivities, which were similarly affected in all three MS subgroups. The uncinate fasciculus had lower FA only in the secondary progressive subgroup, and the cingulum had no DTI differences in any MS subgroup. The FA and/or volumes of these tracts correlated negatively with larger total lesion volume. The only DTI-cognitive correlation was lower right cingulum FA and greater depression over the entire MS cohort.

CONCLUSIONS

Diffusion tractography identified abnormalities in the fornix that appears to be affected early and consistently across all three primary MS phenotypes of RRMS, SPMS, and PPMS regardless of Expanded Disability Status Scale, time since diagnosis, or cognitive scores.

Investigation of grey matter abnormalities in multiple sclerosis patients by combined use of double inversion recovery sequences and diffusion tensor MRI at 3.0 Tesla

AIM

To investigate the grey matter abnormalities in multiple sclerosis (MS) patients by combined use of double inversion recovery (DIR) sequences and diffusion tensor (DTI) magnetic resonance imaging (MRI) at 3 T.

MATERIALS AND METHODS

Twenty relapsing-remitting MS (RRMS) patients and 20 healthy control were enrolled in this study. All participants underwent DIR and DTI MRI and completed the Mini-Mental State Examination (MMSE) and Expanded Disability Status Scale (EDSS). The cortical lesions and normal-appearing grey matter (NAGM) of the patient group, as well as the NAGM of the control group were quantitatively analysed using the DIR and DTI images. The average NAGM mean diffusion (MD) and fractional anisotropy (FA) values of the patient group and control group were measured and compared. The correlation between NAGM MD and FA values and the number of cortical lesions, cognitive impairment, as well as the degree of nerve damage were analysed.

RESULTS

The NAGM of the patient group had average MD and FA values that were significantly different compared with the control group. In addition, the NAGM FA values of the MS patients were negatively correlated with the MMSE score, but positively correlated with the EDSS score. The NAGM MD values of the MS patients were also negatively correlated with the MMSE score, but positively correlated with the EDSS score.

CONCLUSIONS

The NAGM of MS patients has microstructural damages. The extent of such damage was correlated with the number of cortical lesions. The severity of the damage also correlated with increased severity of cognitive impairment and neural defects.

Urgent challenges in quantification and interpretation of brain grey matter atrophy in individual MS patients using MRI

Atrophy of the brain grey matter (GM) is an accepted and important feature of multiple sclerosis (MS). However, its accurate measurement is hampered by various technical, pathological and physiological factors. As a consequence, it is challenging to investigate the role of GM atrophy in the disease process as well as the effect of treatments that aim to reduce neurodegeneration. In this paper we discuss the most important challenges currently hampering the measurement and interpretation of GM atrophy in MS. The focus is on measurements that are obtained in individual patients rather than on group analysis methods, because of their importance in clinical trials and ultimately in clinical care. We discuss the sources and possible solutions of the current challenges, and provide recommendations to achieve reliable measurement and interpretation of brain GM atrophy in MS.

Lesion accumulation is predictive of long-term cognitive decline in multiple sclerosis

OBJECTIVE

To investigate the long-term progression of cognitive dysfunction and its neuroanatomical correlates and predictors in multiple sclerosis (MS).

METHODS

A cohort of 37 MS patients reflecting five decades of disease duration and all subtypes was followed over 17.5 years. Matched controls were recruited at the last follow-up. Global cognitive functioning was assessed using a principal component cognitive index based on comprehensive neuropsychological testing. During the last 8.5 years of the study, brain MRI was performed to analyze normalized volumetrics of three global tissue compartments (white and gray matter, lesions) and strategic regions (corpus callosum, thalamus, hippocampus).

RESULTS

Cognitive decline progressed continuously throughout the study paralleled by atrophy and lesion accumulation. The cognitive index partly correlated with Expanded Disability Status Scale (ρ = -0.47, p < 0.001) and was mainly associated with the lesion fraction (β = -0.48, p < 0.001) and callosal fraction (β = 0.39, p = 0.002) in multiple linear regression analysis. The lesion fraction was an independent predictor of the cognitive performance 8.5 years later (β = -0.35, p = 0.008). Symbol Digit Modalities Test was most frequently abnormal (40%), while Rey-Osterrieth Complex Figure Test was more sensitive to detect cognitive decline.

CONCLUSIONS

Cognitive impairment progresses continuously in MS, associated with atrophy and lesion accumulation, suggesting that interventions targeting these processes could be beneficial at all disease stages. Widespread cognitive functions are more profoundly affected, associated with lesions and corpus callosal atrophy, supporting the idea of an underlying disconnection mechanism for cognitive decline in MS.

Spatial Correlation of Pathology and Perfusion Changes within the Cortex and White Matter in Multiple Sclerosis

BACKGROUND AND PURPOSE

The spatial correlation between WM and cortical GM disease in multiple sclerosis is controversial and has not been previously assessed with perfusion MR imaging. We sought to determine the nature of association between lobar WM, cortical GM, volume and perfusion.

MATERIALS AND METHODS

Nineteen individuals with secondary-progressive multiple sclerosis, 19 with relapsing-remitting multiple sclerosis, and 19 age-matched healthy controls were recruited. Quantitative MR perfusion imaging was used to derive CBF, CBV, and MTT within cortical GM, WM, and T2-hyperintense lesions. A 2-step multivariate linear regression (corrected for age, disease duration, and Expanded Disability Status Scale) was used to assess correlations between perfusion and volume measures in global and lobar normal-appearing WM, cortical GM, and T2-hyperintense lesions. The Bonferroni adjustment was applied as appropriate.

RESULTS

Global cortical GM and WM volume was significantly reduced for each group comparison, except cortical GM volume of those with relapsing-remitting multiple sclerosis versus controls. Global and lobar cortical GM CBF and CBV were reduced in secondary-progressive multiple sclerosis compared with other groups but not for relapsing-remitting multiple sclerosis versus controls. Global and lobar WM CBF and CBV were not significantly different across groups. The distribution of lobar cortical GM and WM volume reduction was disparate, except for the occipital lobes in patients with secondary-progressive multiple sclerosis versus those with relapsing-remitting multiple sclerosis. Moderate associations were identified between lobar cortical GM and lobar normal-appearing WM volume in controls and in the left temporal lobe in relapsing-remitting multiple sclerosis. No significant associations occurred between cortical GM and WM perfusion or volume. Strong correlations were observed between cortical-GM perfusion, normal appearing WM and lesional perfusion, with respect to each global and lobar region within HC, and RRMS and SPMS patients (R² ≤ 0.96, P < .006 and R² ≤ 0.738, P < .006).

CONCLUSIONS

The weak correlation between lobar WM and cortical GM volume loss and perfusion reduction suggests the independent pathophysiology of WM and cortical GM disease.

The Role of Advanced Magnetic Resonance Imaging Techniques in Multiple Sclerosis Clinical Trials

Magnetic resonance imaging has been crucial in the development of anti-inflammatory disease-modifying treatments. The current landscape of multiple sclerosis clinical trials is currently expanding to include testing not only of anti-inflammatory agents, but also neuroprotective, remyelinating, neuromodulating, and restorative therapies. This is especially true of therapies targeting progressive forms of the disease where neurodegeneration is a prominent feature. Imaging techniques of the brain and spinal cord have rapidly evolved in the last decade to permit in vivo characterization of tissue microstructural changes, connectivity, metabolic changes, neuronal loss, glial activity, and demyelination. Advanced magnetic resonance imaging techniques hold significant promise for accelerating the development of different treatment modalities targeting a variety of pathways in MS.

The association between intra- and juxta-cortical pathology and cognitive impairment in multiple sclerosis by quantitative T2* mapping at 7 T MRI

Using quantitative T₂* at 7 Tesla (T) magnetic resonance imaging, we investigated whether impairment in selective cognitive functions in multiple sclerosis (MS) can be explained by pathology in specific areas and/or layers of the cortex.

Thirty-one MS patients underwent neuropsychological evaluation, acquisition of 7 T multi-echo T₂* gradient-echo sequences, and 3 T anatomical images for cortical surfaces reconstruction. Seventeen age-matched healthy subjects served as controls.

Cortical T₂* maps were sampled at various depths throughout the cortex and juxtacortex. Relation between T₂*, neuropsychological scores and a cognitive index (CI), calculated from a principal component analysis on the whole battery, was tested by a general linear model.

Cognitive impairment correlated with T₂* increase, independently from white matter lesions and cortical thickness, in cortical areas highly relevant for cognition belonging to the default-mode network (p < 0.05 corrected). Dysfunction in different cognitive functions correlated with longer T₂* in selective cortical regions, most of which showed longer T₂* relative to controls. For most tests, this association was strongest in deeper cortical layers. Executive dysfunction, however, was mainly related with pathology in juxtameningeal cortex. T₂* explained up to 20% of the variance of the CI, independently of conventional imaging metrics (adjusted-R²: 52–67%, p < 5.10^− 4).

Location of pathology across the cortical width and mantle showed selective correlation with impairment in differing cognitive domains. These findings may guide studies at lower field strength designed to develop surrogate markers of cognitive impairment in MS.

•

Cognitive deficit in multiple sclerosis is associated with cortical T₂* increase.

•

Location of clusters of correlation varies upon affected cognitive domains.

•

Global cognitive deficit was associated with T₂* increase in deepest cortical layers.

•

Executive dysfunction was associated with T₂* increase in outer cortical layers.

•

Regional T₂* explained up to 20% of the variance of cognitive performance in MS.