A multi-shell multi-tissue diffusion study of brain connectivity in early multiple sclerosis.

BACKGROUND: The potential of multi-shell diffusion imaging to produce accurate brain connectivity metrics able to unravel key pathophysiological processes in multiple sclerosis (MS) has scarcely been investigated. OBJECTIVE: To test, in patients with a clinically isolated syndrome (CIS), whether multi-shell imaging-derived connectivity metrics can differentiate patients from controls, correlate with clinical measures, and perform better than metrics obtained with conventional single-shell protocols. METHODS: Nineteen patients within 3 months from the CIS and 12 healthy controls underwent anatomical and 53-direction multi-shell diffusion-weighted 3T images. Patients were cognitively assessed. Voxel-wise fibre orientation distribution functions were estimated and used to obtain network metrics. These were also calculated using a conventional single-shell diffusion protocol. Through linear regression, we obtained effect sizes and standardised regression coefficients. RESULTS: Patients had lower mean nodal strength (p = 0.003) and greater network modularity than controls (p = 0.045). Greater modularity was associated with worse cognitive performance in patients, even after accounting for lesion load (p = 0.002). Multi-shell-derived metrics outperformed single-shell-derived ones. CONCLUSION: Connectivity-based nodal strength and network modularity are abnormal in the CIS. Furthermore, the increased network modularity observed in patients, indicating microstructural damage, is clinically relevant. Connectivity analyses based on multi-shell imaging can detect potentially relevant network changes in early MS

APOE-e4-related differences in left thalamic microstructure in cognitively healthy adults

APOE-ε4 is a main genetic risk factor for developing late onset Alzheimer’s disease (LOAD) and is thought to interact adversely with other risk factors on the brain. However, evidence regarding the impact of APOE-ε4 on grey matter structure in asymptomatic individuals remains mixed. Much attention has been devoted to characterising APOE-ε4-related changes in the hippocampus, but LOAD pathology is known to spread through the whole of the Papez circuit including the limbic thalamus. Here, we tested the impact of APOE-ε4 and two other risk factors, a family history of dementia and obesity, on grey matter macro- and microstructure across the whole brain in 165 asymptomatic individuals (38–71 years). Microstructural properties of apparent neurite density and dispersion, free water, myelin and cell metabolism were assessed with Neurite Orientation Density and Dispersion (NODDI) and quantitative magnetization transfer (qMT) imaging. APOE-ε4 carriers relative to non-carriers had a lower macromolecular proton fraction (MPF) in the left thalamus. No risk effects were present for cortical thickness, subcortical volume, or NODDI indices. Reduced thalamic MPF may reflect inflammation-related tissue swelling and/or myelin loss in APOE-ε4. Future prospective studies should investigate the sensitivity and specificity of qMT-based MPF as a non-invasive biomarker for LOAD risk

Application of neurite orientation dispersion and density imaging (noddi) to a tau pathology model of alzheimer's disease

Increased hyperphosphorylated tau and the formation of intracellular neurofibrillary tangles are associated with the loss of neurons and cognitive decline in Alzheimer's disease, and related neurodegenerative conditions. We applied two diffusion models, diffusion tensor imaging (DTI) and neurite orientation dispersion and density imaging (NODDI), to in vivo diffusion magnetic resonance images (dMRI) of a mouse model of human tauopathy (rTg4510) at 8.5months of age. In grey matter regions with the highest degree of tau burden, microstructural indices provided by both NODDI and DTI discriminated the rTg4510 (TG) animals from wild type (WT) controls; however only the neurite density index (NDI) (the volume fraction that comprises axons or dendrites) from the NODDI model correlated with the histological measurements of the levels of hyperphosphorylated tau protein. Reductions in diffusion directionality were observed when implementing both models in the white matter region of the corpus callosum, with lower fractional anisotropy (DTI) and higher orientation dispersion (NODDI) observed in the TG animals. In comparison to DTI, histological measures of tau pathology were more closely correlated with NODDI parameters in this region. This in vivo dMRI study demonstrates that NODDI identifies potential tissue sources contributing to DTI indices and NODDI may provide greater specificity to pathology in Alzheimer's disease

Imaging brain microstructure with diffusion MRI: practicality and applications

This article gives an overview of microstructure imaging of the brain with diffusion MRI and reviews the state of the art. The microstructure-imaging paradigm aims to estimate and map microscopic properties of tissue using a model that links these properties to the voxel scale MR signal. Imaging techniques of this type are just starting to make the transition from the technical research domain to wide application in biomedical studies. We focus here on the practicalities of both implementing such techniques and using them in applications. Specifically, the article summarizes the relevant aspects of brain microanatomy and the range of diffusion-weighted MR measurements that provide sensitivity to them. It then reviews the evolution of mathematical and computational models that relate the diffusion MR signal to brain tissue microstructure, as well as the expanding areas of application. Next we focus on practicalities of designing a working microstructure imaging technique: model selection, experiment design, parameter estimation, validation, and the pipeline of development of this class of technique. The article concludes with some future perspectives on opportunities in this topic and expectations on how the field will evolve in the short-to-medium term

Probing brain microstructure with multidimensional diffusion MRI: Encoding, interpretation, and the role of exchange

Diffusion MRI (dMRI) is a non-invasive probe of human brain microstructure. It is a long-standing promise to use dMRI for ‘in vivo histology’ and estimate tissue quantities. However, this faces several challenges. First, the microstructure models used for dMRI data are based on assumptions that may cause erroneous interpretations. Also, probing neurites in gray matter assumes high microscopic diffusion anisotropy in both axons and dendrites, which is not supported by evidence. Furthermore, dMRI data analysis typically ignores diffusional exchange between microscopic environments. This thesis investigates and addresses these challenges using ‘multidimensional’ dMRI techniques that vary additional sequence encoding parameters to obtain new information on the tissue. In Paper I, we optimized an acquisition protocol for filter exchange imaging (FEXI). We found slow rates of diffusional exchange in normal brain tissue. In patients with gliomas and meningiomas, faster exchange was tentatively associated with higher tumor grade. In Paper II, we used tensor-valued diffusion encoding to test the NODDI microstructure model. The NODDI assumptions were contradicted by independent data and parameter estimates were found to be biased in normal brain and in gliomas. The CODIVIDE model combined data acquired with different b-tensor shapes to remove NODDI assumptions and reduce the susceptibility to bias. In Paper III, we used tensor-valued diffusion encoding with multiple echo times to investigate challenges in estimating neurite density. We found that microscopic anisotropy in the brain reflected axons but not dendrites. We could not separate the densities and T2 values of a two-component model in normal brain, but we did detect different component T2 values in white matter lesions. Microstructure models ranked regions from normal brain and white matter lesions inconsistently with respect to neurite density. In Paper IV, we optimized an acquisition protocol for tensor-valued diffusion encoding with multiple echo times. The data allowed removing all assumptions on diffusion and T2 relaxation from a two-component model. This increased the measurable parameters from two to six and reduced their susceptibility to bias. Data from the normal brain showed different component T2 values and contradicted common model assumptions. In Paper V, we used tensor-valued diffusion encoding in malformations of cortical development. Lesions that appeared gray matter-like in T1- and T2-weighted contrasts featured white matter-like regions with high microscopic diffusion anisotropy. We interpreted these regions as myelin-poor white matter with a high axonal content. By primarily reflecting axons and not dendrites or myelin, microscopic anisotropy may differentiate tissue where alterations to myelin confound conventional MRI contrasts. In Paper VI, we used SDE with multiple diffusion times in patients with acute ischemic stroke. Subacute lesions exhibited elevated diffusional exchange that predicted later infarction. MD reduction was partially reversible and did not predict infarction. Diffusional exchange may improve definition of ischemic core and identify additional patients for late revascularization

Hallucination Proneness and Musical Aptitude: Functional and Microstructural Underpinnings

The current thesis aimed to explore links between hallucination proneness and musical aptitude utilising a variety of brain imaging methodologies to characterise associated functional and microstructural individual variabilities. A further aim was to investigate whether a short duration of musical training could be used to modulate functional activity and microstructure in regions associated with hallucinatory experiences. It was hypothesised that hallucination proneness and musical aptitude would be negatively associated with each other and inversely related to underlying functional activity and microstructure within a shared network of brain regions. Moreover, it was hypothesised that musical training would lead to changes in functional activity and microstructure within this shared network of regions. Measures of musical aptitude and hallucination proneness were assessed in conjunction with diffusion imaging models which enabled the characterisation of the microstructural features of the corpus callosum. Results revealed an inverse relationship between musical aptitude and hallucination proneness, with a mediating effect of musical aptitude on hallucination proneness through the microstructure of the corpus callosum. The use of a multi-shell biophysical model, based on neurite orientation dispersion density imaging, further revealed that the relationship between hallucination proneness and musical aptitude was primarily due to callosal neurite orientation dispersion rather than neurite density. With the addition of functional connectivity MRI the degree of callosal neurite orientation dispersion also shown to impact on the functional connectivity during a musical categorisation task, such that higher neurite alignment was associated with increased ROI- ROI fronto-temporal functional connectivity. Hallucination proneness was shown to be negatively associated with performance on a speech perception task and functional connectivity between the left IFG and the superior temporal gyrus (STG) (bilaterally) during task completion. Dendritic complexity within the STG grey matter was also found to be negatively associated with individual variability in propensity to hallucinate. Investigations of the effects of exposure to a short musical training session (learning to tap polyrhythms for one hour) provided evidence of an increase in ROI-ROI function within a bilateral network of fronto-temporal regions following training. Moreover, using three distinct but complimentary diffusion imaging models, polyrhythm training was shown to facilitate a decrease in extra-axonal space diffusion in the central portions of the CC which correlated with performance gains on the polyrhythm discrimination task. The overall results of this thesis therefore support the hypothesis that musical aptitude and hallucination proneness are linked and associated with the underlying microstructure of the CC. Moreover, musical aptitude was shown to be positively associated with task based functional fronto-temporal connectivity whereas hallucination proneness was shown to be negatively associated. Hallucination proneness was further shown to be related to microstructure of the STG with orientation dispersion deemed the most sensitive metric for assessing this relationship. Importantly, results offer evidence that musical training may offer a novel approach for improving fronto-temporal functional connectivity and the microstructure of the corpus callosum, providing an initial foundation for investigation of future novel interventions for hallucinatory experiences

Investigating Glymphatic Function In Alzheimer’s Disease Pathology

Alzheimer’s disease is fast becoming the greatest healthcare challenge of our time, with no known cure to-date. Brought about by the toxic formation of plaques of amyloid-β and tangles of tau in the brain, much is still unknown about the precise mechanisms that initiate these protein accumulations, thought to occur decades before clinical manifestation of symptoms. One theory is that an imbalance between the production of these proteins and their removal from the brain promotes retention that eventually aggregates into entities that devastate molecular and cellular machinery. Thus, targeting waste clearance mechanisms responsible for removing cerebral metabolites, including amyloid-β and tau, present novel, enthralling research targets. The glymphatic system is one such pathway that has been recently characterised. Considered a surrogate for lymphatics which are largely lacking in the brain, this fluid network relies on the water channel aquaporin-4, expressed highly on glia, thus being named “glymphatics”. In this work, first, a surgical protocol was established in the mouse brain to facilitate the delivery of tracer molecules into the cerebrospinal fluid. Direct, single time-point, histological assessment of fluorescent tracer distribution was performed to check consistency with previous characterisation of glymphatics in the mouse brain. Glymphatics were then visualised dynamically across the whole brain using magnetic resonance imaging. Glymphatic patterns were investigated in real-time by imaging fluid dynamics in the brain alongside a potent blocker of aquaporin-4. Next, imaging was used to characterise glymphatic changes and aquaporin-4 profiles in mouse models of Alzheimer’s pathology. This revealed a time-dependant relationship between glymphatics and tau accumulation. Finally, the findings were extrapolated onto humans by studying aquaporin-4 modifications in subjects with and without cognitive deficits. Here, the crucial relationship between aquaporin-4 and pathological aggregates of tau and amyloid-β was determined. Furthermore, dystrobrevin, a membrane protein linked to aquaporin-4, was also profiled in the setting of aging and amyloid-β pathology. The work presented herein elucidates the role of glymphatic perturbances in the context of Alzheimer’s disease and clarifies the implications of aquaporin-4 mediated clearance in neurodegeneration

Synaptic loss in the primary tauopathies of Progressive Supranuclear Palsy and Corticobasal Degeneration

In this thesis I address the debilitating symptom of cognitive dysfunction in the primary tauopathies of Progressive Supranuclear Palsy (PSP) and Corticobasal Degeneration (CBD). Both PSP and CBD are associated with an accumulation of 4-repeat tau in cortical and subcortical areas. As well as movement disorders, they impair cognitive function, even where there is minimal atrophy. Neurophysiological studies have also identified electrophysiological changes associated with cognitive dysfunction, in areas without atrophy. I propose that synaptic loss prior to cell loss contributes to these effects of disease. Chapter two summarises my cohort and principal methods. I quantify synaptic density in vivo with dynamic [11C]UCB-J PET, and molecular pathology with [18F]AV1451 PET. Brain structural changes are quantified by MRI. Disease severity and cognition are assessed with the PSP rating scale, and neuropsychological tests. Patients with CBD are negative on amyloid-imaging ([11C]PiB PET) to exclude those with Alzheimer’s pathology. In chapter three, [11C]UCB-J PET reveals widespread loss of synapses in PSP and CBD including areas with minimal atrophy. The loss of synapses correlated with cognition and disease severity. In chapter four, I test whether presynaptic changes (from [11C]UCB-J PET) are correlated with postsynaptic abnormalities (i.e. changes to postsynaptic dendritic microstructural integrity quantified by MRI using the Neurite Orientation and Dispersion Index, NODDI). In accordance with in vitro and animal models, I confirm that loss of dendritic complexity is tightly coupled with presynaptic density, over and above the effects of atrophy. In chapter five, I test the relationship between the molecular pathology in primary tauopathies (tau burden) and synaptic loss, using [18F]AV-1451 and [11C]UCB-J PET. The use of the “tau” ligand [18F]AV-1451 has become controversial in PSP. With due consideration to the caveats, I report that brain regions with a higher synaptic density have higher [18F]AV-1451 binding, consistent with the hypothesis of connectivity-based progression of tauopathy. I further show that accrual of pathology in any given area is associated with loss of synapses, consistent with synaptic injury from tauopathy. I conclude my thesis in chapter 6, by discussing and highlighting the importance of synaptic density in primary tauopathies. The findings are relevant to other neurodegenerative disorders, and support early interventional studies targeting synaptic maintenance and restoration.Association of British Neurologists - Patrick Berthoud Charitable Trus

Functional and structural substrates of increased dosage of Grik4 gene elucidated using multi-modal MRI

Grik4 is the gene responsible for encoding the high-affinity GluK4 subunit of the kainate receptors. Increased dosage of this subunit in the forebrain was linked to an increased level of anxiety, lack of social communication, and depression. On the synaptic level, abnormal synaptic transmission was also reported. The manifestations of this abnormal expression have not been investigated at the circuit level, nor the correlations between those circuits and the abnormal patterns of the behavior previously reported. In this line of work, we aspired to use different non-invasive magnetic resonance imaging (MRI) modalities to elucidate any disturbance that might stem from the increased dosage of Grik4 and how those changes might explain the abnormal behaviors. MRI offers a noninvasive way to look into the intact brain in vivo. Resting-state functional MRI casts light on how the brain function at rest on the network level and has the capability to detect any anomalies that might occur within or between those networks. On the microstructural level, the diffusion MRI is concerned with the underlying features of the tissues, using the diffusion of water molecules as a proxy for that end. Moving more macroscopically, using structural scans, voxel-based morphometry can detect subtle differences in the morphology of the different brain structures. We recorded videos of our animals performing two tasks that have long been linked to anxiety, the open field and the plus-maze tests before acquiring structural and functional scans. Lastly, we recorded blood-oxygenationlevel dependent (BOLD) signals in a different set of animals during electrical stimulation of specific white matter tracts in order to investigate how neuronal activity propagates. Our analysis showed a vast spectrum of changes in the transgenic group relative to the animals in the control group. On the resting-state networks level, we observed an increase in the within-network strength spanning different structures such as the hippocampus, some regions of the cortex, and the hypothalamus. The increased internal coherence or strength in the networks contrasted with a significant reduction in between-networks connectivity for some regions such as parts of the cortex and the hypothalamus, suggesting long-range network decorrelation. Supporting this idea, major white matter (WM) tracts, such as the corpus callosum and the hippocampal commissure, suffered from substantial changes compatible with an important reduction in myelination and/or a decrease in the mean axonal diameter. Macrostructurally speaking, the overexpression of GluK4 subunit had a bimodal effect, with expansion in some cortical areas in the transgenic animals accompanied by a shrinkage in the subcortical regions. Upon stimulating the brain with an electrical current, we noticed a difference in activity propagation between the two hemispheres. In transgenic animals, the evoked activity remained more confined to the stimulated hemisphere, again consistent with an impaired long-range connectivity. The structural changes both, at the micro and macro level, were in tight correlation with different aspects of the behavior including markers of anxiety such as the time spent in the open arms vs the closed arms in the plus-maze test and the time spent in the center vs the corners in the open field test. Our findings reveal how the disruption of kainate receptors, or more globally the glutamate receptors, and the abnormal synaptic transmission can translate into brain-wide changes in connectivity and alter the functional equilibrium between macro-and mesoscopic networks. The postsynaptic enhancement previously reported in the transgenic animals was here reflected in the BOLD signal and measured as an increase in the within-network strength. Importantly, the correlations between the structural changes and the behavior help to put the developmental changes and their behavioral ramifications into context. RESUMEN Grik4 es el gen responsable de codificar la subunidad GluK4 de alta afinidad de los receptores de kainato. El aumento de la dosis de esta subunidad en el prosencéfalo se relacionó con un mayor nivel de ansiedad, falta de comunicación social y depresión. A nivel sináptico, también se informó una transmisión sináptica anormal. Las manifestaciones de esta expresión anormal no se han investigado a nivel de circuito, ni las correlaciones entre esos circuitos y los patrones anormales de la conducta previamente informada. En esta línea de trabajo, aspiramos a utilizar diferentes modalidades de imágenes por resonancia magnética (MRI) no invasivas para dilucidar cualquier alteración que pudiera derivarse del aumento de la dosis de Grik4 y cómo esos cambios podrían explicar los comportamientos anormales. La resonancia magnética ofrece una forma no invasiva de observar el cerebro intacto in vivo. La resonancia magnética funcional en estado de reposo arroja luz sobre cómo funciona el cerebro en reposo en el nivel de la red y tiene la capacidad de detectar cualquier anomalía que pueda ocurrir dentro o entre esas redes. En el nivel microestructural, la resonancia magnética de difusión se ocupa de las características subyacentes de los tejidos utilizando la difusión de moléculas de agua como un proxy para ese fin. Moviéndose más macroscópicamente, utilizando escaneos estructurales, la morfometría basada en vóxeles puede detectar diferencias sutiles en la morfología de las diferentes estructuras cerebrales. Grabamos videos de nuestros animales realizando dos tareas que durante mucho tiempo se han relacionado con la ansiedad, el campo abierto y las pruebas de laberinto positivo antes de adquirir escaneos estructurales y funcionales. Por último, registramos señales dependientes del nivel de oxigenación de la sangre (BOLD) en un grupo diferente de animales durante la estimulación eléctrica de tractos específicos de materia blanca para investigar cómo se propaga la actividad neuronal. Nuestro análisis mostró un amplio espectro de cambios en el grupo transgénico en relación con los animales en el grupo de control. En el nivel de las redes de estado de reposo, observamos un aumento en la fuerza dentro de la red que abarca diferentes estructuras como el hipocampo, algunas regiones de la corteza y el hipotálamo. La mayor coherencia interna o fuerza en las redes contrastó con una reducción significativa en la conectividad entre redes para algunas regiones como partes de la corteza y el hipotálamo, lo que sugiere una descorrelación de redes de largo alcance. Apoyando esta idea, los grandes tractos de materia blanca (WM), como el cuerpo calloso y la comisura del hipocampo, sufrieron cambios sustanciales compatibles con una importante reducción de la mielinización y / o una disminución del diámetro axonal medio. Macroestructuralmente hablando, la sobreexpresión de la subunidad GluK4 tuvo un efecto bimodal, con expansión en algunas áreas corticales en los animales transgénicos acompañada de una contracción en las regiones subcorticales. Al estimular el cerebro con una corriente eléctrica, notamos una diferencia en la propagación de la actividad entre las dos hemiesferas. En los animales transgénicos, la actividad evocada permaneció más confinada al hemisferio estimulado, de nuevo consistente con una conectividad de largo alcance deteriorada. Los cambios estructurales, tanto a nivel micro como macro, estaban en estrecha correlación con diferentes aspectos de la conducta, incluidos marcadores de ansiedad como el tiempo pasado con los brazos abiertos frente a los brazos cerrados en la prueba del laberinto positivo y el tiempo pasado en el centro vs las esquinas en la prueba de campo abierto. Nuestros hallazgos revelan cómo la interrupción de los receptores de kainato, o más globalmente los receptores de glutamato, y la transmisión sináptica anormal pueden traducirse en cambios de conectividad en todo el cerebro y alterar el equilibrio funcional entre las redes macro y mesoscópicas. La mejora postsináptica informada anteriormente en los animales transgénicos se reflejó aquí en la señal BOLD y se midió como un aumento en la fuerza dentro de la red. Es importante destacar que las correlaciones entre los cambios estructurales y elcomportamiento ayudan a contextualizar los cambios en el desarrollo y sus ramificaciones conductuales