DTI - Diffusion Tensor Imaging

PRIMARY OBJECTIVE:

To investigate sustained structural changes in the long-term (>1 year) after mild traumatic brain injury (mTBI) and their relationship to ongoing post-concussion syndrome (PCS).
RESEARCH DESIGN:

Morphological and structural connectivity magnetic resonance imaging (MRI) data were acquired from 16 participants with mTBI and nine participants without previous head injury.
MAIN OUTCOMES AND RESULTS:

Participants with mTBI had less prefrontal grey matter and lower fractional anisotropy (FA) in the anterior corona radiata and internal capsule. Furthermore, PCS severity was associated with less parietal lobe grey matter and lower FA in the corpus callosum.
CONCLUSIONS:

There is evidence for both white and grey matter damage in participants with mTBI over 1 year after injury. Furthermore, these structural changes are greater in those that report more PCS symptoms, suggesting a neurophysiological basis for these persistent symptoms.

Background Localization of brain function is a fundamental
requisite for the resection of eloquent-area brain
tumors. Preoperative functional neuroimaging and diffusion
tensor imaging can display cortical functional
organization and subcortical anatomy of major white
matter bundles. Direct cortical and subcortical stimulation
is widely used in routine practice, however, because
of its ability to reveal tissue function in eloquent regions.
The role and integration of these techniques is still a
matter of debate. The objective of this study was to
assess surgical and functional neurological outputs of
awake surgery and intraoperative cortical and subcortical
electrical stimulation (CSES) and to use CSES to
examine the reliability of preoperative functional magnetic
resonance (fMRI) and diffusion tensor imaging
fiber tracking (DTI-FT) for surgical planning.
Patients and methods We prospectively studied 27 patients
with eloquent-area tumors who were selected to undergo
awake surgery and direct brain mapping. All subjects
underwent preoperative sensorimotor and language fMRI
and DTI tractography of major white matter bundles. Intraand
postoperative complications, stimulation effects, extent
of resection, and neurological outcome were determined.
We topographically correlated intraoperatively identified
sites (cortical and subcortical) with areas of fMRI activation
and DTI tractography.
Results Total plus subtotal resection reached 88.8%.
Twenty-one patients (77.7%) suffered transient postoperative
worsening, but at 6 months follow-up only three
(11.1%) patients had persistent neurological impairment.
Sensorimotor cortex direct mapping correlated 92.3%
with fMRI activation, while direct mapping of language
cortex correlated 42.8%. DTI fiber tracking underestimated
the presence of functional fibers surrounding or
inside the tumor.
Conclusion Preoperative brain mapping is useful when
planning awake surgery to estimate the relationship between
the tumor and functional brain regions. However, these
techniques cannot directly lead the surgeon during resection.
Intraoperative brain mapping is necessary for safe and
maximal resection and to guarantee a satisfying neurological
outcome. This multimodal approach is more aggressive, leads
to better outcomes, and should be used routinely for resection
of lesions in eloquent brain regions.

The analysis of the structural architecture of the human brain in terms of connectivity between its subregions has provided profound insights into its underlying functional organization and has coined the concept of the “connectome”, a structural description of the elements forming the human brain and the connections among them. Here, as a proof of concept, we introduce a novel group connectome in standard space based on a large sample of 169 subjects from the Enhanced Nathan Kline Institute-Rockland Sample (eNKI-RS). Whole brain structural connectomes of each subject were estimated with a global tracking approach, and the resulting fiber tracts were warped into standard stereotactic (MNI) space using DARTEL. Employing this group connectome, the re- sults of published tracking studies (i.e., the JHU white matter and Oxford thalamic connectivity atlas) could be largely reproduced directly within MNI space. In a second analysis, a study that examined structural connectivity between regions of a functional network, namely the default mode network, was reproduced. Voxel-wise struc- tural centrality was then calculated and compared to others' findings. Furthermore, including additional resting- state fMRI data from the same subjects, structural and functional connectivity matrices between approximately forty thousand nodes of the brain were calculated. This was done to estimate structure–function agreement in- dices of voxel-wise whole brain connectivity.
Taken together, the combination of a novel whole brain fiber tracking approach and an advanced normalization method led to a group connectome that allowed (at least heuristically) performing fiber tracking directly within MNI space. Such an approach may be used for various purposes like the analysis of structural connectivity and modeling experiments that aim at studying the structure–function relationship of the human connectome. More- over, it may even represent a first step toward a standard DTI template of the human brain in stereotactic space. The standardized group connectome might thus be a promising new resource to better understand and further analyze the anatomical architecture of the human brain on a population level.

Background: The relationship between cerebrovascular disease (CVD) risk factors and cognitive impairment or dementia has been widely studied with significant variability in findings between groups. We hypothesized that chronic small vessel injury in the form of arteriolar sclerosis, measured quantitatively (using MRI T2 volumetric analysis of white matter hyperintensity (WMH) volumes), would identify specific CVD risk factors and patterns of cognitive decline, associated with mild cognitive impairment of the cerebrovascular type (MCI-CVD), that represent the core features of vascular cognitive impairment in our Kentucky “stroke-belt” population, at risk for cognitive decline and the future development of dementia.
Methods: A Cross-sectional analysis of clinical and quantitative MRI data on 114 subjects with normal cognitive function (n=52) and mild cognitive impairment (MCI; n=62) was performed. Quantitative total WMH volumes were examined in relation to potentially causative CVD risk factors and resultant test scores across cognitive domains using linear regression models adjusted for age, gender, and education.
Results: Among CVD risk factors analyzed, age (p< 0.001), education (p= 0.003), hypertension (p= 0.012), and hyperlipidemia (p= 0.008) demonstrated the strongest associations with WMH volumes. Conversely, CVD risk factors such as diabetes, smoking, history of heart attacks, atrial fibrillation, and history of stroke that have shown associations with CVD pathology on imaging in other studies were not statistically associated with increased WMH in this cohort. Unexpectedly, Framingham risk scores (FRS) and modified (short version) Hachinski Ischemic Scale (HIS) measures were not statistically associated with quantitative WMH volumes in this study. WMH volumes were associated with decrease performance on the Trial Making Test type A & B and long delayed free recall on the California Verbal Learning Test.
Conclusion: Our findings suggest similarities and yet differences in comparison to other studies, regarding the impact of CVD risks and associations with cognitive performance with the development of CVD pathology measured by WMH on MRI. Hypertension and hyperlipidemia appear to represent common shared risks across geographically disparate groups. Our findings, like others, suggest CVD pathology impact processing speed and executive function and provide further evidence for CVD effects on short-term memory in those at risk for cognitive decline and the future development of dementia in our Kentucky “stroke-belt” population.

This research questions whether the measures of brain structural connectivity proceed correctly when they correlate the entire tract with coefficients of cognitive measures. Like other anatomical structures that have a nutritive part, a protective part, and a sensitive part, the cerebral white matter tracts could have a sensitive part and a percentage of the tract could be not related to cognitive algorithms. The diffusion data, which allow the DTI, CSD models, is based on the detection of coherence of diffusion of water within the white matter. It is an indirect measure of nerve transmission efficiency; however, it is the standard measure of structural brain connectivity. This work presents the association between the Inferior Longitudinal Fascicle and a measure of Semantic Executive Control with the addition of the deconstruction of the white matter tract in constitutive rings. The results showed that performance in the cognitive task was associated with a higher correlation strength with the central ring of the Inferior Longitudinal Fasciculus. Therefore, two of the three deconstructed rings of the lower Longitudinal Fascicle were not related to the semantic executive control process. This finding, along with others that are emerging, justify that the orthotropic-radial perspective is supported to explore brain structural connectivity, possibly in replacement of the classical theory of homogeneity of the white matter. Structural brain connectivity studies are based on MRI-diffusion data. The design of that kind of studies is correlational, since the diffusion data sequence requires that the participants have their heads fixed during the moment of the scan, and do not any activity inside the scanner (Soares et al., 2013). At any rate, diffusion data does not inform any about changes related to online behavioural-cognitive processes. The measurement obtained is the diffusion of water within the brain's white matter. That is indirect, since the connectivity of the tract is an inference from the coherence of the movement detected by the scanner (Beaulieu, 2002). The coherence of the movement

The hereditary spastic paraplegias are a group of genetically heterogeneous neurodegenerative disorders, characterized by progressive spasticity and weakness of the lower limbs. Although conventional brain MR imaging findings are normal in patients with pure hereditary spastic paraplegia, microstructural alteration in the cerebral WM can be revealed with DTI. Concomitant investigation of multiple intrinsic diffusivities may shed light on the neurobiologic substrate of the WM degeneration pattern in patients with pure hereditary spastic paraplegia across the whole brain. Tract-based spatial statistics analysis was performed to compare fractional anisotropy and mean, axial, and radial diffusivities of the WM skeleton in a group of 12 patients with pure hereditary spastic paraplegia and 12 healthy volunteers. Data were analyzed counting age and sex as nuisance covariates. The threshold-free cluster-enhancement option was applied, and the family-wise error rate was controlled by using p...

Retrograde tracer injections in 29 of the 91 areas of the macaque cerebral cortex revealed 1,615 interareal pathways, a third of which have not previously been reported. A weight index (extrinsic fraction of labeled neurons [FLNe]) was determined for each area-to-area pathway. Newly found projections were weaker on average compared with the known projections; nevertheless, the 2 sets of pathways had extensively overlapping weight distributions. Repeat injections across individuals revealed modest FLNe variability given the range of FLNe values (standard deviation <1 log unit, range 5 log units). The connectivity profile for each area conformed to a lognormal distribution, where a majority of projections are moderate or weak in strength. In the G29 × 29 interareal subgraph, two-thirds of the connections that can exist do exist. Analysis of the smallest set of areas that collects links from all 91 nodes of the G29 × 91 subgraph (dominating set analysis) confirms the dense (66%) structure of the cortical matrix. The G29 × 29 subgraph suggests an unexpectedly high incidence of unidirectional links. The directed and weighted G29 × 91 connectivity matrix for the macaque will be valuable for comparison with connectivity analyses in other species, including humans. It will also inform future modeling studies that explore the regularities of cortical networks.

Grey matter (GM) damage is a clinically relevant feature of multiple sclerosis (MS) that has been previously assessed with diffusion tensor imaging (DTI). Fractional anisotropy (FA) of the basal ganglia and thalamus might be increased in MS patients, and correlates with disability scores. Despite the established role of the striatum and thalamus in motor control, mood and cognition, the impact of DTI changes within these structures on motor and neuropsychological performance has not yet been specifically addressed in MS. We investigated DTI metrics of deep GM nuclei and their potential association with mobility and neuropsychological function. DTI metrics from 3T MRI were assessed in the caudate, putamen, and thalamus of 30 MS patients and 10 controls. Sixteen of the patients underwent neuropsychological testing. FA of the caudate and putamen was higher in MS patients compared to controls. Caudate FA correlated with Expanded Disability Status Scale score, Ambulation Index, and severity of depressive symptomatology. Putamen and thalamus FA correlated with deficits in memory tests. In contrast, cerebral white matter (WM) lesion burden showed no significant correlation with any of the disability, mobility and psychometric parameters. Our findings support evidence of FA changes in the basal ganglia in MS patients, as well as deep GM involvement in disabling features of MS, including mobility and cognitive impairment. Deep GM FA appears to be a more sensitive correlate of disability than WM lesion burden.

Abstract: The degree of correlation of resting state activity within the brain is a measure of the functional connectivity (FC) of those regions. One such resting state network is the default mode network (DMN). The DMN consist of the posterior cingulate/Precuneus cortex (PCC), the left inferior parietal/angular gyrus (lIPC), the right inferior parietal/angular gyrus (rIPC) and medial prefrontal/anterior cingulate cortex (mPFC). The PCC is thought to form the central core of the DMN and  is believed to be involved in the maintenance of consciousness within the alert resting brain. Numerous studies have been performed to investigate the FC of the DMN, but less is known about the structural connectivity (SC). The aim of this study was to: 1) demonstrate structural connections between nodes (mPFC,lIPC, rIPC) of the DMN and the PCC. 2) To determine if there was a significant variation in the strength of structural and functional connections to the PCC from each node of the DMN. 3) To determine if there was a significant relationship between SC and FC of the nodes of the DMN to the PCC. We used functional magnetic resonance imaging (fMRI), group independent component analysis (GICA) and diffusion tensor imaging (DTI) to produce FC correlation maps and structural white matter tracts to demonstrate structural connections of the nodes of the DMN to the PCC. Repeated measures one-way ANOVA statistical tests were performed to determine variation in the strength of FC and SC from each of the individual nodes of the DMN to the PCC. Bivariate Pearson’s correlation analysis was used to measure the degree of overall correlation between FC and SC. Pairwise structural connections were demonstrated in all 15 subjects for mPFC-PCC regions, and in 11 of 15 subjects for the rIPC-PCC regions and in 8 of 15 subjects for the lIPC-PCC regions in our study. FC of all nodes of the DMN  was found with respect to the PCC. The weakest FC correlation to the PCC was the lIPC, which was found to be significant (p=0.018). The degree of SC to the PCC demonstrated a similar pattern to the FC, although the SC variations were not found to be significant (P=0.414). FC-SC correlation with respect to the PCC  was found to be significant at (r=0.46) P= < 0.01. We can conclude our findings have demonstrated structural connections between the functionally connected regions of the DMN and a significant relationship between DMN FC and SC to the PCC. This reinforces the view that the PCC forms the central core of the DMN. A better understanding of how structural connections relate to functional connectivity is imperative in order to enhance our understanding of long term changes of SC and FC as a consequence of Psychiatric pathology and sleep disorders.

Aiming at exploring the brain's structural organisation underlying successful second language learning, we investigate the anatomy of the perisylvian language network in a group of healthy adults, consisting of participants with high and average language analytical abilities. Utilising deterministic tractography, six tracts per participant (left and right long direct segment, left and right indirect anterior segment and left and right indirect posterior segment) were virtually dissected and measurements pertaining to their microstructural organisation were collected. Our results obtained by means of linear discriminant analysis pointed to mean diffusivity (MD) values of three tracts (right anterior, left long and left anterior segments) as best discriminating between the two groups. By far the highest coefficient was obtained for the MD values of the right anterior segment, pointing to the role of the right white matter fronto-parietal connectivity for superior language learning abilities. The results imply the importance of attentional processes and reasoning abilities for successful L2 acquisition, and support previous findings concerning right-hemispheric involvement in language learning.

Goal of the study: Diffusion MRI based tractography allows for in-vivo reconstruction of brain connectivity pathways. Furthermore, it is possible to segment specific white matter fibers and obtain indices related to their microstructural properties. Most tractography studies compute one mean index value for each white matter fiber, despite the existence of great index variation within the tract. Recent work developed techniques to assess these indices along the tract, to provide more information of these structures. The goal of this study is to present novel findings regarding radial or across-tract variation and a new toolbox designed for its measuring: JACtree. Methods: We performed diffusion MRI processing, tractography dissection of the right inferior fronto-occipital fasciculus and radial-longitudinal analysis in a simple of 161 healthy volunteers (M = 20.28, DE = 2.47, 97 women, 64 men) from York (Yorkshire, UK). Results: We observed significant effects of the radial and longitudinal dimensions on fiber connectivity (measured by fractional anisotropy). Specifically, fractional anisotropy increased from periphery to its center (radially) and from the frontal to the occipital lobe. The interaction between these effects was significant as well. Finally, we also observe that the relation between fractional anisotropy and mean diffusion varied through the radial dimension. Conclusions: In this work, we present novel findings and the tools required to study the radial dimension of white matter fibers connectivity.
Key words: white matter fibers, diffusion weighted MRI, brain connectivity, fractional anisotropy, radial analysis.

Resumen Objetivo: La tractografía basada en datos de difusión de resonancia magnética permite estudiar in-vivo las vías de conectividad cerebral. Además, es posible diseccionar fibras de sustancia blanca de manera específica y obtener índices vinculados a sus propiedades microestructurales. La mayoría de los estudios de tractografía utiliza un valor promedio por cada fibra de sustancia blanca, pese a que existe una gran variación en estos índices. Estudios recientes han desarrollado técnicas para evaluar estos índices a lo largo de la fibra, y así proveer mayor información sobre estas estructuras. El objetivo de este estudio es presentar nuevos hallazgos preliminares respecto a la variación radial o 'cross-tract', y un novedoso toolbox diseñado para su medición: JACtree. Métodos: Se realizó postprocesamiento de datos de difusión, disección tractográfica del fascículo fronto-occipital inferior derecho y análisis radial-longitudinal en una muestra de 161 participantes sanos (M = 20.28, DE = 2.47, 97 mujeres, 64 hombres) de York (Yorkshire, Reino Unido). Resultados: Se observaron efectos significativos sobre la conectividad, medida con anisotropía fraccional, de las dimensiones radial y longitudinal de la fibra. Específicamente, la anisotropía fraccional aumentaba desde la periferia hacia el centro, y desde el lóbulo frontal al occipital. La interacción entre estos efectos también fue significativa. Por último, adicionalmente se observó que la relación entre anisotropía fraccional y difusión media varió en función de la dimensión radial. Conclusiones: En este trabajo presentamos hallazgos novedosos y herramientas necesarias para el estudio de la dimensión radial de la conectividad de las fibras de sustancia blanca. Palabras clave: fibras de sustancia blanca, datos de difusión, conectividad cerebral, anisotropía fraccional, análisis radial. Resumo Objetivo: a tractografia baseada em dados de difusão de ressonância magnética permite estudar in vivo as formas de conectividade cerebral. Além disso, é possível dissecar fibras de substância branca de maneira específica e obter índices ligados às suas propriedades microestruturais. A maioria dos estudos tractográficos usam um valor médio para cada fibra de substância branca embora haja grande variação nesses índices. Estudos recentes desenvolveram técnicas para avaliar esses índices fora da fibra e, portanto, fornecem mais informações sobre essas estruturas. Dessa forma, o objetivo desse estudo é apresentar novas medidas preliminares relativas à variação radial do 'trato transverso', além de uma nova caixa de ferramentas projetada para seu medicamento: JACtree. Método: Realizou-se pós-processamento dos dados da difusão, dissecção tractográfica do fascículo fronto-occipital inferior direito e análise radial-longitudinal em uma amostra de 161 participantes saudáveis (M=20,28, dp=2,47, sendo 97 mulheres e 64 homens) de York (Yorkshire, Reino Unido). Resultados: Foram observados efeitos significativos na conectividade, medida com anisotropia fracionada, com as dimensões radial e longitudinal da fibra. Especificamente a anisotropia fracionada aumentou da periferia para o centro e do lobo frontal para o occipital, mostrando-se significativa. Por fim, também foi observado que a relação entre a anisotropia fracionada e a difusão média varia de acordo com a dimensão radial. Conclusões: Nesse trabalho apresentou-se novas e interessantes ferramentas para o estudo da dimensão radial do suporte de fibra da substância branca. Palavras-chave: fibras de suporte, substância branca, dados de difusão, conectividade cerebral, anisotropia fracionada, análise radial. Artículo recibido: 21/10/2020; Artículo revisado: 16/03/2021; Artículo aceptado: 26/04/2021. Toda correspondencia relacionada con este artículo debe ser enviada a Julián Marino Dávolos,

Degeneration of cerebral white matter (WM), or structural disconnection, is one of the major neural mechanisms driving age-related decline in cognitive functions, such as processing speed. Past cross-sectional studies have demonstrated beneficial effects of greater cardiorespiratory fitness, physical activity, cognitive training, social engagement, and nutrition on cognitive functioning and brain health in aging. Here, we collected diffusion magnetic resonance (MRI) imaging data from 174 older (age 60–79) adults to study the effects of 6-months lifestyle interventions on WM integrity. Healthy but low-active participants were randomized into Dance, Walking, Walking + Nutrition, and Active Control (stretching and toning) intervention groups (NCT01472744 on ClinicalTrials.gov). Only in the fornix there was a time × intervention group interaction of change in WM integrity: integrity declined over 6 months in all groups but increased in the Dance group. Integrity in the fornix at baseline was associated with better processing speed, however, change in fornix integrity did not correlate with change in processing speed. Next, we observed a decline in WM integrity across the majority of brain regions in all participants, regardless of the intervention group. This suggests that the aging of the brain is detectable on the scale of 6-months, which highlights the urgency of finding effective interventions to slow down this process. Magnitude of WM decline increased with age and decline in prefrontal WM was of lesser magnitude in older adults spending less time sedentary and more engaging in moderate-to-vigorous physical activity. In addition, our findings support the anterior-to-posterior gradient of greater-to-lesser decline, but only in the in the corpus callosum. Together, our findings suggest that combining physical, cognitive, and social engagement (dance) may help maintain or improve WM health and more physically active lifestyle is associated with slower WM decline. This study emphasizes the importance of a physically active and socially engaging lifestyle among aging adults.

One possible explanation for the mismatch between tissue damage and preservation
of neurological functions in patients with benign multiple sclerosis (BMS) is
that the pathophysiology differs from that occurring in other multiple sclerosis
(MS) phenotypes. The objective of this study was to identify pathologically
specific patterns of tissue integrity/damage characteristics of patients with
BMS, and markers of potential prognostic value. The pyramidal system was
investigated in 10 BMS patients and 20 controls using voxel-based morphometry to
assess grey matter (GM) atrophy, and diffusion tractography and quantitative
magnetization transfer to quantify the microstructural damage in the
corticospinal tracts (CSTs). Widespread reductions in GM volume were found in
patients compared with controls, including the primary motor cortex. A
significant decrease was observed in the mean macromolecular pool ratio (F) of
both CSTs, with no fractional anisotropy (FA) change. GM volume of the primary
motor areas was associated with clinical scores but not with the CST parameters.
The mismatch between F and FA suggests the presence of extensive demyelination in
the CSTs of patients with BMS, in the absence of axonal damage. The lack of
correlation with GM volume indicates a complex interaction between disruptive and
reparative mechanisms in BMS.

"Bimanual actions involve coordinated motion but often rely on the movements performed with each hand to be different. Older adults exhibit differentially greater variability for bimanual actions in which each hand has an independent movement goal. Such actions rely on interhemispheric communication via the corpus callosum, including both facilitatory and inhibitory interactions. Here, we investigated whether age differences in callosal structure and interhemispheric function contribute to this selective movement difficulty. Participants performed 3 force production tasks: 1) unimanual, 2) bimanual simultaneous, and 3) bimanual independent. Older adults had significantly greater interhemispheric facilitation during voluntary muscle activation. We also report a fundamental shift with age in the relationship between callosal tract microstructural integrity and interhemispheric inhibition (IHI). Specifically, older adults with relatively greater callosal tract microstructural integrity have less IHI. Furthermore, greater IHI was related to poorer bimanual
performance (assessed by dominant hand force variability) in older adults on all tasks, whereas this relationship was only observed in young adults for the bimanual independent condition. These findings indicate changes in interhemispheric communication with advancing age such that older adults may rely on bilateral cortical cooperation to a greater extent than young adults for manual actions."

Carpal tunnel syndrome (CTS) leads to distortion of axonal architecture, demyelination and fibrosis within the median nerve. Diffusion tensor imaging (DTI) characterises tissue microstructure and generates reproducible proxy measures of nerve 'health' which are sensitive to myelination, axon diameter, fiber density and organisation. This meta-analysis summarises the normal DTI values of the median nerve, and how they change in CTS. This systematic review included studies reporting DTI of the median nerve at the level of the wrist in adults. The primary outcome was to determine the normal fractional anisotropy (FA) and mean diffusivity (MD) of the median nerve. Secondarily, we show how the FA and MD differ between asymptomatic adults and patients with CTS, and how these differences are independent of the acquisition methods. We included 32 studies of 2643 wrists, belonging to 1575 asymptomatic adults and 1068 patients with CTS. The normal FA was 0.58 (95% CI 0.56, 0.59) and the normal MD was 1.138 × 10-3 mm 2 /s (95% CI 1.101, 1.174). Patients with CTS had a significantly lower FA than controls (mean difference 0.12 [95% CI 0.09, 0.16]). Similarly, the median nerve of patients with CTS had a significantly higher mean diffusivity (mean difference 0.16 × 10-3 mm 2 /s [95% CI 0.05, 0.27]). The differences were independent of experimental factors. We provide summary estimates of the normal FA and MD of the median nerve in asymptomatic adults. Furthermore, we show that diffusion throughout the length of the median nerve becomes more isotropic in patients with CTS.

Anisotropía Fraccional es una medida de conectividad estructural cerebral. Se calcula a partir de adquisiciones de Resonancia Magnética-Datos de Difusión. Se basa en la modelización de la difusión del agua dentro del sistema nervioso. La articulación entre estos conceptos está desarrollada en el artículo.

La relación entre Anisotropía Fraccional (AF) y Difusión Media (DM) se considera significativa inversa. A mayor AF, menor DM. Esto se relacionó con posibles procesos biológicos, con impacto en la cognición (Kelly et al., 2018). La mayor AF se vincula a la coherencia fibrilar, y la mayor DM a la pérdida de estructuración (Lebel et al., 2017), que podría estar causada por injuria axonal, pérdida de mielina u otros factores como cruzamiento masivo de fibras y presencia de organelos (Beaulieu, 2002). En sentido matemático ambos índices provienen de los valores de Lambda de una lectura (readout) de Resonancia Magnética (RM), secuencia Datos de Difusión, modelos de Tensores de Difusión (DTI, acrónimo en inglés). En los modelos DTI hay 3 valores de Lambda, que modelizan la difusión con vectores ortogonales dentro de un tiempo (DT) y un espacio (Dv). Los vectores-Lambda tienen un autovalor (eigenvalue) y se organizan de este modo: L1>L2>L3. En la DM la fórmula es la siguiente: 1. (L1+L2+L3) /3 Este cálculo esta cerca del readout de RM y su valor implica una cantidad de difusión. La AF es una organización mediante un coeficiente fraccional de los valores de Lambda. Cada Lambda se resta a la DM y se eleva al cuadrado. Se suman entre sí, y al resultado de esta sumatoria se calcula la raíz cuadrada: 2. Ѵ (L1-DM) 2 +(L2-DM) 2 +(L3-DM) 2 La fórmula (2.) es el numerador del coeficiente de AF. El denominador eleva al cuadrado cada Lambda, hace una sumatoria, y luego calcula la raíz cuadrada de esa sumatoria: 3. Ѵ (L1) 2 +(L2) 2 +(L3) 2 Para calcular la AF se multiplica la fracción creada por (2. / 3.) por la constante = 1,2247. Queda un coeficiente fraccional que se extiende en el rango de 0 a 1, y su valor calculado

Executive cognitive functions, including working memory, cognitive flexibility, and inhibition, are impaired in schizophrenia. Executive functions rely on coordinated information processing between the prefrontal cortex (PFC) and thalamus, particularly the mediodorsal (MD) nucleus. This raises the possibility that anatomical connectivity between the PFC and MD thalamus may be: 1) reduced in schizophrenia; and 2) related to deficits in executive function. The current investigation tested these hypotheses.