BACKGROUND AND PURPOSE: Resistance to blood flow in the cerebral drainage system may affect cerebral hemodynamics. The objective of the present study was to use phase-contrast MRA to quantify resistance to drainage of blood across branches of the venous sinus tree and to determine whether the resistance to drainage values correlated with internal jugular vein outflows. MATERIALS AND METHODS: We performed whole-head phase-contrast MRA and 2D phase-contrast MR imaging in 31 healthy volunteers. Vascular segmentation was applied to the angiograms, and the internal jugular vein velocities were quantified from the flow images. Resistance to drainage across branches of the venous sinus tree was calculated from the segmented angiograms, by using the Poiseuille equation for laminar flow. Correlations between the values of resistance to drainage and internal jugular vein outflow measurements were assessed by using the Spearman ρ. RESULTS: The overall mean resistance to drainage of the venous sinus tree was 24 ± 7 Pa s/cm3. The mean resistance to drainage of the right side of the venous sinus tree was 42% lower than that of the left side (P < .001). There were negative correlations between the values of resistance to drainage and internal jugular vein outflows on both the left side of the venous sinus tree (R = −0.551, P = .002) and the right side (R = −0.662, P < .001). CONCLUSIONS: Phase-contrast MRA is a noninvasive means of calculating the resistance to drainage of blood across the venous sinus tree. Our approach for resistance to drainage quantification may be of value in understanding alterations in the cerebral venous sinus drainage system.

An effective connectivity study was carried out on 16 young, healthy subjects performing an alertness task. The objective of this study was to develop and to evaluate a putative network model of alertness by adapting structural equation modeling to fMRI data. This study was designed to evaluate the directed interactivity of an attentional network during intrinsic and phasic alerting tasks. On the basis of theoretical hypotheses, clinical observations, behavioral data and neuroimaging studies, it was hypothesized that neural circuits in the right hemisphere including the dorsolateral prefrontal cortex (DLPFC), anterior cingulate cortex, inferior parietal cortex and the thalamus were involved. The results of this study support the existence of a common network of activated areas with significant path coefficient differences between intrinsic and phasic alertness. Functional interactivity was significantly reinforced during the phasic alertness task and appeared to preferentially involve activity in the DLPFC region, whereas the path coefficients of the model were well-balanced during intrinsic alertness. This study highlights the predominant role of the DLPFC region in maintenance of a state of alertness and in temporal preparation during an alertness task.

Après un bref rappel sur les bases physiologiques et physiques du signal blood oxygen level-dependent (BOLD), nous décrivons les récentes approches d’exploration fonctionnelle de l’activité et de l’interactivité cérébrales mesurées en IRMf, permettant d’appréhender le fonctionnement cérébral normal et ses dysfonctionnements. Nous présentons différentes méthodes d’analyse de l’interactivité cérébrale applicables en clinique et nous détaillons les concepts de connectivité fonctionnelle et effective qui permettent d’étudier la plasticité cérébrale qui se produit chez l’enfant au cours de la maturation (ex. dyslexie), chez l’adulte au cours du vieillissement (ex. démence de type Alzheimer) ou encore dans le cas de la schizophrénie ou de la maladie de Parkinson. L’étude de circuits spécifiques en réseaux doit permettre de cerner de façon plus réaliste le fonctionnement dynamique du système nerveux central qui sous-tend de nombreuses fonctions, tant dans des conditions physiologiques normales que pathologiques, et donc d’améliorer le diagnostique et la prise en charge des pathologies neurologiques.

Purpose. - This fMRI study investigated phonological and lexicosemantic processing in dyslexic and in chronological age- and reading level-matched children in a pseudoword reading task.Materials and methods. - The effective connectivity network was compared between the three groups using a structural model including the supramarginal cortex (BA 40; BA: Brodmann area), fusiform cortex (BA 37) and inferior frontal cortex (BA 44/45) areas of the left hemisphere.Results. - The results revealed differences in connectivity patterns. In dyslexic patients, in contrast with chronological age- and reading level-matched groups, no causal relationship was demonstrated between BA 40 and BA 44/45. However, a significant causal relationship was demonstrated between BA 37 and BA 44/45 both in dyslexic children and in the reading level-matched group.Conclusions. - These findings were interpreted as evidence for a phonological deficit in developmental dyslexia. (C) 2008 Elsevier Masson SAS. All rights reserved.

After having provided a brief reminder of the principle of the blood oxygen level-dependent (BOLD) contrast effect, the physiological bases of brain activity and the concepts of functional integration and effective connectivity, we describe the most recent approaches, which permit to explore brain activity and putative networks of interconnected active areas in order to examine the normal brain physiology and its dysfunctions. We present various methods and studies of brain activity analysis clinically applicable, and we detail the concepts of functional and effective connectivity, which allow to study the cerebral plasticity which occurs at the child's during the maturation (e.g., dyslexia), at the adult during the ageing (e.g., Alzheimer disease), or still in schizophrenia or Parkinson disease. The study of specific circuits in networks has to allow defining in a more realistic way the dynamic of the central nervous system, which underlies various cerebral functions, both in physiological and pathological conditions. This connectivity approach should improve the diagnostic and facilitate the development of new therapeutic strategies.

Aim.- To validate, through functional magnetic resonance imaging (fMRI) from spectral analysis of time series during a visuomotor task, a model of functional connectivity mainly constituted by the pre-supplementary motor area (pre-SMA), the supplementary motor area proper (SMA-proper) and the primary motor cortex (M1).Materials and methods.- The paradigm that was tried out in young subjects (n = 5) consisted of a preparation task of motor movement. We firstly proceeded with an estimate in the frequency domain of coherency coefficients and values of phase shift between these three areas. Secondly, the estimated coherency coefficients were integrated to a model of functional connectivity. Two interaction coefficients were calculated, one for the related M1 and pre-SMA regions, the other one for the related M1 and SMA-proper regions.Results and conclusion.- Our results demonstrate hemodynamic activity that definitely occurred earlier in the pre-SMA area during the preparatory period of the task. In the same way, a more important interaction was found between M1 and pre-SMA areas, which corroborates the assumption of the prevalent role played by these two areas in the case of a preparation task of a motor movement. Thus, this study has allowed highlighting a functional dissociation between the two portions of the SMA. (c) 2007 Elsevier Masson SAS.

Functional magnetic resonance imaging (fMRI) was used to assess the contributions of movement preparation and execution of a visuomotor task in a cerebral motor network. The functional connectivity of the voxel time series between brain regions in the frequency space was investigated by performing spectral analysis of fMRI time series. The regional interactivities between the two portions of the supplementary motor area (pre-SMA and SMA-proper) and the primary motor cortex (M1), defined as a seed region, were evaluated. The spectral parameter of coherence was used to describe a correlation structure in the frequency domain between two voxel-based time series and to infer the strength of the functional interaction within our presumed motor network of connections. The results showed meaningful differences of the functional interactions between the two portions of the SMA and the M1 area depending on the task conditions. This approach demonstrated the existence of a functional dissociation between the pre-SMA and SMA-proper subregions. We therefore conclude that spectral analysis is useful for identifying functional interactions of brain regions and might provide a powerful tool to quantify changes in connectivity profiles associated with various components of an experimental task.

L'alerte est le processus permettant de préparer à l'imminence d'un événement ce qui optimise les performances. Elle peut être extrinsèque, déclenchée par la présence d'un signal avertisseur ou intrinsèque. Notre objectif était d'étudier les mécanismes d'alerte en IRMf par une tâche de temps de réaction simple (TRS), et de comparer alerte intrinsèque et extrinsèque. Nous avons évalué 16 sujets droitiers (28 ans). Le protocole d'activation cérébrale comprenait 2 tâches de TRS utilisant des stimuli visuels élémentaires, l'une sans signal avertisseur (alerte intrinsèque), l'autre avec signal avertisseur (alerte extrinsèque) et une tâche sensori-motrice (contrôle). Les analyses d'imagerie fonctionnelle utilisaient le logiciel SPM version 2 localisant les régions cérébrales activées par le processus d'alerte et ensuite, les relations existant entre ces régions de manière qualitative. Nos résultats étaient en faveur d'un réseau commun d'activation essentiellement fronto-pariétale (cortex frontal dorso-latéral (BA 10, 46, 8), lobule pariétal inférieur (BA 40), gyrus cingulaire antérieur (GCA) (p < 0,05) à prédominance droite. Il existait aussi une activation significative temporale inférieure (BA 21, 20) et du gyrus fusiforme. Les analyses d'interaction entre ces régions d'intérêt ont montré que le GCA jouait un rôle prépondérant dans ce réseau particulièrement pendant l'alerte extrinsèque. L'alerte intrinsèque et extrinsèque partagent un réseau commun d'activation fronto-pariétale à prédominance droite. L'activation de l'aire temporale inférieure et du gyrus fusiforme, plus surprenante peut être liée à la discrimination des stimuli visuels, à l'implication d'un processus mnésique ou au processus attentionnel. Alerte extrinsèque et intrinsèque semblent cependant différer en terme d'interactions entre ces aires impliquées. La poursuite de l'analyse en réseau des aires impliquées devrait permettre de préciser ces résultats.

AIM:To validate, through functional magnetic resonance imaging (fMRI) from spectral analysis of time series during a visuomotor task, a model of functional connectivity mainly constituted by the pre-supplementary motor area (pre-SMA), the supplementary motor area proper (SMA-proper) and the primary motor cortex (M1).MATERIALS AND METHODS:The paradigm that was tried out in young subjects (n=5) consisted of a preparation task of motor movement. We firstly proceeded with an estimate in the frequency domain of coherency coefficients and values of phase shift between these three areas. Secondly, the estimated coherency coefficients were integrated to a model of functional connectivity. Two interaction coefficients were calculated, one for the related M1 and pre-SMA regions, the other one for the related M1 and SMA-proper regions.RESULTS AND CONCLUSION:Our results demonstrate hemodynamic activity that definitely occurred earlier in the pre-SMA area during the preparatory period of the task. In the same way, a more important interaction was found between M1 and pre-SMA areas, which corroborates the assumption of the prevalent role played by these two areas in the case of a preparation task of a motor movement. Thus, this study has allowed highlighting a functional dissociation between the two portions of the SMA.

Functional magnetic resonance imaging (fMRI) permits to obtain physiological information about MRI signal, which is modulated by electrical, biochemical, and physiological properties of the cerebral. tissue. It is possible to characterize the brain interactions from an fMRI signal. Particularly, the use of a spectral. analysis at a given frequency allows access to the time series chronology, which occurs within various activated areas of the brain. Thus, spectral. parameters such as coherency and phase shift may be calculated from presupposed stationary stochastic signals and of an estimate of the cross-spectral power density function. Coherency describes a correlation structure in frequency domain between signals and thus allows obtaining an accurate estimate of the phase retation (time delay), which connects the signals between them. We describe in the last part of the article a calculation method integrating spectral information obtained previously and which makes it possible to evaluate the intensity of the existing interaction between two distinct cerebral areas. (c) 2007 Elsevier Masson SAS.

Evaluer l’effet d’une leucoaraïose cérébrale importante sur les cartographies d’activation obtenues par IRMf lors d’une tâche de fluence verbale. Une séquence d’IRMf a été réalisée chez 8 (76,4 ± 3,0 ans) et 10 patients (67,6 ± 4,2 ans) respectivement avec et sans leucoaraïose. Un paradigme en blocs d’une durée de 2 minutes 33 secondes a été utilisé. Une analyse statistique de deuxième ordre, réalisée avec le logiciel SPM99 après correction des mouvements, normalisation dans l’espace de Talairach, lissage des images avec un filtre gaussien de 8 mm, a permis d’identifier sur la base du maximum local les régions significativement activées dans les 2 groupes. Le nombre de voxels activés dans les régions frontales (seuil arbitraire de valeur de p < 0,005) était 10 fois plus faible dans le groupe avec leucoaraïose que dans le groupe contrôle et le nombre de foyers activés divisé par trois. Les variations de signal BOLD mesurées chez les patients avec leucoaraïose apparaissaient réduites. Les modifications détectées par IRMf chez les patients avec leucoaraïose marquée suggèrent une souffrance frontale.