Development of cortical folds in the human brain: An attempt to review biological hypotheses, early neuroimaging investigations and functional correlates

The folding of the human brain mostly takes place in utero, making it challenging to study. After a few pioneer studies looking into it in post-mortem foetal specimen, modern approaches based on neuroimaging have allowed the community to investigate the folding process in vivo, its normal progression, its early disturbances, and its relationship to later functional outcomes. In this review article, we aimed to first give an overview of the current hypotheses on the mechanisms governing cortical folding. After describing the methodological difficulties raised by its study in fetuses, neonates and infants with magnetic resonance imaging (MRI), we reported our current understanding of sulcal pattern emergence in the developing brain. We then highlighted the functional relevance of early sulcal development, through recent insights about hemispheric asymmetries and early factors influencing this dynamic such as prematurity. Finally, we outlined how longitudinal studies have started to relate early folding markers and the child's sensorimotor and cognitive outcome. Through this review, we hope to raise awareness on the potential of studying early sulcal patterns both from a fundamental and clinical perspective, as a window into early neurodevelopment and plasticity in relation to growth in utero and postnatal environment of the child.

Biomarkers of Age-Related Frailty and Frailty Related to Diseases: An Exploratory, Cross-Sectional Analysis from the MAPT Study

BACKGROUND

Frailty may in most cases result from two main causes: the aging process (age-related frailty) and diseases (evolving chronic conditions or acute medical illnesses - disease-related frailty). The biological determinants characterizing these two main causes of frailty may be different.

OBJECTIVES

The aim of this study is to compare the biological and neuroimaging profile of people without frailty, those with age-related frailty, and subjects with disease-related frailty in community-dwelling older adults.

MATERIAL AND METHODS

We performed a secondary, cross-sectional analysis from the Multidomain Alzheimer Preventive Trial (MAPT). We included 1199 subjects without frailty throughout the 5-year follow-up, 82 subjects with incident age-related frailty, and 53 with incident disease-related frailty. Available blood biomarkers involved nutritional (eg, vitamin D, omega-3 fatty acids), inflammatory-related (IL-6, TNFR1, GDF15), neurodegenerative (eg, beta-amyloid, neurofilament light chain) and neuroimaging markers (MRI, Amyloid-PET).

RESULTS

Although not statistically significant, the results of the unadjusted model showed increasing gradients for inflammatory markers (GDF15, TNFR1) and decreasing gradients for nutritional and neuroimaging markers (omega 3 index, hippocampal volume) from age-related frailty participants to individuals with disease-related frailty. Considering the linear models we observed higher GDF15 values in disease-related frailty group compared to age-related frailty individuals [β = 242.8 (49.5, 436.2)]. We did not find any significant difference between subjects without frailty and those with age-related frailty. Subjects with disease-related frailty compared to subjects without frailty had lower values of DHA [β = -2.42 (-4.76, -0.08)], Omega 3 Index [β = -0.50 (-0.95, -0.06)] and hippocampal volume [β = -0.22 (-0.42,-0.02)]. They also had higher values of GDF15 [β = 246.1 (88.9, 403.4)] and TNFR1 [β = 157.5 (7.8, 307.2)].

CONCLUSION

Age-related frailty and disease-related frailty may represent different degrees of frailty severity on a biological level. Further research is needed to identify biomarkers potentially able to distinguish these classifications of frailty.

Shape variability of the central sulcus in the developing brain: a longitudinal descriptive and predictive study in preterm infants

Despite growing evidence of links between sulcation and function in the adult brain, the folding dynamics, occurring mostly before normal-term-birth, is vastly unknown. Looking into the development of cortical sulci in infants can give us keys to address fundamental questions: what is the sulcal shape variability in the developing brain? When are the shape features encoded? How are these morphological parameters related to further functional development? In this study, we aimed to investigate the shape variability of the developing central sulcus, which is the frontier between the primary somatosensory and motor cortices. We studied a cohort of 71 extremely preterm infants scanned twice using MRI - once around 30 weeks post-menstrual age (w PMA) and once at term-equivalent age, around 40w PMA -, in order to quantify the sulcus's shape variability using manifold learning, regardless of age-group or hemisphere. We then used these shape descriptors to evaluate the sulcus's variability at both ages and to assess hemispheric and age-group specificities. This led us to propose a description of ten shape features capturing the variability in the central sulcus of preterm infants. Our results suggested that most of these features (8/10) are encoded as early as 30w PMA. We unprecedentedly observed hemispheric asymmetries at both ages, and the one captured at term-equivalent age seems to correspond with the asymmetry pattern previously reported in adults. We further trained classifiers in order to explore the predictive value of these shape features on manual performance at 5 years of age (handedness and fine motor outcome). The central sulcus's shape alone showed a limited but relevant predictive capacity in both cases. The study of sulcal shape features during early neurodevelopment may participate to a better comprehension of the complex links between morphological and functional organization of the developing brain.

Association between Red Blood Cells Omega-3 Polyunsaturated Fatty Acids and White Matter Hyperintensities: The MAPT Study

OBJECTIVES

The association between circulating biomarkers of red blood cells (RBC) omega-3 polyunsaturated fatty acids (PUFAs) and cerebral white matter hyperintensities (WMH) on the brain MRI remains yet unclear. We investigated the cross-sectional and prospective associations of RBC omega-3 PUFAs with WMH in dementia-free older adults with subjective memory complaints.

DESIGN

Participants were 234 older adults with assessments for both PUFA and MRI near to baseline; among them, 79 also had an MRI assessment at 3-year follow-up. The measurement of WMH volume was obtained by an automated segmentation algorithm. We related individual or combinational baseline RBC omega-3 PUFAs levels with baseline WMH volumes and WMH evolution over 3 years. We carried out multiple (cross-sectional) and mixed-effect (prospective analysis, with random effects at participant's level) linear regressions with adjustment for age, sex, time interval between date of blood draw for measurement of fatty acids and date of brain MRI, the status of APOE e4 carrier, body mass index, and vascular risk factors. Associations were considered significant at p ≤ 0.006 to take into account multiplicity (8 comparisons).

RESULTS

None of the eight RBC omega-3 PUFAs tested was significantly associated with WMH at both cross-sectional and prospective analyses.

CONCLUSIONS

We did not find any association between omega-3 PUFAs and WMH in non-demented older adults with memory complaints. A longer longitudinal study with data on omega-3 PUFAs and WMH would contribute important information to this field.

Physical Activity and Changes in White Matter Hyperintensities over Three Years

OBJECTIVES

Since physical activity (PA) has demonstrated benefits for cardiovascular health, it is possible to hypothesize that higher or increasing PA slows the progression of white matter hyperintensities (WMH). We investigated the association between PA and the progression of WMH in non-demented older adults with memory complaints.

DESIGN

We included 152 participants (mean age 74.7±3.8 years; 63.8% women) in the analyses, in whom information on self-reported PA and MRI was available at both baseline and 3-year follow-up. From the PA questionnaire, the baseline metabolic equivalent of task (MET-minute/week) and changes in MET-minute/week over three years were separately calculated for overall, leisure-time, and non-leisure time PA. WMH volume at baseline and 3-year follow-up was obtained by using an automated segmentation algorithm.

RESULTS

Mixed-effect linear regression models showed that none of the baseline PA variables was associated with progression of WMH over time. People who had decreased their PA levels over three years tended to show greater progression of WMH compared with those who had maintained PA levels of ≥1200 MET-min/week (roughly equivalent to ≥300 minutes of brisk walking) in the unadjusted model (β±SE=4.85±2.42, p=0.045); however, this association was no longer significant after adjustment for confounders (β±SE =3.63±2.18, p=0.096).

CONCLUSIONS

We did not find any significant association between PA and WMH in non-demented older adults with memory complaints. However, decrease over time in PA levels tended to be associated with progression of WMH. A larger longitudinal study with data on PA assessed using objective measures would provide important information in this field.

Evolution of the central sulcus morphology in primates

The central sulcus (CS) divides the pre- and postcentral gyri along the dorsal-ventral plane of which all motor and sensory functions are topographically organized. The motor-hand area of the precentral gyrus or KNOB has been described as the anatomical substrate of the hand in humans. Given the importance of the hand in primate evolution, here we examine the evolution of the motor-hand area by comparing the relative size and pattern of cortical folding of the CS surface area from magnetic resonance images in 131 primates, including Old World monkeys, apes and humans. We found that humans and great apes have a well-formed motor-hand area that can be seen in the variation in depth of the CS along the dorsal-ventral plane. We further found that great apes have relatively large CS surface areas compared to Old World monkeys. However, relative to great apes, humans have a small motor-hand area in terms of both adjusted and absolute surface areas.

MAPT STUDY: A MULTIDOMAIN APPROACH FOR PREVENTING ALZHEIMER'S DISEASE: DESIGN AND BASELINE DATA

OBJECTIVE

The Multidomain Alzheimer Preventive Trial (MAPT study) was designed to assess the efficacy of isolated supplementation with omega-3 fatty acid, an isolated multidomain intervention (consisting of nutritional counseling, physical exercise, cognitive stimulation) or a combination of the two interventions on the change of cognitive functions in frail subjects aged 70 years and older for a period of 3 years. Ancillary neuroimaging studies were additionally implemented to evaluate the impact of interventions on cerebral metabolism (FDG PET scans) and atrophy rate (MRIs), as well as brain amyloïd deposit (AV45 PET scans).

DESIGN PATIENTS

1680 subjects (mean age: 75.3 years; female: 64.8 %), enrolled by 13 memory clinics, were randomized into one of the following four groups: omega-3 supplementation alone, multidomain intervention alone, omega-3 plus multidomain intervention, or placebo. Participants underwent cognitive, functional and biological assessments at M6, M12, M24 and M36 visits. The primary endpoint is a change of memory function at 3 years, as assessed by the Free and Cued Selective Reminding test. All participants will be followed for 2 additional years after the 3-years intervention (MAPT PLUS extension study).

INTERVENTIONS

1/Omega-3 supplementation: two soft capsules daily as a single dose, containing a total of 400 mg docosahexaenoic acid (DHA), i.e., 800 mg docosahexaenoic acid per day, for 3 years. 2/ Multidomain intervention: collective training sessions conducted in small groups (6-8 participants) in twelve 120-minute sessions over the first 2 months (two sessions a week for the first month, and one session a week the second month) then a 60-minute session per month in the following three areas: nutrition, physical activity, and cognition until the end of the 3 years. In addition to the collective sessions, individualized preventive outpatient visits exploring possible risk factors for cognitive decline are performed at baseline, M12 and M24.

BASELINE POPULATION

For cognition, the mean MMSE at baseline was 28.1 (± 1.6). About 58% and 42% of participants had a CDR score equal to 0 and 0.5, respectively. Regarding mobility status, 200 (11.9%) had a 4-m gait speed lower or equal to 0.8 m/s. According to the Fried criteria, 673 (42.1%) participants were considered pre frail, and 51 (3.2%) frail. The red blood cell DHA content was 26.1 ± 8.1 µg/g. Five hundred and three participants underwent baseline MRI. AV45 PET scans were performed in 271 individuals and preliminary results showed that 38.0% had a cortical SUVR > 1.17, which gave an indication of significant brain amyloïd deposit. DISCUSSION: The MAPT trial is presently the first largest and longest multidomain preventive trial relevant to cognitive decline in older adults with subjective memory complaints. The multidomain intervention designed for the MAPT trial is likely to be easily implemented within the general population.

Genetics of primary cerebral gyrification: Heritability of length, depth and area of primary sulci in an extended pedigree of Papio baboons

Genetic control over morphological variability of primary sulci and gyri is of great interest in the evolutionary, developmental and clinical neurosciences. Primary structures emerge early in development and their morphology is thought to be related to neuronal differentiation, development of functional connections and cortical lateralization. We measured the proportional contributions of genetics and environment to regional variability, testing two theories regarding regional modulation of genetic influences by ontogenic and phenotypic factors. Our measures were surface area, and average length and depth of eleven primary cortical sulci from high-resolution MR images in 180 pedigreed baboons. Average heritability values for sulcal area, depth and length (h(2)(Area)=.38+/-.22; h(2)(Depth)=.42+/-.23; h(2)(Length)=.34+/-.22) indicated that regional cortical anatomy is under genetic control. The regional pattern of genetic contributions was complex and, contrary to previously proposed theories, did not depend upon sulcal depth, or upon the sequence in which structures appear during development. Our results imply that heritability of sulcal phenotypes may be regionally modulated by arcuate U-fiber systems. However, further research is necessary to unravel the complexity of genetic contributions to cortical morphology.

Primary cortical folding in the human newborn: an early marker of later functional development

In the human brain, the morphology of cortical gyri and sulci is complex and variable among individuals, and it may reflect pathological functioning with specific abnormalities observed in certain developmental and neuropsychiatric disorders. Since cortical folding occurs early during brain development, these structural abnormalities might be present long before the appearance of functional symptoms. So far, the precise mechanisms responsible for such alteration in the convolution pattern during intra-uterine or post-natal development are still poorly understood. Here we compared anatomical and functional brain development in vivo among 45 premature newborns who experienced different intra-uterine environments: 22 normal singletons, 12 twins and 11 newborns with intrauterine growth restriction (IUGR). Using magnetic resonance imaging (MRI) and dedicated post-processing tools, we investigated early disturbances in cortical formation at birth, over the developmental period critical for the emergence of convolutions (26-36 weeks of gestational age), and defined early 'endophenotypes' of sulcal development. We demonstrated that twins have a delayed but harmonious maturation, with reduced surface and sulcation index compared to singletons, whereas the gyrification of IUGR newborns is discordant to the normal developmental trajectory, with a more pronounced reduction of surface in relation to the sulcation index compared to normal newborns. Furthermore, we showed that these structural measurements of the brain at birth are predictors of infants' outcome at term equivalent age, for MRI-based cerebral volumes and neurobehavioural development evaluated with the assessment of preterm infant's behaviour (APIB).

Human subinsular asymmetry studied by diffusion tensor imaging and fiber tracking

BACKGROUND AND PURPOSE

Our aim was to improve our understanding of the subinsular white matter microstructural asymmetries in healthy right-handed subjects. Structural brain asymmetries could be related to functional asymmetries such as hemisphere language dominance or handedness. Besides the known gray matter asymmetries, white matter asymmetries could also play a key role in the understanding of hemispheric specialization, notably that of language.

MATERIALS AND METHODS

White matter asymmetries were studied by diffusion tensor imaging at 1.5T (41 diffusion-gradient directions; b-value set to 700 s/mm(2); matrix, 128(2); in-plane resolution, 1.875 x 1.875 mm; section thickness, 2.0 mm) and fiber tracking (BrainVISA software). The main white matter bundles passing through the subinsular area were segmented, and fractional anisotropy (FA) was measured along each of the segmented bundles.

RESULTS

In line with published results, we found an asymmetry of the arcuate fasciculus and the subinsular white matter, namely left-greater-than-right FA in right-handed controls. Furthermore, by segmenting major tracts coursing through this region, we showed that the subinsular portions of the uncinate fasciculus (UF) and the inferior occipitofrontal fasciculus (IOF) contribute to this FA asymmetry. Those tracts have been reported to be likely implicated in the language network.

CONCLUSION

Because the left hemisphere hosts language functions in most right-handers, the significant leftward asymmetry observed within the arcuate fasciculus, the subinsular part of the UF and IOF may be related to the hemispheric specialization for language.

Mapping the early cortical folding process in the preterm newborn brain

In the developing human brain, the cortical sulci formation is a complex process starting from 14 weeks of gestation onward. The potential influence of underlying mechanisms (genetic, epigenetic, mechanical or environmental) is still poorly understood, because reliable quantification in vivo of the early folding is lacking. In this study, we investigate the sulcal emergence noninvasively in 35 preterm newborns, by applying dedicated postprocessing tools to magnetic resonance images acquired shortly after birth over a developmental period critical for the human cortex maturation (26-36 weeks of age). Through the original three-dimensional reconstruction of the interface between developing cortex and white matter and correlation with volumetric measurements, we document early sulcation in vivo, and quantify changes with age, gender, and the presence of small white matter lesions. We observe a trend towards lower cortical surface, smaller cortex, and white matter volumes, but equivalent sulcation in females compared with males. By precisely mapping the sulci, we highlight interindividual variability in time appearance and interhemispherical asymmetries, with a larger right superior temporal sulcus than the left. Thus, such an approach, included in a longitudinal follow-up, may provide early indicators on the structural basis of cortical functional specialization and abnormalities induced by genetic and environmental factors.

Real-time MR diffusion tensor and Q-ball imaging using Kalman filtering

Magnetic resonance diffusion imaging (dMRI) has become an established research tool for the investigation of tissue structure and orientation. In this paper, we present a method for real time processing of diffusion tensor and Q-ball imaging. The basic idea is to use Kalman filtering framework to fit either the linear tensor or Q-ball model. Because the Kalman filter is designed to be an incremental algorithm, it naturally enables updating the model estimate after the acquisition of any new diffusion-weighted volume. Processing diffusion models and maps during ongoing scans provides a new useful tool for clinicians, especially when it is not possible to predict how long a subject may remain still in the magnet.

Automatic inference of sulcus patterns using 3D moment invariants

The goal of this work is the automatic inference of frequent patterns of the cortical sulci, namely patterns that can be observed only for a subset of the population. The sulci are detected and identified using brainVISA open software. Then, each sulcus is represented by a set of shape descriptors called the 3D moment invariants. Unsupervised agglomerative clustering is performed to define the patterns. A ratio between compactness and contrast among clusters is used to select the best patterns. A pattern is considered significant when this ratio is statistically better than the ratios obtained for clouds of points following a Gaussian distribution. The patterns inferred for the left cingulate sulcus are consistent with the patterns described in the atlas of Ono.

MR diffusion-based inference of a fiber bundle model from a population of subjects

This paper proposes a method to infer a high level model of the white matter organization from a population of subjects using MR diffusion imaging. This method takes as input for each subject a set of trajectories stemming from any tracking algorithm. Then the inference results from two nested clustering stages. The first clustering converts each individual set of trajectories into a set of bundles supposed to represent large white matter pathways. The second clustering matches these bundles across subjects in order to provide a list of candidates for the bundle model. The method is applied on a population of eleven subjects and leads to the inference of 17 such candidates.

Anatomically constrained surface parameterization for cortical localization

We present here a method that aims at defining a surface-based coordinate system on the cortical surface. Such a system is needed for both cortical localization and intersubject matching in the framework of neuroimaging. We propose an automatic parameterization based on the spherical topology of the grey/white matter interface of each hemisphere and on the use of naturally organized and reproducible anatomical features. From those markers used as initial constraints, the coordinate system is propagated via a PDE solved on the cortical surface.

Modulation of language areas with functional MR image-guided magnetic stimulation

Repetitive transcranial magnetic stimulation (rTMS) can interfere with linguistic performance when delivered over language areas. At low frequency (1 Hz), rTMS is assumed to decrease cortical excitability; however, the degree of TMS effect on cortical language areas may depend on the localization of the stimulation coil with respect to the inter-individual anatomo-functional variations. Hence, we aimed at investigating individual brain areas involved in semantic and phonological auditory processes. We hypothesized that active rTMS targeted over Wernicke's area might modify the performance during a language-fragment-detection task. Sentences in native or foreign languages were presented to 12 right-handed male healthy volunteers during functional magnetic resonance imaging (fMRI). 3D-functional maps localized the posterior temporal activation (Wernicke) in each subject and MRI anatomical cortical landmarks were used to define Broca's pars opercularis (F3Op). A frameless stereotaxy system was used to guide the TMS coil position over Wernicke's and F3Op areas in each subject. Active and placebo randomized rTMS sessions were applied at 1 Hz, 110% of motor threshold, during the same language-fragment-detection task. Accuracy and response time (RT) were recorded. RT was significantly decreased by active rTMS compared to placebo over Wernicke's area, and was more decreased for native than for foreign languages. No significant RT change was observed for F3Op area. rTMS conditions did not impair participants' accuracy. Thus, low-frequency rTMS over Wernicke's area can speed-up the response to a task tapping on native language perception in healthy volunteers. This individually-guided stimulation study confirms that facilitatory effects are not confined to high-frequency rTMS.

Fiber tracking in q-ball fields using regularized particle trajectories

Most of the approaches dedicated to fiber tracking from diffusion-weighted MR data rely on a tensor model. However, the tensor model can only resolve a single fiber orientation within each imaging voxel. New emerging approaches have been proposed to obtain a better representation of the diffusion process occurring in fiber crossing. In this paper, we adapt a tracking algorithm to the q-ball representation, which results from a spherical Radon transform of high angular resolution data. This algorithm is based on a Monte-Carlo strategy, using regularized particle trajectories to sample the white matter geometry. The method is validated using a phantom of bundle crossing made up of haemodialysis fibers. The method is also applied to the detection of the auditory tract in three human subjects.

Object-based morphometry of the cerebral cortex

Most of the approaches dedicated to automatic morphometry rely on a point-by-point strategy based on warping each brain toward a reference coordinate system. In this paper, we describe an alternative object-based strategy dedicated to the cortex. This strategy relies on an artificial neuroanatomist performing automatic recognition of the main cortical sulci and parcellation of the cortical surface into gyral patches. A set of shape descriptors, which can be compared across subjects, is then attached to the sulcus and gyrus related objects segmented by this process. The framework is used to perform a study of 142 brains of the International Consortium for Brain Mapping (ICBM) database. This study reveals some correlates of handedness on the size of the sulci located in motor areas, which was not detected previously using standard voxel based morphometry.

Brain Anatomy in Turner Syndrome: Evidence for Impaired Social and Spatial-Numerical Networks

Analysis of brain structure in Turner syndrome (TS) provides the opportunity to identify the consequences of the loss of one X chromosome on brain anatomy and to characterize the neural bases underlying the specific cognitive profile of TS subjects which includes deficits in spatial-numerical processing and social cognition. Fourteen subjects with TS and fourteen controls were investigated using voxel-based analysis of high resolution anatomical and diffusion tensor images and using sulcal morphometry. The analysis of anatomical images provided evidence for macroscopical changes in cortical regions involved in social cognition such as the left superior temporal sulcus and orbito-frontal cortex and in a region involved in spatial and numerical cognition such as the right intraparietal sulcus. Diffusion tensor images showed a displacement of the grey-white matter interface of the left and right superior temporal sulcus and revealed bilateral microstructural anomalies in the temporal white matter. The analysis of fiber orientation suggests specific alterations of fiber tracts connecting posterior to anterior temporal regions. Last, sulcal morphometry confirmed the anomalies of the left and right superior temporal sulci and of the right intraparietal sulcus. Our results thus provide converging evidence of regionally specific structural changes in TS that are highly consistent with the hallmark symptoms associated with TS.

Object-based strategy for morphometry of the cerebral cortex

Most of the approaches dedicated to automatic morphometry rely on a point-by-point strategy based on warping each brain towards a reference coordinate system. In this paper, we describe an alternative object-based strategy dedicated to the cortex. This strategy relies on an artificial neuroanatomist performing automatic recognition of the main cortical sulci and parcellation of the cortical surface into gyral patches. A set of shape descriptors, which can be compared across subjects, is then attached to the sulcus and gyrus related objects segmented by this process. The framework is used to perform a study of 142 brains of the ICBM database. This study reveals some correlates of handedness on the size of the sulci located in motor areas, which seem to be beyond the scope of the standard voxel based morphometry.

A primal sketch of the cortex mean curvature: a morphogenesis based approach to study the variability of the folding patterns

In this paper, we propose a new representation of the cortical surface that may be used to study the cortex folding process and to recover some putative stable anatomical landmarks called sulcal roots usually buried in the depth of adult brains. This representation is a primal sketch derived from a scale space computed for the mean curvature of the cortical surface. This scale-space stems from a diffusion equation geodesic to the cortical surface. The primal sketch is made up of objects defined from mean curvature minima and saddle points. The resulting sketch aims first at highlighting significant elementary cortical folds, second at representing the fold merging process during brain growth. The relevance of the framework is illustrated by the study of central sulcus sulcal roots from antenatal to adult age. Some results are proposed for ten different brains. Some preliminary results are also provided for superior temporal sulcus.

Visualizing the neural bases of a disconnection syndrome with diffusion tensor imaging

Disconnection syndromes are often conceptualized exclusively within cognitive box-and-arrow diagrams unrelated to brain anatomy. In a patient with alexia in his left visual field resulting from a posterior callosal lesion, we illustrate how diffusion tensor imaging can reveal the anatomical bases of a disconnection syndrome by tracking the degeneration of neural pathways and relating it to impaired fMRI activations and behavior. Compared to controls, an abnormal pattern of brain activity was observed in the patient during word reading, with a lack of activation of the left visual word form area (VWFA) by left hemifield words. Statistical analyses of diffusion images revealed a damaged fiber tract linking the left ventral occipito-temporal region to its right homolog across the lesioned area of corpus callosum and stopping close to the areas found active in fMRI. The behavioral disconnection syndrome could, thus, be related functionally to abnormal fMRI activations and anatomically to the absence of a connection between those activations. The present approach, based on the "negative tracking" of degenerated bundles, provides new perspectives on the understanding of human brain connections and disconnections.

What is the best similarity measure for motion correction in fMRI time series?

It has been shown that the difference of squares cost function used by standard realignment packages (SPM and AIR) can lead to the detection of spurious activations, because the motion parameter estimations are biased by the activated areas. Therefore, this paper describes several experiments aiming at selecting a better similarity measure to drive functional magnetic resonance image registration. The behaviors of the Geman-McClure (GM) estimator, of the correlation ratio, and of the mutual information (MI) relative to activated areas are studied using simulated time series and actual data stemming from a 3T magnet. It is shown that these methods are more robust than the usual difference of squares measure. The results suggest also that the measures built from robust metrics like the GM estimator may be the best choice, while MI is also an interesting solution. Some more work, however, is required to compare the various robust metrics proposed in the literature.

Multimodal anatomic, functional, and metabolic brain imaging for tumor resection

OBJECTIVE

Improvement of neurosurgical techniques with a more detailed description of brain tumors and their functional environment.

METHODS

We performed: (1) anatomical magnetic resonance imaging (MRI) for reference, (2) functional sequences dedicated to the adjacent cortical structures (sensorimotor, visual, language paradigms), and (3) thallium 201 cerebral tomoscintigraphy to visualize active tumor invasion. Data were transferred to a workstation for automatic registration.

RESULTS

All data were combined into one synthetic image showing the foci of high proliferative activity, which have to be completely resected, and the peritumoral functional structures, which have to be spared in order to minimize postoperative sequelae. This trimodal image is entered into a surgical neuronavigation computer for preoperative planning in order to outline tumoral target and functional risk areas. All this information is displayed in the operative microscope (Zeiss MKM) optically linked to MR images. This multimodality technique diminishes operative time by reducing electrocorticography and improves the operative short-term outcome.

CONCLUSION

Multimodal imaging is useful for optimization of neurosurgical tumor resection.

A multiresolution framework to MEG/EEG source imaging

A new method based on a multiresolution approach for solving the ill-posed problem of brain electrical activity reconstruction from electroencephaloram (EEG)/magnetoencephalogram (MEG) signals is proposed in a distributed source model. At each step of the algorithm, a regularized solution to the inverse problem is used to constrain the source space on the cortical surface to be scanned at higher spatial resolution. We present the iterative procedure together with an extension of the ST-maximum a posteriori method [1] that integrates spatial and temporal a priori information in an estimator of the brain electrical activity. Results from EEG in a phantom head experiment with a real human skull and from real MEG data on a healthy human subject are presented. The performances of the multiresolution method combined with a nonquadratic estimator are compared with commonly used dipolar methods, and to minimum-norm method with and without multiresolution. In all cases, the proposed approach proved to be more efficient both in terms of computational load and result quality, for the identification of sparse focal patterns of cortical current density, than the fixed scale imaging approach.

Role of the supplementary motor area in motor deficit following medial frontal lobe surgery

OBJECTIVE

Patients undergoing surgical resection of medial frontal lesions may present a transient postoperative deficit that remains largely unpredictable. The authors studied the role of the supplementary motor area (SMA) in the occurrence of this deficit using fMRI.

METHODS

Twenty-three patients underwent a preoperative fMRI before resection of medial frontal lesions. Tasks included self-paced flexion/extension of the left and right hand, successively. Preoperative fMRI data were compared with postoperative MRI data and with neurologic outcome.

RESULTS

Following surgery, 11 patients had a motor deficit from which all patients recovered within a few weeks or months. The deficit was similar across patients, consisting of a global reduction in spontaneous movements contralateral to the operated side with variable severity. SMA activation was observed in all patients. The deficit was observed when the area activated in the posterior part of the SMA (SMA proper) was resected.

CONCLUSIONS

fMRI is able to identify the area at risk in the SMA proper whose resection is highly related to the occurrence of the motor deficit. The clinical characteristics of this deficit support the role of the SMA proper in the initiation and execution of the movement.

Diffusion tensor imaging study of subcortical gray matter in cadasil

BACKGROUND AND PURPOSE

In cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL), water diffusion changes suggestive of microstructural tissue alterations have been recently reported in abnormal- and normal-appearing white matter as seen on T2-weighted images. In the subcortical gray matter, typical lacunar infarcts are repeatedly observed. Whether microstructural tissue changes are also present outside these lesions within the putamen or thalamus remains unknown.

METHODS

We used diffusion tensor imaging, an MRI method highly sensitive to cerebral microstructure, in 20 CADASIL patients and 12 controls. Both the trace of the diffusion tensor [Tr(D)] and an anisotropic diffusion index (volume ratio) of diffusion were measured within the putamen and thalamus outside typical lacunar infarcts as detected on both T1- and T2-weighted images.

RESULTS

A significant increase in Tr(D) and a decrease in anisotropy were observed in the putamen and thalamus in patients. The right/left indices of Tr(D) in the thalamus, but not in the putamen, were strongly correlated with the corresponding indices calculated in the white matter of the centrum semiovale. In addition, the diffusion increase in the thalamus was positively correlated with Tr(D) and with the load of small deep infarcts within the white matter and negatively correlated with the Mini-Mental State Examination score.

CONCLUSIONS

Our results suggest that microstructural tissue alterations are present in the putamen and thalamus, outside the typical lacunar infarcts in CADASIL. In the thalamus, these microstructural changes appear constant and are even observed in asymptomatic subjects. Some of these thalamic changes appear to result from degeneration of thalamocortical pathways secondary to ischemic white matter damage. The importance of this degenerative phenomenon in the pathophysiology of CADASIL requires further investigation.

Motion correction algorithms may create spurious brain activations in the absence of subject motion

This paper describes several experiments that prove that standard motion correction methods may induce spurious activations in some motion-free fMRI studies. This artifact stems from the fact that activated areas behave like biasing outliers for the difference of square-based measures usually driving such registration methods. This effect is demonstrated first using a motion-free simulated time series including artificial activation-like signal changes. Several additional simulations explore the influence of activation amplitude and extent. The effect is finally highlighted on an actual time series obtained from a 3-T magnet. All the experiments are performed using four different realignment methods, which allows us to show that the problem may be overcome by methods based on a robust similarity measure like mutual information.

Cerebral mechanisms of word masking and unconscious repetition priming

We used functional magnetic resonance imaging (fMRI) and event-related potentials (ERPs) to visualize the cerebral processing of unseen masked words. Within the areas associated with conscious reading, masked words activated left extrastriate, fusiform and precentral areas. Furthermore, masked words reduced the amount of activation evoked by a subsequent conscious presentation of the same word. In the left fusiform gyrus, this repetition suppression phenomenon was independent of whether the prime and target shared the same case, indicating that case-independent information about letter strings was extracted unconsciously. In comparison to an unmasked situation, however, the activation evoked by masked words was drastically reduced and was undetectable in prefrontal and parietal areas, correlating with participants' inability to report the masked words.

Diffusion tensor imaging: concepts and applications

The success of diffusion magnetic resonance imaging (MRI) is deeply rooted in the powerful concept that during their random, diffusion-driven displacements molecules probe tissue structure at a microscopic scale well beyond the usual image resolution. As diffusion is truly a three-dimensional process, molecular mobility in tissues may be anisotropic, as in brain white matter. With diffusion tensor imaging (DTI), diffusion anisotropy effects can be fully extracted, characterized, and exploited, providing even more exquisite details on tissue microstructure. The most advanced application is certainly that of fiber tracking in the brain, which, in combination with functional MRI, might open a window on the important issue of connectivity. DTI has also been used to demonstrate subtle abnormalities in a variety of diseases (including stroke, multiple sclerosis, dyslexia, and schizophrenia) and is currently becoming part of many routine clinical protocols. The aim of this article is to review the concepts behind DTI and to present potential applications.

Detection of fMRI activation using cortical surface mapping

A methodology for fMRI data analysis confined to the cortex, Cortical Surface Mapping (CSM), is presented. CSM retains the flexibility of the General Linear Model based estimation, but the procedures involved are adapted to operate on the cortical surface, while avoiding to resort to explicit flattening. The methodology is tested by means of simulations and application to a real fMRI protocol. The results are compared with those obtained with a standard, volume-oriented approach (SPM), and it is shown that CSM leads to local differences in sensitivity, with generally higher sensitivity for CSM in two of the three subjects studied. The discussion provided is focused on the benefits of the introduction of anatomical information in fMRI data analysis, and the relevance of CSM as a step toward this goal.

Evaluation of inverse methods and head models for EEG source localization using a human skull phantom

We used a real-skull phantom head to investigate the performances of representative methods for EEG source localization when considering various head models. We describe several experiments using a montage with current sources located at multiple positions and orientations inside a human skull filled with a conductive medium. The robustness of selected methods based on distributed source models is evaluated as various solutions to the forward problem (from the sphere to the finite element method) are considered. Experimental results indicate that inverse methods using appropriate cortex-based source models are almost always able to locate the active source with excellent precision, with little or no spurious activity in close or distant regions, even when two sources are simultaneously active. Superior regularization schemes for solving the inverse problem can dramatically help the estimation of sparse and focal active zones, despite significant approximation of the head geometry and the conductivity properties of the head tissues. Realistic head models are necessary, though, to fit the data with a reasonable level of residual variance.

Temporal lobe dysfunction in childhood autism: a PET study. Positron emission tomography

OBJECTIVE

The nature of the underlying brain dysfunction of childhood autism, a life-long severe developmental disorder, is not well understood. Although researchers using functional brain imaging have attempted to contribute to this debate, previous studies have failed to report consistent localized neocortical brain dysfunction. The authors reasoned that early methods may have been insensitive to such dysfunction, which may now be detectable with improved technology.

METHOD

To test this hypothesis, regional cerebral blood flow was measured with positron emission tomography (PET) in 21 children with primary autism and in 10 nonautistic children with idiopathic mental retardation. Autistic and comparison groups were similar in average age and developmental quotients. The authors first searched for focal brain dysfunction in the autistic group by using a voxel-based whole brain analysis and then assessed the sensitivity of the method to detect the abnormality in individual children. An extension study was then performed in an additional group of 12 autistic children.

RESULTS

The first autistic group had a highly significant hypoperfusion in both temporal lobes centered in associative auditory and adjacent multimodal cortex, which was detected in 76% of autistic children. Virtually identical results were found in the second autistic group in the extension study.

CONCLUSIONS

PET and voxel-based image analysis revealed a localized dysfunction of the temporal lobes in school-aged children with idiopathic autism. Further studies will clarify the relationships between these temporal abnormalities and the perceptive, cognitive, and emotional developmental abnormalities characteristic of this disorder.

Structural group analysis of functional activation maps

We present here a new method for cerebral activation detection over a group of subjects. This method is performed using individual activation maps of any sort. It aims at processing a group analysis while preserving individual information and at overcoming as far as possible limitations of the spatial normalization used to compare different subjects. We designed it such that it provides the individual occurrence of the activations detected at a group level. The localization can then be performed on the individual anatomy of each subject. The analysis starts with a hierarchical multiscale object-based description of each individual map. These descriptions are then compared, rather than comparing the images directly. The analysis is thus performed at an object level instead of voxel by voxel. It is made using a comparison graph, on which a labeling process is performed. The label field on the graph is modeled by a Markov random field, which allows us to introduce high-level rules of interrogation of the data. The process has been evaluated on simulated data and real data from a PET protocol.

Clinical severity in CADASIL related to ultrastructural damage in white matter: in vivo study with diffusion tensor MRI

BACKGROUND AND PURPOSE

CADASIL is a newly recognized cause of subcortical ischemic strokes that progressively leads to dementia associated with pseudobulbar palsy and severe motor disability. This deleterious progression and the severity of clinical presentation are widely variable among affected subjects. The exact role played by MRI white-matter abnormalities, a hallmark of the disease, in the severity of the clinical phenotype remains poorly understood.

METHODS

To address this issue, we used diffusion tensor imaging (DTI), a new MRI technique highly sensitive to white-matter microstructural changes, in 16 symptomatic patients and 10 age-matched controls. Mean diffusivity and anisotropy of diffusion were measured within hyperintensities identified on T2-weighted images (T2WI) and outside these lesions on 4 slices at the level of centrum semiovale.

RESULTS

We found a 60% increase of water mean diffusivity and a parallel loss of diffusion anisotropy in hyperintensities identified on T2WI. The same pattern of diffusion changes, but of lesser intensity, was found in the normal-appearing white matter on T2WI. Mean diffusivity in regions with increased signal on T2WI was higher in patients with severe clinical disability compared with those with no or mild deficit (1.33+/-0.11 versus 1.13+/-0.11 10(-3) mm(2)/s, P<0.01). Furthermore, diffusion measured within T2 hyperintensities correlated with both the Mini-Mental State Examination and Rankin scale scores. In patients with a severe clinical status, the increase of water diffusion in these regions exceeded 70% in comparison with values obtained in the normal white matter in control subjects.

CONCLUSIONS

These results indicate that DTI is able to detect important ultrastructural changes in regions with increased signal on T2WI and within the normal-appearing white matter in CADASIL. The diffusion changes might be related to both neuronal loss and demyelination. The degree of the underlying ultrastructural alterations is related to the severity of the clinical status with a possible threshold level of white-matter damage above which severe neurological impairment may occur in this disease. DTI appears to be a promising technique for monitoring disease progression in CADASIL.

Use of subtraction ictal SPECT co-registered to MRI for optimizing the localization of seizure foci in children

Ictal SPECT studies are increasingly used to localize seizure foci in children with refractory epilepsy, but few studies have reported on ictal-interictal subtraction images co-registered to MRI at this age.

METHODS

Twenty-seven children with partial epilepsy (aged 3 mo-18 y) underwent ictal ethyl cysteinate dimer (ECD) SPECT (20 mCi/1.73 m2) combined with video-electroencephalography (EEG) and interictal ECD SPECT followed 2 d later by three-dimensional MRI. Ictal-interictal and interictal-ictal subtraction images were computed by registering and normalizing the ictal to the interictal SPECT scans for each child. The ictal, interictal SPECT and subtraction images were registered to each child's MRI. Difference images (ictal-interictal) were then superimposed on MRI for anatomic localization of the perfusion changes. Intra- and interobserver reproducibility and "facility of interpretation" of overlay images were compared with standard analysis of the non-coregistered ictal and interictal scans.

RESULTS

Overlay images allowed the detection of at least one hyperperfused focus in 93% of the children, compared with 74% using ictal and interictal scans separately. Seizure onset was suspected clinically, on EEG or on MRI in 20 children. Overlay images were concordant (n = 11) or larger (n = 7) than the suspected focus in 18 of 20 (90%), whereas these images failed to show any abnormality in 1 child and were discordant with MRI in another patient. In the remaining 7, images showed cortical localization in 6 patients. Among the 5 patients who underwent electrocorticography, overlay images were concordant in 3, larger in 1 and absent in 1. The intra- and interobserver reproducibility and facility of interpretation were significantly higher using overlay images than standard analysis, even when ictal and interictal SPECT were co-registered.

CONCLUSION

The co-registration of ictal-interictal subtraction SPECT images to MRI seems to be a helpful technique in localizing the onset of seizure and guiding the intracranial recording in childhood epilepsy. Moreover, this method improves sensitivity, enhances intra- and interobserver reproducibility and makes interpretation easier.

Effect of selenium on rat thioredoxin reductase activity: increase by supranutritional selenium and decrease by selenium deficiency

Thioredoxin reductase is a newly identified selenocysteine-containing enzyme that catalyzes the NADPH-dependent reduction of the redox protein thioredoxin. Thioredoxin stimulates cell growth, is found in dividing normal cells, and is over-expressed in a number of human cancers. Redox activity is essential for the growth effects of thioredoxin; thus, thioredoxin reductase could be involved in regulating cell growth through its reduction of thioredoxin. In rats fed a selenium-deficient diet (<0.01 ppm) for up to 98 days, thioredoxin reductase activity was decreased, compared with that of rats fed a normal selenium diet (0.1 ppm), in lung, liver, and kidney, while thioredoxin reductase activity in the spleen and prostate was unaltered. Rats fed a high selenium diet (1.0 ppm) exhibited a 1.5-fold increase in kidney and a 2.0-fold increase in lung thioredoxin reductase activity that began to return to control values after 20 and 69 days, respectively. Liver showed a 2.1-fold increase in thioredoxin reductase activity at 20 days only. Thioredoxin reductase protein levels measured by western blotting using an antibody to human thioredoxin reductase were decreased in rats fed the selenium-deficient diet and did not increase in rats fed the high selenium diet. Rat thioredoxin reductase was shown to incorporate 75Selenium. Thus, in some tissues at least, the increase in thioredoxin reductase activity of rats fed a high selenium diet appears to be due to an increase in the specific activity of the enzyme, possibly caused by increased selenocysteine incorporation without an increase in thioredoxin reductase protein synthesis.

Three-dimensional reconstruction of the human central sulcus reveals a morphological correlate of the hand area

One way to improve our understanding of cortical anatomy is to visualize the three-dimensional (3D) shape of the cerebral sulci which is normally hidden. Here, we reconstructed the 3D morphology of the central sulcus (CS) in 17 normal subjects, using conventional magnetic resonance images and dedicated software. We found that the 3D morphology was remarkably consistent in all central sulci. Our analyses revealed three different regions (upper, middle and lower), which were easily identifiable by morphological criteria and sharply interconnected in the reconstructed CS. These morphological regions appear to have a strong functional significance, since the middle region corresponded precisely to the 'hand area', as verified by hand vibration positron emission tomography activation studies in eight cases. These data suggest that the 3D anatomy of the cerebral cortex may facilitate sulcal recognition, and sulcal subdivision into smaller morphological elements, bearing remarkable relationships with functional cortical maps.

In vivo imaging of human cerebral acetylcholinesterase

We report here the first positron emission tomography (PET) images showing the in vivo regional distribution of acetylcholinesterase (AChE) in human brain. The study was carried out in eight healthy human volunteers using as a tracer [11C]-physostigmine ([11C]PHY), an inhibitor of AChE. After intravenous injection of [11C]PHY, radioactivity was rapidly taken up in brain tissue and reached maximal uptake within a few minutes, following a regional pattern mostly related to cerebral perfusion. After the peak, the cerebral radioactivity gradually decreased with a half-life varying from 20 to 35 min, depending on the brain structure. [11C] PHY retention was higher in regions rich in AChE, such as the striatum (half-life, 35 min), than in regions poor in AChE, such as the cerebral cortex (half-life, 20 min). At later times (25-35 min postinjection), the cerebral distribution of [11C]PHY was typical of AChE activity: putamen-caudate > cerebellum > brainstem > thalamus > cerebral cortex, with a striatal to cortex ratio of 2. These results suggest that PET studies with [11C]PHY can provide in vivo brain mapping of human AChE and are promising for the study of changes in AChE levels associated with neurodegenerative diseases.

Clinical correlates of [18F]fluorodopa uptake in five grafted parkinsonian patients

To assess the efficacy of fetal mesencephalic grafts in Parkinson's disease, it is important to know if the grafted cells survive and are functional. Positron emission tomography (PET) and [18F]fluorodopa ([18F]dopa) have been used to demonstrate the survival of the grafted cells, but the relationship of [18F]dopa uptake changes in the grafted striatum to motor function remains unclear. We investigated this question with 16 serial PET scans in 5 severe parkinsonian patients unilaterally grafted in whom we found a significant and progressive increase of the [18F]dopa uptake in the grafted putamen. The number of patients was too small to assess the sensitivity of [18F]dopa PET scans in individual patients. Yet, by analyzing the 16 serial PET scans we found a correlation between the [18F]dopa uptake (Ki) in the grafted putamen and the percentage of daily time spent "on," suggesting that Ki changes have a functional meaning. In addition, the Ki values were correlated with the contralateral finger dexterity to the same extent in both the grafted and nongrafted putamen. These results indicate that [18F]dopa uptake reflects the motor function of the opposite side of the body, similarly in the grafted and ungrafted putamen, at least in terms of these tasks. Finally, extrapolating from these correlations offers the suggestion that clinical optimal results of the graft could be achieved if the graft brings the Ki values in the putamen to about two standard deviations of mean control values.

Is interictal temporal hypometabolism related to mesial temporal sclerosis? A positron emission tomography/magnetic resonance imaging confrontation

The mechanism of interictal glucose hypometabolism remains unclear, but this abnormality occurs more frequently in temporal lobe epilepsy (TLE) than in other types of partial epilepsy. Therefore temporal hypometabolism has been suggested to reflect mesial temporal sclerosis (MTS). To investigate this, we selected 22 patients with refractory partial epilepsy of mesial temporal lobe origin (MTLE) who had hippocampal atrophy based on magnetic resonance imaging (MRI) volumetric analysis. We then analyzed the metabolic correlates of unilateral hippocampal atrophy. Thirteen temporal regions of interest (ROI) were defined on MRI scans for each individual and then applied to high-resolution FDG-positron emission tomography (PET) images obtained parallel to the long axis of the hippocampus. The most hypometabolic regions were the temporal pole and the hippocampal region. When we analyzed ensembles of temporal regions grouped into related networks, the temporolimbic network, which included the hippocampal region and the temporal pole, was abnormal in 95% of the patients at a 3-SD threshold. PET hypometabolism was highly correlated with the degree of hippocampal atrophy in this network, but not in other parts of the temporal lobe, which were less frequently hypometabolic. These data indicate that hypometabolism is a consequence of MTS in the temporolimbic region but not necessarily in the other parts of the temporal lobe. Our results also suggest that the combination of PET and MRI may facilitate the noninvasive diagnosis of MTLE.

Generic model for the localization of the cerebral cortex and preoperative multimodal integration in epilepsy surgery

In cerebral cortex neurosurgery and particularly in epilepsy surgery, the recognition of individual cortical anatomic landmarks is necessary for presurgical evaluation and planning. To overcome the classical great variability in the superficial aspect of the human cerebral cortex, we have developed an original generic description of the cerebral cortex in a three-dimensional structure. The result is a generic model matching the cortical topography of each brain regardless of inter- and intra-individual variability. In this model, sulcal roots (fundi) are separated by more or less superficial annectent gyrus, and buried by operculization during the gyrification. This original generic model of the cerebral cortex enabled us to develop a mathematical methodology for the construction of a structural representation of the cortical topography from a T1-weighted three-dimensional MR image. This program can provide automatic detection and recognition of the main cortical sulci.

Fast nonsupervised 3D registration of PET and MR images of the brain

We propose a fully nonsupervised methodology dedicated to the fast registration of positron emission tomography (PET) and magnetic resonance images of the brain. First, discrete representations of the surfaces of interest (head or brain surface) are automatically extracted from both images. Then, a shape-independent surface-matching algorithm gives a rigid body transformation, which allows the transfer of information between both modalities. A three-dimensional (3D) extension of the chamfer-matching principle makes up the core of this surface-matching algorithm. The optimal transformation is inferred from the minimization of a quadratic generalized distance between discrete surfaces, taking into account between-modality differences in the localization of the segmented surfaces. The minimization process is efficiently performed via the precomputation of a 3D distance map. Validation studies using a dedicated brain-shaped phantom have shown that the maximum registration error was of the order of the PET pixel size (2 mm) for the wide variety of tested configurations. The software is routinely used today in a clinical context by the physicians of the Service Hospitalier Frédéric Joliot (> 150 registrations performed). The entire registration process requires approximately 5 min on a conventional workstation.