G. Salamon

Construction of a 3D probabilistic atlas of human cortical structures

We describe the construction of a digital brain atlas composed of data from manually delineated MRI data. A total of 56 structures were labeled in MRI of 40 healthy, normal volunteers. This labeling was performed according to a set of protocols developed for this project. Pairs of raters were assigned to each structure and trained on the protocol for that structure. Each rater pair was tested for concordance on 6 of the 40 brains; once they had achieved reliability standards, they divided the task of delineating the remaining 34 brains. The data were then spatially normalized to well-known templates using 3 popular algorithms: AIR5.2.5s nonlinear warp (Woods et al., 1998) paired with the ICBM452 Warp 5 atlas (Rex et al., 2003), FSLs FLIRT (Smith et al., 2004) was paired with its own template, a skull-stripped version of the ICBM152 T1 average; and SPM5s unified segmentation method (Ashburner and Friston, 2005) was paired with its canonical brain, the whole head ICBM152 T1 average. We thus produced 3 variants of our atlas, where each was constructed from 40 representative samples of a data processing stream that one might use for analysis. For each normalization algorithm, the individual structure delineations were then resampled according to the computed transformations. We next computed averages at each voxel location to estimate the probability of that voxel belonging to each of the 56 structures. Each version of the atlas contains, for every voxel, probability densities for each region, thus providing a resource for automated probabilistic labeling of external data types registered into standard spaces; we also computed average intensity images and tissue density maps based on the three methods and target spaces. These atlases will serve as a resource for diverse applications including meta-analysis of functional and structural imaging data and other bioinformatics applications where display of arbitrary labels in probabilistically defined anatomic space will facilitate both knowledge-based development and visualization of findings from multiple disciplines.

Diffusion tensor MRI shows abnormal brainstem crossing fibers associated with ROBO3 mutations

Horizontal gaze palsy with progressive scoliosis (HGPPS) is caused by mutations in the ROBO3 gene, critical for the crossing of long ascending medial lemniscal and descending corticospinal tracts in the medulla. Diffusion tensor imaging in a patient with HGGPS revealed the absence of major pontine crossing fiber tracts and no decussation of the superior cerebellar peduncles. Mutations in the ROBO3 gene lead to a widespread lack of crossing fibers throughout the brainstem.

L'imagerie de susceptibilité magnétique : théorie et applications

Resume L’imagerie de susceptibilite magnetique (ou SWI pour Susceptibility Weighted Imaging) est une nouvelle technique d’imagerie par resonance magnetique exploitant l’effet BOLD et les differences de susceptibilite magnetique entre tissus. Elle utilise une sequence 3D en echo de gradient caracterisee d’une part, par un temps d’echo long, choisi de facon a annuler de facon optimale le signal des veines et d’autre part un post-traitement specifique faisait intervenir les images de phase comme source de contraste supplementaire. Elle est particulierement utile pour la detection des veines normales ou pathologiques. Elle detecte les hemorragies, notamment celles de petite abondance mieux que les sequences en echo de gradient conventionnel. Son utilisation est encore limitee du fait d’un temps d’acquisition long et de la persistance d’artefacts.

The human cerebral cortex on MRI: value of the coronal plane

The evaluation of different cortical areas of the cerebral cortex has been analyzed using MRI of 50 normal subjects without any neurological symptoms. This analysis has been made with different spin echo and gradient echo in T1 or T2 in three different planes: horizontal, sagittal and coronal. The most accurate plane to define important cortical areas such as Broca area, Wernicke area, temporal cortex at the level of the superior temporal sulcus, angular gyrus, supra marginal gyrus, hippocampal and parahippocampal cortices as well as that of the parieto or temporo –occipital areas is the coronal plane. Evidently it must be correlated with the other orthogonal planes. To be compared with the main Atlas of Neuroanatomy these sections must be perpendicular or parallel to the plane passing through the anterior and posterior commissures. MRI of patients with neurological disorders must have, as a routine, a series of MR sections performed in the coronal plane, as well as in horizontal and sagittal ones. The coronal plane is certainly the most precise to evaluate these areas involved in language, memory, visuo spatial or behavioral functions. It must be always compared with the rest of the neuroradiological examination and correlated with the clinical neurological signs.

L’exploration en IRM des démences de type Alzheimer. Que peut-on voir sur un examen de routine ?

Resume Les IRM de 20 patients diagnostiques comme etant atteints d’une demence de type Alzheimer ont ete comparees a des examens normaux de reference. L’examen IRM conventionnel etait base sur la lecture de coupes frontales, sur l’analyse de ce plan aux autres plans orthogonaux en utilisant une methode d’examen simultanee des trois plans de l’espace. Le plan de reference choisi etait le plan commissural. Toutes les coupes coronales etaient perpendiculaires au plan CA-CP tel qu’il a ete defini par Talairach et Tournoux (1988). Dans les demences de type Alzheimer, les lesions etaient toujours asymetriques et interessaient le complexe amygdalo-hippocampique. D’autres lesions tres nettes etaient visibles au niveau du cortex dorso-frontal, au niveau des aires parietales correspondant au gyrus angulaire et au lobule parietal superieur, au niveau du lobe temporal a la jonction T1-T2, au niveau du cortex insulaire. Il existait dans tous les cas une integrite morphologique de l’aire sensori-motrice et de l’aire visuelle primaire.