KEYWORDS: Education and training, Double patterning technology, Data modeling, Magnetic resonance imaging, Voxels, Mathematical optimization, Brain, 3D modeling, Databases, 3D image processing
The human cerebral cortex has many bumps and grooves called gyri and sulci. Even though there is a high interindividual consistency for the main cortical folds, this is not the case when we examine the exact shapes and details of the folding patterns. Because of this complexity, characterizing the cortical folding variability and relating them to subjects’ behavioral characteristics or pathologies is still an open scientific problem. Classical approaches include labeling a few specific patterns, either manually or semi-automatically, based on geometric distances, but the recent availability of MRI image datasets of tens of thousands of subjects makes modern deep-learning techniques particularly attractive. Here, we build a self-supervised deep-learning model to detect folding patterns in the cingulate region. We train a contrastive self-supervised model (SimCLR) on both Human Connectome Project (1101 subjects) and UKBioBank (21070 subjects) datasets with topological-based augmentations on the cortical skeletons, which are topological objects that capture the shape of the folds. We explore several backbone architectures (convolutional network, DenseNet, and PointNet) for the SimCLR. For evaluation and testing, we perform a linear classification task on a database manually labeled for the presence of the ”double-parallel” folding pattern in the cingulate region, which is related to schizophrenia characteristics. The best model, giving a test AUC of 0.76, is a convolutional network with 6 layers, a 10-dimensional latent space, a linear projection head, and using the branch-clipping augmentation. This is the first time that a self-supervised deep learning model has been applied to cortical skeletons on such a large dataset and quantitatively evaluated. We can now envisage the next step: applying it to other brain regions to detect other biomarkers. The GitHub repository is publicly available on https://github.com/neurospin-projects/2022 jchavas cingulate inhibitory control.
This paper presents a new cortical parcellation method based on group-wise connectivity and hierarchical clustering. A preliminary sub-parcellation is performed using intra-subject and inter-subject fiber clustering to obtain representative bundles among subjects with similar shapes and trajectories. The sub-parcellation is obtained by intersecting fiber clusters with cortical meshes. Next, mean connectivity and mean overlap matrices are computed over the sub-parcels to obtain spatial and connectivity information. To hierarchize the information, we propose to weight both matrices, to obtain an affinity graph, and then a dendrogram to merge or divide parcels by their hierarchy. Finally, to obtain homogeneous parcels, the method computes morphological operations. By selecting a different number of clusters over the dendrogram, the method obtains a different number of parcels and a variation in the resulting parcel sizes, depending on the parameters used. We computed the coefficient of variation (CV ) of the parcel size to evaluate the homogeneity of the parcels. Preliminary results suggest that the use of representative clusters and the integration of sub-parcel overlap and connectivity strength provide useful information to generate cortical parcellations at different levels of granularity. Even results are preliminary, this novel method allows researchers to add group-wise connectivity strength and spatial information for the construction of diffusion-based parcellations. Future work will include a detailed analysis of parameters, such as the matrix weights and the number of sub-parcel clusters, and the generation of hierarchical parcellations to improve the insight into the cortex subdivision and hierarchy among parcels.
Currently, there are many methods for processing diffusion MRI (dMRI) tractography data, with the aim to identify the main white matter connections. However, methods like fiber clustering lack ground truth, making the evaluation of the effectiveness of different clustering algorithms problematic. An alternative to evaluate the performance and test the efficacy of these algorithms is to use simulated fiber datasets. Nevertheless, the simulation of this data is not trivial due to brain fibers’ irregular and complex shape. Although many fiber bundle simulators exist, they have been developed for other purposes, such as validating tractography algorithms or local diffusion models. In addition, these simulators usually use simple fiber bundle configurations without considering complex bundle shapes. With this in mind, the main goal of this work is to implement a simulator of brain fiber bundles based on exponential curves for validating fiber clustering methods. This representation uses bundle centroids and shape parameters to obtain a more realistic appearance of the fascicles. The simulator was validated using a deep white matter fiber bundle atlas, obtaining a good percentage of intersection between the original and simulated bundles, of up to 82%. Furthermore, we used groups of simulated bundles for the whole brain to evaluate the performance of a fiber clustering algorithm (QuickBundles) when using different distance thresholds, showing the utility of the proposed simulator.
The description of the superficial white matter (SWM) functional and structural organization is still an unachieved task. In particular, their shape has not been assessed in detail using diffusion Magnetic Resonance Imaging (dMRI) tractography. This work aims to characterize the different shapes of the short-range association connections present in an SWM multi-subject bundle atlas derived from probabilistic dMRI tractography datasets. First, we calculated a representative centroid shape for each atlas bundle. Next, we computed a distance matrix that encodes the similarity between every pair of centroids. For the distance matrix computation, centroids were first aligned using a streamline-based registration, reducing the 3D spatial separation effect and allowing us to focus only on shape differences. Then, we applied a hierarchical clustering algorithm over the affinity graph derived from the distance matrix. As a result, we obtained ten classes with distinctive shapes, ranging from a straight line form to U and C arrangements. The most predominant shapes were: (i) short open U, (ii) short closed U, and (iii) short C. Moreover, we used the shape information to filter out noisy streamlines in the atlas bundles and applied an automatic segmentation algorithm to 25 subjects of the HCP database. Our results show that the filtering steps help to segment more dense bundles with fewer outliers, improving the identification of the brain’s short fibers.
Parkinson's disease (PD) is a progressive neurodegenerative disorder in which patients show progressively worsening motor symptoms, often followed by cognitive impairment and dementia. Brain MRI can be used to identify patterns of neurodegeneration that are characteristic of PD, but the spatial pattern of brain abnormalities is still not well understood. “Sulcus-based morphometry” provides measures of the cortical fissures of the brain that reflect degenerative changes in relation to neuropsychiatric disease. Extracting sulci requires good contrast between the gray matter and the CSF, and less well-defined sulci may be difficult to extract reliably. Before embarking on a study of sulcal abnormalities in PD, we set out to determine the reliability of measures from 123 sulci, defined by an existing atlas, using publicly available test-retest data from 8 cohorts. Of the 123 atlas-defined sulci, several major sulci were broken down into smaller regions (e.g., the superior temporal sulcus was divided into the main STS, the anterior terminal ascending branch of STS and the posterior terminal ascending branch of STS); we assessed reliability in each individually, and after merging the portions of the sulci together, in a newly defined, concatenated atlas. For 467 subjects from the PPMI cohort (http://www.ppmiinfo. org ;age range: 61.5 ± 10.1 years), we segmented and labeled major sulci and extracted 4 shape descriptors for each: length, depth, surface area, and width. We then aimed to establish the profile of case-control differences for 3 candidate sulci of interest: the central sulcus, superior temporal sulcus and the calcarine fissure. These sulci were among the more robust in terms of reproducibility; we found that the calcarine width was associated with PD, offering new features for genetic and interventional studies of PD.
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