The consequence of fiber orientation downsampling on the computation of white matter tract-related deformation

Incorporating neuroimaging-revealed structural details into finite element (FE) head models opens vast opportunities to understand brain injury mechanisms. Recently, growing efforts have been made to integrate the fiber orientation from diffusion tensor imaging into the FE models to compute white matter (WM) tract-related deformation. Commonly used approaches often downsample the spatially enriched fiber orientation to match the resolution of FE meshes, resulting in an element-wise orientation implementation. However, the validity of downsampling and the consequences on the computed tract-related strains remain elusive. To address this problem, the current study proposed a new voxel-wise approach to integrate fiber orientation into FE models without downsampling. By setting the voxel-wise orientation responses as the reference, we then evaluated the reliability of two existing downsampling approaches on tract-related strains using two FE models with varying element sizes. The results showed that, for a model with a large mesh-image resolution dismatch, the downsampling orientation exhibited an absolute difference over 30 degree across the WM/gray matter interface and pons regions and further negatively affects the computation of tract-related strains with the normalized root-mean-square error up to 20% and peaking tract-related strains underestimated by 5%. This downsampling-induced effect was lower in FE models with finer meshes. Thus, this study yields insights on integrating neuroimaging-revealed fiber orientation into FE models and may better inform the computation of WM tract-related deformation, which are crucial for advancing the etiological understanding and computational predictability of brain injury.

Population-Based Tract-To-Region Connectome of the Human Brain and Its Hierarchical Topology

Connectome maps region-to-region connectivities but does not inform which white matter pathways form the connections. Here we constructed the first population-based tract-to-region connectome to fill this information gap. The constructed connectome quantifies the population probability of a white matter tract innervating a cortical region. The results show that ~85% of the tract-to-region connectome entries are consistent across individuals, whereas the remaining (~15%) have substantial individual differences requiring individualized mapping. Further hierarchical clustering on cortical regions revealed their parcellations into dorsal, ventral, and limbic networks based on the tract-to-region connective patterns. The clustering results on white matter bundles revealed the connectome-based categorization of fiber bundle systems in the association pathways. This new tract-to-region connectome provides insights into the connective topology between cortical regions and white matter bundles. The derived hierarchical relation further offers a connectome-based categorization of gray matter and white matter structures.

Loneliness is linked to specific subregional alterations in hippocampus-default network co-variation

Social interaction complexity makes humans unique. But in times of social deprivation this strength risks to expose important vulnerabilities. Human social neuroscience studies have placed a premium on the default network (DN). In contrast, hippocampus (HC) subfields have been intensely studied in rodents and monkeys. To bridge these two literatures, we here quantified how DN subregions systematically co-vary with specific HC subfields in the context of subjective social isolation (i.e., loneliness). By co-decomposition using structural brain scans of ~40,000 UK Biobank participants, loneliness was specially linked to midline subregions in the uncovered DN patterns. These association cortex signatures coincided with concomitant HC patterns implicating especially CA1 and molecular layer. These patterns also showed a strong affiliation with the fornix white-matter tract and the nucleus accumbens. In addition, separable signatures of structural HC-DN co-variation had distinct associations with the genetic predisposition for loneliness at the population level.

White Matter Tract Disruptions Predict Less Affected Hand Impairment Following Stroke: A Longitudinal Diffusion MRI Study

Although less-affected hand (LAH) deficits following unilateral stroke are well documented, many aspects of LAH impairment mechanisms remain unresolved. To provide a better understanding of these mechanisms, we used diffusion MRI to examine the disruptions of white matter structural connections. Based on the redundancy theory, we hypothesized that a summation of motor-related tract disruptions would characterize LAH impairment. We assessed LAH impairment and fractional anisotropy (FA) in 28 patients at one-month post-stroke (baseline), and 6 and 24 months later. LAH impairment was assessed with the Purdue Pegboard Test (PPT), handgrip strength, and movement time. FA was estimated in the CST, Anterior- Corona Radiata (ACR), and Limb of Internal Capsule (ALIC), Superior Longitudinal Fasciculus (SLF), and corpus callosum (CC). We used Linear Mixed Models to determine the tracts associated with LAH impairment over time. Baseline PPT, grip, and movement time were impaired in 43%, 61%, and 25%, respectively. PPT was modeled by baseline ipsilesional-CST (t=3.75; p<0.001), ipsilesional-SLF (t=3.19; p=0.002), contralesional-ALIC (t=-4.89; p<0.001), and lesion volume (t=-3.18; p=0.004); handgrip by baseline ipsilesional-CST (t=3.39; p=0.001), contralesional-ALIC (t=-3.91; p<0.001) and sex (t=-1.43; p=0.007); movement time by baseline ipsilesional-SLF (t=-3.64; p=0.001), CC (t=4.00; p=<0.001), and lesion volume (t=3.03; p=0.006). In conclusion, white matter tract disruptions determine the LAH impairment profile, with ipsilesional-CST related to motor and ipsilesional-SLF to visuomotor processing. LAH impairment was associated with the summation of several tract disruptions, supporting the concept of cerebral redundancy. These results provide a theoretical basis for integrating LAH in rehabilitation programs and for treatment interventions such as neuromodulation.

Abstract 1122‐000191: White Matter Tract Integrity After Vascular Insult: Longitudinal Analysis of Hemorrhagic vs Ischemic Lesions

Introduction : White matter tract (WMT) injury occurs in patients with acute cerebrovascular disorders. In this study, we elucidate longitudinal differences in mechanism of injury and repair between ischemic stroke (ISC) and intracerebral hemorrhage (ICH). Methods : Twenty patients (10 ISC and ICH) were prospectively imaged at 1, 3, and 12 months of onset on a 3T MRI. 3D anatomical and DTI images were obtained and integrity of the corticospinal tract (CST) assessed at the ipsi and contralesional posterior limb of internal capsule (PLIC). Fractional anisotropy (FA), mean diffusivity (MD) and pixel volume were recorded. A linear regression model was applied for statistical analysis. Results : ISC group had 4 men, 6 women whereas ICH group had 7 men, 3 women, both with average age 52. Baseline NIHSS in ISC was 11 (IQR = 4.5–20) and ICH 6 (IQR = 2‐13). All lesions were unilateral, hemispheric, completely subcortical or with a significant subcortical component. The average lesion and hematoma volume at 1 month was 37 and 39 cc in ISC and ICH, respectively. The MD in the PLIC of the ISC increased from 1 to 3m (P <0.05) then plateaued, whereas it decreased in ICH over the entire 12m (Fig 1A). The rFA showed a similar pattern of initial injury and then improvement over time in both ISC and ICH (Fig 1B). The ISC group showed 12% WM atrophy in the PLIC at 12m, wheras 13% expansion (P < 0.05) in ICH over this period, after an initial contraction of 14% at 1m (fig 1C‐D). Structural changes of the PLIC correlated with changes in mRS/NIHSS (p<0.05). Conclusions : ISC and ICH display unique patterns of WMT changes over one year in which ICH injury reflects a compression of the CST that resolves over time, while in ISC our data show degeneration and microstructural injury. These changes reflect different mechanisms of injury and remodeling on a cellular level. A better understanding of these changes could improve recovery therapies. Larger studies are needed to better characterize long term WMT changes in IS and ICH.

FiberNeat: unsupervised streamline clustering and white matter tract filtering in latent space

Whole-brain tractograms generated from diffusion MRI digitally represent the white matter structure of the brain and are composed of millions of streamlines. Such tractograms can have false positive and anatomically implausible streamlines. To obtain anatomically relevant streamlines and tracts, supervised and unsupervised methods can be used for tractogram clustering and tract extraction. Here we propose FiberNeat, an unsupervised streamline clustering and tract filtering method. FiberNeat takes an input set of streamlines that could either be unlabeled clusters or labeled tracts. Individual clusters/tracts are projected into a latent space using nonlinear dimensionality reduction techniques, such as t-SNE and UMAP, to find spurious and outlier streamlines. In addition, outlier streamline clusters are detected using DBSCAN and then removed from the data in streamline space. Quantitative comparisons with expertly delineated tracts show the promise of the approach. This approach can be deployed as a filtering step after tracts are extracted.

Neurovisualization features of brain anatomy in children with spastic cerebral palsy revealed by magnetic resonance tractography

Aim. To perform quantitative evaluation of the degree of white matter tract abnormalities in children with spastic cerebral palsy by magnetic resonance tractography to determine severity of the disease, as well as to carry out a dynamic assessment of treatment effectiveness.Materials and methods. The study included 46 children (32 males, 14 females; average age 5.4 ± 1.1 years). The participants were divided into two groups. The experimental group consisted of 23 children with spastic cerebral palsy. The control group included 23 children without any neurological disorder. Examination of the brain was performed on the Siemens Essenza 1,5 Т system (Siemens, Germany) and included magnetic resonance tractography to reconstruct the major white matter tracts. The number of fibers, average fractional anisotropy value, apparent diffusion coefficient, and coefficient of myelination of major white matter tracts in the brain were calculated and analyzed.Results. We found a significant difference in the above-stated parameters between the groups. The experimental group showed a decrease in the absolute number of fibers at the central and posterior segments of the corpus callosum, corticospinal tracts, and left inferior longitudinal fasciculus. Besides, we detected a decrease in fractional anisotropy at 2–5 segments of the corpus callosum and right lateral corticospinal tract, an increase in the apparent diffusion coefficient at 2, 4, and 5 segments of the corpus callosum and left lateral corticospinal tract, and a decrease in the myelination coefficient in all the examined tracts, except for superior longitudinal fasciculus. We revealed a positive correlation between the intensity of the motor disturbance and the coefficient of myelination at the anterior corpus callosum and inferior longitudinal fasciculus.Conclusion. Magnetic resonance tractography is an informative technique for unbiased evaluation of white matter tract anatomy, as well the level and degree of motor tract damage. The most useful characteristics of white matter tract anatomy are the absolute number of fibers in the tract, fractional anisotropy, and coefficient of myelination. Some of them correlated with the intensity of motor disturbance, so they can be regarded as potential predictors of rehabilitation potential.