Diffusion tensor imaging reveals microstructural alterations in corpus callosum and associated transcallosal fiber tracts in adult macaques with neonatal hippocampal lesions

Application of diffusion tensor imaging and functional alterations in evaluating brain alterations related to heatstroke in a rat model

OBJECTIVES

To assess the alterations in resting-state functions and neural structures in the brain of a heatstroke rat model and explore the underlying relationship.

METHODS

In total, 17 male Sprague Dawley rats were randomly divided into a control group (CTRL, n = 7) and a heatstroke group (HS, n = 10). All rats underwent 7.0 T magnetic resonance imaging (MRI). T2-weighted imaging, resting-state functional MRI (rs-fMRI), and diffusion tensor imaging (DTI) were obtained. On day 25, the surviving HS group rats (the follow-up group, FU, n = 7) were scanned again.

RESULTS

Heatstroke resulted in functional alterations and structural damage in the cerebellar molecular layer (CML), right perirhinal area (PA), pretectal region (PR), right dentate gyrus, and external cortex of the inferior colliculus (ECIC). Further functional changes occur in the right temporal associative cortex (TAC), left retrosplenial cortex (RC), and CML during convalescence. The fractional anisotropy values were significantly positively correlated with the amplitude of low-frequency fluctuation (ALFF) (HS-CML: r = 0.746, p = 0.034; right PR: r = 0.648, p = 0.049; FU-right PA: r = 0.817, p = 0.025)/regional homogeneity (ReHo) ratio (HS-CML: r = 0.833, p = 0.008; ECIC: r = 0.678, p = 0.045) and negatively correlated with the ALFF (FU-left RC: r = -0.818, p = 0.024; right TAC: r = -0.813, p = 0.049).

CONCLUSION

DTI and rs-fMRI allow meticulous monitoring of the progression of neurological and functional alterations in the brain after heatstroke.

Ultra-high field (10.5T) diffusion-weighted MRI of the macaque brain

Diffu0sion-weighted magnetic resonance imaging (dMRI) is a non-invasive imaging technique that provides information about the barriers to the diffusion of water molecules in tissue. In the brain, this information can be used in several important ways, including to examine tissue abnormalities associated with brain disorders and to infer anatomical connectivity and the organization of white matter bundles through the use of tractography algorithms. However, dMRI also presents certain challenges. For example, historically, the biological validation of tractography models has shown only moderate correlations with anatomical connectivity as determined through invasive tract-tracing studies. Some of the factors contributing to such issues are low spatial resolution, low signal-to-noise ratios, and long scan times required for high-quality data, along with modeling challenges like complex fiber crossing patterns. Leveraging the capabilities provided by an ultra-high field scanner combined with denoising, we have acquired whole-brain, 0.58 mm isotropic resolution dMRI with a 2D-single shot echo planar imaging sequence on a 10.5 Tesla scanner in anesthetized macaques. These data produced high-quality tractograms and maps of scalar diffusion metrics in white matter. This work demonstrates the feasibility and motivation for in-vivo dMRI studies seeking to benefit from ultra-high fields.

Combining brain perturbation and neuroimaging in non-human primates

Brain perturbation studies allow detailed causal inferences of behavioral and neural processes. Because the combination of brain perturbation methods and neural measurement techniques is inherently challenging, research in humans has predominantly focused on non-invasive, indirect brain perturbations, or neurological lesion studies. Non-human primates have been indispensable as a neurobiological system that is highly similar to humans while simultaneously being more experimentally tractable, allowing visualization of the functional and structural impact of systematic brain perturbation. This review considers the state of the art in non-human primate brain perturbation with a focus on approaches that can be combined with neuroimaging. We consider both non-reversible (lesions) and reversible or temporary perturbations such as electrical, pharmacological, optical, optogenetic, chemogenetic, pathway-selective, and ultrasound based interference methods. Method-specific considerations from the research and development community are offered to facilitate research in this field and support further innovations. We conclude by identifying novel avenues for further research and innovation and by highlighting the clinical translational potential of the methods.

Diffusion Tensor Imaging-Based Studies at the Group-Level Applied to Animal Models of Neurodegenerative Diseases

The understanding of human and non-human microstructural brain alterations in the course of neurodegenerative diseases has substantially improved by the non-invasive magnetic resonance imaging (MRI) technique of diffusion tensor imaging (DTI). Animal models (including disease or knockout models) allow for a variety of experimental manipulations, which are not applicable to humans. Thus, the DTI approach provides a promising tool for cross-species cross-sectional and longitudinal investigations of the neurobiological targets and mechanisms of neurodegeneration. This overview with a systematic review focuses on the principles of DTI analysis as used in studies at the group level in living preclinical models of neurodegeneration. The translational aspect from in-vivo animal models toward (clinical) applications in humans is covered as well as the DTI-based research of the non-human brains' microstructure, the methodological aspects in data processing and analysis, and data interpretation at different abstraction levels. The aim of integrating DTI in multiparametric or multimodal imaging protocols will allow the interrogation of DTI data in terms of directional flow of information and may identify the microstructural underpinnings of neurodegeneration-related patterns.