Super-Resolution Track-Density Imaging Reveals Fine Anatomical Features in Tree Shrew Primary Visual Cortex and Hippocampus

Diffusion-weighted magnetic resonance imaging（d MRI） is widely used to study white and gray matter（GM） micro-organization and structural connectivity in the brain. Super-resolution track-density imaging（TDI） is an image reconstruction method for d MRI data, which is capable of providing spatial resolution beyond the acquired data, as well as novel and meaningful anatomical contrast that cannot be obtained with conventional reconstruction methods. TDI has been used to reveal anatomical features in human and animal brains. In this study, we used short track TDI（st TDI）, a variation of TDI with enhanced contrast for GM structures, to reconstruct directionencoded color maps of fixed tree shrew brain. The results were compared with those obtained with the traditional diffusion tensor imaging（DTI） method. We demonstrated that fine microstructures in the tree shrew brain, such as Baillarger bands in the primary visual cortex and the longitudinal component of the mossy fibers within the hippocampal CA3 subfield, were observable with st TDI,but not with DTI reconstructions from the same d MRI data.The possible mechanisms underlying the enhanced GM contrast are discussed.

Probabilistic tractography of the posterior subthalamic area in Parkinson’s disease patients

Deep brain stimulation (DBS) is a non-pharmacological treatment for Parkinson’s disease (PD), and its efficacy depends largely on which anatomical structure (target) is stimulated. The subthalamic nucleus (STN) is one of the most commonly used targets, but stimulation of new targets within the posterior sub-thalamic area (PSA), comprising a group of white matter fibers known as prelemniscal radiations (Raprl), as well as the caudal zonaincerta nucleus (Zic), have proven to be superior at improving certain clinical symptoms. Despite their clinical usefulness, their anatomical connectivity has not been completely described in humans. We performed constrained spherical deconvolution of the signal in diffusion-weighted images and subsequent tractography as a means to non-invasively define the connectivity of the Raprl and Zic in a group of five patients with PD. Further, we used track-density imaging, a novel method to improve the spatial resolution of the acquired images, in order to visualize the small subregions that comprise the PSA with a voxel resolution of 0.2 × 0.2 × 0.2 mm3. Both Raprl and Zic demonstrated high probability of connectivity with the dorsal brainstem, cerebellum, subcortical nuclei (globus pallidum ventral, lateral thalamic nuclei), and cortical areas (orbitofrontal cortex, primary and supplementary motor cortex areas). The connectivity patterns were re-producible between patients and were discretely organized as the tracts entered/exited the PSA, depending on their end points. These findings indicate that the PSA is part of the neuronal circuitry controlling movement, and the precise characterization of its connectivity will aid in our understanding of the net-works involved in PD and how they can be modulated with DBS in order to alleviate symptoms.