Mapping neuronal fiber crossings in the human brain

Obtaining information about the anatomical connectivity of the human brain, noninvasively, is a difficult challenge facing neuroscientists. The adult human brain contains tens of billions of neuronal cells, each with multiple cell contacts that form a complex web. Moreover, higher-order structures, termed neural tracts or fiber bundles, form a complicated 3D network within and connecting different brain regions. The distinct connectivity pattern of a given brain region determines how it processes information and functions. Being able to map these complex neural patterns in vivo is essential for understanding the fundamental basis of many developmental disorders as well as defining how the brain’s structure relates to its function. Diffusion-weighted (DW) magnetic resonance imaging (MRI) is a unique, noninvasive technique capable of quantifying the flow of water molecules in biological tissues such as the human brain.1 The success of DW-MRI comes from its unique capability to accurately describe the geometry of the underlying tissue microstructure as it constrains diffusion. From the raw DW images, diffusion tensor imaging (DTI)2 models the 3D movement of water molecules and has proved to be extremely useful in reconstructing the principal diffusion orientation needed to obtain fiber bundles and in the study of fiber-bundle connectivity in both normal and pathological brain tissues. However, DTI is most notably limited in regions of complex fiber crossings. This becomes a significant constraint when trying to map areas of the brain with complex internal structures (see Figure 1). The limitation is an important one, since the resolution of DTI images is between 1 and 27mm3, while the physical diameter of fibers ranges between 1 and 30μm. Overcoming the limitations of the DTI model and recovering fiber-crossing information is essential for constructing highresolution maps of the human brain. To do so, high angular resolution diffusion imaging (HARDI) reconstruction techniques3 have been used to measure DW images along several direcFigure 1. Processing high angular resolution diffusion imaging (HARDI) from local estimations of water molecule diffusion phenomena to the segmentation and fiber tractography used to recover complex fiber-crossing configurations.