Imaging myeloarchitecture of the brain using MRI

COEX 

The human brain is a marvel of complexity, with its functioning reliant upon a myriad of microstructures and connections. Among the crucial elements of the microstructures, myelin plays a pivotal role. Myelin, which is a fatty substance that envelops nerve fibers, acts as an insulator, facilitating efficient signal transmission within the nervous system. Furthermore, myelin has shown to be critically associated with development, learning, degeneration and various neurological disorders including multiple sclerosis. Therefore, a comprehensive understanding of the spatial distribution and temporal dynamics of myelin is essential for exploring brain structures and functions. Histological studies of myelin distribution in the cortex dates back to 1840, revealing variations in myelin density along cortical depth and among cortical regions. This observation created the concept of “myeloarchitecture”, which refers to intracortical laminar profiles of myelin content across the cortex, similarly to the way cytoarchitecture is used to parcellate the cortex.

In recent years, the field of neuroimaging has witnessed rapid advancements in magnetic resonance imaging techniques, enabling researchers to visualize the in-vivo distribution of myelin in just a few minutes or tens of minutes of scan time. Several methods have been developed, including myelin water imaging (MWI), magnetization transfer (MT) imaging and its variations (MT saturation, and inhomogeneous MT), relaxometric imaging (T1 map and T1 over T2 weighted image ratio), and magnetic susceptibility imaging (quantitative susceptibility mapping, R2* map, and χ-separation). Just like fMRI, where understanding the biophysics behind the technique is important for proper interpretation of the results, it is essential to understand the biophysical foundations of these techniques. In particular, each method has different sensitivity and specificity to myelin and often suffers from various complication factors (e.g., not only myelin but also iron affecting T1 and susceptibility). Therefore, the resulting myelin distribution map inevitably contains errors that vary from one method to another, requiring careful interpretation. No method is considered as a gold standard and prevails. During this presentation, the fundamental principles of these myelin imaging techniques will be described, followed by their technical details, including limitations. After that, validations and key applications of the methods will be included to provide a complete overview of the techniques. Lastly, requirements for an “ideal” myelin imaging technique will be suggested in terms of its need for sensitivity, resolution, etc., for the reliable creation of myelin distribution in future myelin imaging.

Supported by these techniques, several studies have been explored in-vivo myeloarchitecture of the brain, revealing well-known distributions of myelin from histology. I will demonstrate topographical distributions of myelin across cortical regions, illustrating organizational characteristics and also comparing the maps from various myelin imaging techniques against histological results to scrutinize their validity and potential roles for applications including brain parcellation. This examination also unveils the normative distribution of myelin in the healthy brain, offering a chance to compare it with developing, aging and diseased brains. In particular, a few studies that explored such distributions in adolescents and aging brains will be highlighted to understand age-dependent myeloarchitectural changes and their potential impact in brain functions.

In summary, this presentation will provide an in-depth overview of the latest MRI techniques for imaging myelin and myeloarchitecture in the in-vivo human brain. By the conclusion of the talk, attendees will understand the roles of myelin and myelin imaging techniques, currently available myeloachitecture maps, and their applications in exploring neurodevelopment and neurodegenerative diseases. We will navigate through the complex and delicate brain's myeloarchitecture, elucidating its role in our lives, and appreciate how advanced MRI techniques can unlock the secrets it holds.