7 Tesla Diffusion MRI and Tractography to Probe Human Brain Organoid Microstructure.

Human brain organoids are three-dimensional cellular models that mimic architectural features of a developing brain. Generated from human induced pluripotent stem cells (hiPSCs), these organoids offer an unparalleled physiologically relevant in vitro system for diseases modeling and drug screening. The present study endeavors to establish a foundation for an MRI-based label-free imaging system that offers high-resolution capabilities for deep-tissue imaging of whole-organoids. An 11.7 Tesla Bruker/89mm MicroImaging system was used to collect high-resolution multi-shell 3D diffusion images of two iPSC-derived human hippocampal brain organoids. The MRI features identified in the study were interpreted on the basis of similarities with immunofluorescence microscopy. MRI microscopy at <40 μm isotropic resolution provides a 3D view of organoid microstructure. T2-weighted contrast shows a 'rosette-like' internal structure and a protruding spherical structure that correlates with immunofluorescence staining for choroid plexus. Diffusion tractography methods can be used to model tissue microstructural features and possibly map neuronal organization. This approach complements traditional immunohistochemistry imaging methods without the need of tissue clearing. This proof-of-concept study shows, for the first time, the application of using high-resolution diffusion MRI microscopy to image 2-mm diameter spherical human brain organoids. Application of ultra-high field MRI and diffusion tractography is a power modality for whole-organoid imaging and has the potential to make significant impact for probing microstructural changes in brain organoids used to model psychiatric disorders, neurodegenerative diseases, and viral infection of the human brain, as well as for assessing neurotoxicity in drug screening.