Functional imaging of hippocampal layers using VASO and BOLD on the Next Generation (NexGen) 7T Scanner

Spatial accuracy and venous biases are a central concern in mesoscale fMRI, with subcortical brain regions facing additional challenges due to lower contrast-to-noise ratio (CNR), high physiological noise, and complicated vasculature. Here, we optimized CBV VASO on the NexGen 7T scanner for layer-specific investigations of the hippocampus. The presence of venous biases in VASO and BOLD (from the same acquisition) was then compared by using an established autobiographical memory task. While VASO and BOLD based activation patterns converged at macroscale, layer-specific differences emerged in the hippocampal subiculum, consistent with venous bias in the inner layers of the subiculum which can be explained by the unique two-sided venous drainage. Further, both VASO and BOLD showed sensitivity to short blocks (elaboration > construction), revealing an anterior-posterior distinction consistent with stronger involvement of the posterior hippocampus. Hippocampal cortical connectivity revealed brain circuitry between subcortical and cortical regions. Thus, hippocampal fMRI allows mapping layer function with high accuracy, made possible by sequence timing optimization on the high performance NexGen 7T scanner. The improved MR imaging has been developed to enable precision mapping of subcortical brain gray matter. By capturing changes of neural information flow within and across the microcircuitry of the hippocampus, it can provide deeper insights into a number of neuropsychological phenomena and the early changes occurring in Alzheimer's disease (AD) and mild cognitive impairment (MCI).