Recently, much attention has been focused on the definition and structure of the hippocampus and its subfields. While important, the projections from the hippocampus have been relatively understudied. Throughout the majority of its anterior to posterior extent, the hippocampus is enveloped on its superior surface by white matter emanating from within the hippocampus. These afferent myelinated fibers of the alveus and fimbria coat the hippocampus and contour its trajectory through the Medial Temporal Lobe. Near the tail of the hippocampus, the alveus and fimbria coalesce and curve superiorly and anteriorly to form the fornix. Projections then reach the anterior nuclei of the thalamus via the mammillary bodies prior to their ascent into higher cortical regions. It is the extra-hippocampal white matter regions of the alveus, fimbria, and fornix that have received significantly less attention in the literature.

​

Here, we present a set of novel atlases for extra-hippocampal white matter regions. These atlases feature manual segmentations of the alveus, fimbria, fornix and mammillary bodies using a reliable and detailed segmentation protocol. Furthermore, atlas segmentations have been completed on high-resolution magnetic resonance images that are complementary to our previous definitions of the hippocampal subfields. In this way, these novel extra-hippocampal white matter atlases can be used in conjunction with our previous hippocampal subfield atlases.

​

In addition, we have validated the use of these extra-hippocampal white matter and hippocampal subfield atlases in our automatic segmentation pipeline, MAGeT Brain. Furthermore, we have also demonstrated its application by analyzing the volumetric trajectory of these regions in healthy aging (OASIS), Alzheimer’s disease, and mild cognitive impairment (ADNI) datasets.