An axis-referenced morphometric model reshaping the hippocampus following its longitudinal organization

The anterior-posterior gradient of the human hippocampus on coronal slices is commonly referred to as the long axis, which exhibits morphological heterogeneity related to aging and neurodegeneration. However, the actual longitudinal organization follows a laminar architecture along a longitudinal trajectory that terminates not in the absolute anterior tip of the hippocampus, but rather traces the folding uncus. This study proposes an axis-referenced morphometric model (ARMM) to reshape the hippocampus based on its anatomical nature. An atlas of the hippocampus derived from large samples of 7T ex-vivo MRI and histology serves as the prototype of the model. We set up an orthogonal curvilinear coordinate system within the hippocampus, where the internal coordinate lines correspond naturally to longitudinal and transversal axes of the hippocampus through conformal mapping, providing reference when measuring structural features like thickness, width, length, etc. The inverse mapping allows the hippocampal grey matter to unfold onto a two-dimensional rectangle, facilitating point-wise morphological correspondence across individuals. To evaluate the ARMM, we perform a detailed shape representation of the hippocampus from 7T-MRI and compare it with three state-of-the-art shape models. The results show that the ARMM performs best in metrics measuring shape similarity between the reshaped hippocampus and the ground truth, particularly in head folds (Dice=0.9934). Moreover, our examination of ARMM on longitudinal 7T images revealed its ability to capture subtle structural variations due to neurodegeneration. The ARMM offers a unique anatomically motivated morphometric model of hippocampus, and sheds light on discovering new image markers for diseases associated with hippocampal damage. Graphic abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=109 SRC="FIGDIR/small/531443v1_ufig1.gif" ALT="Figure 1"> View larger version (34K): org.highwire.dtl.DTLVardef@1e39698org.highwire.dtl.DTLVardef@b4119forg.highwire.dtl.DTLVardef@1f0f6b3org.highwire.dtl.DTLVardef@d4a25d_HPS_FORMAT_FIGEXP M_FIG C_FIG