Hippocampal subfield volumetry from structural isotropic 1 mm3 MRI scans: A note of caution

Spurred by availability of automatic segmentation software, in vivo MRI investigations of human hippocampal subfield volumes have proliferated in the recent years. However, a majority of these studies apply automatic segmentation to MRI scans with approximately 1 x 1 x 1 mm(3)resolution, a resolution at which the internal structure of the hippocampus can rarely be visualized. Many of these studies have reported contradictory and often neurobiologically surprising results pertaining to the involvement of hippocampal subfields in normal brain function, aging, and disease. In this commentary, we first outline our concerns regarding the utility and validity of subfield segmentation on 1 x 1 x 1 mm(3)MRI for volumetric studies, regardless of how images are segmented (i.e., manually or automatically). This image resolution is generally insufficient for visualizing the internal structure of the hippocampus, particularly the stratum radiatum lacunosum moleculare, which is crucial for valid and reliable subfield segmentation. Second, we discuss the fact that automatic methods that are employed most frequently to obtain hippocampal subfield volumes from 1 x 1 x 1 mm(3)MRI have not been validated against manual segmentation on such images. For these reasons, we caution against using volumetric measurements of hippocampal subfields obtained from 1 x 1 x 1 mm(3)images.

Automated summary

In recent years, in vivo MRI investigations of human hippocampal subfield volumes have increased due to the availability of automatic segmentation software. However, the majority of these studies use automatic segmentation on MRI scans with resolutions of approximately 1 x 1 x 1 mm(3), which is insufficient for visualizing the internal structure of the hippocampus. As a result, the findings of these studies are often contradictory and surprising, particularly regarding the involvement of hippocampal subfields in normal brain function, aging, and disease.