Growing Folds: Fetal MR Imaging Anatomy of the Hippocampal Head Digitations

The adult hippocampal head (HH) is characterized by sulci and digitations visible both histologically and on MRI. The aim of the study was to describe the emergence and progression of HH digitations in the fetal brain across different gestational ages (GAs) using MRI.

A retrospective assessment was performed on 383 fetal brain MRIs (19-39 weeks' GA) acquired on 1.5T (n = 351) and 3T (n = 32) scanners. Imaging included single-shot fast spin-echo T2, T1-weighted sequences, and axial DWI. The fetal HH was identified on coronal T2 images, and HH morphologic variants were classified using an established adult classification system: class 0 (no sulci, 1 digitation), class 1 (1 sulcus, 2 digitations), and class 2 (2 sulci, 3 digitations). MRIs with brain anomalies identified prenatally or postnatally, abnormal sulcation, or severe artifacts were excluded. Variant frequencies in both hemispheres were grouped by GA. Correspondence analysis and cumulative link mixed models were used to assess the association between GA and class distribution.

Two hundred seventy-one fetuses were included, grouped into 8 GA categories. Overall frequencies were 33.6% for class 0, 48.9% for class 1, and 17.5% for class 2. With increasing GA, class 0 frequency decreased, while classes 1 and 2 increased. Between 23 and 25 weeks' GA, detection of more digitated HH variants rose sharply from 1% to 64%. Follow-up in 30 fetuses showed an increase in HH digitations, with no observed reductions. Regression analysis revealed greater digitation complexity in the right hemisphere.

Digitations in the fetal HH begin to appear around 24 weeks' GA, reaching frequencies similar to those observed in adults by the later stages of gestation. The right hemisphere shows higher complexity, possibly reflecting faster growth. These findings contribute to the understanding of hippocampal development and may serve as a biomarker of fetal brain maturation.