Development of the hippocampal region demonstrated by fetal MRI D. Bajic, N. Canto Moreira, J. Wikström, R. Raininko

Hippocampal development Starts at gestation week (GW) 8 At GW 10 a shallow hippocampal sulcus (HS) visible on the medial side of the temporal lobe (TL) Thereafter begins inversion in the TL

1 = Dentate gyrus 2 = Cornu ammonis 3 = Subiculum 4 = Parahippocampal gyrus HCS = Hippocampal sulcus (based on Humphrey 1967, Kier et al 1995) Schematic diagram of the fetal development of the hippocampal formation

Completing of the hippocampal development Morphological development completed at around 21 st (GW) Humphrey (1967), Kier (1995) Fusion of the hippocampal sulcus continues until the GW 30 Arnold and Trajanowski (1996)

Adult HC has an ovoid shape on the coronal MRI slices About 19% has a non-ovoid shape in the coronal MRI slices (incomplete hippocampal inversion??) Hippocampal shape

Ovoid and non-ovoid hippocampal shapes

Previous studies Studies on aborted fetuses Autopsy studies MRI studies of the formalin fixed brain slices Small number of cases Deformation of the brain during preparation Fetal MRI Hippocampal shape not discussed

Aim To analyze the development of the hippocampal region on fetal MRI

Subjects 3 post mortem MRI - aborted for extracranial pathology 37 in utero fetal MRIs Indications of MRI : - suspected body malformations, - CNS abnormalities suspected on previous ultrasound (US) - abnormal screening laboratory tests

Subjects Inclusion criteria: Hippocampal regions were well seen No brain pathology on MRI, repeated US or clinical studies

MRI technique Field strength 1.5 T Coronal slices analyzed Sequence types: Slice thickness: Post mortem: T2-w 2D FSE,2 mm T1-w 3D MP GRE 1 mm In utero: T2-w SS FSE3-4 mm

Methods One observer Evaluation of: Hippocampal sulcus Existence Grade of closure Shape of the inverted hippocampus Non-ovoid Ovoid, horizontal long axis Existence of the collateral sulcus

Fetal MRI Post mortem image (17 GW, open hippocampal sulcus)

Fetal MRI In utero images 27 GW with closed hippocampal sulcus, Axis of the hippocampus a) Vertical b) horizontal ab

Results Ages of the fetuses: Post mortem examinations : GW In utero examinations: GW Image quality higher in the post mortem examinations

Results Hippocampal sulcus Visible ≥ GW 17 Closed earliest at GW 24 Open latest at GW 31 Asymmetry in 14/40 fetuses

Shape of the inverted hippocampus Evaluable at earliest at 24 GW Shape: Non-ovoid ≥ GW 24 Ovoid, with a horizontal long axis ≥ GW 29 Asymmetry in 8/18

Collateral sulcus Visible ≥ GW 17 post mortem exam. ≥ GW 22 in utero exam. Seen in all ≥ GW 29 Asymmetry in one fetus (GW 27)

Conclusions

There is wide temporal variation in the development of the hippocampal region Morphological development of the hippocampus is not fulfilled up to GW 21 as presumed in earlier studies

There is temporal variation even between the right and left sides in one individual. Collateral sulcus did not show the same developmental asymmetry.

Evaluation of the hippocampal region is difficult on fetal MRI. Post mortem examinations are of higher quality ← no motion artifacts, a longer acquisition time