Normal morphometry of fetal posterior fossa at midtrimester: brainstem-tentorium angle and brainstem-vermis angle.

Objective: To standardize the evaluation of normal tentorium insertion and normal rotation of the cerebellar vermis over the brainstem, using two novel measurements: the brainstem-tentorium angle (BT angle) and the brainstem-vermis angle (BV angle). We also aimed to test the reproducibility of these measurements. Methods: Prospective observational study including normal fetuses at routine anomaly scan with confirmed normal follow-up. Three-dimensional volumes of the fetal head were acquired starting from standard trans-cerebellar views. In the sagittal plane, obtained by multiplanar reconstruction, the angle between the brainstem and the tentorium insertion in the fetal skull, and between the brainstem and the lower edge of the vermis were measured twice by two independent operators. Intraobserver and interobserver variations were calculated. Results: Eighty cases were included. The estimated BT and BV angles lie in a wide interval among normal midtrimester fetuses with a median value (min-max) of 25.65 (20.13-47.39) and 9.29 (3.87-19.36) respectively. Intraobserver and interobserver variation were good for both measurements. Conclusion: The BT and BV angle may be of help in assessing the fetal posterior fossa at midgestation and gives a standardized and reproducible measurement of normal tentorium insertion and normal rotation of the cerebellar vermis over the brainstem. © 2012 John Wiley & Sons, Ltd.