Early second-trimester three-dimensional transvaginal neurosonography of the fetal midbrain and hindbrain: normative data and technical aspects.

OBJECTIVES to provide a detailed ultrasound developmental description of various fetal midbrain-hindbrain (MBHB) structures during early second trimester, and to analyze the performance of different transvaginal (TVUS) transducers emission frequencies in their recognition. METHODS we retrospectively analyzed 3D volumetric datasets of the fetal MBHB acquired through the posterior fontanelle on the midsagittal view. By a multiplanar approach, we measured the following structures: tectum thickness and length, aqueductal thickness, tegmentum thickness and width and the Blake's pouch neck height. In addition, we assessed the existence of early vermian fissures, the linear shape of the brain stem and the components of the fastigium. We performed a sub-analysis for the performance of different transducers emission frequencies. RESULTS Sixty brain volumes were included in the study, at a mean gestational age of 16.24 weeks (range 14.14-19.0), at a TCD range of 13.0-19.8 mm. Good correlation was found between fetal age and all measurements, with a high intra and inter observer correlations. We show that the Blake's pouch neck is patent in all fetuses between 14-18 weeks with decreasing diameter, and the aqueductal size is significantly smaller at ≥ 18 weeks compared to < 16 weeks. The early vermian fissures, and the linear shape of the brain stem are present in all fetuses from 14 weeks. We found that at early second trimester, the horizontal arm of the presumed fastigium evolves from the 4th ventricular choroid plexus and not the posterior vermis. both normal and high-resolution ultrasound probes perform similar in the assessment of MBHB anatomy. CONCLUSIONS A detailed early second trimester assessment of the MBHB is feasible by transvaginal neurosonography and provide reference data which may help in the early detection of brain pathologies involving the MBHB. This article is protected by copyright. All rights reserved.