OP04.09: New insights in the longitudinal evaluation of the cerebellar vermis from 12 to 22 weeks of gestation

To compare the rate of visualisation of the 4th ventricle choroid plexus (4V-CP) separately from the inferior border of the vermis using different transvaginal (TV) or transabdominal (TA) US transducers and operating frequencies in 186 normal fetuses between 12 and 22 weeks. In all cases TV or TA ultrasound examinations of the posterior fossa (PF) were performed. The images were obtained using standard frequency TA probe (1-7 MHz) in 60 cases, HF TA transducer (2-9 MHz) in 45 and TV high (6-12 MHz) and standard frequency (5-9 MHz) probes in 81 cases. The rates of differentiation of the 4V-CP from the inferior border of the vermis from 12 to 22 weeks either by TA or TV approach, using standard and HF probes and the significance of the difference between the measurements of the vermian area obtained with standard frequency TA transducer and the measurements acquired with the other probes were compared. The 4V-CP was visible and it was seen separate from the vermis in 29/60 cases (48.3%) examined by TA standard frequency transducer, which was significantly lower (p < 0.00001) compared to 43/45 cases (95.5%) examined by TA HF probe. In all 81 cases examined with TV transducers the medial portion of 4V-CP was visible and was seen as a small echogenic oval shape structure apparently attached to the vermis. In 31/60 patients (51.7%) examined by TA standard frequency transducer the total area of the complex vermis-4V-CP was measured and this was significantly higher compared to the vermian area measured separately from the 4V-CP in fetuses in which the 2 anatomical structures could be differentiated (p < 0.0001), suggesting an overestimation of the true vermian area in these cases. Either standard and HF TV transducers and HF TA transducer allowed to visualise separately the CP and the vermis. In addition in cases in which the 4V-CP could not be seen separately from the vermis, its area appears to be significantly overestimated. These results have an important clinical impact for the accurate interpretation of the PF US findings.