Purpose or Case Report: To develop reference ranges for cerebellum and posterior fossa measurements by fetal MRI and to investigate the reproducibility and diagnostic performance of quantitative MR parameters for the differentiation of fetal posterior fossa abnormalities.
Methods & Materials: We systematically evaluated the posterior fossa of normal fetuses by MRI, including presence of fastigial point (FP), visualization of cerebellar vermis (CV) fissures, tegmento-vermian (TV) angle, proportion of CV above/below the fastigium-declive live, CV anteroposterior (AP) diameter, CV height, CV area, transverse cerebellar diameter and cisterna magna (CM) diameter. Measurements were performed on sagittal and axial 2-3 mm T2 HASTE (Half-Fourier Acquisition Single-shot Turbo Spin Echo) sequences. Regression analysis was performed and normal reference ranges constructed for each parameter. Intra- and interobserver reliability, repeatability, and agreement were evaluated by intra-class correlation coefficient (ICC), Bland-Altman plots and kappa statistics. Cases of posterior fossa anomalies were evaluated using these parameters.
Results: 116 normal fetuses and 13 fetuses with posterior fossa abnormalities were examined by fetal MRI between July 2005 and September 2015. Technically acceptable sagittal images of the CV were available for 95 of the 116 fetuses (81.8%). Intra- and inter-rater agreement were almost perfect (kappa > 0.80) for identification of the FP, primary and pre-pyramidal CV fissures. Intra- and inter-rater reliabilities were excellent (ICC > 0.75) for CV area, AP and CC diameters, and pons AP diameter. Reliability was poor for the proportion of CV above/below the fastigium-declive line. TV angles were significantly different among the13 abnormal cases: Dandy-Walker malformation (DWM) (n=3, 55-82.6 degrees), Blake's pouch cyst (BPC) (n=5, 22.3-41.8 degrees), mega cisterna magna (MCM) (n=4, 0-22.3 degrees), and arachnoid cyst (AC) (n=1, 1 degree) (Kruskal-Wallis test p=0.0001) (Figure 1). DWM cases were also more likely to have CV area and AP diameter < 5th percentile for gestational age (Figures 2 and 3). In addition, FP and primary cerebellar fissure were not visualized in DWM.
Conclusions: Reference values for quantitative evaluation of posterior fossa may help differentiate among DWM, BPC, AC, and MCM. DWM is characterized by lack of visualization of the FP and primary CV fissure, large TV angle, as well as CV AP diameter and CV area below the 5^th percentile for gestational age.