The Development Of Fetal Cerebellum Assessed By MR Images

Background:The cerebellum is one of the most important structure in posterior cranial fossa, which develops over a long period, and it is one of the first structures in the brain to begin to differentiate but one of the last to mature. Investigation of cerebellar development can support early data and anatomy foundation for assessment of cerebellum.Ultrasound, with the advantages of reliable, low cost, portable and real-time, is still remains the main technique in evaluating fetal brain. Some abnormality in posterior cranial fossa could be identified by ultrasound since second-trimester such as cerebellar tonsillar hermia.3D-ultrasound is used to measure cerebellum. Rutten et al. have measured fetal cerebellar volume with 3D-ultrasound from 20th GA determined the side discrepancy between the left and the right. However, ultrasound has its own limitations compared with MRI, the relatively poor contrast resolution and even anatomy delineation of brain structures in particular. In recent years fetal MRI has been proposed as a second level prenatal diagnosis method. It has been demonstrated that MRI is highly accurate to determine the normal and pathological morphologic changes of fetal brain, and becomes especially important when ultrasonography is inconclusive or doubtful. Hated et al. have measured vitro fetal cerebellar volume with MRI, and then the results were compared with that obtained by 3D-ultrasonography. The consequence demonstrated that MRI measurement was more accurate without attenuation shadowing, but its drawback was that fetal movement influenced the MRI acquisition.In previous histological study, the procedure tragectory of cerebellar development has been described in details. There are still some discrepancies between the data of histological and that of vitro fetal MRI, and specimen must be cut so it can't used for following work. With specimen scan, we can obtain clear cerebellar images with small layer without specimen destroyed, and it supports a new way to study fetal cerebellum. In this study, we selected 40 formalin-fixed normal fetal specimens with gestational ages ranging from 14 to 22 weeks and attempted to establish MRI templates of normal cerebellar development. And 7.0T MRI was used to collect clearer images and more accurate templates. The results are approximately similar to those of histological study, which could be used for assessing development of cerebellum at second trimester as a bridge and a foundation.Objective:To investigate the developmental characteristics of fetal cerebellum ranging in age from 14 to 40 gestational weeks by measuring cerebellar volume and diameter.Methods:A total of 87 fetuses(14-40 gestational weeks) without morphologically disorder were scanned by 3.0T MR, then 40 fetuses(14-22 gestational weeks) were scanned by 7.0T MR. And we investigated cerebellar structural development based of coronal, transverse, and sagital images. Amira software was used to determine morphological parameters about fetal cerebellum, including the cerebellar volume (CV), cerebellar transverse diameter (TCD), vermis length and width, and the relationship between gestational age (GA) and these measurements.Results:(1)Results o f3.0T MRIThe TCD, A-PD and vermis length increased in the pattern of straight line as the fetuses grew[TCD=0.165FA-1.410; A-PD=0.091FA-0.717; (vermis length)=0.108FA-0.832]. At the same time, the relationship between cerebellar volume (in cubic centimeters) and FA (in weeks) was well described by a single exponential function [CV=0.034 exp (FA/5.918)]; CV doubling time is 4.1 weeks.(2)Results of 7.0T MRIThe primary fissure is visible at the 14th GA. By the 16th week, the prepyramidal fissure and the secondary fissure and the dentate nucleus could be identified. The posterolateral fissure and the fourth ventricle form at the 17th week, while tentorium of cerebellum forms at the 20th week. The relationship between GA and The CV, TCD, vermian width vermian length was well described by a second-order polynomial regression curve The rate of TCD and vermian length and the rate of TCD and vermian width decrease with GA.[CV=0.0022GA2 + 0.0888GA-1.4941, TCD=-0.0073GA2+0.3804GA-2.5735(r2=0.768, p<0.05)vermian width=-0.0022 GA2+0.1422 GA-1.2698(r2=0.858, p<0.05), vermian height=0.0005 GA2+0.0733 GA-0.7529(r2=0.818,p<0.05)].Conclusion:Images of 3.0T and 7.0T MRI could clearly show the trajectory of cerebellar development, which promote the understanding of cerebellar development of normal fetal, and facilitate the posential usefulness of MR in research on healthy and pathological intrauterine cerebellar changes.