| 1. Background and subject sourcesUltrasound plays an extremely important role in eugenics. The software or computer-aided diagnosis system of fetal imaging may help doctors to diagnosis malfomations. Expert system needs a mass of measurements of normal fetuses. In the context, we make a research on a part of the National Natural Science Foundation of topics "Prenatal ultrasound diagnosis of fetal malformation of expert system".2. Objective and significancePerinatal medicine is interested in how to evaluate the fetal growth accurately and how to diagnose malformations early. With the development of ultrasound technology, doctors pay more and more attention to the development of the fetal brain. Posterior fossa structures play an important role in prenatal ultrasound diagnosis. In prenatal ultrasound examination, posterior fossa structures including length, width and shape of cerebellar hemispheres and vermis and depth of cisterna magna (CM), which show features on the imaging of ultrasound. The structures not only related to the situation of growth, but also gave clues on diagnosis of central nervous system malformations and chromosomal abnormalities. At present, clinical work normally measure cerebellar diameter by the two-dimensional ultrasound. The purpose of this study is that through measurements of normal fetal access to the development of different gestational age and comparison anormal fetuses to confirm the normal clinical value of measurements.3. Research data and methodsData of 534 patients who were examined in our hospital from February 2007 to December 2007 were analysed. Mean maternal age were (28±4) years. Inclusion criteria consisted of (1) Singleton. (2) The end of the menstrual was clear. The menstrual cycle was (28-32)-day and the first trimester ultrasound examination determined gestational age, ultrasound testing indicators included the crown-rump length or biparietal diameter. (3) Patients were 17-37 weeks pregnant. (4) Pregnant women without abnormal history of pregnant and other pregnancy risk factors, these included mothers clear virus infection, severe anemia, hypertension, diabetes, heart disease, kidney disease and other complications of pregnancy. (5) The fetuses were without structures or chromosomal abnormalities. (6)Every pregnant woman was examed only once because of cross-sectional study. (7) Follow-up to ascertain newborns were healthy.Measuring method: The Sequia512 color ultrasound equipment, with a 3.5- to 6.0-MHz transabdominal mechanical transducer, was used for fetal scanning. Additional ultrasound measurements obtained included biparietal diameter, head circumference, abdominal circumference, femur length, humerus length. Sonographically estimated fetal weight and growth centile were calculated for every case. In the fetal cerebellum standard plane, we measured the posterior fossa structures. Include: (1) Cerebellum curtain area(Area under the tentorium)(2) Cerebellum hemisphere area, independent of the cerebellar vermis (3) Cerebellar vermis area(4) Cerebellar vermis length (5) Cerebellar vermis width(6) Cisterna magna depth(7) The cerebellum diameter. Plane standards: Cranium ring was clear. The cavity of septum pellucidum, the fourth ventricle, the cisterna magna and tentorium of cerebellum were showed at the same time. Cerebellar hemispheres were symmetry and cerebellar vermis link in the middle. Each case was measured three times and recorded the average by one doctor, then was checked by the other doctor.Judgement of clinical value as follow: (1) From January 2007 to November 2007, 33 cases that were examed abnormal of posterior fossa structures and these outcome were clear in our hospital, 8 cases that were complicating with chromosomal abnormality, we made a retrospective study of them. (2) From January 2004 to January 2007, there were 129 cases of abnormal depth of CM cases, we tracing and revisit these outcome.Statistical methods: Use statistical software packages of SPSS11.5. P<0.05 was considered significant throughout. We carried out analysis of variance test and the normal inspection. The study were calculation normal reference ranges on the index and were using linear regression to analyze the relevance of gestational age. According to different gestational age, we divided different groups. In each group, we calculated 95% confidence interval, 10,25, 50, 75, 90 percentile.(α=0.05)Repeatability test: One observers measured one case for 15 times , the coefficient of variation were 1.9%, 3.9%, 12.3%, 1.4%, 4.8%, 10.2%, 8.4%. Two observers measured the same group of 10 patients individually, Paired t-test showed that repeated measurement error had no statistical significance. (t=-1.412, P=0.192) .4. Result4.1.1 The development features of posterior fossa structures4.1 The change of posterior fossa structures in different gestational age From Table 1 we can see that the posterior fossa structures were increase as gestational age increased.4.1.2 The relevant of the posterior fossa structures and gestational ageArea under the tentorium(r=0.988), cerebellar hemispheric area(r=0.976), cerebellar vermis area(r=0.934), the cerebellar vermis length(r=0.915), cerebellar vermis width(r=0.876) and the depth of CM(r=0.484) were all relative to gestational age. We made scatter diagrams to discover the linear relationship between gestational age and posterior fossa structures.4.1.3 Established multi-factors regressive equationTreating gestational age as the dependent variable and treating the posterior fossa structures as the independent variables, we obtained the posterior fossa structures to predict gestational age of multi-parameter regression equation: y = 11.654 +1.164 (area under the tentorium)—0.418 (cerebellum area)—0.852 (cerebellar vermis area) +1.384 (cerebellar diameter) +0.838 (vermis Minister degrees) +0.857 (vermis width) +1.099 (posterior fossa pool).4.1.4 The normal reference of the posterior fossa structuresWe calculated different gestational age of the 95% confidence interval and percentile. (Table 2 ~ 7)4.1.5 We drew boxplot diagrams to observe the outliers and extremes. Though diagrams we found the measurements were stable.4.2 Abnormal cases of posterior fossa structures4.2.1 Dandy-Walker malformation (DWM). There were 2 typical types and 12 variants.4.2.2 Cerebellum dysplasia. There were 2 cases.4.2.3 Arnold-Chiari deformity. There was 1 case.4.2.4 Spina Bifida. There were 16 cases.4.2.5 Chromosome Abnormality. There were 7 cases of trisomy 18 and 1 case of trisomy 13, which with abnormal of posterior fossa structures.The above abnomal cases were diagnosised from January 2007 to November 2007. The measurements of DWM, cerebellum dysplasia and Arnold-Chiari deformity cases had great differences from normal fetuses' measurements. Spina Bifida cases were relation to CM disappear close.4.2.6 The abnormal of depth of CM. There were 129 cases. CM disappearance highly indicates the abnormality. With the increase of CM expansion, the incidence rate of malformationgs was significantly increased (x~2=19.3 77, P<0.01). Enlarged CM was the indication for a further systematic sonographic exam and follow-up.5. ConclusionThe research data showed that fetal posterior fossa structures were linear growth as gestational age increasing. The depth of CM were (0.27-0.97) cm .With the assessment of fetal posterior fossa structures may be useful in detecting brain malformation and relevant syndromes prenatally as well as dating gestational age.
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