MRI Quantitative Evaluation Of Fetal Posterior Fossa Structure

Objective:To explore the quantitative evaluation of the fetal posterior fossa structure with MR midline sagittal area.Measure the main structure of the normal fetus posterior fossa(about vermis cranio-caudalr length,antero-posterior length,surface area,brainstem-vermis angle and brainstem-tentorium angle)and analyse the relationship between the measurements and gestation age.Provide the reference data of normal fetus to help assess the fetal malformations in posterior fossa.Compare the fetus malformations of posterior fossa to normal fetus and explore the magnetic resonance imaging features of the fetus with posterior fossa malformations.Methods:A total of 123 normally developed and mild ventriculomegaly fetuses at 24-35 gestational weeks were examined by fetal brain MRI and US from January 2013 to December 2016 in Ningxia medical university General Hospital.The morphology and anatomy of fetal posterior fossa development were observed on the midline sagittal area,and the fetal cerebellar vermis cranio-caudalr length,antero-posterior length,surface area,BV angle,BT angle were measured and their correlations with the gestational weeks were analyzed.Over the same period,including 25 cases of suspected DWM,IVH and MCM detected by MRI and US.The morphology and anatomy of fetal posterior fossa were observed and measured in them.The results were compared to normal group at the same gestational week.Results:1.In normal fetuses,the shape of posterior fossa was well-stacked on mid-sagittal view,cerebellar vermis was located in the behind of brainstem,covering the fourth ventric,brainstem and vermis was almost parallel.The primary fissure could be observed after 22 gestational weeks,roughly on the mid-sagittal plane of the vermis at 2 o'clock.The secondary fissure could be observed after 25 gestational weeks,roughly on the mid-sagittal plane of the vermis at 5 o'clock.The sublobes and fissures could reveal clearer after 30 gestational weeks.2.The total of 123 normal fetuses,vermis cranio-caudalr length,antero-posterior length and surface area were positively correlated with gestational weeks and increased with gestational weeks.Correlation coefficient(R2)was 0.911,0.839,0.947 respectively.The mean angle of BV and BT angles were 5.77°±2.73°,28.58°± 6.60°respectively.The BV and BT angles were not correlated with gestational weeks and correlation coefficient(R2)was 0.011,0.043 respectively,all P > 0.05.3.Twenty-five cases of posterior fossa malformations included 3 cases of DWM,7 IVH,and 15 MCM.In the mid-sagittal plane,the Z score of the vermis cranio-caudalr length,antero-posterior length and surface area were less than-5 in fetuses with DWM,and the BV and BT angles were more than 10,2 respectively.The Z score of the vermis cranio-caudalr length,antero-posterior length and surface area were less than-3 in fetuses with IVH,and the BV angle was more than 5,the BT angle was from 0 to 1.96.The Z score of the vermis cranio-caudalr length,antero-posterior length,surface area,BV and BT angles were from-1.96 to 1.96.Conclusions:1.The brain mid-sagittal plane of fetal MRI can accurately show the shape,detailed structures of posterior fossa and the surrounding relations,which can provide more MRI features of the fetus with posterior fossa malformations.2.In normal fetus,the vermis cranio-caudalr length,antero-posterior length and surface area increases with gestational weeks,which can help assess the fetal development of vermis.The mean angle of BV and BT angles were 5.77°±2.73°,28.58°± 6.60°respectively.Vermian rotation was defined as the BV angle more than 11.12°,and the elevated tentorium as the BT angle greater than 41.51°.3.The result of measurement about the main structure of the DWM fetus posterior fossa showed the vermis area was smaller significantly than that of normal control,the vermis was upward rotated obviously,the cerebellar tentorium was superior displaced;The result of4.measurement about the main structure of the IVH fetus posterior fossa showed the vermis area was smaller than that of normal control,the vermis was upward rotated slightly,the cerebellar tentorium was superior displaced unobvious;The result of measurement about the main structure of the MCM fetus posterior fossa showed the vermis areal,the position of the vermis and cerebellar tentorium were in the normal group corresponding measurement range.5.It is helpful to make diagnosis and differential diagnosis of fetal posterior fossa malformations with careful observation of fetal cerebellar vermis and the adjacent tissue morphology,identification of posterior fossa detailed structures,measurements of cerebellar vermis cranio-caudalr length,antero-posterior length,surface area,BV and BT angle on the mid-sagittal plane of fetal MRI.