Imaging of the fetal central nervous system

Abstract

Introduction : Ultrasound and MR imaging of the fetal central nervous system (CNS) develop at an ever-increasing rate. Theoretically, the two modalities should be synergistic, but a literature review revealed the difficulties of determining the merit of either technique and revealed gaps in our knowledge of imaging normal development of the fetal CNS. Physiology: We obtained three dimensional (3D) images of the embryonic ventricular system with the aid of software developed to chart ovarian follicle development. These images had previously only been produced in research settings. At the same time, ultrasound exposure was limited by using 3D technology. We charted the normal fetal cortical, ventricular and cerebellar development longitudinally, with emphasis on the prevalence and degree of physiological asymmetry. 3D ultrasound proved equal to or better than two dimensional (2D) ultrasound, while limiting the scanning and total examination time. Longitudinal growth curves were generated using multilevel analysis. Pathology: We evaluated the prognostic value of prenatal ultrasound in a retrospective cohort of infants with spina bifida. Multivariate regression analysis showed that lesion level and head circumference were independent predictors of demise. Motor or mental functioning at five years of age could not be predicted by prenatal parameters. The accuracy of lesion level determination improved over time. We studied the prognostic value of Doppler ultrasound measurement of middle cerebral artery pulsatility index (mcaPI) in fetuses with CNS abnormalities and found that fetuses with an increased resistance to flow had an increased risk of adverse neurodevelopmental outcome. A rapid rise in mcaPI in fetuses with hydrocephalus was associated with a high risk of demise. An increased mcaPI which subsequently decreased was seen in fetuses with malformations of cortical development, microcephaly and West’s syndrome. A case of an infant that developed malformations of cortical development after intra-uterine transfusion for Parvovirus B19 infection is presented as introduction to the evaluation of our experience with the relative value of ultrasound and MRI. For this evaluation, we reviewed the diagnostic accuracy of ultrasound, MRI, and multidisciplinary discussions before and after MRI in a retrospective cohort. The diagnostic accuracy of the multidisciplinary discussion was 62% before and 73% after MRI. The amniotic fluid lactate-creatinine-ratio predicts fetal lacticemia, which is of interest for future research in predicting fetal compromise with MR spectroscopy. Conclusions: It is possible to obtain clear pictures of the cerebral ventricle system at eight gestational weeks, to grade fetal cortical development using a simple scoring system, to limit examination time by using 3D ultrasound and to visualize both lateral ventricles with transabdominal ultrasound by simple 2D and 3D techniques. Asymmetry is a feature of normal fetal brain development. The fetus with a suspected fetal central nervous system lesion should be discussed in a multidisciplinary meeting. Prenatal ultrasound parameters are useful to predict survival, but not functioning, in spina bifida. Abnormal cerebral blood flow in a fetus with a CNS abnormality is associated with an increased risk of adverse outcome. Prospective evaluation is needed to determine the diagnostic value of ultrasound and MRI in fetal CNS abnormalities