MRI Versus Four Dimensional Ultrasound in Detection of CNS Fetal Congenital Anomalies

Early detection of congenital CNS anomalies gives time available for the clinician and parents to plan about the outcome of pregnancy. Prenatal ultrasound has been well established for decades as the primary technique for evaluating the developing fetus in normal as well as in high risk cases. Advantages of US include widespread availability, relatively low cost and quick, lack of harmful effect to fetus or mother and real time imaging. Although ultrasound can characterize many anomalies accurately, it has many limitations as operator dependent, small field of view, and relatively poor soft-tissue contrast, beam attenuation by maternal adipose tissue and fetal bone, limited visualization of posterior fossa after 33 weeks gestation because of calvarial calcification. Also, ultrasound relies heavily on fetal positioning and presence of sufficient amniotic fluid to provide an adequate acoustic window fetus So, US findings are occasionally incomplete or inconclusive to guide treatment choices. Fetal brain MRI became embraced as a clinically important imaging technique useful for fetal assessment, which is helpful in formulating prognosis and perinatal management and can detect occult abnormalities in up to 50% of cases for certain indications. It can overcome many of ultrasound limitations as it is not limited by maternal obesity, fetal position, or oligohydramnios. Additionally, it has better soft tissue contrast resolution, as well as the ability to distinguish individual fetal structures such as brain, lung, liver, kidney, and bowel. In addition, visualization of the brain is not restricted by the ossified skull. Moreover, MRI provides multiplanar imaging as well a larger field of view, facilitating examination of fetuses with large or complex anomalies.