Fetal neuroimaging: ultrasound, MRI, or both?

Much has been written recently about the relative merits and demerits of fetal imaging with ultrasound and magnetic resonance imaging (MRI). Unfortunately, the arguments often generate more heat than light. We attempted to identify the strengths and weaknesses of each modality from the literature, and to arrive at some practical recommendations on when to use which imaging modality. In conclusion, combining neurosonography and MRI is mostly redundant, but occasionally complementary. Both are operator dependant and neither technique obviates the need for a thorough knowledge of normal and abnormal neuroanatomy. In early pregnancy, and where repeated assessment is needed, ultrasound has the obvious advantage. Where ultrasound is difficult, as in the obese patient or a patient with severe oligohydramnios, better images might be obtained by MRI examination, although a special MR system is required for patients weighing more than 150 kg. MRI might also identify fetal ischemic lesions early after an insult such as severe maternal trauma or death of a monochorionic co-twin. There is a synergy between ultrasound and MRI for the diagnosis of certain conditions, such as congenital cytomegalovirus infection or cerebellar telangiectasis. Local conditions and expertise obviously influence the accuracy of both modalities. Both ultrasound and MRI should be performed to the highest possible standard, and the final diagnosis should be made in a multidisciplinary setting. TARGET AUDIENCE: Obstetricians & Gynecologists, Family Physicians. LEARNING OBJECTIVES: After completion of this article, the reader should be able to summarize the limitations of ultrasound and MRI in fetal CNS imaging, appropriately prepare and counsel patients regarding the best choice of imaging modality, and modify utilization of resources.