Moving beyond the diatribe on ultrasound vs MRI

Abstract

In their recently published article1, van der Knoop et al. showed that, in a very small cohort of fetuses at high risk for brain damage, multiplanar neurosonography had better diagnostic accuracy compared to standard axial ultrasound, and no additional value was found for fetal magnetic resonance imaging (MRI) compared to axial ultrasound or neurosonography. A thorough multiplanar ultrasound assessment of the fetal brain according to International Society of Ultrasound in Obstetrics and Gynecology (ISUOG) guidelines2 is paramount either in cases of an isolated anomaly3–5 or in fetuses at high risk of brain injury, as described by van der Knoop et al.1. However, there are a few issues that we believe should always be taken into account when discussing the role of neurosonography and fetal MRI in fetuses at risk for brain abnormalities. Recent studies showed that the additional value of fetal MRI is generally lower than reported previously in the literature when neurosonography is accomplished properly by experienced operators, but there is still a small number of cases in which fetal MRI might detect additional anomalies missed on ultrasound, such as neuronal migration disorders or acquired anomalies3–5. We believe that fetal MRI should be considered as a complementary imaging tool that might improve management and counseling, and which may therefore be offered judiciously in those fetuses at high risk for brain damage in order to help parents decide on the future of their pregnancy. In the study of van der Knoop et al., some of the included cases already showed multiple anomalies on neurosonography. In this scenario, neurosonography is able to anticipate objectively the prognosis, and the value of MRI is less relevant. Conversely, for those conditions in which the presence of an associated anomaly may impact the outcome, MRI should be considered even when neurosonography is negative, especially because the most commonly reported associated anomalies detected only on MRI are those involving cortical development of the brain, which can be easy to detect on MRI. Therefore, we think that a more objective statement would be that ‘the contribution of MRI is less significant in determining outcome when multiple anomalies are identified on neurosonography’. Second, the authors found that the accuracy of the neurosonogram was not influenced by cephalic or breech fetal position, thus speculating on the usefulness of transvaginal ultrasound in cases of cephalic position. However, ISUOG guidelines on sonographic examination of the fetal central nervous system report that ‘vaginal probes have the advantage of operating at a higher frequency than do abdominal probes and therefore allow a greater definition of anatomical details’ and provide a satisfactory assessment of the fetal brain from the end of the first trimester2, such that even an external cephalic version might be reasonable when evaluating some breech presenting fetuses6. Third, the emphasis of fetal MRI should be on structures that are more difficult to assess with ultrasound, as also recommended by ISUOG guidelines7. In this scenario, the findings from one of the largest cohorts designed to ascertain the role of MRI in detecting additional anomalies in fetuses undergoing neurosonography confirmed this suggestion, as the large majority of anomalies detected exclusively on MRI were those that usually become evident only later during pregnancy, such as malformations of cortical development, migration disorders and hemorrhage3. Aside from these considerations, we think that it is time to look at the overall diagnostic performance of prenatal imaging techniques in detecting fetal anomalies, rather than insisting on the never-ending conflict between ultrasound and MRI. The main aim of prenatal diagnosis of fetal anomalies, especially those involving the brain, should be to provide a more accurate description of the anomaly, which in turn should allow objective counseling to be provided on the shortand long-term prognosis of the newborn. The fetal medicine specialist should be an expert in prenatal imaging and not only in ultrasound, and should integrate the information from different imaging techniques in order to have a better understanding of an anomaly. There is no debate that ultrasound is the best screening modality for fetal brain anomalies and that it has a comparable performance to MRI in detecting these anomalies. However, MRI can also provide insight into brain function by describing cerebral metabolism, axonal organization or neuronal connectivity, thus potentially improving our ability to formulate a prognosis. Integration of such imaging modalities is the future of fetal neurology, which will allow us to move from diagnostic to prognostic medicine. To conclude, we congratulate the authors for having highlighted important issues in their interesting research. However, the small sample size of the study population inevitably affects the robustness of the results, and we