A call for increased awareness and training within the United States

Routine evaluation of the fetal brain is performed during the second-trimester anatomical survey. This screening is conducted by transabdominal scan in 3 axial planes, namely, the transventricular, transthalamic, and transcerebellar planes.¹ Targeted neurosonography, however, is a dedicated, detailed, and diagnostic examination of the fetal brain that is preferably performed with high-resolution transvaginal ultrasound via a transfontanelle approach, providing multiplanar assessment of the brain anatomy. Like fetal echocardiography in the context of suspected cardiac malformation, neurosonography provides greater diagnostic capacity for fetal brain malformations compared to the routine transabdominal screen in the axial planes.

Neurosonography involves extensive evaluation in multiple successive coronal planes (Figure 1), the midsagittal/median plane (Figure 2), as well as successive parasagittal planes (side to side) to provide high-resolution imaging of detailed brain anatomy. These include structures such as the cavum septi pellucidy and cavum vergae, corpus callosum, vermis, 3^rd and 4^th ventricles, vein of Gallen, ganglionic eminence, the caudate nuclei and brain stem, the fetal brain cortex, gyration, sulcation, and parenchyma as well as detailed evaluation of the entire ventricular system and periventricular tissue.²

The use of 3D ultrasound is also frequently utilized to facilitate expert neurosonographic evaluation, obtain the diagnostic planes, and use display modalities, which may further enable the diagnostic process.³ This technique has been used to adequately diagnose multiple fetal brain pathologies including birth defects, fetal infections, brain tumors, vascular insults, AV malformations, and destructive lesions.

Given that the anatomy of the fetal brain evolves and changes throughout gestation, correlation of the anatomy to the gestational age is a key element required by experts in neurosonography. Thus, different pathologies in the development of the fetal brain can be appropriately detected at different gestational ages. For example, whereas a major malformation such as alobar holoprosencephaly can be reliably detected in the first trimester, most abnormalities of the corpus callosum and cerebellar vermis are reliably diagnosed during the second-trimester scan, while malformations of cortical development, migrational disorders, and some tumors and destructive lesions may not be appropriately detected until the third trimester.

Despite its great diagnostic strength, fetal neurosonography is not commonly practiced in the US. Most providers who provide fetal anatomy scans are not adequately trained to perform transvaginal transfontanelle brain scans, interpret fetal brain images in the nontraditional axial planes (such as the coronal and sagittal planes), or correlate these images with the evolution of the brain anatomy throughout the different gestational ages. Therefore, in some centers, the mere suspicion of a fetal brain malformation may result in immediate referral for a fetal MRI. Although MRI is a complementary method to image the fetal brain that in expert hands may provide valuable information to neurosonography, it is a second-line imaging modality, which is far more expensive and less accessible. Importantly, like neurosonography, fetal MRI is also highly operator-dependent, requiring a high level of expertise in both obtaining the appropriate sequences as well as interpreting the images and correlating them with the gestational age. Moreover, the value of fetal MRI increases in the third trimester when evaluation of the cortex and parenchyma is feasible, whereas neurosonography provides superior images during the first- and second-trimester evaluations.⁴

Of note, current American guidelines for neurosonography are limited to evaluation of neonates and infants⁵ rather than fetuses. The most comprehensive guidelines for fetal neurosonography are published by the International Society of Ultrasound in Obstetrics and Gynecology (ISUOG).⁶  These guidelines also define the indications for detailed neurosonography: such as suspicion of brain malformation on routine screening ultrasound or nuchal translucency scan, family history or prior pregnancy affected by brain malformation, fetal congenital heart disease, monochorionic twins, suspected congenital intrauterine infection, exposure to teratogens affecting neurogenesis, and microarray findings of unknown significance.

Not only does neurosonography facilitate accurate diagnosis of a large variety of brain malformations, it also enables us to reassure many anxious patients in which malformation was suspected on a basic scan whereas detailed neurosonography confirmed normal brain development with no pathology.

Therefore, increased awareness of the value of fetal neurosonography and appropriate utilization may result in the referral of patients with appropriate indications to centers with expertise in neurosonography, as well as highlighting the need for specific education and training. Additionally, there is no specific Current Procedural Terminology (CPT®) code for fetal neurosonography in the US. Creation of such a code will facilitate the acceptance of this practice for indicated cases and help solidify training programs and providers’ interest in becoming proficient.

References:

1. Malinger G, Paladini D, Haratz KK, Monteagudo A, Pilu G, Timor-Tritsch IE. ISUOG Practice Guidelines (updated): sonographic examination of the fetal central nervous system. Part 1: performance of screening examination and indications for targeted neurosonography. Ultrasound Obstet Gynecol 2020; 56:476–484.
2. Timor-Tritsch IE, Monteagudo A. Transvaginal fetal neurosonography: standardization of the planes and sections by anatomic landmarks. Ultrasound Obstet Gynecol 1996; 8:42–47.
3. Bornstein E, Monteagudo A, Santos R, Strock I, Tsymbal T, Lenchner E, Timor-Tritsch IE. Basic as well as detailed neurosonograms can be performed by offline analysis of three-dimensional fetal brain volumes. Ultrasound Obstet Gynecol 2010 Jul; 36(1):20–25. doi: 10.1002/uog.7527. PMID: 20069671.
4. Malinger G, Paladini D, Pilu G, Timor-Tritsch IE. Fetal cerebral magnetic resonance imaging, neurosonography and the brave new world of fetal medicine. Ultrasound Obstet Gynecol 2017; 50:679–680.
5. AIUM practice parameter for the performance of neurosonography in neonates and infants. J Ultrasound Med 2020; 39: E57–E61. https://doi.org/10.1002/jum.15264.
6. Paladini D, Malinger G, Birnbaum R, Monteagudo A, Pilu G, Salomon LJ, Timor IE. ISUOG practice guidelines (updated): sonographic examination of the fetal central nervous system. Part 2: performance of targeted neurosonography. Ultrasound Obstet Gynecol 2021; 57: 661–671. https://doi.org/10.1002/uog.23616.

About the Author

Eran Bornstein, MD, FACOG, is an associate professor of Obstetrics & Gynecology in the Zucker School of Medicine/HOFSTRA, and the Director of the Center for Maternal Fetal Medicine and Ultrasound in OBGYN, at Lenox Hill Hospital, Northwell, in New York.

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Interested in learning more about fetal neurosonography? Check out the following articles from the American Institute of Ultrasound in Medicine’s (AIUM’s) Journal of Ultrasound in Medicine (JUM). Members of AIUM can access them for free after logging in to the AIUM. Join the AIUM today!

• Thrombosis of an Ectatic Torcular Herophili: Anatomic Localization Using Fetal Neurosonography
• Interobserver Reproducibility of Transabdominal 3-Dimensional Sonography of the Fetal Brain
• Sonographic Examination of the Fetal Vermis