How to… Practical advice on imaging-based techniques and investigations with accompanying slides and videoclips online The corpus callosum (CC) is the largest commissural pathway connecting the two cerebral hemispheres. It develops relatively late during cerebral ontogenesis, not assuming its definitive shape until 20 weeks of gestation, and continues to grow well after delivery1. Therefore, a proper prenatal sonographic evaluation can be performed only after 20 weeks. Ideally, the CC is assessed on ultrasound by direct visualization. It is a thin band of white-matter fibers and is not depicted using a standard axial plane. It can be seen in the coronal plane, but is only demonstrated in its entire length by using mid-sagittal views, that represent the gold standard for diagnosing abnormalities of this structure1, 2. Visualization of coronal and mid-sagittal planes requires technical skill, and is not recommended in standard examinations of low-risk pregnant patients3, 4. Reference ranges of fetal CC dimensions have been published and can be used to assess normal and deviant development5. There is a general consensus that diagnosing CC abnormalities is difficult. In standard examinations, absence of the CC may be detected because of either indirect cerebral findings, such as ventriculomegaly, absence of the cavum septi pellucidi or widening of the interhemispheric fissure, or associated extracranial findings6. The sensitivity of screening exams is, however, unknown, but is probably limited1. The interested reader is referred to a recent comprehensive review6. The aim of the present article, together with the accompanying slides and videoclips, is to serve as a practical guide for non-expert operators in their attempts to achieve regularly a satisfactory ultrasonographic assessment of the fetal CC. Adequate demonstration of the CC in the second trimester can often be achieved by standard transabdominal ultrasonography. However, in vertex fetal presentation, a transvaginal scan with a high-frequency transducer provides better resolution. In breech presentation, a transfundal approach is the only possibility1. In coronal and mid-sagittal views, the CC appears as a thin anechoic space, bordered superiorly and inferiorly by echogenic lines6, 7. The mid-sagittal view is certainly the most useful view2, 8. The ultrasound beam crosses the large midline acoustic window, formed from anterior to posterior by the frontal or metopic suture, the bregmatic fontanel and the sagittal suture, and this allows good resolution of the brain structures of the midline. For a proper assessment of the CC, good-quality two-dimensional (2D) gray-scale imaging is essential and is generally sufficient. Color Doppler may, however, play a complementary role, particularly in early gestation. In cases of difficult visualization, three-dimensional (3D) ultrasound may be helpful. Two practical approaches may be of help in order to achieve adequate visualization of the CC: Once the mid-sagittal plane of the fetal brain has been obtained, applying color Doppler will demonstrate the course of the pericallosal artery. This may be helpful, particularly in early gestation and in dubious cases. Proper adjustment of pulse repetition frequency (main cerebral arteries have velocities in the range of 20–40 cm/s during intrauterine life) and signal persistence enhances visualization of small vessels3. The main advantage of 3D imaging is the possibility of obtaining a ‘virtual’ mid-sagittal plane reconstructed from an axial approach, thus avoiding the need to align the transducer with the midline cranial sutures9. However, direct 2D visualization allows images of much superior quality. Furthermore, 3D ultrasound only allows visualization of the external contour of the CC and therefore does not allow identification of abnormalities of CC thickness10, 11. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
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Jan 1, 2016
Achour Radhouane + 2
Open Access
