How to image the fetal corpus callosum Aly Youssef, Tullio Ghi, Gianluigi Pilu Department of Obstetrics and Gynecology University of Bologna Italy

How to: image the fetal corpus callosum Indication: ultrasonographic evaluation of the fetal corpus callosum When? ideal gestational age is ≥ 20 weeks Machine settings: routine obstetric anomaly scan Ideal fetal lie: on back or one side; the transabdominal approach is facilitated by a breech or transverse presentation; the transvaginal approach provides excellent resolution but requires a cephalic presentation

a b c Acoustic window: 3D images of the fetal skull (a,b) demonstrate the wide acoustic windows provided by the frontal suture (1), bregmatic fontanel (2) and sagittal suture (3). By aligning the acoustic beam with these windows, excellent resolution of cerebral structures including the corpus callosum is possible (c). As the acoustic window runs along the entire calvarium, the corpus callosum can be imaged by anterior, superior and posterior approaches (arrows).

a b Fetal profile approach Obtain a mid-sagittal fetal profile view (a) Move the transducer cranially and angulate in a caudal direction in order to use the acoustic window of the anterior fontanel and the frontal suture Slight lateral movements of the probe will be required to locate the mid-sagittal plane thus demonstrating the corpus callosum (b, highlighted in yellow). b

a Coronal approach Obtain a coronal section of the anterior horns of the lateral ventricles Move the probe to orientate the cavum septi pellucidi (arrow) vertically (a). Rotate the transducer 90° obtaining a midline brain section and visualizing the corpus callosum (b, highlighted in yellow). b

a Pericallosal artery The pericallosal artery (arrows) is demonstrated by color Doppler in the mid-sagittal view Identification of the pericallosal artery highlights the presence of the corpus callosum Appearance of the pericallosal artery in a normal fetus at 20 weeks’ gestation (a) Partial agenesis of the corpus callosum inferred by a partial Doppler signal from the pericallosal artery where the presence of the corpus callosum was difficult to assess in b-mode (b) b

3D assessment of the corpus callosum Acquisition plane: the standard BPD axial plane

3D assessment of the corpus callosum Acquire a 3D volume The multiplanar mode displays the axial (A), coronal (B), and sagittal (C) planes Position the intersection point of the planes in the cavum septi pellucidi B A C

3D assessment of the corpus callosum The arrows indicate the direction of the rotation required to align the midline with the C-plane B A C

3D assessment of the corpus callosum Once the A- and B-planes have been rotated so that the midline is aligned with the C-plane, a mid-sagittal view of the brain is demonstrated, in which it is possible to recognize the comma-shaped anechoic complex formed by the combination of the corpus callosum (CC) and the cavum septi pellucidi (CSP) B A C CC-CSP

3D assessment of the corpus callosum Activate the volume contrast imaging (VCI) 3D mode in order to improve the image quality B B A C

3D assessment of the corpus callosum The intersection point may need to be moved towards either wall of the CSP to optimize the image of the CC B A C

3D assessment of the corpus callosum Another simple 3D technique is the Omniview-VCI (GE Healthcare) Following 3D volume acquisition, activation of the OmniView task Draw a line horizontally on the midline across the cavum septi pellucidi Automatically, a reconstructed mid-sagittal plane is displayed on the right side of the screen A B CC-CSP