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Practical advice on imaging-based techniques and investigations with accompanying slides and videoclips online

BACKGROUND

  1. Top of page
  2. BACKGROUND
  3. PRACTICAL POINTS
  4. REFERENCES
  5. Supporting Information

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 delivery[1]. 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 structure[1, 2]. Visualization of coronal and mid-sagittal planes requires technical skill, and is not recommended in standard examinations of low-risk pregnant patients[3, 4]. Reference ranges of fetal CC dimensions have been published and can be used to assess normal and deviant development[5].

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 findings[6]. The sensitivity of screening exams is, however, unknown, but is probably limited[1]. The interested reader is referred to a recent comprehensive review[6].

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.

PRACTICAL POINTS

  1. Top of page
  2. BACKGROUND
  3. PRACTICAL POINTS
  4. REFERENCES
  5. Supporting Information

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 possibility[1]. In coronal and mid-sagittal views, the CC appears as a thin anechoic space, bordered superiorly and inferiorly by echogenic lines[6, 7]. The mid-sagittal view is certainly the most useful view[2, 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-dimensional ultrasound

Two practical approaches may be of help in order to achieve adequate visualization of the CC:

image

Figure 1. Fetal profile approach. From a standard mid-sagittal view of the fetal profile (a), the corpus callosum is visualized (b) through the acoustic window of the anterior fontanel and frontal suture, following angulation of the transducer.

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Fetal profile approach (Figure 1, Videoclip S1)

This approach is usually feasible transabdominally.

  • Obtain a standard mid-sagittal view of the fetal profile (Figure 1a).
  • Angulate the transducer in order to use the acoustic window of the frontal suture and the anterior fontanel, thus demonstrating the CC (Figure 1b).
  • Fine side-to-side movements may be needed in order to achieve an ideal image of the CC.
image

Figure 2. Coronal approach. Coronal section of anterior horns of lateral ventricles and cavum septi pellucidi (arrow), with the latter oriented strictly vertically. The transducer is then rotated 90°, obtaining a midline brain section and visualizing the corpus callosum (see Videoclip S2).

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Coronal approach (Figure 2, Videoclip S2)

This approach is feasible both transabdominally and transvaginally.

  • From a coronal section through the anterior fontanel, obtain a coronal section of the anterior horns of the lateral ventricles and the cavum septi pellucidi. The latter should be oriented as close to vertically as possible, with the anterior horns on the same horizontal level (Figure 2).
  • Rotate the transducer 90°, obtaining the mid-sagittal plane of the fetal brain.
  • Fine side-to-side movements may be needed in order to achieve an ideal image of the CC.
image

Figure 3. Color Doppler demonstration of cerebral circulation (mid-sagittal view) in a normal fetus at 20 weeks' gestation: the pericallosal artery (arrow) highlights the corpus callosum.

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Color Doppler visualization of the pericallosal artery (Figure 3 and Videoclip S3)

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 vessels[3].

image

Figure 4. Three-dimensional imaging. Axial view at the level of the biparietal diameter (left) and reconstructed orthogonal plane showing the mid-sagittal view (right).

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Three-dimensional assessment of the corpus callosum (Figure 4)

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 sutures[9]. 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 thickness[10, 11].

This approach is usually feasible transabdominally.

  • Obtain a standard axial view of the head at the level of the biparietal diameter, with the intersection point of the planes positioned in the cavum septi pellucidi.
  • Activate the volume contrast imaging (VCI) 3D mode to display the B- and C-planes of the fetal head.
  • The CC should be visible in the C- or orthogonal plane which displays the reconstructed mid-sagittal view of the fetal brain.
  • The intersection point may need to be moved towards either wall of the cavum septi pellucidi to optimize the image of the CC.

REFERENCES

  1. Top of page
  2. BACKGROUND
  3. PRACTICAL POINTS
  4. REFERENCES
  5. Supporting Information

Supporting Information

  1. Top of page
  2. BACKGROUND
  3. PRACTICAL POINTS
  4. REFERENCES
  5. Supporting Information
FilenameFormatSizeDescription
uog12367-sup-0001-HowTo-CCslides.pptWord document4252KCC slides
uog12367-sup-0002-Clip1.wmvapplication/media5162KClip 1 axial to midline 21.11.2012
uog12367-sup-0003-Clip2.wmvapplication/media4310KClip 2 coronal to midline 21.11
uog12367-sup-0004-Clip3.wmvapplication/media9670KClip 3 Doppler 20.11.2012

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