Ultrasound Imaging of the Fetal Palate

3.1. Two-dimensional ultrasound

Before the advent of 3D ultrasound, facial clefts were diagnosed by the 2D oblique face view (Pilu et al., 1986)(Fig. 3). The presence of cleft lip detected in this view leads to further assessment of the fetal palate.

The conventional assessment of the fetal face and anterior part of the palate involves the evaluation of these structures in the axial and coronal planes, which could be obtained in 68-95% of fetuses in the mid-trimester (Pretorius 1995, Babcook 1996). The structures to be assessed in the orthogonal planes are as follows:

The sagittal view (or the facial profile) is not always helpful especially in the case of unilateral facial cleft because the view could be very much near normal.

The primary palate and part of the secondary palate are bony. The soft palate including the uvula, is made up of soft tissue. The primary palate is superficial, lying just behind the fetal lip. In contrast, the secondary palate has a dome further inside the head, enclosed by the facial bones (Fig. 2). There are soft tissues present close to the secondary palate in the area e.g. the fetal tongue and the pharynx. The presence of fluid in the oral and pharyngeal cavity and the movement of the tongue may help to define the palate. However, it is known that the hard palate is more difficult to define consistently and the soft palate could not be recognised discretely on 2D ultrasound (Filly & Feldstein, 2000).

3.2. Three-dimensional ultrasound

Ulm et al (1998) used 3D ultrasound to evaluate fetal tooth germs and found it superior to 2D ultrasound with a detection rate of 88-94% versus 56-62%, and noted its usefulness for characterization of facial clefts (Uml et al., 1999).

Fig. 4 and Fig. 5 show the axial plane of the palate in same fetus in 2D and 3D respectively. The application of 3D ultrasound allows the clear visualization of the alveolar ridge in details. The application of 3D ultrasound allows the clear visualization of the alveolar ridge.

Johnson et al. (2000) pointed out the advantages of 3D ultrasound:

1. The fetal face may be viewed in a standard orientation.

2. Interactive display allows the manipulation and scrutinization without the concern for fetal movement

3. Allow artefacts to be detected

4. Allow serial views

5. Allow exact location of the planar images be identified in relation to the rendered image

6. The rendered image allows the family to see the fetal abnormality and the associated abnormality.

Up to this time, shadowing by the alveolar ridge and the facial bones is still an issue for viewing the secondary palate even on 3D ultrasound. Campbell & Lees (2003) introduced the use of three-dimensional reverse face (3D RF) view for the diagnosis of cleft palate (Fig. 6). The usefulness of this technique in the antenatal diagnosis of cleft palate was confirmed

in 8 cases of suspected craniofacial clefts except in a case of a cleft in the soft palate (Campbell et al., 2005). The face is turned through 180 degrees to be viewed in the reverse direction after a 3D volume is obtained on the fetal face. The shadowing by the alveolar ridge could be largely avoided with clearer demarcation of the edge of the facial cleft with this method.

To avoid acoustic shadowing from the alveolar ridge, Pilu & Segata (2006) suggested that the secondary palate to be insonated at a 45 degrees angle in the sagittal plane and 3D ultrasound to be used to construct axial and coronal planes (Fig. 7).

With this method, they were able to obtain satisfactory views of the secondary palate in 10 of 15 cases between 19-28 weeks. They examined the tomographic ultrasound images (TUI) in the orthogonal planes. They confirmed that the axial plane (Fig. 8) allowed the assessment of alveolar ridge defect in cleft lip and palate. However, the tongue might still obscure the edges of the defect in facial cleft. Tomographic ultrasound images of the secondary palate in the coronal plane (Fig. 9) allowed the examination of the secondary palate in serial sections along its length and the edges of the defect appeared to be demarcated. This echoes the finding of Campbell et al. (2003). Although part of the soft palate could be visualized in the sagittal plane, it was not demonstrated in the axial and coronal planes. The limitation with this technique is unfavourable fetal position. (Pilu & Segata 2006)

Platt et al. (2006) demonstrated the use of the “flipped face” view in antenatal diagnosis of facial clefts. Following acquisition of the 3D volume of the fetal face, the volume is rotated to view the palate from below in a thick slice (Fig. 10). They commented that with this method, the full length and width of the mouth and palate in the axial plane could be examined in 2 minutes.

Wong et al (2007, 2008, 2009, and 2010) proposed a rotational method to view the secondary palate especially the soft palate, by rotating the orthogonal planes including the “flipped face” view and the reverse face view to the oblique planes (Fig. 11). With this method, one could view the surfaces of the soft palate at different angles from above or below or from the side with a stored 3D volume of the face. The uvula allows the soft palate to be identified (Fig. 5). The soft tissue around the palate does not appear to affect the view. Similar to the reverse face view, the shadowing by the facial bones and alveolar ridge could largely be avoided because the insonation plane is different from the examination plane.The authors reported the usefulness of this technique in assessing the extent of fetal cleft palate (Wong et al., 2010). However, the application of this method in assessing isolated clefts in the soft palate remains to be tested as this is a less common entity.