THREE-DIMENSIONAL ULTRASOUND IN THE EXAMINATION
OF THE FETAL SKELETON:

The three dimensional and multiplanar ultrasound allows the operator to have numerous computed graphic windows to conduct a very accurate study of the different parts of the fetal skeleton.

Graphic window of multiplanar ultrasound.
This allows a multiplanar comparison of an acquired measurement. It is especially useful to:

• Study defects of the upper jaw and evaluate the extent of palatal damage, thus providing the plastic surgeon and perinatologist the means to schedule the most suitable correction procedure once the baby is born.
• Study the defects of the neural tube. The multiplanar comparison highlights the relationship of the pathognomonic ultrasonographic report of the sagittal scan (double interrupted track) with the transverse scan (open vertebral ring).
• Evaluate limb abnormalities. Lee has reported a case of phocomelia accurately detected by three-dimensional ultrasound examination.

Graphic windows from '3D rendering'
This allows an in-depth study of the pathologies above mentioned, and provides graphic instruments that may be used to illustrate and explain the pathology to the parents.
There are various filters available for the acquisition of the different details :

• Maximum transparency filter - This is most appropriate to study the skeleton, as it is able to detect the highest echogenicity of the structure;
• X-ray filter: Gives a radiographic effect to the reconstruction, thus showing the gradations or areas of different echoic response;
• Surface filter: Allows the maximum definition of the proximal points but does not provide the 'depth effect'.

Our group has studied the application of 3D ultrasound for the evaluation of cranial sutures and fontanelles (M. Iaccarino, E. Vavarigos et al - ' Three dimensional ultrasound in the study of fetal cranial sutures and fontanelle', published in 'Giornale Italiano di Ostetricia e Ginecologia - CIC Edizioni Internazionale, 1999) and has shown how three-dimensional ultrasound is useful in the evaluation of various parts of the fetal cranium.

Fig, 1a Pregnancy at 19th week gestation. 3D rendering of calvarial vault which put in evidence the bregmatic and lambdoidean fontanels.

Fig. 1b Schematic illustration showing the upper view of the cranium showing the sutures and the fontanelle.

Fig. 2a Pregnancy at 17th week gestation. Transabdominal volume examined in 3D rendering. Lateral vision of cranial suture and fontanelle.

Fig. 2b Schematic illustration of calvarial vault-lateral view: identification of fontanelle and sutures.

Fig. 2c Same case as in previous figure. 3D rendering with surface filter showing how the external area of the acoustic orifice in fig 2a corresponds to the auricle of the ear.

Fig. 3a Pregnancy at 17th week gestation. Normal upper jaw evaluated by multiplanar rendering.

Fig. 3b Pregnancy at 17th week gestation. Normal lower jaw evaluated by multiplanar rendering.

Fig. 4a Pregnancy at 23rd week gestation. Cleft palate. Multiplanar ultrasound applied to the examination of the pathologic upper jaw.

Fig. 4b Pregnancy at 23rd week gestation. Cleft palate. Stereotaxic voxel is now positioned on the physiologic lower jaw.

Fig. 5 Pregnancy at 26th week gestation. Multiplanar examination of physiologic rachis.

Fig. 6 Pregnancy at 24th week gestation. Multiplanar examination of vertebral anterior and posterior ossification centres.

Fig. 7 Pregnancy in 22nd week. Transabdominal volume examined in 3D rendering with surface filter. Fetal rachis.