A comparative study was conducted on 4D sonographic and tangential coronal view of 2D sonography coronal plane in the assessment of fetus face mainly the mouth and nose anatomy and their movements. Most of the reviewed literature assessed only fetal behavior using 4D ultrasound. Only the study of Ocal et al. compared 4D and 2D ultrasound concerning the detection of fetal anomalies [4].
To the best of our knowledge, the current research is the first study that compares 4D with 2D in the tangential anterior coronal plan. The view of the nose and lips is obtained by 2D scanning of the face in an anterior coronal plane, in which the tip of the nose and the details of the lip are seen in the same view [7].
This study concluded that 2D sonography tangential views are better and more rapid and accurate in comparison with 4D examination in the anatomical delineation of the mouth and nose due to the following causes.
The first cause
The tangential view obtained by coronal, direct, or oblique views in more or less tangential axis to the face by slow movements to get a slice thickness to include the nose and mouth in details with to and fro movement getting more information about the structures and morphology and with sustained images will observe the movement of the lips, mouth opening, and tongue movement and size, as well the overall facial expressions, if any. It is real-time imaging.
The 4D sonographic image presents a surface rendering image of the structures by the reflection of the amniotic fluid around the fetus. The presence of amniotic fluid is essential to obtain a clear and informative image. Increasing the amniotic fluid helps the operator to get more detailed findings, so we obtained the image of a fluid, not the face itself.
The observation of fetal faces by 4D sonography was hampered as the images were only near-real-time [9, 10]. Azumendi et al. ensured that the examination of the fetal face is facilitated by the presence of the surrounding fluid [11].
The second cause
One of 4D examination preconditions is sufficient amniotic fluid around the examined part to obtain an optimal 4D image. Thus, the lack of amniotic fluid around the intra-fetal structures makes it impossible to obtain diagnostic 4D ultrasound examination [4].
In coronal tangential images, we do not need much amniotic fluid to clarify the target site, even minimal or trace amount; 22 cases showing decreased amniotic fluid [AFI 7–9 cm] and 8 showing oligohydramnios [AFI 1–5 cm] in about 26 cases. 4D sonography was less informative and showed poor image quality in comparison with 2D, which was also less informative in 7 cases only.
The third cause
4D sonography gets enface image after the adjustment of the probes to en-profile images. We need mid-sagittal images to get a clear 4D image, which looks tedious in many cases either due to low lying or deeply impacted head, or posterior enfacement of the face toward the posterior wall not toward the anterior side or otherwise marked flexion of the neck or complex head direction. All are difficult positions that need a long time to access the mid-sagittal view and for optimizing fetal position and lastly may not be accessed.
The case is different in 2D images. We just access the head in the coronal view and move toward the face, even when image quality may be suboptimum, but we will get some diagnostic data. From 17 cases in whom the posterior deep fetal head orientation or deeply seated fetal head at the pelvis, 4D could not show a clear diagnostic image in 14 cases, compared to only 11 cases in the 2D, but with a non-significant difference (P value 1.000 and 0.564, respectively).
Fetal lie, presentation, and position can all affect the configuration of standard diagnostic planes. Informative images can be obtained successfully when the fetus offers an optimal window (e.g., spine down); however, when the fetus moves to a different position (e.g., spine up), the same informative images may not be obtained [12].
The fourth cause is close proximity to adjacent structures, such as hand, limbs, umbilical cord, placenta, or uterine wall to the face. Therefore, the amniotic fluid layers are thinned to obtain a good 4D image. We do not need excess amniotic fluid to obtain coronal 2D views. From 36 fetuses’ structures that intervene in the face, 4D could not show clear images in 27 fetuses, compared to 11 cases for 2D. The most common cause is close proximity to hand and limbs seen in 13 fetuses from whom 4D examinations failed in 9 cases, and 2D examinations could not present clear images in four cases only with a significant difference (P value = 0.049).
Clear 4D ultrasound examination needs the field around the fetal face empty of any fetal parts or cord, just amniotic fluid in adequate amount [4]. Kanenishi et al. published an observation that the fetal face was difficult during 4D examination when fetal extremities or the umbilical cord were in front of the face, or the fetal face was facing the uterine wall or the placenta [13].
The fifth cause
Volumetric sonography requires dedicated software and a special transducer. In contrast, 2D is not dependent on specific ultrasound platforms and applicable by the commonly curved array transducer [12].
The sixth cause is the assessment of the movements of the lips and tongue as well some of the facial expressions. 4D examination results in some image degradation by the movement of any structures that affect the stagnation of the amniotic fluid, so with lips or tongue movements the amniotic fluid moves, and the acquisition of the reconstructed image is of variable quality in comparison with the 2D image, which is not affected by the movement of the structure or the adjacent amniotic fluid. Thus, the image is not hazy. So 2D ultrasound presents a good sufficient job as regards facial expression.
The seventh (last) cause is the wide availability and the low cost of 2D sonographic machines relative to 4D, making 2D examination more available and easily accessible for a very pregnant lady.
In this study of 220 fetuses, the face is not seen in 21 by 2D and 39 by 4D. This finding agrees with Pretorius et al. who studied 71 fetuses of whom faces were seen in 68 and not seen in 3 by either 2D or 3D sonography [14].
In this study, the mouth movement is the commonest facial expression. It agreed with the findings of Kanenishi et al. that the mouthing movement was the most common facial expression at 20–34 weeks of gestation followed by tongue expulsion [13]. Fetal mouthing is considered the most frequent facial expression observed by 4D ultrasound [15].
Still, 4D sonography is superior in some cases and conditions regarding the face. The entire face assessment of 4D is well impressive for facial expression rather than 2D coronal view.
According to Lebit et al., the most benefits of 4D ultrasound could be a real-time assessment of fetal face, grimacing, breathing movements, swallowing, mouthing, isolated eye-blinking, and the direction of the limbs [15]. In seven studies, 2D was more diagnostic in comparison with 3D, whereas in another four studies, both 2D and 3D had the same diagnostic capability [4].
Öcal et al. reported that 2D US was significantly better than 4D USG in detecting anomalies (P < 0.001). However, 4D US was superior to 2D US in terms of intelligibility among the cases with a superficial anomaly (P < 0.005) [4].