Imaging Small Fetal Parts by High-resolution Ultrasound

INTRODUCTION

Ultrasound imaging technology has remarkably improved and aided in our evaluation of fetal morphology. In particular, small fetal organs that were previously invisible can now be observed using high-resolution ultrasound.

Fetuses have unique morphological features not found in adult humans due to still being in the developmental stage of life. Typical organs different from adults include the central nervous system, thymus, and adrenal gland. The fetal central nervous system undergoes dramatic changes as the fetus develops, and the thymus and adrenal glands are significantly larger in fetuses than in adults.

This review article describes several fetal organs now able to be visualized with high-resolution ultrasound equipment that are not often noticed in clinical practice.

Ultrasound Equipment

Ultrasound images were generated using a high-resolution transabdominal transducer (Voluson E10® with C2-9D, RM7C; GE Health Care, Milwaukee, WI, USA). Single crystals are used as oscillators for these transducers. Compared with conventional polycrystalline oscillators, single-crystal oscillators can visualize ultrasound images with not only high resolution but also high penetration.

Thymus

During the fetal period, the thymus is a relatively large organ that extends upward from the superior thoracic opening to the base of the neck. The thymus regresses somewhat from childhood to puberty and is almost invisible by adulthood due to fat infiltration into the cortex.¹

The fetal thymus is visualized as a quadrangular structure in front of the three vessels at the level of the three-vessel view.² It is difficult to distinguish between the thymus and lungs using conventional ultrasound equipment, but the two can be clearly distinguished using high-resolution ultrasound equipment.

Figure 1 shows a fetal thymus at 29 weeks of gestation. The thymus is visualized slightly less brightly than the lungs. A coarse, fine, linear, and high-intensity echo can be seen inside the thymus. The size of the fetal thymus is significantly correlated with gestational age, and multiple two-dimensional (2D) sonographic measurements of the fetal thymus have already been reported.^3–6 Furthermore, the thymus volume calculated by three-dimensional (3D) ultrasound has also been reported.²

The fetal thymus is often misdiagnosed as a mediastinal tumor if its characteristics are unrecognized by the examiner.

Adrenal Glands

The fetal adrenal glands are 10–20 times larger than the adult adrenal glands in terms of body weight. The substantial size of the fetal adrenal glands is due to the large fetal zone, which is the fetal cortex. The fetal adrenal glands lose about two-thirds of their weight in the first few weeks after birth. The fetal adrenal cortex shrinks markedly in the first year of life, whereas the adrenal medulla remains relatively small after birth.⁷

Figure 2 shows a fetal right adrenal gland at 38 weeks of gestation. The adrenal gland is depicted in a leaf-like manner on axial sections, with the central medulla depicted in high brightness and the fetal cortex depicted in low brightness. Although the adrenal glands shrink after birth, the fetal adrenal gland is significantly correlated with gestational age.^8–10

Pancreas

Figure 3 shows a fetal pancreas at 27 weeks of gestation. Visualization of the pancreas by ultrasound in adults is relatively difficult due to the influence of the surrounding bowel air. In fetuses, there is no air around the pancreas, so it is easier to visualize than in adults, but visualization is still difficult because the brightness with the surrounding tissue is similar. The visualization rate of the fetal pancreas has been reported to be approximately 60%.¹¹ The fetal pancreas size is significantly correlated with gestational age.¹² In axial sections, the spleen blood vessels are a marker for finding the pancreas. The pancreas is visualized on the ventral side of the splenic blood vessels with a slightly higher brightness than the surrounding tissue.

Thyroid

Figures 4 and 5 show a fetal thyroid gland at 25 weeks of gestation in the axial view and coronal view, respectively. The thyroid isthmus is located in front of the trachea, and the thyroid lobes are located on either side of the trachea. They are visualized as a high-brightness structure. Two-dimensional sonographic measurements of the fetal thyroid, as well as the volume, have been reported for a relatively long time.^13–17 Since the thyroid gland can be clearly visualized with high-resolution ultrasound, it is expected that more accurate measurements will be obtained in the future.

Uvula

The fetal uvula is a very small structure, but it can be visualized with ultrasound. The uvula is visualized as an equal sign, and the absence of an equal sign has been reported to aid in the diagnosis of cleft palate.^18,19Figure 6 shows the fetal uvula in an axial section at the level of the nasopharynx. The uvula is easily observable when amniotic fluid is present in the pharynx.

Parotid Gland

Figure 7 shows a fetal parotid gland at 25 weeks of gestation. The parotid gland is visualized as a circular structure with relatively high brightness. Familiarity with the location and normal imaging findings of the fetal parotid gland may help diagnose parotid tumors.

The fetal parotid gland has received little attention, so there are no reports of its measurements.

Trigeminal Nerve

The trigeminal nerve is the largest of the cranial nerves. It exits the brain from the lateral edge of the pons. Figure 8 shows a fetal trigeminal nerve exiting Meckel’s cave at 27 weeks of gestation in the axial transcerebellar plane.

CONCLUSION

Advances in ultrasound diagnostic equipment have made it possible to visualize small fetal organs that were previously invisible. Since the morphology of fetal organs differs from that of adults due to fetuses still developing, we need to be familiar with fetal-specific morphologies. By observing fetal organs, including the small parts, we can better understand the development and anatomy of normal fetuses.

References

1. Steinman GG. Chaneges of the human thymus during ageing. In: Muller-Hermelink HK, ed. The human thymus. Histopathology and pathology. Current topics in pathology, vol.75, 1986. p.43.

2. Li L, Bahtiyar MO, Buhimschi CS, et al. Assessment of the fetal thymus by two- and three-dimensional ultrasound during normal human gestation and in fetuses with congenital heart defects. Ultrasound Obstet Gynecol 2011;37(4):404–409. DOI: 10.1002/uog.8853.

3. Felker R, Cartier M, Emerson D, et al. Ultrasound of the fetal thymus. J Ultrasound Med 1989;8(12):669–673. DOI: 10.7863/jum.1989.8.12.669.

4. Cho J, Min J, Lee Y, et al. Diameter of the normal fetal thymus on ultrasound. Ultrasound Obstet Gynecol 2007;29(6):634–638. DOI: 10.1002/uog.3979.

5. Zalel Y, Gamzu R, Mashiach S, et al. The development of the fetal thymus: an in utero sonographic evaluation. Prenat Diagn 2002;22(2):114–117. DOI: 10.1002/pd.257.

6. Jeppesen D, Hasselbalch H, Nielsen S, et al. Thymic size in preterm neonates: a sonographic study. Acta Paediatr 2003;92(7):817–822. DOI: 10.1111/j.1651-2227.2003.tb02539.x.

7. Pepe GJ, Albrecht ED. Regulation of the primate fetal adrenal cortex. Endocr Rev 1990;11(1):151–176. DOI: 10.1210/edrv-11-1-151.

8. Hata K, Hata T, Kitao M. Ultrasonographic identification and measurement of the human fetal adrenal gland in utero. Int J Gynaecol Obstet 1985;23(5):355–359. DOI: 10.1016/0020-7292(85)90143-2.

9. Jamigorn M, Phupong VJ. Nomograms of the whole foetal adrenal gland and foetal zone at gestational age of 16-24 weeks. Obstet Gynaecol 2017;37(7):867–871. DOI: 10.1080/01443615.2017.1308324.

10. van Vuuren SH, Damen-Elias HA, Stigter RH, et al. Size and volume charts of fetal kidney, renal pelvis and adrenal gland. Ultrasound Obstet Gynecol 2012;40(6):659–664. DOI: 10.1002/uog.11169.

11. Kivilevitch Z, Achiron R, Perlman S, et al. The normal fetal pancreas. Ultrasound Med 2017;36(10):1997–2005. DOI: 10.1002/jum.14233.

12. Hata K, Hata T, Kitao M. Ultrasonographic identification and measurement of the human fetal pancreas in utero. Int J Gynaecol Obstet 1988;26(1):61–64. DOI: 10.1016/0020-7292(88)90197-x.

13. Bromley B, Frigoletto FD, Cramer D, et al. The fetal thyroid: normal and abnormal sonographic measurements. J Ultrasound Med 1992;11(1):25–28. DOI: 10.7863/jum.1992.11.1.25.

14. Ho SS, Metreweli C. Normal fetal thyroid volume. Ultrasound Obstet Gynecol 1998;11(2):118–122. DOI: 10.1046/j.1469-0705.1998.11020118.x.

15. Bernardes LS, Ruano R, Sapienza AD, et al. Nomograms of fetal thyroid measurements estimated by 2-dimensional sonography. J Clin Ultrasound 2008;36(4):193–199. DOI: 10.1002/jcu.20434.

16. Gietka-Czernel M, Dębska M, Kretowicz P, et al. Fetal thyroid in two-dimensional ultrasonography: nomograms according to gestational age and biparietal diameter. Eur J Obstet Gynecol Reprod Biol 2012;162(2):131–138. DOI: 10.1016/j.ejogrb.2012.02.013.

17. Barbosa RM, Andrade KC, Silveira C, et al. Ultrasound measurements of fetal thyroid: reference ranges from a cohort of low-risk pregnant women. Biomed Res Int 2019;2019: 9524378. DOI: 10.1155/2019/9524378.

18. Wilhelm L, Borgers H. The ‘equals sign’: a novel marker in the diagnosis of fetal isolated cleft palate. Ultrasound Obstet Gynecol 2010;36(4):439–444. DOI: 10.1002/uog.7704.

19. Liberty G, Boldes R, Shen O, et al. The fetal larynx and pharynx: structure and development on two- and three-dimensional ultrasound. Ultrasound Obstet Gynecol 2013;42(2):140–148. DOI: 10.1002/uog.12358.