MRI of the fetal face: Anatomy of the eyes and orbits

Abstract

The present paper reviews the normal developmental anatomy of the eyes and orbits and corresponding imaging findings during the period covered by fetal magnetic resonance imaging (MRI), i.e. 20 gestational wk (GW) to birth. Several aspects of orbital and ocular anatomy have to be considered that are peculiar to the fetus. There is normal fetal exophthalmos, since the growth of the eye outpaces that of the orbit. The fetal orbital roof has a typical slanting shape. Eyeball shape changes from elliptic to more circular in axial cross-section between 20 and 25 GW. The vitreous is clearly depicted, while the retina, choroid, sclera and Tenon's capsule together appear as a hypointense T2-weighted rim lining it. Transient structures like the hyaloid arterial system and primary vitreous are too small or offer insufficient contrast to be depicted with fetal MRI. The initially spherical lens gains its elliptical shape by the end of the second trimester. Although the current resolution of fetal MRI is high, several structures can only be depicted when they reach sufficient size. This applies to the anterior chamber and the cornea both of which can only be demonstrated in the third trimester. Visualization of the optic nerve and extraocular muscles depends on the signal intensity of the intraorbital fat, which changes in the third trimester. Because of frequent fluid filling, the lacrimal sacs can be demonstrated following 25 GW.