Sonography of the Fetal Cochlea in the Early Second Trimester of Pregnancy

Objectives To examine the feasibility of imaging the fetal cochlea in the early second trimester. Methods This study included 42 healthy fetuses of low‐risk pregnancies between 14 and 16 weeks. The coronal head sections via sagittal and lateral acoustic windows were used for cochlear visualization. The coronal plane was directed across the thalamus, oriented to the petrous part of the temporal bones. All fetuses were examined by 2‐dimensional(2D) transvaginal sonography. Transabdominal scans were added in 11 fetuses at 16 weeks. Three‐dimensional (3D) multiplanar reconstruction was used for coronal plane reformatting in cases with inappropriate head positioning for 2D scanning. Results Because of temporal bone hypomineralization in the early second trimester, sonographic depiction of the cochlear labyrinth was possible. On coronal imaging, the cochleas showed ringlike echogenic borders containing hypoechoic liquid content. The cochlear modiolus appeared as a central echogenic round spot (“cochlear target sign”). The cochleas were observed caudad to the temporal lobes, separated by the clivus. Shifting of the coronal plane showed progressive coiling of the basal cochlear turn toward the second turn (“cochlear whirlpool sign”). Two‐dimensional transvaginal scanning showed at least 1 cochlea in 79% (33 of 42) of the cases. Adding 3D multiplanar reconstruction, at least 1 cochlear target sign was observed in 83% (35 of 42). The cochlear whirlpool sign on at least one side was shown in 57% (24 of 42). The maximal cochlear diameter was measured in fetuses with clear cochlear margin visualization. In this subgroup, the mean gestational age ± SD was 15.8 ± 0.5 weeks, and the mean maximal cochlear diameter was 6.4 ± 0.8 mm. Conclusions Prenatal sonographic inner ear depiction was previously considered impossible. Using a unique time window in the early second trimester, we achieved feasible fetal cochlear imaging.