FOURTH VENTRICLE

• Anatomy.
• Extends from the Aqueduct of Sylvius to the Foramen of Magendie.
• Separates the brainstem from the cerebellum.
• Lateral walls formed by cerebellar peduncles.
• Roof formed by cerebellar peduncles, medullary velum and cerebellar nodules.
• Floor is delineated by the surface of the pons and medulla oblongata.
• Posterolateral extention, known, as the posterior superior recesses are continuous anteriorly with the lateral recesses which pass forward around the brainstem (this results in the boomerang shape).
• Imaging Plane.
• Angle transducer (3.5 or 5 Mhz) caudally from the standard plane used to obtain the traditional axial view of the cerebellum and cisterna magna. May require considerable effort to visualize.

ULTRASOUND

• Hypoechoic.
• Anteromedial portion of the cerebellum.
• Shape = ovoid or round, triangular or boomerang shaped. Shape does not correlate with gestational age except that the boomerang shape is more common in the 3rd trimester.
• Size * Increases with gestational age.
• Mean AP diameter = 3.5mm ± 1.3mm.
• Mean width = 3.9mm ± 1.7mm.
• First seen at 16 weeks GA but best visualized at 21-25 weeks GA
• May be difficult to image prior to the middle of the 2nd trimester or late in the 3rd trimester.

REFERENCES

1. Baumeister LA, Hertzberg BS, McNally PJ et.al. Fetal Fourth Ventricle: US appearance and frequency of depiction. Radiology 1994;192:333-336.