The foramen magnum: the underutilized acoustic window to the posterior fossa.

Visualization of the posterior fossa during neonatal cranial sonography through the anterior fontanelle can at times be limited. To improve visualization of the posterior fossa, the foramen magnum was used as an acoustic window. Scans through the foramen magnum were obtained in addition to routine scans via the anterior fontanelle in 21 preterm infants. A subjective and retrospective evaluation was performed comparing clarity and definition of posterior fossa structures using the anterior fontanelle and foramen magnum. Images of the fourth ventricle, pons, vermis, cerebellar hemispheres, and cisterna magna were considered superior in clarity and definition when obtained via the foramen magnum. The tentorium was visualized equally well with both windows. Foramen magnum scanning demonstrated improved clarity of two Dandy-Walker variants in comparison to anterior fontanelle scanning. The use of the foramen magnum in addition to other acoustic windows can improve perceived clarity and definition of posterior fossa structures and aid in the detection of abnormalities and anomalies occurring in this location.