Which structure is not typically visible in a PA projection of the skull?

In a posteroanterior (PA) projection of the skull, the frontal bone is prominently displayed due to its position at the front of the skull. Similarly, the ethmoidal sinuses, located between the orbits, are often visualized in this view as they lie within the cranial cavity. The dorsum sellae, which is part of the sphenoid bone, can also be appreciated as it’s positioned centrally within the skull base.

In contrast, the occipital bone, located at the back of the skull, is not typically visible in a PA projection due to the nature of this particular imaging angle. The PA view focuses on structures in the anterior and middle cranial fossa and does not adequately capture the posterior structures like the occipital bone, which lies behind the viewer's line of sight. Thus, the anatomy aligned with the posterior aspect of the skull tends to be obscured in this projection.