Final answer:
The central ray (CR) for a PA projection of the skull is perpendicular to the image receptor, entering at the nasion and exiting at the inion, highlighting the greater wings of the sphenoid bone and the skull's lateral projections.
Step-by-step explanation:
The angle of the central ray (CR) when a posteroanterior (PA) projection of the skull is performed is perpendicular to the image receptor. The CR should enter approximately at the level of the nasion, which is the junction of the frontal and the two nasal bones, and pass through the skull exiting at the level of the inion, which is the most prominent point of the external occipital protuberance. This technique ensures that the greater wings of the sphenoid bone, lateral projections of the sphenoid bone that form the anterior wall of the middle cranial fossa and an area of the lateral skull, are appropriately demonstrated.