Which projection of the facial bones requires the central ray to exit at the acanthion?

The parietoacanthial projection, commonly known as the Waters method, is specifically designed for imaging the facial bones with the central ray directed to exit at the acanthion. This positioning effectively demonstrates the orbits, maxillae, zygomatic arches, and nasal bones, providing a clear view of the facial structures.

In this projection, the patient's chin is elevated, so the orbitomeatal line (OML) is placed perpendicular to the film. This orientation allows the X-ray beam to travel through the facial structures at an angle that minimizes superimposition, which is crucial for obtaining detailed images of the facial anatomy.

The requirement for the central ray to exit at the acanthion is significant because it positions the mid-sagittal plane and the OML optimally for the visualization of specific anatomic details. Other projections, like the Caldwell method, lateral, and submentovertex, have different central ray exit points and purposes, making them less suitable for this particular requirement.