How is the central ray positioned for a parietoacanthial projection (Waters method)?

The central ray for a parietoacanthial projection, commonly known as the Waters method, is positioned perpendicular to the image receptor and exits at the acanthion. This alignment is crucial for accurately demonstrating the maxillary sinuses and providing a clear view of the facial structures.

When performing this projection, the patient's orbitomeatal line (OML) should be adjusted so that it is at a 37-degree angle with the image receptor. This orientation allows the central ray to pass through the acanthion, which is located at the junction of the base of the nose and the upper lip. Positioning the central ray in this manner ensures that the projection captures the required anatomy in the best possible detail, minimizing distortion and allowing for optimal visualization of the sinuses.

Other positioning options do not align with the specific requirements of the Waters method. For example, a horizontal placement, or an angle directed toward different anatomical landmarks, would not properly demonstrate the maxillary sinuses as intended in this projection. Hence, the correct choice illustrates the proper technique essential for achieving the desired imaging outcome.