What is the angle of the central ray (CR) when a posteroanterior (PA) projection of the skull is performed?

In a posteroanterior (PA) projection of the skull, the central ray (CR) is positioned perpendicular to the image receptor (IR). This orientation is crucial for capturing a clear and accurate representation of the skull structures. When the CR is perpendicular, it ensures that there is minimal distortion of the anatomical features, allowing for an accurate diagnostic evaluation.

In the PA projection, the x-ray beam travels from the posterior aspect of the skull through to the anterior part, entering the back of the head and exiting at the forehead. This approach is standard and typically avoids the need for angulation that may be required in other projections, making it essential to position the patient correctly to achieve this perpendicular alignment.

Other methods of angulation (such as cephalad or caudad) are utilized in different projections for various anatomical purposes, but in the case of the PA projection, maintaining a perpendicular angle is key to obtaining the best image quality.