American Society of Radiologic Technologist (ASRT) Practice Exam

In a PA axial projection of the mandibular rami, the central ray is angled 20 degrees cephalically and should exit at the acanthion. The acanthion is the anatomical landmark located at the junction of the nasal spine and the upper lip, serving as a reference point for imaging the mandible.

This specific angling of the central ray is critical as it allows for better visualization of the mandibular rami, effectively projecting them away from the paranasal sinuses and minimizing superimposition. The cephalic angle helps to obtain a clearer image by altering the perspective of the mandible, aiding in diagnostic evaluations.

Other angles such as 20 degrees caudal typically do not provide the same degree of clarity for the structures of interest in this particular projection, and exits at different landmarks (like the mental point) would not target the appropriate anatomy as effectively. Thus, the chosen angle and exit point in the correct response facilitate optimal imaging of the mandibular rami in a PA axial projection.