CAMRT Radiography Practice Exam

What central ray orientation is recommended for an anteroposterior (AP) knee projection if the ASIS measurement is 22 cm?

1. 3 - 5 degrees caudad

2. Perpendicular to the long axis of the patella

3. 3 - 5 degrees cephalad

4. Perpendicular to the IR

The correct answer is: Perpendicular to the IR

For an anteroposterior (AP) knee projection, the central ray should be directed perpendicular to the image receptor (IR) in cases where the measurement from the anterior superior iliac spine (ASIS) to the tabletop is around 22 cm. This orientation ensures that the central ray adequately penetrates the knee joint and captures a clear and diagnostic image without introducing distortion. When the ASIS measurement is at this length, it typically indicates that the patient is of average stature, and therefore a perpendicular central ray will align well with the anatomy. This orientation helps prevent magnification and minimizes potential artifacts that could affect the resulting image. In contrast to other choices that involve angling the central ray either caudad or cephalad, the perpendicular orientation allows for a direct view of the joint space. This is especially important since unnecessary angling can obscure anatomical details or lead to misinterpretation during the image evaluation process. Utilizing the perpendicular method simplifies the process and ensures consistent results across similar patient measurements.