Question

Which of the following is the guideline for central ray angulation for an AP projection of the knee of a patient with thin thighs and buttocks?

1) 0-degree central ray angle
2) 3 to 5 degrees caudal angle
3) 3 to 5 degrees cephalic angle
4) 5 to 7 degrees cephalic angle

Answer 1

For an AP projection of the knee, a 3 to 5 degrees caudal angle is advised for the central ray, especially when considering the Q-angle which indicates the femur's lateral angulation.

Step-by-step explanation:

The guideline for central ray angulation for an AP projection of the knee of a patient with thin thighs and buttocks is a 3 to 5 degrees caudal angle. This is because the femur naturally angles laterally away from the vertical plane, which is indicated by the Q-angle, leading to a diagonal orientation within the thigh. The Q-angle, generally ranging from 10-15 degrees, plays a significant role in the central ray angulation as it accounts for the natural angle of the femur, especially noticeable in females who tend to have a wider pelvis and a larger Q-angle.

By using a 3 to 5 degrees caudal angle for the central ray during an AP knee x-ray, the radiographic beam compensates for the diagonal orientation of the femur, ensuring a more accurate representation of the knee structure. It is important to tailor the central ray angle to the patient's body habitus to obtain the best radiographic visualization.