What is the tube tilt required for the AP projection of the sacrum?

The AP projection of the sacrum requires a 15° cephalad tube tilt. This tilt is necessary because the sacrum is curved and positioned in alignment with the normal anatomic structure. A cephalad angle helps to align the x-ray beam parallel to the long axis of the sacrum, which leads to improved visualization of the sacral anatomy and reduces the risk of distortion in the image. By angling the tube cephalad, the path of the x-ray beam is directed toward the sacral region, enhancing the clarity of the resulting radiograph and ensuring that important structures are captured correctly.

In this context, a 0° angle would not provide the necessary alignment for optimal imaging of the sacrum, and angles like 15° caudad or 30° caudad would direct the beam in a way that does not properly visualize the anatomy in question. Therefore, the 15° cephalad angle is the appropriate choice for this projection.