For an AP projection, open-mouth projection of the cervical spine, the central ray should be positioned at the:

• A. Center of the dens
• B. Center of the open mouth
• C. Center of the nasal cavity
• D. Center of the oropharynx

Answer: (B)

In this projection, the central ray is aimed through the center of the open mouth to place the beam right along the midsagittal plane and through the dens. The odontoid process sits just behind the upper teeth, so directing the CR at the center of the open mouth ensures the dens, atlas, and axis are projected clearly in the middle of the image without the teeth or skull obscuring them. This alignment is essential for an accurate view of the atlantoaxial relationship and any potential pathology.

Centering the ray elsewhere would misplace the dens on the image or compromise visibility—through the center of the dens would not align with the mouth entry, through the nasal cavity would be too high and miss the dens, and through the oropharynx would be too low and distort the anatomy.