What is the CR entrance point for a properly positioned AP axial Towne skull projection?

• A. 2 inches superior to the nasion
• B. 1 inch above the glabella
• C. 3 inches superior to the glabella
• D. 2.5 inches superior to the glabella

Answer: (D)

The AP axial Towne projection is designed to pass the central ray through the foramen magnum, so the entrance point must be set so that the caudal angle will place the beam right through that opening. With the skull positioned so the OML is perpendicular to the IR and the CR angled caudally about 30 degrees, the CR should enter about 2.5 inches above the glabella. This fixed landmark locations centers the foramen magnum in the image; measurements from other landmarks would misplace the beam and obscure the target area.