Which central ray enters on the midline for AP axial projections of both sacrum and coccyx?

• A. MSP
• B. ASIS
• C. Pubic symphysis
• D. Iliac crest

Answer: (A)

Directing the central ray along the midline (the mid-sagittal plane) is essential because the sacrum and coccyx lie on the body's midline. By entering the CR through the midline, you center the beam on the structures of interest and use the correct cephalad or caudad angulation without introducing body rotation from pelvic tilt. For an AP axial sacrum projection, you angle the beam cephalad and aim the CR about 2 inches above the pubic symphysis, through the midline. For an AP axial coccyx projection, you angle the beam caudad and aim about 2 inches below the pubic symphysis, also through the midline.

Choosing a lateral landmark like the ASIS, pubic symphysis, or iliac crest would place the beam off the midline, risking pelvic rotation, distortion, and off-center projection of the sacrum or coccyx.