Which features are used to determine the level of S3 in the lateral image?

• A. Shadow of the greater sciatic notch, sacral promontory, tip of the sacrum
• B. Pubic symphysis, iliac crest, sacral ala
• C. Coccyx, sacral hiatus, L5 vertebral body
• D. Spinous process of T12, iliac crest, sacral promontory

Answer: (B)

Understanding how to map sacral levels on a lateral image comes down to using dependable pelvic reference points. The pubic symphysis is a stable anterior landmark you can locate on a pelvic image, and the iliac crest provides a reliable vertical reference line for aligning the pelvis in the lateral view. The sacral ala, the wing-like sides of the sacrum, gives a clear lateral contour of the sacrum itself. Together, these three landmarks let you anchor the sacral segment you’re looking at and estimate where mid-sacral levels like S3 lie relative to the pelvis.

Other options rely on landmarks that don’t align as consistently with S3 on a lateral view—things like the sacral promontory (more related to S1 anteriorly), coccyx or sacral hiatus (lower or variable regions), or thoracic landmarks such as the spinous process of T12. These are less useful for pinpointing S3, whereas using the pubic symphysis, iliac crest, and sacral ala provides the most reliable reference framework for locating S3.