Bontrager's Radiographic Positioning Practice Exam

To see the zygapophyseal joints in the thoracic spine, rotate the patient so the joints lie parallel to the image receptor. The thoracic facet joints are oriented obliquely, and about seventy degrees of rotation brings the joints into profile and opens the joint spaces while minimizing overlap from the vertebral bodies.

If you rotate less (around sixty degrees), the joints don’t open as well and remain overlapped by the vertebrae, making the joints hard to assess. If you rotate to a true lateral (ninety degrees), you don’t get the characteristic oblique view that best demonstrates the zygapophyseal joints, and you lose the joint visibility due to different overlapping structures. A shallow obliquity (about twenty degrees) would likewise fail to open the joints adequately.