Not just bone. Anything that alters the space
normally occupied by the fluid will affect the
shadow of the opaque dye. So it will show a
herniated disc, some cancers, some aneurisms,
any partitioning of upper and lower canal spaces,
and [I'm unsure.] a "cavitation" of the cord.
The most common of these is probably the disc.
Alton, who has had a few