Typical MR changes in ankylosing spondylitis depend on the disease stage.
Early stages: Signs of disease activity in the form of edema and/or contrast enhancement in the subchondral bone. The changes are often bilateral, but may be unilateral and vary from side to side over time. To establish an AS diagnosis there need to be accompanying structural changes in the form of erosion, joint space alteration and/or fatty deposition in the bone marrow to fulfill the radiographic New York criteria.
Middle stages: Is often dominated by structural changes (erosion, joint space alteration in the form of narrowing or widening and fat deposition in the bone marrow). During active stages the structural changes are mixed with signs of activity (edema and/or contrast enhancement), which can vary from side to side.
Late stages: Increasing joint destruction with partial or complete fusion (ankylosis) of the joints, which can appear as "ghost joints" (ankylosis with visible joint contours).