Figure 2: A female patient with three studies showing progression from equivocal to unilateral to advanced bilateral sacroiliitis. (a) The initial equivocal CT, obtained at age 26, shows mild asymmetric sclerosis on the iliac side of the right sacroiliac joint, with a suggestion of a single erosion and slightly poor definition of the iliac subchondral cortex. (b, c) After two years, the suspected changes have progressed into definite unilateral sacroiliitis, with broad erosions and dense inflammatory sclerosis surrounding the right sacroiliac joint. (d) After an additional 11 years, the disease has progressed into bilateral advanced sacroiliitis, with widespread erosions and inflammatory sclerosis affecting both the sacral and iliac sides of the joints.