A transverse (a) and dorsal (b) multiplanar reconstruction CT image at the level of the sacroiliac joint using bone windowing (window width, WW = 2,500, window level, WL = 500). There is asymmetric widening of the left sacroiliac joint with an irregular loss of sharp cortical margins with evidence of mild bony destruction of the ilium, and to a lesser extent, the lateral aspect of the sacrum (block arrows). There is subtle amorphous periosteal proliferation of the left ilium immediately caudal to the sacroiliac joint (curved arrow). Cd, caudal; Cr, cranial; D, dorsal; L, left; R, right; V, ventral