OBJECTIVEIn aid of MR imaging diagnosis of ankylosing spondylitis (AS), MRI findings of sacroiliac (SI) joints in volunteers were compared with those in patients with AS, with special reference to the fat findings.

METHODS18 normal volunteers and 52 AS patients underwent the MRI examinations with the same protocols, i.e., SE T1WI, FSE T2WI, GE T2*WI sequences were taken using a 0.3T imager. All the images were observed and analyzed by two radiologists.

RESULTSSymmetric or patchy distribution fat component either in sacrum and/or ilium was detected in 52.8% of SI joints of volunteers. Thin zone distribution of fat was seen in 35.6% of SI joints of AS patients, which was adjacent to the erosion and sclerosis. In another 25.0% of SI joints of AS patients, the distribution of fat component was found similar to that of the normal volunteers. Cartilage changes were detected in 85.6% of the SI joints of AS patients. In addition, bone erosion, sclerosis and subchondral bone marrow edema were also detected.

CONCLUSIONSubchondral fat infiltration associated with erosion and sclerosis should be considered as sacroiliitis. The knowledge of the characteristics of fat component could be helpful for evaluating sacroiliac joints.