The onset of ankylosing spondylitis begins in the sacroiliac joint, which is commonly referred to as the hip. Patients may experience pain on both sides of the hip, alternating pain, which is worse in the morning, and therefore mainly appears as sacroiliac arthritis on the MRI. On the MRI, the patient can see bone marrow edema in the sacrum or iliac bone of the sacroiliac joint, which means that there is whitening and signal enhancement on the T2 image. In the sacroiliac joint, there is fluid accumulation between the joints, which means that the T2 image and the T2 compression lipid image are bright, white, and have an enhanced signal, which is the earliest manifestation of sacroiliac arthritis in ankylosing spondylitis. Further calcification of the bone will occur, that is, calcification of the ligaments, and at this point, you can see that there is bone formation.