Classical erosion (T1SE sequence). HLA B27 positive, 18-year-old female AS patient with inflammatory back pain for two years. Erosion in the left ilium (arrows) is characterized by loss of visualization of the cortical bone. The erosions are well visualized because they are surrounded by sclerosis as evidenced by very dark signal in the adjacent bone marrow. HLA-B27, human leucocyte antigen B27; T1SE, T1-weighted spin echo sequence.