Fig. 1

MRI studies of a 22-year-old man, human leucocyte antigen B27-positive, with a 4-month history of inflammatory back pain. The radiographic modified New York criteria were not met, and C-reactive protein was 7 mg/l (reference limit ≤5 mg/l). The upper and lower rows show short tau inversion recovery (STIR) and corresponding T1 spin echo (T1SE) sequences, respectively. The left panel represents an MRI slice from the cartilaginous compartment (solid white arrow on T1SE), the right panel from the ligamentous compartment (broken white arrow on T1SE), and the central panel is a transitional slice containing parts of both the cartilaginous and ligamentous compartment (solid and broken white arrows on T1SE). The black and white pattern of the space between the ilium and the sacrum in the ligamentous compartment has its origin on the T1SE images in sacroiliac ligaments which have low MRI signal intensity surrounded by fat tissue which is bright on T1SE. On the STIR sequence, the ligamentous compartment is comprised of low signal ligaments and fat tissue as the fat signal is suppressed on STIR, and the bright signal relates to fluid in the lumen of blood vessels. In this patient with nonradiographic axial spondyloarthritis, bone marrow edema (curved white and black arrows on STIR) predominantly in the iliac part of the right sacroiliac joint extends from the cartilaginous into the ligamentous compartment