Persistent red marrow versus chronic sacroiliitis in spondyloarthritis on a fluid-sensitive MRI of the sacroiliac joints

(A) Fluid-sensitive MRI of the sacroiliac joints of a 13-year-old male with HLA-B27-positive SpA. The bright periarticular signal in the sacrum (dashed arrows) was interpreted as being consistent with active SpA. However, the distribution of the signal along the edges of the sacrum, following the contours of the sacrum along the superior edge of the unfused ala, is more consistent with persistent red marrow.

(B) Coronal STIR images of a 14-year-old HLA-B27-positive male with back pain and stiffness who has findings consistent with chronic sacroiliitis. Prominent bone marrow edema is observed bilaterally (*), as well as irregularity of the joint margins (arrowheads). Normal metaphyseal equivalent signal is also seen (arrows).