Teenager with low back pain

MRI of sacroiliitis
Axial (above left) and coronal (above right) STIR MRI without contrast of the sacroiliac joints shows a linear area of bright signal in the left iliac bone just lateral to the left sacroiliac joint. Axial (below left) and coronal (below right) T1 MRI with contrast shows enhancement in the same area.

The diagnosis was left sacroiliitis.