The 51 patients had a median age of 15 years, and 57% were male. Nineteen patients (22 joints) were diagnosed as having sacroiliitis based on postcontrast imaging, and none had synovitis in the absence of bone marrow edema. All 22 joints demonstrated bone marrow edema on both fluid-sensitive and postgadolinium T1-weighted fat-saturated sequences. Eighteen percent of joints with sacroiliitis had capsulitis, which was observed on both noncontrast and postcontrast imaging