MR imaging of septic sacroiliitis

Objective. To investigate the diagnostic value of magnetic resonance (MR) i maging in detecting septic sacroiliitis and to determine whether the MR cha racteristics allow this entity to be differentiated from sacroiliitis in sp ondylarthropathy (SpA). Patients and design. The imaging findings of 11 patients with septic sacroi liitis were retrospectively analyzed by two experienced radiologists. Radio graphic surveys of the pelvis as well as computed tomography (CT) and MR im ages of the sacroiliac joints were available in all cases. Seven of the pat ients additionally underwent a follow-up MR examination. The MR imaging pro tocol comprised combinations of coronal and transverse T1-weighted spin-ech o (SE) or fast SE sequences, T2-weighted gradient-echo (GE) sequences and s hort tau inversion recovery sequence (STIR) sequences as well as dynamic co ntrast-enhanced T1-weighted acquisitions. Results. Three patients with a short disease history showed anterior and/or posterior subperiosteal infiltrations ("lava cleft phenomenon"), transcaps ular infiltrations of juxta-articular muscle layers, which obscured the fas ciae, and periarticular bone marrow edema. The eight patients with more adv anced stages of sacroiliitis additionally showed abscess formation, sequest ration, and erosion. At follow-up MR examination (n=7) under systemic antib iotic treatment, the morphologic characteristics showed progression (n=1), regression (n=4), unchanged findings (n=1), or a mixed response (n=1). Clin ical improvement precedes resolution of the MR findings. Conclusions. Anterior and/or posterior subperiosteal infiltrations and tran scapsular infiltrations of juxtaarticular muscle layers were depicted in al l patients. These MR imaging findings are characteristic of septic sacroili itis and may be used to differentiate this entity from sacroiliitis in SpA.