MR imaging of septic sacroiliitis

To investigate the diagnostic value of magnetic resonance (MR) imaging in detecting septic sacroiliitis and to determine whether the MR characteristics allow this entity to be differentiated from sacroiliitis in spondylarthropathy (SpA).

The imaging findings of 11 patients with septic sacroiliitis were retrospectively analyzed by two experienced radiologists. Radiographic surveys of the pelvis as well as computed tomography (CT) and MR images of the sacroiliac joints were available in all cases. Seven of the patients additionally underwent a follow-up MR examination. The MR imaging protocol comprised combinations of coronal and transverse T1-weighted spin-echo (SE) or fast SE sequences, T2-weighted gradient-echo (GE) sequences and short tau inversion recovery sequence (STIR) sequences as well as dynamic contrast-enhanced T1-weighted acquisitions.

Three patients with a short disease history showed anterior and/or posterior subperiosteal infiltrations ("lava cleft phenomenon"), transcapsular infiltrations of juxtaarticular muscle layers, which obscured the fasciae, and periarticular bone marrow edema. The eight patients with more advanced stages of sacroiliitis additionally showed abscess formation, sequestration, and erosion. At follow-up MR examination (n=7) under systemic antibiotic treatment, the morphologic characteristics showed progression (n=1), regression (n=4), unchanged findings (n=1), or a mixed response (n=1). Clinical improvement precedes resolution of the MR findings.

Anterior and/or posterior subperiosteal infiltrations and transcapsular infiltrations of juxtaarticular muscle layers were depicted in all patients. These MR imaging findings are characteristic of septic sacroiliitis and may be used to differentiate this entity from sacroiliitis in SpA.