Osteomyelitis of the foot: Relative importance of primary and secondary MR imaging signs

PURPOSE: To determine the usefulness of primary and secondary magnetic resonance (MR) imaging signs of osteomyelitis. MATERIALS AND METHODS: MR imaging at 1.5 T was performed in 73 feet (62 patients) with clinical concern for osteomyelitis. Images were reviewed retrospectively and separately by two reviewers in a blinded fashion for primary (abnormal marrow signal intensity) and secondary (ulcer, cellulitis, soft-tissue mass, abscess, sinus tract, cortical interruption) signs associated with osteomyelitis. RESULTS: Of the 73 feet, 43 had osteomyelitis. Discordant marrow signal intensity between individual MR sequences was observed by reader 1 in six (8%) feet and by reader 2 in 15 (21%) feet. For primary signs, fast spin-echo short inversion time inversion-recovery and gadolinium-enhanced fat-suppressed T1-weighted images had the highest sensitivity, and T1-weighted and gadolinium-enhanced fat suppressed T1-weighted images had the highest specificity and least interobserver variability. Signs of cutaneous ulcer, sinus tract, and cortical interruption had the highest positive predictive value for osteomyelitis; signs of soft-tissue mass and cortical interruption had the highest negative predictive value. All had good interobserver agreement except cellulitis. CONCLUSION: When osteomyelitis of the foot is suspected, marrow signal intensity can differ on different types of MR images. Identification of secondary signs may augment diagnostic confidence when abnormal marrow signal intensity is seen.