Diagnostic challenges in pyogenic spinal infection: an expanded role for FDG-PET/CT.

In a preliminary investigation of FDG-PET/CT for assessment of therapy response of pyogenic spine infection, it was concluded that activity confined to the margins of a destroyed or degenerated joint with bone-on-bone contact represents nonseptic inflammation, regardless of the intensity of uptake. Only activity in bone, soft tissue, or within the epidural space represents active infection. The purpose of this investigation was to assess the performance of these pattern-based interpretation criteria in a series of problem cases of proven or suspected spine infection. Eighty-two FDG-PET/CTs were done for initial diagnosis because other imaging failed to provide a definitive diagnosis and 147 FDG-PET/CTs were done to assess treatment responses. Pattern-based interpretations were compared with the clinical diagnosis based on bacterial cultures and outcomes after cessation or withholding of antibiotic therapy. Pattern-based interpretation criteria achieved a sensitivity and specificity of 98 and 100%, respectively, for initial diagnosis and a specificity of 100% for assessment of treatment response. The same data was analyzed using intensity of activity as the primary factor. Sensitivity and specificity using the intensity-based criteria were 93 and 68%, respectively, for initial diagnosis, and the specificity of a negative interpretation for therapy response was 55%. Differences from pattern-based criteria are highly significant. Pattern-based criteria perform well in problem cases with equivocal MR and for treatment response because they correctly eliminate activity from nonspecific inflammation associated with destroyed joints with bone-on-bone contact. Response occurs within a timeframe that is useful for managing antibiotic therapy.

Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography Interpretation Criteria for Assessment of Antibiotic Treatment Response in Pyogenic Spine Infection.

PURPOSE: The objective of the study was to determine if fluorodeoxyglucose positron emission tomography/computed tomography ((18)F-FDG PET/CT) can assess the response of patients with pyogenic spine infection to antibiotic treatment in a clinically useful time frame. METHODS: Twenty-eight patients with suspected pyogenic spine infection had baseline (18)F-FDG PET/CT. Patients with proven or probable infection were divided into good and poor responders to antibiotic therapy based on clinical criteria. These patients had a follow-up (18)F-FDG PET/CT 6-8 weeks later. RESULTS: Six of 28 patients were deemed negative for infection based on (18)F-FDG PET/CT. Two patients were excluded because of discrepancies in interpretation. Of the 20 patients deemed positive for infection, 13 had a pathogen isolated and all showed (18)F-FDG uptake in bone and/or soft tissue at baseline. Patients with a poor clinical response to treatment had persistent (18)F-FDG uptake in bone and/or soft tissue on follow-up. Patients with good clinical response had uptake confined to the margins of the destroyed disc. None of these patients had recurrent infection, even if antibiotics had already been discontinued at the time of the follow-up scan. CONCLUSIONS: (18)F-FDG uptake confined to the margins of a destroyed disc after antibiotic therapy of pyogenic spine infection must not be considered indicative of persistent infection and likely represents mechanically induced inflammation. (18)F-FDG uptake in bone or soft tissue does indicate active infection. Quantification of activity could not reliably differentiate patients with active infection from those without active infection and those who had had a successful response to therapy. The pattern of activity is critical to accurate interpretation.