Bone formation rather than inflammation reflects Ankylosing Spondylitis activity on PET-CT: a pilot study

Introduction: Positron Emission Tomography - Computer Tomography (PET-CT) is an interesting imaging technique to visualize Ankylosing Spondylitis (AS) activity using specific PET tracers. Previous studies have shown that the PET tracers [F-18]FDG and [C-11](R)PK11195 can target inflammation (synovitis) in rheumatoid arthritis (RA) and may therefore be useful in AS. Another interesting tracer for AS is [F-18]Fluoride, which targets bone formation. In a pilot setting, the potential of PET-CT in imaging AS activity was tested using different tracers, with Magnetic Resonance Imaging (MRI) and conventional radiographs as reference. Methods: In a stepwise approach different PET tracers were investigated. First, whole body [F-18]FDG and [C-11](R)PK11195 PET-CT scans were obtained of ten AS patients fulfilling the modified New York criteria. According to the BASDAI five of these patients had low and five had high disease activity. Secondly, an extra PET-CT scan using [F-18]Fluoride was made of two additional AS patients with high disease activity. MRI scans of the total spine and sacroiliac joints were performed, and conventional radiographs of the total spine and sacroiliac joints were available for all patients. Scans and radiographs were visually scored by two observers blinded for clinical data. Results: No increased [F-18]FDG and [C-11](R)PK11195 uptake was noticed on PET-CT scans of the first 10 patients. In contrast, MRI demonstrated a total of five bone edema lesions in three out of 10 patients. In the two additional AS patients scanned with [F-18]Fluoride PET-CT, [F-18]Fluoride depicted 17 regions with increased uptake in both vertebral column and sacroiliac joints. In contrast, [F-18]FDG depicted only three lesions, with an uptake of five times lower compared to [F-18]Fluoride, and again no [C-11](R)PK11195 positive lesions were found. In these two patients, MRI detected nine lesions and six out of nine matched with the anatomical position of [F-18]Fluoride uptake. Conventional radiographs showed structural bony changes in 11 out of 17 [F-18]Fluoride PET positive lesions. Conclusions: Our PET-CT data suggest that AS activity is reflected by bone activity (formation) rather than inflammation. The results also show the potential value of PET-CT for imaging AS activity using the bone tracer [F-18]Fluoride. In contrast to active RA, inflammation tracers [F-18]FDG and [C-11](R)PK11195 appeared to be less useful for AS imaging.