Determining rheumatoid arthritis disease activity in the sacroiliac joint with FDG- and NaF-PET/CT

Objectives: The sacroiliac (SI) joint is one of the largest synovial joints in the body and can be involved in rheumatoid arthritis (RA). ¹⁸F-sodium fluoride (NaF) is a PET tracer that deposits in sites of new bone formation. Inflammation can be imaged by ¹⁸F-fludeoxyglucose (FDG)-PET/CT. The aim of this study was to determine the utility of the FDG and the NaF mean standardized uptake value (SUVmean) in the SI joints of RA patients.

Methods: Eighteen RA patients underwent prospective imaging by FDG-PET/CT and NaF-PET/CT. The anatomical boundaries of the SI joint were defined as a rectangular region with the lateral and medial borders forming 2 cm laterally and medially from the articulation of the sacrum and the ilium. The superior border and inferior border were defined as the superior and inferior ends of the articulation, respectively. CT segmentation with a lower threshold of 150 HU and an upper threshold of 1500 HU, followed by morphological closing, was performed on the fused PET/CT image. The SUVmean was calculated separately for left and right SI joints on FDG-PET/CT and NaF-PET/CT images. Paired t-tests were used to determine differences in uptake between left and right SI joints. Linear regressions were used to compare averaged left and right PET parameters with clinical data.

Results: No significant correlation was found between the uptake of FDG and NaF. Both the NaF SUVmean and the FDG SUVmean showed significantly higher uptake in the right SI joint compared to the left SI joint. FDG SUVmean was significantly inversely correlated with patient global visual analogue scale (VAS-PtGlobal) (p = 0.003). VAS-PtGlobal uses a 100 mm visual analog scale (VAS) to represent the patient’s subjective assessment of disease severity. NaF SUVmean was significantly positively correlated with BMI and leptin (p = 0.03 and p = 0.01, respectively), but there were no significant associations observed between NaF SUVmean and traditional indicators of RA disease activity like sedimentation rate (ESR), C-reactive protein and Disease Activity Score in 28 joints.

Conclusions: Uptake of NaF was associated with BMI and leptin, suggesting that joint degeneration influences NaF uptake in the SI joint of RA patients. No significant associations were observed between NaF SUVmean and traditional indicators of RA disease activity like ESR sedimentation rate and Disease Activity Score in 28 joints calculated with C-reactive protein, while FDG SUVmean was inversely correlated with VAS-PtGlobal. This indicates that uptake of either tracer at the SI joint seems to be independent of overall disease activity. Future studies evaluating a large number of joints in RA patients using FDG- and NaF-PET/CT are necessary to determine their utility in evaluating RA disease activity.