Correlation between 18F-FDG and 18F-NaF uptake in joint of lower limbs and muscle metabolism in patients with rheumatoid arthritis.

Objectives Rheumatoid arthritis (RA) is a systemic disease characterized by chronic inflammation that primarily affects peripheral joints. However, during the course of the disease, many other body organs including muscles, bone marrow and arterial system are adversely affected by this disease. Positron emission tomography/computed tomography (PET/CT) has become a valuable tool for evaluating musculoskeletal disorders. PET/CT imaging with 18F-Fluorodeoxyglucose (FDG) can assess and quantify physiological and disease activities of the affected sites. Similarly, bone scanning with 18F-Sodium Fluoride (NaF) PET/CT detects the osseous reaction to inflammation, or other destructive processes in the skeleton. The purpose of this study was to assess the correlation between FDG and NaF uptake in joints with FDG uptake in muscles of the lower extremities.

Methods Thirteen RA patients were prospectively enrolled into this research study and underwent separate PET/CT imaging after administration of FDG and NaF within two weeks interval. FDG and NaF uptakes were measured in joints using volumetric PET/CT indices. Muscle metabolism was quantified by measuring FDG uptake in each trans-axial slice of PET/CT scan as an indicator of global muscle activity.

Results There were thirteen (male 9) patients with mean age of 56.3 (SD: 13.34) and disease duration of 7.9 years. Muscle FDG uptake activity in the thigh was not correlated with FDG uptake activity (Rho: 0.018 P: 0.954) and with 18F-NaF uptake activity (Rho: 0.413 P: 0.161) in joints of the lower extremities. Similar results were observed between muscle FDG uptake in leg and joint FDG (Rho: -0.030 P: 0.923) and NaF uptake activity (Rho: 0.330 P: 0.271) in the lower limbs. There was also significant correlation between FDG uptake of femur bone marrow and FDG uptake of joints of lower limbs (Rho: 0,735 P: 0.004). Patients with highter CRP were more likely to have elevated bone marrow FDG uptake (Rho: 0.591 P: 0.033).

Conclusions The approaches employed for analyzing the data from this research study will play a role in assessment of the muscles and joints involved in patients with RA. We have proposed a novel technique for measurement of muscle metabolism, which might be useful for accurate quantification of muscle disease in this population and could be of value as an indicator of this major complication. Altered activities at the cellular and molecular levels as detected by PET will be of importance in the management of RA patients.