Role of FDG-PET/CT in measuring global thigh muscle metabolism in rheumatoid arthritis and control subjects

Introduction: Rheumatoid arthritis (RA) is a chronic inflammatory autoimmune disease that affects synovial joints and results in swelling, pain, and stiffness. ¹⁸F-fluorodeoxyglucose (FDG) is a PET tracer that identifies glucose metabolism. FDG accumulates in inflammatory tissue, thereby functioning as an indicator of inflammation and arthritis. PET imaging is also capable of describing muscle activity. In this study, we predicted that rheumatoid arthritis patients would present with lower global metabolism of the thigh muscle, indicative of decreased muscle use.

Methods: Prospective FDG-PET/CT scans were collected from 16 RA patients and 16 control subjects. The anatomical regions of interest (ROI) were determined by manually delineating superior and inferior boundaries followed by using a growing region algorithm with a lower threshold of 1 hounsfield unit and an upper threshold of 150 hounsfield units, thereby segmenting all the muscle tissue within our preset anatomical boundaries. The superior boundary was set 5 cm below the greater trochanter and the inferior boundary 5 cm above the intercondylar fossa. Age, gender and weight were considered as potential confounding factors, and linear regression analysis was performed to account for those variables. The metabolically active product (MAP) as a surrogate for global metabolism was obtained for each patient by computing the product of volume (cm³) and SUVmean. Linear regression was performed to assess significance.

Results: In the right thigh, RA patients show a significantly higher mean SUVmax than that of the control group as assessed by linear regression (p<0.001). RA patients also show a significantly higher mean SUVmax than the control in the left thigh (p<0.001). RA patients also show a significantly lower muscle volume in both the left and right thigh (p<0.001). Furthermore, linear regression analysis displayed a statistically lower MAP in RA patients in comparison to control in both the left and right thigh (p<0.001).

Conclusions: In both the left and right thigh of RA patients, there was significantly lower muscle volume. Lower muscle volume could be attributed to the atrophy associated with RA, resulting from decreased muscle use. In both the left and right thigh, there was significantly higher mean SUV max in RA compared to controls, suggesting higher level of background inflammation. Furthermore, RA patients have significantly lower global muscle metabolism than the controls. One explanation for this finding could be muscle atrophy in RA patients due to decreased muscle use. Future studies should be aimed at examining FDG uptake in other areas of the body as well as expanding to larger prospective studies to study the impact of RA on muscle metabolism.