[18F]NaF PET-MRI provides direct in-vivo evidence of the association between bone metabolic activity and adjacent synovitis in knee osteoarthritis: a cross-sectional study.

Summary Objective Synovitis is hypothesized to play a role in the development and growth of osteophytes. Our objectives were to use hybrid positron emission tomography-magnetic resonance imaging (PET-MRI) to (1) determine whether synovitis adjacent to peripheral bone subregions with increased metabolic activity is greater than adjacent to regions without increased metabolic activity and (2) assess the association between subregional bone metabolic activity and adjacent synovitis. Design We recruited 11 participants (22 knees) with a diagnosis of OA in at least one knee. Simultaneous bilateral knee PET-MRI was performed. We quantified bone metabolic activity using the radiotracer [18F]sodium fluoride ([18F]NaF) with calculation of maximum standardized uptake values (SUVmax). Synovitis was quantified using dynamic contrast-enhanced MRI with calculation of Ktrans. Bone subregions were coded as osteophyte (OP), focal increased [18F]NaF uptake without osteophyte (FIU), or normal (no osteophyte or FIU). We used robust linear mixed effects models to assess differences in adjacent Ktrans between different subregion types and to assess association between Ktrans and adjacent SUVmax. Results 94 OPs were detected (59 MOAKS grade 1, 30 grade 2, 5 grade 3), along with 28 FIU and 18 normal subregions. Ktrans was higher adjacent to FIU (adjusted mean[95% CI] = 0.06[0.03,0.09]) and OPs (0.08[0.05,0.11]) when compared to normal bone subregions (0.03[0.00,0.09]). PET SUVmax was positively associated with adjacent Ktrans (β[95% CI]=0.018[0.008,0.027]). Conclusions Synovitis is more intense adjacent to peripheral bone regions with increased metabolic activity than those without, although there is some overlap. Subregional bone metabolic activity is positively associated with intensity of adjacent synovitis.