¹⁸F-sodium fluoride PET/CT as a complementary measure of osteoblastic disease burden in axial spondyloarthritis: a prospective exploratory study

PURPOSE: To determine whether SIJ ¹⁸F-NaF uptake is associated with MRI-defined inflammation and structural damage and whether it provides a complementary measure of osteoblastic disease burden. METHODS: In this prospective exploratory study, 43 adults with chronic low back pain underwent pelvic radiography, sacroiliac joint (SIJ)-MRI, and whole-body ¹⁸F-NaF PET/CT and were classified as having radiographic axial spondyloarthritis or ankylosing spondylitis (AS; n = 29), non-radiographic axial spondyloarthritis (nr-axSpA; n = 5), or degenerative joint disease (DJD; n = 9). MRI was evaluated using SPARCC inflammation and structural scores. SIJ uptake was assessed visually and quantified using SUVmax. The primary endpoint was the correlation between SIJ SUVmax and the SPARCC structural score. RESULTS: SIJ SUVmax correlated with the SPARCC inflammation (ρ = 0.629) and structural (ρ = 0.606) scores (both p < 0.001) on SIJ-MRI; both associations persisted after mutual adjustment. Median SIJ SUVmax differed across AS, nr-axSpA, and DJD (14.7 [11.2-22.4], 5.5 [4.4-9.9], and 5.7 [4.0-9.2], respectively; p = 0.001). Visually positive uptake occurred in 23/29 (79.3%) patients with AS, including 10/16 without MRI-defined bone marrow edema; no patient with AS had edema without increased uptake. Peripheral joint and entheseal uptake were frequent but non-discriminatory. Exploratory AUCs were 0.841 for AS versus DJD and 0.848 for AS versus nr-axSpA. CONCLUSION: SIJ ¹⁸F-NaF uptake was associated with both MRI-defined inflammation and structural burden in patients with AS, including those without active inflammation on MRI. SIJ SUVmax warrants evaluation as a complementary quantitative imaging biomarker of osteoblastic disease burden, but all derived thresholds require external validation.