Bone-Seeking PET Tracer Reveals Hidden Remodeling in Ankylosing Spondylitis

Chronic low back pain is one of the most common and frustrating complaints in medicine, and few diagnoses are harder to pin down than axial spondyloarthritis, the inflammatory condition that includes ankylosing spondylitis. The disease attacks the sacroiliac joints, where the spine meets the pelvis, and over years it can fuse the vertebrae into a rigid column. The trouble is that early on, its symptoms overlap almost perfectly with ordinary mechanical back pain, and the standard imaging tools each capture only part of the story. A new prospective study published in the European Journal of Nuclear Medicine and Molecular Imaging suggests that a radioactive fluoride tracer, long used in bone scanning, may finally offer a way to measure the disease’s most destructive process directly: the runaway bone formation that ultimately welds joints together.

The research team, led by investigators at Tri-Service General Hospital and the National Defense Medical University in Taiwan, enrolled 43 adults with chronic low back pain who were being evaluated for suspected axial spondyloarthritis. Every participant underwent the full diagnostic workup: plain pelvic radiography, magnetic resonance imaging of the sacroiliac joints, and a whole-body positron emission tomography scan using fluorine-18 labeled sodium fluoride. Based on the combined clinical, laboratory, and imaging assessment, 29 patients were classified as having radiographic ankylosing spondylitis, 5 as having non-radiographic axial spondyloarthritis, and 9 as having degenerative joint disease, a non-inflammatory comparator group. The design was exploratory, but it was prospective and carefully standardized, with all three imaging studies completed within a two-month window.

The chemistry behind the tracer is what makes it so revealing. The fluoride ion exchanges with hydroxyl groups on the surface of newly deposited hydroxyapatite, the mineral scaffold of bone, converting it to fluoroapatite. In other words, wherever osteoblasts, the bone-forming cells, are actively laying down mineral, the tracer accumulates. Compared with older technetium-based bone agents, fluorine-18 sodium fluoride binds less to plasma proteins, reaches bone faster, and clears from the blood more quickly, producing sharper images. And because it is imaged on a PET scanner rather than a gamma camera, it delivers far better spatial resolution, standardized quantitative uptake values, and whole-body coverage in a single thirty-minute acquisition. Where MRI shows where marrow is inflamed, the fluoride signal shows how hard bone is actually responding.

The study’s primary question was whether the quantitative uptake in the sacroiliac joints, measured as the maximum standardized uptake value, or SUVmax, tracked with structural damage scored on MRI using the validated SPARCC system. It did. Sacroiliac SUVmax correlated moderately with the SPARCC structural score, with a Spearman coefficient of 0.606, and even more strongly with the SPARCC inflammation score, at 0.629, both highly significant. Because inflammation and structural damage scores were themselves correlated, the team ran partial rank correlations adjusting each association for the other, and both survived. This is the first patient-level demonstration that a single quantitative PET measure runs in parallel with both inflammatory and structural MRI domains in this disease.

The most striking finding concerned patients whose MRI showed no active inflammation at all. Bone marrow edema, the bright signal on fluid-sensitive MRI sequences that defines active sacroiliitis, was present in only 13 of the 29 ankylosing spondylitis patients. Yet visually increased fluoride uptake appeared in 23 of the 29, or 79.3 percent, including 10 of the 16 patients with no edema whatsoever. Not a single patient showed bone marrow edema without increased uptake. That asymmetry matters: it means the tracer was sensitive to disease activity that MRI classified as quiescent, consistent with the idea that bone remodeling continues even when inflammatory water signals fade. The median SUVmax in the ankylosing spondylitis group was 14.7, roughly two and a half times the values seen in either the non-radiographic or degenerative groups, which clustered around 5.5 to 5.7.

The diagnostic implications are tantalizing but come with heavy caveats. In exploratory receiver operating characteristic analyses, sacroiliac SUVmax separated ankylosing spondylitis from degenerative joint disease with an area under the curve of 0.841, and from non-radiographic disease with an area of 0.848. A threshold of 10.8 yielded 79.3 percent sensitivity and 88.9 percent specificity against degenerative disease. After bootstrap optimism correction, performance held up reasonably well, though the authors are emphatic that these thresholds were derived and tested in the same small cohort and must not be used as diagnostic cutoffs until validated externally. The non-radiographic subgroup, with only five patients, was too small for any firm conclusion, and its uptake values resembled the degenerative group’s more than the ankylosing spondylitis group’s.

Not everything the tracer lit up proved useful. Whole-body imaging revealed frequent uptake in peripheral joints and entheses, the attachment points of tendons and ligaments, across all three diagnostic groups, concentrated at mechanically loaded sites such as the feet and knees. These signals did not distinguish ankylosing spondylitis from degenerative disease, and in fact non-sacroiliac joint uptake was numerically more common in the degenerative group. Previous work in psoriatic arthritis has shown that most fluoride-positive peripheral sites are clinically silent, suggesting they reflect subclinical bone turnover rather than active inflammation. The message is clear: the whole-body scan’s real diagnostic value in this disease lies in the sacroiliac joints, not in the periphery.

Equally instructive was what did not correlate. Blood inflammatory markers, erythrocyte sedimentation rate and C-reactive protein, showed no significant relationship with sacroiliac uptake or with either SPARCC score, reinforcing a long-standing observation that systemic blood tests poorly reflect local disease in the axial skeleton. HLA-B27, the classic genetic marker, was far more prevalent in the ankylosing spondylitis group but showed no association with imaging measures within it. A single post hoc correlation between entheseal uptake and sedimentation rate failed to survive correction for multiple testing and was reported only as hypothesis-generating. Taken together, the data portray sacroiliac fluoride uptake as a measure of local osteoblastic burden, a dimension of disease that blood tests, genetics, and even MRI each capture only partially.

The study has honest limitations. The comparator groups were small, the degenerative group was defined by exclusion rather than a positive diagnostic standard, and the PET and MRI studies were not always performed concurrently because imaging occurred during routine care disrupted by the COVID-19 pandemic. Reliability of the MRI readings was not formally re-tested, and fluorine-18 sodium fluoride is fundamentally not inflammation-specific: it marks mineralization and perfusion regardless of trigger, so degenerative remodeling can mimic disease. Still, with excellent inter-reader agreement on the PET measurements, an intraclass correlation of 0.961 for SUVmax, and a biologically coherent pattern of results, the study makes a compelling case that fluoride PET/CT measures something MRI cannot see. Larger multicenter trials with concurrent imaging and validated clinical activity indices will determine whether this bone-seeking tracer earns a place beside MRI in the diagnostic arsenal, offering patients earlier and more complete answers about the disease quietly remodeling their spines.

Subject of Research: Fluorine-18 sodium fluoride PET/CT measurement of osteoblastic disease burden in axial spondyloarthritis

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

Article References: ¹⁸F-sodium fluoride PET/CT as a complementary measure of osteoblastic disease burden in axial spondyloarthritis: a prospective exploratory study. (n.d.). https://doi.org/10.1007/s00259-026-08190-w

Image Credits: AI Generated

DOI: 10.1007/s00259-026-08190-w

Keywords: ankylosing spondylitis, axial spondyloarthritis, PET/CT, fluorine-18 sodium fluoride, sacroiliac joints, MRI, SPARCC score, bone remodeling, osteoblastic activity, medical imaging, rheumatology, biomarkers