Assessing the role of FDG- and NaF-PET/CT in evaluating inflammatory spine disorders

Introduction: To assess the feasibility and value of FDG- and NaF-PET/CT imaging in inflammatory low back pain, specifically in ankylosing spondylitis (AS).

Methods: Original studies and reviews examining clinical practices of PET/CT and PET/MRI in AS were gathered from large databases like PubMed, Google Scholar, and Web of Science. Keywords used included FDG, NaF, fluoride, PET, CT, MRI, ankylosing spondylitis, sacroiliac joint, spine, vertebrae. PRISMA-ScR guidelines were used to conduct the scoping review eligibility criteria included number of patients, quality of methods, and recency of publications.

Results: While FDG PET/CT was less commonly used in AS, NaF PET/CT was often the tracer of choice and showed accuracy in detecting and diagnosing AS, consistent with the clinical indexes relating to AS disease activity like BASDAI and ASDAS. NaF PET/CT was able to evaluate whole body lesions in a single examination and predicted therapeutic response better than quantitative pharmacokinetic parameters. NaF PET/CT was also able to identify typical AS positive findings like enthesopathy, syndesmophytes, and symmetric sacroiliitis. Furthermore, dual phase NaF-PET/MRI of the sacroiliac joint was also effective in detecting osteoblastic activity. On the other hand, FDG-PET/CT detected far fewer lesions than NaF with an uptake of five times lower compared to NaF PET/CT.

Conclusions: Because NaF reflects osteo-proliferative processes and AS is characterized by excessive bone formation, NaF PET/CT is a more suitable option for visualizing axial bony lesions in AS patients. On the other hand FDG-PET/CT, which represents inflammation, was not a very sensitive and specific marker for AS. This supports the idea that AS disease activity is reflected by bone formation rather than inflammation, in contrast to rheumatoid arthritis (RA) where inflammation tracers appear to be more useful. Not only did NaF-PET/CT show axial bone lesions with bone formation, but it was also a useful marker for predicting and monitoring treatment response in patients with AS receiving anti-TNF-a drugs. While the studies support the immense value of NaF PET-CT imaging in AS, there is a need for larger longitudinal and prospective data to further understand the utility of this imaging modality, especially comparing the effectiveness in different joints of the spine and lower back region.