Applications of FDG- and NaF-PET in musculoskeletal disorders

Objectives: 1. Discuss the role of PET in imaging inflammatory and infectious disorders that affect the musculoskeletal system. 2. Describe the utility of PET in assessing bone turnover and its applications in metabolic bone disease.

Methods: Although the introduction of magnetic resonance imaging (MRI) in musculoskeletal imaging has resulted in tremendous advantages in the assessment of soft tissue and bone marrow compared to the offerings of computed tomography (CT), the emergence of PET provides even greater diagnostic utility in the assessment of molecular processes related to musculoskeletal disorders. ¹⁸F-fluorodeoxyglucose (FDG), a glucose analogue and PET tracer, localizes to sites of inflammation and infection due to the high metabolic activity of inflammatory cells, while ¹⁸F-sodium fluoride (NaF) accumulates in areas with new bone formation. By providing molecular information before structural changes are detectable by conventional modalities, PET-based methods facilitate early diagnosis, allowing timely initiation of interventions aimed at reversing disease progression. Furthermore, PET imaging can provide critical information in conditions when MRI is limited by susceptibility artifact or contraindicated due to the presence of foreign bodies near critical organs. Morphological information from CT and MRI can be combined with PET in the hybrid modalities PET/CT and PET/MRI, respectively, with significant improvements in diagnostic utility.

Results: Autoimmune-mediated inflammation in synovial joints caused by rheumatoid arthritis can be detected and quantified using FDG-PET. Involvement of the spine and sacroiliac joint in ankylosing spondylitis, another autoimmune disease affecting the joints, has been successfully detected by both FDG- and NaF-PET. In both of these inflammatory conditions, PET has been found to have utility in baseline assessment as well as in evaluation of response to treatment, allowing clinicians to identify non-responders and change ineffective medications sooner than by clinical assessment alone. Osteoarthritis and degenerative disorders of the spine are also associated with increased uptake of FDG and NaF, likely due to associated inflammation and bone turnover. Infectious disorders such as osteomyelitis and diabetic foot can be imaged with FDG-PET, which should be considered as part of the workup in cases of fever of unknown origin. By showing signs of abnormal activity years before detectable changes in bone mineral density appear, NaF-PET is uniquely suited to diagnose and monitor metabolic bone diseases such as osteoporosis and osteopenia and to evaluate changes in response to treatment with bisphosphonates, teriparatide, denosumab, and novel therapeutics. Condylar hyperplasia in young adolescents represents a potential novel use of NaF-PET, which offers superior precision in quantification and can help determine appropriateness and timing of maxillo-facial surgery.

Conclusions: Hybrid modalities with FDG- and NaF-PET are powerful diagnostic tools in the evaluation of musculoskeletal disorders and may assist in improving patient outcomes by allowing for earlier diagnosis and informing choice of therapy. PET-based molecular imaging techniques provide unique information compared to MRI and can overcome conditions in which metallic objects result in MRI contraindication or artifacts.