Background The current paradigm of radiographic progression in ankylosing spondylitis involves resolution of an inflammatory lesion and its replacement by fat tissue followed by cartilage metaplasia and endochondral ossification. However, syndesmophytes may also develop at sites without inflammation on baseline and follow up MRI, potentially through noninflammatory pathways. High bone turnover assessed by nuclear medicine methods may detect osteoproliferative processes regardless of their inflammatory or noninflammatory origin.
Objectives To compare the frequency and distribution of increased activity on 18F-fluoride PET/CT with the presence of bone marrow edema on whole body MR imaging in patients with active ankylosing spondylitis.
Methods 10 patients (6 men and 4 women, between 32 and 58 years old (median 44) with active ankylosing spondylitis were prospectively examined with both whole-body MRI and 18F-Fluoride-PET/CT. Patients fulfilled the modified New York criteria for ankylosing spondylitis and had a Bath Ankylosing Spondylitis Disease Activity Index of at least 4. Increased radiotracer uptake in PET/CT and bone marrow edema in whole body MRI of spine and sacroiliac joints were scored independently by two blinded observers for each modality. Kappa statistics were used to analyze interobserver agreement as well as consensus reading of the two imaging modalities.
Results Analysis of interobserver agreement for PET/CT yielded a kappa value of 0.68 for spinal lesions and of 0.88 for SIJ lesions. The corresponding kappa values for MRI were 0.64 and 0.93 respectively. More spinal lesions were detected by MRI in comparison to PET/CT (68 vs. 38 out of 240), whereas a similar number of SIJ quadrants scored positive in both modalities (19 vs. 17 out of 80). Analysis of agreement of lesion detection between both imaging modalities yielded a kappa value of only 0.25 for spinal lesions and of 0.64 for SIJ lesions.
Conclusions Increased 18F-fluoride uptake in PET/CT correlates only modestly with bone marrow edema on MRI in the spine of patients with AS. The additional information provided by F-PET/CT imaging may complement MRI for disease assessment and prediction of spinal radiological progression.
Disclosure of Interest None Declared
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