Background/Purpose: to assess increased SIJ uptakes on 18-FNa (an osteoblastic tracer) PET/CT according to a qualitative and quantitative approach and to compare with MRI SIJ assessments for inflammation and with CT-scan for structural damage in a population of 23 patients with SpA (IDRCB: 2012-A00568-35).

Methods: This single-center prospective study included 23 patients with active SpA according to ASAS and/or modified mNY criteria (males 43.5%, HLA-B27 30%, years 44, symptom duration 7.7 years, median CRP 8mg/L, median BASDAI 6.1). All patients had a pelvic AP-view radiograph, MRI of the SIJ and 18-FNa PET/CT examinations during the same month. For MRI, SIJ were assessed for the presence/absence of inflammation according to ASAS criteria and were quantitatively assessed according to SPARCC method for scoring inflammation. CT-scans were read by three readers blinded to MRI images. Structural lesions were scored in consecutive slices in SIJ quadrants (erosion, sclerosis) or SIJ halves (ankylosis) on a dichotomous basis (present/absent) using the same anatomical principles for defining SIJ quadrants as developed for the SPARCC MRI SIJ inflammation and structural scores. On the 18-FNa PET, SIJ were scored blinded to MRI and CT images by two nuclear physicians according to a slice by slice approach performed from the anterior to the posterior part of the joint. A positive PET was defined when unilateral (sacral or iliac part) uptake was observed on 2 consecutive slices or when bilateral uptake was depicted on a single slice. As for SPARCC MRI methods, quantitative assessment on 18-FNa PET was performed according to SIJ quadrants for six consecutive slices through the cartilaginous region of the joint (PET-activity score). The Standardized Maximal Uptake Value (SUV-max) was measured for each SIJ, corresponding to the highest uptake value of the SIJ.

Results: 7 patients had radiographic sacroiliitis, 9 had inflammatory sacroiliitis on MRI (mean SPARCC 7.65). On CT-scan, inter-reader reliabilities for ankylosis, erosion and sclerosis were excellent to mild (ICC=0.95; ICC=0.81; ICC=0.39) respectively. The concordance between the two readers for a positive PET was good (73.9%) as well as the inter-reader reliabilities for the PET-activity score (ICC= 0.69 (95%CI: 0.40 to 0.86)). 18 patients had a positive PET with a mean PET-activity score of 15.7 (± 14). The mean SUV-max for a positive PET was 1.91 versus 1.27 for a negative one. According to a binary approach, a positive PET did not correlate to a positive MRI (ASAS criteria) or to a structural sacroiliitis on CT-scan. The PET-activity score (r=0.57, p=0.005) and SUV-max (r= 0.02, p=0.046) correlated with the SPARCC inflammation score but not with erosion, ankylosis and both ankylosis-erosions scores on CT-scan (ICC=-0.04; p=0.85), (ICC=-0.30; p=0.16) and (ICC=-0.19; p=0.37) respectively.

Conclusion: In axial SpA, the frequency of a positive 18-FNa PET (78.3%) was higher than the frequency of an ASAS positive MRI for inflammatory sacroiliitis (39.1%) and was also higher than the frequency of structural damages on CT-scan (39.1%). PET activity score had a good correlation with inflammatory sacroiliitis but not with structural lesion on CT-scan.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/performance-of-18fluoride-sodium-positron-emission-tomography-with-computed-tomography-to-assess-inflammatory-and-structural-sacroiliitis-respectively-on-magnetic-resonance-imaging-and-computed-tomogr/