Comparing Semiquantitative and Qualitative Methods of Vascular 18F-FDG PET Activity Measurement in Large-Vessel Vasculitis

The study rationale was to assess the performance of qualitative and large-vessel vasculitis. Methods: Patients with giant cell arteritis or Takayasu arteritis underwent independent clinical and imaging assessments within a prospective observational cohort. 18F-FDG PET/CT scans were interpreted for active vasculitis by central reader assessment. Arterial 18F-FDG uptake was scored by qualitative visual assessment using the PET vascular activity score (PETVAS) and by semiquantitative assessment using SUVs and target-to-background ratios (TBRs) relative to liver or blood activity. The performance of each scoring method was assessed by intrarater reliability using the intraclass correlation coefficient (ICC) and areas under the receiveroperating-characteristic curve, applying physician assessment of clinical disease activity and reader interpretation of vascular PET activity as independent reference standards. The Wilcoxon signed-rank test was used to analyze change in arterial 18F-FDG uptake over time. 43) contributed 212 18F-FDG PET studies. The ICC for semiquantitative evaluation (0.99 [range, 0.98-1.00]) was greater than the ICC for qualitative evaluation (0.82 [range, 0.56-0.93]). PETVAS and target-tobackground ratio metrics were more strongly associated with reader interpretation of PET activity than SUV metrics. All assessment methods were significantly associated with physician assessment of clinical disease activity, but the semiquantitative metric liver tissue-to-background ratio (TBRLiver) achieved the highest area under the receiveroperating-characteristic curve (0.66). Significant but weak correlations with C-reactive protein were observed for SUV metrics (r = 0.19, P < 0.01) and TBRLiver (r = 0.20, P < 0.01) but not for PETVAS. In response to increased treatment in 56 patients, arterial 18F-FDG uptake was significantly reduced when measured by semiquantitative (TBRLiver, 1.31-1.23; 6.1% change; P < 0.0001) or qualitative (PETVAS, 22-18; inflammation.