Diagnostic Accuracy of Cranial and Large Vessel Giant Cell Arteritis with Digital 18FFDG PET/CT

18 FFluorodeoxyglucose ( 18 FFDG) PET/CT has become a standard imaging modality for assessment of large vessel vasculitis. Digital PET/CT enhanced performance may improve evaluation of cranial arteries in giant cell arteritis (GCA). This study assesses digital 18 FFDG PET/CT for the combined evaluation of cranial and large vessel-GCA. 18 FFDG PET/CT scans of patients > 50 years with suspected GCA and elevated inflammatory markers were retrospectively reviewed. Exclusion criteria were corticosteroid therapy > 3 days pre-scan or known vasculitis. Images were visually graded by nuclear physicians blinded to clinical data with guideline-proposed criteria and considered positive for cranial-GCA with grade ≥ 1 uptake (above surrounding tissues) and grade 3 (> liver uptake) for large vessel-GCA. Whole-body semi-quantitative analysis was also performed. Reference standard was final clinical diagnosis after ≥ 6-month follow-up based on clinical course, biochemical evolution, imaging and biopsy results, or if followed for < 6 months, definitive non-GCA diagnosis such as infection or spontaneously resolved symptoms with normalization of inflammatory markers. Fifty-six patients (mean age 78 ± 12 years, 36 female) were included. Among 24 patients (43%) with final GCA diagnosis, 21 scans (38%) were true positives (62% cranial-GCA, 5% large vessel-GCA, 33% combined). Three false negatives included patients receiving intravenous corticosteroids with altered 18 FFDG biodistribution (demonstrating decreased brain uptake). Four false positives included grade 3 aortic or grade 1-2 vertebral artery uptake. Eight out of 32 patients (25%) without GCA had alternative PET/CT diagnoses. Combined cranial and large vessel-GCA assessment yielded 88% sensitivity and specificity (95% CI 68-97% and 71-96% respectively). Reclassifying grade-2 large vessel uptake as negative improved specificity from 63% to 88% without reducing sensitivity. Target-to-background-ratios outperformed SUV max on evaluation of large vessels and combined assessments but had similar accuracy for cranial arteries. Digital 18 FFDG PET/CT demonstrated high diagnostic accuracy for cranial and large vessel-GCA while identifying alternative aetiologies of inflammation, highlighting its potential as a first-line imaging modality.