How to combine CTA, 99m Tc-WBC SPECT/CT, and [ 18 F]FDG PET/CT in patients with suspected abdominal vascular endograft infections?

PURPOSE: We aimed at comparing 99m Tc-HMPAO white blood cells (99m Tc-WBC) scintigraphy, 18fluorine-fluorodeoxyglucose ([18 F]FDG) positron emission tomography/computed tomography (PET/CT) and CT angiography (CTA) in patients with suspected abdominal vascular graft or endograft infection (VGEI). Moreover, we attempted to define a new visual score for interpreting [18 F]FDG PET/CT scans aiming at increasing its specificity.

METHODS: We prospectively compared 99m Tc-WBC SPECT/CT, [18 F]FDG PET/CT, and CTA in 26 patients with suspected abdominal VGEI. WBC scans were performed and interpreted according to EANM recommendations. [18 F]FDG PET/CT studies were assessed with both qualitative (Sah's scale and new visual score) and semi-quantitative analyses. CTA images were interpreted according to MAGIC criteria. Microbiology, histopathology or a clinical follow-up of at least 24 months were used to achieve final diagnosis.

RESULTS: Eleven out of 26 patients were infected. [18 F]FDG PET/CT showed 100% sensitivity and NPV, with both scoring systems, thus representing an efficient tool to rule out the infection. The use of a more detailed scoring system provided statistically higher specificity compared to the previous Sah's scale (p = 0.049). 99m Tc-WBC SPECT/CT provided statistically higher specificity and PPV than [18 F]FDG PET/CT, regardless the interpretation criteria used and it can be, therefore, used in early post-surgical phases or to confirm or rule out a PET/CT finding.

CONCLUSIONS: After CTA, patients with suspected late VGEI should perform a [18 F]FDG PET/CT given its high sensitivity and NPV. However, given its lower specificity, positive results should be confirmed with 99m Tc-WBC scintigraphy. The use of a more detailed scoring system reduces the number of 99m Tc-WBC scans needed after [18 F]FDG PET/CT. Nevertheless, in suspected infections within 4 months from surgery, 99m Tc-WBC SPECT/CT should be performed as second exam, due to its high accuracy in differentiating sterile inflammation from infection.