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

Abstract

Purpose We aimed at comparing 99mTc-HMPAO white blood cells (99mTc-WBC) scintigraphy, 18fluorine-fluorodeoxyglucose ([18F]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 [18F]FDG PET/CT scans aiming at increasing its specificity. Methods We prospectively compared 99mTc-WBC SPECT/CT, [18F]FDG PET/CT, and CTA in 26 patients with suspected abdominal VGEI. WBC scans were performed and interpreted according to EANM recommendations. [18F]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. [18F]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). 99mTc-WBC SPECT/CT provided statistically higher specificity and PPV than [18F]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 [18F]FDG PET/CT given its high sensitivity and NPV. However, given its lower specificity, positive results should be confirmed with 99mTc-WBC scintigraphy. The use of a more detailed scoring system reduces the number of 99mTc-WBC scans needed after [18F]FDG PET/CT. Nevertheless, in suspected infections within 4 months from surgery, 99mTc-WBC SPECT/CT should be performed as second exam, due to its high accuracy in differentiating sterile inflammation from infection.

Imaging of Vascular Graft/Endograft Infection with Radiolabeled White Blood Cell Scan and [18F]FDG PET/CT

“…Unfortunately, further studies on the use of serial imaging techniques during the follow-up to document the long-term evolution of a graft infection or the normalization of a physiologic post-surgical inflammation over time are lacking. Nevertheless, these data support the use of WBC scans to monitor the response to treatment and to promptly assess the healing process even in early phases after surgery, in which other modalities may still detect non-specific signs of inflammation [10,18,20]. To date, no guidelines have been published on the timing of imaging during the follow-up of patients with VGEI; therefore, at the moment, the decision to perform or not perform a CTA, a WBC scan, or [ 18 F]FDG PET/CT during the follow-up is based on clinical aspects, local availability, and waiting times.…”

“…To date, no study has directly compared patients with and without antimicrobial treatment for VGEI. However, in the abovementioned studies, antimicrobial therapy at the time of the study did not seem to affect the diagnostic accuracy of the WBC scans [13,15,18,19]. Consequently, the EANM evidence-based guidelines stated that antimicrobial therapy has no influence on the diagnostic accuracy of WBC scans in detecting VGEI [10].…”

“…In the past decades, several papers on the utility of WBC scans in VGEI have been published [13][14][15][16][17][18][19][20] (Table 2).…”

“…Overall, WBC scans were demonstrated to be extremely reliable and accurate in differentiating VGEI from a sterile post-surgical inflammation. If standardized acquisition and interpretation protocols are used, this ability also is preserved in early post-surgical phases (1-4 months from surgery) [10,18,20] and even in low-grade infections [13,14], in which CTA and [ 18 F]FDG PET/CT exhibit their own limits.…”

“…However, in clinical practice, the majority of patients studied with [ 18 F]FDG do not fit into any of these two patterns; instead, they show diffuse and heterogeneous [ 18 F]FDG uptake. This pattern is particularly challenging since it is common to both infected and non-infected patients [18,21], and despite many efforts being devoted to better defining solid and reproducible interpretation criteria, at the moment, none of these has been widely validated and universally adopted.…”