Thrombus and Aortic Wall ¹⁸F-FDG Positron Emission Tomography Uptake in Abdominal Aortic Aneurysms

Most abdominal aortic aneurysms (AAA) present intraluminal thrombus (ILT). Whether its role is detrimental or protective for AAA evolution and patients outcomes is debated. ¹⁸F-fluorodeoxyglucose positron emission tomography analysis in patients with AAA has shown contradictory results and has not widely explored ILT. The aim of this cross-sectional study was to investigate the colocalized association between ¹⁸F-fluorodeoxyglucose and ILT presence and characteristics in AAA.

Consecutive patients with infrarenal AAA were prospectively enrolled and underwent positron emission tomography/magnetic resonance imaging. From the abdominal aorta segmentation, an aortic wall volume was defined and discretized into 28 standardized aortic patches, permitting colocalized interpatient and intrapatient analyses. The target-to-background ratio (TBR) was extracted in each patch, and extremely low (coldspots) and high (hotspots) absorption areas were computed. Local AAA diameters and ILT characteristics were computed in each patch. Linear mixed-effect models were used to test the association between AAA and ILT characteristics and ¹⁸F-fluorodeoxyglucose uptake.

Thirty-six patients (median age, 73 [interquartile range, 68-77] years, 31 [86.1%] men) completed the protocol. The AAA maximum diameter was 49.6 (45.0-56.2) mm, and most patients (28 [77.8%]) presented ILT. Patients with thrombus presented higher coldspot area and lower hotspot area (P<0.001). Patches in nondilated regions presented higher TBR compared with AAA patches (P<0.001), which showed a negative association of TBR with local diameter (β=-0.006) and thrombus presence (β=-0.003). TBR decreased with increased ILT coverage (β=-0.002), while coldspot area increased (β=0.046). In both the ILT and thrombus-covered wall, TBR decreased with increasing ILT thickness.

Decreased metabolic activity as per 18-fluorodeoxyglucose positron emission tomography is observed in the AAA wall compared with the nondilated abdominal region. This effect is exacerbated by local thrombus presence and thickness, suggesting that ILT contributes to the further degradation of the aortic wall in the diseased region.