High Structural Stress and Presence of Intraluminal Thrombus Predict Abdominal Aortic Aneurysm 18F-FDG Uptake: Insights From Biomechanics

Abstract

Background: Abdominal aortic aneurysm (AAA) wall inflammation and mechanical structural stress may influence AAA expansion and lead to rupture. We hypothesized a positive correlation between structural stress and fluorine-18-labeled 2-deoxy-2-fluoro-d-glucose (¹⁸F-FDG) positron emission tomography-defined inflammation. We also explored the influence of computed tomography-derived aneurysm morphology and composition, including intraluminal thrombus, on both variables.

Methods and results: Twenty-one patients (19 males) with AAAs below surgical threshold (AAA size was 4.10±0.54 cm) underwent ¹⁸F-FDG positron emission tomography and contrast-enhanced computed tomography imaging. Structural stresses were calculated using finite element analysis. The relationship between maximum aneurysm ¹⁸F-FDG standardized uptake value within aortic wall and wall structural stress, patient clinical characteristics, aneurysm morphology, and compositions was explored using a hierarchical linear mixed-effects model. On univariate analysis, local aneurysm diameter, thrombus burden, extent of calcification, and structural stress were all associated with ¹⁸F-FDG uptake (P<0.05). AAA structural stress correlated with ¹⁸F-FDG maximum standardized uptake value (slope estimate, 0.552; P<0.0001). Multivariate linear mixed-effects analysis revealed an important interaction between structural stress and intraluminal thrombus in relation to maximum standardized uptake value (fixed effect coefficient, 1.68 [SE, 0.10]; P<0.0001). Compared with other factors, structural stress was the best predictor of inflammation (receiver-operating characteristic curve area under the curve =0.59), with higher accuracy seen in regions with high thrombus burden (area under the curve =0.80). Regions with both high thrombus burden and high structural stress had higher ¹⁸F-FDG maximum standardized uptake value compared with regions with high thrombus burdens but low stress (median [interquartile range], 1.93 [1.60-2.14] versus 1.14 [0.90-1.53]; P<0.0001).

Conclusions: Increased aortic wall inflammation, demonstrated by ¹⁸F-FDG positron emission tomography, was observed in AAA regions with thick intraluminal thrombus subjected to high mechanical stress, suggesting a potential mechanistic link underlying aneurysm inflammation.

Keywords: abdominal aortic aneurysm; fluorodeoxyglucose F18; inflammation; mechanical stress; positron-emission tomography; thrombosis.

Figure 1.

In vivo images and the calculated structural stress of an abdominal aortic aneurysm: (A) contrast-enhanced computed tomography (CT), sagittal view; (B) fluorine-18-labeled 2-deoxy-2-fluoro-d-glucose (¹⁸F-FDG) positron emission tomography (PET), sagittal view; (C) structural stress plotted on the 3-dimensional geometry (both normalized and absolute values); (D) contrast-enhanced CT, transverse view; (E) ¹⁸F-FDG PET, transverse view; and (F) structural stress plotted on the transverse plane (both normalized and absolute values).

Figure 2.

Scatter plot between the maximum standard uptake value (SUV_max) of fluorine-18-labeled 2-deoxy-2-fluoro-d-glucose positron emission tomography and morphological features or structural stress: (A) SUV_max vs intraluminal thrombus (ILT) ratio; (B) SUV_max vs normalized stress in all regions; and (C) SUV_max vs normalized stress in regions with ILT ratio >0.67.

Figure 3.

Comparison of maximum standardized uptake value (SUV_max) in regions with and without local intraluminal thrombus (ILT).

Figure 4.

Heat map showing the relation between maximum standardized uptake value (SUV_max), intraluminal thrombus (ILT) ratio, and normalized stress. The value of SUV_max is indicated by the color of each grid.

Figure 5.

Serial receiver-operating characteristic curve analyses comparing the capability of differentiating regions with high (maximum standardized uptake value [SUV_max]>2) and low (SUV_max≤2) fluorine-18-labeled 2-deoxy-2-fluoro-d-glucose uptake. The structural stress lead to an improved prediction compared with abdominal aortic aneurysm morphology and composition, and this further improved with the combination of the extent of local intraluminal thrombus (ILT). AUC indicates area under the curve.

Figure 6.

Comparison of maximum standardized uptake value (SUV_max) in regions with low (≤0.45) and high (>0.45) structural stress. This comparison was performed in the regions with intraluminal thrombus (ILT) ratio >0.67, and the stress threshold was identified earlier in the receiver-operating characteristic curve analyses.

Figure 7.

Relation between normalized stress and local intraluminal thrombus (ILT) ratio within each octant (the figure in the brackets blow the x axis showed the number of octants).