Clinical Research
A Positron Emission Tomography/Computed Tomography (PET/CT) Evaluation of Asymptomatic Abdominal Aortic Aneurysms: Another Point of View

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Background

To assess the prevalence of increased 18F-fluorodeoxyglucose (FDG) uptake in aneurysmal walls, adopting a case–control approach in a population of asymptomatic patients with abdominal aortic aneurysm (AAA).

Methods

This study included 40 males (mean age: 74 years, range: 59–93 years), consecutive, white Caucasian patients, with asymptomatic infrarenal AAA. The mean diameter of AAA was 4.9 cm (range: 4.8–5.4 cm), detected by computed tomography (CT) scan. Control Subjects: 44 age-matched controls subjects (mean age: 71 years, range: 59–85 years, 24 males, 20 females) who were selected according to a case–control criterion among a population of patients without any clinical evidence of atherosclerotic disease. Patients and controls underwent simultaneous FDG-positron emission tomography (PET) and CT imaging from the skull base to the femoral neck by using an integrated PET/CT scanner. PET/CT studies were analysed both visually and quantitatively. For quantitative analysis, circular CT-based regions of interest (ROIs) were drawn on the AAA, on all the aortic segments, and on the large vessel included in the study (carotid, subclavian, and iliac arteries). FDG uptake was quantified by calculating the mean and maximum standardized uptake values (SUVs) within each ROI and normalizing for the blood-pool SUV to obtain the final target-to-background ratio. Arterial calcium load was graded according to a semiquantitative five-point scale based on calcification of the arterial ring.

Results

Metabolic activity in the aneurysmal aortic segment was even lower with respect to both the adjacent—nonaneurysmal—samples of patient group and the corresponding arterial segments of control subjects (P < 0.001 and P < 0.01, respectively). In visual analysis, no patients showed an increased focal uptake of degree adequate to identify the aneurysmal arterial wall. AAA patients showed significantly higher values of total calcium load (ACL) than controls in ascending aorta and subclavian and iliac arteries (P < 0.01), and only in AAA patient group, a significant correlation was present between values of ACL in both iliac arteries and abdominal aorta on one side and wall metabolic activity in the same arteries on the other (P < 0.05).

Conclusions

In conclusion, our results suggest that FDG hot spot, as well an increased diffuse uptake of FDG, in PET/CT studies is an extremely rare finding in patients with AAA of diameter close to surgical indications.

Introduction

Rupture is an acute and life-threatening complication of abdominal aortic aneurysm (AAA). Its risk increases with aortic diameter; however, it is not negligible in relatively smaller AAAs (sizing: 4.0–5.5 cm) in which its incidence ranges from 1% to 5 % per year. Thus, although the time of surgery is currently delayed until the aneurysm reaches 5.5 cm in diameter,1 predicting AAA rupture risk remains a major issue in vascular medicine. On one side, this task implies a model, that is, a complete comprehension of the factors contributing to the precipitation of this syndrome, of their interplay, and temporal sequence. On the other side, it also asks for the availability of methods able to describe the presence of these same factors and their activation in each single patient at risk. According to these considerations, several authors investigated the role of inflammation because these pathways underlie many of acute complications of atherosclerotic disorders and can worsen the effect of biomechanical forces acting on the diseased aorta. Actually, AAA development in experimental models is paralleled by the appearance of inflammatory infiltrates2, 3 whose release of matrix metalloproteinases (MMPs) and proinflammatory cytokines ultimately results in an accelerated proteolysis of elastin and collagen4 of aortic wall. In recent years, this inflammatory paradigm of AAA rupture has been extended to the clinical arena, as the increased availability of positron emission tomography/computed tomography (PET/CT) imaging systems permit the detection of focal arterial inflammation as areas of increased 18F-fluorodeoxyglucose (FDG)5, 6, 7 caused by the elevated insulin-independent glucose consumption of infiltrating leukocytes.8 Actually, the high content of activated macrophages justifies the use of PET/CT imaging to identify vulnerable plaques in a variety of acute complications of atherosclerosis.9 However, the pathologic features of AAA seem relatively unsuitable for this purpose, as the progression of aortic dilation is paralleled by a reduction in cell density and thus by a local reduction in the elements able to trap FDG.10 This consideration implies that PET/CT might represent a relatively inaccurate method to identify the inflammatory activation in asymptomatic patients with AAA characterized by even a relatively low-diameter dilatation.

Accordingly, in the present study, we planned to verify the prevalence of visible FDG uptake in aneurysmal walls, adopting a case–control approach. To identify the possible role of this technique in the clinical setting, selection criteria were focused on those patients in whom the question about risk of rupture represents the main clinical factor able to identify surgical correction. Accordingly, patients with symptoms as well as patients with any evidence of aortic wall fissuration were excluded a priori.

Section snippets

Patient Population

This study included 40 males (mean age: 74 years, range: 59–93 years), consecutive, white Caucasian patients, with asymptomatic infrarenal AAA, selected by the outpatient clinic of Vascular and Endovascular Surgery Division of our University Hospital between January 2009 and 2010. All subjects were candidates to open surgical repair of AAA according to conventional morphological criteria: saccular aneurysm, aortic diameter ≥5 cm, or diameter increase >1 cm in the last 6 months (Table I). Within

Clinical Data

The main clinical and metabolic characteristics of the study group are reported in Table II, Table III, respectively.

Surgical intervention was successfully completed in 35 patients, and no complication occurred in the postoperative period.

PET/CT Evaluation of AAA

On visual inspection, no patient showed an increased focal uptake of degree adequate to identify the aneurysmal arterial wall.

Main descriptors of systemic FDG kinetics were similar in the patient and control groups (liver average SUV: 2.3 ± 0.5 vs. 2.2 ± 0.4, P

Discussion

The present study was designed to investigate whether imaging of the aneurysmal metabolism enables one to identify specific patterns among candidates to surgical abdominal repair of AAA. No patient of this series showed any visible focal tracer uptake of degree adequate to identify the AAA. Similarly, the metabolic activity in the aneurysmal aortic segment was even lower compared with both the adjacent—nonaneurysmal—samples of the same patient and corresponding arterial segments of control

Conclusion

In conclusion, our results suggest that FDG uptake is most often very low and does not result in either hot spots or diffuse tracer retention in asymptomatic patients with AAA of diameter close to surgical indications. The possible accuracy of metabolic imaging in patients with relatively smaller AAA cannot be defined by the present data and remains an open question.

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