Role of molecular imaging with positron emission tomographic in aortic aneurysms

Abstract

Aortic aneurysms (AA) are often asymptomatic before the occurrence of acute, potentially fatal complications including dissection and/or rupture. Beyond aortic size, the ability to assess aortic wall characteristics and processes contributing to aneurysm development may allow improved selection of patients who may benefit from prophylactic surgical intervention. Current risk stratification for aneurysms relies upon routine noninvasive imaging of aortic size without assessing the underlying pathophysiologic processes, including features such as inflammation, which may be associated with aneurysm development and progression. The use of molecular imaging modalities with positron emission tomographic (PET) scan allows characterization of aortic wall inflammatory activity. Elevated uptake of Fuorine-2-deoxy-D-glucose (FDG), a radiotracer with elevated avidity in highly-metabolic cells, has been correlated with the development and progression of both abdominal and thoracic AA in a number of animal models and clinical studies. Other novel PET radiotracers targeting matrix metalloproteinases (MMPs), mitochondrial translocator proteins (TSPO) and endothelial cell adhesion molecules are being investigated for clinical utility in identifying progression of disease in AA. By further defining the activation of molecular pathways in assessing aortic regions at risk for dilatation, this imaging modality can be integrated into future clinical decision-making models.

Keywords: Aortic aneurysms (AA); molecular imaging; positron emission tomographic (PET).

Figure 1

FDG PET/CT imaging (A and B) and finite element analysis (C and D) of two patients with AAA showing discordances between the two measures. The first patient (A) has a region of increased FDG uptake in AAA with increased wall stress (D, wall stress =161 kPa) in the corresponding region. The second patient has no FDG uptake in AAA (B) and a large area of increased wall stress (C, arrow, wall stress =315 kPa). The intensity scale ranges from deep blue to red. (E) Event-free survival curves in FDG PET-positive and PET-negative patients according to their last examination (22).

Figure 2

Pathophysiology of abdominal aortic aneurysm (AAA). Note the possible targets of molecular imaging including inflammation and proteolysis (45).

Figure 3

18F-FPPRGD2 PET imaging of AAA and normal aorta (48). (A) AAA mouse model with aortic diameter of 1.89 mm on ultrasound (US) showing increase PET uptake in the AAA lesion (yellow arrows) compared to control mouse aorta that had a diameter of 1.09 mm on US that did not have any uptake. A, anterior; L, left; P, posterior; R, right; (B) quantitative analysis showing both the percentage-injected dose per gram and target to background ration (TBR) that were significantly higher in AAA than normal control aorta. P value <0.05. *, P=0.003 versus control; †, P=0.0008 versus control. PET, positron emission tomographic; AAA, abdominal aortic aneurysm.