The objective of this study is to use non-invasive ultrasound imaging to characterize aortic wall dynamics in abdominal aortic aneurysms (AAAs). We induced AAAs (n=5) in apolipoprotein-E deficient mice by subcutaneously implanting an osmotic pump, which systemically delivers angiotensin II at a rate of 1000 ng/kg/min for 28 days, in the back of each mouse. Once aneurysm formations were confirmed using a high frequency small animal ultrasound system (Vevo2100, Visualsonics; 40 MHz central frequency), we implemented our cardiac- and respiratory-gated volumetric ultrasound technique to acquire 3D AAA geometries throughout a cardiac cycle. Figure 1 summarizes the results obtained from a representative mouse with a suprarenal AAA. A 3D volume rendering of an abdominal aorta, with the aneurysm positioned in the center, is illustrated next to a color bar, which provides position references for both the maximum circumferential Green-Lagrangian (GL) strain and the instantaneous strain plots. Qualitative inspections of both strain plots depicted in Figure 1 demonstrate that maximum GL circumferential strain occurs during systole and is smallest within the AAA. Quantitative assessments of GL circumferential strain for all five AAAs were performed with similar results indicating that maximum GL strain decreased significantly within the aneurysm (1.1±0.2%) compared to proximal (9.3±5.3%) and distal (4.0±2.1%) regions (p<0.05). This decrease in GL circumferential strain within the aneurysmal portion of the abdominal aorta corroborates the idea that an increase in vessel stiffness is associated with the degradation of elastin and increased collagen turnover in the AAA wall. Interestingly, in all five cases, we observed a stiffer distal region, when compared to regions proximal to the aneurysm, suggesting that AAA development may also contribute to extracellular matrix remodeling of the aortic wall downstream of the aneurysm site.
Development of a Novel Index to Characterise Arterial Dynamics Using Ultrasound Imaging