Pulse wave imaging in murine abdominal aortas: A feasibility study

Abstract

One of the most crucial aspects of abdominal aortic aneurysm (AAA) diagnosis lies in the early detection of the aneurysm and its propensity for rupture. This study aims at determining whether the estimation of the aortic wall stiffness of the natural mechanical, pulsating motion of the aorta is feasible using high- resolution and high-frame-rate imaging in a murine model. Twelve wild-type (WT) mice were anesthetized, and underwent laparotomy. The abdominal aortas of five normal mice were scanned using a high-resolution (30 MHz) Vevo 770 (Visualsonics, Inc., Ontario, Canada) system and acquiring the RF signals. A composite frame rate of 8 kHz was achieved through ECG triggering on the RF acquisition over several cardiac cycles, which increased the tracking ability of the elastographic technique. Crosscorrelation techniques using windows of 150-micron size and 90% overlap were applied to estimate motions on the order of tens of microns between successive frames. The strain and the velocity of the pulsive wave in the aorta were estimated and imaged using the gradient and phase shift estimation techniques, respectively. Finally, the Young's modulus of the aortic wall was derived using the pulsive wave velocity based on the Moens-Korteweg equation. The pulsive wave during pulsatile flow was imaged by mapping the wall displacements consecutively in a ciné-loop format. High wall displacements were observed 10.3 ms after the R-wave peak of the ECG and a pulsive wave was initiated traveling from the heart's side along the aortic wall in less than 3 ms within the image view. The phase velocity of the pulse wave was computed at the frequency of 200 Hz and a velocity of 2.60 ± 0.57 m/s was found. The radius of the aorta and the wall thickness (measured on the B-mode image) were found equal to R=0.47 mm and h=0.12 mm, respectively. Using these measured parameters, the Young's modulus of the aortic wall was found equal to E = 50.9 ± 20.0 kPa. This value is in agreement with previously reported Young's moduli in the adventitia layer of the porcine descending aorta. In this study, we demonstrated that pulse wave imaging is feasible and that it can be used for mapping the propagation of the pulsive wave along the wall of the abdominal aorta in a murine model in order to determine its elasticity. Ongoing studies aim at determining the potential for this technique to be used in the detection of aneurysms based on their associated change of mechanical properties of the aortic wall. © 2006 IEEE.