In vivo assessment of blood flow patterns in abdominal aorta of mice with MRI: implications for AAA localization

Abstract

Abdominal aortic aneurysms (AAA) localize in the infrarenal aorta in humans, while they are found in the suprarenal aorta in mouse models. It has been shown previously that humans experience a reversal of flow during early diastole in the infrarenal aorta during each cardiac cycle. This flow reversal causes oscillatory wall shear stress (OWSS) to be present in the infrarenal aorta of humans. OWSS has been linked to a variety of proatherogenic and proinflammatory factors. The presence of reverse flow in the mouse aorta is unknown. In this study we investigated blood flow in mice, using phase-contrast magnetic resonance (PCMR) imaging. We measured blood flow in the suprarenal and infrarenal abdominal aorta of 18 wild-type C57BL/6J mice and 15 apolipoprotein E (apoE)-/- mice. Although OWSS was not directly evaluated, results indicate that, unlike humans, there is no reversal of flow in the infrarenal aorta of wild-type or apoE-/- mice. Distensibility of the mouse aortic wall in both the suprarenal and infrarenal segments is higher than reported values for the human aorta. We conclude that normal mice do not experience the reverse flow in the infrarenal aorta that is observed in humans.

Fig. 1.

Blood flow waveforms from a healthy human volunteer (right) taken at the axial locations indicated in the accompanying image (left). Note the triphasic flow pattern due to the reversal of flow in the infrarenal aorta (arrow).

Fig. 2.

An image of an abdominal aortic aneurysm (AAA) from a mouse model. Note the suprarenal location of the aneurysm. This is the most common site of AAA formation in mice based on a review of all AAA mouse models.

Fig. 3.

Phase-contrast magnetic resonance (PCMR) magnitude (left) and phase (right) images from the mouse aorta at peak systole. In the phase image, pixel intensity corresponds to the direction and magnitude of the velocity. Aorta (Ao) and inferior vena cava (IVC) are noted in both images. Also noted are the heated water supply (S) and return (R) tubes situated with the cradle housing the animal. These are used to keep the animal warm during the imaging scan.

Fig. 4.

Coronal scout image indicating positions of suprarenal and infrarenal blood flow and velocity measurements in the mouse.

Fig. 5.

Abdominal aortic blood velocity and flow averaged over all normal mice (n = 18). Blood velocity (A) and flow (B) values for the suprarenal and infrarenal aorta are shown. Note the lack of reversal (negative flow) of infrarenal blood flow. This is in contrast to the flow waveform in humans, which shows reversal of flow in late systole (Fig. 1).

Fig. 6.

Abdominal aortic blood velocity and flow averaged over all apolipoprotein E (apoE)−/− mice (n = 15). Blood velocity (A) and flow (B) values for the suprarenal and infrarenal aorta measured in apoE−/− mice are shown.