Vector Velocity Imaging in AAA Patients

Currently a AAA diameter > 5.5 cm is the cut-off for treatment with the rationale that rupture risk outweighs risk of surgery at that point. Although in large cohorts such thresholds adequately reflect the repair indication, they fail in case of patient-specific decision-making. This is reflected by the fact that both small AAA rupture and large AAA's have been documented to be stable. Moreover, recently introduced biomechanical parameters show promising results with respect to improved rupture risk prediction. A key parameter is intraluminal thrombus (ILT) formation at the vessel wall, which is observed in 75% of AAA's. Recent work has shown ILT to be promoting rupture risk, outweighing its protective effect by reducing wall stress. An explanation lies in the fact that the vessel wall is primarily oxygenated due to luminal oxygen diffusion and ILT decreases oxygen supply to the AAA wall by as much as 95%. This, in turn, leads to hypoxia, inflammation and vessel wall weakening, promoting AAA rupture. Others have also shown that small AAA's (< 6 cm) that ruptured have a significantly higher volume percentage of ILT. Biomechanics behind the formation of ILT indicate that aneurysmal blood flow plays a role, in combination with the complex and patient-specific AAA geometry, inducing blood flow perturbations, e.g. vortices and stagnation zones. These local blood flow perturbations facilitate platelet adhesion and ILT formation in regions of slow and stagnant flow, in turn increasing AAA growth and the associated rupture risk. This project is a first step to quantify in vivo AAA blood flow, using high frame rate ultrasound techniques, to perform both better risk stratification and risk management.

Conventional color Doppler imaging is performed using focused ultrasound beams, which limits the achievable frame rates to approximately 25 frames per second (fps). Recent developments in computer hardware and software has enabled a shift towards unfocused imaging, where anatomical ultrasound images can be made using only a single unfocused ultrasound pulse. With the new technology, the frame rate limit is suddenly increased by a factor of 500, which results in increased flexibility when designing imaging sequences. Extremely high frame rates can now be traded for increased complexity, enabling new information to be obtained.

In this project, the investigators will use continuous, high frame rate, ECG gated, 3D slice acquisition to quantify blood flow at the inflow of the aneurysm. This will result in a vector flow imaging dataset, including the through plane component. The received signal is based on the in vivo scattering properties of blood. However, the technique might benefit from the addition of ultrasound contrast agents, therefore a second set of measurements will be obtained after intravenous injection of a contrast agent (SonoVue).

The hypothesis is that high frame rate 3D ultrasound can be used to image aneurysm blood flow. From these measurements the investigators expect that travel distance analysis shows different outcomes between AAA patients with and without intraluminal thrombus. In turn this is another piece of the puzzle to stratify patients having a higher risk of aneurysm rupture.