A longitudinal feasibility study on AAA growth vs. ultrasound elastography

To improve risk stratification of abdominal aortic aneurysms, ultrasound elastography could provide insight in changes in mechanical properties of the aneurysmal wall. In this feasibility study, 2-D US elastography was performed in AAA patients in a longitudinal study. Several patients with non-growing, slow growing and fast growing AAAs were imaged using 2D ultrasound. The RF-data were processed to estimate the mechanical properties, e.g., compliance (C), distensibility (D) and incremental Young's modulus (E_inc). An improved RF-based tracking algorithm was used to estimate the volume changes over time, assuming rotational symmetry of the aneurysm. The estimated parameters and possible changes were compared between the three groups. Incremental moduli ranging from 1- 8 MPa were found. The distensibility ranged from 2 to 10 MPa^-1. The relative increase in E_inc was -10 to 100% for the patients with no growth, 25% -125% for the medium group and 75% to 700% for the patients with fast growth. Distensibility also decreased by a factor 5. The largest increase in stiffness corresponded to the largest increase in diameter. These preliminary findings imply that large changes in mechanical properties occur in the aneurysmal wall in periods of considerable growth. Automated segmentation and higher frame rates might decrease the intra-subject variability. Finally, the inclusion of more patients is required to strengthen the evidence found.