Asymmetric aortic expansion of the aneurysm neck: Analysis and visualization of shape changes with electrocardiogram-gated magnetic resonance imaging

Abstract

Electrocardiogram (ECG)-gated imaging offers insight into aortic shape changes throughout the cardiac cycle. Morphologic changes of the anchoring zones may influence stent graft fixation and sealing and may have serious implications for endograft design and durability. We used multiphase magnetic resonance imaging (MRI) scans to evaluate the asymmetric aspect of aortic shape changes in the aneurysm neck before and after endovascular aneurysm repair (EVAR). Eleven patients were scanned before and after EVAR using ECG-gated balanced gradient-echo MRI with 16 reconstructed phases. Transverse scan planes were planned perpendicular to the aorta in the coronal and sagittal planes. Three levels were studied: 3 cm above the lowest renal artery, between the renal arteries, and 1 cm below the lowest renal artery. After segmentation of the aortic area in the images, aortic radius changes during the cardiac cycle were determined over 360 axes and plotted. Radii were measured from the center of mass of the aortic lumen to the vessel wall. An ellipse was fitted over the plots allowing determination of radius changes over the major and minor axis, and the most prominent direction of distention. The difference between radius change over the major and minor axis was significant preoperatively and postoperatively (P </= .002) at all levels, indicating asymmetric expansion. The pre-EVAR mean radius change over the major vs minor axis was infrarenal, 0.9 +/- 0.2 vs 0.6 +/- 0.1 mm; juxtarenal, 1.0 +/- 0.2 vs 0.8 +/- 0.1 mm; and suprarenal, 1.3 +/- 0.4 vs 0.9 +/- 0.2 mm. At all levels, there was no significant difference (P > .05) between pre-EVAR and post-EVAR radius changes. Pre-EVAR, the ratio of the radius change over the major vs minor axis ranged from 1.10 to 1.82. The pre-EVAR and post-EVAR asymmetry ratios did not differ significantly (P > .1). Preoperatively, the suprarenal direction of distention showed a tendency to right-anterior; for infrarenal, the tendency was to left-anterior. We measured the asymmetric aspect of earlier reported pulsatile aortic shape changes. The rate of asymmetric distention varied by patient and level. Asymmetric aortic expansion may have consequences for endograft design because it probably affects endograft sealing, especially in patients with high radius changes and asymmetry ratios. Asymmetric expansion remained preserved after stent graft placement. The stent grafts with Z-stent rings used in the study participants seem to adapt to the aortic shape changes well.