2.2. Measurements

We modified the methodology of a previous study^[10] to definitively confirm the usefulness of AWE. The previous study defined AWE as present when there was an increase of >20 Hounsfield units (HU) in the mean CT values at 1 month after EVAR. In our practice, we evaluated the AWE value using a volumetric analysis of CT images at 2 time points. The previous study defined expansion and shrinkage as a change of >5 mm in the maximal aneurysm diameter; however, we measured the sac volume itself to determine the rate of change.

The CT images were transferred to a three-dimensional image analysis workstation (Osirix MD, v. 11.0.2) to measure the CT value and aneurysm sac volume. We selected 1 slice of the axial transverse view image that showed the maximal aneurysm diameter after EVAR. We manually circumscribed the aneurysm wall, excluding the calcified lesions, and the average CT value (HU) of the region of interest (ROI) was automatically calculated with Osirix software. We calculated the difference in the CT values of the contrast-enhanced images and plain images to determine the AWE value (Fig. ​(Fig.1).1). We also measured the preoperative and postoperative volume of the AAA sac by setting the ROI using Osirix. The extent of the change in the AAA sac was adjusted by comparing the axial CT images before and after EVAR. We defined the maximum change in the sac volume as the sac shrinkage rate.

Definition of aortic wall enhancement. CT = computed tomography.