Elective preventive surgery of abdominal aortic aneurysm (AAA) has a more favorable outcome than treatment after rupture. For that reason, many countries have population-based ultrasound (US) screening programs to identify AAA before rupture. AAA with smaller diameters are surveilled in intervals determined by aneurysm size. For that reason, an accurate assessment of rupture risk is crucial. There have been reports of the discrepancy between US and CT, the two modalities used for surveillance of AAA. This study aims to evaluate the absolute difference in maximum diameter between the US and CT and to analyze the potential effect on surveillance intervals.
453 men diagnosed with AAA (diameter ≥30 mm) in the regional population-based US screening program between July 1st 2010 and June 30th 2018 were identified in the screening register. This study, which is a series of comparative image analysis, included 104 patients who had an available (+/- 6 months) high-quality CT showing the AAA (ordered on any indication). US diameter was measured according to the "leading-to-leading" edge principle by experienced sonographers with a very low reported inter-variability in internal validation testing. On CT, the maximal orthogonal (perpendicular to blood flow, fig 1) AAA diameter was measured according to the "outer-to-outer" edge principle using a manual multiplanar reconstruction tool (MPR). Agreement between US and CT diameter measurements were illustrated by a Bland-Altman plot and correlation with The Pearson's correlation coefficient (r). Collection of data from the screening program has been approved by the local ethics committee.
The mean orthogonal CT diameter was 2 mm larger than the mean "leading-to-leading" US diameter (SD 4 mm, r=0.97). In the considerable part of the cohort (43%) US and CT measurements differed over 2 mm, and about 10% had a disagreement over 5 mm. In a local clinical setting the discrepancy between the two methods would result in a shorter AAA surveillance interval for 13% of the patients if they were assigned a surveillance interval based on maximal orthogonal CT diameter instead of US diameter. Furthermore, 4 patients would have been considered for immediate pre-operative work up (≥ 50 mm) and 6 patients would have been scheduled for surgical repair (≥ 55 mm). Median AAA diameter in included patients was 41 mm (IQR 54 - 32) on ultrasound and 43 mm (IQR 57-35) on orthogonal CT.
Our data support the usefulness of US in screening of AAA for the majority of patients. However, despite relatively good agreement between the methods, there is a potential clinical benefit of including CT as a complementary evaluation of the diameter, illustrated by the significant number of patients that would have been re-considered regarding surveillance intervals or even scheduled for surgery earlier.
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