Is Ultrasound More Accurate than Axial Computed Tomography for Determination of Maximal Abdominal Aortic Aneurysm Diameter?

Abstract

Objective(s). Clinical assessment of maximal abdominal aortic aneurysm (AAA) diameter assumes clinical equivalency between ultrasound (US) and axial computed tomography (CT). Three-dimensional (3D) CT reconstruction allows for the assessment of AAA in the orthogonal plane and avoids oblique cuts due to AAA angulation. This study was undertaken to compare maximal AAA diameter by US, axial CT, and orthogonal CT, and to assess the effect that AAA angulation has on each measurement.Methods. Maximal AAA diameter by US (USmax), axial CT (axialmax), and orthogonal CT (orthogonalmax) along with aortic angulation and minor axis diameters were measured prospectively. Spiral CT data was processed by Medical Media Systems (West Lebanon, NH) to produce computerized axial CT and reformatted orthogonal CT images. The US technologists were blinded to all CT results and vice versa.Results. Thirty-eight patients were analyzed. Mean axialmax (58.0 mm) was significantly larger (P<0.05) than USmax (53.9 mm) or orthogonalmax (54.7 mm). The difference between USmax and orthogonalmax (0.8 mm) was insignificant (P>0.05). When aortic angulation was ≤25°, axialmax (55.3 mm), USmax (54.3 mm), and orthogonalmax (54.1 mm) were similar (P>0.05); however, when aortic angulation was >25°, axialmax (60.1 mm) was significantly larger (P<0.001) than USmax (53.8 mm) and orthogonalmax (55.0 mm). The limits of agreement (LOA) between axialmax and both USmax and orthogonalmax was poor and exceeded clinical acceptability (±5 mm). The variation between USmax and orthogonalmax was minimal with an acceptable LOA of −2.7 to 4.5 mm.Conclusion. Compared to axial CT, US is a better approximation of true perpendicular AAA diameter as determined by orthogonal CT. When aortic angulation is greater than 25° axial CT becomes unreliable. However, US measurements are not affected by angulation and agree strongly with orthogonal CT measurements.