The proper projection of the aorta for measurement is the sagittal plane (solid line in image below), since the axial plane measurement (dashed line) may overestimate the diameter when the aorta is not parallel to the skin.
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File:Ultrasonography of abdominal aortic aneurysm in sagittal plane, annotated.jpg
File:Ultrasonography of abdominal aortic aneurysm in axial plane, annotated.jpg