When measuring the abdominal aorta (or any vessel diameter for aneurysm evaluation), calipers should be placed from outer wall to outer wall to ensure inclusion of the full vessel diameter, including any mural thrombus. This is the standard method accepted by professional societies.

According to AIUM and SRU Guidelines:

“Vessel diameter measurements should be performed from outer wall to outer wall to avoid underestimation of aneurysm size.”