In 2D mode, with the transducer in the suprasternal notch, the long axis of the aortic arch is visualized with a 30°-45° leftwards (clockwise) rotation of the transducer. The anonymous, left carotid and left subclavian supra-aortic branches are often visualized. The proximal (A, just before the anonymous branch), intermediate (B, after the left carotid branch), and distal (C, just distal to the left subclavian branch) diameters of the aortic arch are measured between the superior and inferior inner borders (intima) of the aortic walls.
AA: distal thoracic ascending aorta.

In color Doppler mode, the “pseudo-angiographic” effect of color Doppler flow (caution: gain should be lowered so that the color velocities do not “overflow” over the aortic wall tissue) may facilitate the measurement of the aortic arch diameters.
In the example (arrow), the measurement of the distal aortic arch is facilitated by addition of color Doppler imaging (compare to figure B above).
AA: distal thoracic ascending aorta.

In 2D mode, apical modified 2-chamber view (transducer rotated to allineate with the long axis of the descending thoracic aorta), the diameter of the aorta is measured between the anterior and posterior inner borders (intima) of the aortic walls.
AO: descending thoracic aorta; LA: left atrium; LV: left ventricle.

Montgomery’s Classification of Aortic Atheromas.

Class I. Normal or mild thickening of intima.

Class II. Extensive intimal thickening.

Class III. Atheroma < 5 mm.

Class IV. Atheroma > 5 mm. Class V. Mobile lesion.

A. Class IV atheroma, ascending thoracic aorta.

2D mode, parasternal long axis view of the aortic root and ascending aorta.
The arrow points at an atheroma, located at the posterior sino-tubular junction, which protrudes more than 5 mm into the aortic lumen, and which is not mobile (Class 4).
AO: ascending aorta; LA: left atrium.