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In M-mode, parasternal long axis view, the antero-posterior diameter of the aortic root is measured with the "leading-edge to leading-edge" method (A), synchronous with the Q wave of the EKG.
B: antero-posterior diameter of the left atrium (LA), measured at maximum atrial expansion in end-systole.
LA: left atrium; RVOT: right ventricular outflow tract.

In the example, a moderate to severe dilatation of the aortic root (54 mm).

B. Aortic root and sino-tubular junction diameters, 2D.

In 2D mode, parasternal long axis view, the maximum antero-posterior aortic root (A, between the anterior right coronary and posterior non coronary sinus of Valsalva) and the minor sino-tubular junction (B) diameters are measured between the internal borders (intima) of the aortic walls , in end-diastole (EKG Q-wave).
LA: left atrium; LV: left ventricle; RV: right ventricle, outflow chamber.

The 2D measurement of the aortic root is smaller than the corresponding M-mode measurement (see: figure A), but it can be compared to the corresponding CAT scan measuremnt.
In the example, a normal exam.

C. Left ventricular outflow tract / aortic valve annulus diameter, 2D.

In 2D mode, parasternal long axis view, the outflow tract / aortic valve annulus diameter (A) is measured in systole between the outflow tract anterior and posterior endocardiums immediately below the attachments of the anterior (right coronary) and posterior (non coronary) aortic valve leaflets.
AO: aortica root; LA: left atrium; LV: left ventricle; RV: right ventricle.

This measurement is used to calculate with the continuity equation the functional area of the native aortic valve (aortic stenosis) or of the aortic valve prosthesis.

D. Aortic valve diameter for the implant of a percutaneous / transapical biologic aortic valve, 2D.

In 2D mode, parasternal long axis view, the annulus (A) is measured between the anterior and posterior most external borders of the native calcific (stenotic) aortic valve , in systole. For comparison purposes, the measurement of the standard LV outflow / aortic annulus (B) used in the calculation of the continuity equation (see also figure C).
AO: aortic root; LA: left atrium; LV: left ventricle.

The measure is quite demanding, and requires the differential visualization of the anterior aortic root wall and the calcific aortic valve tissue. (the arrow points at the calcific anterior right coronary aortic valve cusp).