= Aortic annulus
The \b[aortic annulus] is an anatomical or operational reference associated with attachment of the <aortic valve> leaflets at the base of the <aortic root>. The term requires a convention. The three-dimensional semilunar leaflet attachment lines form a crown-like arrangement; they are not simply one flat circular line of tissue. An expert anatomical account explicitly distinguishes these attachments from the junction between ventricular and arterial walls.<The surgical anatomy of the aortic root by Robert H. Anderson>[Abstract, semilunar attachments and ventriculo-aortic junction]{ref}
For an explicitly defined geometric measurement, one can construct a <virtual basal ring> from basal leaflet-attachment landmarks. Such a reference <plane> and its enclosed outline should not be silently identified with the entire crown-shaped attachment path. The <sinotubular junction> is a different, more distal boundary of the <aortic root>.
An original area calculation explains why a reported diameter also needs a definition. Suppose the chosen reference outline is an <ellipse> with semiaxes $a=14$ mm and $b=10$ mm. Its area is $A=\pi ab=140\pi$ mm$^2$, while its largest and smallest diameters are 28 and 20 mm. The diameter of an equal-area circle is
$$
D_A=2\sqrt{\frac A\pi}=2\sqrt{ab}\simeq23.66\ \mathrm{mm}.
$$
The three numbers describe different features of the same outline. Selecting one two-dimensional chord does not determine its entire area or the spatial leaflet attachments.
This is a geometry example, not a device-sizing recommendation. An imaging account should identify the landmarks, reference <plane>, phase of the <cardiac cycle> and reported descriptor. Those choices connect a measured outline to a reproducible anatomical question; they do not turn every structure called annulus into the same physical ring.
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