Aortic annulus

The 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.^[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 isThe 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.

The virtual basal ring is the geometric reference formed from the lowest attachment points of the three aortic valve leaflets. It specifies a basal plane of the aortic root; it is not the entire crown-shaped tissue attachment represented by the anatomical aortic annulus.^[ref]

Three noncollinear landmarks determine a plane. For landmark position vectors ${\mathbf{p}}_1,{\mathbf{p}}_2,{\mathbf{p}}_3$, a normal isThe reference plane then obeys $\mathbf{n}\cdot (\mathbf{x}-{\mathbf{p}}_1)=0$. Near-collinear landmarks make $\Vert \mathbf{n}\Vert$ small, illustrating geometrically why landmark identification affects orientation. These equations construct a reference, not a ring of newly discovered tissue.

A plane alone also supplies no unique anatomical outline. Three points can define a circumcircle, but choosing that circle is a modeling convention; the actual lumen outline in that plane can be noncircular. For an original ellipse with semiaxes 15 and 10 mm, the enclosed area is $150\pi$ mm${}^2$ and the equal-area circular diameter is $2\sqrt{150}\simeq 24.49$ mm. Neither the 30 mm major diameter nor the 20 mm minor diameter equals that number.

The virtual basal ring is useful because it gives different images a specified basal reference. A measurement must still identify its landmarks, outline, cardiac cycle phase and diameter or area convention. It should not silently substitute the sinotubular junction or the longer three-dimensional leaflet attachment path for this basal reference.