= Virtual basal ring
The \b[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>.<Surgical anatomy of the aortic valve and root—implications for valve repair by Ruggero De Paulis and Andrea Salica>[General considerations; Figures 1–2, basal landmarks and crown-shaped leaflet attachment]{ref}
Three noncollinear landmarks determine a <plane>. For landmark position vectors $\boldsymbol p_1,\boldsymbol p_2,\boldsymbol p_3$, a normal is
$$
\boldsymbol n=(\boldsymbol p_2-\boldsymbol p_1)\times(\boldsymbol p_3-\boldsymbol p_1).
$$
The reference <plane> then obeys $\boldsymbol n\mathbin{\cdot}(\boldsymbol x-\boldsymbol p_1)=0$. Near-collinear landmarks make $\lVert\boldsymbol n\rVert$ 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 $2\sqrt{150}\simeq24.49$ 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.
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