Suppose that the nadir of each cusp is in the middle of the cusp itself and remember the orientation of each nadir (e.g., nadir of RCC at 12 o’clock, nadir of LCC at 4 o’clock, nadir of NCC at 8 o’clock in the Fig. above).

In axial plane, drag the axes toward the middle of the RCC (Fig. 2.1) and then scroll images toward LVOT until RCC disappears (Fig. 2.2).

In axial plane, rotate the blue axis toward the nadir of the LCC (at 4:00, Fig. 2.3) and check in the corresponding plane (coronal) if the purple axis touches the nadir of the LCC; if not, rotate the axis itself in the coronal plane until it touches the nadir of the LCC, so you should see the LCC disappearing in the axial plane (Fig. 2.4).

Back to the axial plane, rotate the orange axis toward the nadir of the NCC (at 8:00, Fig. 2.5) and check in the corresponding plane (sagittal) if the purple axis touches the nadir of the NCC; if not, rotate the purple axis itself in the sagittal plane until it touches the nadir of the NCC, so you should see the NCC disappearing in the axial plane (Fig. 2.6).

Back to the axial plane, double check the result repeating the above steps for the other two cusps:

For example, LCC first:

Then repeat for the NCC:

At the end of this process, the annulus plane should be identified on the axial plane (purple plane). To double check the findings, scroll images in the axial plane from annulus to STJ, and check if the cusps of the aortic valve appear at the same time (or inversely, scroll from the STJ to the LVOT and check if the cusps of the aortic valve disappear at the same time).

Once confident with the plane, start measuring diameters (with the length tool) and perimeter (with either the closed polygon tool or the pencil tool); the latter will give also measure of the area of the annulus (Fig. 2.13).