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Figure 4 Schematic drawing of a stenotic mitral valve and the image planes that were used to study the effects of image plane variations on mitral valve areas. The optimal C plane (A) through the mitral valve orifice was defined. Using this plane, x- and y-axis were varied in 3° increments over a range of –6° to +6° to create 24 image planes from which MVA were calculated. This procedure was repeated for image planes separated by 1 mm (B) and 2 mm (C) from the optimal image plane.