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Figure 1 Schematic of PFC technique. As flow approaches a finite orifice, it forms isovelocity shells which by conservation of mass can be used to calculate the regurgitant flow rate. The regurgitant flow rate equals the surface area of the isovelocity shell times its velocity. Hence, flow rate equals (2r²)v where r is the radius of a hemispheric shell. An M-mode cursor is aligned along the maximal radius (r) of the PFC region to obtain a color Doppler M-mode of the PFC region (upper right); instantaneous ROA is then obtained as instantaneous flow rate/orifice velocity (obtained by continuous-wave Doppler).