BACKGROUND The purpose of the present study was to develop a new method of measuring heart valvular regurgitation based on control volume theory and to verify its accuracy in vitro and in vivo. Current methods of quantifying valvular regurgitation rely too much on assumptions about the flow field and therefore are difficult to apply in vivo. In particular, the proximal isovelocity surface area (PISA) method oversimplifies the proximal velocity field by assuming hemispherical isovelocity contours proximal to the orifice. This severely limits the applicability of the PISA method. Use of the basic control volume theory, however, removes the need to assume the manner in which the proximal flow accelerates toward the regurgitant orifice, the shape and size of the orifice, the shape of the orifice plate, and the non-newtonian behavior of the fluid. Apart from a correction that is necessary if the orifice plate is moving, the control volume method assumes only the incompressibility of the fluid and therefore is a potentially more accurate approach. In addition, the use of magnetic resonance imaging (MRI) precludes the need for an acoustic window. RESULTS MRI has been used to measure the three-dimensional velocity field proximal to regurgitant orifices, including single and multiple orifices and a cone-shaped orifice plate. Both steady (0 to 7.5 L/min) and pulsatile (2 and 3 L/min) flows were used. By intergrating this velocity over a control volume surrounding the orifice, we calculated the flow rate through the orifice. As a validation, the cardiac output of a 50-kg pig also was measured and was compared with thermodilution measurements. It was found that MRI could be used to measure the three-dimensional flow proximal to regurgitant orifices. This enabled the calculation of the flow rate through the orifice by integrating the velocity over the surface of a control volume covering the orifice. This flow rate correlated well with the actual rate (0.992; correlation line slope, 1.01). Care had to be taken, however, to exclude from the integration regions of aliased velocity. The cardiac output of the pig measured using MRI was in close agreement with the themodilution measurements. CONCLUSIONS Our new method of measuring valvular regurgitation has been shown to be very accurate in vitro and in vivo and therefore is a potentially accurate way to quantify valvular regurgitation.