Doppler color flow “proximal isovelocity surface area” method for estimating volume flow rate: Effects of orifice shape and machine factors

Abstract Previously described Doppler color How mapping methods for estimating the severity of valvular regurgitation have focused on the distal jet. In this study, a newer Doppler color flow technique, focusing on the flow proximal to an orifice, was used. This method identifies a proximal isovelocity surface area (PISA) by displaying an aliasing interface. Volume flow rate (cm 3 /s) can be calculated as PISA (cm 2 ) × aliasing velocity (cm/s). For planar circular orifices, a hemi-elliptic model accurately approximated the shape of PISA. Clinically, however, orifice shapes may be noncircular. In vitro flow experiments (n = 226) using orifices of various shapes (ellipse, square, triangle, star, rectangle) were performed. Volume flow rate calculated using a hemi-elliptic model for PISA was accurate, with average percent differences from actual flow rate = +4.3% for a square, −4.2% for a triangle, −4.7% for a star, −4.5% for an ellipse and −2.8% for a rectangle. However, average percent differences for calculated volume flow rates using a hemispheric model for PISA shape ranged from −11.6% (square) to −34.8% (rectangle). In addition, to evaluate whether PISA is influenced by machine factors, in vitro studies (n = 83) were performed. For a volume flow rate of 13 liters/min, the color aliasing radius was not affected by: 1) system gain (radius = 9.9 mm at −20 dB versus 10.4 mm at +20 dB); 2) wall filter (10.8 mm at high versus 11.2 mm at tow); 3) frame rate (11.2 mm at 6/s versus 10.6 mm at 22/s); 4) transmit power (10.3 mm at high versus 10.1 mm at low); and 5) packet size (10.6 mm at 4 samples/line versus 10.5 mm at 8 samples/line). The aliasing radius was lower at higher aliasing velocities. However, the calculated volume flow rate was not affected by changes in aliasing velocity. It is concluded that differences in planar orifice shape do not affect volume flow rate calculated using a hemi-elliptic model. Furthermore, changes in machine factors do not alter volume flow rate calculated using the PISA method. The PISA method may have advantages over previous Doppler color flow methods in calculating volume flow rate and may be useful clinically in estimating valvular regurgitant or shunt volume.