Determination of most appropriate Nyquist velocity for applying hemispherical flow convergence equation to calculate flow rate using the transorifice pressure gradient: Digital computer analysis of doppler color flow convergence region

Summary

Nyquist velocity and transorifice pressure gradient dramatically influence color aliasing shape and accuracy of simple hemispherical flow convergence equation for calculation of flow rate. The present in vitro study was performed to determine whether the value of Nyquist velocity, at which the shape of proximal isovelocity surface is best fit for a given shape assumption in different orifice size, and the flow rate may be a determinable and orifice size independent function of clinically measurable peak velocity or transorifice pressure gradient. Steady flow was driven through circular discrete orifices with diameter of 3. 8 mm, 5.5 mm and 10 mm and flow rate ranging from 2.88 L/min to 8.2 8 L/min. For every flow rate, Doppler color encoded M-mode images through the center of flow convergence region were transferred into the microcomputer in their original digital format. The continuous wave Doppler traces of maximal velocity through the orifice were performed for the calculation of pressure gradient. Direct numerical spatial velocity measure using color pixel intensity was obtained from the transferred color encoded M-mode images with computer software. The shape of isovelocity surface was determined by the ratio of calculated flow rate with hemispherical flow convergence equation to the actual flow rate. Both the flow rate and orifice size influence the position of the velocity profile curve. The shape of isovelocity surface is not constant and changes with the velocities used for the calculation of flow rates for a given flow rate and orifice size or pressure gradient and also changes with the flow rate or transorifice pressure gradients for a constant Nyquist velocity and orifice size. It was found that for a given ratio of calculated flow rate to actual flow rate (0. 7 and 1) the velocities used for the calculation of flow rate with hemispherical flow convergence equation were correlated well with the pressure gradient for a given orifice size and the differences in velocities among different orifice sizes adjusted for the covariance pressure gradients were not statistically significant (P = 0.794 for ratio = 0.7 andP = 0.81 for ratio = l). Our present study provides an orifice size independent quantitative method with which to select the most suitable Nyquist velocity for applying simple hemispherical flow convergence equation according to clinically measurable pressure gradients ranging from 5.32 kPa to 26.60 kPa, and offers a correcting factor for the hemispherical flow convergence equation when pressure gradient is less than 5.32 kPa.