Two-dimensional flow index mapping for hemodynamic imaging: a feasibility study

The resistive index (RI) is a functional hemodynamic index used to measure flow resistance within an organ, such as the kidney, to assess if there is any vascular disease associated with that organ. Currently, the measurement of RI values using spectral Doppler ultrasound (US) is performed in one location at a time. As a result, RI values are typically obtained only from a few locations. To visualize the entire two-dimensional distribution of RI values, we developed a method called two-dimensional flow index mapping (2D-FIM). To evaluate this method, we performed a feasibility study using 16 kidney data sets, with eight from healthy volunteers with normal native kidneys and eight from kidney transplant recipients. Quadrature-demodulated baseband Doppler data were used to calculate the spatial distribution of RI values using 2D-FIM. For comparison, the intrarenal RI values at different levels of renal arterial branches were measured with conventional spectral Doppler US. The RI values determined from 2D-FIM were compared with those from spectral Doppler US, which led to a Pearson correlation coefficient of 0.84. There was no significant difference between 2D-FIM- and spectral Doppler US-produced RI values. With the ability to visualize RI in more than one dimension, 2D-FIM could help guide placement of the spectral Doppler range gate to regions of abnormal RI, thus potentially reducing exam times.