Nyquist velocity extension in ultrafast color Doppler

Conventional color Doppler is limited by a low frame rate (~15-20 fps in echocardiography) as a consequence of the focused transmit beamforming approach. Another inconvenience in color Doppler is the maximum unambiguous velocity that can be estimated. Aliasing occurs in color Doppler when the flow speed exceeds the Nyquist velocity (V_N). Such Nyquist velocity is proportional to the pulse repetition frequency (PRF): either the maximum image depth or the Nyquist velocity can be increased at the expense of the other one. In this study, we use ultrafast ultrasound imaging (with diverging circular beams) along with a staggered multiple-PRF scheme to obtain color Doppler images at very high frame rates and solve the velocity ambiguity dilemma. The ultrafast multiple-PRF scheme for color Doppler was tested in two in vitro models. Staggered dual- or triple-PRF sequences were used to delay transmits. RF signals were dynamically focused and demodulated, then the 2-D autocorrelator was used to provide Doppler velocity estimates. The Nyquist velocity was extended using the full information provided by the dual- or triple-PRF velocity data. The de-aliased (unambiguous) velocity estimates were compared to ground truth values: for the spinning disc set up, the NRMSE ranged between 8.2% (at 94 cm.s^-1) and 4.8% (at 140 cm.s^-1), showing an accurate agreement. An adequate fit (r² = 0.97) was also obtained between the estimated and theoretical maximum velocities for the free water jet set up. This study shows that ultrafast staggered-PRF can increase the frame rate and extend the Nyquist velocity of color Doppler imaging.