2D tracking doppler for cardiac jet flow velocity estimation

The maximum velocity of jet flow though an insufficient heart valve gives important information on the pressure gradient, and is usually measured by continuous wave Doppler. With this method, correct estimation of the maximum velocity relies on a small (<; 20°) beam-to-flow angle. Angle correction is not recommended due to difficulties in estimating the correct flow angle, and increasing transit-time effect. 2D tracking Doppler is a recently proposed method which has been shown to produce robust estimates of the true velocities even at high (50°-80°) beam-to-flow angles. The method reduces transit-time broadening by summation of the signal along the trajectory of the blood scatterers. In previous work, the use of 2D tracking Doppler has been successfully demonstrated in vivo for carotid stenosis with a linear array. In this simulation study the potential of the tracking Doppler technique with a phased array probe for cardiac applications is investigated. The Computational Fluid Dynamics (CFD) software ANSYS Fluent was used to simulate a jet from a 5 mm opening with different maximal velocities. Using the resulting velocity fields, the received signals were simulated using Field II, for CW Doppler with flow angle 0°, and PW Doppler with multiple receive beams with flow angles between 0° and 80°. Plane wave transmissions were used in combination with parallel beams on receive. Due to diffraction effects, transmit apodization using a Tukey window was used to obtain a more homogeneous field in the depth range of interest (5-10 cm). Simulating flow with maximum velocity 4 m/s, and using a -6dB threshold, the spectral broadening is less than 1% for tracking Doppler for tracking angles between 0° and 60°, clearly outperforming PW Doppler and being comparable to CW Doppler with 0° beam-to-flow angle. It was found that transmit apodization had minor impact on the quality of the spectra for the relevant scan depths. Additionally, the maximum spectral amplitude was found when the tracking is performed in the flow direction, indicating the feasibility of automatic angle correction. The scan sequence was also implemented on a modified GE scanner, and in vivo results are shown.