Estimation of Velocity Vectors in Synthetic Aperture Ultrasound Imaging

Abstract

A method for determining both velocity magnitude and angle in a synthetic aperture ultrasound system is described. The approach uses directional beamforming along the flow direction and cross correlation to determine velocity magnitude. The angle of the flow is determined from the maximum normalized correlation calculated as a function of angle. This assumes the flow direction is within the imaging plane. Simulations of the angle estimation method show both biases and standard deviations of the flow angle estimates below 3 degrees for flow angles from 20 degrees to 90 degrees (transverse flow). The method is also investigated using data measured by an experimental ultrasound scanner from a flow rig. A commercial 128 element 7-MHz linear array transducer is used, and data are measured for flow angles of 60 degrees and 90 degrees . Data are acquired using the RASMUS experimental ultrasound scanner, which samples 64 channels simultaneously. A 20-micros chirp was used during emission and eight virtual transmit sources were created behind the transducer using 11 transmitting elements. Data from the eight transmissions are beamformed and coherently summed to create high-resolution lines at different angles for a set of points within the region of flow. The velocity magnitude is determined with a precision of 0.36% (60 degrees) and 1.2% (90 degrees), respectively. The 60 degrees angle is estimated with a bias of 0.54 degrees and a standard deviation of 2.1 degrees. For 90 degrees the bias is 0.0003 degrees and standard deviation 1.32 degrees. A parameter study with regard to correlation length and number of emissions is performed to reveal the accuracy of the method. Real time data covering 2.2 s of the carotid artery of a healthy 30-year-old male volunteer is acquired and then processed offline using a computer cluster. The direction of flow is estimated using the above mentioned method. It is compared to the flow angle of 106 degrees with respect to the axial direction, determined visually from the B-mode image. For a point in the center of the common carotid artery, 76% of the flow angle estimates over the 2.2 s were within 10 degrees of the visually determined flow angle. The standard deviation of these estimates was below 2.7 degrees. Full color flow maps from different parts of the cardiac cycle are presented, including vector arrows indicating both estimated flow direction and velocity magnitude.