Synthetic aperture flow imaging using a dual stage beamformer approach

A new technique for synthetic aperture flow imaging is presented in this paper. The objective is to increase the frame rate and still maintain a beamforming quality sufficient for flow estimation that is possible to implement in a commercial scanner. A dual stage beamformer is used to beamform the data. In the first stage data are beamformed with a fixed transmit and receive focal point to obtain conventional RF lines. Then the fully dynamic second beamformer uses the data from the first stage to beamform the full image. High resolution image (HRI) lines obtained from the second stage beamformer are input to the velocity estimator. There are four emissions in each emission sequence, which repeat over time. In each emission 32 elements are excited for transmission and 64 elements are used for receiving signals. Thus, four low resolution lines generated in the first stage are input to the second stage beamformer to generate a HRI. Velocities are estimated by cross correlating RF lines n and n + N, where n is the global emission index and N is the number of emissions in one sequence. Furthermore, the velocity profile can be obtained by dividing RF lines into small segments, thus, they are calculated as a function of depth. The performance of this approach has been studied using simulations by Field II and different parameters are studied to reveal the robustness of this approach. For different spacings between emission lines [2, 6, 10, 14]λ, the standard deviation and the bias of the axial velocity component were [1.92, 2.0, 2.07, 2.21]% and [0.61, 1.07, 1.29, 1.5]%, respectively, compared to the peak velocity. With a spacing of 10λ, varying the number of emission sets [1, 2, 4, 6, 8] used for averaging resulted in standard deviations of [7.14, 2.28, 2.07, 1.92, 1.36]% and bias [11.43, 2.36, 1.29, 1.29, 1.22]%. The parameter study showed that larger spacings gave an increased standard deviation and bias. Furthermore, performance improves if more emissio- - ns sets are used for averaging. The performance of the simulation indicates that this dual beamformer approach was able to estimate the flow velocity.