Multiple zone beamforming in FOCUS

Software tools for diagnostic ultrasound simulations are under development in FOCUS, the `Fast Object-oriented C++ Ultrasound Simulator´ (http://www.egr.msu.edu/~fultras-web). Previously [Zhu et al, IUS 2012], the accuracy of FOCUS was demonstrated in comparisons with Field II, which is a widely accepted reference for simulations of diagnostic ultrasound. These initial evaluations used single zone beamformers for the transmit and receive apertures. To leverage the fast, accurate, and memory efficient algorithms in FOCUS for ultrasound imaging simulations, multiple zone beamformers and other new features are needed. The present work demonstrates an efficient approach for simulating multiple zone beamforming in FOCUS. After decomposing the input pulse, intermediate transient signals are computed for the transmit and receive apertures with the fast nearfield method combined with time-space decomposition. These intermediate signals are stored and then repeatedly reused, which greatly improves the computational efficiency of the beamformer. The contributions from each decomposed signal are superposed, and the results are then convolved for each combination of transmit and receive zones. B-mode images are simulated in FOCUS and in Field II using 5 transmit and receive zones. The FOCUS and Field II simulations are evaluated for temporal sampling frequencies of 40MHz, 160MHz, and 320MHz. The effects of the discretization of the time delays applied to the transmit and receive apertures are also evaluated. For each simulation, A-lines are computed and B-mode images are formed for a linear arrangement of 20 point scatterers. The results demonstrate that the images and the A-lines obtained with these two programs are in excellent agreement.