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In order to improve the lateral resolution and extend the image view of the previously developed 16-channel digital beamformer for a 30MHz ultrasound linear array, the design of an ultrasound imaging system with a higher frame rate and increased channel count is reported in this paper. The system is composed of 256 channels of analog front-end pulser/receiver, 64 channels of Time-Gain Compensation (TGC), 64 channels of high-speed digitizer as well as a beamformer. A PC is used as user interface to display the real time images. This system is designed for imaging using up to 256 elements linear arrays. The system provides 64 channels of excitation pulsers while receiving simultaneously at a 20 MHz-500 MHz sampling rate with 12-bit resolution. The digitized data of all channels are first fed through FPGAs, and then stored in memories. Those raw data are accessed by the beamforming processor to rebuild the image or download to the PC for further processing. The beamformer that applies delays to the echoes of each channel is implemented with the strategy that combines coarse and fine delays. The coarse delays are integer multiple the sampling clock rate. They are achieved by controlling write enable pin of First-In-First-Out memory to obtain valid beamforming data. The fine delays are accomplished with interpolation filters. The design allows a maximum frame rate of 50 frames per second to be achieved. The wire phantom images show the spatial resolution of 146 μm (lateral) and 54 μm (axial). Images with excised rabbit and pig eyeball as well as mouse embryo were acquired.