We designed and implemented a flexible real-time volumetric ultrasound imaging system for capacitive micromachined ultrasonic transducer (CMUT) arrays, consisting of an ultrasound data acquisition system, an FPGA board, and a host PC. The system works with arbitrary-shaped CMUT arrays and non-standard beamforming methods, as well as with regular-shaped CMUT arrays and conventional beamforming methods. In this paper, we present the system design and real-time imaging results obtained using this system with a ring array, a rectangular array, and a linear array. In synthetic phased array (SPA) imaging with a 64-element ring array, we could display 3 image planes with a total of about 70,000 pixels in real time, at a frame rate of 9 frames per second (fps) which was limited by the computational load on the CPU required for synthetic beamforming. On the other hand, the frame rate in classic phased array (CPA) imaging is limited by the data transfer time. In CPA imaging with a 16×16-element rectangular array, a frame rate of 5.4 fps was achieved for 1,250 acquisitions per frame and a 2.5-cm imaging depth. The frame rate can be increased by reducing the number of pixels processed in SPA, or by reducing the number of beams received in CPA, at the expense of degraded image quality or reduced field of view.
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