FPGA based digital high frequency beamformers for arrays [ultrasonic eye imaging applications]

We have developed two digital beamformers for eye imaging using 30-50 MHz transducer arrays: a 6-channel digital beamformer for a six-ring annular array ultrasound transducer (43 MHz) and a 16-channel digital beamformer for a linear array ultrasound transducer (30 MHz) of 48-64 elements. The hardware allows the flexibility for multiple transmit focusing, dynamic receive focusing, amplitude apodization, and dynamic aperture control. Individual channel delays are applied using a combination of coarse and fine delays. The coarse delays are integer multiples of the clock periods achieved by using a FIFO structure, and the fine delays are applied with interpolation of a 4-tap fractional delay (FD) FIR filter. The dynamic receive focusing is implemented based on a state machine which updates delay values and the FD filter coefficients according to the depth of the echoes. The aperture apodization is set according to the focusing zone. Cross-sectional wire target images, using a 20 μm diameter wire, from both arrays are shown. The images of an excised pig eye clearly show the fine structures of the anterior segment, including cornea and lens.