A Low Cost Receive Beamformer for a High Frequency Annular Array

High-frequency ultrasound imaging systems can be costly to develop due to the more stringent analog design requirements and higher sampling rates. Advances in consumer electronics and computer processors, however, have allowed the development of beamformers which are lower-cost and easily adapted to custom applications. We have developed a software beamformer which uses a new interpolation scheme to reduce the sampling rate while maintaining secondary lobes that are suppressed by approximately 60 dB with respect to the main lobe. A 7-channel, 20 MHz array beamformer was implemented using an Analog Devices AD9272 octal ADC evaluation board and ADC-HSC-EVALC FPGA acquisition board costing <$1200. Data was sampled at 80 MHz and 12 bits and transferred to a PC using a USB cable. Digital filters and amplifiers built into the AD9272 were controlled using software, supplied by the manufacturer, running on the host PC. The sampled data was interpolated using a 20 MHz sine-wave. By using look-up-tables and by selecting a sampling frequency equal to four times the center frequency of the pulse, the computation cost of the interpolation calculation was similar to that of linear interpolation. The interpolation algorithm and subsequent beamforming were coded in MATLAB, allowing application specific customization. The performance of the software based receive beamformer was evaluated initially using data simulated using the sine-wave interpolation and linear interpolation. Simulated data for a linear array with a 64-element active aperture focused at f/2 was also processed. A 5 dB decrease in secondary lobes was found for an annular array using the sine interpolation compared to linear interpolation while a 10 dB decrease in secondary lobes were found for a linear array. The software beamformer was capable of 20 Frames/sec. To test the hardware, a 7-element 20 MHz annular array was connected to the beamformer and a series of wire targets were imaged in a water tank. The RF data obtained displayed 60 dB of dynamic range and the beamformer operated at 10 Frames/second, limited only by the USB interface.