Parametric Beamformer for Synthetic Aperture Ultrasound Imaging

In this paper a parametric beamformer, which can handle all imaging modalities including synthetic aperture imaging, is presented. The image lines and apodization coefficients are specified parametrically, and the lines can have arbitrary orientation and starting point in 3D coordinates. The beamformer consists of a number of identical beamforming blocks, each processing data from several channels and producing part of the image.

A number of these blocks can be accommodated in a modern field-programmable gate array device (FPGA), and a whole synthetic aperture system can be implemented using several FPGAs. For the current implementation, the input data is sampled at 4 times the center frequency of the excitation pulse and is match-filtered in the frequency domain.

In-phase and quadrature data are beamformed with a sub-sample precision of the focusing delays of 1/16th of the sampling period. Each line is completely specified by 3 input parameters. The focusing delays are calculated iteratively in a 8-stage deep pipeline, and focusing information for 8 different lines is interleaved to produce delays at every clock cycle.

The apodization is specified using piecewise linear approximation with 255 levels. A beamforming block uses input data from 4 elements and produces a set of 10 lines. Linear interpolation is used to implement sub-sample delays. The VHDL code for the beamformer has been synthesized for a Xilinx V4FX100 speed grade 11 FPGA, where it can operate at a maximum clock frequency of 167.8 MHz.

Each beamformation block requires 12 multipliers, 5 buffers for parameters, 8 buffers for input data and 32 buffers for output data (I and Q). Furthermore double-buffering is used for the input data, thus simplifying the synchronization. Up to six beamforming blocks can fit in one FPGA. Clocked at 150 MHz they produce 900 x 106 I and Q samples/second.

Assuming a pulse repetition frequency of 5000 Hz, these blocks can be configured to beamform in real time 256 B-mode lines of synthetic aperture data from 4 transducer elements, or 64 lines from 16 elements.