Nikolov Svetoslav Ivanov, Jensen Jørgen Arendt, Tomov Borislav Gueorguiev
Dept. of Electr. Eng., Tech. Univ. of Denmark, Lyngby.
IEEE Trans Ultrason Ferroelectr Freq Control. 2008 Aug;55(8):1755-67. doi: 10.1109/TUFFC.2008.860.
This paper describes the design and implementation of a real-time delay-and-sum synthetic aperture beamformer. The beamforming delays and apodization coefficients are described parametrically. The image is viewed as a set of independent lines that are defined in 3D by their origin, direction, and inter-sample distance. The delay calculation is recursive and inspired by the coordinate rotation digital computer (CORDIC) algorithm. Only 3 parameters per channel and line are needed for their generation. The calculation of apodization coefficients is based on a piece- wise linear approximation. The implementation of the beamformer is optimized with respect to the architecture of a novel synthetic aperture real-time ultrasound scanner (SARUS), in which 4 channels are processed by the same set of field-programmable gate arrays (FPGA). In synthetic transmit aperture imaging, low-resolution images are formed after every emission. Summing all low-resolution images produces a perfectly focused high-resolution image. The design of the beamformer is modular, and a single beamformation unit can produce 4600 low-resolution images per second, each consisting of 32 lines and 1024 complex samples per line. In its present incarnation, 3 such modules fit in a single device. The summation of low-resolution images is performed internally in the FPGA to reduce the required bandwidth. The delays are calculated with a precision of 1/16th of a sample, and the apodization coefficients with 7-bit precision. The accumulation of low-resolution images is performed with 24-bit precision. The level of the side- and grating lobes, introduced by the use of integer numbers in the calculations and truncation of intermediate results, is below -86 dB from the peak.
本文描述了一种实时延迟求和合成孔径波束形成器的设计与实现。波束形成延迟和加权系数采用参数化描述。图像被视为一组独立的线，这些线在三维空间中由其起点、方向和样本间距定义。延迟计算是递归的，灵感来源于坐标旋转数字计算机（CORDIC）算法。每条通道和线的生成仅需3个参数。加权系数的计算基于分段线性近似。波束形成器的实现针对新型合成孔径实时超声扫描仪（SARUS）的架构进行了优化，其中4个通道由同一组现场可编程门阵列（FPGA）处理。在合成发射孔径成像中，每次发射后都会形成低分辨率图像。将所有低分辨率图像相加可产生完美聚焦的高分辨率图像。波束形成器的设计是模块化的，单个波束形成单元每秒可产生4600幅低分辨率图像，每幅图像由32条线组成，每条线有1024个复数样本。在当前版本中，3个这样的模块可安装在一个设备中。低分辨率图像的相加在FPGA内部进行，以减少所需带宽。延迟计算的精度为样本的1/16，加权系数的精度为7位。低分辨率图像的累加精度为24位。计算中使用整数以及中间结果的截断所引入的旁瓣和栅瓣电平比峰值低86 dB以下。
