The Study And Optimization On Digital Beamforming Of Ultrasound System

Ultrasonic imaging technology has been widely used in clinical diseases diagnosis,such as the cardiovascular, visceral and urinary system. Beamforming is the core part ofultrasonic imaging system in the ultrasonic imaging process, and directly determines thequality of medical ultrasonic imaging. In this part, the dynamic focusing, dynamicaperture and coded excitation are the keys to improve the main-lobe width and side-lobeamplitude, and enhance the signal to noise ratio (SNR) and detection depth. In dynamicfocusing process, the lateral resolution improves with the decrease of focal distance, andthen the main-lobe width and side-lobe amplitude improve. Dynamic aperture canreduce the maximum delay, increase the focal depth in the near field, and reduce thedead zone. Coded excitation can not only improve the axial resolution, also enhancesignal penetration, which has significant effect on SNR, detection depth and systemsensitivity to the weak signal. Although these methods can greatly improve the imagequality, their inherent characteristics has limited the wide application in the engineeringto some extent, the specific performance are as follows:①Point-by-point dynamic focus, has faced many problems, such as so manydelay parameters and huge storage capacity. It has seriously hindered the wideapplication of high precision dynamic focusing in the engineering. At the same time,without dynamic aperture, the maximum delay will not be reduced, and the dead zoneexists in adjacent scan areas, where the beam cannot cover in the common scan mode.②Conventional pulse compression (CPC) is typically located after beamforming,and it can lower computational complexities to some degree. However, CPC stillrequires a few hundred filter taps to obtain sufficient SNR improvement. The operationrequirement is strict, and the computational complexity is still a burden to modernultrasonic imaging systems.Then, for the two defects, the main research work in this dissertation is as follows:1) A focus delay parameters compression method combined with dynamic apertureis put forward. On the basis of relative acoustic path difference and dynamic aperturecontrol, the compression storage of focus delay parameters is accomplished by storingthe channel-opening positions, its quantized initial values and values’ variation positions.High precision focus can be achieved by decompressing the stored data with dynamicaperture control in the process of focusing. The data analysis shows that this method greatly improves the focusing delay parameters compression efficiency with thecombination between dynamic aperture and dynamic focusing, and the imaging qualityalso improves. Point-by-point dynamic focus of high precision can be realized easilyin a FPGA with small capacity and low density.2) The study of coded excitation combined with downsampling is carried out.Considering the real-time problem of coded excitation in engineering, complexbaseband pulse compression combined with downsampling (CBPCD) method isproposed. First, the echo spectrum is shifted to high and low frequency components,then the low frequency component is extracted and downsampled to complex basebanddata with the low-pass filter before pulse compression in CBPCD algorithm. A newcomplex baseband compression filter is put forward in compression process. This filtercan be designed by demodulating, low-pass filtering and downsampling the originalpulse compression filter. Finally the imaging data is obtained after pulse compression.The experiments of imaging simulation with the Chirp codes, Barker codes and Golaycodes show that this method is useful for lowering the computational complexity on thebasis of reserving the various characteristics of code signals.