Study On Beamforming Of Digital Ultrasound Imaging System

The modern medical ultrasound imaging beamforming algorithms directlydetermine not only the system’s structural design, but also the quality of imagingsystems. Therefore, as soon as possible to come up new and efficient imagingbeamforming algorithms, will significantly increase the overall quality of domesticmedical ultrasound equipment.The traditional and conventional delay-and-sum beamfoming is the simplest andused most widely in ultrasound imaging. While the imaging mode based on traditionaland conventional delay-and-sum beamfoming limits the further improvement of theimaging quality, the reasons are as follows:①When imaging with the single point-to-point beamforming method, because ofthe lower imaging frame rate and the large storage capacity of point-to-point focusingdelay parameters, it is difficult to realize high-speed and high-precision point-to-pointfocusing.②The resolution of image is always decreasing with respect to the increasing ofinvestigation depth when using traditional delay-and-sum beamforming, viz thecontradiction between the resolution and investigation depth cannot be effectivelyresolved. The weightings of apodization are fixed window function values. Afterapodization, the side-lobe level was decreased while the lateral beam width wasincreased, the lateral resolution became worse.Then, for the two problems in the beamforming process of ultrasound imaging, themain research works in this dissertation are as follows:①A compression algorithm for four-beam high-precision point-to-point focusingdelay data is proposed. Firstly, the calculation of four-beam focusing delay data issimplified to that of single-beam focusing, and then the delay data are decomposedquantified, compressed and stored. At last, in point-to-point receive focusing, absolutefocusing delay data can be generated by decompressing the stored data for each channel.Performance analysis shows this method can improve the image frame rate andsignificantly reduce the storage of delay parameters.②The virtual element dual beamforming and its fusion with adaptive weightingand Chirp-coded are carried out. Considering the resolution and investigation depthdeficiency, lower contrast and noise immunity of traditional delay-and-sum beamforming, the new algorithms are proposed, such as Dual focusing beamformingbased on virtual element(DFB) and Robust dual focusing ultrasonic imaging algorithmbased on Chirp-coded and adaptive beamforming fusion(CARDFB).DFB algorithm: Firstly, single fixed focus point was used in the first focusing;and then the delay parameters used in the second focusing were calculated with theconcept of virtual element; in the end the second focusing was carried out bydelay-and-sum point-to-piont focusing to obtain the imaging data. The simulation resultindicates that DFB can resolve the contradiction between resolution and investigationdepth in some extent and effectively improve the imaging quality.CARDFB algorithm: Firstly, using Chirp-coded technology to improve theemission energy and filtering the received echo signals with matched filter; and thengenerating real-time weighting values with adaptive beamforming algorithm; in the end,the high quality beamformed signals were obtained by dual focusing beamformingbased on virtual element. The experimental results show that CARDFB has high lateralresolution, good main-to-side lobe ratio and robustness to noise compared to traditionalultrasonic imaging algorithm.