Research And Implementation Of Broadband Beamforming For Medical Ultrasound

Medical ultrasound imaging has become one of the important clinical diagnosis methods for human soft tissue and flow state tissue with merits, such as safe noninvasive, real time, low cost and portable, etc. As one of the key technologies of medical ultrasound imaging system, beamforming method affects the quality of imaging quality directly. Although the application of dynamic focusing, dynamic aperture and dynamic Apodization has improved the beam characteristics of traditional delay and sum beamformer technology, however, the side lobe of the synthesized beam is still high and the main lobe is wide, which ultimately result in insufficient of spatial resolution and contrast resolution of the images. Based on the problem of the high side-lobe caused by the deficiency of dynamic focusing in the digital time delay precision, and the issue of broad main-lobe caused by traditional window function apodization, the main work of this dissertation are as follows:1. The basis of the ultrasound imaging system and its performance evaluation, and the theory of the beamforming and three different implementation methods are introduced. The relationship between the time delay precision and the imaging performance, and the broadband beamforming are also discussed.2. Variable fractional delay Farrow filter is introduced for dynamic focusing. Three different methods of designing prototype fractional delay filter are evaluated by computing and comparing the proposed error parameters for each design method. Finally, the effect on ultrasound beamforming caused by the variable fractional delay Farrow filter is analyzed through simulation.3. Based on the criterion of general cystic resolution metric, the constrained least squares(CLS) optimal apodization method is employed to design the coefficient vector of broadband FIR beamformer. The simulation results show that the proposed broadband FIR can achieve lower side lobe and narrower main lobe at the same time.4. The implementation of broadband beamformer based on FPGA is presented. The structure of the beamformer and dual input Farrow filter are studied. Improved methods for implementation of Farrow filter and realization for broadband apodization technology are presented, which effectively reduce the usage of multiplier resources.