| Ultrasound imaging has been widely used in medical area for its safety, convenient, real time and low cost. Ultrasound imaging algorithms play a key part in the whole ultrasound imaging process, and the beamforming technique is the core role in ultrasound imaging algorithm. The minimum variance(MV) dynamically calculates the weighted vectors on the basis of the received data. It can maintain a good resolution as well as suppressing the side lobe level. The synthetic transmit aperture could realize dual focusing both in the transmitting and receiving process, such the spatial resolution is improved greatly. Both of the two methods can improve the image quality, still some defects lie in them:(1) The minimum variance beamforming has its own limit in improving image quality, such as the resolution and contrast ratio(CR). What’s more, its robustness against the noise is low.(2) The synthetic transmit aperture would retard the time to obtain the data when using all elements to transmit and receive pulse echo, which makes it hard to meet the criteria of fast imaging.For the purpose of overcoming the abovementioned flaws, this thesis has made the following study:(1) In order to improve the image resolution, CR and the robustness against the noise of MV, here we proposed two beamforming algorithms, which is the eigenstructure generalized side lobe canceller beamforming(ES-GSC) algorithm and the ES-GSC combining with the sign coherence factor(ESGSC-SCF) algorithm.The ES-GSC beamforming algorithm is constructed according to the minimum variance principle. Firstly, the canceller divides the weighting vector into non-adaptive and adaptive weights two parts, then the eigenstructure subspace is established according to the covariance matrix of the received signal, and the renewed weighting vector is achieved at last by projecting the weighting vector into the left singular space of the eigenstructure subspace. Both the simulation and experiment results show that the ES-GSC could effectively enhance the image quality.The ESGSC-SCF algorithm processes is as follows: obtaining the ES-GSC weighted vector first, then refer to the polarity of the received data to calculate the SCF weighted vector according to variance principle, combining the ES-GSC and SCF at last to obtain the final weights. Both the simulation and experiment results indicate the combined method can improve the image quality further.(2) A modified sparse synthetic transmit aperture image algorithm is proposed. This method is based on the principle of effective aperture, use selected elements instead of the full array to fire ultrasound signal. Appropriate weighting process is added to the received data to obtain the equivalent effective aperture, thus the point spread function(PSF) in the far field area is identical. Second weighting process with SCF can be used to eliminate the artifacts. The simulation has demonstrated that the proposed method can obtain the image quickly, after combining with the SCF, the image quality can be improved a lot without slowing the imaging speed. |
