Research On Method Of Ultrasound Computed Tomography Using Array Detectors

Ultrasound has such characteristics as no ionizing radiation, hurtless in available frequency scale and inexpensive transmitter-receiver equipment, so it has great application potential in nondestructive testing. The ultrasond computed tomography testing is gradually becoming a hotspot in research because it can reflect the inner quality of materials by color or grayscale images.Creating a tomographic device with ultrasound is a new target for researchers.The linear array has some shortcomes such as lack of data,too much interference factor, big error and so on. A method of surround array is proposed according to the shortcomes.The method has two merits:(1) Sensors can be arranged with random shape;(2) The area which ultrasound goes through is more extensive.The acquired data is more .Ignoring the weak diffraction in materials, we introduced the ray theory into the algorithm of ultrasound computed tomography.The linear travel-time interpolation ray tracing algorithm has a drawback: rays can't go round the low velocity area if the launching point, the receiving point and the low velocity area distribute on the same column (row). Aiming at the drawback, an improved algorithm is proposed on the basis of researching reconstruction theories. The given algorithm adopts a cross-scanning strategy to carry out forward processing and uses discrete points as the secondary sources to simplify backward processing. The numerical simulation experiments show that the rays can go round the low velocity areas located in the same column (row) of the launching points and the receiving points. The simultaneous iterative reconstruction techniques is derived through the least squared criterion. The algorithm of four boundaries scanning is proposed to get correlation matrixes.We can acquire the correlation matrix faster and more convenient by judging the relationship between the intersections and the four boundary points of the pixel.The experimental results show that the images can be more precise through SIRT algorithm in the same iterations.In the situation of big relative density about diffraction, a precise wave equation is deduced based on the propagation characteristics of ultrasound. The discretization algorithm about the scattering filed and the whole field is discussed with the method of momen.The expersimental model is designed based on surround array.Whether the model can be regularized is judged with the Picard criterion. The nonlinear of equations is solved with distorted Born iIterative.The ill-posedness of equations is solved with TSVD regularization. As a result, the convergence rate is accelerated. The imaging in spatial domain is no longer based on a approximation.The method can be applied in strong scattering field. The images are clearer. On the basis of wavelet threshold noise reduction, we analyzed the shortcomings about soft and hard thresholding of Dohono.A new wavelet threshold function based on hyperbola is proposed. It can acquire better estimate of wavelet coefficient, overcoming the weakness of uncontinuous hard threshold function and imprecise soft threshold function. A new idea of image segmentation is adopted based on image histogram and region growing.A new algorithm of image segmentation based on fuzzy mathematics and clustering analysis is applied to ultrasound CT images. It better solves uncertain question in the process of image processing.