Ultrasonic Tomographic Detection Methods Based On Ring-shaped Array

Ultrasound imaging has been widely used in industrial and medical detection area due to its non-radiative characteristic and the popular prices of the detection equipment. The tomographic detection technique can be used to reconstruct the structure of the tested objects by detecting their scattering signals. Its visualized images allow the non-specialists identify the un-known tested objects and this technique is becoming a hot topic gradually.In order to meet the needs of industrial detection of the two-phase flow and the biological tissue imaging in weak scattering conditions, 4 types of reconstruction algorithms which have potential applicable values in the corresponding areas are presented. Fristly, the advantages of the immersion ring-shaped array ultrasonic imaging model is presented in this paper. Not only the detection range of the effective signals(especially in the condition of wide beam spread angle of the transducer) can be extended by this model,but also the better image quality can be obtained by altering the number and the combination of the array elements according to the different objects. After that, on the basis of the wave theory, Comsol Multiphysics® software is adopted to study the acoustic propagation characteristics under the circumstances of the gas/liquid and liquid/liquid coupling. According to the characteristics of the scattering wave from different tested objects, the effective detection algorithms are specified. The imaging processes under the condition of gas/liquid or solid/liquid two-phase coupling are studied and analyzed. At last, the data acquisition equipment based on immersion ring-shaped array is established by combining the phased array pulse transceiver and the self-designed flexible transducer array. The images of the solid materials immersed in the water and the bubbles suspended in the liquid ultrasonic couplant are reconstructed successfully. The main research contents of this paper are as follows:The dynamic simulations of ultrasonic propagation in gas / liquid, liquid / liquid condition are conducted. The different effects of the size and material properties of the tested objects are analyzed and the imaging algorithms for the corresponding case are specified inspectively. After that, the theory of ultrasonic tomographic imaging is studied. The reconstruction methods based on the ray theory and the mathematical expressions of the scattering sound field based on the wave theory are derived step-by-step in detail. From these derivations, the applicable conditions of various algorithms studied in this dissertation are given.For the case of great acoustic impedance difference between the tested objects and the coupling medium, the traditional elliptic and hyperbolic positioning algorithms based on reflected and refracted wave are improved by introducing the principle of all-time-domain waveform back-projection. This principle is successfully integrated with the elliptic and hyperbolic algorithms. Several bubbles in the gas/liquid condition and solid ojbects with different shapes, positions and sizes in the solid/liquid condition are imaged by the two new algorithms. A localized signal probabilistic algorithm is presened. This algorithm can be used to reconstruct the corss-section image of the two-phase couping mixture. As for the significant phase shift between the detection signals and the reference signals, the localized signal difference coefficient is employed to realize the probabilistic algorithm in transmission-mode.For the case of minor acoustic impedance difference between the tested objects and the coupling medium, a back-projection reconstruction algorithm based on the ultrasonic slowness difference is presented. Multi-physics sound field propagation simulations are conducted in the weak scattering condition and the images of the biological materials are reconstructed through transmission-mode. Cross-correlation calculation is used here to improve the image quality.In the end, all-time-domain waveform elliptic algorithm, hyperbolic algorithm and the probabilistic algorithm are applied respectively to reconstruct the cross-sectional images of the solid objects immersed in the water and the bubbles suspended in the liquid ultrasonic couplant, thus, the feasibility of the detection model and the imaging algorithms presented in this paper are verified. During this process, the effects on image reconstruction resulted from different numbers and different combinations of the elements are discussed while the comparison reconstruction images are given. The resolving power, minimum detection value, detection coverage area and the corresoponding detection sensitivity curves are all demonstrated.