(A) study on grating lobe elimination in synthetic focusing imaging and its application to three dimensional ultrasonic imaging system

Abstract

The ultrasonic synthetic focusing(SF) is the best way to improve the azimuthal resolution, and it requires relatively simple data acquisition hardware comparing to the dynamic focusing. But it suffers from grating lobes(GLs) which results in unpredictable artifact in final image. And when a two-dimensional(2-D) phased array is used in the 3-D imaging system, the complexity of the front-ends is proportional to the total number of array elements. For example, if we use 64 by 64 2-D phased array, the system requires 4096 front-ends, which is not realizable size. In the first part of this dissertation, the generalized SF schemes, (M, N, P), are mathematically formulated and strict sense GLs-free conditions are derived. The conditions require that the inter-element spacing must be less than equal half wavelength of driving pulse and the number of receiving elements, N, must be integer multiple of shifting elements, P, i.e., N= 2nP. But the first condition can be relaxed according to the level of GLs and the types of imaging application. The conditions imply that the generalized schemes can increase the data acquisition rate by increasing the P. The simulational and experimental results show the scheme can speed up the data acquisition rate without degradation of the resolution as well as eliminate the GLs effectively. In the second part, we propose new 3-D imaging scheme to adopt the generalized SF schemes to reduce the complexity and to increase the data acquisition rate. It is mathematically formulated and verified by simulations. In the simulations 3-D phantom is defined and it is reconstructed by the proposed scheme in different view angles. A real-time hardware architecture is designed to speed up the synthetic focusing procedure and the timings are verified using currently available commercial devices. It shows that the architecture accomplishes the 32-MHz sampling rate.