| Summary: | 碩士 === 國立臺灣大學 === 電機工程學研究所 === 89 === The beamformer is one of the most complicated units in an ultrasound array imaging system. To optimize image quality, dynamic receive focusing is required. Dynamic focusing involves extensive real-time computations and thus the system cost and size can not be reduced unless major redesign of the system is implemented. It is the main purpose of this thesis to explore new design strategies such that complexity of the receiver can be reduced without sacrificing image quality. The thesis consists of three main components:
(I) A new delta-sigma based beamformer is proposed. The new system reduces complexity, size, and cost while maintaining a sufficient signal-to-noise ratio due to the single-bit architecture. A well-known problem in delta-sigma based beamformers is that the system introduces noises during dynamic focusing. Several approaches were proposed to solve this problem at the price of some advantages of delta-sigma modulators. The new method proposed in this thesis is based on the symmetry of focusing using an array transducer. By proper delay-control and selection of the inserted samples during dynamic focusing, no additional noise is added to the beamforming results and the image background level is not increased. Using real ultrasound data, it is shown that the technique offers image quality comparable to a conventional system.
(II) Sound velocity inhomogeneities in the human body produce phase aberrations and degrade image quality. The delta-sigma based beamformer can also be used to correct the aberrations using a correlated-based approach. Results show that the performance is equivalent to the same approach implemented in a conventional system with a greatly simplified system architecture. The different dynamic focusing control schemes discussed in part (I) are also studied in the context of phase aberration correction. It is shown that the delta-sigma based single bit dynamic focusing approach does not affect the overall performance of phase aberration correction.
(III) The third topic is the investigation of dynamic focusing using 2-D arrays. The dynamic focusing architecture becomes more complicated when a 2-D array is used for real-time 3-D imaging. Size and cost of the receiving beamformer becomes enormous if conventional methods are used. In this thesis, a new technique utilizing the spatial and value characteristics of the focusing delay pattern is proposed. It is shown that grouping of delay-change-patterns can reduce the number of delay controllers, and non-uniform quantization of delay-change-patterns can simplify the delay control architecture. Simulation results show that dynamic focusing can be implemented using a simple architecture with a slight degradation in the near field focusing quality.
It is the conclusion of the thesis that the various beamforming architectures proposed in this thesis can be used for size and cost reduction of a digital imaging system, adaptive imaging and real-time 3-D imaging using 2-D arrays. An ASIC-design of the delay controller is also included in the appendix.
|
|---|
