B-scan is the most widely used modality in current medical ultrasound imaging. The visuality of the B-scan image is primarily determined by the spatial resolution which is mainly dependent on the focusing capability. In all the commercial B-scan equipments using analogue delay-line system for focusing, however, the poor resoluion especially along the lateral direction has known to be the major limiting factor. The conventional systems can provide only one or few focal points, which is in practice undesirable by the fact that there exists direct conflict between the lateral resolution and the depth-of-focus in a spherically lensed system.
Hence, there have been many works for carrying out line focusing which produce a relatively narrow beam width over a large depth of field. Among these efforts, the most successful approaches to maintain a good lateral resolution over a large depth of field in real-time imaging situations, may be to carry out dynamic focusing in receive, which requires too complex circuitry for delay controls to be realized in the conventional analogue imaging system. As another approach, some investigators have described nonspherical lenses such as a cone and a horn which can provide transmit line focusing but can not be used as both the transmitter and the receiver due to the resulting high sidelobe levels.
In this thesis, a new focusing method for ultrasonic annular array imaging systems is presented. The technique minimizes the lateral resolution for a specified sidelobe suppression over the depth of interest, using a nonspherical lens for line focusing in transmit in combination with continuous dynamic focusing in receive. For this purpose, a method is presented for efficient calculation of the wideband ultrasound field distribution generated by a circular transducer. The approach is based on the derivation of a closed-form solution for the impulse response to reduce the computation time, in contrast to the direct numerical solution which involves a double numerical integration. The closed-form impulse response is derived under the paraxial assumption for a lensed aperture, not only in the focal plane but also in the nonfocal plane, and an exact on including the obliquity factor is derived for a planar aperture. To reduce the approximation error, the paraxial assumption is applied only to the phase term but not to the magnitude term of the diffraction formula. The validity of this paraxial approximation is examined and the procedure of field calculation is presented in detail. By computer simulations using the proposed approach, the effects of delay and array quantizations on focusing and amplitude apodizing for an annular array are investigated. The results suggest how to effectively control the phase and amplitude of the annular array transducer for the sake of reducing the quantization errors.
Using this analysis method, an iterative technique is than applied to obtain an optimum transmit delay profile when continuous receive focusing is employed at all imaging points along the depth of view. The delay profile is optimum in the sense that the lateral beam width is minimized under a specified sidelobe suppression at three depths of the imaging field. Experimental results show the validity of the analysis model and the suggested optimization techniques. The continuous receive focusing is realized in a new digital focusing which enables arbitrary control of transmit time delays and dynamic focusing in the receive mode.
The system employs the Pipelined-Sampled-Delay-Focusing (PSDF) scheme to eliminate the bulky memory addressing and interpolation circuits which are needed in conventional digital imaging systems. To reduce hardware requirements and the bandwidth required for digital processings, we describe an efficient method to generate the sampling clocks for dynamic focusing and some modified bandwidth sampling techniques to reduce the sampling rate for signal digitization. The system can also be applied to real-time sector scanning to achieve dynamic focusing and steering simultaneously.
본 논문은 임상용 초음파 B-스캔 영상장치의 가장 중요한 문제인 측방향 해상도 증대를 위한 새로운 방법에 관한 연구이다. 측방향 해상도 증대를 위하여 수신시에 연속적인 빔집속을 수행하면서 영상을 얻고자 하는 모든 영역에서 sidelobe를 어느 값 이하로 억제하면서 측방향 빔폭을 최소화하는 송신의 지연 패턴을 최적화 방법을 사용하여 계산하였다. 새로운 초음파 집속방식은 실험을 통하여 기존의 집속방식보다 우수한 특성을 가지고 있음이 증명되었다.
이를 위하여 모든 관찰점에서의 초음파 음장의 크기를 계산할 수 있는 새로운 음장 해석 방법을 제안하였으며 수신시의 연속적 빔 집속을 위한 새로운 구조의 디지털 빔 집속장치를 설계하였다. 본 논문에서 제안된 음장해석 방법은 광대역 초음파 여기 시스템의 모든 가능한 집속방식에 의한 모든 관찰점에서의 음장계산을 할 수 있다. 새로운 시스템은 이 분야의 가장 중요한 문제인 양자화 주파수와 회로의 복잡도를 크게 줄일 수 있는 구조를 가지고 있다. 특히, 섹터 스캔을 위한 phased array 시스템에 적용하여 빔의 스티어링(steering)과 연속집속을 동시에 수행할 수 있다는 것을 실험을 통하여 확인할 수 있었다.
