The ultrasound B-scan system is the most successful imaging system in the field of diagnostic ultrasound and nondestructive testing. The B-scan system offers real time tomographic imaging at relatively low cost and does not produce any harmful effects to the patient. The B-scan is, in principle, a map of the back scattering efficiency from every point in the imaging volume. But a conventional B-scan system can provide only a crude approximation to a back scattered image due to 1) diffraction effect, 2) nonlinear propagation effect, and 3) B-scan system artifact.
The major limiting factor on the use of the B-scan system is the poor resolution of the imaging, especially along the lateral direction caused by a finite transducer size. To improve the resolution, a number of studies have been reported including linear inverse filtering, nonlinear filtering, transducer design, and various focusing methods. Perhaps, dynamic focusing may be the most practical approach in real time imaging. However, employing dynamic focusing in phased array system has many problems to overcome.
One of the most inherent problem is that the delay function should be quantized. The resultant quantization error is one of the troublesome problems in ultrasonic imaging system. Until now, the results of previous studies examining the effects of the delay function quantization error in analytic fashion can only be applied in narrow band system. But the B-scan phased array system used in medical imaging is wide band system, hence we need a new theory to examine the delay function quantization error for the wide band phased array imaging system.
And the other problem is the implementation of delay function. Because in a phased array system, continuous dynamic focusing combined with steering operation caused the maximum delay time be some tens of microseconds, while the minimum tap of delay function should be less than several nanoseconds.
In this thesis, first, we have developed a new focusing method, to be called the pipelined-sampled-delay-focusing (PSDF), is proposed. In the recently proposed sampled-delay-focusing (SDF) process which completely eliminates the use of analog L-C delay lines for the beam focusing in the ultrasound B-mode imaging systems, the product of sampling rate and maximum time delay is required to be less than unity. To remove this constraint, we propose first-in-first-out pipelining technique in this thesis, whereby it is now possible in the sector scanning system to perform beam steering and dynamic focusing simultaneously on the resolution-cell basis and in a digital fashion without the use of analog L-C delay lines.
Second, a new theory of quantitative analysis of the effect of delay function quantization error. It is based on the time domain analysis technique, while conventional theory is based on the frequency domain. From the result of the proposed theory, good estimation of performance degradation due to delay quantization can be obtained in wide band phased array imaging system.
초음파 영상 장치중 Pulse-Echo 방식의 B-SCAN 시스템은 가장 성공적으로 발전해 왔다. 그러나 낮은 해상도, 비선형적 진행효과 및 시스템 구성상의 제 문제가 남아 있다.
본 논문에서는 이를 향상 시키기 위하여 주력하였으며 다음의 세가지 문제에 관하여 새로운 제안을 하였다. 첫째, 초음파 집속방식에서 기존의 지연-합-샘플 방식에서 문제가 되어온 아나로그 지연선을 제거하는 샘플-지연-합 방식의 제한조건을 파이프라인 개념을 도입하여 해결하였으며 모든깊이 및 관찰 각에 대한 연속집속이 가능하게 되었다.
둘째, 연속집속시 지연시간의 양자화에 의한 오차분석을 시간영역에서 행하였으며 마지막으로 첫번째의 파이프라인 방식의 연속집속을 할수있는 시스템에 있어서 성능에 큰 변화를 주지않고 동작주파수를 가능한한 낮추어 줄수 있는 방법을 제안하였다.
