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 principal, a map of the backscattering efficiency from every point in the imaging volume. But a conventional B-scan system can provide only a crude approximation to a backscattered image due to (a) diffraction effect (b) non-linear propagation effect (c) B-scan system artifact.
The major limiting factor on the use of the B-scan system is the poor resolution, especially along the lateral direction caused by a finite transducer aperture size. To improve the resolution, a number of studys have been reported to date including linear inverse filtering, non-linear filtering, transducer design, and focusing methods. Perhaps, dynamic focusing may be the most practical approach in real time imaging.
The non-linear propagation effects can alter the backscattering efficiency estimation in a B-scan system. To get a quantitative B-scan image, all the phenomena that can alter the efficiency estimation should be taken into account. The propagation effects encountered in a diagnostic ultrasound are: inhomogeneous attention, beam softening by frequency dependent attention, and different velocity distribution in tissue.
The B-scan system also changes the image estimation. In a digital sector scanner, a number of artifacts occurs during the coordinate transformation, known as scan conversion. The reduction of the artifact is very difficult because of the real time requirement.
In this thesis, we propose several new methods to improve the B-scan performance. In part Ⅰ, a new focusing method, to be called the sampled delay focusing (SDF), is proposed to improve the B-scan resolution. In SDF, sampling-sum process can replace the conventional delay-sum-sampling process, and thus the analog delay lines are no longer necessary. This algorithm offers continuous dynamic focusing on the resolution cell basis with greatly reduced hardware.
In part Ⅱ, we discuss the scan conversion algorithm for a digital sector scanner. To eliminate the artifacts related to the coordinate transformation, we proposed a quite different algorithm, namely uniform-ladder algorithm. Since the sampling space is the same as the display space in ULA, the artifact concerned with the scan conversion can be removed. The implementation requires only two frequency synthesizers and a low pass filter.
In part Ⅲ, the non-linear propagation effects are investigated. Among the non-linear parameters, estimation of the frequency dependent attention is the most important. In this thesis, we suggest a new estimation procedure based on Wiener, Kalman, and median filtering. The simulation and experimental results shows a better performance compared to the previous algorithms.
초음파 영상 장치중 PULSE-ECHO 방식의 B-SCAN 시스템은 가장 성공적으로발전해 왔다. 그러나 낮은 해상도, 비 선형적 진행효과 및 시스템 구성상의 제문제가 남아 있다. 본 논문에서는 이를 향상시키기 위하여 주력하였으며, 크게 세가지 분야에서 새로운 제안을 하였다. 첫째 초음파 집속 방식에서 기존의 지연-합-샘플 방식에서 문제가 되어온 아나로그 지연선을 제거하는 샘플-지연 방식을 제안하였으며 모든 깊이에 대한 연속 집속이 가능하게 되었다.
둘째 섹터형 초음파 B-SCANNER에서 문제가 되어온 내삽 방식을 확기적으로 개선하는 균일사다리 방식을 제안하였다. 제안된 방식은 내삽 오차가 거의 없으며 실현 회로가 매우 간단하다.
마지막으로 비 선형 진행 효과중 가장 문제가 되는 감쇄 계수를 측정하는 새로운 방법으로서 최소 평균자승 여파기 및 중간값 여파기를 사용한 방식을 제안하였으며 기존 시스템에 비하여 우수한 특성을 보여 주고 있다.
