(A) new digital continuous dynamic focusing technique and analysis of delay quantization errors in thereal-time ultrasound imaging systems

Abstract

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 mpa 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 teh 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 a new focusing m...