(A) study on resolution improvements in the ultrasonic B-scan imaging system

Abstract

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 dircetion 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 i...