서강대학교

초록/요약

Plane wave (PW) imaging has been widely used for various applications in the field of medical ultrasound imaging. Numerous researches have been published regarding ultrafast ultrasound imaging, blood flow imaging, and transient tissue motion imaging. The PW imaging (PWI) can provide a superior imaging quality by synthesizing multiple PWs steered at different angles. Nevertheless, PWI was mostly considered only as an ultrafast imaging technique. In this thesis, to examine and utilize the superior focusing quality of PWI, PW synthetic focusing (PWSF) is mathematically formulated and its beam pattern is analyzed. Based on the analysis, several methods are proposed to realize and optimize the PWSF to address the current challenges in two-dimensional (2D) and three-dimensional (3D) ultrasound imaging. For the realization of 2D PWSF with a curved array, the number and the angle of the PWs and the transmission and compounding schemes are optimized considering the geometry of transducer, field-of-view, and desired spatial resolution. It is demonstrated by phantom experiments that the optimized PWSF using a convex array provides a higher image quality in terms of resolution, contrast and signal-to-noise ratio even at a higher frame rate compared to the conventional focusing. In addition, on the basis of beam pattern analysis, simple methods for grating lobe reduction are proposed and verified using simulations. For high-precision 3D ultrasound image, aperiodic PW direction patterns are proposed to enhance the contrast of the image. The aperiodic PW patterns allow for the use of fewer PWs for the same contrast and therefore provides faster scanning. Simulation studies show that the aperiodic patterns enhance the contrast of B-mode images by approximately 3–6 dB over all depths. In addition, for the same contrast and resolution, the aperiodic PW patterns require approximately 2 to 6 times fewer PWs than the periodic pattern. Phantom study also demonstrates that the proposed patterns provide approximately 2.3 dB better contrast of images without compromising spatial resolution than the periodic pattern. Compared with conventional focusing and multiline transmit focusing, the proposed methods also provided the improved resolutions and volume rates with comparable contrast. The practical considerations for the implementation of PWSF are also included in the thesis. Baseband in-phase/quadrature beamformer (IQBF) is investigated for a realization of the massive beamforming for PWSF. Optimization method of delay resolution in IQBF is also suggested to minimize the implementation cost without compromising the focusing quality.