| Summary: | 博士 === 國立臺灣大學 === 電子工程學研究所 === 103 === Ultrasonic imaging system provide diagnostic information like tissue images and blood velocity. Compared to other medical imaging systems such as X-ray, computed tomography and magnetic resonance imaging, ultrasonic imaging system has features like non-invasive, non-radioactive, low cost, high frame rate and portable. With the progress of VLSI technique, portable ultrasound imaging systems have become a trend for tens of years. They are easily carried to where patients are, which largely decrease the inconvenience and pain for patients. Moreover, portable systems become more indispensable in supporting immediate diagnosis for emergency rescue to increases the survival rate.
Currently, beamforming is the main applications in ultrasound system to generate the B mode imaging which expresses the power of the received echo. Since the natural characteristics of the propagation wave, beamforming plays a vital role to focus the received echoes. The oldest but the most important beamforming method is to delay and sum for received echoes alignment. Since DAS beamformer has a wide main lobe and higher sidelobe levels, adaptive minimum variance directionless response (MVDR)-based beamformers, or also known as MV-based beamformers, are proposed to enhance the image quality of ultrasound imaging. However, it is not suitable for MV beamformer to implement in ultrasound imaging system due to its high computational complexity. Therefore, how to design a low-complexity MV beamformer become an important research issue.
Beamforming could be roughly classified into real aperture and synthetic aperture. Compared to real aperture, synthetic aperture is more suitable in high frame rate ultrasound imaging system due to lower complexity and cost. However, the output image of synthetic aperture is formed with accumulating series of low resolution images (LRIs) which is obtained from multiple probes, therefore it is susceptible to motion, which will cause the inhomogeneous LRIs. In the normal clinics, motion can be reduced by holding breath or compensated by existing off-line algorithm. However, when the system is used on the ambulance, battlefield, or children, motion is difficult to avoid and it will degrade the image quality severely.
There are three main topics in this work. In the first part, the low-complexity MV beamformer is proposed to reduce the computational complexity of traditional MV beamformer. We applied Approximate Sample Covariance Matrix (ASCM) and the matrix inversion lemma to derive a new formula to perform MV beamforming without computing matrix inversion. Compared with traditional MV beamformer, the proposed method reduce the computational complexity from O(L3) to O(L) with similar image quality. In the second part of this work, a low-complexity two-dimensional motion compensation algorithm is proposed. The proposed method can reduce computational complexity significantly by geometry characteristics of synthetic transmit aperture, and generate high quality images. In the third part, a low-complexity linear array delay-and-sum architecture is proposed. The hardware of the proposed algorithms is also implemented in 90 nm technology. The implementation results of beamforming engine have 42 fps (frames per second).
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