| Summary: | 碩士 === 國立臺灣大學 === 生醫電子與資訊學研究所 === 106 === Photoacoustic (PA) imaging combines light excitation and ultrasound detection. It can achieve a larger imaging depth than that of conventional optical imaging. Recent studies in the literature show that the use of contrast agents can enhance specificity and sensitivity of PA detection. There are different forms of PA imaging. To achieve better spatial resolution, optical resolution PA microscopy (OR-PAM) can be used. However, it has a limited imaging depth. Another approach is the acoustic resolution photoacoustic microscopy (AR-PAM), which can achieve deeper penetration at the price of degraded spatial resolution. In this study, we propose the use of AR-PAM and contrast agents to improve the resolution while maintaining the imagine depth. The proposed method is based on the tracking of the centroid of an isolated particle. By tracking and accumulating the particles, one is able to enhance the lateral resolution. In the first part of this study, experiments were performed for proof of concept by using a 532 nm Nd:YAG laser and a 128 channel ultrasound array system. Microspheres with a size of 53~63um were used as the contrast agent. The experimental results show that this method is capable of resolving two adjacent tubes placed 150um apart, which is beyond the theoretical diffraction limit of the imaging system. In the second part, gold nanodroplets were used as the contrast agent. Nonetheless, instead of the thermal expansion effect that generates the PA signal, the vaporization signal was used to achieve higher SNR. Experimental results show that a concentration of the nanodroplets about 〖10〗^7~〖10〗^8droplets/mL is required for sufficient signal generation. Given that the droplets have to be sufficiently sparse (i.e., concentration lower than 〖10〗^5droplets/mL) for the proposed super resolution approach, it is difficult to achieve a feasible tradeoff between SNR and the sparsity required for super resolution imaging. On the other hand, we also demonstrated that the resulting gaseous phase of the vaporized droplets can also be used as the contrast agent for ultrasound imaging. For future works, we will focus on how to quantify the vaporization effect of gold nano-droplets using this approach.
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