Multilayer films and their applications in all optical ultrasound/photoacoustic imaging

• Main Authors: Shi-Yao Hung, 洪士堯
• Other Authors: Pai-Chi Li
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/99554729418701838540

碩士 === 國立臺灣大學 === 生醫電子與資訊學研究所 === 102 === Multimodality ultrasound (US) and photoacoustic (PA) imaging has received wide research attention for biomedical research. It is an efficient way to combine two complementary imaging mechanisms within one single system. Nevertheless, like many other multimodality imaging systems, generally different energy sources and detectors need to be used. Thus, image registration and temporal alignment become a critical issue limiting the imaging performance in certain applications. In this study, we propose a new imaging method that requires only a single laser pulse to concurrently perform US and PA imaging. Specifically, we propose a thin film with multiple optically absorbing layers which is also partially transparent to light. With this multilayer thin film, the transmission light can be used to perform PA imaging, whereas the absorbed light energy can generate US for US imaging. As the PA and US images are created by the same laser pulse, the image registration and temporal alignment problems practically no longer exist. In our previous study, a similar approach was taken but with a single layer film. Because of the broadband nature of the generated US, the US signal and the PA signal are spectrally overlapped, thus making it difficult to be separated. By using the multilayer film proposed in the current study, the generated US signal has a relatively narrower bandwidth and thus it is spectrally separable from the PA signal. Characteristics of the generated US signal can also be tuned by adjusting the optical absorption coefficient of light-absorbing layers as well as thickness of the layers. In our designs, the US signals generated by the multilayer films typically have the center frequency ranging from 14 to 28MHz. The bandwidth is typically around 30%, compared to the 100% bandwidth from the single layer films. The PA signal, on the other hand, generally is most sensitive around 10MHz or lower for biological tissues. Thus, the US signal and the PA signal can be separated using a filter at the receiver end. In addition, when using a microring to detect both the US signal and the PA signal, the US and PA imaging system becomes all optical. A thin film phantom and a cyst-like phantom were used to test imaging performance of this approach. The feasibility is demonstrated. The lateral resolution and SNR can be further improved by applying the synthetic focusing technique.