Development of photoacoustic imaging with near-infrared pulse laser to improve depth resolution

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Commercial imaging systems, such as computed tomography (CT) and magnetic resonance imaging (MRI), are frequently used powerful tools for observing structures deep within the human body. However, they cannot precisely visualized several-tens micrometer-sized structures for lack of spatial resolution. In this research, we proposed a multiphoton excitation-assisted photoacoustic tomography (MEAPAT) as a means of improving depth resolution. Since the multiphoton absorption occurs at only the focus point and the employed infrared pulses deeply penetrate living tissues, it enables us to extract characteristic features of structures embedded in the living tissue. When nanosecond pulses from a 1064-nm Nd:YAG laser were focused on Rhodamine B solution (absorption peak: 540 nm), the peak intensity of the generated photoacoustic signal detected by a 10 MHz transducer was proportional to the square of the input pulse energy. This result shows that the photoacoustic signals can be induced by the two-photon absorption of infrared nanosecond pulse laser and also can be detected by a commercial low-frequency MHz transducer. This means that MEAPAT overcomes the depth limitation of existing one-photon photoacoustic tomography caused by the need of a high-frequency transducer to improve depth resolution. Furthermore, in order to evaluate the depth resolution of MEAPAT, we compared the image of MEAPAT with that of one-photon photoacoustic imaging of a vascular phantom in a water bath. As a result, we found that the depth resolution of MEAPAT (1064 nm) is greater than that of one-photon photoacoustic imaging (532 nm).We conclude that evolving multiphoton-photoacoustic imaging technology renders feasible the investigation of biomedical phenomena at the deep layer in living tissue.