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

商业成像系统，如计算机断层扫描（CT）和磁共振成像（MRI），是经常使用的强大工具，用于观察人体深处的结构。然而，由于缺乏空间分辨率，它们不能精确地显示几十微米大小的结构。在这项研究中，我们提出了一种多光子激发辅助光声层析（MEAPAT）作为提高深度分辨率的手段。由于多光子吸收仅发生在焦点处，并且所采用的红外脉冲深深地穿透活组织，因此它使我们能够提取嵌入活组织中的结构的特征。当纳秒脉冲从1064 nm的Nd：YAG激光器聚焦在罗丹明B溶液（吸收峰：540 nm），所产生的光声信号的峰值强度检测到的10 MHz的换能器是成比例的输入脉冲能量的平方。这一结果表明，红外纳秒脉冲激光的双光子吸收可以产生光声信号，也可以用商业低频MHz换能器检测。这意味着MEAPAT克服了现有的单光子光声层析成像的深度限制所造成的需要的高频换能器，以提高深度分辨率。此外，为了评估MEAPAT的深度分辨率，我们比较了在水浴中的血管体模的MEAPAT与单光子光声成像的图像。结果表明，MEAPAT的深度分辨率（1064 nm）高于单光子光声成像的深度分辨率（532 nm），说明多光子光声成像技术的发展为研究活体组织深层生物医学现象提供了可能。

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.