Development of Sound Speed Microscope for Nano-scale Imaging of Biological Tissue Using Pico-second Ultrasonic Pulse

利用超声显微成像技术观察生物组织具有非破坏性、非接触性等优点，可以定量获取声速、衰减等物理参数。我们一直在开发一种用于生物组织的超声波声速显微镜。将薄片样品放置在玻璃基板上，将聚焦的宽带频率的超声脉冲的反射解释为声速等参数。当聚焦光束光斑被机械扫描时，可以观察到2D图像。目前正在考虑将其应用于手术中的组织病理学检查。这些数据对评估临床超声成像的来源也很重要。然而，传统仪器的横向分辨率曾低至15微米，尽管从生物和医学角度来看，更高分辨率的仪器有望带来重要信息。在本研究项目中，我们的目标是研制一种分辨率为亚微米级的超声显微镜。组装了一台使用100-500 MHz宽带脉冲的超声显微镜。为了获得亚微米级的分辨率，采用了压电级。从培养的大鼠海马神经细胞中清晰地观察到强度和声速图像，并阐明了当声波从底物的背面传播时，可以测量到底物与组织交界处的反射波。因此，反射被可视化为声阻抗的二维剖面，以便在不准备薄层标本的情况下观察组织。通过声阻抗图像清晰地观察到一只大鼠小脑组织的结构。

Observation of biological tissue by ultrasonic micro-imaging has advantages of being non-destructive and non-contacting, making it possible to quantitatively obtain physical parameters like sound speed and attenuation. We have been developing an ultrasonic sound speed microscope for biological tissues. A thin sliced specimen is placed on a glass substrate, and the reflection of a focused ultrasound pulse with a wide band frequency is interpreted into sound speed and other parameters. A 2D image can be observed when the focal beam spot is mechanically scanned. Its clinical application to histopathological examination during surgery is being considered. The data are also important to assess the origin of clinical echographic imaging. However, the lateral resolution of conventional one used to be as poor as 15 micrometers, although one with a higher resolution was expected to bring important information in biological and medical point of view. In this research project, we aimed to develop an ultrasonic microscope with a resolution of sub-micron level. An ultrasonic microscope using a wide-band pulse spreading from 100-500 MHz was assembled. A piezo-electric stage was employed in order to obtain a resolution of sub-micron level. Cultured nerve cells taken from the hippocampus of a rat was clearly observed as both intensity and sound speed images.In addition, it was clarified that the reflected wave at the interface between the substrate and tissue can be measured when the acoustic wave was transmitted from the rear side of the substrate. Therefore the reflection was visualized as two-dimensional profile of acoustic impedance, in order to observe the tissue without preparing a thin sliced specimen. The structure of the cerebella tissue of a rat was clearly observed as an acoustic impedance image.